Atkinson cycle

Design

Atkinson produced three different designs that had a short compression stroke and a longer expansion stroke. The first Atkinson-cycle engine was named the "Differential Engine". This engine used opposed pistons. The second, and most well-known design, was called the "Cycle Engine", which used an over-center arm to create four piston strokes in one revolution of the crankshaft. Reciprocating engine allowed 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 avoid infringing certain patents covering Otto-cycle engines. Atkinson's third and final engine was called the "Utilite Engine" and it operated much like any two-stroke engine. The common thread throughout Atkinson's designs is the engines have an expansion stroke that is longer than compression stroke and by this method the engine can achieve greater thermal efficiency than a traditional piston engine. Atkinson's engines were produced by the British Gas Engine Company and also licensed to other overseas manufacturers. Many modern engines now use unconventional valve timing to produce the effect of a shorter compression stroke/longer power stroke, Miller applied this technique to the four-stroke engine, so it is sometimes referred as the Atkinson/Miller cycle, US patent 2817322 dated Dec 24, 1957. In 1888, Charon filed a French patent and displayed an engine at the Paris Exhibition in 1889 . The Charon gas engine (four- stroke) also used a similar cycle to Miller but without a supercharger. It is referred to as the "Charon cycle" Modern engine designers are realizing the potential fuel-efficiency improvements the Atkinson-type cycle can provide.

Atkinson "Differential Engine"

The first implementation of the Atkinson cycle was in 1882; unlike later versions, it was arranged as an opposed piston engine, the Atkinson differential engine. In this, a single crankshaft was connected to two opposed pistons through a toggle jointed linkage that had a nonlinearity; for half a revolution, one piston remained almost stationary while the other approached it and returned, and then for the next half revolution, the pistons changed over which piston was almost stationary and which piston approached and returned. Thus, in each revolution, one piston provided a compression stroke and a power stroke, and then the other piston provided an exhaust stroke and a charging stroke. As the power piston remained withdrawn during exhaust and charging, it was practical to provide exhaust and charging using valves behind a port that was covered during the compression stroke and the power stroke, and so the valves did not need to resist high pressure and could be of the simpler sort used in many steam engines, or even reed valves.

Atkinson "Cycle Engine"

The next engine designed by Atkinson in 1887 was named the "Cycle Engine" This engine used poppet valves, a cam and an over-center arm to produce four piston strokes for every revolution of the crankshaft. The intake and compression strokes were significantly shorter than the expansion and exhaust strokes. The "Cycle" engines were produced and sold for several years by the British Engine Company. Atkinson also licensed production to other manufactures. Sizes ranged from a few up to 100 horsepower.

Atkinson "Utilite Engine"

Atkinson's third design was named the "Utilite Engine". Atkinson realized an improvement was needed to make his cycle more applicable to a high-speed engine. The final engine produced by the British Gas Engine company was the Utilite Engine. With this design, Atkinson was able to make a more conventional engine yet preserve the efficiency of having a short compression stroke and a long expansion stroke in a rather ingenious way. It operates much like a standard two-stroke except that the exhaust port is located at about the middle of the stroke. During the expansion stroke, a valve (which remains closed until the piston reaches the end of the stroke) prevents pressure from escaping as the piston moves past the exhaust port. Once the valve is opened, it remains open as the piston heads back toward compression, letting fresh air charge the cylinder and exhaust escape until the port is covered. A rich fuel/air mixture is injected by a small piston pump after the start of compression. This resulted in a two-stroke engine with a short compression and longer expansion stroke. The Utilite Engines were tested and found to be even more efficient than Atkinson's previous designs. Very few Utilite Engines were produced and none is known to survive. The British patent is from 1892, #2492. No US patent for the Utilite Engine is known.

Ideal thermodynamic cycle

The ideal Atkinson cycle consists of:

Modern Atkinson-cycle engines

Recently, the term "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 simulated Atkinson cycle is most notably used in the widespread Toyota 1NZ-FXE engine. The effective compression ratio is reduced (for the 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. Due to a smaller portion of the compression stroke being 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 that use the same type of intake valve motion but with a turbocharger 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 the use of alternative fuels such as diesel and hydrogen. Disadvantages of this design include the requirement that rotor tips seal very tightly on the outer housing wall and the mechanical losses suffered through friction between rapidly oscillating parts of irregular shape. See external links below for more information.

Vehicles using Atkinson-cycle engines

While a modified Otto-cycle piston engine using the Atkinson cycle provides good fuel efficiency, 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. This drive-train first entered production in late 1997 in the Japanese-market Toyota Prius.

At this writing, many production full hybrid-electric vehicles use Atkinson-cycle theories:

Summary of the patent

The 1887 patent (US 367496) describes the mechanical linkages necessary to obtain all four strokes of the four-stroke cycle for a gas engine within one revolution of the crankshaft. There is also a reference to an 1886 Atkinson patent (US 336505) which describes an opposed-piston gas engine. The British patent for the "Utilite'" is from 1892 (#2492).