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 density and is beginning to see use in modern hybrid electric applications.

Design
The original Atkinson-cycle piston 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. Due to the unique crankshaft design of the Atkinson, the expansion ratio may differ from the compression ratio. By adjusting the linkage to allow a power stroke that is longer than the compression stroke, the engine can achieve greater efficiency than with the Otto cycle engine. While Atkinson's original engine design is no more than a historical curiosity, the Atkinson cycle, where the power stroke is longer than the compression stroke, is increasing in popularity due to the increase in fuel economy it provides.

Ideal thermodynamic cycle
The ideal Atkinson cycle consists of following operations:


 * 1) Isentropic or reversible adiabatic compression.
 * 2) Isochoric heating.
 * 3) Isentropic expansion.
 * 4) Isobaric cooling.

Modern "Atkinson-cycle" engines
Recently Atkinson cycle has been used to describe a modified Otto cycle 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 is more like a Miller cycle engine than an actual Atkinson cycle engine. The effective compression ratio is reduced (for a time the air is escaping the cylinder freely rather than being compressed) but the expansion ratio is unchanged. This means the compression ratio is smaller than the expansion ratio. Heat gained from burning fuel increases the pressure, thereby forcing the piston to move, expanding the air volume beyond the volume when compression began. The goal of the modern Atkinson cycle is to allow the pressure in the combustion chamber at the end of the power stroke to be equal to atmospheric pressure; when this occurs, all the available energy has been obtained from the combustion process. For any given portion of air, the greater expansion ratio allows more energy to be converted from heat to useful mechanical energy meaning the engine is more efficient.

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 compression 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 a supercharger to make up for the loss of power density 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 scavenging. 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 modified Otto cycle engine using the "Atkinson cycle" provides good fuel economy, it is at the expense of a lower power-per-displacement as compared to a traditional four-stroke engine. If demand for more power is intermittent, the power of the engine can be supplemented by an electric motor during times when more 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.

Several production vehicles use "Atkinson-cycle" engines:


 * Toyota Prius hybrid electric (front-wheel drive) with a (purely geometric) 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
 * Ford Fusion Hybrid/Mercury Milan Hybrid hybrid electric (front-wheel drive) with a compression ratio of 12.3:1
 * Toyota Camry Hybrid hybrid electric (front-wheel drive) with a compression ratio of 12.5:1
 * Chevrolet Tahoe Hybrid hybrid electric (four-wheel drive) with a compression ratio of 10.8:1
 * Lexus RX 450h hybrid electric (front-wheel drive)
 * Mercedes ML450 Hybrid hybrid electric
 * Mercedes S400 Blue Hybrid hybrid electric
 * Lexus HS250h

Note that the compression ratios shown above reflect the expansion ratio, which is the ratio of the combustion chamber volumes when the piston is at Bottom dead centre and Top dead centre. 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.