CHAMPCAR/CART: Mercedes' Pinwheel of Performance

MONTVALE, N.J. (Aug. 5, 1998) -- If you can imagine a child, lips puckered, cheeks puffed out, blowing on a pinwheel, you already understand the basics of turbocharging. The Garrett turbochargers which will assist Mercedes-Benz...

MONTVALE, N.J. (Aug. 5, 1998) -- If you can imagine a child, lips puckered, cheeks puffed out, blowing on a pinwheel, you already understand the basics of turbocharging.

The Garrett turbochargers which will assist Mercedes-Benz racing engines in propelling eight FedEx Championship Series drivers to speeds in excess of 190 mph at the Texaco/Havoline 200 at Road America in Elkhart Lake, Wis., Aug. 14-16, are a far cry from children’s toys, but the principle is the same. "The turbocharger, its design and its interaction with the engine, was one of the fundamental elements we considered when designing and developing the Mercedes-Benz IC108E Champ Car engine," said Paul Ray, vice president of Ilmor Engineering, the race engine design and building arm of Mercedes-Benz. "At the boost levels we’re running in CART, an inch of boost (0.49 p.s.i.) can be worth just over 20 horsepower. That means a properly functioning turbocharger can increase power output by 35 percent. If you get it wrong, you give some portion of that up, and then it’s absolutely impossible to be competitive."

What it Is and What it Does

A turbocharger is basically two pinwheels connected by a shaft. One wheel, known as the hot wheel, is a turbine. The other wheel, known as the cold wheel, is an air compressor. Like the child blowing on the pinwheel to make it spin, hot, high-pressure exhaust gases leaving the engine are ducted to blow on the hot wheel. When that exhaust spins the turbine (hot wheel), the connecting shaft spins the compressor (cold wheel) at the same rate -- up to 80,000 rpm. This action forces fresh air into a pressurized tank on top of the engine, known as the plenum.

"The power an engine can produce is basically limited by how much air you can get into the engine," explained Doug Milliken, manager, motorsports turbochargers, for AlliedSignal Inc., Turbocharging Systems, manufacturers of Garrett turbochargers. "A turbocharger basically takes the energy in the exhaust, which would normally be wasted, and harnesses it to power an air compressor, force-feeding air into the engine. However much you increase the air flow into the engine, you can proportionately increase fueling, resulting in a more powerful combustion event and more horsepower."

Boost Regulation

Because turbocharging has such a profound effect on engine power output, the Champ Car governing body, CART, very carefully regulates the boost pressure generated by the turbo. The manifold pressure relief valve (a.k.a. popoff valve) sits atop the plenum and is designed to open, bleeding off air should the pressure inside the plenum exceed the maximum allowable 40 inches of Mercury (19.65 p.s.i. Absolute).

"In the early days of turbocharged Champ Cars (the first methanol-fueled, turbocharged Champ Car appeared at the Indianapolis 500 in 1966), the popoff valve was the only regulator of the boost pressure," said Milliken. "But when the popoff valve would blow, you’d lose so much power that you could spoil a qualifying lap or lose a race in the process."

Teams needed a way to more finely control the boost without "blowing the valve." The answer was wastegates, spring-loaded valves which divert some of the exhaust gases away from the turbine and out through the exhaust pipe. Just as the pinwheel would slow down if the child didn’t blow as hard, the hot wheel slows as less of the exhaust flows to the turbine. Consequently, the cold wheel slows, forcing less air into the plenum.

Initially, the wastegates were regulated manually by a knob in the cockpit. A tube ran from the engine compartment to the driver’s helmet, allowing him to listen for a telltale whistle of air escaping as the popoff valve began to lift off of its seat on the plenum. Upon hearing that sound, the driver would "dial back" the boost using the boost control knob.

These days, Mercedes-Benz Trackside Support Engineers monitor the boost pressure and popoff valve by watching telemetry data in the pits and advising the team on boost adjustments. The adjustments are still made by the driver but are now accomplished electronically -- through the Magneti Marelli engine management system -- by pushing a button on the steering wheel, rather than mechanically, by turning a knob in the cockpit.

All this from a little pinwheel.

Mercedes-Benz Notes: Turbocharging vs. Supercharging

The terms supercharger and turbocharger are related but not synonymous. A turbocharger is a type of supercharger, but a supercharger is not necessarily a turbocharger. Generally speaking, a supercharger is driven by the engine crank shaft, while a turbocharger is driven by exhaust gases.

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Series IndyCar