Under Pressure to Raise the Bar

By Jeff Yirak, P.E., LEED AP BD+C, O+M

I try to hit one bar on the way to work every day. Now before you sign me up for a 12 step program, hear me out.

My car has a turbocharger.  Have you ever tried to determine the correct size turbocharger for your internal combustion application? This is something most people don’t actually think about (unless you’re a car person like me), but the phenomenon involved is around all of us every day (literally). The atmosphere, thanks to gravity, exerts about 14.7 pounds of force over every square inch (pounds per square inch is abbreviated as psi) of all things at sea level.  At higher elevations, the atmospheric pressure is less, due to gravity; at 5,000 feet, the static atmospheric pressure is about 12.2 psi. This change affects the boiling point of water, the apparent level of inflation of sealed containers, and contributes to altitude sickness because the air is “thinner”.

Back to turbochargers. Too often, the focus is on the maximum pressure to be developed, or the boost!  The turbo goes from zero to some number, usually between 10 and 20 for a gasoline engine, and maybe has high as 50 or 60 for a diesel engine.  This number is the pressure developed in the intake manifold, measured in pounds per square inch.

This VDO gauge measures boost in psi. (Image credit: VDO-Gauges)

Mine is set to provide an additional 15 psi in my intake manifold. Since this makes the total pressure roughly double normal atmospheric pressure, this correlates to roughly double the amount of air that would otherwise be drawn into my engine.  Having double the air lets me mix in about double the fuel, making the engine about twice as powerful as it would be without the turbocharger.  This is a good thing.  This measurement, however, doesn’t tell the whole story.

What, you don’t have a pair of turbochargers at your desk too?

If you’ve ever inflated a tire, you’ve probably seen pressure gauges that read zero when not hooked up to the Schrader valve, and then change or pop out to a number between 0 and 100 when pressed onto the valve.  These gauges are lying.  The pressure is not zero when the gauge is not being used; there’s the air pressure of 14.7 psi (or otherwise, depending on your elevation). These gauges are actually measuring something called “gauge pressure”, which subtracts the 14.7 psi from the absolute pressure being measured.  Since you’re probably only interested in the relative pressure anyway, your tire gauge indicating that your tires are inflated to 38 psi, which matches the manufacturer’s recommendation, is correct.  But it would be more correct to specify that they are inflated to 38 psi gauge, or psig, to indicate that this includes the additional 14.7 psi required to be psi actual, or psia.

This gauge is reading zero.  This is correct, in terms of psig, but it is not the actual pressure, psia. (Image credit: Made-in-China.com)

Most of the pressures you encounter are gauge pressure, not absolute pressure. There is another unit of measure for gauge pressure that doesn’t have an absolute equivalent, and that’s called bar.  Bar was developed as a metric unit, but it is not an SI unit like gram and meter.  It happens to be equivalent to 14.7 psi. When my manifold pressure hits maximum, I’m at one bar.  And it’s a good place to be. So now you see that hitting a bar on the drive to work is a good thing!

Follow Jeff on Twitter @JYirak_WH