What is correlation between vacuum and barometric pressure?

[Bihili]

Sometimes I can't find the answers on Google so maybe the mechanical engineers here can help me.

This is a layman describing this so please have patience.

Most carburetor engines have approximately 20 pounds of vacuum.
I am assuming the vacuum is created by the down stroke of the piston of a four stroke engine.

Barometric pressure is measured differently than vacuum.
As a layman I think pressure would be the opposite of vacuum so the measurement would be pounds of pressure as compared to pounds of vacuum. But it is not.

With barometric pressure pumping air into an engine isn't the vacuum gauge really measuring the two, barometric pressure and vacuum?

Why do I ask,
With my small blower on my engine I have vacuum until the blower is spun enough to produce boost. With boost I am producing more pressure than barometric pressure , right?
It is measured in pounds of boost.

[Rick_L]

Barometric pressure is usually expressed in either pounds per square inch (PSI) or in inches of mercury.

Standard atmospheric pressure is 14.7 psi, which is 29.92" Hg (mercury). So, 1 psi is 2.03" Hg.

Now comes the trickier part. There are actually two common ways to state pressure. One is absolute pressure, where 0 psi is a perfect vacuum, and standard atmospheric pressure is 14.7 psi. The other is gauge pressure, where the indicated or gauge pressure is the pressure relative to atmospheric pressure. In this scheme a perfect vacuum is -14.7 psi of vacuum, or 29.92" Hg of vacuum.

Now let's look at a couple of examples. Your oil pressure is measured as gauge pressure. If you have 50 psi oil pressure, that's relative to atmospheric pressure because that's what the gauge references. If you connect a typical hand held or dash mounted vacuum gauge to your intake manifold, it might read 20 inches of mercury. That would be 20" Hg less than atmospheric pressure. The absolute pressure would be (29.92 - 20) = 9.92" Hg.

Most of the common gauges and other pressure measuring devices use gauge pressure, the lone everyday exception being the barometric pressure on the weather report - it's absolute pressure.

When boost is stated in "pounds", what's really meant is pounds per square inch or psi. Same as having so many pounds of air in your tire, it's really psi.

The automotive exception to the gauge pressure is the sensor used to measure intake manifold pressure in most EFI applications. The MAP sensor used for most EFI applications is based on absolute pressure - MAP stands for "manifold absolute pressure". Most MAP sensor pressure is reported in KPa which is kilopascals. Atmospheric pressure is 100 KPa. If the MAP sensor indicates less than 100 KPa, you have a vacuum in the intake manifold. If it indicates more than 100 KPa, you have boost.

Hope this helps, or is enough for more questions.

[NickP]

Good stuff Rick!

[Bihili]

Thanks Rick for the explanation. But I have a follow up question.

Standard atmospheric pressure is 14.7 psi and this is used for our air fuel ratio of 14.7 to 1.
To run a richer mixture it is expressed as 14.2 to 1 or 13.8 to 1.
As a layman it seems to me the air fuel mixture would be expressed as 14.7 to 1.1 or 14.7 to 1.2 since to make the mixture richer we increase the fuel not decrease the atmospheric pressure with our carburetor.
Why is it expressed this way?

[Rick_L]

Bill, it's not clear what you are asking here.

An air fuel ratio of 14.7 to 1 is what's called the "stoichiometric" ratio. This means that all the fuel combines with all the oxygen in the air when it burns. When the mixture is "rich" (more fuel than the air will combine with) the a/f ratio is less than 14.7:1, and there will be unburned hydrocarbons or carbon monoxide in the exhaust. When the mixture is lean (more air than there is fuel with combine with), there is still uncombined oxygen in the exhaust.

The fact that atmospheric pressure is 14.7 psi and the fact that the "ideal" air/fuel ratio is 14.7:1 is merely a coincidence.

The 14.7 ratio applies to gasoline. Ethanol is about 9:1. This means that a gasoline/ethanol mix is slightly less than 14.7:1. Methanol has an even lower ideal ratio, about 6:1 if I remember correctly.

There is another way of expressing the air/fuel ratio. It's the air/fuel equivalence ratio, called "lambda" which is a Greek symbol. There, the ideal a/f ratio is lambda = 1. Variations above and below lambda =1 are lean (lambda >1) or rich (lambda <1). So that's pretty close to what you're suggesting.

The EFI guys often use lambda instead of a numerical a/f ratio. It's also useful with mixed fuel like gasoline/ethanol.

[Bihili]

Bill, it's not clear what you are asking here.
The fact that atmospheric pressure is 14.7 psi and the fact that the "ideal" air/fuel ratio is 14.7:1 is merely a coincidence.


Thanks Rick. I just assumed the 14.7 was not a coincidence and there was a relationship between the two.

[chevynut]

As a layman it seems to me the air fuel mixture would be expressed as 14.7 to 1.1 or 14.7 to 1.2 since to make the mixture richer we increase the fuel not decrease the atmospheric pressure with our carburetor.
Why is it expressed this way?

Mathematical ratios are typically expressed as something to 1 instead of some other number. Sure, you could express 2:1 as 4:2 or 6:3, but they don't. You could express it as you mentioned, but that's not done...it's simply convention. Like Rick said, the 14.7 used for both the stoichiometric ratio and air pressure is purely a coincidence.

[Maddog]

"What is correlation between vacuum and barometric pressure? "


One sucks the other blows

[5Clint7]

One sucks the other blows
Best answer I've seen. To the point.

[shua57]

Ahhh...Thermodynamics. Love this stuff. When I had Thermodynamics we spent alot of time using a rather simple equation used in alot of calculations:
Pabs=Patm+Pgage
Pabs = Pressure absolute (aka pressure measure in a perfect vacuum)
Patm = Atmoshperic pressure (the 14.7psi mentioned above)(this is also refered to as Barometric pressure)
Pgage = well, gage pressure or measure of the pressure difference of a system and the surrounding atmosphere

On the flip side....really just any pressure below 14.7psi (760mmHg) (aka a vacuum) you end up with
Pabs=Patm-Pvac

Pretty cool how much these types of equations are being calculated in a cars computer or the design of an engine.
Josh