External Combustion Engines

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The first true micro CHP systems were based on external combustion technology as their characteristics are best suited to this stationary, constant running application.  External combustion engines separate the combustion process (which is the energy input to the engine) from the working gas, which undergoes pressure fluctuations and hence does useful work.  As the combustion process is used to provide a continuous heat input to the working gas, it is inherently more controllable and, in theory at least, more efficient, cleaner and quieter than internal combustion engines. 

External combustion engines have the potential for long life and service intervals similar to the annual maintenance of a gas boiler, typically around 3,000 operational hours.  However, the inherent benefits of this technology have yet to be fully realised; early products have suffered from reliability problems, and have failed to deliver the expected efficiencies.

The two most common external combustion micro CHP technologies are the Stirling engine, in which a working gas is alternately heated and cooled to create pressure changes which in turn drive a power piston, and the Rankine engine in which a fluid is heated to evaporate and expand against a piston or turbine, and then cooled and condensed prior to the next heating and cooling cycle.

 For further discussion of the relative merits of engine types see section on PAPERS; greater detail and description of operating principles can be found in the book, micro CHP

For information on products under development, click on the respective RANKINE and STIRLING links below.


Page updated 6th November 2013



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This site was last updated on 01 January 2015  Jeremy Harrison