If you ever suspect that you’re getting a little too comfortable with your own level of knowledge of the world, there’s a nice simple trick you can try to humble yourself, quickly and decisively. When some fact that you know pops into your head, take a moment and ask “why is that?” I bet it won’t take too long before you hit one that stumps you. I know it didn’t take long for me, and the fact that stopped me was “diesel engines are only fuel injected, never carb’d.” Okay, why is that? I realized I wasn’t exactly certain, so I reached out to someone smarter than me.

Well, not just smarter than me—you can find those sorts of people pretty much anywhere; throw an open bag of chili into a crowd and I bet most of the people screaming at you would qualify. No, I needed someone smarter than me on the subject of diesel engines, and luckily we had one: Gabriel Moreno, a mechanical engineer, with all these letters after his name: BSMET, MSIE, and Sys. Eng. Cert., so you know he’s not playing around.

Gabriel works for a very well-known diesel engine maker (since he’s not speaking for the company, I won’t mention which one) but he absolutely knows what he’s talking about regarding diesel engines.

Okay, so, here’s his explanation as to why diesels have never used carburetors:

As you know, gasoline engines are spark ignited, internal combustion, reciprocating engines. They rely on a spark to jump from electrode to ground strap at a precise moment to ignite an air/fuel mixture within the cylinder. Diesel engines, on the other hand, are compression ignition, internal combustion, reciprocating engines. This means that the air/fuel mixture within the cylinder is ignited not by a spark, but by the heat created by compressing the air/fuel mixture within the cylinder. This is why diesel engines have much higher compression ratios compared to gasoline engines, and why their thermal efficiency is greater as well.

So, now that the fundamental difference between gasoline and diesels is laid out, let’s get to your question: why can’t a carburetor be used on a diesel? Well, because the air/fuel mixture is ignited by the heat of compression, we must have a way to time the start of ignition. We do this in a gasoline engine using ignition timing, but without a spark plug in a diesel, we do this by timing the injection of fuel. If we tried to run a diesel using a carburetor, it would run very poorly, because at each intake stroke we’d be introducing air and fuel at the same time into the cylinder. The cylinder would fire as soon as the mixture became hot enough, but this would be in an extremely advanced condition. Rather, a diesel must use a high pressure fuel system that injects fuel at the precise time, and it must be high pressure so that fuel pressure can overcome cylinder pressure and flow out of the injector, despite fueling at a point in the cycle where cylinder pressure is high as the piston nears top dead center. Using a high-pressure injector, we can control fuel timing (and therefore engine speed), and controlling the amount of fuel flow through the injector determines how much cylinder pressure is created (and therefore torque).

Think of diesel fuel injection timing akin to a spark curve created by a distributor on a carbureted engine. Without the ability to control diesel fuel timing, we could not get the engine to rev up or produce power. A carburetor on a diesel engine would only let fuel flow constantly with no control over fuel timing.

Ahhh, this makes sense! I knew that diesels were compression-ignition engines, but the connection I failed to make was that that would mean, inherently, that the combustion timing would all depend on when the fuel was injected into the cylinder!

If there are no spark plugs controlled by a distributor, how else would you be able to control when a given cylinder should hit its power stroke? You’d have to time it by controlling when you inject that diesel in there. Of course! This also means, as Gabriel clarified for me, that the fuel injection must be some form of direct injection, since each cylinder has to be handled individually, so a throttle body-type setup wouldn’t work.

(I know there are indirect injection diesel engines that don’t inject directly into the combustion chamber, but they’re still directed to particular cylinders, so they’re not like a generalized throttle body setup.)

This also means, as Gabriel clarified for me, that the fuel injection must be some form of direct injection, since each cylinder has to be handled individually, so a throttle body-type setup wouldn’t work.

This also means that stomping on the go pedal of a diesel introduces a richer fuel/air mixture into the cylinder, and if that mixture is too rich without enough air mass flow, leading to poor combustion, then you get a lot of black particulate matter which is how you get “rolling coal,” which is how you get lots of people angry.

So, look, we even figured out how changing the fuel/air mixture in an engine can produce power, heat, and anger!

I’ve known that diesel fuel injection systems had absurdly high fuel pressure—think between 10,000 and 30,000 psi, compared to gasoline fuel injection systems that run at 10 to 60 psi—but now, finally, thanks to Gabriel’s explanation, this all makes sense.

Because diesels have really high compression ratios—generally between 14:1 and 23:1—the fuel has to be injected at really high pressure to overcome the already high pressure inside the cylinder at about maximum compression, which is when the fuel is injected in for combustion to take place.