Diesel engine and no vacume

[Richard]

Why is it a diesel engine doesn't produce any vacume?

Or, would it be better asked if I said why DOES a gasoline?

This is one of those things I've always wondered and finally thought to ask.

 

[Jimbrown]

Because there is no throttle plate. The induction system is a full atmospheric pressure at all times. Most gas engines have either a throttle plate in the carburettor or if fuel injected in a throttle body. The plate is almost always partially closed so a vacuum is created in the inductions system. In engines that are run at full throttle there is no vacuum. Large stationary engines and airplanes are some examples.

Speed/power of a diesel is controlled by how much fuel the injectors put into the cylinders. In most gas engines speed/power is controlled by how much air the throttle plate lets into the engine.

 

[DieselPower]

And on top of what Jim said many diesel engines have turbos and some like older Detroit Diesels might even have a super charger which makes the intake manifold pressurized. Ford pickups and vans with diesel engines had drive belt driven vacuum pumps for years to overcome this so they had vacuum operated items like brake boosters and HVAC controls.

 

[john_bud]

Gas engine needs vacuum to pull the fuel out of the carb. The carb needs to keep the air:fuel ratio about constant at 14:1 to 12:1. To do that at low fuel demand (idle etc), it needs to limit the total amount of air, if not it would be too dilute with air and would not burn.

Diesel fuel can burn at very high air to fuel ratios. So, the total amount of air is not restricted, but the amount of fuel is restricted. Full air supplied = no throttle plate = no vacuum.

The energy it takes to create that large volume of vacuum is one of the reasons that gas engines use more fuel. It takes a LOT of energy to draw that much suck. For a 5 liter V8, it is like dozens of vacuum cleaners all going at the same time.