Is the concept of a supercharged diesel, quite pointless?
I can see the benefits of turbo'ing a diesel, but with a supercharger's inherent low-down power delivery, would that just be silly on a diesel anyway?
I read in Evo that someone said that Mazda had done a supercharged diesel 626 some years back, with little commercial success. Maybe a supercharger & turbocharger combo like the Golf GT would be an option, but adding more weight to an already heavy diesel lump, could upset the dynamics of the car?
D.
A supercharged diesel is silly, but for different reasons than you cite.
There were a number of supercharged diesel trucks back before turbo technology was available, but I doubt anyone would bother building one now. And an supercharged diesel car would be an especially dumb idea.
Here's the reason why.
A supercharger is an air pump driven by the engine. It takes power to drive according to how much air it is pumping. A supercharged petrol engine is only pumping lots of air, and therefore drawing lots of power from the crank, at full throttle. So a supercharged petrol engine isn't much less economical than a turbo petrol engine. But a diesel has no throttle. Its like a petrol engine at wide open throttle all the time. It varies it power by varying the amount of fuel that's injected. So the supercharger would be drawing lots of power even when the diesel only needs to produce a little bit. A supercharged diesel would waste lots of power, and therefore fuel, driving the supercharger. That'd defeat the purpose of a diesel.
A turbo is an air pump driven by the exhaust gases. But here's the bit that people have trouble understanding. The engine doesn't have to use fuel to gernerate more power to push the exhaust out harder to drive the turbocharger. The turbo runs on the thermal energy in the exhaust gases. It cools the exhaust down. So a turbo diesel doesn't waste any fuel driving the turbo to pump more air into the engine.
The result is a turbo diesel is way more economical that a supercharged diesel. Especially in a car diesel where the diesel engine is at part or low throttle most of the time.
