How Smaller Engines Offer More Power: Superchargers Vs Turbos

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Turbochargers have been with us in volume production only since the Seventies, but superchargers have a long and honorable automotive history.

They were used on the Blower Bentleys that won Le Mans 80 years ago, and have appeared ever since on high-performance engines, often larger ones.

A boost for boost

Now, propelled by new and stricter emissions and fuel economy laws, both types of boost are poised to appear in much greater numbers as carmakers seek to make smaller engines far more efficient.

Ford in particular has branded its latest line of turbo engines as EcoBoost, applying both direct injection and turbocharging across the board, from 1.6-liter fours to 3.5-liter V-6s.

But on a recent visit to Eaton, the automotive supplier that makes more superchargers than any other company, executives argued that supercharging offers numerous benefits over turbos.

Eaton EVS supercharger rotor

Eaton EVS supercharger rotor

Enlarge Photo

Two kinds of pumps

First, a bit of engineering background: Both turbos and superchargers are pumps that compress air and feed it at high pressure into the engine’s intake manifold. That allows more fuel to be injected to keep the fuel-air mixture constant, meaning the engine produces more power.

But superchargers are mechanical air pumps driven off the engine’s crankshaft, via chains, belts, or gears. Turbochargers, on the other hand, are small turbines propelled by the pressure of the hot exhaust gases.

Parasitic loss vs turbo lag

Each has its pros and cons. Superchargers operate even at low engine speeds, but use engine power to do it, meaning they impose a parasitic loss across the engine’s rev range.

Eaton technologies test day, Marshall, Michigan, Sept 2010

Eaton technologies test day, Marshall, Michigan, Sept 2010

Enlarge Photo

They are also usually more compact than turbos, and in a V-formation engine, they can sit in the vee between the cylinder banks.

Turbochargers, on the other hand, have historically operated best at higher engine speeds when the flow of the exhaust is strongest—leading to the phenomenon known as ‘turbo lag,’ in which there’s a short delay before power is delivered while the turbo spools up to its operating speed.

Two turbos required

Turbos are also usually hung off the exhaust manifold, which on all but a very few new engines is on the outside of the cylinder bank.

For a V-formation engine, this means a pair of turbos is required—one per bank. Each outside turbo, along with its associated plumbing, increases the size of the complete engine package.

(Some manufacturers, including GM and BMW, have designed new V-8 engines with their manifolds reversed—intake on the outside, exhaust in the vee—so a single turbo can be mounted between the banks. This is a clever adaptation, but exhaust manifolds between two banks of cylinders can pose enormous challenges in managing such a concentration of engine heat.)

Eaton R410 supercharger

Eaton R410 supercharger

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Adding Audi to Cadillac and Corvette

Eaton believes that as makers seek to make smaller engines work more efficiently, they will find advantages not only in downsizing, but also in “downspeeding”—keeping engine speeds lower to reduce friction losses at higher revs.

And that, says Eaton, is where superchargers win out. The company points to Audi, which extensively tested both turbocharging and supercharging for a new 3.0-liter high-performance V-6 engine program.

In the end, Audi decided that a supercharger provided the best mix of immediate power, low-speed torque, and compact packaging against the pair of turbos that would have been needed.

Its supercharged 3.0-liter engine first appeared in the 2008 Audi A6, and this year, the engine will be fitted to no fewer than eight Audi models, including its S4 and S5 high-performance models.

Supercharging takes the 3.0-liter V-6 from 290 to 430 horsepower, and Audi uses it to replace V-8s of both 3.6- and 4.2-liter displacements. The new engine produces the same performance as the larger V-8s, with fuel efficiency 17 to 24 percent higher.

High-performance hybrids too

An adaptation of the supercharged Audi 3.0-liter engine is also fitted to the 2011 Porsche Cayenne S Hybrid and 2011 Volkswagen Touareg Hybrid, the first hybrid from each maker. An Audi Q5 Hybrid is expected to follow.

In laps around Eaton’s Marshall, Michigan, test track, a current-year supercharged Audi A6 provided a surge of torque from virtually any speed, with the engine characteristics matched by an automatic gearbox that kept the engine operating largely below 4000 rpm.

No comparison or non-supercharged car was available for comparison, but the basic point came across.

Other performance cars that use Eaton superchargers are the Chevrolet Corvette ZR-1; the Cadillac CTS-V sedan, coupe, and station wagon; and the Jaguar XFR and XJR sedans, along with certain Range Rover models.

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