Compound Systems
There are many compound systems and configurations, but there are two basic types, according to how HP and LP piston strokes are phased and hence whether the HP exhaust is able to pass directly from HP to LP (Woolf compounds) or whether pressure fluctuation necessitates an intermediate "buffer" space in the form of a steam chest or pipe known as a receiver (receiver compounds).
In a single-expansion (or 'simple') steam engine, the high-pressure steam enters the cylinder through a cut-off valve (archaically known as a regulator). The piston moves down the cylinder, and when it is at about 25%–33% of its stroke, the cut-off valve shuts and the steam expands, pushing the piston to the end of its stroke, the exhaust valve opens and expels the depleted steam to the atmosphere, or to a condenser. As steam expands in a high-pressure engine, its temperature drops; because no heat is released from the system, this is known as adiabatic expansion and results in steam entering the cylinder at high temperature and leaving at low temperature. This causes a cycle of heating and cooling of the cylinder with every stroke which is a source of inefficiency. The steam cut-off point when using a slide valve is less than 30% of the stroke. Early cut-off causes the turning moment on the shaft to be more uneven, requiring a larger flywheel to smooth this out.
Read more about this topic: Compound Engine
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