The quality of operation of a car's internal combustion engine depends on many factors, such as power, efficiency, and cylinder volume.
The valve timing in an engine is of great importance, and the efficiency of the internal combustion engine, its throttle response, and stability at idle speed depend on how the valves overlap.
In standard simple engines, timing timing is not changed, and such engines are not highly 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 camshafts as the number of revolutions in the internal combustion engine changes.
Valve timing diagram of a two-stroke engine
A two-stroke engine differs from a four-stroke engine in that its operating cycle takes place in one revolution of the crankshaft, while on a 4-stroke internal combustion engine it takes place in two revolutions. The valve timing in the internal combustion engine is determined by the duration of the opening of the valves - exhaust and intake; the valve overlap angle is indicated in degrees of c/v position.
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 45-65º, and in some internal combustion engines even later (up to one hundred degrees), after the piston has passed bottom point. The total intake duration 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 approximately 120-150º at the crankshaft turn, and purging also lasts less, so filling with the working mixture and cleaning exhaust gases 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, K-175 engine. 
Two-stroke engines are used infrequently in cars, as they have lower efficiency, worse efficiency and poor purification of exhaust gases from harmful impurities. The last factor is especially relevant - due to tightening environmental standards, it is important that the engine exhaust contains a minimum amount of CO.
But still, 2-stroke internal combustion engines also have their advantages, especially diesel models:
- power units are more compact and lighter;
- they are cheaper;
- A two-stroke engine accelerates faster.
Many cars in the 70s and 80s of the last century were mainly equipped with carburetor engines with a “trawler” ignition system, but many leading car companies even then began to equip engines with an electronic engine control system, in which all the main processes were controlled by a single block (ECU). Now almost all modern cars have an ECM - the electronic system is used not only in gasoline, but also in diesel internal combustion engines.
Modern electronics contain various sensors that monitor engine operation, sending signals to the unit about the state of the power unit. Based on all the data from the sensors, the ECU makes a decision - how much fuel needs to be supplied to the cylinders at certain loads (speeds), what ignition timing should be set.
The valve timing sensor has another name - the camshaft position sensor (CPS), it determines the position of the timing belt relative to the crankshaft. Its readings determine the proportion in which fuel will be supplied to the cylinders, depending on the number of revolutions and ignition timing. If the DPRV does not work, it means that the timing belt 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) is supplied to all cylinders simultaneously, the engine operates inconsistently, and on some car models 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 operating principle of such a system is as follows:
- the camshaft is connected to a hydraulic coupling;
- the camshaft is also connected to this clutch;
- at idle and low speeds, the cam gear with the camshaft is fixed in the standard position, as it was installed according to the marks;
- when the speed increases 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 used 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 began to be used, with the help of which the position of the exhaust and intake camshafts was already regulated. 
What advantage does a timing adjuster provide? At idle, valve timing overlap 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 being completely burned. 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 timing clutch makes it possible to effectively fill the cylinders with the working mixture, which means increasing the efficiency of the engine and increasing its power. At the same time, at idle, the r/shafts with the clutch are in their original state, and combustion of the mixture occurs in full. It turns out that the phase regulator increases the dynamics and power of the internal combustion engine, while fuel consumption is quite economical.
The variable valve timing system (VPV) provides lower fuel consumption, reduces the level of CO in the exhaust gases, and allows for more efficient use of internal combustion engine power. Different global automakers have developed their own SIFG, which applies not only changes in the position of the camshafts, but also the level of valve lift in the cylinder head. For example, Nissan uses a CVTCS system that 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. The opening valve increases the pressure in the system, and the higher it is, the greater the angle the camshafts move. 
It should be noted that SIFG are mainly used on engines with two camshafts, where 4 valves are installed in the cylinders - 2 intake and 2 exhaust.
Tools for setting valve timing
In order for the engine to operate without interruption, it is important to set the timing timing correctly and install the camshafts in the desired position relative to the crankshaft. On all engines, the shafts are aligned according to marks, and a lot depends on the accuracy of the installation. If the shafts are not aligned correctly, various problems arise:
- the engine is unstable at idle;
- The internal combustion engine does not develop power;
- There are shots in the muffler and popping noises in the intake manifold.
If the marks are wrong 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 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 incorrect, the shafts should be reinstalled. The device 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 at 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°, and the intake cam should be slightly higher than the exhaust camshaft cam. 
There are also special devices for installing camshafts on BMW engines of the M56/ M54/ M52 models. The kit for installing the valve timing of the internal combustion engine of the BVM includes:
Malfunctions of the variable valve timing system
The valve timing can be changed in various ways, and recently the most common is turning the r/shafts, although the method of changing the amount of valve lift and using camshafts with cams of a modified profile are often used. From time to time, various malfunctions arise in the gas distribution mechanism, due to which the engine begins to work intermittently, becomes “stupid”, and in some cases does not start at all. The causes of problems may be different:
- the solenoid valve is faulty;
- the phase change clutch is clogged with dirt;
- the timing chain has stretched;
- The chain tensioner is faulty.
Often when malfunctions occur in this system:
- Idle speed decreases, 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, you have to turn it with the starter several times;
- a chirping sound is heard coming from the SIFG coupling.
By all indications, the main cause of problems with the engine is the failure of the SIFG valve; usually, computer diagnostics reveal an error in this device. It should be noted that the Check Engine diagnostic lamp does not always light up, so it is difficult to understand that failures are occurring specifically in the electronics.
Often, timing problems arise due to clogging of the hydraulics - bad oil with abrasive particles clogs the channels in the coupling, and the mechanism jams in one of the positions. If the clutch “wedges” in its original position, the internal combustion engine runs quietly at idle, but does not develop speed at all. If the mechanism remains in the maximum valve overlap position, the engine may not start well. 
Unfortunately, SIFG is not installed on Russian-made engines, but many motorists are engaged in tuning the internal combustion engine, trying to improve the characteristics of the power unit. The classic option for upgrading an engine is to install a “sports” camshaft, whose cams are shifted and their profile is changed.
This r/shaft has its advantages:
- the engine becomes responsive and clearly responds to pressing the gas pedal;
- The dynamic characteristics of the car improve, the car literally tears out from under itself.
But this tuning also has its disadvantages:
- idle speed becomes unstable, you have to set it 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 subject to tuning. The problem with 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 of installing an automatic tensioner instead of the standard factory one. 
Often, a single-row chain is installed on VAZ-2101-07 and 21213-21214 engine models: the engine runs quieter with it, and the chain wears out less - its service life is on average 150 thousand km.
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 the distribution body. There are several holes made on the surface of the engine cylinder. They are called windows, and they are essential for the push-pull cycle. The purpose of the intake and exhaust ports is quite obvious - the intake port allows the air-fuel mixture to enter the engine for subsequent combustion, and the exhaust port ensures the removal of gases resulting from combustion from the engine. The purge channel serves to ensure flow from the crank chamber, into which it previously entered, into the combustion chamber, where combustion occurs. 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 on top by a piston, which means that the volume of this chamber, and, consequently, the pressure inside it, changes as the piston moves back and forth 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 downward, the volume decreases and the pressure becomes higher than atmospheric).
The intake port on the cylinder wall is closed most of the time by the piston skirt; it opens when the piston approaches the top of its stroke. The vacuum created draws 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 distribution body for obvious reasons, is the simplest type of two-stroke engine; the number of moving parts in it is not significant. In many ways this is a significant advantage, but leaves much to be desired in terms of efficiency. At one time, in almost all two-stroke engines, the piston served as a distribution element, but in modern designs this function is assigned to more complex and efficient devices
Improved two-stroke engine designs
Effect on gas flow One of the reasons for the inefficiency of the two-stroke engine described above is incomplete purification of exhaust gases. Remaining in the cylinder, they prevent the penetration of the entire volume of fresh mixture, and, therefore, reduce power. There is also a related issue with this: fresh mixture from the purge port window flows directly into the exhaust port, and as mentioned earlier, to minimize this, the purge port window directs the mixture upward.
Pistons with deflector
Cleaning efficiency and fuel efficiency can be improved by creating moreefficient gas flow inside the cylinder. Early improvements in two-stroke engines were achieved by specially shaping the piston crown to deflect the mixture from the intake port to the cylinder head - a design called a deflector piston.” However, the use of deflector pistons on two-stroke engines was short-lived due to piston expansion problems. The heat generated in the combustion chamber of a two-stroke engine is usually higher than that of a four-stroke engine because combustion occurs at twice the rate, and the head, top of the cylinder and piston are the hottest parts of the engine. This leads to problems associated with thermal expansion of the piston. In fact, the piston is shaped when manufactured to be slightly out of circle and tapered at the top (oval-barrel profile), so that when it expands with temperature changes, it becomes round and cylindrical. Adding an asymmetrical metal protrusion in the form of a deflector on the bottom of the piston 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 weight with a mass shift from the axis of symmetry. This disadvantage became much more apparent as engines were improved to operate at higher rotational speeds.
Types of two-stroke engine purges
Loop blowing
Because the deflector piston has too many disadvantages and a flat or slightly rounded bottom Since the piston is not greatly affected by the movement of the incoming mixture or the flowing exhaust gases, another option was needed. It was developed in the 30s of the 20th century by Dr. E. Schnurle, who invented and patented it (although admittedly he originally designed it for a two-stroke diesel engine). The purge windows are located opposite each other on the cylinder wall and are directed at an angle upward and backward. Thus, the incoming mixture encounters the rear wall of the cylinder and is deflected upward, then, forming a loop at the top, falls onto the exhaust gases and promotes their displacement through the exhaust port. Consequently, good cylinder purging can be achieved by selecting the location of the purge windows. It is necessary to carefully consider the shape and size of the channels. If the bore is made too wide, the piston ring, bypassing it, may fall into the window and jam, thereby causing failure. Therefore, the size and shape of the windows is made in such a way as to guarantee the shock-free passage of the track past the windows, and some wide windows are connected in the middle by a jumper 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, which have played a large role in increasing the power of two-stroke engines. Some engines are equipped with scavenge windows that serve the sole purpose of improving scavenging and are opened shortly before the opening of the main scavenge windows, which supply most of the fresh mixture. But that's all for now. what can be done to improve gas exchange without using expensive parts to produce. To continue to improve performance, the filling phase needs to be controlled more precisely.
Suzuki Lets TW Reed Valve 
Reed valves
In any two-stroke engine design, improving efficiency and fuel economy means that the engine must operate more efficiently, this requires burning the maximum amount of fuel (hence producing maximum power) on 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. Until gas exchange processes are improved within an engine with a piston as a distribution organ, complete purification of the exhaust gases remaining in the cylinder cannot be guaranteed, and the volume of the incoming fresh mixture cannot be increased to facilitate the displacement of the exhaust gases. A solution may be to fill the crank chamber with more mixture by increasing its volume, but in practice this leads to less efficient purging. Increasing the efficiency of purging requires reducing the volume of the crank chamber and thus limiting the space available for filling with the mixture. So a compromise has already been found, and other ways to improve performance should be looked for. In a two-stroke engine, in which the piston acts as the timing element, some of the air-fuel mixture supplied to the crank chamber will inevitably be lost as the piston begins to move downward during the combustion process. This mixture is forced back into the intake port and is thus lost. A more efficient way to control the incoming mixture is needed. Mixture loss can be prevented by using a reed valve, a disc valve, or a combination of both.
The reed valve consists of a metal valve body and a seat mounted on its surface withsynthetic rubber seal. Two or more reed valves are attached to the valve body and under normal atmospheric conditions these reeds are closed. In addition, to limit the movement of the petal, restrictor plates are installed, one for each valve petal, which serves to prevent its breakage. Thin valve petals are usually made of flexible (spring) steel, although exotic materials based on phenolic resin or fiberglass are becoming increasingly popular.
The valve is opened by bending the petals up to the restrictor plates, which are designed to open as soon as a positive pressure difference occurs between the atmosphere and the crank chamber; this occurs when the upward moving piston creates a vacuum in the crankcase. When the mixture is fed into the crank chamber and the piston begins to move downward, the pressure inside the crankcase increases to atmospheric pressure and the lobes are pressed, closing the valve. This ensures that the maximum amount of mixture is supplied and any backflow is prevented. The additional mass of the mixture fills the cylinder more completely, and purging occurs more efficiently. Reed valves were first adapted for use on existing piston-actuated engines, leading to significant improvements in engine efficiency. In some cases, manufacturers chose a combination of two designs: one - when the engine had a piston as a gas distribution element. complemented by a reed valve to continue the filling process through additional channels in the crank chamber after the piston has closed the main channel, if the crankcase pressure level allows this. In another design, windows were made on the surface of the piston skirt to finally get rid of the control that the piston has over the ports; in this case, they open and close solely under the influence of the reed valve. Development of this idea meant that the valve and intake port could be moved from the cylinder to the crank chamber. Fearful warnings that valve petals would crack and could become trapped inside the engine turned out to be largely unfounded. Moving the inlet port provides a number of benefits, the main one being: that the flow of gas into the crankcase cavity becomes more free and, therefore, more of the mixture can enter the crank chamber. This is aided to some extent by the momentum (speed and weight) of the incoming mixture. By moving the inlet port out of the cylinder, efficiency can be further improved by repositioning the purge port(s) to the optimal purge position. Of course, the basic arrangement of reed valves has come under scrutiny in recent years, and complex designs have emerged. containing two-stage petals and multi-leaf valve bodies. Recent developments in reed valves relate to the materials used for the reeds and the location and size of the reeds.
Disc valves (spool valve)
A 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 like this. so that in the normal state the channel is blocked by the disk. In order for 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 port, allowing the mixture to enter directly into the crank chamber. The passage is then blocked by a disc, preventing mixture from being blown back into the carburetor as the piston begins to move down.
The obvious advantages of using a disc valve include more precise control of the beginning and end of the process (the section, or sector, of the disc that passes 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). Also, the disc valve allows the use of a large diameter inlet port and guarantees unhindered passage of the mixture entering the crank chamber. Unlike a reed valve with a fairly large valve body, a disc valve does not create any obstructions in the intake passage, and therefore gas exchange in the engine is improved. Another advantage of the disc valve on sport bikes is the time it takes to change it to match engine performance to different trails. The main disadvantage of the disc valve is the technical difficulty of requiring small manufacturing tolerances and lack of adaptability, that is, the valve's inability to respond to changing engine needs like a reed valve. In addition, all disc valves are vulnerable to debris entering the engine with air (fine particles and dust settle on the sealing grooves and scratch the disc). Despite this. In practice, disc valves work very well and usually provide a significant increase in power at low engine speeds compared to a conventional engine with a piston as the timing element.
Combined use of reed and disc valves
The inability of the disc valve to respond to changing engine needs has led some manufacturers to consider using a combination of disc and reed valves to obtain high engine flexibility. Therefore, when conditions require it, crankcase pressure closes the reed valve, thereby closing the crank chamber side intake port, even though the cut section of the disc may still open the carburetor side intake port.
Using a crankshaft cheek as a disc valve
An interesting variant of the disc valve was used for several years on a number of scooter engines Vespa. Instead of using a separate valve unit to perform its role, manufacturers used a standard crankshaft. The plane of the right cheek of the flywheel is machined with very high precision so that when the crankshaft rotates, the gap between it and the crankcase is several thousandths of an inch. The intake port is located directly above the flywheel (on these engines the cylinder is horizontal) and is thus covered by the edge of the flywheel. By machining a recess in part of the flywheel, the port can be opened at a given point in the engine cycle in a similar way to a traditional butterfly valve. Although the resulting intake port is less straight than it could be, in practice this system works very well. As a result, the engine produces useful power over a wide range of engine speeds, and remains technically simple.
Exhaust window location
In many ways, the intake and exhaust systems on a two-stroke engine are very closely related. In the previous paragraphs we discussed methods for supplying the mixture and removing exhaust gases from the cylinder. Over the years, designers and testers have discovered that exhaust timing can have just as significant an impact on engine performance as intake timing. The exhaust phases are determined by the height of the exhaust 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 position that covers 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 window is different at low and high engine speeds, and upon closer examination we can say 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 the characteristics of the exhaust systems changing during engine operation to match changing rotation frequencies. Such systems appeared in (YPVS), (ATAS). (KIPS), (SAPC), Cagiva(CTS) and Aprilia(RAVE). The following systems are described: , and .
Yamaha Power Rivet System - YPVS
At the heart of this system is the power valve itself, which is essentially a rotary valve mounted in the cylinder liner so that its lower edge matches the upper edge of the exhaust port. At low engine speeds, the valve is in the closed position, limiting the effective window height: this improves low- and mid-range performance. When the engine speed reaches a set level, the valve opens, increasing the effective window height, which improves high-speed performance. The position of the power valve is controlled by a servomotor using a cable and pulley. YPVSi control unit - receives data on the valve opening angle from the potentiometer on the servomotor and data on engine speed from the ignition control unit; this data is used to generate the correct signal to the servomotor drive mechanism (see Fig. 1.86). Note: The company's off-road motorcycles use a slightly different version of the system due to the low power of the battery: 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. A vertical rod connects the drive mechanism to the control rod of the power valve installed in the cylinder liner. Two such power valves are located in auxiliary passages on either side of the main inlet port and are connected to the drive rod by means of a gear and rack. As the drive rod moves from side to side, the valves rotate, opening and closing auxiliary passages in the cylinder and resonator chamber located on the left side of the engine. The system is designed so that at low speeds 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 exhaust gases leaving, thus increasing the volume of the expansion chamber. At high rpm, the valves rotate to open both auxiliary passages and increase the duration of the passage opening, hence 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 duct height and larger volume of the exhaust system, and at high speeds by increasing the height of the exhaust port and smaller volume of the exhaust system. The system was further improved by introducing an intermediate gear between the drive rod and one of the valves, which ensures rotation of the valves in counter directions, as well as the addition of a flat power valve on the leading edge of the exhaust port. On larger displacement models, starting and low speed performance has been improved by adding a nozzle profile at the top of the valves.
Torque boost 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. A mechanism consisting of a rack and a roller transmits force from the regulator to the ATAC valve installed in the cylinder liner. The HERP (High Energy Resonance Tube) chamber is opened by the ATAC valve at low engine speeds and closed at high 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 of high fuel consumption and harmful emissions, is to use a fuel injection system. However, if the fuel is not supplied directly to the combustion chamber, inherent problems with the charging phase and engine efficiency still remain. The problem with direct injection of fuel into the combustion chamber is that... that fuel can only be introduced after the intake ports are closed, hence there is little time left for the fuel to atomize and fully mix with the air present in the cylinder (which comes from the crank chamber, as in traditional two-stroke engines). This creates 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 simply will 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 using 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 stream of fuel into a separate closed auxiliary chamber containing compressed air (supplied either from a separate compressor or via a check valve duct from the cylinder]. After the exhaust port is closed, the auxiliary chamber communicates with the combustion chamber through a valve or nozzle, and the mixture is fed directly to the spark plug.Aprilia claims an 80% reduction in emissions, achieved by reducing oil consumption by 60% and fuel consumption by 50%, in addition, the speed of the scooter with this system is 15% higher the same scooter with a standard carburetor.
The main advantage of using direct injection is. that, compared to a conventional two-stroke engine, there is no need to pre-mix fuel with oil to lubricate the engine. Lubrication is improved because the oil is not washed away from the bearings by the fuel and therefore less oil is required, resulting in reduced toxicity. Fuel combustion is also improved, and carbon deposits on the pistons, piston rings and exhaust system are reduced. Air is still forced through the crank chamber (its flow is determined by the throttle valve linked to the motorcycle's throttle) This means that oil is still being burned in the cylinder and lubrication and lubrication are not as effective as desired. However, the results of independent tests speak for themselves. All that is now necessary is to provide an air supply, bypassing the crank chamber.
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The time intervals from the beginning of the opening of the engine valves until they are completely closed relative to the dead points of the piston movement are called the valve timing. Their influence on engine operation is very great. Thus, the efficiency of filling and cleaning the cylinders during engine operation depends on the duration of the phases. This directly determines fuel economy, power and torque.
The essence and role of valve timing
At the moment, there are motors in which the phases cannot be changed forcibly, and motors equipped with mechanisms (for example, CVVT). For the first type of engine, the phases are selected experimentally during the design and calculation of the power unit.
Unregulated and variable valve timing
Visually, they are all displayed on special valve timing diagrams. Top and bottom dead centers (TDC and BDC, respectively) are the extreme positions of the piston moving in the cylinder, which correspond to the largest and smallest distance between an arbitrary point of the piston and the axis of rotation of the engine crankshaft. The starting points for valve opening and closing (phase length) are shown in degrees and are considered relative to the rotation of the crankshaft.
The phases are controlled using a timing belt, which consists of the following elements:
- cam camshaft (one or two);
- chain or belt drive from the crankshaft to the camshaft.
Gas distribution mechanism Always consists of strokes, each of which corresponds to a certain position of the valves at the inlet and outlet. Thus, the beginning and end of the phase depend on the angle of the crankshaft, which is connected to the camshaft, which controls the position of the valves.
For one revolution of the camshaft, the crankshaft makes two revolutions and its total angle of rotation during the operating cycle is 720°.
Circular valve timing diagram Let's consider the operation of valve timing for a four-stroke engine using the following example (see picture):
- Inlet. At this stage, the piston moves from TDC to BDC, and the crankshaft rotates 180º. The exhaust valve is closed and the intake valve is subsequently opened. The latter occurs with an advance of 12º.
- Compression. The piston moves from BDC to TDC, and the crankshaft makes another rotation of 180º (360º from the initial position). The exhaust valve remains closed and the intake valve remains open until the crankshaft rotates 40º.
- Working stroke. The piston moves from TDC to BDC under the influence of the ignition force of the air-fuel mixture. The intake valve is in the closed position, and the exhaust valve opens ahead of time when the crankshaft has not yet reached 42º BDC. At this stroke, the full rotation of the crankshaft is also 180º (540º from the initial position).
- Release. The piston moves from BDC to TDC and at the same time pushes out exhaust gases. At this moment, the intake valve is closed (it will open 12º before TDC), and the exhaust valve remains in the open position even after the crankshaft reaches TDC another 10º. The total amount of crankshaft rotation at this stroke is also 180º (720º from the starting point).
Timing timing also depends on the profile and position of the camshaft cams. So, if they are the same at the inlet and outlet, then the duration of opening of the valves will also be the same.
Why is valve actuation delayed and advanced?
To improve the filling of the cylinders, as well as to ensure more intensive cleaning of exhaust gases, the valves operate not at the moment the piston reaches the dead points, but with a slight advance or delay. Thus, the intake valve opens until the piston passes TDC (from 5° to 30°). This allows for more intensive injection of fresh charge into the combustion chamber. In turn, the closing of the intake valve occurs with a delay (after the piston has reached bottom dead center), which allows the cylinder to continue filling with fuel due to inertial forces, the so-called inertial boost.
The exhaust valve also opens early (from 40° to 80°) until the piston reaches BDC, which allows the majority of the exhaust gases to escape under its own pressure. Closing of the exhaust valve, on the contrary, occurs with a delay (after the piston passes the top dead center), which allows inertial forces to continue removing exhaust gases from the cylinder cavity and makes its cleaning more effective.
The advance and retard angles are not common to all engines. More powerful and high-speed ones have larger values of these intervals. Thus, their valve timing will be wider.
The stage of engine operation in which both valves are open simultaneously is called valve overlap. As a rule, the amount of overlap is about 10°. Moreover, since the duration of the overlap is very short and the opening of the valves is insignificant, no leakage occurs. This is a fairly favorable stage for filling and cleaning the cylinders, which is especially important at high speeds.
At the beginning of the intake valve opening, the current pressure level in the combustion chamber is higher than atmospheric pressure. As a result, the exhaust gases move very quickly towards the exhaust valve. When the engine switches to the intake stroke, a high vacuum will be established in the chamber, the exhaust valve will close completely, and the intake valve will open to a cross-sectional area sufficient for intensive filling of the cylinder.
Features of adjustable valve timing
At high speeds, the car engine requires more air volume. And since in unregulated timing valves the valves can close before a sufficient amount of it enters the combustion chamber, the operation of the engine turns out to be ineffective. To solve this problem, various methods of adjusting valve timing have been developed.
Valve timing control valve The first motors with a similar function allowed step adjustment, which made it possible to change the phase length depending on the motor reaching certain values. Over time, stepless designs have emerged to allow for smoother, more optimal tuning.
The simplest solution is a phase shift system (CVVT), implemented by rotating the camshaft relative to the crankshaft at a certain angle. This allows you to change the timing of the opening and closing of the valves, but the actual duration of the phase remains unchanged.
To directly change the duration of a phase, a number of cars use multiple cam mechanisms, as well as oscillating cams. For precise operation of regulators, complexes of sensors, controllers and actuators are used. The control of such devices can be electrical or hydraulic.
One of the main reasons for the introduction of timing control systems is the tightening of environmental standards regarding the level of exhaust gas toxicity. This means that for most manufacturers, the issue of optimizing valve timing remains one of the most important.
The exhaust valve begins to open at the end of the expansion process ahead of the b.m.t. by angle φ o.v. = 30h-75° (Fig. 20) and closes after T.M.T. with a delay by the angle φ s.v., when the piston moves during the filling stroke in the direction to ground level. The beginning of the opening and closing of the intake valve is also shifted relative to the dead points: the opening begins before TDC. advanced by angle φ 0 . vp, and closing occurs after n.m.t. with a delay by the angle φ W.W. at the beginning of the compression stroke. Most of the release and filling processes occur separately, but around the b.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 s.v + φ o.vp, is short for piston engines (Fig. 20, a), but for combined engines it can be significant (Fig. 20, b). The total duration of gas exchange is φ o.v + 360 o + φ w.vp = 400-520 o; for high-speed engines it is greater.
Gas exchange periods in two-stroke engines
In a two-stroke engine, gas exchange processes occur when the piston moves near ground level. and occupy part of the piston stroke during the expansion and compression strokes.
In engines with a loop gas exchange scheme, both the intake and exhaust windows are opened by the piston, so the gas distribution phases and cross-sectional area diagrams of the windows are symmetrical relative to the b.m.t. (Fig. 24, a). In all engines with direct-flow gas exchange schemes (Fig. 24, b), the opening phases of the exhaust ports (or valves) are asymmetrical relative to the b.m.t., thereby achieving better filling of the cylinder. Typically, inlet ports and outlet ports (or valves) close at the same time or with slight angle differences. It is also possible to implement asymmetrical 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° crankshaft rotation angle, which is 3-3.5 times less than in four-stroke engines. Opening angle of exhaust windows (or valves) φ r.o. = 50-90° BC, and the angle of advance of their opening φ pr = 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 exhaust and filling processes occur mostly together - with the intake (scavenge) and exhaust ports (or exhaust valves) open simultaneously. Therefore, air (or a combustible mixture) enters the cylinder, as a rule, provided that the pressure in front of the inlet windows is greater than the pressure behind the outlet windows (valves).
Literature:
Nalivaiko V.S., Stupachenko A.N. Sypko S.A. Methodological instructions for conducting laboratory work on the course “Marine internal combustion engines”, Nikolaev, NKI, 1987, 41 p.
Marine internal combustion engines. 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, 1983 – 372 pages.
Vanscheidt V.A. Marine internal combustion engines. L. Shipbuilding, 1977.-392 p.
Types of purging the combustible mixture of an internal combustion engine.
There are two main types of blowing: deflector (transverse) and deflectorless (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. In Fig. Figure 44 shows a diagram of deflector purge.
The mixture compressed in the crankcase enters the cylinder through the purge channel and window, 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 window, displacing exhaust gases through it from the cylinder. With such a purge system, the exhaust window is located opposite the purge window, which to some extent contributes to an increase in the loss of the working mixture through the exhaust window during purge of the cylinder. Engines with deflector scavenging have increased fuel consumption. The presence of a deflector at the bottom of the piston increases its weight and worsens the shape of the combustion chamber. However, for a number of design reasons, deflector blowing is widely used for outboard motors: for example, the Moskva motor with a power of 10 hp is designed like this. With.
Somewhat greater efficiency is achieved by using deflectorless blowing. The scheme of return, two-channel purge is shown in Fig. 45.
In this case, the piston is made with a flat or slightly convex bottom. The scavenging streams collide and rise up along the cylinder wall, displacing exhaust gases into the exhaust port. Based on the number of purge channels and the nature of the movement of the mixture, this type of purge is called two-channel, loop.
Return loop purge can be three- or four-channel; in the latter case, the purge channels are located side by side, in pairs or crosswise.
Rice. 45. Scheme of return (loop) deflectorless blowing
Return, two-channel blowing is more common. ZIF-5M and Strela outboard motors have this type of purging.
The use of deflectorless purge makes it possible to obtain high compression ratios with the most advantageous shape of the combustion chamber, which makes it possible to extract more liter power from the engine. Racing two-stroke engines with crank-chamber purge, as a rule, have a two- or three-channel return loop purge.
The process of purging and filling the crankcase of a two-stroke engine with 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 opening and closing of the cylinder intake, scavenge and exhaust ports, as well as the duration of intake, scavenge and exhaust, expressed in degrees of crankshaft angle, can be seen on the engine valve 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 intake window, is called the intake phase. The 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.
In Fig. 46 shows the gas distribution diagram of the Strela engine. For this engine, the valve timing, expressed in degrees of crankshaft rotation angle, is: intake phase into the crankcase - 120°, purge phase - 110° and exhaust phase - 140°.
The diagram shows that relative to the axis passing through the dead points, the right and left parts of the diagram are symmetrical. This means that if the intake window begins to open with the piston 60° before TDC, then it will close 60° after TDC. Opening and closing of the exhaust and purge windows occurs in the same 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 engine.
The symmetrical valve timing of a two-stroke engine with crank-chamber scavenging negatively affects its liter power and efficiency.
Rice. 46. Gas distribution diagram of engines of outboard boat motors ZIF-5M and "Strela"
A short duration of the intake phase reduces crankcase filling and, therefore, engine power. Increasing the height of the intake 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 its loss due to the mixture being thrown back into the carburetor through the open window during the downward movement of the piston. The duration of the intake phase depends on the engine speed. If the engine makes no more than 3000-4000 rpm, the intake phase usually does not exceed 110-120° of the crank angle. In racing engines developing 6000 rpm or more, it reaches 130-140°, but when operating at low speeds, such an engine experiences the mixture being thrown back into the carburetor.
The exhaust phase of high-speed engines is also increased and amounts to 150-160°. In this case, the height of the exhaust window is 7-8 mm greater than the purge window. The need to expand the phases for racing multi-turn engines is explained by the fact that at high speeds the time (duration) of opening the windows decreases, as a result of which the filling of the cylinders with the working mixture and engine power drop.
Rice. 47. Diagram of two-stroke engines with spool valve timing: a- with a disc spool on the crank; b- with a driven cylindrical valve (crane)
The crankcase filling of a two-stroke engine can be increased by using an intake system through a rotating spool or reed valves.
In the first case, a disk with a hole is installed on the crankshaft journal, inside the crankcase, to allow the working mixture to be sucked into the crankcase. The second hole is in the upper wall of the crankcase, to which the spool is pressed by a spring. As the crankshaft rotates, 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. Diagrams 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 increase the intake phase to 180-200° crankshaft rotation angle. The mixture enters the crankcase as soon as the upper edge of the piston closes the purge window. The intake ends after 40-50°, having passed TDC (Fig. 48).
The intake phase diagram of such an engine is asymmetrical.
Rice. 48. Gas distribution diagram of a two-stroke engine with spool control for the release of a combustible mixture into the crankcase
