![]() ![]() In an external combustion engine, such as a steam engine, the control of the valve timing is by the valve gear. Tappet clearance is measured by an instrument called feeler gauge. If tappet clearance is more, then valve will open late and close early. If tappet clearance is less, then valve will open early and close late. ![]() The valve timing of a diesel engine also depends on tappet clearance of the inlet and exhaust valves. However, some supercharged two-stroke diesel engines (such as the Wilksch aero-engine) do have a cylinder head and poppet valves, similar to a four-stroke cycle engine. Many two-stroke cycle and all wankel engines do not have a camshaft or valves, and the port timing can only be varied by machining the ports, and/or modifying the piston skirt (two-stroke applications). Two-Stroke engine showing ports in the cylinder walls. Primary cooling is accomplished by dissipating heat to the valve seats. Valve overlap is a secondary means to cool exhaust valves with intake air during valve overlap. The amount in crankshaft by which the valves close after top dead centre or bottom dead centre is reached is known as valve lag. The amount in crankshaft degrees by which the valves open before top dead centre or bottom dead centre is reached is known as valve lead. In the diagram, the valve overlap periods are indicated by the overlap of the red and blue arcs.Įither valve opens before the piston head reaches top dead centre or bottom dead centre. Engine designers aim to close the exhaust valve just as the fresh charge from the intake valve reaches it, to prevent either loss of fresh charge or unscavenged exhaust gas. The intake valve is opened before the exhaust gases have completely left the cylinder, and their considerable velocity assists in drawing in the fresh charge. With traditional fixed valve timing, an engine will have a period of "valve overlap" at the end of the exhaust stroke, when both the intake and exhaust valves are open. Valve timing diagram for a 4-stroke engine ![]()
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