Turbochargers vs. superchargers: Your intro to forced induction

The principal

Essentially, both turbochargers and superchargers work by delivering more air to the engine. Oxygen is one leg of the combustion pyramid, along with fuel and a source of ignition – aka “spark.” For combustion to happen most efficiently, there needs to be a careful balance between oxygen, fuel, and spark.

Finding this balance has long been a problem for engine designers. For most of the history of internal combustion, the problem lay in delivering enough fuel. Thanks to the recent development of super high-pressure direct fuel injection, this has basically been solved. Now it is a question of providing enough air for the engine to breathe.

A really high-performance engine will need to suck in roughly a child’s bedroom worth of air every minute – think Dodge Hellcat. The laws of physics don’t ordinarily allow this because the only force pulling air into an engine is the low pressure created when the engine’s cylinders withdraw — just like what happens when the plunger is pulled back on a syringe.

This force just isn’t powerful enough to keep up with the amount of fuel modern engines can inject into their cylinders. Turbochargers and superchargers both attempt to address this by a process called forced induction, which amounts to a fancy term for forcing more air into the engine. Each does this in different ways, though, with separate advantages and disadvantages.

Turbochargers

The most common form of forced induction is turbochargers. Automakers love them because, at least in theory, they can be used to improve both power and economy.

It would seem that anything capable of doing that must be incredibly complex, but turbos are actually deceptively simple. Beneath their oddly aquatic looking shells, turbochargers are essentially a pair of fans linked by an axle.

Hot engine exhaust spins the first fan that in turn drives the second one, which is used to compress air. This compressed air is then forced back into the engine. As discussed above, this added air means that more fuel can be injected and that whatever fuel is there can be burned more completely.

From a performance standpoint, the benefits are clear. Turbochargers increased air volume, allowing more fuel to be used in each combustion cycle, which leads to more power. In essence, this allows a small engine to pretend it is much larger.

When it comes to efficiency, the benefits of turbocharging are a little harder to understand. To begin, turbochargers can offer some inherent benefits. Turbochargers can help improve efficiency by insuring there is enough air that each combustion is complete. They can also help improve the thermodynamic efficiency of an engine by increasing its operating temperature. Best of all, because turbos are driven by essentially “free” energy (engine exhaust), their presence doesn’t hurt efficiency at all.

The real reason automakers love turbochargers has less to do with engineering and more to do with human behavior. The average driver uses full throttle less than one percent of the time. In a big, naturally aspirated engine, this means a lot of power is going to waste, while the sheer size of the engine still drags down efficiency.

By contrast, turbochargers are driven by engine exhaust, which is only produced in sufficient quantities when the engine is working hard. This means when the driver isn’t hard on the throttle, the turbocharger isn’t adding more air, and the engine isn’t adding more fuel. So one way to think about a turbocharger is as creating an engine that can be as big or as small as the situation calls for.

This is great in theory, in that a car has the potential to be efficient and powerful at the same time. Reality, as it often is, is not as rosy. Turbocharged engines can be expected to deliver either their power figures or their economy figures, but not both at the same time. Drive full throttle on a turbocharged car and the fuel economy won’t be any better than a car with a larger engine. Drive it slowly and the turbo simply isn’t being used.

So perhaps the best way to think of turbochargers is as giving drivers flexibility. They can choose how efficient they want to be, and how much fun they want to have. Sadly, turbos aren’t magic so they can’t always provide both at the same time.

Superchargers

Superchargers may work off of the same principal as turbochargers, but they are a bit more complicated.

Rather than being driven by exhaust gases, superchargers are mechanically driven. Typically superchargers are connected to the engine’s crankshaft via a chain or belt. The engine’s power is then used to drive one of two types of compressor.

Most superchargers use some variation of the “Roots” type blower. These superchargers use twin, overlapping rotors to compress air. The Roots design is relatively simple and can be made to a wide variety of specifications. Advanced Eton built superchargers can be found on a wide variety of vehicles such as the outgoing Cadillac CTS-V and the Audi S4.

The less common design is the “Lysholm” impeller. This design forces air between two overlapping Archimedes screws, compressing it to tremendously high pressures. Impeller superchargers are efficient and deliver power across a wide array of engine rpms, but they are expensive to build. The twin screws must mesh perfectly, and this requires incredibly fine manufacturing tolerances. For this reason Lysholm superchargers tend to be found in very high performance applications such as Mercedes AMG cars or the Dodge Hellcats.

In either case, superchargers deliver several important advantages when compared to exhaust driven turbos. Superchargers don’t have lag, as they are driven directly by the drive shaft. This is especially important on large displacement low revving engines, which is why high performance American V8s are often supercharged rather than turbocharged.

Superchargers also deliver torque over a wider range of engine rpms, making them attractive from a performance standpoint.

Unfortunately, there are also disadvantages. Superchargers tend to be mechanically more complex than turbochargers, due to their direct connection with the engine. Simply put, superchargers use power to create power. They are also large, heavy, and generally need to be mounted directly on top of the engine, the latter of which is a serious problem for modern automakers, for whom space is at a premium.

Unlike turbochargers, superchargers reduce engine efficiency, as a supercharger is constantly using some of the engine’s power to turn. When not much power is needed, this energy is essentially wasted. This is why superchargers are rarely — if ever — found outside of performance applications.

Conclusion

Anyone buying a car in the next few years is likely going to face the choice of whether or not to get a turbocharged model. There is no simple answer. It is worth noting, though, that while they offer a great many advantages, particularly in performance, they won’t always live up to their promise of efficiency.

Turbochargers may not be a silver bullet, but they can be an excellent compromise. They offer added power when it is needed — or just wanted — while being able to keep up the mpg when driving normally. Superchargers, on the other hand, are all about 24/7 performance.

For buyers looking to experience big displacement V8 power, a supercharger is an excellent option. Consider for a moment that some of the most exciting cars this year all come with big blowers attached: the new Cadillac CTS-V, the Corvette Z06, and the mighty Dodge Hellcats.

Ultimately, the decision to get a turbocharger, a supercharger, or neither is going to depend on the car and driving style of the buyer. Regardless, forced induction has a lot to offer for nearly any sort of driver.