Very high boost running with WOT.
These tend to be small displacement with compound turbo-supercharging.
There are diminishing returns to high rpm as the pumping losses go up.

Of course you can look up the US Army work on adiabatic diesel project over the years.

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A couple of automotive diesels hit 42.6%. The best automotive gasoline engines is just under 37%. In general the use of turbos on gasoline engine does little to improve their maximum efficiency. The efficiency of modern automotive diesels is compromised because of NOx or other emissions.

Cheers

Greg Locock


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Toyota's Prius engine breaks in to the 40% range. Maybe boost isn't so important. In the hybrid configuration the engine can be operated at maximum efficiency at all times during charging.

Formula 1 at 50+%.
Highly supercharged, turbo-compound, operating in the ultra-lean region at all loads.
Mercedes built a road supercar using this technology but didn't get into production. The ultra high efficiency is little benefit in a road car as it is only seen at high load.

je suis charlie

Just thinking about it all production engines for cars these days have their efficiency capped because of emissions. A 3 way cat more or less forces you to operate in a fairly tight band of air/fuel ratios.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

All practical designs tend to have compromises, driven by conflicting requirements like, "as much mileage as possible, BUT, don't pollute the air."

Therefore, the question about "most ...ly efficient" is misplaced, and the only question can be "most ...ly efficient, given X, Y, and Z."

So, what's your X, Y, and Z?

TTFN (ta ta for now)
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The biggest driver of inefficiency in automotive engines is the lug curve, its one big compromise for the sake of drivability. Constant speed engines are significantly more efficient bc you only have to design and calibrate combustion around two points - idle and operation, the rest of the RPM range be damned. Quite a few certified stationaries today have advertised cycle efficiencies close to 50% and can be pushed quite a bit higher under the correct conditions. Much as I enjoy a manual throttle and transmission, preferably in a tiny sports car, a constant speed engine and generator in a hybrid setup are vastly more efficient.

Original poster has vanished. It would be nice to know why the question is being asked.

And also why in this topic area, and not the Engine topic area?