CSAs, 36-volt batteries, and 42-volt electrical systems are the current of the not-too-distant future.
BMW Gets Juiced
by Bill Siuru
As more electric power-hungry goodies are added to BMWs, it is clear that today's fourteen-volt DC electrical systems and twelve-volt batteries will not be able to handle all the power demands. Also, luxury and high-performance cars now carry nearly 100 pounds of batteries, and this could grow - hardly in keeping with the goal of weight reductions to achieve more mpgs. Further compounding the problem will be the many pollution-reducing technologies like catalytic converters that are preheated to reduce emissions at startup, and electromagnetically controlled valve trains that will require substantial amounts of electrical power.
According to Axel Diecke, who heads up automotive electronics at BMW AG, today's vehicles need around two kilowatts of electric power. A big demand comes when the engine is not running and all power is supplied to the battery. Almost as demanding is stop-and-go driving in heavy traffic with the engine idling or running at low rpm; under these conditions, today's fourteen-volt systems are at their capacity limits. Also, continuous cyclic draining of the battery when the alternator cannot handle the load and then recharging when the car is moving reduces battery life.
Being belt driven, current alternators at best can supply between two and three kilowatts. Larger alternators, though possible, result in more inertia: unacceptable because its reduces an engine's ability to rev freely. Alternators are typically only 60 to 70 percent efficient, with the remaining 30 to 40 percent wasted in the form of heat - which adds to the already heavy heat load found into today's engine compartments.
One solution is the crankshaft starter / alternator (CSA), which is driven directly off the crankshaft, eliminating the belt drive. BMW's prototype CSA, which is located between the engine and transmission, received a "Best of What's New" award from Popular Science in 1998. The CSA concept is definitely not new to BMW; they experimented with "dynamo-starters" in the mid-1930s and used one, called DynaStart, in the Isetta, the 600, and the 700. DynaStart was used in other contemporary cars with small engines like the DKW, Berkeley, and Messerschmitt.
BMW engineers says CSAs can operate at more than 80% efficiency. They provide quieter starts because there is no starter pinion that must mesh with the flywheel teeth. The CSA also helps reduce start-up emissions, the focus of much current emissions R&D, because it can much more quickly rev the engine up to operating speeds with much lower emissions. Finally, the CSA is just the ticket for the stop-start engines that will be appearing soon; one way to reduce fuel consumption and emissions is to shut the engine off when not needed, even for short periods such as at traffic lights.
Even bigger changes in electrical systems are coming - significantly higher voltage systems. Recognizing future electric-power shortfalls, the MIT / Industry Consortium on Advanced Automotive Electrical / Electronic Components and Systems was established a few years ago to find solutions. Sponsored by the Society of Automotive Engineers, the consortium includes 44 automakers, including BMW, and their suppliers. There is a similar group in Europe. The consortium is advocating a 42-volt system with a 36-volt battery.
Besides making more power available for new technologies like electrically controlled valves and electric suspension systems, 42 volts could bring big changes in how things are done under the hood. Forty-two volt systems could eliminate power-robbing and heat-producing belt-driven water and power steering pumps as well as the air conditioning compressor. Future steer-by-wire and brake-by-wire systems with electric motors replacing hydraulic pumps would consume power only when turning and stopping. Electric water pumps would pump coolant at low rpms, or even when the engine is off, when cooling is usually needed most. Electrically powered air conditioning and heating will provide climate control even when the vehicle is parked with the engine off. Belt-less systems would also allow designers much more flexibility because they could be placed just about anywhere in the vehicle, not just in specific locations in crowded engine compartments. As an added benefit, wiring cable bulk and weight can be reduced by half to two-thirds because the higher-voltage system's lower current means smaller-diameter wiring. Dr. Peter Thoma, Director of the Electric / Electronic Development Department at BMW, predicts that an all-electric vehicle is possible before the end of the decade. This, of course, would be a true BMW with an internal combustion engine under the hood, but with everything else powered by electricity - not a golf-cart-like EV.
Switching to 42 volts requires changing virtually every electric component from connectors and wiring to alternators and switches. A complete switch to a 42-volt system immediately is not practical; initially, dual or hybrid systems will probably be used with 42 volts available for "power hungry" systems - the Audi A8 already has a 42-volt system for its windscreen heating - and fourteen for lighting and low-power electronic components and AM/FM/stereo systems.
There are a couple of ways to do this. BMW favors a 42-volt system with a single 36-volt battery; DC/DC converters would be used to drop voltage to fourteen volts. Other automakers are considering dual systems that would require both twelve-volt and 36-volt batteries. Axel Diecke estimates that BMW will introduce the first 42-volt system around 2003. Undoubtedly, it will be in a top-of-line model like the 7 Series where the power demand is the greatest - and buyers are willing to pay a premium to get it. |
