The Discharge of the combustion gas from the cylinder and the intake of fresh air are closely related to the combustion of the fuel, and therefore to the engine output.
It is desirable to maintain a high intake efficiency (volume effeciency) through out the intire range of engine speed. Intake eficiency (volume efficiency) ; The amount of air that is actually. sucked in by an engine can never be 100 percent o the stroke volume. The ratio of the actual amount of air intake to the sroke volume is called intake efficiency. However, when timing that results in high intake efficiency in the high speed range is used, intake eficiency decreases in the low speed range. And when timing that is good in the low speed is used, efficiency tends to be poor in the high speed range. Therefore the timing is determined through consideration of the operating conditions of the engine. Some engine has Variable Valve Timing (VVT) to prevent this.
Generally, the inlet valve and the exhaust valve open early and close with some delay in relation to the top dead center and bottom dead center of the piston, as ilustrated in the valve timing diagram.
The valve timing is explained as follows.
1. Advance opening angle of the inlet valve.
The opening area of inlet valve is very small at the moment the valve leaves the seat. If the inlet valve were to begin to open at top dead center. the opening area would not be sufficiently large when the piston began to go down. The intake efficiency would be low because the intake resistance would be high. Therefore, as shown in the figure the inlet valve is opened slightly earlier than that so that the intake area will be large enough when the piston begins to move down and the piston speed increases.
The advance opening angle of inlet valve depends on the caracteristics of the engine, but it is generally betwen 15 degree and 30 degree before top dead center.
2. Delayed Closure angle of the inlet valve.
Even when the piston has reached bottom dead center during the intake stroke, the air pressure in the cylinder is still lower than atmospheric pressure because of intake resistance. Air will flow into the cylinder as long as the air pressure in the cylinder is lower than the pressure inside intake manifold. Therefore, air intake can be increased by delaying closure of the inlet valve until after the piston reaches the bottom dead center. This delay angle is related to the rotational speed and the cam shape. It is advantageous for the delayed closure angle to be small in the low speed range, but a large angle is advantageous in the high speed range. However, some of the intake air will be discharged if the delay is too large. Therefore, the delayed closure angle is generally betwen 40 degree and 60 degree after botton dead center.
3. Advance opening angle of the exhaust valve.
In the combustion stroke, the combustion pressure could theoritically be used most effectively by keeping the exhaust valve closed until the piston reaches bottom dead center. However because of the resistance of the exhaust gas pressure (back pressure), after the piston passes bottom dead center (after the exhaust stroke begins), the pump loss of the piston increases. Therefore, it is advantageous to open the exhaust valve early to decrease the back pressure. For this reason, the exhaust valve is given a suitable advance opening angle. This angle depends on the characteristic of the engine, but it is generally betwen 40 degree and 60 degree before bottom dead center.
4. Delayed clossure angle of the exhaust valve.
The inlet valve and the exhaust valve are both open for some period of time when the piston is near top dead center, because ofthe advance opening angle of the inlet valve and the delayed closure angle of the exhaust valve. This is called Valve Overlap. In this state, because of the inertia of the intake air and exhaust gas, air can be cusked in and the remaining exhaust gas can be discharged. Therefore, the exhaust gas can be replaced by fresh air. Generally, the delayed closure angle of the exhaust valve is between 15 degree and 30 degree after top dead center
During engine operation if a valve hits the rocker arm due to thermal expansion, the valve timing will be disturbed. Valve clearence is provided to prevent this. Excessively large or small valve clearence will upset the valve timing.
It is desirable to maintain a high intake efficiency (volume effeciency) through out the intire range of engine speed. Intake eficiency (volume efficiency) ; The amount of air that is actually. sucked in by an engine can never be 100 percent o the stroke volume. The ratio of the actual amount of air intake to the sroke volume is called intake efficiency. However, when timing that results in high intake efficiency in the high speed range is used, intake eficiency decreases in the low speed range. And when timing that is good in the low speed is used, efficiency tends to be poor in the high speed range. Therefore the timing is determined through consideration of the operating conditions of the engine. Some engine has Variable Valve Timing (VVT) to prevent this.
Generally, the inlet valve and the exhaust valve open early and close with some delay in relation to the top dead center and bottom dead center of the piston, as ilustrated in the valve timing diagram.
The valve timing is explained as follows.
1. Advance opening angle of the inlet valve.
The opening area of inlet valve is very small at the moment the valve leaves the seat. If the inlet valve were to begin to open at top dead center. the opening area would not be sufficiently large when the piston began to go down. The intake efficiency would be low because the intake resistance would be high. Therefore, as shown in the figure the inlet valve is opened slightly earlier than that so that the intake area will be large enough when the piston begins to move down and the piston speed increases.
The advance opening angle of inlet valve depends on the caracteristics of the engine, but it is generally betwen 15 degree and 30 degree before top dead center.
2. Delayed Closure angle of the inlet valve.
Even when the piston has reached bottom dead center during the intake stroke, the air pressure in the cylinder is still lower than atmospheric pressure because of intake resistance. Air will flow into the cylinder as long as the air pressure in the cylinder is lower than the pressure inside intake manifold. Therefore, air intake can be increased by delaying closure of the inlet valve until after the piston reaches the bottom dead center. This delay angle is related to the rotational speed and the cam shape. It is advantageous for the delayed closure angle to be small in the low speed range, but a large angle is advantageous in the high speed range. However, some of the intake air will be discharged if the delay is too large. Therefore, the delayed closure angle is generally betwen 40 degree and 60 degree after botton dead center.
3. Advance opening angle of the exhaust valve.
In the combustion stroke, the combustion pressure could theoritically be used most effectively by keeping the exhaust valve closed until the piston reaches bottom dead center. However because of the resistance of the exhaust gas pressure (back pressure), after the piston passes bottom dead center (after the exhaust stroke begins), the pump loss of the piston increases. Therefore, it is advantageous to open the exhaust valve early to decrease the back pressure. For this reason, the exhaust valve is given a suitable advance opening angle. This angle depends on the characteristic of the engine, but it is generally betwen 40 degree and 60 degree before bottom dead center.
4. Delayed clossure angle of the exhaust valve.
The inlet valve and the exhaust valve are both open for some period of time when the piston is near top dead center, because ofthe advance opening angle of the inlet valve and the delayed closure angle of the exhaust valve. This is called Valve Overlap. In this state, because of the inertia of the intake air and exhaust gas, air can be cusked in and the remaining exhaust gas can be discharged. Therefore, the exhaust gas can be replaced by fresh air. Generally, the delayed closure angle of the exhaust valve is between 15 degree and 30 degree after top dead center
During engine operation if a valve hits the rocker arm due to thermal expansion, the valve timing will be disturbed. Valve clearence is provided to prevent this. Excessively large or small valve clearence will upset the valve timing.
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