Hybrids Give Atkinson Engine New Life

By dancurranjr On March 26th, 2009

atkinsonmodelOriginally developed by British engineer James Atkinson to get around patents granted to Nicholas Otto in 1876 for the invention of the four-stroke internal combustion engine, the Atkinson cycle engine has been granted a new lease on life.

We are seeing mention of it with reference to most hybrid vehicles because the Atkinson cycle provides greater efficiency than the traditional Otto cycle.

Before we get into the nitty-gritty of how it does this, let’s have a brief review of the four-stroke internal combustion engine – whether operating on the Otto or Atkinson cycle. In either case, a single rotation of the crankshaft allows the intake, compression, power and exhaust strokes to occur – in different cylinders.

As the rotating crankshaft pulls the piston down in any particular cylinder, the intake valve or valves open and the suction created by the withdrawing piston draws air into the cylinder. As the piston starts to move back up, the intake valve(s) close, sealing the chamber and the rising piston compresses the air/fuel mixture.

As the piston reaches the top of its stroke, an electric charge arcs across the spark plug, igniting the mixture. The resulting explosion forces the piston back down on what is referred to as the power stroke.

Lastly, as the piston reaches the bottom of the power stroke and comes back up, the exhaust valve(s) open and the spent gases are forced out by the rising piston. All of this happens in each of the three, four, six, eight, 10 or 12 cylinders several thousand times every minute.

The original Atkinson cycle engine utilized an altered linkage and a unique crankshaft to deliver a power stroke that was longer than the compression stroke, resulting in improved efficiency.

The problem was that this came at the expense of power output. That complicated original design was dropped soon after it was invented. But modern electronic controls, especially the ability to alter valve timing, have allowed the Atkinson cycle to be revisited in the search for less fuel consumption and lower emissions – i.e. efficiency.

The modern Atkinson cycle engine uses this ability to alter valve timing – when they open and close – to hold the intake valve open slightly longer than with the Otto cycle engine. Instead of closing when the piston reaches the bottom of the intake stroke, it stays open briefly as it starts back up on the compression stroke. The intake valve then closes as the piston continues on the compression stroke.

Because the engine does not have to force the piston up as much on the compression stroke, less energy is consumed. On the other hand, because a smaller portion of the intake stroke is devoted to compressing the incoming air/fuel mixture, less of it is available to ignite and generate power.

This loss of efficiency and reduced power output is offset somewhat by allowing higher compression ratios and the use of electric motors or forced induction. Since the Atkinson cycle works best at lower rpm where the gases in the cylinder have time to change directions, an electric motor, which produces its maximum torque or power at low rpm, is an ideal companion.

Mazda engineers developed an Atkinson cycle engine for the Millenia luxury car more than a decade ago – augmented by a supercharger that allowed the tiny little 2.5-litre V-6 to produce both impressive power and fuel economy.

More recently, companies have used an electric motor to compensate for the slight loss of power from the Atkinson cycle engine in hybrid vehicles.

SOURCE: Globe and Mail

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