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What difference does the "cycle" make?

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Old 05-19-2006, 12:04 PM
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Default What difference does the "cycle" make?

I've read about certain cars having either an Atkinson cycle, Miller cycle, or Otto cycle, but I don't really know what these terms mean or how it affects performance or fuel usage. Does anyone here know?
 
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Old 05-19-2006, 12:27 PM
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Default Re: What difference does the "cycle" make?

The Otto cycle is what a conventional four-stroke engine uses, which involves compression and expansion strokes of the same length. A true Atkinson cycle runs much like an Otto cycle, except that the crank mechanism allows for an expansion stroke that is longer than the compression stroke. (It's hard to explain but there are illustrations on the 'net.) The advantage of this is increased thermodynamic efficiency of the powerplant at the expense of some power. The Miller (or Modified Atkinson) cycle does the same thing in a different way. Rather than making the piston travel two different lengths (complicated and unconventional) it uses a standard crank/con rod set up and uses the valves to chage the effective length of the compression stroke. The intake valve closes late, allowing some of the intake charge to exit before anything is compressed. The expansion stroke then uses the full length of the piston stroke. Again, power is reduced because the quantity of air/fuel burned in each cycle is reduced. But the amount of work produced is greater than what you would get from that quantity of air/fuel in an Otto cycle engine.
 
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Old 05-19-2006, 01:10 PM
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Default Re: What difference does the "cycle" make?

http://www.keveney.com/Atkinson.html

This helped me alot in figuring out how these different gas engine cycles work.

Cheers,

Curt.
 
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Old 05-19-2006, 01:53 PM
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Default Re: What difference does the "cycle" make?

I thought a Miller cycle is what Mazda used with a turbo. Do they not leave the intake open a bit longer continuing to force more fuel/air in even during the first part of compression?
 
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Old 05-19-2006, 01:59 PM
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Default Re: What difference does the "cycle" make?

Ahhh here we go..Miller Cycle:

http://en.wikipedia.org/wiki/Miller_cycle

http://auto.howstuffworks.com/question132.htm

Atkinson Cycle:

http://en.wikipedia.org/wiki/Atkinson_cycle

http://auto.howstuffworks.com/ford-escape4.htm

The Atkinson cycle may also refer to a four stroke engine in which the intake valve is held open longer than normal to allow a reverse flow of intake air into the intake manifold. This reduces the effective compression ratio and, when combined with an increased stroke and/or reduced combustion chamber volume, allows the expansion ratio to exceed the compression ratio while retaining a normal compression pressure.
 
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Old 05-20-2006, 04:59 AM
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Default Re: What difference does the "cycle" make?

The simplest explanation is that the EFFICIENCY of the cycle largely depends on the EXPANSION RATIO of the engine. So, roughly speaking....

Cycle type - Expansion ratio - Peak thermal efficiency of engine (%)

Otto (gasoline) - 10:1 - 28% efficient

Miller/Atkinson - 13:1 - 35% efficient

Diesel cycle - 20:1 - 43% efficient

Marine diesel - 25:1 - 50+% efficient


So an old otto cycle engine extracts only about half the energy from the fuel as a big marine diesel.
 
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Old 05-20-2006, 08:43 AM
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Thumbs up Re: What difference does the "cycle" make?

Originally Posted by clett
The simplest explanation is that the EFFICIENCY of the cycle largely depends on the EXPANSION RATIO of the engine. So, roughly speaking....

Cycle type - Expansion ratio - Peak thermal efficiency of engine (%)

Otto (gasoline) - 10:1 - 28% efficient

Miller/Atkinson - 13:1 - 35% efficient

Diesel cycle - 20:1 - 43% efficient

Marine diesel - 25:1 - 50+% efficient

So an old otto cycle engine extracts only about half the energy from the fuel as a big marine diesel.
I would observe that the Miller/Atkinson cycle expansion ratio is not constrained by detenation ignition of the fuel-air mix. This means any of the traditional engine rebuilding techniques to increase compression ratio should work perfectly fine and result in a more efficient engine. Just change the cam intake period to minimize detonation risk.

Bob Wilson
 
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Old 05-22-2006, 09:27 AM
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Default Re: What difference does the "cycle" make?

Wouldn't it be nice if the 1.9L TDI VW engine was teamed up with the Synergy Drive. That should make 80 MPG routine.

https://www.greenhybrid.com/compare/mileage/car/2647/signature.png
 

Last edited by pcflorida; 05-22-2006 at 10:44 AM.
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Old 05-22-2006, 12:35 PM
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Default Re: What difference does the "cycle" make?

Audi did pair up the 1.9L TDI VW engine with a 30 hp electric motor for the DUO plug-in hybrid about 10 years ago - not sure what the consumption was though.

More recently, VW have been pairing up the 1.4L TDi with a mild-hybrid set up and are seeing about 75 mpg in tests so far. No word yet as to when it's coming to market, though the system looks pretty much production ready.
 
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