Vacuum principle in cars

Level 5 User Joined: 28 Dec 2004 Posts: 110

Is there anywhere I could look on the net to give information abou the vacuum principle in cars? WHen reading about it in the Haynes for example it gives reference to vacuum hoses connecting a varietry of parts together, would like to know more about how it works. Does it just ensure a steady supply of e.g. fuel?

Forum Moderator Joined: 02 Mar 2004 Posts: 19978

Well what exactly do you want to know?

Vacuum has nothing to do with fuel supply - really.

There is a pump for that but yes the vacuum pulls the fuel into each cylinder.

Level 5 User Joined: 28 Dec 2004 Posts: 110

no, i wasn't talking about the fuel supply, it was like you said, pulling it onto the cylinder.

Really I just wanted to know how it works on a car, and the main parts it works on. I see vacuum hoses connected to the airbox, rocker cover, inlet mani, MAP sensor etc. and wondered if there is an underlying principle that would say what it does for each part it goes to.

Forum Moderator Joined: 02 Mar 2004 Posts: 19978

read my efi article! I cover it in there.

But basically there is a vacuum formed in the inlet as the throttle plate is closed (causing a restriction) and the pistons descend pullign mixture in, thus creating a vacuum.

Vacuum varies, at idle its nice and steady a set value about 20inhg. When the throttle floored the vacuum drops away as the pressure tries to equalise with atmospheric pressure.

When you are driving and come off the throttle the vacuum is the highest it will go, the throttle plate is closed and the engine is spinning quickly thus higher vacuum.

Air pressure is what determines power output.

At full throttle you want atmospheric air pressure cramming in as much air into the cyl's as possible, more air means more fuel which means bigger bang which means more go. In road cars its very rare to get abouve 80/90% air pressure in the inlet. This is whats called volumetric efficiency. The higher the VE the more efficient the design is.

NA road cars will always have a vacuum in the inlet.

race engines and super bikes using tuned runner lengths can get 105% VE.

Turbos cram in even more air above atmospheric pressure thats why they make more power.

In theory assuming everything else being equal a car that makes 100bhp at atmospheric pressure, will at 1 BAR of positve boost will make 200bhp, its rarely that clear cut though.

Turbo cars as you can imagine often have 200% VE and above.

Site Subscriber Joined: 07 Oct 2004 Posts: 2281

Volumetric efficiency governs both bhp and torque.

Naturally aspirated Touring-car spec motors have VE of ~ 130-140%, with your typical road-going car making somewhere around 60-70% VE.

Easy way to reference this is 100bhp/litre, hence why the Honda engines are well renowned - I think the 1.6 VTEC made nearly 160bhp so it had near perfect efficiency, although the 100bhp/litre is not strictly a true reference.

Torque is derived from the volume of air consumed on each rev, brake horsepower is the byproduct of torque and is best described as the rate at which an engine can process air.

Forum Moderator Joined: 02 Mar 2004 Posts: 19978

ben don't try and look smart when you have limited knowledge on a subject.

Site Subscriber Joined: 07 Oct 2004 Posts: 2281

Is what I said wrong?