Atkinson Cycle Engines & Hybrids

[Big O Dave]

I have come to the conclusion that the reason most hybrids use Atkinson Cycle engines is that it works like a compression release such as was used on old large single cylinder motorcycle engines - it make the engine easier to start.

This would be important because the ICE in a hybrid is constantly being turned off and re-started.

The attendant loss of low-rpm power is compensated for with the electric motor.

What say you?

 

[oggb]

The Atkinson cycle isn't helpful in that regard by itself - most hybrid powerplants still have moderate effective compression ratios. I think what really helps is use of the variable valve timing mechanisms as a way to reduce compression during the "spin up" to build oil pressure before operating the engine under load - IE leaving the exhaust valve open slightly during the compression stroke during spin-up.

 

[Shannow]

It's inlet valve isn't it ?

Atkinson and Miller "cycles" are to make a longer power stroke for a given compression, getting more work out of it.

Nissan mucked with an engine years ago that used linkages to have a (say) 2 litre engine on induction and compression, with a 3L "power and exhaust"....cam makes it easier.

 

[friendly_jacek]

yap, increased fuel efficiency at the expense of power in Atkinson.
Miller is the best of of both worlds but complex design.

 

[meep]

much more fuel efficient. it kills low rpm torque, but the electric motor picks up that duty.

the latest VVT technologies, if the intake and exh valves are modulated separately, can also go pseudo-atkins on cruise to aid in better mpg.

 

[Kiwi_ME]

It's all about increased expansion ratio, nothing to do with starting.
http://en.wikipedia.org/wiki/Atkinson_cycle

 

[artificialist]

Originally Posted By: friendly_jacek
yap, increased fuel efficiency at the expense of power in Atkinson.
Miller is the best of of both worlds but complex design.


Miller cycle engines weren't all that great. During the lifecycle of the Mazda Millenia 2.3 S/C, the Nissan Maxima 3.0 DOHC had about equal power and fuel effeciency from a NA engine. The Mazda was like paying $5000 for almost no gain.

The Mazda might have made financial sense in places where displacement determined how heavily the car was taxed, and the size of registration fees, but since the USA didn't use that, the Millenia was far outsold by the Maxima.

 

[morris]

in the early 70s i worked for Davis mfg. later J.I. Case. i was in the experimental shop building prototype machines, i worked on the small machines that use Kohler engines and one i cant remember, but not B & S. they came out with an easy starting model. my boss told me that it had a set up that unloaded one of the valves, making it easy to start, it seamed to worked very good. we got so used to them, when me got an old one we wasn't happy. is this the system you cats a talking about?

 

[bdcardinal]

Originally Posted By: artificialist
Originally Posted By: friendly_jacek
yap, increased fuel efficiency at the expense of power in Atkinson.
Miller is the best of of both worlds but complex design.


Miller cycle engines weren't all that great. During the lifecycle of the Mazda Millenia 2.3 S/C, the Nissan Maxima 3.0 DOHC had about equal power and fuel effeciency from a NA engine. The Mazda was like paying $5000 for almost no gain.

The Mazda might have made financial sense in places where displacement determined how heavily the car was taxed, and the size of registration fees, but since the USA didn't use that, the Millenia was far outsold by the Maxima.


Not to mention the Miller Cycle Mazda's are an absolute nightmare to work on. Nothing is really hard, just very time consuming. Funny parts is parts are cheap, but the labor his horrendous. Figure about 12 hours to do spark plugs since you have to pull the supercharger and all the associated gaskets.

 

[yonyon]

I couldn't agree with you more on that one, bdcardinal. Some part of me really wants one of those Millenia S engines to put in one of those little bathtub sized fishing boats, though. [censored] my empty wallet and lazy nature!