How does a turbocharger work step by step?

May. 06, 2024

How Does A Turbocharger Work?

Very simply, a turbocharger is a kind of air pump taking air at ambient pressures (atmospheric pressure), compressing to a higher pressure and passing the compressed air into the engine via the inlet valves. For cars and vans, generally, turbos have been more commonly used on diesel engines as a way of boosting performance but, to meet ever-tightening emissions control, there is now a move towards the turbocharging of production petrol engines.

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As all engines are dependent on air and fuel we know that increases in these elements within set limits will increase power from the engine, but if we increase the fuel we must be capable of burning off all of it or the mixture becomes too rich which can have various issues. Likewise, too much air is known as running too lean and can be quite destructive.

To meet our requirements for power, this requires air; putting in more air presents far more problems than putting in more fuel. Air is around us all the time and is under pressure, (at sea level this pressure is about 15 p.s.i.) and it is this, when combined with the induction stroke of the engine, forces air into the cylinders. To increase the airflow further, an air pump (turbocharger) is fitted and compressed air is blown into the engine. This air mixes with the injected fuel allowing the fuel to burn more efficiently so increasing the power output of the engine.

One other side of turbocharging, which may be of interest, is an engine which works regularly at high altitudes, where the air is less dense and where turbocharging can restore some of the lost power caused by the drop in air pressure. An engine's power at 8,000 feet is only 75% of its power at sea level.

Capture

Instead of escaping through the exhaust pipe, hot gases produced during combustion flow to the turbocharger. The cylinders inside an internal combustion engine fire in sequence (not all at once), so exhaust exits the combustion chamber in irregular pulses. Conventional single-scroll turbochargers route those irregular pulses of exhaust into the turbine in a way that causes them to collide and interfere with one another, reducing the strength of the flow. In contrast, a twin-scroll turbocharger gathers exhaust from pairs of cylinders in alternating sequence.

Spin

The exhaust strikes the turbine blades, spinning them at up to 150,000 rpm. The alternating pulses of exhaust help eliminate turbo lag.

Vent

Having served their purpose, exhaust gases flow through an outlet to the catalytic converter, where they are scrubbed of carbon monoxide, nitrous oxides and other pollutants before exiting through the tailpipe.

Compress

Meanwhile, the turbine powers an air compressor, which gathers cold, clean air from a vent and compresses it to 30% above atmospheric pressure, or nearly 19 pounds per square inch. Dense, oxygen-rich air flows to the combustion chamber. The additional oxygen makes it possible for the engine to burn gasoline more completely, generating more performance from a smaller engine. As a result, engines can generate 30% more power than a non-turbocharged one of the same size.

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How do turbochargers work?

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.

Photo: The essence of a turbocharger: two gas fans (a turbine and a compressor) mounted on a single shaft. When one turns, the other turns too. Photo courtesy of NASA Glenn Research Center (NASA-GRC).

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—a closer look

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:

Cool air enters the engine's air intake and heads toward the compressor.
The compressor fan helps to suck air in.
The compressor squeezes and heats up the incoming air and blows it out again.
Hot, compressed air from the compressor passes through the heat exchanger, which cools it down.
Cooled, compressed air enters the cylinder's air intake. The extra oxygen helps to burn fuel in the cylinder at a faster rate.
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.
Waste gas from the cylinder exits through the exhaust outlet.
The hot exhaust gases blowing past the turbine fan make it rotate at high speed.
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.
The exhaust gas leaves the car, wasting less energy than it would otherwise.

In practice, the components could be connected something like this. The turbine (red, right) takes in exhaust air through its intake, driving the compressor (blue, left) that takes in clean outside air and pumps it into the engine. This particular design features an electric cooling system (green) in between the turbine and compressor.

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Artwork: How the turbine and compressor are connected in an electrically cooled turbocharger. From US Patent #7,946,118: Cooling an electrically controlled turbocharger by Will Hippen et al, Ecomotors International, granted May 24, 2011. Artwork courtesy of US Patent and Trademark Office.

Where does the extra power come from?

Turbochargers give a car more power, but that extra power is not coming directly from the waste exhaust gas—and that sometimes confuses people. With a turbocharger, we harness some of the energy in the exhaust to drive the compressor, which allows the engine to burn more fuel each second. This extra fuel is where the car's extra power comes from. All the exhaust gas is doing is powering the turbocharger and, because the turbocharger isn't connected to the car's crankshaft or wheels, it's not directly adding to the car's driving power in any way. It's simply enabling the same engine to burn fuel at a faster rate, so making it more powerful.

How much extra power can you get?

If a turbocharger gives an engine more power, a bigger, better turbocharger will give it even more power. In theory, you could keep improving your turbocharger to make your engine more and more powerful, but you will eventually hit a limit. The cylinders are only so big and there's only so much fuel they can burn. There's only so much air you can force into them through an inlet of a certain size, and only so much exhaust gas you can expel, which limits the energy you can use to drive your turbocharger. In other words, there are other limiting factors that come into play that you have to take into account as well; you can't simply turbocharge your way to infinity!

Advantages and disadvantages of turbochargers

Advantages

Photo: A typical automobile turbocharger. You can clearly see the two fan/blowers (one above the other) and their inlet/outlets. Photo courtesy of US Army.

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 conservation 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.

“Most industry experts expect that by 2027, more than half the vehicles sold in the United States will be powered by one.”

The New York Times, 2018

Disadvantages

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 the turbocharger is 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. In the last few years, leading manufacturers such as Garrett and BorgWarner have been busily developing partly or fully electric turbochargers to solve this issue; Garrett's offering is called the E-Turbo and Borg's is the eBooster®.

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.

Büchi wasn't the only important figure in the story, however. Some years earlier, Sir Dugald Clark (1854–1932), Scottish inventor of the two-stroke engine, had experimented with separating the compression and expansion stages of internal combustion using two separate cylinders. This worked a bit like supercharging, increasing both the air flow into the cylinder and the amount of fuel that could be burned. Other engineers, including Louis Renault, Gottlieb Daimler, and Lee Chadwick, also experimented successfully with supercharging systems.

Artwork: One of Alfred Büchi's turbocharger designs from the late 1920s (the patent was filed in 1927 and granted in April 1934). I've colored it so you can make sense of it quickly. You can see a single cylinder (yellow) and piston, crank, and connecting rod (red) on the left. Exhaust gas from the cylinder feeds around a pipe (green) that drives a turbine. This is connected to the orange "charging blower" (compressor) and cooler (blue box) that pushes air into the cylinder through the blue pipe. There are various other intricate bits and pieces, but I won't go into all the details; if you're interested, take a look at US Patent #1,955,620: Internal combustion engine (served via Google Patents). Artwork courtesy of US Patent and Trademark Office.

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Woodford, Chris. (2010/2020) Turbochargers. Retrieved from https://www.explainthatstuff.com/how-turbochargers-work.html. [Accessed (Insert date here)]