Vacume and diesel engines

[Hooked_on_HP]

I was reading an older post and vacume was being discussed. It was stated that a diesel engine does not produce vacume, and everyone agreed. Could someone explain this to me?On a normaly asperated engine, when the piston is on the downstroke and the intake valve opens it has to produce vacume. If there was no vacume dirt wouldn't get drawn into the air filter. Am I having a brain fart here or what.
Bill

 

[john_bud]

Brain fart.


Kidding....


There is no throttle in the air tract for the diesel, so there is no vacuum generated. Put a vac gauge on a gas engine and watch what happens when you open the throttle full. Vac = 0. A diesel is like that all the time. speed controlled by amount of fuel injected. If you want vacuum for accessories, you need an external pump to produce it.

jb

 

[firedog]

Bill, I think I know what you're asking. The answer is yes and no.
A vacuum but not in the sense of a traditional gasoline engine, where a constant vacuum is maintained at say 14" - 20".
An aspirated (non-turbo) diesel engine, the cylinder moves downward & pulls in air thru an open valve (obviously) and open intake. For air to be pulled in there must be a negative pressure created in the cylinder or as you stated no air would move across the filter. However, that vacuum is minimal at best. I think for the purpose of the diesel discussion, the traditional vacuum for the air/fuel ratio mix is non-existant, thus it is deemed as having no vacuum.
clear as mud??

 

[SnowRidge]

Normally aspirated diesels suck in air by producing a vacuum. Normally aspirated gas engines do the same thing, but they will also produce a high vacuum when the throttle butterfly is closed. Since diesels generally don't have a butterfly or similar restriction in the intake tract, they don't produce a high vacuum in the same manner.

 

[milkman]

There is a vacuum in the cylinder on the intake stroke on that cylinder on a diesel, but since there is no restriction between the air inlet and the cylinder, it produces no manifold vacuum. On a gas engine, there is a vacuum in the intake manifold because the throttle plate restricts the flow of air when it is partially closed and between the throttle plate and the cylinders there is a vacuum created by the piston pulling air on the intake stroke.

 

[Jimbrown]

The piston does not suck the air into the cylinder. Atmospheric pressure nominally 14.7 lbs forces air into the cylinder.

Atmospheric pressure - Wikipedia, the free encyclopedia

 

[Danno1]

.

Nature abhors a vacuum........

.

 

[Soundguy]

I like Jim's answer... AP pressurizes the cyl as it moves down.... thus the manifold must be near AP on a non turbo machine.. IE.. no usefull vacume to run vacume operated equipment.

soundguy

 

[SnowRidge]

The piston does not suck the air into the cylinder. Atmospheric pressure nominally 14.7 lbs forces air into the cylinder.

Atmospheric pressure - Wikipedia, the free encyclopedia

And when you inhale, you don't suck air into your lungs, either. Atmospheric pressure pushes it in.

That's nothing but hair splitting semantics.

 

[milkman]

The piston does not suck the air into the cylinder. Atmospheric pressure nominally 14.7 lbs forces air into the cylinder.

Atmospheric pressure - Wikipedia, the free encyclopedia

You're fighting with an unarmed man, you didn't accomplish much.