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, or total power extracted per unit of displacement per rotation. A modern variation of this approach is used in some modern automobile engines. While originally seen exclusively in hybrid electric applications such as the Toyota Prius, some non-hybrid automobiles now feature engines that can run in the Atkinson-cycle as a part-time operating regime, giving good economy while running in Atkinson cycle, and conventional power density when running as a normal-cycle engine. Mazda Skyactiv models offering this capability include the Mazda 3 and MX-5.

Design

Atkinson gas engine as shown in US Patent 367496
Animation of the Atkinson gas engine

The original Atkinson-cycle piston 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.[1] Due to the unique crankshaft design of the Atkinson, its expansion ratio can differ from its compression ratio and, with a power stroke longer than its compression stroke, the engine can achieve greater thermal efficiency than a traditional piston engine. While Atkinson's original design is no more than a historical curiosity, many modern engines use unconventional valve timing to produce the effect of a shorter compression stroke/longer power stroke, thus realizing the fuel-efficiency improvements the Atkinson cycle can provide.[2]

Atkinson differential engine

Patent drawing of the Atkinson differential engine
Animation of the 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.[3] In this, a single crankshaft was connected to two opposed pistons through a toggle jointed linkage that had a non-linearity; 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.

Ideal thermodynamic cycle

Figure 1: Atkinson Gas Cycle

The ideal Atkinson cycle consists of following operations:

Modern Atkinson-cycle engines

A small engine with Atkinson-style linkages between the piston and flywheel. Modern Atkinson-cycle engines do away with this complex energy path.

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. 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. 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 supercharger to make up for the loss of power density are known as Miller-cycle engines.

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 the use of alternative fuels like 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

2004 Toyota Prius hybrid
2010 Ford Fusion Hybrid (North America)

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.[4] 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[citation needed]:

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.[1] There is also a reference to an 1886 Atkinson patent (US 336505 ) which describes an opposed-piston gas engine.[8]