It's a lot like the combustion in gas turbine engines (except diesels are compression ignition and gas turbines are continuous combustion). The mixture immediately close to the fuel droplets is close to stoichiometric and there is healthy, complete combustion, the heat from which may be diluted by the excess air (50:1 air:fuel ratio isn't uncommon in a gas turbine). It's a bit like a blowtorch in a bucket of air.

Gasoline combustion works a little different in that too lean a mixture won't carry the flame front. When gasoline combustion breaks down from being a little too lean (engine starts to "run rough"), what we think of as the air:fuel ratio is an average, but it that average includes local pockets that are richer and leaner. Getting the combustion event reliably started is another technical challenge; we see evidence of that in how certain spark plugs work better with LOP ops, or we read about technology like stratified charge (Honda in particular has done a ton of R&D on this aspect of combustion).

But gasoline combustion is a lot different than diesel combustion. Diesel fuel generally wants to burn either the moment the air in the cylinder gets hot enough or the moment it hits that already hot air. Rich-lean has very little to do with whether it ignites.

You might be confusing efficiency with power output. Even in a gasoline engine those two peak at different F/A ratios. In any NA ICE potential power output for a given RPM is limited by the amount of air (O2 actually) passing through the engine. If you apply forced induction things change a little but in the short term (where engine overheating isn't the concern) the max power output will be limited by the amount of air pushed through.

In simple terms, max efficiency results from obtaining useful power from as much of the fuel as possible, max power results from getting as much of the air involved with combustion as possible.

Something else that seems to be underappreciated by Peter is the fact that operating at constant efficiency, the more power an engine produces, the more heat it must dissipate. Worse yet if you increase power and simultaneously lower efficiency, the heat output goes up even faster.

Diesels detonate instead of the flame front burning smoothly across the face of the piston. The engines are much stronger to withstand the explosion.

Hi Doug,

actually that's not true. The flame front tends to burn along the leading edge of the fuel spray, but there is no "detonation".

the engine is beefier primarily because of the relatively higher compression ratios and ability to deliver more peak torque

I can agree that they do not run continuously in detonation, I have spent my life around diesel engines and they are often running in detonation. Especially the older ones. The new electronically controlled engines run considerably smoother and quieter.

Here is a clip from a published engineering paper on the subject.

https://www.ndt.net/article/jae/papers/29-078.pdf

2. The Diesel Knock Phenomena
Knock, pinging or detonation are all terms that have been widely used to describe the characteristic “metallic rattling” noise associated with abnormal combustion in spark-ignition engines.
Spark-ignition knock is caused by the spontaneous ignition of gas ahead of the propagating flame
front (the end gas) within the combustion chamber. This spontaneous ignition results in a rapid
release of chemical energy and an accompanying rapid rise in cylinder pressure [15]. Unlike
spark-ignition knock, diesel knock occurs when injected fuel auto-ignites and combusts in the
premixed stage of combustion. Whilst this process is a normal part of diesel engine operation,
various circumstances can lead to excess quantities of fuel combusting in a premixed fashion.
This situation often develops if the parameters governing combustion lead to abnormally long
ignition delay periods. As a consequence, excessive diesel knock can often be a symptom of underlying faults such as poor or contaminated fuels, injection system problems or unsuitable rates
of alternative fuel substitution.

Yeah that’s not really what they are talking about. But it’s not really relevant to this thread. Btw one of the authors of your paper had the distinction of vomiting on my shoes in the bar at a conference after a bit too much celebrating.

Now that's funny! I agree that an Audi diesel is probably not detonating... If it is tuned properly...

That's funny...

With the knock sensors and computer control that engine has, if it’s knocking something is really wrong.

there are no "knock sensors" on a diesel in that sense. Again, that is not a diesel engine phenomenon as such.

As for computer control - he's rolled his own, hasn't he ?

Yes and no. He’s gone for an aftermarket ECU and watched a YouRube video on tuning.

Now you tell me.
I got 2 engineering masters degrees and then spent a couple decades learning on the job from the older engineers, before I really got comfortable with Diesel engine performance.
If we’d had YouTube 30 years ago I could have skipped all that.

So now that he's in the air, who thinks he's going to get the performance promised 5 years ago? And if not, what climb/cruise/fuel burn do you forecast based on his videos so far?

Now you tell me.
I got 2 engineering masters degrees and then spent a couple decades learning on the job from the older engineers, before I really got comfortable with Diesel engine performance.
If we’d had YouTube 30 years ago I could have skipped all that.