Pushing Air: More Power Through Forced Induction

By Zach Bowman

A typical turbocharger

Manufacturers all over the globe are scrambling to get the absolute best fuel economy out of their engines thanks to ever-wavering gas prices and a federal choke-hold on mileage requirements. While car companies in Europe have been slowly tweaking their engines over the past few decades to get the most efficiency possible out of them, the U.S. has lagged behind. The result is a slew of new-to-us tech popping up left and right. There’s more out there than just direct-injection. Automakers are starting to take forced induction seriously as a way to create more power out of smaller engines, thereby saving fuel.

The original MINI Cooper S
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Small European gasoline and diesel vehicles have benefited from forced induction for years as a way to sneak under displacement regulations and still produce the kind of power buyers want. In a forced induction engine, air is forced into the engine’s combustion chamber at a higher pressure than normal. The result is more compression, more oxygen and thereby more power with every stroke of the engine. Though forced-induction engines can be found on everything from massive diesel-powered ships to prop-driven aircraft, there are only three basic methods for internal-combustion engines: the mechanically driven supercharger, the turbocharger and the pressure-wave supercharger.
Of the three, the mechanical supercharger is likely the oldest, with the first patent going to Gottlieb Daimler in 1885. Daimler was a mechanical genius – helping pave the way for the modern internal-combustion engine and eventually founding the forefather of the automotive leviathan currently known as Daimler Auto Group. Daimler was instrumental in the production of the world’s first supercharged production vehicle – a 1923 Mercedes. Thanks to the bit of air-cramming tech on top of the four-cylinder motor, the car boasted a whopping 20 horsepower. That may not seem like much, but it was more than double what the naturally aspirated version produced.
While there are several variations on the mechanical supercharger these days, the principals in each are basically the same. Mechanical units are typically driven by a belt that runs from the vehicle’s crank pulley to the supercharger itself, using power from the engine to turn a set of turbines. Those turbines spin at an incredibly fast rate – sometimes as high as 15 times the normal rotational speed of the engine – to compress the air as it flows into the combustion chamber. 
There are plenty of benefits to this type of system. For one, power is instantaneous because the unit’s drive is directly connected to the motor. Mechanical superchargers are also the most efficient of all the forced-induction options out there – delivering the most power per revolution. Unfortunately there are also drawbacks, the biggest of which is price. Mechanical superchargers are incredibly complex, requiring special materials and the tightest of manufacturing tolerances. All of that equates to a pretty steep price tag that’s sure to keep carmakers shy. The other big issue with mechanical superchargers is the amount of parasitic drag they inflict on the engine. It takes horsepower to turn the compressor, and while the unit eventually overcomes that loss, it’s gusto that will never make it to the ground.
Mechanical superchargers tend to find their way onto high-dollar, high-horsepower sports and muscle cars. Since they tend to operate best on big displacement engines, the V-8 is the tried and true home for this type of forced induction with cars like the Cadillac CTS-V and the Chevrolet Corvette ZR1 boasting the technology. In both of those cases, the blowers (as superchargers are commonly known) exist not to better mileage, but for the sole purpose of making exorbitant horsepower.
Next on the list is the turbocharger. This version of forced induction relies on the speed of spent exhaust gasses to compress air on the intake side. The process recoups some unused energy as air rushes out of the engine. The unit can basically be broken down into two halves, with one containing an exhaust-gas turbine and the other a compressor wheel for fresh air. The turbine is exposed to the exhaust gasses, and spins proportionally as they rocket by on their way toward the tail pipe. On the flip side, the compressor is attached to the turbine by a shaft, and as one spins, so does the other. This compresses the fresh air before it enters the combustion chamber, just like on a mechanical supercharger.
The main difference is that a turbocharger isn’t physically connected to the engine’s revolutions in any way. Instead, it must rely on the speed of the exhaust gasses to generate boost, or higher compression. That means that there can be a delay between when you put your foot down and when the real power kicks in, especially in large units that generate lots of boost. Smaller turbos can keep that performance gap, or lag, to a minimum, and some manufacturers incorporate two turbos (one big, one little) to deal with the problem. 
Turbochargers offer plenty of benefits to tuners and manufacturers alike. For one, the units are simpler compared to their supercharger brethren. Simple means cheap, a fact that everyone can appreciate. What’s more, a turbo doesn’t draw any power from the engine to begin with, it simply adds power, and setting up a motor for turbocharging is also easy and inexpensive.
Domestic manufacturers are just now embracing turbos as a way to get V-8 power with V-6 fuel economy. First and foremost on that front is Ford with its new generation of Ecoboost engines. The company has already created a twin-turbo V-6 version with around 355 horsepower that manages to land fuel economy in the mid-to-high 20 mpgs. What’s more, the company is working on a twin-turbo Ecoboost four-cylinder variant that will be similarly potent and fuel-thrifty.
Last on the list is the pressure-wave supercharger. Ferrari was one of the first companies to experiment with using this type of set up back in the ‘70s, eventually abandoning it for the simpler twin-turbocharged setup on its 126C Formula 1 car. Though only a handful of production cars have employed the pressure-wave model, it remains an important cross between the standard mechanical supercharger and the exhaust-gas driven turbocharger.
There’s no easy way to explain how a pressure-wave supercharger works, so we’ll just dive right in. Think of an empty soda can on its side with a hole approximately the same size as the open pour spout on the bottom, but set off just to the side from the top hole. Now imagine that inside there are four small cylinders the same size as holes in the soda can. The cylinders can move from top to bottom inside the can, but they also rotate on a shaft around the center of the can. This way, each cylinder can open toward the top hole and then the bottom hole has it go ‘round and ‘round.
Now you’ve got the basic idea of a pressure-wave supercharger compressor assembly. A belt (or chain) runs from the engine’s crank shaft to the center shaft on our soda can, spinning the inner cylinders proportionally to the engine speed, just like in a mechanical supercharger. Here’s the main difference: Hot exhaust gasses fly from the exit side of the engine into the top hole on our soda can, compressing the clean air below with the first cylinder. From there, the first cylinder rotates all the way around to the bottom hole, where the fresh air rushes into the combustion chamber. The system can then draw in more uncompressed air and the cycle continues. Complicated enough for you?
Needless to say, the system incorporates lots and lots of moving parts, which are expensive to produce. It’s no surprise that manufacturers aren’t exactly quick to embrace this kind of forced induction.
Though purists will claim that nothing can beat the reliability and pure power of a naturally aspirated motor, there’s no denying the benefits of forced induction. Modern production methods can ensure turbocharger and supercharger units that are as reliable as the engines themselves, and similar power for significantly less fuel will win out every time. It wouldn’t surprise us to see superchargers and turbochargers become more and more common in the coming years. 

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