How to change the valve timing of a two-stroke engine. Types of purging the combustible mixture of an internal combustion engine, the basics of the structure and operation of boat engines of watercraft, how a sports vessel is arranged, repair of a boat, repair of a watercraft, as sd

Engine quality internal combustion car depends on many factors, such as power, efficiency, cylinder volume.

The valve timing is of great importance in the engine, and the efficiency of the internal combustion engine, its throttle response, and the stability of idle speed depend on how the valves overlap.
In standard simple engines, timing changes are not provided, and such motors are not highly efficient. But recently, more and more often on cars of advanced companies such as Honda, Mercedes, Toyota, Audi, power units with the ability to change the displacement of the camshafts as the number of revolutions in the internal combustion engine change.

Valve timing diagram of a two-stroke engine

A two-stroke engine differs from a four-stroke engine in that its operating cycle takes one revolution of the crankshaft, while on a 4-stroke internal combustion engine it takes two revolutions. The gas distribution phases in the internal combustion engine are determined by the duration of the opening of the valves - exhaust and intake, the angle of valve overlap is indicated in degrees of position to / in.

In 4-stroke engines, the cycle of filling the working mixture occurs 10-20 degrees before the piston reaches top dead center, and ends after 45-65 degrees, and in some ICEs even later (up to one hundred degrees), after the piston has passed bottom point. The total intake time in 4-stroke engines can last 240-300 degrees, which ensures a good filling of the cylinders with the working mixture.

In 2-stroke engines, the duration of the intake of the air-fuel mixture lasts approximately 120-150º at the turn of the crankshaft, the purging also lasts less, therefore filling with the working mixture and cleaning exhaust gases two-stroke internal combustion engines are always worse than 4-stroke power units. The figure below shows a diagram of the valve timing of a two-stroke motorcycle engine of the K-175 engine.

Two-stroke engines are rarely used on cars, since they have lower efficiency, worse efficiency and poor cleaning of exhaust gases from harmful impurities. The last factor is especially relevant - due to the tightening of environmental standards, it is important that the engine exhaust contains a minimum amount of CO.

But still, 2-stroke internal combustion engines have their own advantages, especially in diesel models:

• power units are more compact and lighter;
• they are cheaper;
• a two-stroke motor accelerates faster.

On many cars in the 70s and 80s of the last century, carburetor engines with a "trambler" ignition system were mainly installed, but many advanced car manufacturing companies already then began to equip motors with an electronic engine control system, in which all main processes were controlled by a single block (ECU). Now almost all modern cars have ECMs - the electronic system is used not only in gasoline, but also in diesel ICEs.

In modern electronics, there are various sensors that control the operation of the engine, send signals to the unit about the status power unit... Based on all the data from the sensors, the ECU decides how much fuel should be supplied to the cylinders at certain loads (revolutions), what to set the ignition timing.

The valve timing sensor has another name - the camshaft position sensor (DPRV), it determines the position of the timing relative to the crankshaft. It depends on its readings in what proportion the fuel will be supplied to the cylinders, depending on the number of revolutions and the ignition timing. If the DPRV does not work, it means that the timing phases are not controlled, and the ECU does not "know" in what sequence it is necessary to supply fuel to the cylinders. As a result, fuel consumption increases, since gasoline (diesel fuel) is simultaneously supplied to all cylinders, the engine runs erratically, and on some car models the internal combustion engine does not start at all.

Camshaft adjuster

In the early 90s of the 20th century, the first engines with automatic timing change were produced, but here it was no longer the sensor that controlled the position of the crankshaft, but the phases themselves were shifted directly. The principle of operation of such a system is as follows:

• the camshaft is connected to a hydraulic clutch;
• also with this clutch has a connection and a camshaft;
• at idle and low speeds, the camshaft gear with a camshaft is fixed in the standard position, as it was installed according to the marks;
• with an increase in speed under the influence of hydraulics, the clutch turns the camshaft relative to the sprocket (camshaft), and the timing phases shift - the camshaft cams open the valves earlier.

One of the first such developments (VANOS) was applied on BMW M50 engines, the first engines with variable valve timing appeared in 1992. It should be noted that at first VANOS was installed only on the intake camshaft (M50 engines have a two-shaft timing system), and since 1996, the Double VANOS system was used, with which the position of the exhaust and intake p / shafts was already adjusted.

What is the advantage of the timing controller? At idle, overlap of the valve timing is practically not required, and in this case it even harms the engine, since when the camshafts shift, exhaust gases can enter the intake manifold, and some of the fuel will enter the exhaust system without burning out completely. But when the engine is operating at maximum power, the phases should be as wide as possible, and the higher the rpm, the more valve overlap is necessary. The timing change clutch makes it possible to effectively fill the cylinders with the working mixture, which means to increase the efficiency of the motor and increase its power. At the same time, at idle speed, the r / shafts with the clutch are in their original state, and the combustion of the mixture is in full. It turns out that the phase regulator increases the dynamics and power of the internal combustion engine, while fuel is consumed quite economically.

The variable valve timing system (CIFG) provides lower fuel consumption, reduces the level of CO in the exhaust gases, and allows more efficient use of the power of the internal combustion engine. Different world automakers have developed their own CIFG, they apply not only the change in the position of the camshafts, but also the level of valve lift in the cylinder head. For example, Nissan uses CVTCS, which is controlled by a variable valve timing valve ( solenoid valve). At idle, this valve is open and does not create pressure, so the camshafts are in their original state. An opening valve increases the pressure in the system, and the higher it is, the more the camshafts are displaced.

It should be noted that SIFGs are mainly used on engines with two camshafts, where 4 valves are installed in the cylinders - 2 inlet and 2 outlet.

Camshaft timing accessories

In order for the engine to work without interruption, it is important to correctly set the timing phases, to set the camshafts in the desired position relative to the crankshaft. On all engines, the shafts are set according to marks, and a lot depends on the accuracy of the installation. If the shafts are not aligned correctly, various problems arise:

• the motor runs unstable at idle;
• ICE does not develop power;
• there are shots at the muffler and pops in the intake manifold.

If you mistake a few teeth in the marks, it is possible that the valve may bend and the engine will not start.

On some models of power units, special devices have been developed for setting the valve timing. In particular, for engines of the ZMZ-406/406/409 family, there is a special template with which the camshaft angles are measured. The template can be used to check the existing corners and if they are not aligned correctly the shafts must be reinstalled. The attachment for 406 motors is a set consisting of three elements:

• two protractors (for the right and left shaft, they are different);
• protractor.

When the crankshaft is set to TDC of the 1st cylinder, the camshaft cams should protrude above the top plane of the cylinder head at an angle of 19-20 ° with an error of ± 2.4 °, and the intake shaft cam should be slightly higher than the exhaust camshaft cam.

There are also special devices for installing camshafts on BMW motors models M56 / M54 / M52. The kit for installing the valve timing of the internal combustion engine BVM includes:

Malfunctions of the variable valve timing system

It is possible to change the valve timing in various ways, and recently the most common rotation of the p / shafts, although the method of changing the amount of valve lift is often used, the use of camshafts with modified cams. From time to time, various malfunctions occur in the gas distribution mechanism, due to which the engine starts to work intermittently, "dulls", in some cases it does not start at all. The causes of problems can be different:

• defective solenoid valve;
• the phase change coupling is clogged with dirt;
• the timing chain is stretched;
• chain tensioner defective.

Often when malfunctions occur in this system:

• decline idle speed, in some cases, the internal combustion engine stalls;
• fuel consumption increases significantly;
• the engine does not develop speed, the car sometimes does not even accelerate to 100 km / h;
• the motor does not start well, it has to be driven by the starter several times;
• a chirp is heard coming from the SIFG coupling.

By all indications, the main cause of engine problems is the failure of the SIFG valve, usually with computer diagnostics detects the error of this device. It should be noted that the Check Engine diagnostic lamp does not always light up at the same time, so it is difficult to understand that failures occur precisely in the electronics.

Often timing problems arise due to clogged hydraulics - bad oil with abrasive particles clogs the channels in the clutch, and the mechanism jams in one of the positions. If the clutch "wedges" in the initial position, the internal combustion engine quietly works at XX, but does not develop the speed at all. If the mechanism remains in the position of maximum valve overlap, the engine may not start well.

Unfortunately on engines Russian production SIFG is not installed, but many motorists are tuning the internal combustion engine, trying to improve the characteristics of the power unit. The classic version of the engine modernization is the installation of a "sports" camshaft, which has shifted cams, changed their profile.

This p / shaft has its advantages:

• the motor becomes throttle, responds clearly to pressing the gas pedal;
• the dynamic characteristics of the car are improved, the car literally tears from under itself.

But this tuning has its drawbacks:

• idle speed becomes unstable, they have to be set within 1100-1200 rpm;
• fuel consumption increases;
• it is quite difficult to adjust the valves, the internal combustion engine requires careful adjustment.

Quite often, VAZ engines of models 21213, 21214, 2106 undergo tuning. The problem of VAZ engines with a chain drive is the appearance of "diesel" noise, and often it arises from a failed tensioner. Modernization of the VAZ internal combustion engine consists in installing an automatic tensioner instead of the standard factory one.

Often, a single-row chain is installed on the VAZ-2101-07 and 21213-21214 engine models: the engine runs quieter with it, and the chain wears out less - its resource is on average 150 thousand km.

The outlet valve begins to open at the end of the expansion process ahead of LMW. at an angle φ o.v. = 30h-75 ° (Fig. 20) and closes after a.m.t. with a delay by the angle φ z.v., when the piston moves in the filling stroke in the direction towards the N.m.t. Start opening and closing intake valve also shifted relative to the dead center: the opening begins before a.m.t. leading by an angle φ 0. vp, and closing occurs after nm. with a delay by the angle φ c.v. at the beginning of the compression stroke. Most of the processes of release and filling are carried out separately, but near the t.m.t. the inlet and outlet valves are open for a while at the same time. The duration of valve overlap, equal to the sum of the angles φ З.в + φ о.вп, is small for piston engines (Fig. 20, a), and for combined ones it can be significant (Fig. 20, b). The total duration of gas exchange is φ o.v + 360 o + φ z.vp = 400-520 o; it is higher for high-speed engines.

Gas exchange periods in two-stroke engines

In a two-stroke engine, gas exchange processes occur when the piston moves near the borehole. and occupy part of the piston stroke in the expansion and compression strokes.

In engines with a loop gas exchange scheme, both the inlet and outlet ports are opened by a piston, therefore the valve timing and cross-sectional area diagrams of the windows are symmetrical relative to the LMW. (Fig. 24, a). In all engines with direct-flow gas exchange circuits (Fig. 24, b), the opening phases of the exhaust ports (or valves) are performed asymmetric relative to the nominal pressure, thereby achieving better cylinder filling. Typically, inlet ports and outlet ports (or valves) close at the same time or with little angle difference. It is also possible to carry out asymmetric phases in an engine with a loop gas exchange scheme,

if you install (inlet or outlet) additional devices - spools or valves. Due to the insufficient reliability of such devices, they are not currently used.

The total duration of gas exchange processes in two-stroke engines corresponds to 120-150 ° of the angle of rotation crankshaft, which is 3-3.5 times less than in four-stroke. The opening angle of the outlet ports (or valves) φ r.v. = 50-90 ° BC, and the pre-opening angle φ pr = 10-15 0. In high-speed engines with valve exhaust, these angles are larger, and in engines with window exhaust, these angles are smaller.

In two-stroke engines, the exhaust and filling processes take place for the most part together - with simultaneously open inlet (purge) and outlet ports (or exhaust valves). Therefore, air (or a combustible mixture) enters the cylinder, as a rule, provided that the pressure in front of the inlet ports is greater than the pressure behind the outlet ports (valves).

Literature:

Nalivaiko V.S., Stupachenko A.N. Sypko S.A. Methodical instructions for laboratory work on the course "Ship internal combustion engines", Nikolaev, NKI, 1987, 41p.

Internal combustion engines for ships. Textbook / Yu.Ya. Fomin, A.I. Gorban, V.V. Dobrovolsky, A.I. Lukin et al. - L.: Shipbuilding, 1989 - 344 p.: Ill.

Internal combustion engines. Theory of piston and combined engines: Ed. A.S. Orlina, M.G. Kruglova –M .: Mechanical engineering, 1983yu - 372p.

Vansheidt V.A. Internal combustion engines for ships. L. Shipbuilding, 1977.-392s.

The simplest two-stroke engine

The two-stroke engine is the simplest from a technical point of view: in it, the piston performs the work of a distributor. Several holes are made on the surface of the engine cylinder. They are called windows, and they are essential for a two-stroke cycle. The purpose of the inlet and outlet ports is fairly obvious - the inlet port allows the air-fuel mixture to enter the engine for subsequent combustion, and the outlet port allows the combustion gases to be removed from the engine. The purge channel serves to provide overflow from the crank chamber, into which it entered earlier, into the combustion chamber, where combustion takes place. This raises the question of why the mixture enters the crankcase space under the piston, and not directly into the combustion chamber above the piston. To understand this, it should be noted that in a two-stroke engine, the crank chamber plays an important secondary role, being a kind of pump for the mixture.

It forms a sealed chamber, closed from above by a piston, from which it follows that the volume of this chamber, and, consequently, the pressure inside it, changes, since the piston is mixed reciprocally in the cylinder (as the piston moves up, the volume increases, and the pressure drops below atmospheric, a vacuum is created; on the contrary, when the piston moves down, the volume decreases and the pressure becomes higher than atmospheric).

The intake port on the cylinder wall is covered most of the time by the piston skirt and opens when the piston approaches the top of its stroke. The created vacuum sucks a fresh charge of the mixture into the crank chamber, then, as the piston moves down and creates pressure in the crank chamber, this mixture is forced into the combustion chamber through the purge channel.

This design, in which the piston plays the role of a distributor for obvious reasons, is the simplest variety of a two-stroke engine, the number of moving parts in it is not significant. This is a significant advantage in many respects, but leaves much to be desired in terms of efficiency. At one time, in almost all two-stroke engines, the piston played the role of a distribution organ, but in modern designs this function is assigned to more complex and efficient devices.

Improved two-stroke engine designs

Influence on gas flow One of the reasons for the inefficiency of the above-described two-stroke engine is incomplete cleaning of exhaust gases. Remaining in the cylinder, they interfere with the penetration of the entire volume of the fresh mixture, and, therefore, reduce power. There is also a related problem: fresh mixture from the purge port enters directly into the outlet port and, as mentioned earlier, to minimize this, the purge port port directs the mixture upward.

Pistons with deflector

Cleaning efficiency and fuel economy can be improved by creating moreeffective gas flow inside the cylinder. In the early stages, improvements to two-stroke engines were achieved by giving the piston crown a special shape to deflect the mixture from the inlet to the cylinder head - this design is called the piston with a deflector. " However, the use of baffled pistons on two-stroke engines was short-lived due to piston expansion problems. The heat dissipation in the combustion chamber of a two-stroke engine is usually higher than that of a four-stroke engine, because combustion occurs twice as often, in addition, the head, the top of the cylinder and the piston are the hottest parts of the engine. This leads to problems with thermal expansion of the piston. In fact, the piston is shaped during manufacture so that it is slightly different from the circumference and is tapered upward (oval-barrel profile), so when it expands with changes in temperature, it becomes round and cylindrical. The addition of an asymmetric metal protrusion in the form of a deflector on the piston bottom changes the characteristics of its expansion (if the piston expands excessively in the wrong direction, it can jam in the cylinder), and also leads to its heavier weight with a displacement of mass from the axis of symmetry. This disadvantage has become much more apparent as motors have been improved to operate at higher rpm.

Types of two-stroke engine purges

Loop blowing

Since the piston with a deflector has too many flaws, and a flat or slightly rounded bottom the piston is not strongly influenced by the movement of the incoming mixture or the flowing out exhaust gases, another option was needed. It was developed in the 1930s by Dr. E. Schnurle, who invented and patented it (although, admittedly, he originally designed it for a two-stroke diesel engine). The blow-out windows are located opposite each other on the cylinder wall and are directed upward and backward at an angle. Thus, the incoming mixture hits the rear wall of the cylinder and deflects upward, then, forming a loop at the top, falls on the exhaust gases and contributes to their displacement through the outlet window. Consequently, good cylinder blowdown can be obtained by adjusting the position of the blowdown ports. The shape and size of the canals must be carefully considered. If the channel is made too wide, the piston ring, bypassing it, can fall into the window and jam, thereby causing damage. Therefore, the size and shape of the windows is designed to ensure that the track passes the windows without impact, and some wide windows are connected in the middle by a lintel that serves as a support for the rings. Another option is to use more and smaller windows.

At the moment, there are many options for the location, number and size of windows that have played a large role in increasing the power of two-stroke engines. Some engines are equipped with a purge and ports for the sole purpose of improving purge, they open shortly before the main purge ports open, which feed most of the fresh mixture. But that's all for now. what can be done to improve gas exchange without using expensive parts in production. In order to continue to improve performance, it is necessary to more accurately control the filling phase.

Suzuki Lets TW Lobe Valve

Petal valves

In any two-stroke engine design, improved efficiency and fuel economy means that the engine has to run more efficiently, which requires the maximum amount of fuel to be burned (hence maximum power) at each engine stroke. The problem remains of the complex removal of the entire volume of exhaust gas and filling the cylinder with the maximum volume of fresh mixture. As long as the gas exchange processes are improved within the framework of the engine with a piston as a distribution element, it is impossible to guarantee complete purification of the exhaust gases remaining in the cylinder, and the volume of the incoming fresh mixture cannot be increased in order to facilitate the displacement of the exhaust gases. The solution is to fill the crank chamber with more mixture by increasing its volume, but in practice this leads to less efficient blowing. Increasing the purge efficiency requires a decrease in the volume of the crank chamber and thus a limitation of the space intended for filling with the mixture. So a compromise has already been found, and other ways to improve performance should be sought. In a two-stroke engine, in which the role of the valve body is assigned to the piston, part of the air-fuel mixture supplied to the crank chamber will inevitably be lost as the piston begins to move downward during combustion. This mixture is forced back into the intake port and is thus lost. More needed effective method control of the incoming mixture. Mixture loss can be prevented by using a petal or disc (spool) valve, or a combination of both.

The flap valve consists of a metal valve body and a seat fixed on its surface withseal made of synthetic rubber. Two or more petal valves are attached to the valve body, these petals are closed under normal atmospheric conditions. In addition, to restrict the movement of the petal, restrictive plates are installed, one for each valve petal, which serve to prevent its breakage. Thin valve petals are usually made from flexible (spring) steel, although exotic materials based on phenolic resin or fiberglass are becoming more popular.

The valve opens by bending the petals up to the restrictive plates, which are designed to open as soon as there is a positive differential pressure between the atmosphere and the crank chamber; this happens when the upward-moving piston creates a vacuum in the crankcase. When the mixture is fed into the crankcase and the piston starts to move downward, the pressure inside the crankcase rises to atmospheric level and the petals are pressed, closing the valve. In this way, the maximum amount of mixture is supplied and any backflow is prevented. The additional mass of the mixture fills the cylinder more completely, and the blowing is more efficient. At first, petal valves were adapted for use on existing piston engines with valve timing, resulting in significant improvements in engine efficiency. In some cases, manufacturers have chosen a combination of two designs: one - when the engine with a piston in the role of a valve body. complemented by a petal valve to continue the filling process through additional channels in the crank chamber after the piston closes the main channel, if the pressure level in the engine crankcase allows it. In another design, windows were made on the surface of the piston skirt in order to finally get rid of the control that the piston has over the channels; in this case, they are opened and closed solely by the action of the petal valve. The development of this idea meant that the valve and intake port could be transferred from the cylinder to the crankcase. The daunting caveats that valve petals will crack and trap inside the engine have proven largely unfounded. Moving the inlet has a number of advantages, the main one being that. that the flow of gas into the crankcase becomes more free and, therefore, a larger amount of the mixture can enter the crank chamber. This is facilitated to some extent by the momentum (speed and weight) of the incoming mixture. As the inlet is moved out of the cylinder, efficiency can be further improved by mixing the purge port (s) to the optimum purge position. Of course, in recent years, the basic arrangement of petal valves has undergone extensive research and complex designs have emerged. containing two-stage petals and multi-lobe valve bodies. Recent developments in the field of petal valves are related to the materials used for the petals and the position and size of the petals.

Disc valves (spool distribution)

The disc valve consists of a thin steel disc secured to the crankshaft with a key

Or splines in such a way that they rotate together, It is located outside the intake port between the carburetor and the crankcase cover so. so that in the normal state the channel is overlapped by the disk, In order for the filling to occur in the desired area of ​​the engine cycle, a sector is cut out of the disk. As the crankshaft and disc valve rotate, the intake port opens as the cut section passes the channel, allowing the mixture to enter directly into the crankcase. The channel is then sealed off by a disc, preventing the mixture from being ejected back into the carburetor as the piston begins to move downward.

The obvious advantages of using a disc valve include more accurate control of the beginning and end of the process (the section, or sector, of the disc bypasses the channel), and the duration of the filling process (that is, the size of the cut section of the disc, proportional to the opening time of the channel). The disc valve also allows the use of a large inlet diameter and guarantees an unobstructed passage of the mixture entering the crank chamber. Unlike a petal valve with a sufficiently large valve body, the disk valve does not create any obstruction in the intake duct, and therefore the gas exchange in the engine is improved. Another advantage of the disc valve appears on sport bikes is the time it takes to change it to match the engine's performance for a variety of trails. The main disadvantage of a disc valve is technical difficulty, requiring small manufacturing tolerances and a lack of adaptability, that is, the inability of the valve to respond to changing engine demands like a petal valve. In addition, all disc valves are vulnerable to airborne debris entering the engine (fine particles and dust settle on the sealing grooves and scratch the disc). Despite this. disc valves work very well in practice and usually provide a significant increase in power at low engine speeds compared to a conventional piston engine.

Combined use of petal and disc valves

The disc valve's inability to respond to changing engine needs has led some manufacturers to consider using a disc and lobe valve combination to achieve high engine flexibility. Therefore, when conditions dictate, crankcase pressure closes the petal valve, thus closing the crank-side intake port, even though the cut-out section (sector) of the disc may still open the carburetor-side intake port.

Using the crankshaft cheek piece as a disc valve

An interesting version of the disc valve has been used for several years on a number of scooter engines. Vespa... Instead of using a separate valve assembly to fulfill its role, manufacturers used a standard crankshaft. The plane of the right flywheel cheek is machined with very high precision so that when the crankshaft rotates, the clearance between it and the crankcase is a few thousandths of an inch. The intake port is located directly above the flywheel (on these engines the cylinder is horizontal) and thus covered by the edge of the flywheel.Machining a notch in the flywheel portion can open the port at a given point in the engine cycle, just as with a traditional disc valve. Although the resulting inlet is less straight than it could be, in practice this system works very well. As a result, the engine delivers useful power over a wide range of engine speeds and remains technically simple.

Discharge port location

in many ways, the intake and exhaust systems on a two-stroke engine are very closely related. In the previous paragraphs, we discussed the methods of supplying the mixture and removing the exhaust gases from the cylinder. Over the years, designers and testers have found that exhaust phases can have as significant an impact on engine performance as intake phases. The exhaust phases are determined by the height of the outlet port in the cylinder wall, that is, when it is closed and opened by the piston as it moves up and down in the cylinder. Of course, as in all other cases, there is no single provision that would cover all engine modes. Firstly, it depends on what the engine is to be used for, and secondly, how this engine is used. For example, for the same engine, the optimal height of the exhaust port is different at low and high engine speeds, and on closer examination, it can be said that the same applies to the dimensions of the channel and directly to the dimensions of the exhaust pipe. As a result, various systems have been developed in production with variable exhaust system characteristics during engine operation to match the changing rotational speeds. Such systems appeared at (YPVS), (ATAS). (KIPS), (SAPC), Cagiva(CTS) and Aprilia(RAVE). Systems, and are described below.

Yamaha Power Rivet System - YPVS

At the heart of this system is the power valve itself, which is essentially a rotary valve installed in the cylinder liner so that its lower edge matches the upper edge of the outlet port. At low engine speeds, the valve is in a closed position, limiting the effective window height: this improves low and medium performance.When engine speed reaches a predetermined level, the valve opens, increasing the effective window height, which improves performance at high speeds. The position of the power valve is controlled by the servomotor using a rope and a pulley. YPVSi control unit - receives data about the valve opening angle from the potentiometer on the servomotor and data about the engine speed from the ignition control unit; this data is used to generate the correct signal to the servo motor drive mechanism (see Fig. 1.86). Note: The company's off-road bikes use a slightly different version of the system due to the low battery power: the power valve is driven by a centrifugal mechanism mounted on the crankshaft.

Kawasaki Complete Power Valve System - KIPS

The system has a mechanical drive from a centrifugal (ball) regulator mounted on the crankshaft. The vertical link connects the drive mechanism to the control rod of the power valve installed in the cylinder liner. Two such power valves are located in the auxiliary passages on either side of the main intake port and are connected to the drive rod by means of a pinion and a rack. As the drive rod moves "from side to side", the valves rotate to open and close the auxiliary channels in the cylinder and the resonator chamber located on the left side of the engine. The system is designed so that at low speed, the auxiliary channels are closed by valves to ensure a short-term opening of the channel. The left valve opens the resonator chamber to the escaping exhaust gases, thus increasing the volume of the expansion chamber. At high rpm, the valves are rotated to open both auxiliary passages and increase the opening time of the passage, thus providing more peak power. The resonator chamber is closed by a valve on the left side, reducing the overall volume of the exhaust system. The KIPS system provides improved performance at low and medium speeds by reducing the height of the duct and a larger exhaust system, and at high speeds, by increasing the height of the outlet port and a smaller exhaust system. The system was further improved by the introduction of an intermediate gear between the drive rod and one of the valves, which ensures the rotation of the valves in opposite directions, as well as the addition of a flat power valve at the leading edge of the exhaust port. On larger models, start-up and low-speed performance have been improved by the addition of a nozzle profile at the top of the valves.

Torque amplification chamber with automatic control Honda - ATAS

The system used on the company's models is driven by an automatic centrifugal regulator mounted on the crankshaft. The rack and pinion mechanism transfers force from the regulator to the ATAC valve installed in the cylinder liner. The HERP (Resonant Energy Pipe) chamber is opened by the ATAC valve at low engine speeds and closed at high engine speeds.

Fuel injection system

Apparently, the obvious method for solving all the problems associated with filling the combustion chamber of a two-stroke engine with fuel and air, not to mention the problems high flow fuel and harmful emissions, is the use of a fuel injection system. However, if fuel is not fed directly into the combustion chamber, inherent problems with the filling phase and engine efficiency still remain. The problem with direct fuel injection into the combustion chamber is. that fuel can only be delivered after the intake ports have been closed, therefore there is little time left for atomizing and completely mixing the fuel with the air in the cylinder (which comes from the crankcase as in traditional two-stroke engines). This gives rise to another problem, since the pressure inside the combustion chamber after closing the exhaust port is high and it builds up quickly, therefore, the fuel must be supplied at an even higher pressure, otherwise it will simply not flow out of the injector. This requires a fairly large fuel pump, which entails problems associated with increased weight, size and cost. Aprilia solved these problems by employing a system called DITECH, based on a design by an Australian company, Peugeot and Kymmco developed a similar system. The injector at the beginning of the engine cycle delivers a jet of fuel into a separate closed auxiliary chamber containing compressed air (supplied either from a separate compressor or through a channel with a check valve from the cylinder]. After the exhaust port is closed, the auxiliary chamber communicates with the combustion chamber through the valve or a nozzle, and the mixture is fed directly to the spark plug.Aprilia claims to reduce emissions by 80%, achieved by reducing not 60% oil consumption and 50% fuel consumption, in addition, the speed of a scooter with such a system is 15% faster than speed the same scooter with a standard carburetor.

The main advantage of the application direct injection therein. that, in comparison with a conventional two-stroke engine, there is no need to pre-mix the fuel with the oil to lubricate the engine. Lubrication is improved because the oil is not flushed from the bearings by the fuel and therefore less oil is required, resulting in reduced toxicity. Fuel combustion is also improved and carbon build-up on pistons, piston rings and in the exhaust system is reduced. Air is still supplied through the crank chamber (its flow rate is determined throttle connected to the motorcycle throttle) This means that oil is still burning in the cylinder and the lubrication and lubrication is not as effective as desired. However, the results of independent tests speak for themselves. All that is now necessary is to provide air supply, bypassing the crank chamber.

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Kart Design - Forcing Engines

There will be no ready-made recipes for boosting specific types of engines. All engines are different, the dimensions of individual elements (for example, the exhaust system) will change on different chassis, and the characteristics will also change. Therefore, some specific recipes, in which, nevertheless, a lot of white spots will remain, can only lead to useless work.

In particular, the fundamentals of the theory of the processes occurring in the engine will be considered, with special emphasis on those issues that are fundamental when forcing the engine. Of course, in the proposed chapter, only those sections of the theory are considered, the knowledge of which is necessary so that a novice karting fan does not spoil the engine in an effort to squeeze maximum power out of it. General recommendations are also given on the directions in which engine modifications should be carried out in order to achieve positive results. General instructions are illustrated with examples from practical work on boosting kart engines. In addition, a number of comments and practical recommendations are given regarding seemingly small changes, the introduction of which will improve the operation of the engine, increase its reliability, and save us from sometimes costly learning from our own mistakes.

Gas distribution phases

The valve timing is expressed by the angles of rotation of the crankshaft at which the corresponding cylinder windows open and close. In a two-stroke engine, consider three phases: opening the inlet port, opening the outlet port, and opening the bypass ports (Figure 9.3).

The phase of opening a window, for example, an exhaust one, is the angle of rotation of the crankshaft, measured from the moment when the upper edge of the piston opens the exhaust window, until the moment when the piston, moving back, closes the window. Similarly, you can define the phases of the opening of other windows.

Rice. 9.3. Valve timing diagrams:

a-symmetrical; b - asymmetrical; OD and ZD - inlet opening and closing. OP and ZP - bypass opening and closing; OW and ZW - issue opening and closing; a, y- angles opening, respectively, inlet and outlet ports; B - opening angle of bypass windows

Rice. 9.4. Comparison of time-sections (area under curves) for windows of different shapes

In the usual piston engine all windows are opened and closed by a piston, therefore the valve timing diagram is symmetrical (or almost symmetrical) about the vertical axis (Fig.9.3, a). In kart engines, in which the crank chamber is filled with a combustible mixture using a rotating valve, the intake phase may not depend on the movement of the piston, therefore the valve timing diagram is usually asymmetrical (Fig.9.3, b).

Valve timing are comparable values ​​for engines with different piston strokes, that is, they serve as universal characteristics. When comparing engines with the same piston stroke, the valve timing can be replaced by the distances from the windows, for example, to the top plane of the cylinder.

In addition to the valve timing, an important parameter is the so-called time-section. When the window is gradually opened by the piston, the shape of the channel depends on how the open surface of the window increases, depending on the angle of rotation of the crankshaft (or time). The wider the window, the more surface will open when the piston is moved downward. For the same time, a larger amount of combustible mixture will pass through the window. It is advisable that when the window is opened by the piston, its area is immediately as large as possible. In many engines, for this, the window is made extended upward. This achieves the effect of quickly opening the window without increasing its surface.

The diagram of the growth of the open surface of windows of different shapes as a function of time at a constant FW of the engine is shown in Fig. 9.4. The total area of ​​the windows is the same in both cases. The area under the curves of the diagram characterizes the time-section value. For an irregularly shaped window, the time-section is larger.

Cylinder Purge Systems

Rice. 9.10. Diagram of cylinder purging systems and corresponding cylinder mirror sweeps:

a - two-channel system; b - three-channel system; c - four-channel system; d - five-channel system

Cylinder purge systems used in kart engines are schematically shown in Fig. 9.10. Alongside, the location of the bypass windows on the scan of the cylinder mirror is shown for each of the systems: two-, three-, four- and five-channel. In engines where crankcase filling is controlled by a piston, covers and does not close the intake port. In this case, the inlet is not made in the cylinder, and it becomes possible to place an additional bypass channel.

The role of the exhaust system

In a two-stroke engine, the exhaust system plays a huge role, consisting of an exhaust pipe (in the cylinder and behind the cylinder), an expansion chamber and a muffler. At the moment the exhaust port is opened, there is some pressure in the cylinder, which is reduced in the exhaust system. The gas expands, shock waves appear, which are reflected from the walls of the expansion chamber. Reflected shock waves cause a new increase in pressure near the exhaust port, as a result of which some of the exhaust gases again enters the cylinder (Fig.9.11).

Rice. 9.11. Schematic representation of sequential exhaust phases:

a - opening of the outlet window; b - full opening of the window; c - closing the window

It seems that it would be more advantageous to obtain a vacuum at the outlet when it is fully open. This will cause the gases to be pumped out of the cylinder and thus fill the cylinder with a fresh mixture. However, in this case, part of this mixture, together with the exhaust gases, will enter the outlet pipe. Therefore, it is necessary to achieve increased pressure at the outlet port when it is closed. In this case, the combustible mixture that has entered the exhaust pipe along with the exhaust gases will be returned to the cylinder, significantly improving its filling. This happens after the bypass ports are closed by the piston. As in intake system, wave phenomena in the exhaust system give a positive effect only in the vicinity of the resonant PW. By changing the dimensions, and especially the length of the exhaust system, it is also possible to shape the speed characteristics of the engine. The effect of changes in the size of the exhaust system on the performance of the engine is more significant than the change in the size of the intake system.

Combustion Basics

For a better understanding of the operation of the engine, it is necessary to say a few words about the processes occurring in the combustion chamber of the engine. The growth of pressure in the cylinder depends on the course of the combustion process, which determines the engine power.

The results of fuel combustion, perceived as the work of the crank mechanism, primarily depend on the composition of the combustible mixture. The theoretically ideal composition of the combustible mixture is the so-called stoichiometric composition, that is, one in which the mixture contains so much fuel and oxygen that after combustion there is no fuel or oxygen in the exhaust gases. In other words, all the fuel in the combustion chamber will burn, and all the oxygen contained in the combustible mixture will be consumed for its combustion.

If there was an excess of air in the combustion chamber (lack of fuel), then this excess would not be able to help the combustion process. However, it would become an additional mass of gas that must be "pumped" through the engine and heated using heat, which, without this additional mass, would raise the temperature and, therefore, the pressure in the cylinder. A combustible mixture with excess air is called lean.

Lack of air (or excess fuel) is equally unfavorable. This would lead to incomplete combustion of the fuel and, as a result, to less energy. The excess fuel is then passed through the engine and evaporated. A combustible mixture with a lack of air is called rich.

In practice, to obtain the highest power, it is advisable to use a slightly rich mixture. This is due to the fact that local inhomogeneities in the composition of the combustible mixture are always formed in the combustion chamber, which arise due to the fact that it is impossible to achieve ideal mixing of fuel with air. The optimal composition of the mixture can only be determined empirically.

The volume of the combustible mixture sucked into a cylinder each time is determined by the working volume of this cylinder. But the mass of air in this volume depends on the air temperature: the higher the temperature, the lower the density of the air. Thus, the composition of the combustible mixture depends on the air temperature. Because of this, it is necessary to "tune" the engine depending on the weather. On a hot day, warm air enters the engine, therefore, to maintain the correct composition of the combustible mixture, the fuel supply must be reduced. On a cold day, the mass of incoming air increases, so more fuel must be supplied. It should be noted that air humidity also affects the composition of the combustible mixture.

As a result of all this, the temperature of even the ideal mixture composition under these conditions significantly affects the degree of filling of the crank chamber. In a constant crankcase volume at a higher temperature, the mass of the combustible mixture will be less and, therefore, after its combustion, there will be a lower pressure in the cylinder. Because of this phenomenon, they try to give the engine elements such a shape, especially the crankcase (ribbing), in order to achieve their maximum cooling.

The combustion of the mixture in the combustion chamber occurs at a certain speed; during the combustion, the crankshaft rotates at a certain angle. The pressure in the cylinder builds up as the mixture burns. It is advisable to obtain the highest pressure at the moment when the working stroke of the piston has already begun. To achieve this, the mixture must be ignited a little earlier, with a certain advance. This advance, measured by the crankshaft angle, is called the ignition timing. It is often more convenient to measure the ignition timing by the distance that the piston has to travel to top dead center.

Range of modifications

Before we start working on the engine, we need to decide what figure we want to achieve. In five-, six-speed motors of the racing category, we can strive to increase the CV, although it is known that as a result of this CV of the maximum torque approaches the CV of maximum power; we reduce the range of working revolutions, seeking more power in return.

In engines of the popular category, and these are Damba engines with a volume of 125 cm 3 with a three-stage gearbox, one should not strive to achieve too high CV, it is necessary to achieve the greatest range of operating CV. In such engines (using its own components and assemblies), it is possible to achieve a power of more than 10 kW at a rotational speed of the order of 7000-8000 rpm.

It is also necessary to determine the range of improvements that we are going to perform. You need to know in advance whether this will be the introduction of improvements in the engine under development, or the range of improvements will be so wide that in the end we will get practically new engine keeping several original (but modified) nodes as required by the rules.

Assuming the revision of the engine, preference should be given to those operations that will significantly increase the performance of the engine. However, it is not worth (at least at this stage of work) to provide for the implementation of such operations that require significant labor and which are known in advance that they will give insignificant results. Such operations include the polishing of all cylinder bores of the engine, despite the fact that there is a general belief in the effectiveness of this operation. Bench tests of many engines have shown that polishing the cylinder bores increases the engine power by 0.15-0.5 kW. As you can see, the effort spent on doing this work is completely incommensurate with the results.

Here are the operations that will undoubtedly affect the increase in engine performance: increasing the compression ratio; change in valve timing; changing the shape and size of channels and cylinder windows; correct selection of the parameters of the intake and exhaust systems; optimization of the ignition timing.

Changing the compression ratio

An increase in the compression ratio obtained by reducing the volume of the combustion chamber leads to an increase in engine power. An increase in the compression ratio leads to an increase in the combustion pressure in the cylinder by increasing the compression pressure, improving the circulation of the mixture in the combustion chamber and increasing the combustion rate.

The compression ratio cannot be increased to any arbitrary value. It is limited by the quality of the fuel used and the thermal and mechanical strength of the engine components. Suffice it to say that with an increase in the effective compression ratio from 6 to 10, the forces acting on the piston almost double; that is, the load, for example, on the crank mechanism doubles.

Taking into account the strength of engine parts and the detonation properties of available fuels, it is not recommended to use a geometric compression ratio greater than 14. Increasing the compression ratio to this value requires not only removing the gasket (if any), but also shaping the cylinder head and sometimes the cylinder. To facilitate the calculation of the volume of the combustion chamber for different degrees, you can use the diagram shown in Fig. 9.17. Each of the curves refers to a specific cylinder displacement.

Rice. 9.17. Diagram of the dependence of the compression ratio a on the volume of the combustion chamber V 1 = 125 cm 3 and V 2 -50 cm 3

In some engines with a relatively low compression ratio, it can only be significantly increased by machining. In this case, the combustion chamber is melted and processed again. It also allows you to change the shape of the camera. Most modern engines used in karting have a hat-shaped combustion chamber. This shape should not be changed when modifying the engine.

The only way to accurately determine the volume of the combustion chamber is to fill it with engine oil through the spark plug hole (Fig. 9.18) with the piston at top dead center. With this method of measurement, the volume of the plug hole must be subtracted from the volume of the poured oil. The volume of the candle hole for a candle with a short thread is 1-1.1 cm ’1, for a candle with a long thread - 1.7-1.8 cm 3.

Cylinder head gaskets are either not used at all in racing engines, or they are replaced by thin copper rings. In both cases, the joint surfaces of the cylinder and the head must be ground in. The use of gaskets made of a material with a low thermal conductivity coefficient is contraindicated because it will impede the outflow of heat from the upper part of the cylinder liner, which carries a significant thermal load, to the head and its cooling fins. The cylinder head gasket must under no circumstances protrude into the combustion chamber. The protruding edge of the gasket will heat up and become a source of glow ignition.

Rice. 9.18. Determination of the volume of the combustion chamber

Octane number the gasoline used must correspond to the compression ratio. However, it should be borne in mind that the compression ratio is not the only factor that determines the possible detonation of the fuel.

Detonation depends on the course of the combustion process, on the movement of the mixture in the combustion chamber, on the ignition method, etc. The type of fuel for a particular engine is selected empirically. However, it does not make sense to use high-octane fuel for a low compression ratio engine because engine performance is not improved.

Blowing out the cylinder

The selection of the appropriate valve timing in a two-stroke engine is of great importance for removing exhaust gases from the cylinder and filling it with fresh mixture. In addition, it is necessary to direct the jets of the mixture coming from the bypass windows so that they pass through all the nooks of the cylinder and combustion chambers, blowing out the remaining exhaust gases from them and directing them to the outlet window.

To increase the CW of the engine and, as a consequence, its power, it is necessary to significantly expand the exhaust phase, or rather, to increase the difference between the exhaust and purge phases. As a result, the time during which the exhaust gases expand out of the cylinder increases. In this case, at the moment of opening the bypass windows, the cylinder is already empty, the fresh charge entering it only slightly mixes with the residual exhaust gases.

The release phase is increased due to the displacement (cutting) of the upper edge of the window. The release phase in racing engines reaches 190 ° versus 130-140 ° in serial engines... This means that the top edge can be cut down a few millimeters. However, it must be borne in mind that as a result of an increase in the height of the outlet port, the stroke of the piston on which the work is performed decreases. Therefore, an increase in the height of the outlet port pays off only if the losses in the piston operation are compensated for by the improvement in cylinder blowing.

Due to the expediency of achieving the maximum difference between the exhaust and purge phases, the opening angle of the purge ports usually remains unchanged.

The size and shape of the by-pass channels and windows have a significant influence on the quality of the blowdown. The direction of the mixture inlet into the cylinder from the bypass channel must correspond to the adopted purge system (see paragraph 9.2.4, Fig. 9.10). In two- and four-channel blowing systems, the jets of the combustible mixture entering the cylinder are directed above the piston to the cylinder wall opposite the outlet port, and in the four-channel system, the jets emanating from the windows located closer to the outlet port are usually directed toward the cylinder axis. In systems with three or five bypass ports, one port must be located opposite the outlet port, the channel of this port must direct the flow of the combustible mixture upward at a minimum angle to the cylinder wall (Figure 9.19). This is a necessary condition for the effective action of this additional jet, usually obtained by reducing its cross section, as well as the later opening of this window.

The manufacture of an additional (third or fifth) channel is the rule for motors with a rotating spool or diaphragm valve. In engines in which the filling of the crank chamber is controlled by a piston, an intake port is located in place of the classic third (or fifth) bypass. In such engines, there may be additional bypass channels, and the inlet port must have an appropriate shape; a similar solution is shown in Fig. 9.20. In this engine, three additional small bypass ports are made, connected by a common bypass, the entrance to which is located above the inlet port. The required intake phase is ensured here by the corresponding shape of the intake port.

Rice. 9.19. Influence of the shape of the third bypass channel on the movement of the charge in the cylinder:

a - irregular shape; b- correct form

When installed on conventional engine a rotating spool in the cylinder, it becomes possible to make a bypass channel opposite the outlet port. It is convenient to make a strongly curved short canal here (Fig.9.21, a), the flow of the mixture into which is closed for a while by the piston skirt.

The disadvantage of this solution is that the movement of the piston disrupts the normal flow of the combustible mixture, but it has two important advantages: the small volume of the channel only slightly increases the volume of the crank chamber, and the combustible mixture, passing through the piston, cools it perfectly. In practice, such a channel is easy to do as follows. Two holes are made in the cylinder (the bypass window and the entrance to the channel), in this place the ribs are cut out and the lining with the channel cut through it is screwed on (Fig. 9.21.6). You can also try to cut a vertical groove in the cylinder mirror between the channel entrance and the window, the width of the groove being equal to the width of the channel. However, in this case, the downward movement of the piston will cause some turbulization of the combustible mixture in the channel (Fig. 9.21, c).

The bypass channels should taper towards the ports in the cylinder.

Rice. 9.21. Additional bypass channel with the mixture flowing through the piston:

a - principle of action; b - part of the channel passes through the outer pad; c - channel cut in the cylinder mirror

The inlet to the bypass must have an area 50% larger than the bypass. Obviously, the change in the channel cross-section must be performed along its entire length. The corners of the windows and channel cross-sections should be rounded with a radius of 5 mm to increase the laminar flow.

Any errors when joining parts of channels located in different engine parts are unacceptable. This remark primarily concerns the junction of the cylinder with the engine crankcase, where the gasket can become a source of additional turbulence of the mixture, and the joints of the inlet and outlet pipes with the cylinder. Vortices in the flow of the mixture can also occur at the junction of the cast jacket of the cylinder with the cast or pressed sleeve (Fig. 9.22). Discrepancies in sizes in these places must be undoubtedly corrected.

In some engines, the cylinder windows are split by a rib. This applies primarily to intake and exhaust ports. It is not recommended to reduce the thickness of these ribs and, even more so, to remove them when the window area increases. These ribs prevent the piston rings from getting into the wide windows and hence from breaking. It is only permissible to streamline the rib of the intake port, but only on the outside of the cylinder.

Rice. 9.22. Charge movement disturbances caused by incorrect

the relative position of the cylinder liner and the cast cylinder jacket

It is impossible to give an unambiguous recipe for obtaining certain effects of modifications. In general, it can be said that increasing the opening of the outlet window increases the engine power, simultaneously increasing the CW of maximum power and maximum torque, but narrowing the range of working CW. An increase in the size of the windows and cross-sections of the channels in the cylinder has a similar effect.

These tendencies are well illustrated by changes in the speed characteristics of an engine (Fig. 9.23) with a volume of 100 cm (cylinder diameter 51 mm, piston stroke 48.5 mm), obtained as a result of changes in the dimensions and valve timing (Fig. 9.24). In fig. 9.24, a the dimensions of the windows are given at which the engine develops the maximum power (curves N A and M d in fig. 9.23). The exhaust phase is 160 °, the purge phase is 122 °, and the intake phase is 200 °. The inlet window opened at 48 ° from TDC and closed at 68 ° from TDC. The diameter of the carburetor diffuser is 24 cm.

In fig. 9.24, b the dimensions of the windows are shown at which the largest operating range of NW is achieved (see Fig. 9.23, curves N B and M c). The exhaust phase is 155 °, the purge phase is 118 ° and the intake phase is 188 °, the intake is opened at an angle of 48 ° after BDC and closes at an angle of 56 ° after TDC. The diameter of the carburetor diffuser is 22 mm.

It should be noted that relatively small changes in the dimensions and valve timing have significantly changed the characteristics of the engine. At the engine A more power, but it is practically useless at speeds below 6,000 rpm. Option V applicable in a much wider range of CW, and this is the main advantage of an engine without a gearbox.

Although the example considered concerns an engine of a class not used in Poland, it well illustrates the relationship between the shape of the windows and cylinder bores and the parameters of its operation. However, we must remember that whether our modifications have led to the desired results, we will know only after they have been completed and the engine has been checked at the stand (or subjectively during running in). Preparation of a racing engine is an endless cycle of modifications and checks of the results of this work, new modifications and checks, and in fact other engine units (carburetor, exhaust system, etc.) also have a huge impact on engine performance, the optimal parameters of which can only be determined empirically ...

It is also necessary to emphasize the enormous importance of the geometric symmetry of all windows and channels in the cylinder. Even a slight deviation from symmetry will have a negative effect on the movement of gases in the cylinder. A slight difference in the height of the bypass ports on both sides of the cylinder (Fig. 9.25) will cause an asymmetrical movement of the mixture and disrupt the operation of the entire purge system. An excellent indicator that allows you to directly assess the correctness of the direction of the mixture flows coming from the bypass ports are traces on the piston bottom. After some time of engine operation, part of the piston crown is covered with a layer of soot. The same part of the bottom, which is washed by the jets of fresh combustible mixture entering the cylinder, remains shiny, as if it had been washed.

Rice. 9.25. Influence of differences in the height of the bypass windows

on both sides of the cylinder on the symmetry of the charge movement

Piston and piston rings

Rice. 9.28. The dependence of the throughput of the inlet channel of the carburetor on the forums of its section

V modern engines Pistons made of a material with a low coefficient of linear expansion are used, so the clearance between the piston and the cylinder liner can be small. If we assume that the clearance around the circumference and the length of the piston skirt in a heated engine will be the same everywhere, then after cooling the piston will be deformed. Therefore, the piston must obtain the appropriate shape even during machining, which is done in practice. Unfortunately, this form is too complicated, and it can only be obtained on special machines. It follows from this that the shape of the piston cannot be changed by locksmith operations, and all kinds of grinding of the piston skirt with a file or a sharpener, which are used everywhere after the piston is jammed, will lead to the piston losing its correct shape. In case of urgent need, such a piston can be used, but there is no doubt that its interaction with the cylinder mirror will be much worse.

It is necessary to warn against using sandpaper for emergency cleaning of the piston skirt. Grains of abrasive material dig into the soft material of the piston, after which they rinse the entire cylinder mirror. This will result in the cylinder having to be bored to the next oversize.

An approximate temperature distribution on the piston is shown in Fig. 9.29. The highest heat load falls on the bottom and top, especially from the side of the outlet window. The temperature of the lower part of the skirt is lower and depends primarily on the shape of the piston. The shape of the inner surface of the piston should be such that there are no narrowings in the cross section of the piston that impede heat transfer (Fig. 9.30). Heat from the piston to the cylinder is transferred through the piston rings and the contact points of the piston skirt with the cylinder.

To reduce the mass of the piston and, thereby, to reduce the forces that noticeably increase at a high engine speed, it is possible to remove some of the material inside the piston, but only in its lower part. Usually, the lower edge of the piston ends with a shoulder inside, which is the technological base for processing the piston. This bead can be removed, leaving about 1 mm of skirt thickness at this point. The wall thickness of the piston should increase smoothly towards the bottom. You can slightly increase the cutouts in the piston skirt under the bosses. The shape and dimensions of these cutouts must match the cutouts in the bottom of the cylinder liner (Fig. 9.31). To change the time-section, it is easiest to undercut the lower edge of the piston from the side of the intake port, although the selection of the undercut amount is more difficult.

To reduce the heat load on the upper piston ring, it is recommended to make a bypass groove above it 0.8-1 mm wide and 1-2 mm deep. Sometimes a similar groove (or even two) is made between the rings. These notches direct heat flow to the bottom of the piston, reducing the temperature of the piston rings.

In general, we do not have the ability to change the appearance and arrangement of the rings. We can only control the gap in the lock (cut) of the ring, which should not exceed 0.5% of the cylinder diameter. It is also necessary to carefully determine the angular position of the locks so that they never fall on the windows when the piston moves (Fig. 9.32). When carrying out work on the cylinder, it is also necessary to take into account the position of the piston ring locks.

Sometimes a simple method is used to reduce the elasticity of the piston ring by chamfering from its inner edges. This ensures a better fit of the rings to the cylinder bore. This method is especially useful when changing rings without grinding the cylinder.

Crank mechanism

As already mentioned, in the 501 engine -Z3A it is advisable to rearrange the crankshaft cheeks. After disassembling with a press, the following operations must be performed over the shaft.

1. Deepen in the cheeks of the shaft sockets for the lower head of the connecting rod by the thickness of the additional discs attached to the outer surface of the cheeks (Fig. 9.35, size e).

2. Squeeze out the axle shafts from the cheeks to the thickness of additional
disks.

3. Reduce the thickness of the connecting rod (Fig. 9.36) on the grinding machine. Manual processing is used only for finishing.
The thickness can be reduced even to 3.5 mm, provided that the connecting rod is polished. Each scratch on the connecting rod is a stress concentrator from which crack propagation can begin. In addition, all fillets must be done very carefully. When modifying the connecting rod, it is advisable to make slots in the upper and lower heads to improve the access-mixture to the bearings.

4. Shorten the crank pin to size with(Fig. 9.36), equal to the width of the shaft after rearranging the cheeks, but before attaching additional discs. The pin must be shortened on both sides, this will allow the bearing roller raceways to remain in their old place.

5. Weigh the upper and lower connecting rod heads as shown in fig. 9.37.

6. Assemble the crankshaft. Pressing in the crank pin can be done using a press or a large vise.

Of course, after such an assembly, it is difficult to achieve the alignment of the axle shafts. The error can be detected by applying a steel plate to one of the cheeks (Fig. 9.38), which will lag behind the other cheek. This can be corrected by striking one of the cheeks with a mallet (fig. 9.39). More precisely, we will check the runout of the shaft when it rotates in bearings. On the semiaxis covered with chalk, the starter will indicate the places in which the runout must be reduced (Fig. 9.40). When assembling the shaft, remember to maintain a gap between the lower connecting rod head and the shaft cheeks. This gap must be at least 0.3 mm. Too small a clearance in many cases is the cause of the connecting rod bearing seizure.

7. Balance the crankshaft. This is done using a static method. We will support the shaft on prisms and, having hung the weight in the upper head of the connecting rod, we will select the balanced mass (not to be confused with the weight of the weight) so that the shaft remains at rest in any position. The mass of the sinker is the fraction of the masses involved in the reciprocating motion that must be balanced. Suppose that the mass of the upper connecting rod head is 170 g, and the mass of the piston with rings and piston pin is 425 g. The reciprocating mass is 595 g. Assuming that the balance coefficient is 0.66, we obtain that the mass, which must be balanced, is equal to 595X0.66 = 392.7 g. Subtracting from this value the mass of the upper connecting rod head, we obtain the mass of the weight G suspended on the head.

The state of static equilibrium of the crankshaft is achieved by drilling holes in the shaft cheeks on the side that is overtightening.

8. Make additional discs of steel and attach them to the shaft with three MB screws with countersunk bevel heads. Before mounting the discs, it is advisable to lubricate the plane of the joint with the shaft with sealant. Counter the screws by punching.

We add that additional discs can be attached not to the shaft, but motionlessly to the inner walls of the crankcase. However, due to the loose fit of the disc to the wall, heat transfer may deteriorate. It should be noted that the displacement of the crankshaft cheeks does not exclude the use of a thin "horseshoe".

Before starting the modifications to the cylinder, you need to make a tool for measuring the valve timing, using a circular protractor with a 360 ° scale for this purpose (Fig. 9.42). Install the protractor on the engine crankshaft, and attach a wire arrow to the engine.

To unambiguously determine the time of opening and closing the windows, you can use a thin wire inserted through the window into the cylinder and pressed by the piston in the upper edge of the window. The thickness of the wire will hardly affect the measurement accuracy, but this method will facilitate the work. It is especially useful in determining the opening angle of the intake port.

Taking impressions from the cylinder mirror will greatly facilitate the work of changing the valve timing and the size of the channels and windows. Such an impression can be obtained as follows:

put a piece of cardboard inside the cylinder and adjust it so that it exactly lies along the cylinder mirror; its upper edge should coincide with the upper plane of the cylinder;

with the blunt end of a pencil, squeeze out the outlines of all windows;

on the cardboard removed from the cylinder, we get an imprint of the cylinder mirror; cut out the displayed windows in the cardboard along the lines of the prints.

On the resulting scan of the cylinder mirror, you can measure the distance from the edges of the windows to the upper plane of the cylinder and calculate the corresponding valve timing (using the formulas found in every book about engines).

Now let's look at how to fix the new valve timing in a modified engine. To do this, on the goniometer, we alternately set the required angles, measuring each time the distance from the upper edge of the piston to the upper plane of the cylinder. The measured distances are applied to the previously made pattern.

Now we can outline the new shape of the windows, and then cut them out on the pattern. It remains to put the pattern into the cylinder and enlarge the windows so that their shape matches the designed ones. Using a pattern will save us from having to repeatedly check the corners when enlarging the windows.

Rice. 9.42. Simple goniometer for measuring valve timing

Gas distribution phases

The location of the channels and the valve timing of the engine

The reciprocating motion (up and down) of the engine piston allows it to act as an air compressor. Initially, the air / fuel mixture moves into the crankcase under the piston, and then travels into the cylinder (above the piston) where it is compressed and ignited. As soon as the gases are burnt, the temperature and pressure rise rapidly. This pressure propels the piston to the underside of its stroke, where the exhaust gases are ultimately purged out. Sounds simple, but very precise channel design - shape, size, position and timing - is essential if you want to achieve significant engine performance.

The wastegate pass the fresh air / fuel mixture into the cylinder prior to combustion while the exhaust gases are purged through the exhaust port.

THE BASES

If you're curious enough to disassemble your engine, you've probably seen holes in the liner and crankshaft. These holes are known as ducts or holes, and in a two-stroke engine they have 3 functions:

1. Intake - Allows fresh air / fuel mixture to enter the crankcase below the piston.

2. Bypass - movement of the air / fuel mixture from the crankcase to the cylinder above the piston.

3. Exhaust - This is where exhaust gases exit from the engine after combustion.

The holes are opened and closed by the movement of the piston and crankshaft, and unlike engines with mechanical valves, they do not require additional energy from the engine to function.

The holes you see are necessary for the two-stroke engine to function properly.

CHANNEL TYPES

INLET. Car engines use an intake system based on a crankshaft rotary valve. How it works: A bore made in the shaft journal aligns with the air intake hole in the engine housing (under the carburetor) at every revolution of the shaft. The air / fuel mixture passes through an open hole in the surface of the crankshaft journal and then through a channel in the center of the crankshaft and finally into the engine crankcase.

The intake port in the crankshaft "metes out" how much air and fuel is entering the engine. The air / fuel mixture then enters the crankcase through a channel in the center of the crankshaft.

BYPASS HOLES. These holes are made in the cylinder wall and are alternately closed and opened by a piston. The air / fuel mixture from the crankcase (below the piston) moves through the bypass channels outside the cylinder to the bypass ports.

Two-stroke car engines use a variety of bypass combinations. There can be anywhere from two to 10-11 bypass holes of various shapes and sizes - plus an exhaust hole or holes (yes, there might even be multiple exhaust holes).

LOCATION OF SHNURLET CHANNELS: There are many bypass and exhaust port configurations used in two-stroke engines, but self-similar engines use a basic configuration known as the Schnurle duct arrangement, so we will only discuss that option.

In the Schnurle system, the two by-pass holes are directed upward and away from the single exhaust port that is located between them. Fresh fuel mixture is deliberately directed to the point farthest from the exhaust port. At this point, the fresh mixture loops towards the cylinder head and pushes the exhaust gases out through the exhaust port.

Schnurle holes direct the air / fuel mixture away from the exhaust port.

BOOST HOLE: The boost hole is an important improvement in the basic arrangement of the Schnurle channels. It is located opposite the exhaust port and is easily distinguishable from the rest of the cylinder bores by its sharp upward angle. The boost hole not only creates another path through which the air / fuel mixture can enter the cylinder, but it also does so at an angle that directs the mixture towards the glow plug at the top of the cylinder. This contributes to better cylinder filling and improved exhaust gas purging.

The boost port is opposite to the exhaust port. Its sharp upward angle helps direct the fresh air / fuel mixture towards the glow plug at the top of the cylinder.

A LOT - NOT ALWAYS GOOD: More important than the number of ports are valve timing (i.e. when the ports open and close), duration (how long they stay open) and area (port size), so don't be impressed by the number of ports advertised for a given engine. A properly designed 3-channel motor can be more powerful than a poorly designed 7-channel motor.

Properly designed channels help direct the flow of the air / fuel mixture and exhaust gases. More channels sometimes equals more power, but not always.

GAS DISTRIBUTION PHASES

The valve timing indicates the points in the engine cycle at which the holes open and close. These points are usually measured from TDC (top dead center) or BDC (bottom dead center), from the one to which the piston is closer.

In addition to opening and closing the holes, the valve timing tells us how long the hole stays open (duration). This is important in determining the operating speed of an engine, high speed motors move gases longer than low speed motors.

Most experts measure the opening and closing of bores in degrees of crankshaft rotation. Some designers and engineers use a system that measures the opening and closing of bores as a percentage of TDC (TDC). Although there is technical advantages in use latest system, the first is the most used.

To measure the events of the valve timing, a goniometer wheel is attached to the crankshaft. The stationary gauge aligns with the gage wheel and precisely matches the piston position at TDC, providing intake, bypass and exhaust phase measurements.

All you need to start measuring your engine's camshaft timing is a protractor wheel, a pointer and a sturdy engine mount. This method is used by all engine designers to map valve timing and locate potential improvements.

DUCTS AND PURGE

In engine terminology, "purge" means volume scrubbing - in other words, scrubbing the exhaust gas from the cylinder and moving the fresh air / fuel mixture from the crankcase to the cylinder. For an engine designer, cleaning the cylinder from exhaust gases is only half of the problem, while replacing these gases with a fresh air-fuel mixture is another problem.

When the engine is running, some of the fresh mixture transferred to the cylinder mixes with the blown exhaust gases and reduces the efficiency and power of the engine. Many duct systems have been tried over the years to minimize this mixing and fouling, and the design has been improved, but this phenomenon continues to affect the performance of two-stroke engines. The size, position, and direction of these holes determines how successful the blowdown will be and how well the engine will perform.

The air / fuel mixture flows out of the bypass port on the left, fills the cylinder for the next combustion cycle, and helps "blow" the exhaust gases through the exhaust port on the right.

GAS DISTRIBUTION PHASES

In a two-stroke engine, several events occur simultaneously. They overlap and affect each other, and their effect is difficult to track simply by looking at the valve timing. The valve timing diagram makes these numbers easier to understand.

In the example of the diagram, the exhaust port opens at 80 degrees before BDC (BBDC). It is also 100 degrees After TDC (ATDC). As the exhaust port opens closer to BDC, the phase is measured from this position. The total opening time (duration) of any channel is determined by adding individual rotations.

PRACTICAL USE

The Mungen MT12 engine used to drive the Yokomo GT-4R showed flat power despite having a very significant increase in peak power. This was achieved by optimizing the valve timing for racing.

I recently spoke with renowned engine modification expert Dennis Ritchie from Texas. Dennis modified hundreds of engines for his customers' boats and cars every year, in fact, he modified Steve Pond's Mugen MT12 engine for the Yokomo GT-4R, and it worked very well. He kindly set aside his time for a discussion about ducts, valve timing and duct modifications.

Dennis Ritchie sees a significant difference in valve timing philosophy between expensive 12 and 15 displacement engines and 21 displacement engines. According to Denis, small engines have much more conservative valve timing.

Here's a typical example:

• INLET - opens at 40 degrees After BDC, closes at 48 degrees After TDC, duration 188 degrees.
• EXHAUST - opens at 78 degrees Before BDC, closes at 78 degrees After BDC, duration 156 degrees.
• BYPASS - opens at 60 degrees Before BDC, closes at 60 degrees After BDC, duration 120 degrees.

He said, “While the exhaust and bypass times are somewhat low, the greatest increase in performance is high revs obtained by lengthening the admission time. "According to my calculations, if the inlet opening remains unchanged and the close advances to about 65 degrees After ATDC, then the admission time is extended to 205 degrees - a 9% increase. The best engines volume. 21 (3.44 cc) always have advanced valve timing.

Here are some typical times for an advanced 21cc engine. inch (3.44 cc):
- intake 210 degrees;
- exhaust 180 degrees;
- bypass 126 degrees.

Dennis said these engines "safely" use fuel with 30% nitromethane and, after modifications, their peak power is between 33,000 and 34,000 rpm.

The bypass and exhaust ports allow compressed gas to escape from the top and bottom of the piston during engine cycles. Having enough time (phase duration) for this is only half the story. Having a sufficiently large hole (hole area) is the other half. To put it another way: the time it takes to move a certain amount of gas through the hole depends on the area of ​​the hole.

An analogy might be helpful: 50 people have 30 seconds to leave the premises after a fire alarm sounds. If the door is fully open, they will easily leave the room within the allotted time. If the door is faulty and only partially open, people can still exit, but there is a crush on the door, which will allow a maximum of 35 people to leave the premises at the appointed time. Arithmetic shows that a partially open door will allow only 70% of people to leave at the appointed time. A similar situation exists for gases trying to pass through the bypass and exhaust ports. If the flow is too limited, the hole can be widened to increase its area, or it can be made higher to increase both its area and phase duration. Each solution has a different effect. Deciding which one is the best is a subject of long study and experience.

Most engine mods aim to increase power. The easiest way to do this is to get the engine running faster. When the maximum RPM is increased, the channels remain open for a shorter time. Based on experience with a particular motor, the modifier expands the hole or increases its height - or a combination of both. This practice is known as "porting" (modifying channels or holes).

Hole shapes, sizes and positions are very critical to engine performance and you cannot make one change without affecting engine performance elsewhere. It's always a compromise.