Why do diesel engines seem to outlast gasoline engines? Shouldn’t they be built with the same stress safety factors built in and fail at around the same rate?

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Edit: Specifically referring to passenger vehicles and consumer trucks, not commercial vehicles that i assume would be built to go much longer anyway.

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They actually operate at higher pressures and stresses. In some ways, this contributes to reliability because weak parts can’t be used. It also makes them heavier. The reduced complexity of not having spark plugs and spark timing eliminates one set of failure modes.

Gasoline engines usually operate at 25-50% higher RPM and have longer piston strokes further increasing piston speed which causes increased wear on pistons and cylinders as well as stress on the connecting rods. Both rods and pistons also have to be lighter to accommodate the higher speed. Diesel engines on the other hand have to be built stronger to allow higher compression ratios that can auto-ignite the fuel.

That being said I doubt you will see very significant differences in lifespans of modern engines that are properly maintained. Maintenance is probably the biggest factor that increases lifespan of commercial vehicles that have to regularly undergo it while passenger vehicles don’t necessarily do so. And this is probably where the greater robustness of diesel engines might allow them to better cope with lousy maintenance.

my guess is that it has to do with the fact that they are simpler. (no spark plugs, no spark timing, etc) so there are fewer parts that can fail

Diesel engines operate on simpler engine design than gasoline engines. They also run at lower RPM’s since the fuel has more energy. So the main reasons small diesel engines typically outlast their gasoline counterparts is that they **run at lower RPM’s and do not have spark plugs or complex timing mechanisms**.

However, maintenance is key to a long lasting engine. Most diesel engines have turbo’s, and that can often be the weakest link in a poor maintenance scenario. A turbo replacement can cost thousands of dollars. But if it triggers a runaway engine before you can replace it, then you’ll be replacing the entire engine (A runaway diesel is caused by oil leaking from the turbo into the intake, and causing the engine to run on its own oil. The only way to stop it is to cut off the air intake).

To understand the difference, we need to briefly discuss the ideal gas law. It reads that the volume V in a system times the pressure P is equal to the number of molecules of gas N times a “constant” (a multiplier that is slightly different for different gases, don’t worry about this one) r times the temperature T. So shortened, this is PV=NrT. Both engines use this cycle, but in terms of performance gasoline engines are limited by it. Gas engines use the Otto cycle of engines, which is to say, they compress the gas using a piston, then add heat using a spark plug which causes the system to expand and creates work to drive the axle. This is a fine system, but they have to be careful because you can’t just change one dude if the equation in a real engine. The hope would be, “decrease volume, increase pressure” but in reality the temperature will also increase. If a gas engine compresses too much, the increased temperature can cause the gas to ignite itself and move the Pistons early, which causes all kinds of problems.
In a diesel engine however, it uses this pricipal intentionally. Diesel compresses just air (rather than an air fuel mixture) until it’s at a high enough temperature, then injects fuel which automatically ignites. This has several benefits: higher efficiency once the engine is up to temperature, and importantly to your question, much less time with unburnt fuel sitting inside the pistons. Because fuel itself is somewhat “corrosive” for lack of a better term and contributes to weakening parts, particularly the rubber seal in the engines. Even at the much higher pressures in a diesel, pressure is never a real concern for the steel, it would be just fine. The issue is all the other components and how they interact with gasoline. And if a gas engine gets too hot and auto ignites the Pistons fighting each other can tear the engine apart, with no danger of this really happening in diesel.