Atkinson vs. Otto Cycle

[Rockville1]

I'm posting this in the Main Forum (vs. Technical) as I thought this would be of general interest to Prius Owners. I was also surprised to find so few threads about the Atkinson cycle in the Prius Chat Forum (most are 2 or more years old now).

For those who don't know, the Prius uses an Atkinson Cycle combustion engine, combined with the electric motor, to provide power. The Atkinson cycle is much different from the typical Otto Cycle that is found in most internal combustion engine (ICE) cars today. The advantage of the Atkinson cycle is that it is much more efficient than the Otto cycle. The disadvangtage is that it has less power (especially when moving the car from a dead stop). However, combined with the hybrid system, the electric motors provide the necessary power to get the car moving before the ICE is needed.

If you want to learn more about the Otto vs. the Atkinson cycles, read the following two articles at Wikipedia:

Four-stroke engine - Wikipedia, the free encyclopedia

Atkinson cycle - Wikipedia, the free encyclopedia

[ksstathead]

See also...

nice graphical treatment of the subject (& other engine types) at

Keveney.com

[Rockville1]

Cool site Stathead! Thanks.

Current Cars that use the Atkinson Cycle:

Toyota Prius hybrid electric (front-wheel drive) with a (purely geometric) compression ratio of 13.0:1
Ford Escape/Mercury Mariner/Mazda Tribute hybrid electric (front- and four-wheel drive) with a compression ratio of 12.4:1
Toyota Camry Hybrid hybrid electric (front-wheel drive) with a compression ratio of 12.5:1
Chevrolet Tahoe Hybrid hybrid electric (four-wheel drive) with a compression ratio of 10.8:1

Notice that the Lexus hybrids do not use the Atkinson Cycle engines as Lexus has opted to use the hybrid system to boost performance (when combined with an Otto Cycle engine) rather than maximize efficiency (although their hybrid vehicles are still more efficient than the non-hybrid counterparts, e.g., Lexus RX400h vs RX350).

[David Beale]

I do have to disagree with the statement that the Atkinson cycle is "much different" from the Otto cycle. It's only slightly different. All you do is construct the engine with a long stroke. To avoid the high compression this would normally give you (and which is undesirable if you want to burn low octane rated fuel), you hold the intake valves open during the start of the compression stroke.

The result is a long power stroke, allowing more energy to be extracted from the expanding gases, without the high compression complications seen in high power engines and diesels. I'm told efficiency gains are an extra 5-10%, which is quite a bit when you are starting from about 25-30%. The downside is a loss of about 30% of the power output.

BTW, this was done many years ago (probably unintentionally) in the old GM 6 cylinder engine, though most of the low compression was gained there with an inefficient intake path and largish combustion chamber (and therefore high pollution generation).

[Rockville1]

Originally Posted by David Beale

I do have to disagree with the statement that the Atkinson cycle is "much different" from the Otto cycle. It's only slightly different. All you do is construct the engine with a long stroke.

Yes, my mistake for using the phrasing "much different." I guess I was just trying to point out that due to the rather ingenious design (IMO) of the Atkinson engine, there are a number of differences that result in a more efficient, albeit less powerful engine. There are several notable differences that orignally allowed Mr. Atkinson (in 1882) to get around the patents on Mr. Otto's engine, inlcuding the longer power stroke vs. intake and ehaust strokes and the fact that all four strokes are completed with one cycle of the crankshaft.

What's most interesting is that this efficient design was never able to succeed commercially due to the low power output until electric motors were able to merge successfully with the internal combustion engine.

[qbee42]

Originally Posted by David Beale

I do have to disagree with the statement that the Atkinson cycle is "much different" from the Otto cycle. It's only slightly different. All you do is construct the engine with a long stroke. To avoid the high compression this would normally give you (and which is undesirable if you want to burn low octane rated fuel), you hold the intake valves open during the start of the compression stroke.<snip>

The differences are (or were) originally greater. A true Atkinson cycle engine uses an entirely different crank system than an Otto cycle engine. It is more complicated but allows for a longer power stroke verses the compression stroke, even without playing with the valve timing. This was originally done to avoid infringing on Otto's patent, but the side effect was the ability to have a slightly more efficient cycle. In more recent times, someone realized they could get the same cycle effect by holding open a valve and pushing part of the charge back out of the cylinder, which was a lot easier to do than the complex gearing on a true Atkinson cycle engine. If you add a super-charger you get a Miller cycle engine.

Tom

[bredekamp]

Please explain how an engine can be more efficient and less powerful at the same time. Does it have something to do with the amount of twisting force (torque) the engine can provide?

[kohnen]

Originally Posted by bredekamp

Please explain how an engine can be more efficient and less powerful at the same time. Does it have something to do with the amount of twisting force (torque) the engine can provide?

Easy.

Efficiency = work out / fuel consumed

Power = work out / time

So, if you had an engine that puts out only 1/2 the power, but uses 1/3 the fuel, then the engine would be MORE efficient but LESS powerful.

[Rockville1]

Originally Posted by bredekamp

Please explain how an engine can be more efficient and less powerful at the same time. Does it have something to do with the amount of twisting force (torque) the engine can provide?

In very basic terms (because I'm not an engineer!), the Atkinson design reults in increased fuel efficiency but at a loss of low-speed torque and high-end power. Without the electric assist of the hybrid, an Atkinson engine just doesn't produce sufficient low-speed torque to get the car moving fast enough. Traditional ICE engines do but they are very inefficient (energy wise) at low torque. Once the vehicle is up to speed it takes very little power to keep the car moving. Thus traditional ICE or Otto cycle engines are efficient at highway speeds (where low power is needed) but terribly inefficient at low speeds (high power).

See link below for more technical description:

Atkinson Cycle@Everything2.com

[hill]

It's a thread revisited. About 1, or 2 years ago the topic was gone over ... in fact even more recently. Got a little controversial, as I remember, in at lease one of the dozen or so threads.