Gas distribution mechanism (timing)

4.5.1 Gas distribution mechanism 

The gas distribution mechanism serves for the timely admission of a combustible mixture (for example, gasoline and air) into the cylinder and the release of exhaust gases. At least two valves are located in the cylinder head - intake and exhaust ones.

Note
While speaking of valves, we mean valve groups that consist of valves, springs and push rods.

The valves are driven by a camshaft, which in turn is connected through a chain, gear or belt to the crankshaft. A combustible mixture or air enters the cylinder through the intake valve. Exhaust gases escape through the exhaust valve into atmospheric air through the exhaust system.

4.5.2 The device and principle of operation of the gas distribution mechanism

Nowadays, a valve-type gas distribution mechanism with an overhead valve arrangement is mainly used in gasoline and diesel engines. This means that the valves are located on top, inside the cylinder head, as shown in Figure 4.8.

Thus, in case of an overhead arrangement, the valves with springs and their fastening parts are installed in guide bushings in the cylinder head, in which the intake and exhaust channels are also cast.

Figure 4.8 Cylinder head with gas distribution mechanism.

The force from the cams of camshaft, which is located here - in the cylinder head, is transmitted to the valves using push rods and/or rocker arms. The rocker arms are pivotally mounted on an axis, which is fixed to the cylinder head. The cylinder head is covered with a cover.

4.5.3 About expansion gap

There must be a gap (the so-called expansion gap) between the valve rod and the push rod or the end of the rocker arm of the gas distribution mechanism, which is necessary for compensating for the elongation of the valve rod when it heats up without disturbing the tightness of the valve seating in the valve seat. In other words, if there was no gap between the camshaft cam and the valve, then the valve in the process from heating to high temperature would increase in length and cease to fit tightly against the seat in the cylinder head.

The value of the expansion gap for engines of different brands is set for intake valves in a cold state in the range of 0.15-0.30 mm, and for exhaust valves that are exposed to greater heating - in the range of 0.20-0.40 mm. However, for some manufacturers, this gap may differ from the above values. 

Adjusting devices are installed on this mechanism in order to adjust the size of this gap. Although the word "device" would be too loud for an adjusting bolt and lock nut (figure 4.9) or washers of different thicknesses (Figure 4.10).

Figure 4.9 Adjustment of the expansion gap using the bolt.

Figure 4.10 Adjustment of the expansion gap using nuts (А – cylinder head without camshaft; B – cylinder head with camshaft).

Nowadays, a design with hydraulic lifters is very common. Under the influence oil pressure it pushes the rocker arm or push rod to the camshaft cam, thereby removing the negative effect of the expansion gap, namely, the impact of the cam on the push rod during operation. But it is worth mentioning that the installation of hydraulic lifters increases the cost of the design of the cylinder head and increases its requirements for the quality of the engine oil, which is used and for the frequency of its replacement, since the oil channels of the lifter can be clogged with wear products.

Note
Hydraulic lifters are described in more details below.

4.5.4 Preliminary about the camshaft

Note
Why preliminary? Because for the integrity of the perception of this section, it is necessary to say a few words about the camshaft, and a more detailed description of this detail will be provided below.

The correctness of the alternation of various strokes in the engine cylinders is achieved due to the appropriate location of the cams on the camshaft, as well as by the correct installation of the gas distribution mechanism drive: engagement of the timing gears/pulleys with the drive gear/crankshaft pulley. In a four-stroke engine, the working cycle in all cylinders is completed within two crankshaft revolutions. During this time the intake and exhaust valves in each cylinder must open and close once. This occurs for each revolution of the camshaft. Thus, the camshaft must rotate twice as slow as the crankshaft. In order to do this, the camshaft gear has twice as many teeth as the crankshaft gear, or the pulley must be twice the diameter of the crankshaft pulley.

4.5.5 Valve timing of a four-stroke engine

The moments of opening and closing of valves in four-stroke engines do not coincide with the positions of the pistons at TDC and BDC for better filling the cylinders with a fresh charge and the most complete cleaning of them from exhaust gases. But they occur with a certain advance or delay. In other words, the inlet valve can close once the piston passes BDC, and the exhaust valve - once the piston passes TDC. 

The moments of opening and closing of valves that are expressed in degrees, corresponding to the angles of rotation of the crank of the crankshaft relative to the dead points, are called valve timing. The valve timing can be plotted on a radius chart, which is called the valve timing diagram, as shown in Figure 4.11.

Perhaps it will be easier to show this with an example. Thus, if one says that the valve opens for 5 ° before TDC, then it started opening at a time when the crankshaft crank, to which the piston connecting rod is attached, was 5 ° before the top dead center.

 Figure 4.11 Four-stroke engine gas distribution diagram 

4.5.5.a Іnlet valve 

The inlet valve starts opening a little earlier than the piston comes to TDC. In this case the valve will already open slightly by the beginning of the downward stroke of the piston during the intake stroke. The intake valve opening advance for engines of different models fluctuates in different ranges. Often, the closing of the intake valve occurs with a certain delay when the piston goes over BDC and starts to move up. In this case, for some time after the transition over the BDC, despite the beginning of a slight upward movement of the piston, filling the cylinder with a charge will continue due to some rarefaction still present in the cylinder, as well as due to the inertia of the charge moving in the intake manifold.

Note
However, it should be mentioned that there are at least two cycles that are called Miller and Atkinson cycles, during which the intake valve does not close in the same way as on conventional internal combustion engines.

Thus, the opening time of the intake valve is longer than the time during which a half revolution of the crankshaft occurs. The duration of the intake increases, and the cylinder is more fully filled with a fresh charge.

4.5.5.b Exhaust valve

The exhaust valve opens before the piston reaches BDC. In this case, the gases that are under high pressure inside the cylinder, quickly start exhausting into the muffler, although the piston is still moving down. Then the piston, upon passing the BDC and moving towards the TDC, will push out the gases remaining in the cylinder. The exhaust valve closes when the piston goes over TDC. Despite the fact that the piston will start moving down a little, the gases will continue to leave the cylinder by inertia and due to the suction effect of the flow of gases moving in the exhaust pipeline. Thus, the opening time of the exhaust valve is longer than the time during which the shaft half turn occurs, and the cylinder is better cleaned of exhaust gases.

Note
The angle of rotation of the crank, which corresponds to the position at which the intake and exhaust valves are simultaneously open, is called the valve overlap angle. Due to the insignificance of this angle and the negligible size of the gap between the valves and sockets, the possibility of leakage of the combustible mixture is excluded. Overlapping of valves is necessary for additional purging of the cylinder in order to better fill it with fresh charge.

A slight decrease in the pressure of gases on the piston, which occurs during the working stroke due to the early opening of the exhaust valve, and the loss of part of the work of the gases. In this case, it is compensated by the fact that the piston, which is moving upward during the exhaust stroke, does not experience much resistance from gases remaining in a small amount in cylinder.

4.5.6 Change of Valve Timing 

With the development of technology, serious opportunities for improving engine efficiency have opened up for designers and engineers. Increasing engine power while reducing fuel consumption has become a new trend in the automotive industry. In order to improve the operation of the internal combustion engine, it is necessary to adjust the valve timing for all load modes - from idle to full load.

4.5.6.a Idling speed

Note
Idling speed is the minimum speed at which the engine can run stably without load. You have started the engine without any movement or impact on the accelerator pedal.

4.5.6.b Ignition timing

How can one change valve timing? — Rotate the camshaft relative to the crankshaft, thereby changing the valve opening moments. We add to this the ignition advance control*, which made it possible to control the start and end of engine strokes and allowed the internal combustion engine to be improved so much that the power and fuel consumption indicators improved many times over.

Note
* Ignition timing. In order for the air-fuel mixture to have time to burn while the piston moves from top dead center to bottom, it shall be ignited a little earlier. The main indicator is the ignition timing, which indicates how many degrees before TDC there will be a breakdown between the spark plug electrodes on the compression stroke. Depending on the crankshaft speed and engine load, the ignition timing must change. This can be done using an ignition distributor or an electronic engine control unit (for more information, see chapter 10 "Electrical Equipment and Electrical Systems", section 10.3 "Ignition System").

4.5.6.c The essence of the system

The essence of this system is simple. A special mechanism, on the outside of which there is a sprocket for the drive chain from the crankshaft, is installed on the camshaft (or shafts). This mechanism is installed so that it can turn the camshaft in the direction of advance or retard, depending on the operating mode of the engine. In more detail, the operation of the variable valve timing mechanism (phase shifter) occurs as described below.

The crankshaft through the drive chain rotates the phase shifter, which is installed on the camshaft. At the moment when it is necessary to shift the opening time of the valves in the direction of delay or advance, the phase shifter turns the camshaft in the appropriate direction.

Figure 4.12 Phase shifter appearance.

Phase shifters are mainly mounted on the intake camshaft (the shaft that only opens the intake valves). But nowadays these mechanisms are often mounted on both camshafts - intake and exhaust ones.

4.5.7 Variable valve height

In modern gasoline engines, the amount of the fuel mixture is regulated by the means of throttle gate: the throttle gate opens, more air flows in, and more fuel is injected accordingly. The air, which is necessary for preparing the fuel-air mixture until it reaches the cylinder, will overcome several very "unpleasant" obstacles: an air filter, a throttle gate, and other valves. These are all losses that directly affect the power of the internal combustion engine. Try breathing in a gas mask yourself while doing exercises under stress… That's how the engine "breathes hard". Throttle gate is one of the obstacles in the path of air, which the designers dreamed of getting rid of. However, how to adjust the amount of intake air then? The solution was again related to valves. They came to the conclusion that it is necessary to adjust the valve opening height. There were systems with stepwise regulation of the valve opening height, namely: the valve could be opened only to three different heights. Then they came up with a system of steeples valve opening with a range from 1 mm to 10 mm. This allowed to get rid of the throttle valve - the engine could "breathe" easier. However, getting rid of the throttle gate by changing the valve opening height is not an end in itself. Valve operation control allows you to further refine the operation of the four-stroke internal combustion engine.

4.5.8 Valve group details

The valve group includes a valve, a valve guide, a valve spring with a plate and fastening parts (they are also called "crackers"). Everything described here is shown in Figure 4.14.

4.5.8.a Valve

The valve is used for closing and opening the intake or exhaust ports in the cylinder head. Head and rod are the main elements of the valve.

Рисунок 4.13 Клапан.

4.5.8.b Valve head

The valve head has a polished tapered working surface - a chamfer (usually at an angle of 45 °), with the help of which the valve is firmly ground to the seat.

4.5.8.c Valve stem

The valve rod is ground and passes through the guide bush. There is a groove or hole at the end of the rod for attaching the spring plate. Opposite valves have heads of different diameters (as a rule, the larger one is at the inlet valve) or are distinguished by special marks.

Figure 4.14 Valve gear

4.5.8.d Guide bush

The guide bushing, in which the valve is installed by the means of rod, ensures a precise fit of the valve into the seat. Bushings are pressed into the cylinder head.

4.5.8.e Valve spring

The valve spring keeps the valve closed. It ensuring a tight fit in the valve seat, and also creates a constant pressure of the push rod against the surface of the camshaft cam. The spring is put on the end of the valve rod, which comes out of the sleeve and fixed on it in a compressed state using a plate with conical split “crackers” that enter the groove on the valve rod. Sometimes two springs are installed on the valve: a spring of a smaller diameter - inside a spring of a larger diameter. This is done in order to avoid resonance of the spring at certain engine operating frequencies, as well as to provide a backup in case of a spring break. Springs with variable pitch are often used. This eliminates the likelihood of vibration of the spring and its breakage at a high speed of the engine crankshaft. In case of installing two springs, they are selected in such a way, that the direction of winding their coils is in different directions, which also eliminates the danger of the occurrence of resonant vibrations of springs.

Rubber valve oil seals are installed on the top of the bushings in order to limit the amount of oil entering the guide bushing and eliminate its suction into the cylinder through the gaps in the bushing.

4.5.8.f Pusher

The push rod is used for transmitting the axial force from the camshaft cam to the valve rod or to the rod. The fact is that it is better to transmit the force from the camshaft cam precisely through the intermediate link - the push rod. Since the valve gear elements wear out over long periods of time when it is time to replace excessively worn parts, it is easier to replace a small push rod than an entire camshaft or valves. 

Figure 4.15 Cylinder head with elements of gas distribution mechanism.

As noted above, the so-called hydraulic lifters are now widely used. They are “hydraulic”, because they work due to the pressure of the engine oil; and “lifters”, because they compensate or, more simply, reduce to naught the gap between the camshaft cam and the push rod during operation.

Push rods in most engines are installed without bushings directly in the bore holes of the cylinder head. Guide bushings that are molded in a section to several cylinders are used ad push rods in some engines.

4.5.8.g Rocker

Rocker arm changes the direction of the transmitted motion. They are often installed when the camshaft is one, and there are two or four valves per cylinder, but they are located in a special way (see Figure 4.16). The rocker arms are installed on bronze bushings or without bushings on the axles that are fixed to the block head with the help of struts. One arm of the rocker arm is located above the valve rod and the other is situated below or above the camshaft cam. In order to adjust the clearance between the valve rod and rocker arm an adjusting screw with a lock nut is screwed into the end of the rocker arm.

Figure 4.16 Valve drive over rocker arm.

4.5.9 Camshaft and its drive

The camshaft ensures timely opening and closing of valves. The shaft has intake and exhaust cams (see figure 4.17) and bearing journals*.

 Figure 4.17 Gas distribution mechanism assembly. 

Note
* Bearing journals are not shown in figure 4.17, since the image is schematic and is given for preliminary acquaintance. One might get an idea of the appearance of the camshafts from figure 4.18.

4.5.9.a Cams

Cams are manufactured as one piece with the shaft. However, there are prefabricated designs when the cams are pressed onto the shaft. Four-stroke engines, depending on the number of valves, have two or more cams for each cylinder: intake and exhaust. Cam shape ensures smooth lifting and lowering of the valve and the corresponding duration of its opening. Cams of the same name for each cylinder (for example, intake cams) are located in four-cylinder engines at an angle of 90 °, in six-cylinder engines - at an angle of 60 ° and in eight-cylinder engines — at an angle of 45 °. Opposite cams (intake and exhaust) are set at an angle, the value of which depends on the valve timing. The tops of the cams are located in the order of operation accepted for the engine, taking into account the direction of rotation of the shaft.

Figure 4.18 Cylinder head with camshafts.

4.5.10 How the camshaft is driven into rotation?

The camshaft is driven by the crankshaft in different ways. The most common are chain and belt drives, less often a gear or combined drive is used.

4.5.10.a Chain drive

Chain drive. Sprockets are installed (like on a bicycle) and a drive chain is put on at the end of the crankshaft and camshaft. A chain damper, which is a long bar along which the chain moves, is additionally installed in order to exclude the beating of the chain Usually the chain tensioner guide is installed on the other side. The chain drive can also be examined in Figures 4.19 and 4.20.

Figure 4.19 Diagram of chain drive of gas distribution mechanism. 

Figure 4.20 Example of chain drive of gas distribution mechanism.

4.5.10.b Belt drive

Belt drive. Toothed pulleys are installed on the crankshafts and camshafts. These pulleys somehow remind sprockets, but much wider than them. Toothed belt is fitted onto these toothed pulleys A belt tensioner (often automatic) is installed for the convenience of removing and installing the drive belt. An example of a camshaft (or shafts) drive using a toothed belt is shown in Figures 4.21 and 4.22.

Figure 4.21 Diagram of belt drive of gas distribution mechanism.

Figure 4.22 Example of belt drive of gas distribution mechanism.

4.5.10.c Gear drive

Gear drive. The camshaft is driven from a gear on the crankshaft through a row of intermediate gears or directly, as shown in Figure 4.23. The gear drive is the most reliable, but it is too bulky and can "create" many problems in case of disassembly and subsequent assembly of the valve train.

Figure 4.23 Gear drive of gas distribution mechanism.

4.5.11 Switchable valves

In pursuit of economy, the designers solved one of their concerns: what to do if the engine is running using only 15-20% of its power. This happens when we are, for example, in a traffic jam or drive along the highway at a cruising speed.

4.5.11.a Cruising speed

Note
Cruising speed is the speed at which optimum fuel efficiency is achieved. This term, of course, is more suitable for the aviation industry. However, if we are driving on the highway in fifth or even sixth gear, then it is quite applicable in this industry.

4.5.11.b Zero height cams

In case if not all the engine power is used, then why should all the engine cylinders work? What if just take and disconnect, for example, for a car waiting in a traffic jam, two of the four cylinders?

After all, a pair of cylinders is quite enough for the engine to idle. The remaining two cylinders are no longer supplied with fuel and, so that they do not waste air through the intake and exhaust manifolds, the intake and exhaust valves are closed. In order to perform such a simple operation, they came up with a relatively simple solution: cams with "zero height" were placed on the camshaft, next to the usual cams. That is, they do not affect the valve push rod in any way.

Thus, during normal operation, the camshaft rotates and all the valves fulfill their purpose. When it is necessary to turn off the latter, a special valve opens. Through this valve the engine oil under pressure acts on the camshaft and displaces it in the direction of the longitudinal axis. Cams with a regular profile opened and still open the valves. At the same time the cams with "zero height" simply do not reach the valves, and they, in turn, stand motionless.

Note
Various companies at different times have suggested several schemes for the implementation of the above operation in order to turn off part of the valves. The aforementioned method is just one of the ways.