Reciprocating Engines

Reciprocating or piston-driven engines have been developed for over 100 years. The technology has improved dramatically over the past three decades due to increased environmental and economic pressures, and the need for power density improvements. These engines offer a low capital cost CHP alternative that is easy to startup, is proven reliability, offers good load-following characteristics, and excellent heat recovery potential. There are two basic types - spark ignition and compression ignition. The former uses natural gas as the preferred fuel, although other fuels like propane, gasoline, etc can be used. The electric efficiency of natural gas fired engines range from 28 - 40% LHV based on size of the engine. The waste heat can be recovered in CHP mode to achieve an overall efficiency of 70-80%. The compression ignition type operates on diesel or heavy fuel and typically has efficiencies in range of 30 - 50% HHV.

Reciprocating engines tend to have higher emissions than other technologies. In some locations, especially where local air quality standards are high, emissions permits could limit engine use, although the use of exhaust catalysts and better combustion design and control has significantly reduced emissions for current technology.

The spark ignition engine operates on the principle of Otto cycle while the compression ignition engine operates on Diesel cycle. For the most part, these two cycles are identical except for the method of igniting the fuel. These engines use a cylindrical combustion chamber in which a close fitting piston travels the length of the cylinder. Complete cycle consists of intake stroke, compression stroke, power stroke, and exhaust stroke. The piston is connected to the crankshaft that transforms the linear motion to the rotary motion.