How do turbos work? How do supercharged engines work? What's the difference, anyway?
We could all use an energy boost from time to time, and engines are no different. With the addition of a turbocharger or supercharger, manufacturers are able to squeeze more power from an engine. That’s why high-performance cars like the Chevrolet Corvette Z06 are often supercharged. It’s also the reason growing numbers of more practical cars are gaining turbocharged engines. Two examples are the EcoBoost engines offered in the Ford Escape and the 2L turbo four-cylinder engine found in the Cadillac ATS.
Even if you're the kind of car buyer who is more interested in economy than performance, you may still wish to purchase a car with an engine that benefits from this boost of power. Read on for an explanation.
A forced induction primer
Turbocharging and supercharging are methods of what engineers call “forced induction.” The idea is to produce more power by pushing more fuel into the cylinders. You can’t just add more gas, though; gasoline burns best when it’s mixed with air at a constant ratio. So rather than just ingest extra gas, the engine needs additional air as well.
Turbochargers and superchargers are air compressors designed to push aerosolized fuel into the cylinders.
A turbocharger is driven by the car’s exhaust gases. As they rush out through the exhaust pipe, they spin a small turbine, which looks like a fan mounted on a shaft. On the other end of the shaft is a second turbine, but this one collects inlet air and pushes it in a compressed stream into the engine where it mixes with more fuel to be burned.
In contrast, a supercharger is driven directly from the engine, usually by a belt. That means it’s pumping air into the engine all the time and not only pumping additional air when the car’s exhaust gases are flowing more quickly.
To complicate matters, some car manufacturers, notably Audi, are starting to use electric turbochargers in their higher-performance vehicles. As these aren’t driven by the exhaust, they’re more like a supercharger, but one that can be turned on and off as needed.
“Lightweighting” for better mileage
Historically, forced induction was about helping cars accelerate faster. Today it’s mostly about improving fuel economy. Vehicle manufacturers are under pressure to improve their cars’ gas mileage, and making engines smaller is one way to do that.
Smaller engines weigh less and burn less gas, both of which improve mileage. The downside is that they make less power. Since no car buyer really wants to be stuck in a slow car, the answer is to add a turbocharger or supercharger. That’s how a five-passenger family car like the 2016 Chevrolet Cruze can successfully pack a compact 1.4 liter engine under its hood. Being small, the Cruze gets up to 42 mpg on the highway, and the turbocharger helps it deliver a healthy 153 horsepower.
Supercharger pros and cons
Another difference between a turbocharger and a supercharger is that the supercharger is always running, and needs engine power (and fuel) to drive it. In contrast, the turbo only runs when the engine is already working hard. That means that under light loads, such as when you’re trundling around town, the supercharger is costing you energy. It only starts producing more power than it takes to run when you really push the pedal down. In short, a supercharger gives you power, but you’ll pay for it at the gas pump.
Turbocharger pluses and minuses
A turbocharged engine not only makes plenty of power—it recycles the waste of the car as a source of energy. Therefore, it gets good mileage too. The Cadillac ATS’s 2L, four-cylinder engine churns out 272 horsepower, which can make for some entertaining driving, yet it delivers 22 mpg in city driving and up to 31 mpg on the highway.
The chief drawback of a turbocharger is that the power it makes isn’t available the instant you mash down on the gas pedal. Instead, it takes a brief moment for the exhaust gases to spin it up to speed. This phenomenon is known as “turbo lag.” In most modern turbocharged vehicles, engineers have minimized lag to where it’s almost imperceptible. It’s still there though, and it’s a source of irritation for some drivers.
A second issue is that many turbocharged engines prefer a diet of premium gas. They’re higher-compression engines, and they need high-test fuel that ignites precisely. They’ll still run on regular, but their engine management systems will dial back the amount of power they produce.
It’s fair to say that twenty years or more ago, turbocharged engines suffered a few problems. However, modern materials coupled with continuous engineering improvements seem to have made turbocharged vehicles much more equal to their non-turbo counterparts.
Stay naturally aspirated?
An engine without forced induction is referred to as being “naturally aspirated.” While a growing number of cars come with turbocharged or supercharged engines, drivers can still find plenty of choices that do not have forced inductionalthough this may mean purchasing a model with a larger engine. For example, the Cadillac ATS has an optional 3.6 liter V6. This makes some 63 horsepower more than the 2L turbo engine, although it’s slightly less torque-y and gets 20 mpg in the city and around 29 on the highway.
Turbocharged or supercharged - which is for you?
Whether or not to go with a turbocharged or supercharged vehicle comes down to the emphasis you place on performance. For acceleration that really pushes you back in your seat, like the kind you’d get in a Chevrolet Camaro ZL1 (available in 2017)or Corvette Z06, it has to be a supercharger. Plus, if you listen carefully you’ll hear a whistle from the supercharger over the engine’s rumble. Gearheads find this very cool—it’s music to their ears. But for those enthusiasts who buy cars with their heads rather than their hearts, go with a turbocharged engine. You’ll get good mileage numbers yet still have power when you need it.