Mommy, where does engine vacuum come from?

[white_2kgt]

Well it looks like you have figured out that the vacuum comes from the pistons moving up and down and the air flowing across the intake manifold (google Manifold Vacuum for more specifics). Now, why that differs from a diesel. In the simple terms (sorry, I'm not an ISAT major) a gas engine is 'air throttled', when you stomp on the 'gas' pedal (incorrectly named) you allow more air into the engine, however a diesel engine is fuel throttled. A turbo-diesel is free to intake as much air as it's intake conduits and turbocharger can supply for the load on the engine. This means that the output of a diesel engine is fuel throttled. So for the most part the diesel engine creates no vacuum while under load, it does produce vacuum at times, but nothing that is really useful like in a gas engine.

-c