The purpose of the overlap is to flush the exhaust gasses using fresh air. What is the purpose of the valve overlap? This will cause both valves to open together with a duration of about 25 degrees.
On the other hand, the suction valve also starts to open even before the piston reaches TDC in the end of exhaust stroke. But the exhaust valve is still open when the piston moves down in the beginning of intake stroke. As explained above, the exhaust valve opens when the piston moves to TDC in the end of combustion stroke. This condition occurs at the end of the exhaust stroke until the beginning of the intake stroke. This ‘overlap’ helps in better ‘scavenging’ or expelling the remaining exhaust gases from the engine cylinder.Valve overlap is a condition where both valves (suction valves and exhaust valves) are both open. At this point, both the inlet & exhaust valves remain open for a very short period of time causing an ‘overlap’. when the piston starts to move down the cylinder in the suction stroke. The exhaust valve closes after a few degrees of piston reaching the TDC, i.e. It releases the excess pressure and helps to reduce pumping losses for the piston when it moves up. As the exhaust valve opens just before BDC, it causes some of the pressurized exhaust gases to escape even before the piston starts it’s upwards stroke. The exhaust valve opens before the piston reaches BDC in its power stroke.
In the exhaust stroke, the piston again moves upwards pushing the exhaust gas out through the open exhaust valve. The engineers precisely calibrate the actual closing point of the inlet valve in such a way that it coincides with the point where the motion of the incoming charge starts to reverse. At this point, the pressure in the cylinder becomes almost equal to the atmospheric pressure. Hence, the inlet valve is kept open until the piston enters its next upward stroke i.e the compression stroke. If the inlet valve closes at this point, the cylinder will receive less charge than its requirement. Therefore, the pressure in the combustion chamber remains below the atmospheric pressure, while the charge is still moving in the direction of the motion of the piston with high velocity. The entire charge cannot enter the cylinder even when the piston reaches the end of its downward stroke because the opening of the inlet valve is small. This air-fuel mixture (also known as charge) has both mass and motion. This is because the downward movement of the piston creates the vacuum (or negative pressure) in the cylinder and the air-fuel mixture gets filled in the empty space. In suction stroke, the air-fuel mixture or charge gets sucked into the cylinder very rapidly. when the piston starts to move up the cylinder in the compression stroke. It closes after quite a few degrees of piston reaching the BDC, i.e. The inlet valve usually opens few degrees before the piston reaches TDC in its exhaust stroke. Engine valve timing is the most critical process of IC engines. The timing of the opening & closing of valves is specified in degrees corresponding to the position of engine’s pistons.
The engine’s camshaft opens and closes the valves at a specific interval. An automotive engine uses valves for it’s ‘breathing’ (inhale/exhale) process. The engine valves are just like the human nose. Firstly, read here about how the engine valves open & close.
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