The Atkinson Cycle Engine is a type of internal combustion engine. Its operation is characterized and differentiated from 'conventional' four-stroke and simplified two-stroke engines in that it actually has five phases over four strokes in its operational cycle. The advantages of the Atkinson cycle engine are realized entirely in increased fuel efficiency; this increase is accompanied by a loss of low-speed torque and high-end power. If, however, efficiency is the primary goal, then excellent results can be had from utilizing this engine type.

An Atkinson engine has five phases of a full engine revolution. They are:

This differs from standard four-stroke engines in the addition of the 'back-flow' phase, which shares the second stroke (compression stroke) of the piston with the compression phase.

In a conventional four-stroke (also known as Otto Cycle) engine, such as the one in the average car, the compression and expansion phases are always of the same ratio. That is, the fresh fuel/air mixture is compressed to a fixed degree (for example, 10:1) and then, after ignition, the chamber area expands by that same ratio, driven by the energy of combustion. The problem is that the best ratio for maximum energy extraction (expansion ratio) is not the same as the maximum ratio allowed by the combustive characteristics of gasoline. If you increase the compression of an engine, you will reach a point where your particular gasoline/air mixture begins to ignite before the most efficient time - that is, when the piston has finished moving upward and is ready to begin the downstroke. This is what is called knocking or detonation and is extremely bad for your engine, in addition to robbing you of power as the burning fuel pushes against the still-rising piston. This point is typically around 10.5:1 for modern gasoline of around 93 octane (this number is variable; Transitional Man corrected my 9.5:1 figure, informing me his Ford Contour SVT runs at 10.2:1 on 91 octane); to go higher, and hence extract more power on the combustion stroke, you need to raise the octane level of your gas. However, the maximum useful expansion ratio of a gas/air combustion is around 25:1, meaning the expanding fuel/air mix reaches ambient pressure at around 25 times its original volume. It reaches a point of diminishing returns at around 17:1. Thus, the normal Otto cycle engine cannot be maximally efficient, as its expansion ratio can never rise above the compression ratio, which is limited to around 10.5:1.

Furthermore, there is the matter of pumping loss. The speed of an Otto cycle engine is regulated by how much air it is allowed to draw into its cylinder; the mixture of fuel and air is typically maintained at or around stoichiometry, which for gasoline/air is approximately 1:14.3 by mass - for every gram of gas burned, the engine will draw in 14.3 grams of air from its surroundings. So in order for an engine to operate below its maximum power setting, the user must restrict the flow of air (and hence the air/fuel mix) into the cylinders. This is done by utilizing a throttle to regulate the volume of air taken in; and since engines rarely if ever operate at absolute maximum power, they tend to take in a fuel/air mixture that is at a lower pressure than ambient. This produces what is called manifold vacuum - the intake manifold's pressure is always lower than that of the surrounding air. Hence, as the piston undergoes an intake stroke, it has ambient air pressure below the piston and lower-than-ambient pressure above. Thus, the engine is doing extra work (and hence, wasting energy) in fighting this imbalance.

A British engineer named James Atkinson, working around the same time as the German Nikolaus Otto, inventor of the four-stroke, came up with a solution to these problems. There is speculation among historians that one reason he did so was to differentiate his design enough to avoid Otto's well-developed patent on the four-stroke engine; however, the efficiency increase might have been his sole motive. In any case, he realized that one could vary the power of the engine by making the intake volume variable; that is, rather than vary the amount of air and fuel in a given volume (and thus changing the pressure inside), you could change the size of the space that was being filled with fuel/air and compressed. His original design did just that. It did so by a system of complex machinery on the crankshaft that allowed the intake and compression stroke of the piston to be of varying lengths depending on the speed of the engine; this meant that the compression ratio of the engine depended on the shorter intake stroke, whereas the expansion depended on the full cylinder size.

This ameliorated both problems mentioned above! The problem of the compression and expansion ratios being different was solved by having a lower compression ratio than the cylinder design was capable of, since the initial volume of the cylinder at the end of the short intake cycle was not the full volume of the cylinder. It also meant that the intake manifold could be kept at ambient pressure, eliminating the pumping losses.

The problem with his original design, however, was that the mechanisms for having different stroke lengths were complex, and increased not only the chances of failure but also increased the losses due to friction inside the engine. As a result, it remained mostly a curiosity until the twentieth century.

In 1947, an American engineer named Miller hit upon an ingenious version of the Atkinson design that solved both problems. Rather than varying the actual stroke length of the intake stroke, he realized that you could simply delay closing the intake valve past the end of the intake stroke. Thus, as the piston travelled back up the cylinder, rather than compressing the fuel mix, it simply pushed it back out into the intake manifold. The compression phase only occured from the moment the valve was closed to the moment the piston reached TDC; thus, if you could dynamically vary the timing on the intake valve, you could effectively change the compression ratio of the engine, dropping it below the total ratio of the cylinder. Hence, the expansion (total) ratio could be closer to the ideal 17:1 required for the most efficient extraction of energy.

The modern version of this engine really ought to be called the Atkinson-Miller Cycle engine, but most references seem to simply call it the Atkinson Cycle, with the Miller Cycle designation used to refer to those versions which use forced induction to make up the power loss. As can be seen, the engine sacrifices top-end power when compared to an Otto cycle engine of the same cylinder displacement since the maximum amount of fuel it can burn per cycle is smaller. Also, because it is difficult to burn enriched fuel/air mixtures (which most Otto cycle engines do in order to achieve better torque and hence acceleration) it is much less powerful at the low end of its RPM range. This engine really needs to run in a narrow RPM band in order to make the best use of its capabilities.

Fortunately, there is an application which fits this perfectly - the gas/electric hybrid vehicle. The gas engine in such a car need only run at its ideal, efficient rate (typically around 55-65% of max rpm) in order to generate electricity rather than direct motion. Also, the lack of low-end power can be offset by the use of extremely torquey but low horsepower electric drive motors. The hybrid Toyota Prius, starting in the 2004 model year, is powered by an Atkinson cycle version of the engine used in the conventional Toyota Echo - running at lower BHP and torque, but at much higher efficiency.

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