November 2002

Why Not a 40-MPG SUV?

Continued from page 1

By Mark Fischetti

Fast Starts

Greater fuel efficiency starts with some seemingly simple ideas, for example, shutting down the engine to eliminate wasteful idling whenever the car is not moving. But for an engine to shut down at every red traffic light, the vehicle would need a high-power device able to restart the engine immediately-far faster than a traditional starter motor-when the driver taps the gas. Sounds straightforward. But early versions were generally too slow or too noisy to keep drivers happy.

That's changing. Several auto suppliers have built prototypes of integrated starter-generators, which replace both the starter motor and the alternator, that are fast enough to crank the engine in less than half a second (see "An End to Idling," below). "The big issue is reducing cost," says Thomas Keim, an MIT electrical engineer who directs an MIT-industry consortium on advanced automotive electronics.

Illustration by John MacNeil

Most designs use induction motors, which must be controlled by expensive electronics that can rapidly switch between the starter-generator's dual roles of starting the engine and acting as the alternator to generate electricity. But the MIT consortium has developed a starter-generator with a simpler version of the electronics, which could reduce the cost by 20 percent. "In the automotive world, a technology that is 20 percent cheaper tends to drive the more expensive choice out of the market," Keim says. Ford has built a prototype of the consortium's design and has completed initial tests. But there's another roadblock: the difficulty of generating enough power to keep such devices humming. Rapid cranking takes lots of power. Like most other experimental starter-generators, the MIT device, built in anticipation of a future where cars employ a higher voltage standard, operates at 42 volts. Trouble is, virtually all of today's cars still use a 12-volt system. (Exceptions include gas-electric hybrids and a 42-volt luxury Toyota sedan sold only in Japan.) Tethering a starter-generator to a 12-volt system is theoretically possible, but the starter-generator is just one of a growing number of advanced automotive electrical components on industry drawing boards. Taken together they exceed the system's limits. Some industry experts say the voltage shift will take years: replacing an entire electrical system-which in a new car generally costs as much as the engine and transmission combined-is an expensive and complex task. But even without a complete replacement, a stopgap approach could provide a voltage boost. Manufacturers are crafting ways to install 42-volt systems beside existing 12-volt systems. The existing low-power system would remain in place, continuing to provide electricity to familiar light-duty equipment such as lights, radios, power seats, and window motors. The new high-power system would serve only heavy-duty equipment such as starter-generators and electric compressors. The dual system "would obviously add cost," says Xingyi Xu, an engineer at Ford Research Laboratory in Dearborn, MI. Automakers would need to see considerable gains in fuel economy and consumer conveniences, he says, so "at the present time it is difficult to make that justification." Even if justifiable, such systems would take at least five years to reach market, Xu says. But because they can provide the power needed to transform the average car from a mechanical into a more efficient electromechanical machine, 42-volt systems represent an enabling technology.