Monday, July 21, 2008

Tweaking the Hemi

Ralph Gilles, chief designer on the 2009 Dodge Ram, and his team turned to aerodynamics to boost the pickup's mileage. An associate says Gilles has made aerodynamics a "religion" in Chrysler LLC's design department.

The truck, redesigned for 2009, emerged from more than 200 hours in a wind tunnel with a coefficient of drag of .419 for the Ram 1500 Crew Cab 4x4. That compares with a coefficient of .463 for the 2008 Ram Quad Cab 4x4. For some pickups, coefficient of drag can be 0.5 or higher.

Chrysler engineers also tweaked the Hemi engine's cylinder cutoff system so it operates in a wider rpm range, enabling the vehicle to run longer in four-cylinder mode.

"There's more torque in the four-cylinder mode," says Mike Cairns, Ram chief engineer. "So you're able to propel the vehicle in four-cylinder mode more often."

Also, compression is up, and the efficiency of air and fuel flowing through the engine has been improved. The Hemi added the fuel-saving feature of variable valve timing, which changes the time the valves open and close to let the engine run at peak efficiency.

Then the pickup went on a diet. Engineers cut out 80 pounds, which might not seem like much on a 6,700-pound vehicle. But the weight loss, engine tweaks and improved aerodynamics combined to achieve a 1 mpg improvement in fuel economy. That may seem paltry, but as Chrysler spokesman Bryan Zvibleman says, "We're fighting for every mile per gallon in this ultracompetitive segment."

Little changes add up, says Derrick Kuzak, Ford group vice president of global product development. He says he tells his team, "We need to treat every joule of energy in a vehicle like a precious commodity."

That means improving "every element of a vehicle, whether it's aero, rolling resistance, powertrain," driveline efficiency, or energy drains that don't move the vehicle, such as power windows or heated seats, he says.

"All of those, we're working on; we benchmark; we're improving."

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