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Mr. Green Car: Putting Atkinson cycle engines to good use
By Allen Penticoff
This column may be a bit dry. It is about hidden technology lurking in the engines of hybrid cars. The name “Atkinson Cycle” popped up in a review I was reading about a car — and as is often the case, my curiosity demanded that I investigate. Especially since I know a lot about all the various means of locomotion, was I missing one?
In1882 — 130 years ago — James Atkinson designed an engine with a unique crankshaft that enabled the engine to complete all four of the cylinder operations of a four-stroke engine within one revolution of the crankshaft. In part, Atkinson did this to circumvent the patents of the “Otto Cycle” engines (pretty much what everyone is driving around with to this day).
A normal Otto cycle four-stroke engine draws in fuel and air on a downward “intake stroke” of the piston, up it goes on the “compression stroke” — squeezing the fuel-air mixture — that’s one revolution. The compressed fuel-air mixture is ignited, the expanding gas pushes the piston down the “power stroke,” and then the piston comes back up on the “exhaust stroke,” pushing the spent gases out — ending at the second revolution of the crankshaft. This process happens thousands of times per minute in each of the cylinders of your engine, gas or diesel. It amazes me, and I thoroughly understand it! It’s almost magic.
A two-cycle engine (like a chain saw) condenses all four operations down to one revolution per cycle, but two-cycle engines do this by overlapping operations/strokes, i.e., power/exhaust and exhaust/intake. This is why they are bad polluters — too much unburned fuel going out the exhaust. But they can be made very light-weight. The Atkinson cycle, on the other hand, allows the expansion ratio to vary from the compression ratio. This allows the power stroke to be longer than the compression stroke. This also allows the engine to burn the fuel-air mixture more completely, and thus be more efficient while reducing un-burned fuel existing in the exhaust. Just the thing when you are after high fuel economy. A more technical way of saying this is that it has better “thermodynamic” efficiency.
Unfortunately, James Atkinson’s design for doing this with a unique crankshaft did not survive to modern times. But some smart engineers figured out how to do the same thing with unconventional valve timing. Your average vehicle does not have variable-valve timing, but recent use of computers in controlling everything going on in an engine, and mechanical inventions that allow changes in both intake and exhaust valve timing, have done some amazing things for engines. Mostly, this technology is used to get a lot of horsepower out of a smaller engine, which is more efficient. But mimicking the Atkinson cycle with valve timing produces an even more efficient engine.
How this happens to not have the same compression and power ratios is that as the piston moves up on the compression stroke, the intake valve is still open, so it is not compressing the mixture yet. This is effectively lowering the compression ratio (and allows use of lower-octane fuel). The goal is to have the cylinder pressure equal atmospheric pressure at the end of the power stroke — in essence, to have completely burned ALL the fuel. The disadvantage of this is now the engine is not drawing in as much fuel-air mixture on the intake stroke, so less power is generated.
The down side of the Atkinson cycle engine — and why it was not so popular — is they don’t have much power at low RPM. So, a car using an Atkinson cycle engine would be pretty slow moving away from a traffic light or getting into a traffic stream. This can be countered by use of a supercharger, creating what is known as a Miller cycle engine. And the Atkinson cycle can also be used very effectively in a rotary engine, but neither of these is common.
Along comes the marriage of gasoline engines and electric motors — the hybrid. Now, engineers can use the more thermodynamically-efficient Atkinson cycle, gaining the benefits of a long power stroke, yet can overcome its weaknesses with the instant power and torque provided by an electric motor. Voila — the hybrid works.
The combination Atkinson cycle engine and electric hybrid first appeared in production with the debut of the 1997 Toyota Prius. Now, most production hybrid vehicles use this technology, as well as some non-hybrids that have high-compression engines (Mazda) but have overcome the disadvantages with technology. The hybrid/Atkinson cycle is so prevalent that it is rarely even mentioned in reviews of vehicles — it is taken for granted. And now you can take for granted this way-back technology that has been modernized and put to good use — lowering your fuel consumption — and that’s good for everyone.
From the Nov. 14-20, 2012, issue