Atkinson mod?

[elhigh]

Guys,
It's my understanding that you could Atkinson-ize pretty much any engine just by installing a custom cam.

I know Atkinson engines aren't supposed to be any great shakes in the power department, especially at low revs, unless you've got some forced induction to make the most of what intake they do get. This is probably a much more expensive mod than I'm willing to make on my aging truck - but if I win the lottery then I might make a few phone calls and make some changes - but just thinking out loud, since I don't drive fast and don't even do that much hauling in my truck, would it in theory be a worthwhile project?

I don't know many particulars about Atkinson cycle engines, except the impressions I've already stated above. It's supposed to make the engine more fuel efficient by permitting an effectively longer expansion stroke than the compression stroke. How much bigger has anyone taken this? Are we seeing a big ratio, like 2:1, or is just a fractional increase?

[theunchosen]

I could have sworn this came straight out of that thread we had. . .like two weeks ago? hold please I'll find a link http://ecomodder.com/forum/showthrea...rsion-955.html

The long short answer is no.

The valve timing is part of it but the expansion stroke is longer than the compression stroke(the crank shaft has to change from what yours is(equiaxed)). Thats why atkinson is more efficient than otto buts its not quite so simple as a cam change.

Its why it never really took off. A little more FE for substantially more complex.

[elhigh]

I guess I'm thinking in terms of the Miller cycle, then. I keep hearing the Prius engine being described as an Atkinson cycle, but when I read the description, I can't help but wonder to myself, how's that different from the Miller? Late-close vs. early-close? I dunno. I know the Prius' engine stroke doesn't change - nobody's done that, yet.

But all the descriptions I read of the Miller specifically mention forced induction. And I see numbers like a 15% improvement in efficiency (compared to...?) which would be nothing to sneeze at. If I could have that, it'd push me over 40mpg.

[theunchosen]

Miller is a force induced atkinson.

The atkinson came in a few cars over the years and they have(this data is being relayed from the other thread) elliptical shaft that allows for 2 strokes to be long and 2 to be short. compression and exhaust are short and intake and expansion are long(down cycles are long up cycles are short). The gain is from scavenging slightly more power from the longer expansion and losing less power on a shorter compression and exhaust.

[evolutionmovement]

There are two types of Atkinson. The Atkinson engine with the odd crank and extra linkages is an old design and not even a 4-cycle engine. The 4-cycle Atkinson found in cars today is just as you asked, a valve timing change. You should be able to apply it to any engine if you can figure out the cam design and ECU tuning. You will lose a lot of power, though, which is why they're used in conjunction with an electric motor. The Miller cycle is the forced induction version that makes up for some of the power loss without losing all the efficiency gains, but Mazda's the only production one I know of and I don't know much else about how well they worked or their reliability, but it's also the only 4-cycle Atkinson-cycle engine I know of that isn't combined with an electric motor. I guess with Atkinson, you pick your poison: electric motor or supercharger (a turbo would probably not work as well as the exhaust energy would be really low).

[elhigh]

Meh. I think if I wanted to mess around with a genuine Atkinson, I'd want it in a big ol' cast iron hit-and-miss engine. I like seeing all the moving parts at those slow speeds, just hypnotic.

But for in the car, it sounds like too much of a hassle and too many extra moving parts. So Mazda can keep their Millenia S and I'll just go how I'm going.