What is valve clearance (sharing valve clearance adjustment method)

1, valve timing definition:

Valve timing is the opening time and opening duration of intake valve and exhaust valve expressed by crank angle, which is usually expressed by ring diagram-valve timing diagram.

2. Theoretical valve timing analysis.

Theoretically, intake, pressure, power and exhaust each account for 180? That is to say, the intake and exhaust valves open and close at the top and bottom dead center for a duration of crank angle 180? . However, the practice shows that simple valve timing is not suitable for practical work and cannot meet the requirements of the engine for intake and exhaust valves. Reason:

① There is a process for opening and closing the valve.

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② Influence of gas inertia

With the movement of the piston, it also causes insufficient air intake and unclean exhaust.

③ Requirements of engine speed

The actual engine crankshaft speed is very high, and the piston stroke is very short. When the speed is 5600r/min, a stroke is only 60/(5600? 2)=0.0054s, that is, the speed 1500r/min, and one stroke is only 0.02s Such a short intake or exhaust process makes the engine have insufficient intake and unclean exhaust.

It can be seen that the theoretical valve timing can not meet the requirements of engine intake, saturation and exhaust, so how does the actual valve timing meet this requirement? Let's analyze it below.

3. Actual valve timing analysis

In order to make the intake air sufficient and the exhaust air clean, besides improving the structure (such as adding intake and exhaust pipes), we can also think of some ways from the valve timing. Can the valve open early and close late to extend the intake and exhaust time?

① Possibility of valve opening early and closing late As can be seen from the indicator diagram, when the piston reaches the intake bottom dead center, the gas pressure in the cylinder is still lower than the atmospheric pressure due to the intake suction, and air can still be intake under the action of atmospheric pressure; In addition, the inertia of intake airflow is large at this time. It can be seen that the late closing of the intake valve can increase the intake air volume.

Opening the intake valve in advance can make the intake port have a larger channel area at the beginning, which can increase the intake air volume.

At the end of the power stroke, the exhaust valve is opened, and the exhaust gas is rushed out of the cylinder at high speed by using the residual pressure of power work, and the exhaust volume accounts for about 50%. The premature opening of the exhaust valve will inevitably lead to power loss, but the loss is not great because of the low air pressure, and the premature opening can reduce the work consumed by exhaust, which is beneficial to the discharge of exhaust gas, so the total power is still improved.

It can also be seen from the indicator diagram that when the piston reaches the top dead center, the exhaust gas pressure in the cylinder is still higher than the external atmospheric pressure, and the inertia of exhaust gas flow makes the exhaust gas cleaner by closing the exhaust valve later.

It can be seen that the valve may open early and close late, so what are the benefits of opening early and closing late for the actual work of the engine?

Early opening of intake valve: increase the opening height of valve at the beginning of intake stroke, reduce intake resistance and increase intake air volume.

Late closing of intake valve: Under the action of atmospheric pressure and gas inertia force, the intake time is prolonged and the intake volume is increased.

Exhaust valve opens in advance: with the help of high pressure in cylinder, exhaust resistance is greatly reduced and exhaust is clean.

The exhaust valve closes late: the exhaust time is prolonged, and the exhaust is clean under the action of exhaust pressure and inertia force.

② Valve overlap

Because the intake valve opens early and the exhaust valve closes late, it is bound to cause two valves to open at the same time. The crank angle at which two valves open at the same time is called valve overlap angle. Will combustible mixture and waste gas be mixed randomly during this period? No, this is because: a. The intake and exhaust airflow has its own flow direction and flow inertia, and the overlapping time is short, so it will not be chaotic, that is, the inhaled combustible mixture will not be discharged with the exhaust gas, and the exhaust gas will not flow back to the intake pipe through the intake valve, but can only be discharged from the exhaust valve; B. there is a depressurization effect near the intake valve, which is beneficial to air intake. More auto repair materials pay attention to the official account of WeChat WeChat, the major of automobile inspection and maintenance.

③ Actual opening and closing time and duration of intake valve and exhaust valve.

Actual intake time and duration: when the exhaust stroke is near the end, before the piston reaches the top dead center, that is, when the crankshaft turns away from the top dead center? The intake valve starts to open and the intake stroke until the piston passes the bottom dead center? When the intake valve is closed. The duration of the whole intake process is equivalent to 180 crank angle? +? +? .

? -Is the intake advance angle average? = 10? ~30?

? -The intake delay angle is average? =40? ~80?

So the crank angle at intake is 230? ~290?

Actual exhaust time and duration: Similarly, when the power stroke is near the end, the front valve of the piston begins to open at the bottom dead center. What is the angle of early opening? Generally 40? ~80? After the piston crosses the bottom dead center? Corner exhaust valve closed. General 10? ~30? The whole exhaust process is equivalent to 180 crankshaft angle? +? +? .

? -Is the exhaust advance angle average? =40? ~80?

? -The intake delay angle is average? = 10? ~30?

So the crank angle in the exhaust process is 230? ~290?

Valve overlap angle? +? =20? ~60?

As can be seen from the above analysis, there is a big difference between the actual valve timing and the theoretical valve timing. In the actual valve timing, the valve should be opened early and closed late, mainly to meet the requirements of sufficient intake and clean exhaust. But in fact, when will the valve open? When is the best time to close it? This is mainly determined by experiments according to various vehicle types, and is guaranteed by the shape, position and valve train of the camshaft.

Second, the valve clearance

1, definition:

Valve clearance refers to the clearance between the end of the valve stem and the rocker arm or tappet when the valve is completely closed (the convex part of the cam does not push the tappet).

2. Function:

Leave room for thermal expansion.

Different models have different valve clearances. According to the experiment, the exhaust valve clearance is larger than the intake valve clearance, and the intake valve clearance is about 0.25 ~ 0.3 mm Generally, the exhaust valve clearance is about 0.3 ~ 0.35 mm in cold state. Excessive clearance: After the intake valve and exhaust valve are opened late, the intake and exhaust time is shortened, and the valve opening height is reduced, which changes the normal valve timing and makes the engine power drop due to insufficient intake and unclean exhaust. In addition, the impact of valve train parts increases and the wear accelerates.

Too small clearance: after the engine works, the parts are heated and expanded, pushing open the valve, which makes the valve not close tightly, causing air leakage, reducing power, causing serious carbon deposition or burns on the sealing surface of the valve, and even the valve hits the piston.

The valve mechanism with hydraulic tappet does not need to leave valve clearance.

When the valve is completely closed, the clearance between the end of the valve stem and the rocker arm is called valve clearance.

The following introduces the adjustment method of valve clearance (taking a single cylinder diesel engine as an example).

The role of valve clearance is to ensure that the intake valve and exhaust valve are tightly closed, leaving room when the parts of the valve and its transmission mechanism expand when heated.

When the diesel engine is working, the intake valve is cooled with fresh air at a temperature between 300℃ and 400℃, and the exhaust valve is washed with high-temperature exhaust gas at a temperature between 600℃ and 800℃. Therefore, the exhaust valve has higher temperature and greater thermal expansion than the intake valve. Therefore, the exhaust valve clearance is generally greater than the intake valve clearance, such as the intake valve clearance of vertical 195 diesel engine is 0. 18 ~ 0.25mm, and the exhaust valve clearance is 0.20~0.27mm.

However, the exhaust valve of some diesel engines is made of materials with small expansion coefficient, or measures are taken to strengthen the heat dissipation of the exhaust valve, so the gap between the intake valve and the exhaust valve is equal. For example, in a 195 diesel engine, the gap between the intake valve and the exhaust valve is 0.4 mm.

During the use of diesel engine, the valve clearance will change due to the wear of parts, the loosening of adjusting screws, the reassembly and disassembly of cylinder head and the tightening of cylinder head nuts. If the valve clearance is too small, the parts will expand and stretch when heated, which will lead to the untight valve closure and the power reduction of diesel engine. At the same time, the high-temperature gas in the cylinder leaks from the clearance, which will overheat the valve and even lead to partial ablation of the valve.

If the valve clearance is too large, the impact between the valve and the valve seat will be intensified, the service life will be shortened, and the duration of valve opening will be shortened, which will affect the intake of fresh air in the cylinder and the discharge of exhaust gas, resulting in the decline of diesel engine power. Therefore, in order to ensure the normal operation of diesel engine, it is necessary to check and adjust the valve clearance regularly.

The specific adjustment method is as follows:

① When the diesel engine is completely cooled, remove the cylinder head cover.

② Turn the flywheel so that the top dead center line of the flywheel is aligned with the pointer line on the radiator, so that the piston is at the top dead center position of the compression stroke.

③ Measure the clearance between the valve tappet end and the rocker arm with a feeler gauge. If the feeler gauge cannot be inserted or there is still a big gap after insertion, the valve clearance must be adjusted. When the S 195 diesel engine is completely cold, the intake valve clearance is 0.3 ~ 0.4 ~ 0.5 mm, and the exhaust valve clearance is 0.4 ~ 0.5 mm. ..

④ Loosen the lock nut of the valve clearance adjusting screw, turn the adjusting screw slightly, measure with a feeler gauge until the measured value is consistent with the specified value, and tighten the lock nut while keeping the adjusting screw still.

⑤ When the clearance between the intake valve and the exhaust valve is adjusted, turn the crankshaft several times and measure the valve clearance again. If there is any change, it should be readjusted.