Eli5: why do diesel engines tend to be turbocharged rather than supercharged or even N/A?
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Diesel engines can work without Boost, but in terms of efficiency they work much better with higher pressure intake air. Diesels work by combusting fuel with high pressure and hot air, so the more high pressure air you have the better.
Superchargers are less efficient on Diesels because they tend to run at lower RPMs. Superchargers also rob power from the engine.
While a turbo is powered by exhaust which diesels make plenty of. It doesn’t rob power from the engine and can be tuned to kick in at lower RPMs.
There are supercharged diesels of course, it’s just that Turbo’s in general tend to be a more efficient way of generating boost.
Diesel engines work by filling the cylinder full of air, compressing it so its really hot, then injecting the desired amount of fuel which combusts and pushes the piston down.
In a diesel the cylinder is *always* filled with air and the amount of fuel inject varies unlike in a gasoline engine where a throttle plate restricts the amount of air in the cylinder. If you can ram more air into the cylinder then you can add more fuel and get more power
For gasoline engines a turbo generally requires lower compression ratios to avoid preignition and a stronger engine block to handle the increased pressures but diesels are starting with high compression ratios and injecting fuel last second so there really isn’t extra strengthening required so a turbo is just a free way to get more power out of a smaller volume
Superchargers are kinda bleh. They suck power from the engine *all the time* so anything meant to run at relatively low load ends up burning significantly more fuel with a supercharger. Their main benefit is low RPM boost pressure for gasoline engines since they’re not relying on large volumes of exhaust to spin them up, but again, a diesel is always filling its cylinders completely with air so it has a large exhaust volume even at low RPMs to spin up the turbo
Diesel engines can work without Boost, but in terms of efficiency they work much better with higher pressure intake air. Diesels work by combusting fuel with high pressure and hot air, so the more high pressure air you have the better.
Superchargers are less efficient on Diesels because they tend to run at lower RPMs. Superchargers also rob power from the engine.
While a turbo is powered by exhaust which diesels make plenty of. It doesn’t rob power from the engine and can be tuned to kick in at lower RPMs.
There are supercharged diesels of course, it’s just that Turbo’s in general tend to be a more efficient way of generating boost.
Diesel engines work by filling the cylinder full of air, compressing it so its really hot, then injecting the desired amount of fuel which combusts and pushes the piston down.
In a diesel the cylinder is *always* filled with air and the amount of fuel inject varies unlike in a gasoline engine where a throttle plate restricts the amount of air in the cylinder. If you can ram more air into the cylinder then you can add more fuel and get more power
For gasoline engines a turbo generally requires lower compression ratios to avoid preignition and a stronger engine block to handle the increased pressures but diesels are starting with high compression ratios and injecting fuel last second so there really isn’t extra strengthening required so a turbo is just a free way to get more power out of a smaller volume
Superchargers are kinda bleh. They suck power from the engine *all the time* so anything meant to run at relatively low load ends up burning significantly more fuel with a supercharger. Their main benefit is low RPM boost pressure for gasoline engines since they’re not relying on large volumes of exhaust to spin them up, but again, a diesel is always filling its cylinders completely with air so it has a large exhaust volume even at low RPMs to spin up the turbo
For gasoline engines, the limit for turbocharging or supercharging is autoignition of fuel, which limits the amount of boost and compression ratio. Diesels don’t really have to worry about that (as autoignition is basically the point), so they can boost up to the mechanical limits of the engine and the block materials. Boosting universally increases efficiency, so there’s no reason *not* to boost.
As for why they use turbos over superchargers; superchargers are less efficient because of mechanical losses and friction, and the only real advantage is in getting around turbo lag. However, with modern control systems and variable-vane turbocharging, plus the low speed range of diesel engines, there’s basically just no advantage to superchargers over turbos. Thus, everything gets a turbocharger.
The vast majority of diesel engines are naturally aspirated. Vast, vast. It’s very rarely that you’ll see a turbocharged diesel generator, or similar. For transportation, you see turbo diesels a lot because apart from being the most fuel-efficient piston engines we have, the amount of torque they offer, and its distribution makes them particularly excellent for utility operation, and ease of use.
For gasoline engines, the limit for turbocharging or supercharging is autoignition of fuel, which limits the amount of boost and compression ratio. Diesels don’t really have to worry about that (as autoignition is basically the point), so they can boost up to the mechanical limits of the engine and the block materials. Boosting universally increases efficiency, so there’s no reason *not* to boost.
As for why they use turbos over superchargers; superchargers are less efficient because of mechanical losses and friction, and the only real advantage is in getting around turbo lag. However, with modern control systems and variable-vane turbocharging, plus the low speed range of diesel engines, there’s basically just no advantage to superchargers over turbos. Thus, everything gets a turbocharger.
The vast majority of diesel engines are naturally aspirated. Vast, vast. It’s very rarely that you’ll see a turbocharged diesel generator, or similar. For transportation, you see turbo diesels a lot because apart from being the most fuel-efficient piston engines we have, the amount of torque they offer, and its distribution makes them particularly excellent for utility operation, and ease of use.
I think one important point that a few people touched on but didn’t outright say is that the advantage of a supercharger is the instant throttle response. Yes modern turbochargers can have very low turbo lag, but if you’re building a car and really want that instant response, superchargers are the way to go. Diesel engines aren’t really meant for high performance driving, they’re made for consistent and efficient power output, so throttle response isn’t important.
There do exist supercharged diesel engines though. Some of Detroit Diesel’s two-stroke 71 series used both a supercharger and a turbocharger, because they worked by creating air pressure in the crankcase and therefore needed the supercharger for that instant boost.
I think one important point that a few people touched on but didn’t outright say is that the advantage of a supercharger is the instant throttle response. Yes modern turbochargers can have very low turbo lag, but if you’re building a car and really want that instant response, superchargers are the way to go. Diesel engines aren’t really meant for high performance driving, they’re made for consistent and efficient power output, so throttle response isn’t important.
There do exist supercharged diesel engines though. Some of Detroit Diesel’s two-stroke 71 series used both a supercharger and a turbocharger, because they worked by creating air pressure in the crankcase and therefore needed the supercharger for that instant boost.
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