Car Engine Vacume

As the piston decends (intake stroke) the volume in the combustion chamber increases.

Until additional air is pushed in by atmospheric pressure, a condition known as "partial vacuum" exists. The filling is slowed by the inertia + viscous resistance through the passages.

Engine power is varied at the throttle plate; a variable resistance, the largest in the system.

What is commonly referred to as 'vacuum' is a pressure drop across this resistance; When you accelerate, you open this valve, reducing its pressure drop.

Thanks for the reply, sidevalveguru. Where is the vacuum hose attached to access this vacuum, at the intake manifold?

Yes, in the manifold; usually in the common plenum, as this reduces the individual pulsations.

Be careful of leaks here, it will make the fuel mixture 'lean'.

Should add: in times past a pressure tap was used, across the carb. venturi.

More pressure drop here is related to 'more power', it's rpm independant.

This signal was intergrated into controls; much as MAF sensor in todays electronic systems.

You could tee-off from the vacuum line to the brake booster, or, on really old engines, the line to the distributor advance diaphragm. How much responce do you need?

Stu.

Q1>MAF sensor??

Q2>distributor diaphragm is old??i just learned that stuff a little while ago!!what do thy use now??

(p.s. i can use smilies now!)

1) MAF: Mass Air Flow sensor, for electronic fuel management systems; a factor in calculating mixture.

2) Distributor diaphragm: Mechanical apparatus to physically move electric contacts ('breaker points') to alter ignition timing. This function is now usually done electronically, by relating multiple sensor input to values programmed into the E(ngine) C(ontrol) U(nit)

Usually fuel + ignition management are in one 'black box'

Hello Stu - I'd just comment that if I remember right (it's a long while since I worked on one) the vacuum advance is a bit more subtle. The vacuum isn't taken from the manifold, but from the carb, tapping point just on the atmosphere side of the throttle disc when closed. So at idle there is no vacuum advance (cos you don't want it at idle speed), but when the disc edge passes the tapping point vacuum starts. Then falls off as the throttle is opened wider.

Cheers....Codey

It does not work in the way you imply, the carburettor comes between the atmosphere and the vacuum take off point. The vacuum is developed by the engine sucking and the throttle being either almost closed or only partially open.

Full throttle usually makes no vacuum, so a car must store a vacuum for a brake servo for example....

Look at this simple sketch:-

Nope!

Codey's right.

I was talking, rather in a limited way, about a couple of specific, old examples.

All breaker point controlled, carburetted, car engines from about early '40s to present are as he says.

Cheers,

Stu.

Yeah Codey, you're right. But REALLY old engines (like when I was a kid ) had simpler vac advances like the ones I mentioned. Or, perhaps, more accurately, the ones I worked on did. Of course the ones before THAT were manually operated. I loved those.

I forgot to mention that some cars had vac operated wipers. Weren't those fun. Pulling a heavy vehicle up a rise in the rain????

Neatest wiper operation, in my opinion, was the one operated by the 'speedo' drive. They figured that if you weren't moving you didn't need to see where you were going. Sorta makes sense really?? Worked a treat going down a cliff. Ah, yes, I did love my old 2CV.

(This could go on forever)

Cheers,

Stu.

Must ask: What are you trying to do?

That will help answer your query.

I am not trying to do anything. I have always been curious how an engine created vacuum when everything is operated by pressure in the cylinders making the engine rotate.

as piston moves from tdc to bdc it creates a suction.once all the air etc. from the intake manifold is sucked in, it is not replaced immediately.atmospheric pressure takes its own time.thatts why the vacuum exists.

as for why its neccesary other than to suck more air/fuel in it also serves to control other systems in the automobile that monitor intake pressure to time themselves.

The vacuum take-off for an automotive engine comes from the intake manifold.

When you accellerate, the throttle opens, the engine gets more mixture, but the manifold pressure rises, so the available vaccum decreases.

All of the above assume one thing which they all forgot to mention:- Petrol engines only and those without a Turbo.....

For Turbo motors, there is often an extra vacuum pump situated somewhere, on older Mitsubishi Diesels, it was built onto the back of the alternator......what is done on other models I cannot say, probably something similar as most modern cars need a vacuum to operate the brake servo.

Turbos mean that the inlet manifold is under more positive air pressure and rarely a vacuum.....

so in said case using turbo,a seperate apparatus whose sole purpose is to create the vacuum would be necessary??just making sure i understand.

since only the brakes require the vacuum in modern automobiles it wouldn be necessary if you werent using vacuum booster assistance right??

The modern automatic transmission does use vacuum?

I thought they used hydraulics, but I would not know as I do not like automatics.....!!

I thought they used hydraulics

Yes of course you are correct the automatic transmission is hydraulic equipment but at one time a vacuum actuated modulator valve is used to aid shifting gears or speed ranges. I was questioning if the modulator were yet used in the fore mentioned fashion.

i assumed that with the pedal shifting availible in normal cars now the had moved to electronically actuated hydraulic systems???

It depends on the manufacturer and application.

For example, GM's TH-350 and TH-400 transmissions do use vacuum modulators, and are still being used in heavy duty applications because they are physically very robust, able to withstand some serious punishment.

Thanks I'll robustly remove a TH-400 or TH-425 from a bus then apply to engine to supplant weak drive train in diesel powered pick up.

Yup. Thats due to the fact that modern brake material is far harder due to no asbestos being allowed, which makes for a hard pedal. Some women (and some men!) would not be able to brake properly, plus, it would eventually break the back of the seat!!!!

I know that because it happened to me many years ago......on a 1963 tuned up mini Cooper with no servo!!!

As stated before the engine vacuum signal changed as you open or close the throttle. That change in vacuum signal is used by the older automatic transmissions to help modify the shift characteristics of the transmission. If the vehicle is driven easy, the vacuum signal is high. As road speed increases the trans will shift early, and soft. As throttle pressure is increased the vacuum is increased at the transmission modulator. This allows the internal spring to push harder against the pressure that builds as the car drives faster. The delay in building sufficient pressure for the shift valve causes a later shift, and a harder shift.

Today these functions are controlled by TPS (throttle position sensors) VSS (vehicle speed sensors) and computers associated with the engine and transmissions.