ElI5 What are the characteristics of Diesel that make it a better fuel for larger engines?



I know that large trucks and commercial equipment use large diesel engines that produce more torque, but what about the diesel makes it better for these applications?


Diesel engines are more efficient, when running at their optimum load and speed. Big industrial applications value this cost benefit a lot, so they like diesel. Cars are often used outside these optimum conditions, so diesel market share isn’t as big there.

The reason we use diesel engines in large equipment is that they generate a lot of torque at low RPMs which means you can use a reasonable gear ratio to move a heavy vehicle. You could use something small and powerful like an F1 engine, but you’d need to strap 6 step down gears in series to get the big truck moving despite the engine having 1000 HP when it hits 15,000 RPM.

If you look at something like the [Cummins ISB 5.9L diesels](https://en.wikipedia.org/wiki/Cummins_B_Series_engine#ISB_5.9) which was used in the diesel Dodge Ram pickups for a while, you can see that it gives you just 325 horsepower but generates 610 ft-lbs of torque at 1600 RPM. Most gasoline cars will have a peak torque figure fairly close to their peak HP.

So **why** does diesel give more torque? Because of how we built it

Diesel engines take in air, squeeze it really hard so it heats up, then inject the fuel which immediately combusts and pushes the piston down. Gasoline engines tend to take in air *and* fuel, then squeeze that mixture, then fire it off with a spark plug. Because most gasoline engines are squeezing the fuel-air mixture they can’t squeeze it as hard (~10:1) as a diesel(up to 23:1). The more you squeeze the air, the more you can let it expand when the fuel combusts, and the more you let it expand the more energy you can remove from it. This lets diesels capture a lot more energy per cycle of the piston, but because they’re dealing with much higher cylinder pressures they often have heavier components which can’t be spun as fast so you can’t get a diesel up to stupid high RPMs for tons of power like you can with a gasoline engine.


I think the simplest answer here is diesel has more potential energy than gasoline. Quick google search says its has 10-15% more energy.

Also, Diesel engines don’t have higher compression just because. In gasoline engines, gas and air are compressed, then ignited by spark. Diesel engines compress air, then inject diesel which causes the ignition without spark.