Heated seats, antilock brakes, navigation systems, traction and stability control. To a driver, they are mere buttons on a panel, but in combination they pose a major headache for a vehicle's electrical system.
Automakers such as BMW AG know the electrical demands in their vehicles are rising and that traditional air-cooled alternators are struggling to keep up.
The electrical equipment on BMW's top models once used less than a kilowatt of power; now they consume more than two kilowatts. Their alternators are running progressively hotter and louder as they are enlarged to meet demand.
So, like its competitors, BMW is studying high-power technologies for the next century, such as dual-voltage electrical systems and flywheel starter-generators. But that hardware is still locked in the lab.
To keep the lights from going dim in the meantime, BMW is equipping all its V-8 and V-12 1999 models with a new liquid-cooled alternator from Robert Bosch GmbH. BMW is the first automaker to use the design.
90 TO 150 AMPS
The new 14-volt alternator is rated to deliver 90 amps of current at idle and up to 150 amps at maximum speed, which makes it capable of generating more than 2.1 kilowatts of steady power, Bosch says.
Unlike most alternators, it is brushless; it relies on noncontact electromagnetic induction to charge the rotor. The components are sealed in a waterproof casing that is bolted directly to the engine block and has cast-in galleries for the engine coolant.
BMW first sampled the unit in 1996, equipping special armor-plated versions of its sedans with Bosch's liquid-cooled alternator. In those vehicles, passengers are shielded from bullets and bombs, but the high-output alternator can bake from the lack of air flow behind the protective plating. The test was successful, so BMW slipped the unit into regular production this year.
The benefits of liquid cooling are multifold, says Tobias Heiter, the Bosch departmental manager who oversaw development of the alternator. Internal temperatures, which can reach 338 degrees Fah-renheit in an air-cooled unit, are held down by 86 to 104 degrees through liquid cooling, he says.
CUTTING THE CLATTER
The design also protects alternator components from the heat generated by catalytic converters, which are creeping closer to the engine to achieve fast light-off.
As alternators grow, the noise generated by their magnetic fields and cooling fans increases. 'In a 150-amp (air-cooled) alternator, you will hear the noise from the fan before you hear the engine,' says Heiter.
BMW's engineers considered clattering electrical components unbecoming in a $60,000 luxury car, so liquid cooling was adopted, partly because it insulates and quiets the alternator.
Another benefit is the cold engine's quick warm-up as it draws warmed coolant from the alternator housing. European regulations now stipulate how quickly the vehicle's defroster must clear the windshield, and the alternator helps.
As for cost, Heiter will say only that it costs more than a conventional air-cooled alternator and less than a starter-generator.
DaimlerChrysler has expressed interest in the design, and BMW plans to use the liquid-cooled alternator next on a Land Rover. The sealed case will keep water and dust out of the componentry, Heiter says. He adds that in two to three years, 'we expect to be making 350,000 pieces per year.'