Reciprocating Engines

Reciprocating engines are the most common and most technically mature of all DER technologies. They are available from small sizes (e.g., 5 kW for residential back-up generation) to large generators (e.g., 7 MW). Reciprocating engines use commonly available fuels such as gasoline, natural gas, and diesel fuel.

Photo Source: Fair Manufacturing, Inc.

Reciprocating Engines Overview
Commercially Available	Yes
Size Range	5 kW – 7 MW
Fuels	Natural gas, diesel, landfill gas, digester gas
Efficiency	25 – 45%
Environmental	Emission controls required for NOx and CO
Other Features	Cogen (some models)
Commercial Status	Products are widely available

A reciprocating, or internal combustion (IC), engine converts the energy contained in a fuel into mechanical power. This mechanical power is used to turn a shaft in the engine. A generator is attached to the IC engine to convert the rotational motion into power.

There are two methods for igniting the fuel in an IC engine. In spark ignition (SI), a spark is introduced into the cylinder (from a spark plug) at the end of the compression stroke. Fast-burning fuels, like gasoline and natural gas, are commonly used in SI engines. In compression ignition (CI), the fuel-air mixture spontaneously ignites when the compression raises it to a high-enough temperature. CI works best with slow-burning fuels, like diesel.

ICEs are also classified as high-speed, medium-speed, or low-speed:

• High-speed units are derived from automotive or truck engines and operate at 1200-3600 rpm. These engines generate the most output per unit of displacement and have the lowest capital costs, but also have the poorest efficiency.
• Medium-speed engines are derived from locomotive and small marine engines, and operate at 275-1000 rpm, have higher capital costs, but also have greater efficiency.
• Low-speed units are derived from large ship propulsion engines and operate at 58-275 rpm. Low-speed engines are designed to burn low-quality residual fuels and are practical only if there is a large price differential between heavy oil and natural gas and there are no environmental restrictions (not in U.S.).