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Machine Design

Supercharging the Family Car

Carmakers are turning to low-cost, quiet, and reliable superchargers for performance.

Ken Streeter
Supercharger Unit
Eaton Corp.
Cleveland, Ohio

Ask a friend what comes to mind when you say “supercharger” and you’ll most likely hear “hot rod” or “top fuel dragster.” Odds are slim to none that your friend will say “fuel-efficient family car.” That’s because most people who have heard of superchargers think they’re noisy, unreliable, fuel-consuming relics of the 60s. But thanks to advances in materials and manufacturing, today’s superchargers are quiet, efficient, reliable, and powerful. And they’re gaining popularity as original equipment on sports cars, trucks, sport-utility vehicles, and even family cars.

What’s boosted this interest in superchargers among OEMs? Demands for more horsepower and increased fuel efficiency, and more stringent emission and engine-displacement regulations. Additionally, continued refinements on the venerable Roots-type blower have helped make supercharging a realistic option for mass-production automobiles.

In the 50s and 60s, American carmakers offered a large variety of V8 engines in their cars, and consumers quickly learned that not all V8s were created equal. A bigger V8 was more desirable than a smaller V8; thus a 351 was better than a 289, and a 428 was better yet. But counting cubic inches became a moot point with the advent of metric nomenclature for engine displacement, so consumers learned to count cylinders. And although engine manufacturers have made great strides wringing more power out of high-revving, multivalve four-cylinder engines, most consumers figure that a six-cylinder engine is better than a four cylinder, and an eight cylinder is even better.

And for nonsupercharged engines, bigger usually is better. But a supercharger changes everything. Studies by Mercedes-Benz, for example, show that a supercharged four-cylinder engine provides more horsepower and better fuel economy than either a nonboosted four or six-cylinder engine. Similarly, supercharged six-cylinder engines outshine normally aspirated eight-cylinder engines when it comes to power, torque, and fuel economy.

R&D has eliminated most of the durability, noise, and efficiency problems associated with superchargers and helped spur renewed OEM interest in them. Supercharger performance, for example, is limited in part by the clearances between the moving rotors and the stationary housing. Tightening up these clearances reduces leakage and losses, but if those clearances are too tight or there’s a manufacturing flaw, the rotors can contact the housing or each other, causing severe wear and failure.

Computer simulation, used extensively in the development of the latest generation of superchargers, let engineers quickly and inexpensively evaluate, and then avoid, clearance conditions that could potentially also traditional problems on early superchargers, have been refined to the point that they now require no external lubrication. They’re also designed and built to last the life of the engine. OEMs usually use stronger crankshafts, connecting rods, valves, and pistons on supercharged engines, and, as a result, generally report fewer warranty claims on boosted engines than with the normally aspirated versions of the same engine.

Early supercharged cars often screamed like a banshee, forewarning all within earshot that something wicked was quickly coming. While appreciated on a hot rod or sports car, most family or luxury car owners would consider such a characteristic an unwelcome addition. This noise has virtually been eliminated by refining the rotor profiles and balancing the airflow at the supercharger inlet and outlet ports. Traditional Roots-type blowers have two twin-lobed rotors to move air. Engineers at Eaton added a third lobe on each rotor and gave each lobe a 60° helical twist. The helix reduces noise and improves efficiency.

To reduce noise even more, engineers studied the port design using prototypes and computer models. They learned that noise could be minimized and efficiency maximized by lengthening the time it takes for the supercharger to compress the air. Proper port design, coupled with longer backflow compression, cut outlet pressure variations in half and greatly reduces noise.

Roots-type blowers are inherently more efficient than other types of superchargers during nonboost conditions because there is no internal compression. Eaton superchargers use a bypass valve to further enhance system efficiency. The valve, actuated by either a vacuum or electric motor, recirculates supercharger airflow when boost isn’t required. When the supercharger isn’t being used, which is about 95% of the time for most drivers, the engine performs like a naturally aspirated engine with minimal loss in fuel economy. The supercharger’s parasitic power loss under these conditions is less than 0.5 hp. Essentially, the Eaton supercharger provides the power-on-demand of a large engine and the fuel efficiency of a small engine.

While big displacement engines are still plentiful and relatively unregulated in North America, regulations in Europe, Asia, and South America place expensive premiums on any engine with more than two liters of displacement. In addition, the high price of fuel in these places encourages consumers to purchase more fuelefficient engines.

To improve performance on smaller engines without adversely affecting fuel economy, engineers are working on a new generation of superchargers designed for engines that displace from 0.6 to 1.3 liters. Eaton’s developmental M20 supercharger, for example, could provide a generous performance boost to the “city cars” that many European OEMs build. Although consumers no longer count cubic inches, Eaton’s supercharger engineers do. The M20 designation means that the supercharger, which is little larger than a paperback book, moves 20 cu in. of air with each revolution of the rotors. Additionally, the M20 housing will be able to use rotors of varying lengths to displace 16, 20 or 24 cu in. of air, making it adjustable for different sized engines.

Another development on the horizon is a supercharger integrated into the intake manifold. Currently, all superchargers are mounted directly on top of the intake manifold or bolted to the engine like an accessory. Air ducts connect them to the manifold. If the intake manifold and supercharger housing are cast as one piece, however, it would have a simpler airintake system. It is also feasible to integrate other components, such as the bypass valve and EGR and PCV hardware, into the supercharger housing. These concepts require further development, as well as direct input from engine designers and builders.

What is a supercharger?
The Roots-type supercharger is a positive-displacement pump that increases air pressure and density in the intake manifold. The supercharger is matched to the engine by its displacement and belt ratio, and can provide excess airflow at any engine speed. Superchargers pack more air into the cylinders, which, when mixed with fuel, yields a more powerful combustion stroke and more engine power.

Supercharger, turbocharger, what’s the difference? Superchargers are driven by a belt connected directly to the crankshaft, while turbochargers are driven by exhaust gases. Superchargers provide improved horsepower and torque at all rpms, including lower ones, by pumping extra air into the engine in direct relationship to crankshaft speed. This direct connection yields instant response. Turbochargers must overcome inertia and take time to spin up to speed as the flow of exhaust gas increases, thus exhibiting the phenomenon known as “turbo lag.”

Eaton superchargers are self-contained, “sealed-for-life” units that require no external lubrication, while turbochargers are usually cooled and lubricated with engine oil. Finally, superchargers have no effect on emissions. Turbochargers, on the other hand, increase cold-start emissions because exhaust-gas energy that’s normally used to

© 2010 Penton Media, Inc.

TAGS: Automotive
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