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There are several advantages to flying at high altitude, like decreased drag, faster true airspeed, and if you're going the right direction, higher tailwinds. But there's one major disadvantage for normally aspirated engines: a lack of oxygen.
As you increase altitude, air pressure decreases, and it decreases quickly. In fact, if you're flying at 18,000 feet, 50% of the atmosphere is below you. That means less air for your engine to burn, and a lot less horsepower that's coming out of the front of your airplane.
Turbochargers have three main components:
It all starts with the turbine, which is driven (turned) by exhaust gas exiting your engine. As exhaust exits through the exhaust manifold, it passes over the turbine and spins it. The more exhaust that passes through, the faster the turbine spins. And that's pretty much it - at least for now.
A shaft connects the turbine and the compressor, so when the turbine starts spinning as the engine is fired up, the compressor starts spinning too.
The compressor is in charge of drawing in air from outside the airplane, compressing it, and then passing it on to the engine. And as you've already read, the compressor is spinning because its connected to turbine, thanks to the shaft.
Now that you know the basics of a turbocharger, there are a few more pieces to cover.
Turbochargers are good at increasing the air pressure in your engine's intake manifold, known as manifold pressure, but sometimes they're a little too good. Turbochargers are capable of producing too much manifold pressure, which can damage or destroy your engine. So how do turbochargers prevent too much air from entering your engine? With something called a wastegate.
Some wastegates are automatic, and others are manually operated by the pilot, but the theory behind them is always the same. Wastegates open and close to regulate the amount of air that passes over the turbine, and prevent the turbine from spinning too fast. Again, the faster the turbine spins, the faster the compressor spins, and the more air that enters the engine.
So how much air can your engine really handle? It depends on the engine, but there are two main types of turbocharging: altitude turbocharging and ground boosting.
Altitude turbocharging, which is sometimes called 'normalizing', keeps your engine running like it's at sea level for as long as possible. It depends on the engine, but most altitude turbochargers keep your manifold pressure between 29-30 inches of mercury (sea level pressure), as you climb in altitude. But eventually, as your altitude increases, your turbocharger isn't able to compress enough air to keep your manifold pressure at sea level. This is called the critical altitude, and it's the highest altitude where your engine can produce the maximum horsepower it's rated for (engine horsepower is rated at sea level). From this point, the higher you climb, the less air will enter your engine, and the less horsepower you'll produce. But it's still much more effective than a normally aspirated engine.
Ground boosting is similar to altitude turbocharging, but it uses much more pressure. Boosted systems typically run at manifold pressures between 31-45 inches of mercury - much more than altitude turbochargers. The idea is simple: more pressure = more air entering the engine = more horsepower output. But the disadvantage is a big one: lots of heat.
As you compress air, it heats up. This is one of the biggest disadvantages for any turbocharger. Aircraft engines are already really hot things, and hot intake air makes them even worse. To solve the problem, many turbochargers use something called an 'intercooler'.
An intercooler is basically a mini air-conditioner that's placed between the turbocharger and the engine. As the hot air moves from the turbo to the engine, it passes through the intercooler, and the temp significantly drops. That cooler air makes your engine much happier, and keeps it running smoothly.
Turbochargers are the key to piston-driven airplanes climbing to high altitude. While they add some complexity to an engine system, they're about the only thing that can get your plane up to the flight levels for strong tailwinds, higher true airspeed, and views like this:
Colin is a Boldmethod co-founder, pilot and graphic artist. He's been a flight instructor at the University of North Dakota, an airline pilot on the CRJ-200, and has directed development of numerous commercial and military training systems. You can reach him at firstname.lastname@example.org.