Not doing so can cause some burnt exhaust gasses to remain inside the combustion chamber and dilute the air-fuel mixture This is to assist in pulling the fresh charge from the intake manifold into the combustion cylinder just like the Siphon Effect. This is no way a manufacturing fault and the opening of the intake valve slightly before TDC and closing of the exhaust valve later after TDC is deliberately done. In the diagram, it is very prominent that when the piston reaches TDC in the exhaust stroke, both the valves, intake and exhaust, are open. If it the exhaust valve were to remain close till the BDC, some engine power had to be wasted in order to assist the piston to move from BDC to the TDC. This also confirms that there is no other pressure exerting on the piston during its motion to BDC. ![]() This releases the excess pressure from the combustion chamber. ‘Blow-down’ is the process where the exhaust valve opens before the piston reaches the BDC. Blow Downīoth the valves remain shut to perform the combustion process efficiently during the compression stroke all the way up to the power or expansion stroke. Now the question comes why isn’t the intake and exhaust valve opening and closing in sync with the pistons TDC and BDC? And why at TDC both the valves are open for some duration? The reason likes in the three crucial factors called Blow Down, Overlap, and Ram Effect. and the cycle repeats itself stroke after stroke.
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