Why are diesels turbocharged . . .

grimmelshanks wrote: i think a good simple way to put it is that superchargers deliver lots and lots of torque and turbos are better for horsepower. diesels aready have enough torque, what they need is more horsepower. also they move so much air that an air driven (exhaust driven, whatever) forced induction system is truly free power

No.

You can size a turbo to give you a huge increase in low end torque but it could run out of breath when the engine gets higher up in the rev band, sacrificing an increase in high end power. You can also size a turbo to not give a lot of boost until you high up in the rev band giving a nice boost in torque at high RPMs (and thus HP) while sacrificing low end torque increases.

A roots or twin screw supercharger feeds the same amount of air into the engine (per engine revolution) at all RPMs so it gives a fairly constant increase in torque across the entire engine speed range. It will have better gains in different parts of the speed range based on the engine's volumetric efficiency at different speeds, the efficiency of the supercharger at different speeds etc. Centrifugal (spelling?) superchargers give very little low end torque increase, but lots up top.

As for why is everything not turbocharged? Cost.

Bob

with the proper match you can make either (supercharger vs turbo) do whatever you want...

With a good VG setup, all the "lag" issues we used to talk about are gone. There are folks working on stand alone controllers for holset VGT's. They'll be perfected soon, probably the older ones that use PWM for control, but soon.. Then match isn't exactly what we need to worry about.. The question is, how strong are your head bolts? :-)

Will wrote:

aircooled wrote: I am pretty sure the 6-71 were used on the exhaust.

A 6-71 is a supercharger. I'm not sure where you're going with this.

Actually, they started life as a scavenge pump on Detroit Diesel 2-strokes.

Hotrodders turned them into superchargers.

Jimmy diesel (Detroit Diesel) two strokes used blowers just to blow the spent exhaust gas out of the cylinder (see above post about a scavenge pump). They didn't necessarily raise the amount of air in the cylinder much above barometric. Blowers (which later became known as superchargers) were necessary for normal operation, not as performance-enhancers. Later, turbos were added for this.

In reply to Trans_Maro:

I never knew that. Thanks for the info.

I have nothing to add except that I miss the sound of our two strokes. The people whose buildings I used to scream past probably not so much.