//// Atkinson cycle Diesel ////
 

Hi All,

Just recently learned how hybrid cars running an atkinson cycle engine gets more fuel mileage but I have several questions...

1) Given the "same" engine (other than the cam to alter the intake valve closing) how much more efficiency does the atkinson style of operation yield over the otto cycle engine ?

2) Has anybody tried making a diesel engine operate on the atkinson cycle? THAT would seem like the ultimate in efficiency (a diesel is more efficient to start with then using an atkinson cycle to boot ? I'm drooling at the possibilities!).

Acutally I wonder ho much a custom can costs to do that ? .... I'll research and see what I can find .... no THAT and making it into a hybrid WOW (I have access to a TDI 1.9 L ALH engine unless my buddy sells it beforehand!).

Andrew

Diesels already rum a thermodynamic cycle optimized for efficiency and are already hybrid if they have a turbo.

HI,

From my understanding , it's not a fuel mixture or air ratio thing that gives the atkinson engine its added efficiency but rather the added energy extraction from an effectively reduced intake charge stroke VS the longer power stroke.

I have looked into a custom camshaft earlier today and hopefully they'll be able to brew one up (long live Benedict Arnold Prius !!) ;-)

Andrew

To my understanding, it isn't so much a added compression that gains efficiency as it is added expansion one gets with a higher static compression ratio - the added compression is really an unwanted side effect. In a gasoline engine you might run a static compression ratio of, say, 16:1, but leave the intake valves open for part of the intake stroke, so part of the charge is pushed back out. For the rest of the cycle it operates with a 16:1 ratio, just with lower cylinder pressures, much like if you were limited to only 3/4 throttle but without the throttle plate pumping losses.

If cylinder pressures weren't limited this way, early detonation of the combustion charge would be inevitable with such high compression (without super high octane fuels), and in most engines that can cause damage and negative work.

Diesels operate solely on detonation. To my knowledge, many already have compression ratios as high as 20:1 and have no need for limiting cylinder pressures. I'm less familiar with how engineers optimize them though.

I believe the reason for the Atkinson Cycle is to use the extra expansion energy to push the piston further down, rather than fighting that extra expansion to waste it into the exhaust manifold.

It would be interesting to try a real Atkinson approach in a Diesel engine. Otherwise, in order to emulate the Atkinson cycle like it's done on gassers, it would need such an outstanding static compression ratio as the 28:1 of the Scania dedicated-ethanol engine.

Is there that much expansive power left in a diesel after the 20:1 expansion?

Not sure if it would be enough to overcome the pumping losses.

Not so sure about it having a much higher efficiency. Fuel consumption by volume is around 50% higher, but it's due to the lower energy density of ethanol. Considering that for spark-ignited engines the rule-of-thumb gives a 30% higher consumption by volume with ethanol, I'm inclined to believe there is no increase in thermal efficiency on compression-ignited ethanol engines compared to their Diesel counterparts.

I guess the advantage is using renewable fuel, probably engineered with no sulfur.