Device at work
At two-stroke engines with crank-chamber purge there is no special gas distribution mechanism. Gas distribution is carried out using a cylinder, a piston and a crankcase, while the crank chamber serves as the body of the scavenge pump.The cylinder has windows that open and close with a moving piston. A combustible mixture from the crankcase enters the cylinder through the windows and exhaust gases exit the cylinder.
In two-stroke engines, loop and direct-flow purge circuits are used. Loop schemes are characterized by the rotation of the combustible mixture as it moves inside the cylinder in such a way that it forms a fly. There are return and transverse loop circuits.
With a once-through scheme, the combustible mixture usually enters from one end of the cylinder, and the combustion products exit from the other end.
Engines with various types gas distribution systems.
On fig. 54a shows a cylinder with a purge port located opposite the outlet port. When blowing, when the piston is near n. m.t., the combustible mixture, pre-compressed in the crankcase, enters the cylinder through the purge window and is directed by the deflector on the piston up to the combustion chamber. Then the combustible mixture goes down, displacing the exhaust gases through the exhaust window, which closes by the end of the purge. When expelled from the cylinder through the exhaust port of the exhaust gases, a slight leakage of the combustible mixture occurs.
The described transverse scavenging is almost never used. More perfect is the reciprocating scavenging, carried out with a conventional piston with a flat or slightly convex head. Such pistons make it possible to use a combustion chamber close in shape to a hemispherical chamber.
With loop-back purge, there are two purge windows in the engine cylinder (Fig. 54, b), directing two jets of combustible mixture at an angle to one another on the cylinder wall located opposite the exhaust window. Jets of the combustible mixture rise up to the combustion chamber and, making a loop, fall down to the outlet window. Thus, the exhaust gases are expelled and the cylinder is filled with a fresh mixture.
The return two-channel purge has the greatest distribution. It is used both in engines of domestic and foreign motorcycles (M-104, Kovrovets-175A, Kovrovets-175B and Kovrovets-175V, IZH Jupiter, Java, Panonia, etc. ).
Three-channel purge (Fig. 54, e) is used, for example, for Tsyundap engines, four-channel purge (Fig. 54, d) - for IZH-56 motorcycle engines, cross-shaped two-channel purge (Fig. 54, e) - for Ardi engines, four-channel (Fig. 54, e) -_. for Villiers engines. 
With all the described purge methods, a single-piston engine has a symmetrical valve timing diagram (Fig. 55). This means that* if the intake phase starts before the piston arrives at c. m. t. (for example, beyond 67.5 °), then its end occurs through 67.5 ° of the angle of rotation of the crankshaft after c. m. t. Also begin and end relative to n. m. t. phases of release and purge. The exhaust phase is greater than the purge phase. Filling the cylinder with a combustible mixture occurs all the time with the outlet port open. This feature of gas distribution with symmetrical phases limits the possibility of increasing the liter engine power. In addition, the compressed working mixture contains a relatively large amount of residual gases. To reduce the amount of residual gases and improve the filling of the cylinder with a combustible mixture, the purge is improved. To do this, sometimes the design of the engine is changed, although it is more advisable to achieve an increase in power from a conventional two-stroke engine without complicating its design. At the Dunelt engine (Fig. 56, a), a stepped piston was used to increase the amount of incoming combustible mixture. The volume described by the lower part of the oversized piston is about 50% larger than the volume of the upper part of the cylinder. 
The Bekamo engine (Fig. 56, b) has an additional large-diameter cylinder with a piston having a small stroke. The piston is driven by a connecting rod from an additional crank on the crankshaft. Such engines, in contrast to engines with superchargers, are called engines with "backup" (engines of this type were installed, in particular, on some domestic sport bikes). In these engines, gas distribution with symmetrical phases is carried out by one piston. However, the outlet window closes later than the purge window. The piston delivers more mixture when the exhaust port is open, so that the cylinder is not filled with a compressed combustible mixture, as is the case in a supercharged engine, in which intake occurs partially with the exhaust port or valve closed.
To increase the filling of the engine with a combustible mixture, spool devices are also used, with the help of which the intake phase is increased. Options spool device are the installation of a spool on the cylinder instead of a pipe for the carburetor (Fig. 57, a) or on the crankcase (Fig. 57, b), as well as the spool proposed by the author in the hollow crankshaft main journal. In the latter case, it is possible to change the valve timing during engine operation (Fig. 57, c) and use its vortex motion in the crankcase to form and stop the combustible mixture jets. Such a design, but without a device for changing the valve timing, is used, in particular, on the D-4 bicycle engine.
Record results are shown by MZ motorcycle engines manufactured in the GDR, in which the combustible mixture is supplied to the central part of the crankcase through a device located in it with a rotating springy spool (Fig. 57, d) made of sheet steel.
Direct-flow scavenging engines with two pistons in two cylinders with a common combustion chamber (the so-called two-piston engines) are distinguished by high power. 
The Junkers engine with direct-flow scavenging has the following device (Fig. 58, a). The cylinder contains two pistons moving towards each other. The middle part of the cylinder between the bottoms of the pistons when they are in c. mt serves as a combustion chamber. It contains a spark plug. The combustible mixture enters through the windows on the right side of the cylinder and displaces the exhaust gases into the exhaust ports located on the left side of the cylinder. In this case, the combustible mixture almost does not mix with the exhaust gases. 
The cylinder can be fed in the usual way using a crank-chamber purge or a separate compressor supplying the mixture with a spool device. Each piston is connected by a connecting rod to a separate crankshaft. The crankshafts are interconnected by gears so that when approaching n. m.t., the left piston opens the exhaust windows approximately 19 ° earlier than the right piston opens the purge windows. The release of exhaust gases begins earlier than in a single-piston engine, and, accordingly, the pressure in the cylinder is lower by the beginning of the purge. When the piston moves from n. m.t.sq. m. t., unlike one piston engines, the exhaust ports close before the purge ports, and the cylinder is filled with the exhaust ports closed for approximately the time corresponding to the crankshaft rotation by 29 *. The asymmetric diagram of the blowdown and exhaust phases with direct-flow blowdown makes it possible to effectively use the supercharger to obtain high power.
The domestic engine of the GK-1 racing motorcycle is similarly arranged.
Engines of this design are complex and expensive to manufacture, not. correspond to the layout adopted in motorcycle construction and therefore have not received mass distribution.
There are direct-flow scavenged engines that are more convenient to place on a motorcycle. In engines with direct-flow scavenging according to the Zoller scheme, two pistons move in a U-shaped cylinder. The combustion chamber is located in the middle. The combustible mixture enters through a window on the right side of the cylinder, and the exhaust gases exit through a window on its left side. The movement of the pistons, which provides asymmetric purge and exhaust phases, is carried out using various crank mechanisms. For DKV engines (Fig. 58, b), one piston is installed on the main connecting rod, and the other on the trailer. The Pooh engine (Fig. 58, c) has a forked connecting rod. For Triumph engines with the Zoller scheme, crankshaft consists of two cranks offset one relative to the other and two connecting rods (Fig. 58, d).
With direct-flow purge, the cylinders can be placed under acute angle- combustion chamber at the top of the corner (Fig. 58, e). In this case, the combustion chamber is less stretched than with a U-shaped cylinder. Otherwise, such an engine is similar to the engine of the Juncker system.
Direct-flow purge and parts of the cylinder located at an angle have domestic engines with superchargers of racing motorcycles S-1B, S-2B and S-3B, which are distinguished by a high liter power.
Service
The gas distribution in a two-stroke engine is most often disturbed when excess air enters it and when the resistance of the exhaust tract increases. It is necessary to monitor the tightness of the crankcase, tighten the connections in a timely manner, change damaged gaskets and seals, and also clean the exhaust windows of the cylinder, pipe and muffler from carbon deposits.The exhaust valve begins to open at the end of the expansion process in advance of the b.w.t. at the angle φ r.v. = 30h-75° (Fig. 20) and closes after t.m.t. with a delay by an angle φ z.v., when the piston moves in the filling stroke in the direction to the n.m.t. The beginning of the opening and closing of the intake valve are also shifted relative to the dead points: the opening starts before TDC. ahead of the angle φ 0 . vp, and closing occurs after n.m.t. with a delay at the angle φ z.vp. at the beginning of the compression stroke. Most of the discharge and filling processes are separate, but about w.m.t. the intake and exhaust valves are open for some time at the same time. The duration of valve overlap, equal to the sum of the angles φ w.v + φ o.vp, is small for reciprocating 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; higher speed engines.
Gas exchange periods in two-stroke engines
In a two-stroke engine, gas exchange processes occur when the piston moves near the n.m.t. and occupy part of the piston stroke in the expansion and compression strokes.
In engines with a loop gas exchange scheme, both intake and exhaust windows are opened by a piston, so the valve timing and window cross-sectional area diagrams are symmetrical with respect to n.m.t. (Fig. 24, a). In all engines with direct-flow gas exchange schemes (Fig. 24, b), the phases of opening the exhaust windows (or valves) are performed asymmetrical with respect to n.m.t., thereby achieving better filling of the cylinder. Typically, inlet ports and outlet ports (or valves) close at the same time or with a slight difference in angle. It is also possible to carry out asymmetric phases in an engine with a loop gas exchange scheme,
if you install (at the inlet or outlet) additional devices - spools or valves. Due to the lack of 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 of the crankshaft, which is 3-3.5 times less than in four-stroke engines. Opening angle of exhaust windows (or valves) φ o.v. \u003d 50-90 ° BC, and the angle of advance of their opening φ pr \u003d 10-15 0 . In high-speed engines with exhaust through valves, these angles are larger, and in engines with exhaust through windows, they are smaller.
In two-stroke engines, the processes of exhaust and filling occur for the most part together - with both intake (purge) and exhaust ports (or exhaust valves) open at the same time. Therefore, air (or a combustible mixture) enters the cylinder, as a rule, provided that the pressure in front of the intake ports is greater than the pressure behind the exhaust ports (valves).
Literature:
Nalivaiko V.S., Stupachenko A.N. Sypko S.A. Guidelines for conducting laboratory work on the course " Marine internal combustion engines”, Nikolaev, NKI, 1987, 41s.
marine engines internal combustion. Textbook / Yu.Ya. Fomin, A.I. Gorban, V.V. Dobrovolsky, A.I. Lukin and others - L.: Shipbuilding, 1989 - 344 p.: ill.
Internal combustion engines. Theory of piston and combined engines: Ed. A.S. Orlina, M.G. Kruglova - M .: Mashinostroenie, 1983 - 372 pages.
Vanscheidt V.A. Marine internal combustion engines. L. Shipbuilding, 1977.-392s.
Types of purge of the combustible mixture of an internal combustion engine.
There are two main types of blowdown: deflector (transverse) and non-deflector (return or loop).
A deflector is a special protrusion - a visor - on the bottom of the piston, which serves to ensure the correct direction of the flow of the combustible mixture entering the cylinder through the purge window. On fig. 44 shows a diagram of a deflector purge.
The mixture compressed in the crankcase through the purge channel and the window enters the cylinder, meeting the deflector on its way. The flow of the mixture is deflected upward into the combustion chamber, and from there it goes down to the exhaust port, forcing the exhaust gases out of the cylinder through it. With such a purge system, the exhaust port is located opposite the purge port, which to some extent contributes to an increase in the loss of the working mixture through the exhaust port during cylinder purge. Engines with deflector scavenging have increased consumption fuel. The presence of a deflector on the bottom of the piston increases its weight and worsens the shape of the combustion chamber. Nevertheless, for a number of design reasons, deflector purge is widely used for outboard motors: for example, the Moskva motor with a capacity of 10 hp is arranged. with.
Somewhat more economical is achieved by using a non-deflector purge. The scheme of the return, two-channel purge is shown in fig. 45.
In this case, the piston is made with a flat or slightly convex bottom. Scavenging streams collide and rise up along the cylinder wall, forcing exhaust gases into the exhaust port. According to the number of purge channels and the nature of the movement of the mixture, this type of purge is called two-channel, loop.
The return loop purge can be three- and four-channel; in the latter case, the purge channels are located side by side, in pairs or crosswise.
Rice. 45. Scheme of return (loop) non-deflector purge
Return, two-channel purge is more common. Outboard motors ZIF-5M and Strela have such a purge.
The use of non-deflector purge makes it possible to obtain high compression ratios with the most advantageous shape of the combustion chamber, which makes it possible to remove a large liter of power from the engine. Racing two stroke motors with a crank-chamber purge, as a rule, they have a two- or three-channel return loop purge.
The flow of the process of purging and filling the crankcase of a two-stroke engine with a fresh working mixture depends to a large extent on the size of the windows and the duration of their opening by the piston. The beginning of the opening and closing of the intake, purge and exhaust ports of the cylinder, as well as the duration of intake, purge and exhaust, expressed in degrees of the crankshaft angle, can be seen on the engine timing diagram (Fig. 46).
The period corresponding to the angle of rotation of the crankshaft, when the crankcase is filled with fresh working mixture through the open inlet window, is called the intake phase. Periods corresponding to the angles of rotation of the crankshaft at opening of the purge and exhaust windows are called the purge and exhaust phases.
On fig. 46 shows the gas distribution diagram of the Strela engine. In this engine, the valve timing, expressed in degrees of the crankshaft angle, is: the inlet phase to the crankcase is 120 °, the purge is 110 ° and the exhaust is 140 °.
It can be seen from the diagram that with respect to the axis passing through the dead points, the right and left parts of the diagram are symmetrical. This means that if the inlet port begins to open with the piston 60° before TDC, then it will close 60° after TDC. The opening and closing of the inlet and purge windows occurs in a similar way. The duration of the exhaust phase is usually 30-35° longer than the duration of the purge phase. The described engine is called a three-window.
The symmetrical valve timing of a two-stroke engine with a crank-chamber purge adversely affects its liter power and efficiency.
Rice. 46. Diagram of gas distribution of outboard engines outboard motors ZIF-5M and Strela
A short duration of the intake phase reduces the filling of the crankcase and, consequently, the engine power. Increasing the height of the inlet window has its limit: it increases the amount of mixture sucked into the crankcase during the upward stroke of the piston, but it leads to losses due to the mixture being thrown back into the carburetor through the open window when the piston moves down. The duration of the intake phase depends on the engine speed. If the engine does not exceed 3000-4000 rpm, the intake phase does not usually exceed 110-120° of the crank angle. For racing engines that develop 6000 rpm or more, it reaches 130-140 °, but when working at low speeds, such an engine is observed to throw the mixture back into the carburetor.
The exhaust phase of high-speed engines is also increased and is 150-160°. At the same time, the exhaust window is 7-8 mm higher than the purge window. The need to expand the phases for racing multi-turn engines is explained by the fact that high speed the time (duration) of opening the windows decreases, as a result of which the filling of the cylinders with the working mixture and the engine power decrease.
Rice. 47. Scheme of two-stroke engines with spool valve timing: a- with a disk spool on the crankshaft; b- with a drive cylindrical spool, (faucet)
It is possible to increase the filling of the crankcase of a two-stroke engine by using an intake system through a rotating spool or reed valves.
In the first case, on the neck of the crankshaft, inside the crankcase, a disk with a hole is installed to pass the working mixture sucked into the crankcase. The second hole is located in the upper wall of the crankcase, against which the spool is pressed by a spring. During the rotation of the crankshaft, the spool rotates with it; when the hole in the spool coincides with the inlet window in the crankcase wall, the mixture fills the internal volume of the crankcase. Schemes of an engine with suction through a rotating spool are shown in fig. 47.
The advantage of such a device is the ability to fully use the upward stroke of the piston and bring the intake phase to 180-200° of the crankshaft angle. The intake of the mixture into the crankcase begins as soon as the upper edge of the piston closes the purge window. The intake ends after 40-50 °, having passed the TDC (Fig. 48).
The intake phase diagram of such an engine is asymmetric.
Rice. 48. Diagram of gas distribution of a two-stroke engine with spool control of the release of a combustible mixture into the crankcase
The quality of the internal combustion engine of a car depends on many factors, such as power, coefficient useful action, volume of cylinders.
The gas distribution phases are of great importance in the engine, and the efficiency of the internal combustion engine, its throttle response, and the stability of idling depend on how the valves overlap.
In standard simple engines, timing change is not provided, and such motors are not very efficient. But recently, more and more often on cars of leading 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 changes more and more often.
Valve timing diagram of a two-stroke engine
A two-stroke engine differs from a four-stroke engine in that the duty cycle takes place in one revolution of the crankshaft, while on a 4-stroke internal combustion engine it occurs in 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 valve overlap angle is indicated in degrees of position to / in.
In 4-stroke engines, the filling cycle of the working mixture occurs 10-20 degrees before the piston reaches top dead center, and ends after 45-65º, and in some internal combustion engines even later (up to one hundred degrees), after the piston has passed bottom point. The total duration of the intake in 4-stroke engines can last 240-300 degrees, which ensures good filling of the cylinders with the working mixture.
In 2-stroke engines, the duration of the intake of the air-fuel mixture lasts at a crankshaft turn of approximately 120-150º, and the purge also lasts less, so filling with the working mixture and exhaust gas purification in two-stroke internal combustion engines is always worse than in 4-stroke power units. The figure below shows the valve timing diagram of a two-stroke motorcycle engine engine K-175. 
Two-stroke engines are rarely used on cars, as they have lower efficiency, poorer efficiency and poor exhaust gas purification from harmful impurities. The last factor is especially relevant - in connection with 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 advantages, especially diesel models:
- power units are more compact and lighter;
- they are cheaper;
- 2-stroke motor accelerates faster.
Many cars in the 70s and 80s of the last century were mainly equipped with carbureted engines with a "trawler" ignition system, but many advanced car manufacturing companies already then began to equip engines with an electronic engine control system, in which a single unit (ECU) controlled all the main processes. Now almost all modern cars have an ECM - electronic system It is used not only in gasoline, but also in diesel internal combustion engines.
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 needs to be supplied to the cylinders at certain loads (revs), which ignition timing to set.
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 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 oil) is simultaneously supplied to all cylinders, the engine runs randomly, and on some models of the car, the internal combustion engine does not start at all. 
Valve timing regulator
In the early 90s of the 20th century, the first engines with automatic timing change began to be produced, but here it was no longer the sensor that controlled the position of the crankshaft, but the phases themselves 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 timing gear;
- at idle and low speeds, the camshaft with the camshaft is fixed in the standard position, as it was set according to the marks;
- with an increase in speed under the influence of hydraulics, the clutch rotates 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's 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 (the M50 engines have a two-shaft timing system), and from 1996 the Double VANOS system began to be used, with which the position of the exhaust and intake r / shafts was already regulated. 
What is the benefit of a timing belt regulator? On the Idling overlapping of the valve timing is practically not required, and in this case it even harms the engine, since when the camshafts are shifted, the exhaust gases can enter the intake manifold, and part of the fuel will enter exhaust system without completely burning out. But when the engine is running at maximum power, the phases should be as wide as possible, and the higher the speed, the more valve overlap is needed. The clutch of the timing change 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, 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 quite economically consumed.
The variable valve timing system (CVG) 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 global automakers have developed their own SIFG, not only changing the position of the camshafts, but also the level of valve lift in the cylinder head is used. For example, Nissan uses a CVTCS system, which is controlled by a variable valve timing (solenoid valve). At idle, this valve is open, and does not create pressure, so the camshafts are in their original state. The opening valve increases the pressure in the system, and the higher it is, the greater the angle the camshafts are shifted. 
It should be noted that SIFGs are mainly used on engines with two camshafts, where 4 valves are installed in the cylinders - 2 intake and 2 exhaust.
Devices for setting the valve timing
In order for the engine to work without interruption, it is important to correctly set the timing phases, install in the desired position camshafts relative to the crankshaft. On all engines, the shafts are set according to the marks, and a lot depends on the accuracy of the installation. If the shafts are set incorrectly, various problems arise:
- the motor is unstable at idle;
- ICE does not develop power;
- there are shots in the muffler and pops in the intake manifold.
If the marks are mistaken by a few teeth, it is possible that the valves 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 position angles are measured. The template can be used to check the existing angles, and if they are not set correctly, the shafts should be reinstalled. The fixture for 406 motors is a set consisting of three elements:
- two goniometers (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 upper plane of the cylinder head at an angle of 19-20º with an error of ± 2.4 °, moreover, the intake roller cam should be slightly higher than the exhaust camshaft cam. 
There are also special tools for installing camshafts on BMW engines models M56/ M54/ M52. The installation kit for the gas distribution phases of the internal combustion engine BVM includes:
Malfunctions of the variable valve timing system
You can change the valve timing different ways, and recently the most common is the rotation of the p / shafts, although the method of changing the amount of valve lift is often used, the use of camshafts with cams of a modified profile. Periodically, various malfunctions occur in the gas distribution mechanism, due to which the motor 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 clutch is clogged with dirt;
- the timing chain has stretched;
- chain tensioner defective.
Often in the event of malfunctions in this system:
- are declining idling, 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 engine does not start well, it has to be driven with a 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 diagnostic lamp check engine At the same time, it does not always light up, so it is difficult to understand that failures occur precisely in electronics.
Often, timing problems arise due to hydraulic clogging - 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 idle, but does not develop speed at all. In the case when the mechanism remains in the position of maximum valve overlap, the engine may not start well. 
Unfortunately, the engines Russian production SIFG is not installed, but many motorists are tuning the internal combustion engine, trying to improve the performance of the power unit. The classic version of the engine modernization is the installation of a “sports” camshaft, in which the cams are shifted, their profile is changed.
This r / shaft has its advantages:
- the motor becomes torquey, clearly responds to pressing the gas pedal;
- are improving dynamic characteristics car, the car literally tears from under itself.
But in such tuning there are also disadvantages:
- idle speed becomes unstable, you have to set them within 1100-1200 rpm;
- fuel consumption increases;
- it is quite difficult to adjust the valves, the internal combustion engine requires careful tuning.
Quite often, VAZ engines of models 21213, 21214, 2106 are subjected to tuning. The problem of VAZ engines with a chain drive is the appearance of “diesel” noise, and often it occurs due to 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 motor runs quieter with it, and the chain wears out less - its resource averages 150 thousand km.
In most designs of two-stroke engines, there is no valve mechanism and the gas distribution is carried out by the working piston through the exhaust, intake and purge ports. The absence of a valve drive simplifies the design of the engine and facilitates its operation. A significant disadvantage of valveless gas distribution is the insufficient cleaning of the cylinders from combustion products during its purge.
Purge systems are divided into two main types: contour and direct-flow. Purge, outlet windows with a contour purge system are located at the bottom of the cylinder. The scavenging air moves up along the cylinder contour, then turns 180° at the cover and goes down, displacing the combustion products and filling the cylinder. With direct-flow purge systems, the purge air moves from the purge windows to the exhaust organs in only one direction - along the axis of the cylinder. The location of the purge and outlet ports, their inclination to the axis of the cylinder are very important for all purge systems.
On fig. 160,hell various purge schemes are shown. Cross-slit blowdowns (schemes a and b) are the simplest and are used in various engines. In the schemeb used in diesel engines of high power, purge windows have an eccentric arrangement in the horizontal plane and are inclined to the vertical plane. This arrangement of windows improves ventilation. Residual gas coefficient 0.1-0.15. The loop-loop purge (scheme c) with a radial arrangement of the purge windows is characterized by the fact that the purge air first enters the piston bottom, and then, having described a loop along the contour, displaces the combustion products into the outlet windows, which are located above the purge windows and have a slope of 10 15° to the axis of the cylinder down. The coefficient of residual gases is 0.08-0.12. Loop purges are used in low-speed and medium-speed engines.
Direct-flow purge systems are valve-slotted (scheme d) and direct-flow slotted (scheme e).
With direct-flow-valve purge, tangentially directed windows are located at the bottom of the cylinder along the circumference. Through the exhaust poppet valves (one to four) is released. The exhaust valves are actuated by the camshaft, which allows you to set the most advantageous valve timing, as well as, if necessary, provide additional charging due to the later closing of the purge windows. The scavenging air, moving in a spiral manner, ensures good displacement of combustion products and mixes well with the atomized fuel. This type of purge is used in powerful low-speed diesel engines of the Bryansk plant, Burmeister and Vine, as well as in high-speed diesel engines. Direct-flow-valve purge is one of the most efficient, the coefficient of residual gases is 0.04-0.06.
Straight-through-slit purge (Fig. 160,d ) are used in engines with oppositely moving pistons. Purge and exhaust ports are located around the entire circumference of the cylinder: exhaust at the top, and purge at the bottom. The purge windows have a tangential arrangement. This type of purge is currently the most efficient. The quality of cylinder cleaning is not inferior to cleaning in four-stroke engines. Residual gas coefficient 0.02-0.06. Direct-flow slotted blowing is used in Doskford engines, in 10D100 engines, etc.