Atkinson Cycle

[2StrokeSteve]

Hello,

I know Atkinson cycle in banded about quite a lot in the world of Toyota hybrids.

But the Anson Engine Museum in the UK has an actual Atkinson engine.

The Atkinson cycle engines employes some clever bit of linkage between the crank shaft and con rod so that complete Otto 4 stroke cycle is completed in one revolution of the flywheel.

Just a tad bit different to the system employed in the Toyota Hybrids, which I think is closer to the Miller cycle?

Cheers, Steve

[FROSTYBALLS]

Extract from Wikipedia:

"Recently, the term "Atkinson cycle" has been used to describe a modified Otto cycle engine in which the intake valve is held open longer than normal to allow a reverse flow of intake air into the intake manifold. The effective compression ratio is reduced (for a time the air is escaping the cylinder freely rather than being compressed) but the expansion ratio is unchanged. This means the compression ratio is smaller than the expansion ratio. Heat gained from burning fuel increases the pressure, thereby forcing the piston to move, expanding the air volume beyond the volume when compression began. The goal of the modern Atkinson cycle is to allow the pressure in the combustion chamber at the end of the power stroke to be equal to atmospheric pressure; when this occurs, all the available energy has been obtained from the combustion process. For any given portion of air, the greater expansion ratio allows more energy to be converted from heat to useful mechanical energy meaning the engine is more efficient.

The disadvantage of the four-stroke Atkinson cycle engine versus the more common Otto cycle engine is reduced power density. Due to a smaller portion of the compression stroke being devoted to compressing the intake air, an Atkinson cycle engine does not take in as much air as would a similarly designed and sized Otto cycle engine.

Four-stroke engines of this type with this same type of intake valve motion but with a supercharger to make up for the loss of power density are known as Miller cycle engines."

[2StrokeSteve]

Basically a Miller cycle without a supercharger.

Cheers, Steve

[Cyker]

A real Atkinson cycle engine is fascinating to watch in action; They look fragile as hell tho' with all those linkages!

One thing I'm interested in is the newer VVTi's they're going to start to using - They claim they are using their quasi-Atkinson/Miller cycle but without the huge torque loss you normally get. Be fascinating to know how they pull that off!!!

I'm wondering if they've found a way of switching the valve-timings on the fly between full-cylinder intake and half-cylinder intake modes or something...? i.e. so it runs in the Atkinson/Miller cycle normally, but can switch back to the normal petrol Otto cycle when you need more grunt? (Would that even work?? You'd have to find a way of compensating for the large change in compression ratio...)

[ervin00]

http://www.gizmag.com/toyota-atkinson-engines-improved-thermal-fuel-efficiency/31615/

[FROSTYBALLS]

See also http://www.toyotaownersclub.com/forums/topic/159623-new-engine-for-the-aygo/

[Bono Estente]

Interesting stuff thanks