How It Works: Twin-charged engines

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Designing any engine has always been a challenge for automakers, but it’s becoming even tougher as they try to balance the fuel efficiency standards they’re required to meet, with the performance buyers want.

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Many are adding turbochargers to smaller-displacement engines to pull more power out of them. But a few are taking it a step further with twin-charging — adding a supercharger along with the turbo. (Note that twin-charging is not the same as twin-turbo or bi-turbo, which indicates two turbochargers.) Lancia, Nissan, and Volkswagen were among the first to use the combination starting back in the 1980s, but it’s not very common today. Among those that still use it is Volvo, which has twin-charging on engines available in many models across its lineup.

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How It Works: Twin-charged engines Back to video

Both turbochargers and superchargers are air pumps, which compress air and push it into the engine, a system called forced-air induction. The difference between them is in how they’re powered, which in turn affects how the engine responds to them.

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Engines burn gasoline, but only after it’s been turned into a vapour by being mixed with air. What many people call the “gas” pedal could more accurately be described as an “air” pedal. When you put your foot down, the engine pulls in more air. Sensors then determine how much, and instruct the fuel system to add the appropriate amount of gasoline to blend the correct air-fuel mixture.

Burning more gasoline will create more power, but if the mixture’s too rich — too much gas and not enough air — the engine won’t run properly. Adding more air would allow for more fuel, but a naturally aspirated engine (one without a turbocharger or supercharger) is limited in how much air it can take in. Each piston creates a vacuum as it moves downward in its cylinder and air rushes in to fill that void, but it’s at atmospheric pressure. When air is pumped in under pressure — the job of those forced-air chargers — more fuel can be delivered, and the engine makes more power.

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A turbocharger is powered by the engine’s exhaust gases. One side is located at the exhaust manifold where spent gases are expelled, and the other at the intake where fresh air comes in. Inside the turbo are two small fans, joined together by a shaft. As exhaust gas flows through the turbo, it spins one of the fans, called the turbine. The shaft turns, spinning the second fan, called the compressor. This fan draws in fresh air, pressurizes it, and forces it into the engine.

A supercharger works similarly in that it compresses air and forces it into the engine, but its fan is driven by the engine. The supercharger has a drive gear and pulley that’s linked by a belt to the engine’s crankshaft. When the crankshaft turns, so does the supercharger’s compression fan. It’s also possible to power the supercharger off the crankshaft with gears, rather than a belt, but that’s mostly used on racing engines.

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Air that’s being compressed heats up, but cool air is more oxygen-dense and provides better combustion when the fuel mixture is ignited. Automakers add a heat exchanger, called an intercooler, which brings down the air’s temperature before it enters the engine.

For every plus to either forced-air system, there’s a minus, of course. A supercharger provides boost almost immediately, but the engine uses some of its energy to drive the unit, known as parasitic loss. For turbochargers, it’s lag: Once you hit the throttle, it takes time for the exhaust to spin the turbine fan to sufficient speed.

One way of getting around turbo lag is to install two turbochargers: A small one that spins up faster, and a larger one that provides more power at higher speeds. Twin-charging does the same. The supercharger kicks in right away to provide the initial boost of power, and then as its performance levels out, the turbocharger is spinning and doing its thing. Together, their combined performance creates smooth power over a wide range of engine speeds.

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So why don’t more engines have twin-charging? The quick answer is that while it works well, it’s complex, and that of course adds to its price. Automakers are more likely to use twin-turbos, or to use a twin-scroll turbocharger — which uses a special engine manifold that combines and directs the exhaust from specific cylinders, creating a smoother airflow to the turbine fan.

And why use turbochargers or twin-charging at all? These forced-air systems used to be set aside almost exclusively for sports and performance cars for speed. Today, automakers are putting them on small-displacement engines to help them meet stricter fuel economy standards. That smaller engine is inherently more fuel-sipping than a larger one, but when power is needed, the forced-air system starts operating to provide it — a four-cylinder feels like a six-cylinder, for example. But remember that when you’re adding more air, you’re also adding more fuel. It’ll still probably use less gasoline than if you had a bigger engine under the hood, but whether it’s from a supercharger or a turbocharger, going fast will always come with a cost.