Engines Exposed: Why Do Diesels Have So Much Torque?


We take a stab at explaining the diesel engine.

Take alook at any diesel engine's specs and more often than, well, ever, there ismore torque than horsepower. Let’s just get one thing out of the way to start.Diesel engines are completely different from gas engines. They both havecrankshafts, pistons and rods, fuel rails and some of them havedirect injection. In fact, some diesel engines have had direct injectiongoing as far back as the 1980s. However that’s where the similarities stop and things get tricky. Well, actually we're not there yet. It'll get trickier later on for sure.

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Gas engines ignite fuel with little things called spark plugs. On the compression stroke the spark plug is activated and fuel ignites, creating little explosions. In a diesel engine, there is no spark. Wait, what?! Yes, that’s right, diesel engines do not ignite fuel on spark. Instead, diesel engines ignite on compression. The piston rises, compressing the fuel until it spontaneously combusts. See, diesel fuel needs to reach a very hot temperature for that to happen, much hotter than regular gasoline. As a result it burns a lot slower. So, in order to ignite it needs an insane compression ratio. Diesel fuel needs at least a 14:1 compression ratio in order to ignite, and that is due to a lot of science talk. Ultimately diesel fuel takes longer to reach combustion.

If you have a cylinder, and the piston is rising inside the cylinder, the point it reaches its apex is the compression stroke, and the space between the valve and the piston is a fraction of the total space inside of the cylinder. Make sense?

Saying 14:1 means that the space between the piston and the valve in the compression stroke is 1/14th of the size of the cylinder. A normal gas engine without direct injection, like say for example a Mustang 5.0-liter from 1995, has a 9:1 compression ratio. That means diesel fuel is compressed a lot more than regular gasoline, to the point of combustion. Because of this, diesel engines have a much longer stroke, or, the distance the piston travels before compression. So the engine has a ton more torque. However, the caveat to this is that the engine revolves a lot slower. So in a gas engine where you’ll see a redline of maybe 7,000 rpms, a diesel engine will hit its redline closer to 4,500. Incidentally, that is also why it makes much less horsepower.

See with a race engine, it hits around 15,000 rpm, so they have a much wider power band. Essentially, diesel engines work less hard, so they have less horsepower. A regular petrol engine has more rpms and so it works harder and therefore it has more horsepower. On the flip side, gas engines have less of a stroke so they have less torque. All of this may sound confusing, but it’s how diesels work. All of this also explains how diesel engines can weigh so much. In order to handle the magnitudes of torque and the insane compression the engine is a lot beefier, so diesel engines typically weigh a lot more than regular gas engines. So why use diesel engines at all, especially in trucks? It’s because you need all that torque to lug trees up hills.

It’s also why diesel engines are used mostly to lug boats around. They're also fairly invincible. So with all this in mind, if you’re thinking of getting a diesel truck be sure to get it for the right reasons, and above all know what you’re getting into.

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