Potential engine mod? Otto to Atkinson conversion

[trebuchet03]

So I was aware of the loss of low end torque and high end power... But I wasn't aware that it caused detonation issues - I'll have to do more research as to why that happens (doesn't seem right as we'd be dealing with lower overall compression).

Oh, and the way I understood it, the Atkinson gets its efficiency from retaining more of its kinetic energy from the compression stroke since it is compressing less volume than an Otto cycle engine.

Keep in mind it's going to have less useful energy in the first place (controlling the rpm variable).... The major pitfall of the otto engine is that it doesn't use all of the available work in the compressed volume of ignited fuel. The expansion ratio for useful work is something like 25:1 (and it diminishes exponentially) for gasoline. Diminishing returns occur around 17:1 - 18:1 expansion ratios.... Obviously, compressing 18:1 on gasoline would cause detonation without some crazy high octane ratings.

Atkinson resolves that issue by having a disproportionate compression and power stroke to take advantage of energy that would otherwise be ejected out of the tailpipe (literally). Hrmm - thinking of it that way... I'm thinking there will be a EGT value....

[AndrewJ]

cat lighting troubles, eh?

the HF, VX? and CX have the cat directly under the manifold, on the front side of the block, so that may help on that front.

[RH77]

I'm thinking that the CAT lighting issues aren't that big of a deal: burned fuel is hot. Start any car -- even a Diesel, and let it idle for less than a minute. The A-pipe will be way too hot to the touch. Even a Prius makes heat within a minute of starting -- Atkinson style, with low emissions. It can be done.

RH77

[Daox]

Originally Posted by AndrewJ

So, there may be problems....

Can't seem to find any difinitive word on the HF's particular cycle (Atkinson/Otto)

This doesn't make sense to me. Why would you have detonation problems at low rpm without a supercharger, but with a supercharger you wouldn't? When you apply higher cylinder pressures is usually when you see more problems with detonation.

I can definitly see why the electric motor works, but why supercharger?

[MetroMPG]

Originally Posted by AndrewJ

I think the Honda D-series (B's to a lesser extent) may be good candidates, they're all undersquare, so good for torque. They have cheap parts, so easy to get the custom grind and bump up compression.

My bro in-law and I have talked lots about Atkinsonizing via cam grinding. This guy does grinds for the Suzukiclones, and would be someone to talk to if a clone driver wanted to try it. (He makes & sells XFi cam copies for the efficiency crowd & hot cams for the speed freaks.)

Not me, though! It would be interesting to try, but so many other things to doooo...

[MetroMPG]

Note: I split off the cylinder deactivation discussion in to a new thread.

[DAN]

a muller cycle motor is a motor starved on the intake. this leaves more room for the hot gas to work. it dates from the 20's was used in big ship motors. and , i think in industrial motors. it cut high revs and power but keeps much of the torke. crane cams sold a cam that let some of the charge blow back. it was made for the old motors that needed hi-test gas. its not legal for cars now. but was sold for trucks. it was used on big motors. cane sold out and i don't know if its still sold.

[DAN]

oh' mazda used the muller cycle on its high line car. they used high boost superchargers on a small, for us, motor. the car mags said it worked great with power and less fuel

[wwest40]

Originally Posted by AndrewJ

So this one hit me as I was looking around wikipedia last night. I seem to recall reading somewhere a long time ago that the CRX HF engine ran on a different "cycle" than a normal engine, partially accounting for it's increased fuel efficiency.

A few minutes of searching turned up the Otto cycle (the "normal" cycle) and the Atkinson cycle (the high FE cycle).

It looks to me as if the only appreciable difference is in the lift duration of the intake cam. If it was possible to have a new cam ground to provide this increased intake duration, you could effectively convert your engine to the Atkinson cycle, thereby raising your FE.

So the question I'm posing for all of you engine gurus out there is this:
Would this be a feasible mod for all us folks out here with Otto cycle engines?

First, what is currently being recognized as an Atkinson Cycle engine is not. Only a close approximation.

The current "Atkinson" implementation, delaying the intake valve closing so that a portion of the cylinder A/F mixture, CHARGE, can be pushed back into the intake manifold, only works satisfactorily with 4 cylinder engines or multiples thereof wherein an "opposite" cylinder is in an "intake" cycle and will threfore "absorb" the A/F charge being "pushed" back into the intake manifold.

Another approach might be to use a "reed" valve, one-way shuttle valve, like that used in 2-cycle engines, to prevent reverse flow out of the intake system.

A more common approach is that developed in teh late forties, called the Miller Cycle, using a positive displacement supercharger to prevent the reverse flow and at the same time provide "make-up" for the lost efficiency HP/torque wise of the Atkinson cycle engine.

And just changing the cam profile isn't enough. The modern implementation of the Atkinson cycle typically uses a "native" cylinder compression ratio of about 13:1. So once you "push" 20-30% of the mixture back out of the cylinder you end up with a "net", effective, compression ratio of 10:1, pretty standard.

[amcpacer]

So we could take a 4 cylinder engine and have the head milled for super high compression then have a reduced duration intake lobe ground in the valve cam. That would be a miller cycle engine?

Please somebody grind a cam for my dodge daytona 2.2 before I take a grinder to my cam! Just kidding.