why does a diesel engine need a vacuum pump
No i dont think they need it because of valetronic. Even when the engine switches to running in valvetronic mode (when you first start the engine there's a throttle body does the throttling) and so the throttle body moves (and remains at) the fully open position, you still have a vacuum in the intake. My understanding is that the vacuum comes from each piston on the intake stroke (piston going down + intake valves open == suction from the intake == negative pressure / vacuum in the intake manifold).
I understand that because of overlap of the cylinders / strokes, there's always one piston sucking in. I believe theres some kind of really fast (faster frequency than the valves) resonance in the air intake because of the rapid switching between cylinders sucking in (shockwaves of some sort) and somehow these also contribute to make the vacuum even greater.
This is how the variable length intake manifolds come into play as well i think. You can tell from the wooly wording i dont know the full score but would be really happy to hear from someone that can put the above right!
The above answer is right on.
I'd like to add that while some diesels use a vacuum pump, a lot use a hydroboost system to power the brake booster. This works by using hydraulic pressure from the power steering pump to power the brake booster. This is what almost all diesel trucks use (besides the large truck with air brakes) because the power steering pump will last much longer than a vacuum pump.
Also you may notice that some gas engine cars that have forced induction will also use a hydroboost or a vacuum pump. This is because the turbo or supercharger is preventing the engine from making enough vacuum to run the booster.
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