![]() In simple terms, the air is rammed inside the combustion chamber. This is a physical phenomenon wherein a large amount of air entering the cylinder rapidly cannot stop itself. Like the others, this is also intentionally done to let in more air into the combustion chamber. This is the situation where the intake valve closes at a few degrees after the BDC. 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. Now through BDC to all the way up to a few degrees more than TDC the exhaust valve is open after which it closes.Coming back to the valve timing, the exhaust valve opens way before the piston reaches the BDC after expansion stroke or power stroke.Let’s save ignition timing for some other day. This also depends on the ignition timing. If the engine is FI (Fuel Injected), the fuel is atomized (sprayed into tiny droplets) inside the combustion chamber before the piston reaches TDC and then fuel is ignited in SI engine (Spark Ignition engine). ![]() This is when both the valves are shut (inlet as well as the exhaust valve).
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