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. Thus the advantage of valve overlap is obtained to achieve maximum engine efficiency.Valve timing diagram for a 4-stroke engine
But, if the closure of exhaust valve is delayed more, it would result into loss of fresh air also through the exhaust valve. The closure exhaust valve is delayed to the piston reaches TDC making fully removal of burnt gases and creating partial vacuum inside. This makes the exhaust gases to escape from the cylinder with its own energy and hence reduces the work to be done during the exhaust stroke. Ideally, the exhaust valve should be opened after the piston reaches BDC on completion of the power stroke, but the exhaust valve starts opening well before the piston reaches BDC. Similarly, the closer of the inlet valve is made later than the BDC when the piston has already stated moving up for the compression stroke, this is done to achieve the maximum volumetric efficiency.
But, if the inlet valves are opened very advanced, it may make some of the exhaust gases to re-enter in the cylinder through the inlet manifold. The opening of inlet valve is advanced to use the depression made by the outgoing gases and hence the fresh air is taken inside the cylinder. The overhead valve arrangement is the most commonly used arrangement these days. The valve mechanism is generally classified by the method of valve arrangement as either side valve or over head valves. The cam shaft which is run through timing gears rotates at half of the number of rotations per minute of the crankshaft. The valves are opened and closed according to the valve timing diagram. The condition when both the inlet and exhaust valves are open simultaneously is called valve overlap. Similarly, the exhaust valve is opened before 46 0 of BDC and closed after 10 0 of TDC. The inlet valves is generally opened before 10 0 of TDC which is known as valve lead and closes almost 46 0 after BDC which refers to valve lag.
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