According to the experts at Wards Auto, light-duty vehicles equipped with turbocharged engines reached 27.6 percent of the U.S. market in spring 2017, up from less than 6.6 percent in 2010. And that number should continue to grow as smaller turbocharged engines will be available in cars ranging from the subcompact Chevrolet Sonic to Lexus’s redesigned flagship, the LS sedan.
The main reason behind this growth is the EPA’s requirement that automakers achieve a fleet-wide fuel economy average of about 54.5 miles per gallon by 2025. To meet the target while maintaining the performance customers have come to expect, automakers are reducing engine size while adding turbocharging and supercharging to meet these conflicting demands.
Let’s look at how these features work—and how turbochargers are different from superchargers.
Think of an engine as the heart of a car. Air is drawn into the system and mixed with fuel and that mixture is compressed by the pistons. The engine’s spark plugs ignite the compressed air/fuel mixture, and the expanding gases drive the cylinders, which push out the exhaust gases—like a person breathing out. This is why the typical internal-combustion powerplant, one lacking a turbocharger or supercharger, is known as a naturally-aspirated engine.
For decades, automakers delivered more power by increasing engine size. More cylinders meant that more fuel could be burned, which produced more power. But larger engines not only use more fuel than smaller ones, they also weigh more, which reduces fuel economy. Tightening fuel economy mandates automakers reduce engine size. In order to maintain increase horsepower, automakers need to burn more fuel power from smaller engines. This is where turbocharging and supercharging prove useful.
Turbochargers and superchargers can force more air into the engine’s cylinders by condensing the air before its mixed with fuel and ignited. Increasing the density of the air allows more fuel to be burned, which increases horsepower.
The Difference Between Turbochargers and Superchargers
While turbochargers and superchargers both condense air, they differ in how the extra air is condensed. With a turbocharger, exhaust gases flow through one of two turbines, which in turn starts the other one spinning. The second turbine acts as an air compressor, sending compressed air into the cylinders.
While a supercharger also uses a turbine to compress air, it’s driven by the engine’s crankshaft via an accessory belt. Popularity-wise, superchargers are found in fewer than 0.5 percent of the light-duty vehicles on sale in this country, far fewer than turbochargers.
Benefits and Disadvantages
Because a turbocharger relies on exhaust gases, there can be a noticeable delay between the time you press the accelerator and when the extra power is delivered. This also means that turbochargers work best at higher engine speeds, which is when the most exhaust is being produced.
Superchargers avoid issues with turbo lag entirely since the supercharger is driven by the engine itself, so extra power is delivered from the second the engine produces power. But this means that even though the engine is gaining power from the supercharger, some of that power isn’t available for driving the wheels — it has to be used to run the supercharger itself. In the end, a supercharger works best at lower rpm, while turbochargers work better at higher rpm. But turbochargers hold the edge in efficiency since the exhaust gases used to power them is essentially free. In contrast, part of a supercharged engine’s energy is used to power the supercharger.
The New Frontiers for Forced Induction
Given their relative advantages, it’s no surprise that the Volvo S90 integrates a supercharger for low-speed performance and a turbocharger for higher speeds in the same 2.0-liter four-cylinder engine. That combination produces 316 horsepower, 295 pound-feet of torque and has EPA rating of 22 mpg city, 31 mpg highway on all-wheel-drive S90s.
Meanwhile, Audi has been experimenting with an electrically-assisted turbocharger in the Audi TT ClubSport Turbo concept car. Producing 591 horsepower and 479 pound-feet or torque from a 2.5-liter turbocharged five-cylinder engine, the TT ClubSport Turbo uses an electrically-driven supercharger to spin the turbines, rather than the engine, negating power losses from an engine-driven supercharger. It’s paired to a conventional turbocharger for higher speed work.
Rather than choosing a turbocharger or supercharger, automakers have found that pairing the two types of blowers delivers the power of a larger power plant in an engine with the fuel economy of a smaller one.