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Q The terms - constant pressure and - constant volume are frequently used in
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technical papers on power units in reference to the combustion cycle. What is the meaning of the two terms?
AA constant volume cycle is one in which the pressure in the cylinder at the start of the combustion stroke (around top dead
centre) rises to its peak value after compression without a material change in volume. The spark ignition engine operates on the constant volume cycle: when the spark ignites the mixture at the end of the compression stroke combustion pressure increases to its maximum with virtually no movement of the piston. It should be remembered that piston movement is very small for a change in crank angle of, say, 10deg before top dead centre to 10deg after top dead centre.
The typical diesel engine operates on a "mixed cycle", although in theory the cycle is of the constant-pressure type. In a diesel the fuel burns as it leaves the injector and the amount of fuel injected is proportional to movement of the crank. During combustion the piston moves down the cylinder for an appreciable distance and because the combustion space is expanding there is no rise in pressure or the increase is small compared with the increase in the case of a constantvolume cycle. In practice there is generally some increase in pressure and the unit is then said to operate on a mixed cycle.