Atkinson cycle

The Atkinson cycle engine is a type of Internal combustion engine invented by James Atkinson in 1882. The Atkinson cycle is designed to provide efficiency at the expense of power, and is beginning to see applications in modern hybrid electric applications.


Design

The original Atkinson cycle engine allows the intake, compression, power, and exhaust strokes of the four-stroke cycle to occur in a single turn of the crankshaft, and was designed to bypass patents covering the existing Otto cycle engines. Owing to the extra linkage in the Atkinson engine, the expansion ratio may differ from the compression ratio. By adjusting the linkage to allow a power stroke longer than the compression stroke, the engine can achieve greater efficiency than with the Otto cycle engine. While Atkinson's engine design is no more than a historical curiosity, the Atkinson cycle, where the power stroke is longer than the compression stroke, is beginning to become common due to the increase in fuel economy it provides.



Four stroke Atkinson cycle engine

The Atkinson cycle may also refer to a four stroke piston engine in which the intake valve is held open longer than normal to allow a reverse flow of intake air into the intake manifold. This reduces the effective compression ratio and, when combined with an increased stroke and/or reduced combustion chamber volume, allows the expansion ratio to exceed the compression ratio while retaining a normal compression pressure. This is desirable for improved fuel economy because the compression ratio in a spark ignition engine is limited by the octane rating of the fuel used. A high expansion ratio delivers a longer power stroke, allowing more expansion of the combustion gases and reducing the amount of heat wasted in the exhaust. This makes for a more efficient engine.

The disadvantage of the four-stroke Atkinson cycle engine versus the more common Otto cycle engine is reduced power density. Because a smaller portion of the intake stroke is devoted to compressing the intake air, an Atkinson cycle engine does not take in as much air as would a similarly-designed and sized Otto cycle engine.

Four stroke engines of this type with this same type of intake valve motion but with forced induction (supercharging) are known as Miller cycle engines.


Rotary Atkinson cycle engine

The Atkinson cycle can be used in a rotary engine. In this configuration an increase in both power and efficiency can be achieved when compared to the Otto cycle. This type of engine retains the one power phase per revolution, together with the different compression and expansion volumes of the original Atkinson cycle. Exhaust gases are expelled from the engine by compressed air scavenge. This modification of the Atkinson cycle allows for the use of alternative fuels like Diesel and hydrogen. See External Links for more information.



Vehicles using Atkinson cycle engines

While a modifed four stroke engine using the Atkinson cycle provides good fuel economy, it is at the expense of a lower power-per-displacement than a traditional four stroke. If the engine is only run at high powers intermittently, then the power of the engine can be supplemented by an electric motor during times when high power is needed. This forms the basis of an Atkinson cycle based hybrid electric drivetrain. These electric motors can be used independently of, or in combination with, the Atkinson cycle engine, to provide the most efficient means of producing the desired power.

Multiple production vehicles use Atkinson cycle engines:

Toyota Prius hybrid electric (front-wheel-drive) with a compression ratio of 13.0:1
Ford Escape/Mercury Mariner/Mazda Tribute hybrid electric (front- and four-wheel drive) with a compression ratio of 12.4:1
Toyota Camry Hybrid hybrid electric (front-wheel drive) with a compression ratio of 12.5:1

Note that any compression ratio shown above only reflects the physical compression ratio, which provides the ratio of the combustion chamber volumes when the piston is at "bottom dead center" versus "top dead center". The effective compression ratio of the air-fuel mixture in an Atkinson cycle engine, with respect to atmospheric pressure, is lower due to the aforementioned delay in closing the intake valve.