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The Atkinson-cycle engine is a type of internal combustion engine
Internal combustion engine

The internal combustion engine is an engine in which the combustion of a fuel occurs in a combustion chamber inside and integral to the engine. In an internal combustion engine it is always the expansion of the high temperature and pressure gases that are produced by the combustion which apply force to the movable component of the engine, such as...
 invented by James Atkinson
James Atkinson (inventor)

James Atkinson of Hampstead was a United Kingdom engineer who invented the Atkinson cycle engine in 1882. By use of variable engine strokes from a complex crankshaft, Atkinson was able to increase the efficiency of his engine, at the cost of some power, over traditional Otto-cycle engines....
 in 1882. The Atkinson cycle is designed to provide efficiency at the expense of power and is beginning to see use in modern hybrid electric applications.

original Atkinson-cycle engine allows the intake, compression, power, and exhaust strokes of the four-stroke cycle
Four-stroke cycle

Today, internal combustion engines in automobile, trucks, motorcycles, aircraft, construction machinery and many others, most commonly use a four-stroke cycle....
 to occur in a single turn of the crankshaft
Crankshaft

The crankshaft, sometimes casually abbreviated to crank , is the part of an engine which translates reciprocation linear piston motion into rotation....
 and was designed to bypass patents covering the existing Otto cycle
Four-stroke cycle

Today, internal combustion engines in automobile, trucks, motorcycles, aircraft, construction machinery and many others, most commonly use a four-stroke cycle....
 engines.






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The Atkinson-cycle engine is a type of internal combustion engine
Internal combustion engine

The internal combustion engine is an engine in which the combustion of a fuel occurs in a combustion chamber inside and integral to the engine. In an internal combustion engine it is always the expansion of the high temperature and pressure gases that are produced by the combustion which apply force to the movable component of the engine, such as...
 invented by James Atkinson
James Atkinson (inventor)

James Atkinson of Hampstead was a United Kingdom engineer who invented the Atkinson cycle engine in 1882. By use of variable engine strokes from a complex crankshaft, Atkinson was able to increase the efficiency of his engine, at the cost of some power, over traditional Otto-cycle engines....
 in 1882. The Atkinson cycle is designed to provide efficiency at the expense of power and is beginning to see use in modern hybrid electric applications.

Design

The original Atkinson-cycle engine allows the intake, compression, power, and exhaust strokes of the four-stroke cycle
Four-stroke cycle

Today, internal combustion engines in automobile, trucks, motorcycles, aircraft, construction machinery and many others, most commonly use a four-stroke cycle....
 to occur in a single turn of the crankshaft
Crankshaft

The crankshaft, sometimes casually abbreviated to crank , is the part of an engine which translates reciprocation linear piston motion into rotation....
 and was designed to bypass patents covering the existing Otto cycle
Four-stroke cycle

Today, internal combustion engines in automobile, trucks, motorcycles, aircraft, construction machinery and many others, most commonly use a four-stroke cycle....
 engines. Due to the unique crankshaft
Crankshaft

The crankshaft, sometimes casually abbreviated to crank , is the part of an engine which translates reciprocation linear piston motion into rotation....
 design of the Atkinson, the expansion ratio may differ from the compression ratio
Compression ratio

The compression ratio of an internal-combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber; from its largest capacity to its smallest capacity....
. 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
    Isentropic process

    In thermodynamics, an isentropic process or isoentropic process is one during which the entropy of the system remains constant. It can be proved that any Reversible process adiabatic process is an isentropic process....
     or reversible
    Reversible process (thermodynamics)

    In thermodynamics, a reversible process, or reversible cycle if the process is cyclic, is a process that can be "reversed" by means of infinitesimal changes in some property of the system without loss or dissipation of energy....
     adiabatic
    Adiabatic process

    In thermodynamics, an adiabatic process or an isocaloric process is a thermodynamic process in which no heat is transferred to or from the working fluid....
     compression (that is, compression with no transfer of heat).
  2. Addition of heat at constant volume.
  3. Isentropic expansion.
  4. Rejection of heat at constant pressure.


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. The 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 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 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
Supercharger

A supercharger is an air Gas compressor used for forced induction of an internal combustion engine. The greater mass flow-rate provides more oxygen to support combustion than would be available in a naturally-aspirated engine, which allows more fuel to be provided and more work to be done per cycle, increasing the power output of the engine...
) are known as Miller cycle
Miller cycle

In engineering, the Miller cycle is a combustion process used in a type of four-stroke internal combustion engine. The Miller cycle was patented by Ralph Miller , an United States engineer, in the 1940s....
 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

Prius2004
While a modified 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
Electric motor

An electric motor uses electrical energy to produce mechanical energy, nearly always by the interaction of magnetic fields and current-carrying conductors....
 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.

Several production vehicles use Atkinson-cycle engines:

  • Toyota Prius
    Toyota Prius

    The Toyota Prius is a hybrid electric vehicle mid-size car developed and manufactured by the Toyota Motor Corporation.The Prius first went on sale in Japan in 1997, making it the first mass-produced hybrid vehicle....
     hybrid electric (front-wheel drive) with a (purely geometric) compression ratio of 13.0:1
  • Ford Escape
    Ford Escape Hybrid

    The Ford Escape Hybrid, launched in 2004, is a Gas-electric hybrid vehicle powered version of the Ford Escape SUV developed by the Ford Motor Company....
    /Mercury Mariner
    Mercury Mariner

    The Mercury Mariner luxury compact SUV was introduced in 2005. It is a sibling of the Mazda Tribute and Ford Escape, although it is slightly more upmarket than the other two....
    /Mazda Tribute
    Mazda Tribute

    The Mazda Tribute is a mid-size car-based Crossover made by Japanese automaker Mazda since 2001. It is jointly developed with Ford Motor Company and based on the front-wheel drive Mazda 626 platform, which is in turn the basis for the similar Ford Escape on the Ford CD2 platform....
     hybrid electric (front- and four-wheel drive) with a compression ratio of 12.4:1
  • 2010 Ford Fusion Hybrid/Mercury Milan Hybrid hybrid electric (front-wheel drive) with a compression ratio of 12.3.1
  • Toyota Camry Hybrid
    Toyota Camry Hybrid

    The Toyota Camry Hybrid is a hybrid vehicle version of the Toyota Camry. The Toyota Camry Hybrid was introduced in May 2006 and has estimated U.S....
     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
  • 2010 Lexus RX 450h hybrid electric (front-wheel drive)


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.

See also

  • History of the internal combustion engine
    History of the internal combustion engine

    Various scientists and engineers contributed to the development of internal combustion engines:File:Benz Patent Motorwagen Engine.jpg*1206: Al-Jazari described a double-acting Reciprocating engine with a crankshaft-connecting rod mechanism....


External links

  • Note that this animation shows the true Atkinson engine, which uses a complex linkage that allows different stroke lengths for intake/compression and power/exhaust. However, the illustration shows the engine with the linkage laid out to generate 4 equal strokes. To alter the ratio of the strokes, the rightmost pivot point (the one that is attaching the horizontal green link to the frame) should be moved downwards along the frame. This will allow more angular movement as the link rotates up, giving a longer piston stroke for power and exhaust, and less angular movement as the link rotates down, giving a shorter piston stroke for intake and compression. In fact, a sliding pivot point at that location would allow the engine to dynamically change the stroke ratios.
  • alternative variable valve timing strategy increases low speed torque obtainable from Atkinson Cycle Engine.
  • , Oak Ridge National Laboratory
    Oak Ridge National Laboratory

    Oak Ridge National Laboratory is a multiprogram science and technology national laboratory managed for the United States Department of Energy by UT-Battelle....
  • Home page of the company developing the rotary Atkinson cycle engine
  • A web page giving details of this engine and comparisons with conventional and Wankel engines.
  • A quick and easy take on how the Prius uses the Atkinson cycle to get better results than an Otto-cycle engine.