Supercharging Basics

Supercharging is a simple concept complicated by a surprising variety of machinery and an entire lexicon surrounding the practice. But pilots need to understand the concepts and some of the nomenclature, so we’ll hit the basics in this article. We can detail the hardware in another article.

Let’s start with the catch-all term “supercharging.” The “super” part comes from Latin and mainly means “above” or “over” or “on top of.” Also going all the way back to Latin, “charge” originally meant to load a wagon, but in more modern times, it was used when loading a firearm. Or, one could “charge one’s glass” with drink prior to a toast. Or, most applicable to our needs, it denotes the air/fuel “charge” captured inside an engine cylinder just before ignition.

Twin screw supercharger on a V6 engine. (Image: PatriciaWrites, CC BY-SA 4.0, via Wikimedia Commons)

Thus, to supercharge an engine means to forcefully load more air/fuel into the cylinders than can be accomplished natively by air pressure. To distinguish this, an engine with no supercharging is naturally aspirated, and one force-fed an air/fuel charge is supercharged.

The big advantage to supercharging aircraft engines is that sea-level or greater-than-sea-level volumes of air can be stuffed into the engine’s cylinders when the airplane is tens of thousands of feet above sea level. This means sea-level or greater amounts of horsepower are generated at a time when the airframe is seeing far less than sea-level aerodynamic drag. Pretty cool, as this is a recipe for speed and fuel economy.

You can also guess supercharging might not be worth the trouble if the engine is going to spend its life below 10,000 feet chasing pancakes or going around and around the local airport pattern.

If you’re familiar with supercharging automobiles, you take it for granted the whole point is to cram greater-than-sea-level air into the engine while still at sea level and make more power. This is often true with aircraft supercharging, but not always. It’s just as common to leave the sea-level power unchanged but to maintain that power for as long as possible as the aircraft ascends. In that case, the engine is “normalized,” as in “turbo-normalized.”

A centrifugal supercharger looks similar to a turbocharger, but it’s belt-driven instead of being powered by exhaust gasses..

Normalizing means the engine should not see greater-than-normal stress on the crankshaft, pistons, and other internal parts, so theoretically, there’s no more wear than if the engine were naturally aspirated. There are nuances to this, but to the first order, the principle is true.

All good things are limited, and there comes a high altitude where supercharging can’t provide sea-level amounts of air. This is the supercharged engine’s critical altitude. Below the critical altitude, the engine can produce its full rated power; above the critical altitude, power drops off with increasing height. This drop-off is at the same rate as if the engine wasn’t supercharged; the loss simply begins much higher than if there had never been a supercharger.

Measuring how much supercharging is taking place is the job of the manifold pressure instrument. At first, this is a rather simple instrument, and in practice, it is pretty simple to understand, but there are nuances to supercharging at least worth exploring to get the bigger understanding.

Several things can affect the pressure inside an intake manifold, and thus, manifold pressure readings. You could be flying very low or very high; that certainly affects manifold pressure. The atmosphere could be unusually warm or cold. There could be ram pressure in the intake system thanks to the airplane’s high speed through the atmosphere. Or, you could have a supercharger, of course.

In the everyday piloting world, the outside air temperature and ram inlets give measurable effects, but they’re secondary to how much supercharging is in play.

To peek down a rabbit hole, manifold pressure instruments read in inches of mercury, or inches m.p. And when the airplane is sitting on the ground, they tend to read “29,” or 29 inches of mercury. Not 0 inches, anyway. This probably rings a bell from your flight training, as 29.92 inches of mercury is sea-level air pressure on a standard day, which might give you the idea a manifold pressure gauge might be similar to a barometer. Well, it essentially is a barometer and reads in the same units. It’s just that the barometric pressure inside a supercharged engine’s intake manifold is a pretty dynamic place.

One great thing about a manifold pressure instrument is that it is calibrated to read the pressure in the intake manifold in absolute terms and not in gauge terms. Absolute readings are simple; whatever the pressure in the intake manifold is, that is what the m.p. instrument reads. It doesn’t matter if the pressure shown is high because the supercharger is really blowing air, or if it’s particularly hot inside the intake and expanding the air, or if you’re flying in a mineshaft (that would be a good trick) and the atmosphere is really thick down there. If the pressure is high in the intake manifold, then the m.p. reading will be high.

A gauge reading is the air pressure inside the intake manifold minus the ambient air pressure (the local barometric pressure, in other words). So 34 inches m.p. absolute at sea level is 5 inches m.p. gauge (34 inches m.p. – 29 inches m.p. = 5 inches m.p.). You could say the engine is enjoying a 5-inch m.p. boost compared to running naturally aspirated.

Where the absolute versus gauge readings take practical effect is most supercharged aircraft employ m.p. instruments giving absolute units, but a few (the English are fond of this) have a “boost gauge.” This instrument reads in pounds per square inch (psi) and ignores the ambient pressure in the atmosphere. So, a boost gauge might say 8 pounds of boost (8 psi), while a manifold pressure instrument would read 45 inches m.p. The 8 psi on the boost gauge does not include the roughly 14 psi of the atmosphere at sea level, while the 45 inches m.p. does include the standard day 29 inches of mercury at sea level.

Automotive hot rodders favor “pounds of boost” as well. In many ways, it boils down to which units you’re more comfortable with, but the absolute readings of a manifold pressure instrument relate directly to naturally aspirated aviation practice and give the pilot an immediate, accurate, no-arithmetic-required sense of how much air the engine is ingesting. In other words, the manifold pressure reading is a surrogate for how much load the engine is under, and thus, how much power it is producing.

Next time, we’ll get into the nuts and bolts of supercharging, whether that be turbocharging, supercharging, or nitrous oxide injection.

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