How turbochargers work

Turbocharger by Rishabh Sharma on Scribd

There's no such thing as a perfect invention: we can always make something better, cheaper, more efficient, or more environmentally friendly. Take the internal combustion engine. You might think it's remarkable that a machine powered by liquid can hurl you down the highway or speed you through the sky many times faster than you could otherwise travel. But it's always possible to build an engine that will go faster, further, or use less fuel. One way to improve an engine is to use a turbocharger—a pair of fans that harness waste exhaust power from the back of an engine to cram more air into the front, delivering more "oomph" than you'd otherwise get. We've all heard of turbos, but how exactly do they work? Let's take a closer look!

What is a turbocharger?

Have you ever watched cars buzzing past you with sooty fumes streaming from their tailpipe? It's obvious exhaust fumes cause air pollution, but it's much less apparent that they're wasting energy at the same time. The exhaust is a mixture of hot gases pumping out at speed and all the energy it contains—the heat and the motion (kinetic energy)—is disappearing uselessly into the atmosphere. Wouldn't it be neat if the engine could harness that waste power somehow to make the car go faster? That's exactly what a turbocharger does.

Car engines make power by burning fuel in sturdy metal cans called cylinders. Air enters each cylinder, mixes with fuel, and burns to make a small explosion that drives a piston out, turning the shafts and gears that spin the car's wheels. When the piston pushes back in, it pumps the waste air and fuel mixture out of the cylinder as exhaust. The amount of power a car can produce is directly related to how fast it burns fuel. The more cylinders you have and the bigger they are, the more fuel the car can burn each second and (theoretically at least) the faster it can go.

One way to make a car go faster is to add more cylinders. That's why super-fast sports cars typically have eight and twelve cylinders instead of the four or six cylinders in a conventional family car. Another option is to use a turbocharger, which forces more air into the cylinders each second so they can burn fuel at a faster rate. A turbocharger is a simple, relatively cheap, extra bit of kit that can get more power from the same engine!

How does a turbocharger work?

If you know how a jet engine  works, you're halfway to understanding a car's turbocharger. A jet engine sucks in cold air at the front, squeezes it into a chamber where it burns with fuel, and then blasts hot air out of the back. As the hot air leaves, it roars past a turbine (a bit like a very compact metal windmill) that drives the compressor (air pump) at the front of the engine. This is the bit that pushes the air into the engine to make the fuel burn properly. The turbocharger on a car applies a very similar principle to a piston engine. It uses the exhaust gas to drive a turbine. This spins an air compressor that pushes extra air (and oxygen) into the cylinders, allowing them to burn more fuel each second. That's why a turbocharged car can produce more power (which is another way of saying "more energy per second"). A supercharger (or "mechanically driven supercharger" to give it its full name) is very similar to a turbocharger, but instead of being driven by exhaust gases using a turbine, it's powered from the car's spinning crankshaft. That's usually a disadvantage: where a turbocharger is powered by waste energy in the exhaust, a supercharger actually steals energy from the car's own power source (the crankshaft), which is generally unhelpful.

 How does turbocharging work in practice? A turbocharger is effectively two little air fans (also called impellers or gas pumps) sitting on the same metal shaft so that both spin around together. One of these fans, called the turbine, sits in the exhaust stream from the cylinders. As the cylinders blow hot gas past the fan blades, they rotate and the shaft they're connected to (technically called the center hub rotating assembly or CHRA) rotates as well. The second fan is called the compressor and, since it's sitting on the same shaft as the turbine, it spins too. It's mounted inside the car's air intake so, as it spins, it draws air into the car and forces it into the cylinders.

Now there's a slight problem here. If you compress a gas, you make it hotter (that's why a bicycle pump warms up when you start inflating your tires). Hotter air is less dense (that's why warm air rises over radiators) and less effective at helping fuel to burn, so it would be much better if the air coming from the compressor were cooled before it entered the cylinders. To cool it down, the output from the compressor passes over a heat exchanger that removes the extra heat and channels it elsewhere.

How a turbocharger works

The basic idea is that the exhaust drives the turbine (the red fan), which is directly connected to (and powers) the compressor (the blue fan), which rams air into the engine. For simplicity, we're showing only one cylinder. Here then, in summary, is how the whole thing works:

1. Cool air enters the engine's air intake and heads toward the compressor.
2. The compressor fan helps to suck air in.
3. The compressor squeezes and heats up the incoming air and blows it out again.
4. Hot, compressed air from the compressor passes through the heat exchanger, which cools it down.
5. Cooled, compressed air enters the cylinder's air intake. The extra oxygen helps to burn fuel in the cylinder at a faster rate.
6. Since the cylinder burns more fuel, it produces energy more quickly and can send more power to the wheels via the piston, shafts, and gears.
7. Waste gas from the cylinder exits through the exhaust outlet.
8. The hot exhaust gases blowing past the turbine fan make it rotate at high speed.
9. The spinning turbine is mounted on the same shaft as the compressor (shown here as a pale orange line). So, as the turbine spins, the compressor spins too.
10. The exhaust gas leaves the car, wasting less energy than it would otherwise.

Who invented the turbocharger?

Whom do we thank for turbochargers? Alfred J. Büchi (1879–1959), an automotive engineer employed by the Gebrüder Sulzer Engine Company of Winterthur, Switzerland. Much like the turbocharger I've illustrated up above, his original design used an exhaust-driven turbine shaft to power a compressor that forced more air into an engine's cylinders. He originally developed the turbocharger in the years before World War I and patented it in Germany in 1905, but continued to work on improved designs until his death four decades later.

Advantages and disadvantages of turbochargers

You can use turbochargers with either gasoline or diesel engines and on more or less any kind of vehicle (car, truck, ship, or bus). The basic advantage of using a turbocharger is that you get more power output for the same size of engine (every single stroke of the piston, in every single cylinder, generates more power than it would otherwise do). However, more power means more energy output per second, and the law of conversation of energy tells us that means you have to put more energy in as well, so you must burn correspondingly more fuel. In theory, that means an engine with a turbocharger is no more fuel efficient than one without. However, in practice, an engine fitted with a turbocharger is much smaller and lighter than an engine producing the same power without a turbocharger, so a turbocharger car can give better fuel economy in that respect. Manufacturers can often now get away with fitting a much smaller engine to the same car (such as a turbocharged V6 instead of a V8, or a turbocharged four-cylinder engine instead of a V6). And that's where turbocharged cars get their advantage: working well, they might save up to 10 percent of your fuel. Since they burn fuel with more oxygen, they tend to burn it more thoroughly and cleanly, producing less air pollution.

More power for the same engine size sounds wonderful, so why aren't all engines turbocharged? One reason is that the fuel economy benefits promised by early turbochargers didn't always turn out as impressively as manufacturers (eager to seize any marketing advantage over their rivals) liked to claim. One 2013 study, by Consumer Reports, found small turbocharged engines giving significantly worse fuel economy than their "naturally aspirated" (conventional) counterparts and concluded: "Don't take turbocharged engines' eco-boasts at face value. There are better ways to save fuel, including hybrids, diesels, and other advanced technologies." Reliability has often been a problem too: turbochargers add another layer of mechanical complexity to an ordinary engine—in short, there are quite a few more things to go wrong. That can make maintenance of turbos significantly more expensive. By definition, turbocharging is all about getting more from the same basic engine design, and many of the engine components have to suffer higher pressures and temperatures, which can make parts fail sooner; that's why, generally speaking, turbocharged engines don't last as long. Even driving can be different with turbos: since they're powered by the exhaust gas, there's often a significant delay ("turbo lag") between when you put your foot on the accelerator and when the turbo kicks in, and that can make turbo cars very different (and sometimes very tricky) to drive.