This is why a diesel engine cannot use a carburetor


Image of article titled Here's Why a Diesel Engine Can't Use a Carburetor

Photo: Volkswagen

If you ever think that you are a little too comfortable with your own level of knowing the world, there is a nice and easy tip that you can try to humiliate yourself, quickly and decisively. When a fact that you know comes to mind, take a moment and ask “why is this?” I bet it won’t take too long before you hit one that crushes you. I know it didn’t take me long, and the fact that stopped me was “diesel engines are only fuel injected, never carbureted”. Okay, why is that? I realized I wasn’t exactly sure, so I contacted someone smarter than me.

Well, not just smarter than me, you can find those kinds of people just about anywhere; Throw an open bag of chili in a crowd and I bet most people yelling at you would qualify. No, I needed someone smarter than me about diesel engines, and luckily we had one: Gabriel Moreno, a mechanical engineer, with all these letters after his name: BSMET, MSIE and Sys. Ing. Sure, so you know he’s not having fun.

Gabriel works for a well-known diesel engine builder (since he’s not speaking for the company, I won’t quote which one) but he knows exactly what he’s talking about when it comes to diesel engines.

Okay, here’s his explanation of why diesels never used carburetors:

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

So, now that the fundamental difference between gasoline and diesels is established, let’s move on to your question: why can’t a carburetor be used on a diesel? Now, because the air / fuel mixture is ignited by the heat of compression, you have to have a way to time the start of ignition. We do it in a gasoline engine using ignition timing, but without a spark plug in a diesel engine, we do it by timing the fuel injection. If we were to try to run a diesel with the help of a carburetor, it would perform very poorly, because with each intake stroke we would put air and fuel into the cylinder at the same time. The cylinder would ignite as soon as the mixture got hot enough, but it would be in an extremely advanced state. On the contrary, a diesel has to use a high pressure fuel system that injects fuel at the precise moment, and it has to be at high pressure so that the fuel pressure can overcome cylinder pressure and flow from the injector, despite refueling at a point in the cycle where the cylinder pressure is high as the piston approaches top dead center. With the help of a high pressure injector, we can control the fuel timing (and therefore the engine speed), and controlling the amount of fuel through the injector determines the pressure in the cylinders created (and therefore the couple).

Think of the diesel fuel injection timing similar to a spark curve created by a distributor on a carbureted engine. Without the ability to control the timing of the diesel fuel, we couldn’t rev up the engine or produce power. A carburetor on a diesel engine would only let fuel flow constantly without any control over the fuel timing.

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

If there are no spark plugs controlled by a distributor, how could you control when a given cylinder should reach its driving stroke? You should time it by controlling when you inject that diesel in there. Sure! It also means, as Gabriel clarified to me, that the fuel injection has to be a form of direct injection, as each cylinder has to be managed individually, so a throttle body type setup would not work. .

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

Image of article titled Here's Why a Diesel Engine Can't Use a Carburetor

Photo: John deere

It also means, as Gabriel clarified to me, that the fuel injection has to be a form of direct injection, as each cylinder has to be managed individually, so a throttle body type setup would not work. .

It also means that depressing the drive pedal of a diesel engine introduces a richer fuel / air mixture into the cylinder, and if that mixture is too much rich without sufficient air mass flow, resulting in poor combustion, then you get a lot of black particles, which makes you get “rolling coal”, which makes a lot of people angry.

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

I knew diesel fuel injection systems had absurdly high fuel pressure – think between 10,000 and 30,000 psi, compared to gasoline fuel injection systems that operate at 10 to 60 psi – but now, well, thanks to Gabriel’s explanation, it all makes sense.

Because diesels have very high compression ratios – typically between 14: 1 and 23: 1 – fuel must be injected at very high pressure to overcome the already high pressure inside the cylinder at around maximum compression, that is, that is, when fuel is injected into for combustion to take place.

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