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?

Question

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

In: Technology

Answer 1

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.