January 2003 Articles & Features
By Mike Dale | January 2003
While 42-volt electrical systems may be further from production than once thought, several innovative products add life to conventional 42-volt systems.
A few years ago, the big Japanese, European and American automakers went to their suppliers with the story that 42-volt electrical systems were the next big thing, and that they had better embrace them if they wanted a share in the future business. The big gamble was that the automakers were expecting the suppliers to fund the R&D costs to invent the new technology.
As we told you more than a year ago, the wheels on the 42-volt bus have a serious case of the wobbles. The expected 20% fuel economy increases didn't happen. When it got down to it, the costs of tooling for whole new products such as the integrated starter alternator were far beyond the resources of either the manufacturers or the suppliers. It didn't help when it was discovered that whole technologies, such as electrical connectors and terminals to withstand the higher arc current potentials, would have to be redesigned.
The aftermath of all of this is a huge problem for suppliers like Delphi, Dana, Denso, Bosch and others. Each of these companies normally devotes a certain portion of its annual revenues toward the research and development of new products. Among them, they've spent billions over the last five years hoping the OEMs were correct in their predictions of the future. What they've got right now is a whole inventory of 42-volt products, without a paying customer in sight.
The good news is that from the ashes of this debacle there are signs of life. At last month's SAE Convergence show on automotive electronics, suppliers showed how they're going to accomplish at least some of the vision of 42 volts within the framework of the current 14-volt system.
The question is how to find the needed electrical power to make things like electrically powered water and oil pumps a reality. The solution has two parts: First, reduce losses in the alternator, especially at idle, so there's more output in the first place; second, actively manage the whole powertrain as a system to best use the electrical power that's available.
One source of power loss and wasted energy in the alternator are the diodes that rectify AC output into DC. You may remember from electronics class that the forward voltage drop (Vf) across a silicon diode is .6 volt. This is a correct number when the current is low. For high-current applications such as the alternator, the voltage drop can be more like 1.2 volts. If you run the math on this (P=IE), the power dissipation in the diode at 75 amps is 90 watts for the time the diode is actually on. When you figure out what all six diodes are losing, you can see not only wasted energy, but unwanted heating of the diodes and the alternator, as well
A number of companies, including Delphi and International Rectifier, are promoting what they call active rectification (AR). In current alternator technology, nothing other than the polarity of the pulse tells the passive diodes when to conduct. In AR, the diodes are replaced with MOSFET transistors that are switched on and off by the energy management computer.
In MOSFET transistors, the equivalent of the forward voltage drop losses in diodes is something called RdsOn-the resistance from drain to source when the device is fully turned on. Squaring the current and then multiplying the result by the MOSFET's resistance tells you what the power loss is going to be.
International Rectifier has something it calls Trench HEXFET technology (read more at www.irf.com). These have RdsOn ratings of 2.3 milliohms at 75 amps. When you run the math (P=I2R), you'll see that the loss for a 75-amp pulse is 12.93 watts. Compared to the 90-watt loss in the diode, this is a huge improvement.
International Rectifier has packaged its new technology into a device called Active Integrated Rectifier Regulator, which is in production now on the new super car from DaimlerChrysler called the Maybach. This device is a combination voltage regulator, rectifier group and control electronics, all in one package. In this application, the alternator has an output of 200 amps at idle and 350 amps at 6000 rpm.
In addition to having much lower electrical losses, the MOSFETs, under the control of a central computer, can be turned on and off at any point in the rectification cycle. This is especially useful at idle, where International Rectifier says it can increase the output by as much as 25% above what's available today.
Siemens VDO showed a new software package at Convergence that controls the electrical loads and the alternator, determining what loads have to be switched off so that power needed for power steering is available. It's called Integrated Power Train Management (IPM). "New alternative fuels and energy generation and storage systems each bring their own advantages to powertrain architecture," said Siemens VDO business development manager Alex Saari. "However, it is only through software control that you can make the whole powertrain strategy greater than the sum of its parts."
Siemens VDO presented its findings from tests performed on a European-built 1.6L vehicle equipped with an integrated starter generator, automated manual transmission and a dual energy storage system that has both a lead-acid battery and a pack of the new super capacitors called DLCs. The purpose of the software is to link all of these pieces together in ways that maximize the performance of each subsystem without harming the performance of the vehicle or any of the subsystems. Siemens VDO said it saw a 16% to 18% improvement in fuel economy as a result.
How the load management software will work can be seen by looking at what Dana was showing at its booth. Dana thinks its intelligent cooling system can be converted back from 42 volts and run in today's 14-volt systems.
The system is made up of five separate pieces-an electric water pump, a multiport proportional flow control valve to replace the thermostat, a cylinder head gasket with built-in thermal sensors, a variable-speed electric cooling fan and a computerized control system. In this system, the head gasket is fitted with temperature sensors that directly monitor the coolant, oil and combustion chamber temperatures. The electronic control system then decides how much pumping needs to be done, how much cooling fan action is needed and the position of flow control valve. The decisions can be made on the basis of need as well as available energy
There are three other advantages besides the reduction in wasted pumping effort by the conventional fan-belt-driven water pump. First of all, by not pumping coolant during warmup, the time for the cylinder to warm up to stable operating temperature (212�F) can be cut from 220 seconds to 120 seconds-good news for emissions performance.
Second, getting rid of the thermostat by using an electronically controlled valve is a good idea. The traditional wax thermostat has major disadvantages in that it's slow to respond and significantly restricts fluid flow even when it's fully open. This restriction adds to the power consumption of the belt-driven pump.
The third advantage is the elimination of something called afterboil. When the engine is shut down after a period of use, there's a spike in the coolant temperature that can last for ten minutes or more and can amount to as much as 59�F over normal coolant temperature. In the case of the electrically controlled cooling system, it's possible to keep the water pump circulating coolant, thus preventing afterboil
It seems unlikely that some of the 42-volt ideas will happen in the 14-volt world. Electronic valve actuation, heated window glass and sealed, electrically driven a/c compressors are probably not feasible. However, there was-and still is-a lot of momentum behind 42 volts. There's some talk about 42-volt vehicles for the 2008-2010 timeframe, so we may still see these features at some point.