The Atkinson Cycle Engine

Besides the diesel and gasoline internal combustion engine, there is another type of engine that was introduced more than a hundred year ago which now gains favor in hybrid electric application. Named after its inventor, the Atkinson engine differs from other combustion engines where efficiency is emphasized over power. In modern applications in electric hybrid vehicles the low power is compensated by the electric motor.

 The engine was designed by James Atkinson in 1882 and like many of the internal combustion engines operating in a four stroke cycle it allows intake, compression, power and exhaust strokes. In the case of the Atkinson engine, all four strokes occur on one crankshaft operation. This is due to the unique crankshaft design that allows an expansion ratio to be different form the compression ratio. The linkage of the crankshaft could be adjusted which allows a longer power stroke than the compression stroke. Hence the engine can achieve greater efficiency and was proven to be more efficient than Otto cycle engines.

In terms of an ideal thermodynamic cycle, the Atkinson engine operation includes an isentropic or reversible adiabatic compression, heat addition at constant volume, isentropic expansion and heat rejection at constant pressure.

 In the Atkinson cycle, the compression ratio is reduced but the expansion ratio is unchanged making the compression ratio smaller than the expansion ratio. The goal of the engine is to allow the pressure in the combustion chamber at the end of the power stroke to be equal to normal atmospheric pressure which, when accomplished, all available energy will be obtained from the combustion process. And since the expansion ratio is greater than the compression ratio, more energy from heat could be converted to mechanical energy. This makes the engine more efficient.

In terms of power the Atkinson engine lag behinds the Otto cycle engine due to the amount of air it takes in; the Atkinson engine does not take in as much air as an Otto cycle engine resulting in greatly reduced power.

The Atkinson currently in use for electric hybrid cars are in modified version. The low power of the engine is compensated by the electric motor. When the engine is running in high power, the extra power is supplemented by the motor. These motors can either be used independently or in combination with the Atkinson cycle engine. Together the desired fuel efficiency is achieved without the loss of power.