Valve Timing Setting. This relation between the valve opening timings to the piston moves from the top dead centre (tdc) to the bottom dead centre (bdc) can be represented on a circle. Engine valve timing is the most critical process of ic engines.
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The most accurate way of doing this is to use a modified spark plug, with a fixed probe in place of the electrode. These are commonly poppet valves in an internal combustion engine, and slide valves or piston valves in a steam engine. To optimise the breathing, engine requires different valve timing at different speed.