[thsksbd]

Not quite. otto cycle is most efficient at full throttle due to throttle valve pump losses.

Diesels are most thermodynamically efficient at low throttle because it takes time to combust the fuel. During this time the piston expands and the compression ratio drops.

You can see the derivation and formulas in Faires (and older edition. Even the 70s edition lacked the chapter). Actually just comparing the the PV diagram of otto and diesel makes it obvious.

[jacquesm]

> Diesels are most thermodynamically efficient at low throttle

I think you meant 'low RPM'. Low throttle doesn't always equate low RPM and VV.

[thsksbd]

See [1] for reference.

First a nitpick that is unfortunately important - diesels' don't have a throttle; what we call throttle is the fuel cutoff ratio, or how long we inject fuel for.

This is important because what distinguishes Otto and Diesel cycles is not the fuel, but the form of ignition. Otto cycle analysis is assumed instant, whereas diesel cannot be assumed instant. Car otto engines are fast enough that a diesel-like cycle is used for advanced analysis.

Low rpm and low throttle improve diesel's efficiency for the same reason - the injected fuel has more time to burn without the piston expanding.

As [1] points out in page 180:

"Study of equation (62) shows that as r e increases, the bracketed factor increases, and the efficiency decreases (Fig 99) Therefore, the lower fuel cutoff ratios are conducive to higher efficiencies but larger ratios result m greater power "

Also, very interesting, Diesel's efficiency is lower than Otto all else being equal. However, due to knocking, gasoline (octane ~ 90) engines have low compression ratios and lower efficiency than Diesels. However an otto methane (octane of 130) engine would kick diesels' butts

[1] https://archive.org/details/in.ernet.dli.2015.134070/page/n1...

[jacquesm]

Diesels running constant RPM will increase throttle with increasing load to keep their RPM. That's a matter of injecting more fuel. It's how every diesel based generator works.

As for high octane kicking diesel's butts: yes, but that fuel is far more expensive so it tends to be diesel that wins that particular contest in the economy department.