I can't decide if this is automotive or non-automotive. They need a half-automotive forum.

On the Rangers, a vacuum "sprout" thing for lack of the correct term, comes out of the intake and has tubes attached to it. Vacuum is provided whenever the engine is running, by the engine, without a pump.

On the airplanes I fly, there is a vacuum pump that provides vacuum whenever it's running. They aren't paticularly reliable, and power some important flight instruments.

Why does the truck engine provide vacuum, yet there has to be a pump to provide vacuum on the airplane?

I've noticed my engine runs a steady 19 inches of mercury vacuum at idle. The flight instruments typically require 5-9 inches of mercury to operate correctly.

At full throttle, my vacuum gauge reads zero. Since most operation in a piston-powered airplane would take place at or near full throttle, would this be the reason?

Thanks for amusing my wondering mind. Hope everyone's havin' a good day.

RP
Zach

 

I believe you've answered your own question. Assuming you are talking about piston engine powered aircraft and not turbine powered craft. The airplane spends most of its time operating at or near full throttle. At this setting, the butterflies are fully open and provide the least restriction to the incoming air. Automobiles spend most of their time at or near no-throttle (or at least less than half throttle). The restriction of the throttle blades being mostly closed creates a vacuum even at highway cruise RPMs.

It is the restriction of the butterflies being closed that creates vacuum. The closed butterflies block airflow into the engine. The motion of the pistons removes air from the intake manifold. If the butterflies are closed and do not allow the air to be replace, a vacuum (lack of air) develops in the intake manifold.

As a point of trivia, diesel engines have no butterflies or even venturies. The intake is completely open from the air filter to the intake valve. Guess what. Diesel engines operate at a constant near zero vacuum situation. In fact, there are sensors that can be installed that if a vacuum is sensed, it throws a red flag to show that the air filter needs to be changed.

 

Passenger vehicles and trucks use the vacuum to operate (vacuum boost) power brakes and other equip on a vehicle. My '75 F-250 Sc has 3 vacuum lines > dizzy, brake booster, and manifold gauge in the dash. "74 - '76 was a transition to emission control.

 

That's what I was thinking, glad I was correct. So diesels have a vacuum pump as well? Thanks guys!

RP
Zach

 

I believe they do if they have vacuum operated accessories.