Steam engine

A steam engine is a heat engine that makes use of the thermal energy that exists in steam, converting it to mechanical work. Steam engines were used in pumps, locomotive trains, steam ships and steam tractors, and were essential to the Industrial Revolution. They are still used for electrical power generation using steam turbines.

Efficiency

To get the efficiency of an engine, divide the number of joules of mechanical work that the engine produces by the number of joules of energy input to the engine by the burning fuel. In general, the rest of the energy is dumped into the environment as heat. No pure heat engine can be more efficient than the Carnot cycle, in which heat is moved from a high temperature reservoir to one at a low temperature, and the efficiency depends on the temperature difference. Hence, steam engines should ideally be operated at the highest steam temperature possible, and release the waste heat at the lowest temperature possible.

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In practice, a steam engine exhausting the steam to atmosphere will have an efficiency (including the boiler) of 5%, but with the addition of a condenser the efficiency is greatly improved to 25% or better. A power station with exhaust reheat, etc. will achieve 30% efficiency. Combined cycle in which the burning material is first used to drive a gas turbine can produce 60% efficiency. It is also possible to capture the waste heat using cogeneration in which the residual steam is used for heating. It is therefore possible to use about 90% of the energy produced by burning fuel - only 10% of the energy produced by the combustion of the fuel goes wasted into the atmosphere.

Related Topics:
Condenser - Combined cycle - Gas turbine - Cogeneration

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One source of inefficiency is that the condenser causes losses by being somewhat hotter than the outside world, although this can be mitigated by condensing the steam in a heat exchanger and using the recovered heat, for example to pre-heat the air being used in the burner of an external combustion engine.

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The operation of the engine portion alone is not dependent upon steam; any pressurised gas may be used. Compressed air is sometimes used to test or demonstrate small model "steam" engines.

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