Atkinson cycle valve timing v compression ratio?

[MWPC]

I would like to build a SB Chevy engine to run as an Atkinson cycle engine. By Atkinson cycle I am referring to high compression with late intake valve closing. Pump premium gas. I am willing to live with reduced power output for a slight increase in efficiency. Can anyone provide a starting point for compression ratio v intake valve closing point? Advertised or @ 0.050" lift. I can taylor the ignition timing for RPM and MAP but I need to stay out of the CI range. Thanks for the help.

 

[SMOKEY44211]

I think what you are trying to achieve is a miller cycle. Crower cam co. marketed a cam + piston kit for sbc. back in the early to mid 70s. I remember one of the magazines assembled an engine with these components and published the results. Low speed torque #'s were dismal. Max power was on par with a stock engine. This was back before EFI was common. Perhaps its worth a revisit with more up to date fueling and ignition timing techniques.-----Phil

 

[MWPC]

Phil,
I have seen it called both. I am not ready for a blower yet thus my choice of terminology. Also Ford and Toyota seem to like the Atkinson term. I am not sure why. I have not been able to find cam timing specs on the Escape Hybrid and the Prius specs that I found did not give the lift that they were measured at. I did a search here on Miller and did find some helpful info. Thanks Ken

 

[ca2sbl]

Ken: the Atkinson Cycle engines used in the Toyota Prius and Ford Escape Hybrids are true Atkinson Cycle engines in which all four cycles: intake, compression,ignition, exhaust occur in only one crankshaft revolution. See

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

for a simple diagram. For all four cycles to be completed in one crank redvolution, the connecting rods are articulated using complex linkage inside the cylander. When the Ford and Toyota engines are running, they sound almost like quiet rotaries. Mazda used a turbo Miller Cycle engine with late intake valve timing recently. Russ

 

[GregLocock]

Do you have any evidence for that at all?

The Prius uses a modified Otto cycle, with a conventional crank and conrod. The valve timing is adjusted to produce an Atkinson/Miller type of expansion, and the crank is offset. But there are no rocker levers or a secondary con rod.

It's interesting to see how people keep mentioning that animation in the context of the Prius.

http://www.cleangreencar.co.nz/page/prius-petrol-engine

has a reasonably accurate description.

The following document includes a cross section through the engine on p14. It could be that Toyota is telling fairy tales. I doubt it.

http://www.toyota.co.jp/en/tech/environment/ths2/SpecialReports_12.pdf

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ca2sbl]

Greg: After looking at the links you posted and a Google search, it looks like the Toyota and Ford do not have pure Atkinson Cycle engines. It looks like either some public relations people or engineers who wanted to put a name on an engine design attached the words "Atkinson Cycle" to a design of an IC engine that had a expansion ratio greater than the compression ratio and ignored the fact that the Atkinson design accomplished four strokes with only one crankshaft revolution instead of the two crankshaft revolutions of the Otto Cycle design. I often drive examples of both hybrids at work and those IC engines do not sound like any other four stroke engine that I have ever heard. I think they should call it a "Miller Cycle" because of the late intake valve timing. It would be interesting to see Ken convert his Chevy engine into a pure "Atkinson Cycle" engine but I think the "Miller Cycle" name fits his idea better. Then again, it is only a name. Russ