Diesels and lean-burn damage

One of the dangers of using a lean air/fuel combustion ration in an IC engine is “burning” pistons and valves; the excess hot oxygen attacks the metal.

How do diesels get away with this?

For that matter, how did those lean-burn gas engines a few years ago do it?

I’m not sure your premise is correct. I run my airplane engine lean of peak at cruise. It’s not fuel injected so there is no computer to contemplate the universe inside the cylinders.

However, to answer your question, Ford uses piston oil squirters on a number of their new engines to keep things cool. Oil is squirted on the back side of the piston head.

Diesel engines are designed from the outset to deal with higher pressures and temperatures inside the cylinders due to the way the diesel cycle works. I would assume any damage potential caused by high partial pressure of oxygen would be factored in, especially since engine brake retarders (“jake brakes”) are a major feature of large truck engines and those work with no fuel at all being injected into the cylinders.

The problem with running lean is not the excess oxygen, it’s the higher operating temps that seem to go along with lean operation. That’s what fries exhaust valves and overheats pistons and cylinder walls in a gasoline engine that’s running leaner than it’s supposed to.

Paradoxically, if you look at a plot of adiabatic flame temperature versus air/fuel ratio, you will see lower peak temperatures for lean combustion than for stoichiometric combustion. Things that make ya go…hmmmmmmm?

The answer is that in lean combustion, the reaction proceeds more slowly than for stoich. In your gasoline engine, the spark timing is selected by the designers so that combustion is completed relatively early in the expansion stroke, and then the remainder of that expansion stroke is used to extract mechanical work from the combustion gases, rapidly reducing the in-cylinder temperatures to a level that doesn’t fry parts. If you’re running lean with the same spark timing, then combustion takes a lot longer: you then have less expansion stroke remaining in which to extract mechanical work from the mixture, resulting in persistence of higher temperatures all the way to the end of the expansion stroke. The engine is less efficient (less mechanical work extracted for a given fuel input), your exhaust temps are higher, and your cooling system load is higher - that is, until you melt the exhaust valves and score the cylinder walls.

The upshot of all this is that you can run an engine on a lean mixture, provided you advance the spark timing to account for it. Magiver hasn’t mentioned it, but I’ll wager he is able to manually adjust spark timing on his aircraft engine when running lean, and is also able to monitor exhaust gas temperatures to assure that he’s achieved a safe engine operating condition.

With lower peak temperatures and a larger throttle opening for a given power output, lean operating can result in more efficient operation. Unfortunately it doesn’t make best use of the catalytic converter, which does its best work with a stoich or near-stoich mixture. This is why you aren’t currently seeing cars on the road with lean-burn technology: manufacturers haven’t found a way to run lean while still meeting current emissions regs.

Since excess oxygen is not the problem, it’s now easier to see why Diesel engines don’t have any difficulty dealing with lean operation.