Many new cars feature engines with turbochargers, which purportedly produce the power of a large engine with the fuel economy of a small engine. What is a turbocharger, and can it really deliver what it promises?
To understand what a turbocharger does, one needs a basic understanding of how gasoline engines work. (Those in the know can skip this paragraph.) A gasoline engine has several tube-shaped cylinders in which metal pistons move up and down. The motion of the pistons pull in a mix of gasoline and air, then compress it; a spark ignites the mixture which drives the piston downward and produces power. The piston then pushes the spent exhaust gasses out of the cylinder, and the cycle repeats.
The key to turbocharging is that first bit, the intake stroke, in which the downward motion of the piston creates a vacuum that draws in the fuel-air mix. Instead of relying on that vacuum, if one could blow the mixture into the cylinder, one could pack in more air and fuel and produce more power.
Mechanically force-feeding an engine is called supercharging, and there are several ways to do it, including an engine-driven blower. A turbocharger is another form of supercharging, one that is powered by the engine’s exhaust. And that is the key to turbocharging: It only does its thing when the engine is working hard.
A turbocharger consists of two turbines (the fans look very similar to those used by commercial jet engines) on a single shaft, but in separate housings. One is plumbed into the exhaust, the other into the intake. When the exhaust pressure is high enough, the turbine spins; the other turbine then works as a pump, pushing air into the intake system. When power demands on the engine are low — say, when idling, or when cruising down the highway — the turbocharger doesn’t do much. But when the driver demands more power from the engine, the exhaust pressure drives the turbo like crazy, pumping more air (and fuel) into the engine (we call this “boost”) and producing more power. With a turbocharger, a 2-liter engine can produce as much power as a 3-liter engine.
So how does that translate into better fuel economy? The answer lies in the on-demand nature of the turbocharger.
Ten years ago, a mid-size sedan might have a 3-liter V6 engine. Such an engine produced strong acceleration, but even when power demands were low, it still needed enough fuel (and air) to fill up all six cylinders. It was, to oversimplify it a bit, always using three liters’ worth of fuel.
Today, that same car might have a 2-liter turbocharged engine. When the driver gives it the beans — say, to accelerate onto the highway — the turbocharger pumps in more air and fuel than the engine could take in on its own. Essentially, the 2-liter engine is producing the same power as a three-liter engine, because it’s able to take in and burn as much fuel as a 3-liter engine!
But when the driver eases off the accelerator, the exhaust pressure drops, and the turbocharger stops producing boost. Now the engine works like the 2-liter that it is, consuming only the fuel it can breathe in on its own — delivering efficiency, much to the delight of consumers.