I was just reading an article on hybrids, and they mentioned that the most efficient ones use a gasoline engine modified to run on the Atkinson cycle as opposed to the Otto cycle.


Um, OK - I know 4 cycle vs 2 cycle, but this is a new one on me.

( I still don't understand how you make an engine 2 stroke vs 4 stroke, or why GE locomotive diesel engines are 2 stroke and yet somehow more efficient than a comparable GM 4 stroke locomotive engine , but that's another matter. )


They go on to say that hybrids oriented towards performance maintain the Otto cycle , and there example was hybrids produced by Toyota.


Anyone here able to shed some light ?

Thanks !


Britt

 

I dont know but this is what google turned up.

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

 

4 cycle means 4 strokes of the piston to complete one power cycle.

1. Piston moves down, intake valve opens, cylinder fills
2. Intake valve closes, piston moves up, intake charge compresses
3. Spark plug ignites mixture, forces piston back down...this is the power stroke
4. Exhaust valve opens, piston moves up again, spent gases exit, valve closes at the end.

That EXTREMELY simplified, but that's a 4 cycle or 4 stroke engine.

A 2 cycle literally does all that in 1 upstroke and 1 downstroke. The downstroke is the exhaust and power stroke and the upstroke is the intake and compression stroke. You can see how you could make more power because in the same amount of time at a given RPM you have twice as many power strokes. Its not as easy to explain but here's an illustration of a 2 stroke. http://science.howstuffworks.com/two-stroke2.htm

Essentially there's no "valves". All diesel 2 strokes are turboed I think, because they won't work w/o a pressurized intake charge.

It appears the Atkinson cycle is also a 4 stroke..the difference is in how the power is transfered to the crankshaft....

 

I've been hearing about that technology for at least a year - maybe more now - but much like Zeroshift and other techs that claim to be the future of the automotive world, I've yet to see any convincing evidence that I'm going to be seeing them at a dealership near me in the next decade.

 

Ford Escape Hybrid and, I think, Camry Hybrid is an Atkinson engine.

 

Wow interesting... I responded quickly to this thread and after reviewing the atkinson cycle design I think I was thinking of a difference engine design - can't remember the name of it. Something definitely more radical.

 

The Prius uses it as well.

The original Atkinson cycle uses two different stroke lengths - one for intake and exhaust, and one for compression and power. With the "connecting rod" linkage required to realize this kinematic, the piston travels twice per one revolution of the crank - thus, the confusion over whether this is a "four stroke" or "two stroke". It's closer to a four stroke.

Modern Atkinson cycle engines simply hold open the intake valve longer than normal, which trades expansion ratio for compression ratio. This kills the power efficiency of the engine (especially down low), so it's really only useful on hybrids.

 

You've actually got it backwards: GE locomotives are 4 stroke and GM are 2 stroke. Actually, GM toyed with using a 4 stroke cycle on its "H" engine, but reliability problems (mostly with the cast, as opposed to fabricated, block) took it out of locomotive use. It still does some duty on stationary platforms. GM eventually figured out how to meet emissions standards with its old 2 stroke motors anyway.

GM's locomotive engines are basically scaled up models of their hugely successful 2-stroke Detroit Diesel engines. Whereas the Detroit Diesels (in their most popular form) displace 71 cubic inches per cylinder, the locomotive engines displace 710 cubic inches per cylinder.

 

There are exhaust valves, four of them, in each cylinder head. The intake valves are "ports", or holes, in the cylinder liners that are uncovered as the piston moves down.

I'm not sure about requiring a supercharger. Certainly they all have them (sometimes both a super- and a turbocharger), and they wouldn't make much power without them, but I don't think they are absolutely required for the engine to run. I've wondered this myself for a long time but never had a definitive answer.

 

So an Otto-cycle is 4 stroke?

What is a Miller-cycle? I am guessing 4 stroke as well.

 

Four stroke with delayed closing of the intake valve. This effectively reduces the dynamic compression ratio while still maintaining a high expansion ratio which is critical for power. Miller cycle engines are almost always used with forced induction of some kind.

 

I don't think I have ever heard of one that didn't use forced induction.

 

I don't think it is possible to do a Miller w/o FI. The whole idea is that the supercharger can compress the air against the rising piston.

 

A diesel two stroke, such as the old 6V71 (still in a bunch of TTC buses!) won't run without the supercharger. The supercharger is used to push in new air and eject the old air after combustion. The valves are exhaust valves only, and they have to close before the intake charge is allowed into the combustion chamber. The turbocharger can offload the supercharger, providing for more efficient operation. The turbos weren't combined with the Detroit two strokes until fairly late -- mostly with the later 6V92 and 8V92 engines, though it was also available with the other configurations.

See http://auto.howstuffworks.com/diesel-two-stroke.htm