Atkinson cycle

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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.

[edit] Design

Figure 1: Atkinson Gas Cycle

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. 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 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.

[edit] Ideal Thermodynamic Cycle

The ideal Atkinson cycle consists of following operations:

1. Adiabatic compression
2. Addition of heat at constant volume.
3. Adiabatic expansion.
4. Rejection of heat at constant pressure.

[edit] 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. Compression is reduced (for a time the air is escaping the cylinder freely rather than being compressed) but the expansion process 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. 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) are known as Miller cycle engines.

[edit] Rotary Atkinson-cycle engine

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.

[edit] Vehicles using Atkinson-cycle engines

Toyota Prius 2004 NHW20

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 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 (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
• 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

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.

[edit] External links

• Animation of Atkinson Cycle Engine 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.
• Modified Atkinson Cycle Engine: alternative variable valve timing strategy increases low speed torque obtainable from Atkinson Cycle Engine.
• COMPARISON OF PRIME MOVERS SUITABLE FOR USMC EXPEDITIONARY POWER SOURCES, Oak Ridge National Laboratory
• Rotary Atkinson cycle engine A web page giving details of this engine and comparisons with conventional and Wankel engines.