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The days of 12 volt systems and conventional air-cooled alternators for cars are numbered – radical changes are just around the corner! By Julian Edgar D uring the last 15 years or so there has been a major change in the electronic architecture of cars. The introduction of engine management, anti-lock brakes, traction control, climate control, automatic stability control and similar systems has meant that some cars have as many as 10 electronic control units. These frequently communicate with one another via Controller Area Network or CAN buses and all have full self-diagnostic ability. But while all of these electronic changes have been occurring, the power generation and distribution system of cars has remained static. An air-cooled alternator charging a single 12V battery, with a running-car voltage of about 14V distributed by copper cables, has been the system employed in all cars. However, even this has now started to change – water-cooled alternators are being fitted to some vehicles, combined starter/alternators are being developed and a standardised 42V electrical system is imminent. New Electrical Loads The electrical loads of modern cars have increased dramatically over the 4  Silicon Chip last 10 or 15 years. Luxury cars – especially – are voracious consumers of electrical power. Fig.1 shows the maximum electrical loads of the current model BMW 750iL, which can reach a staggering 428 amps (5.9kW)! So what on earth draws so much electrical power? In this car over half of the maximum power load is from the short-term electrical heating of the catalytic converters. Because this single load is so great, it’s worth looking at in a little detail. Catalytic converters function most efficiently at cleaning the exhaust when they reach a temperature of about 600°C. This heating is normally provided by the passage of the hot exhaust gases through them. However, in cold start conditions, it takes some time for the catalytic Power Consumers             Maximum Consumption                          (Amps) Electric catalytic converter heating element (30 seconds)............................ 240 Engine.............................................................................................................. 23 Suspension......................................................................................................... 4 Body.................................................................................................................. 3 Secondary air pump......................................................................................... 30 Low beam light................................................................................................ 13 High beam light................................................................................................. 9 Fog light............................................................................................................. 9 Brake light......................................................................................................... 4 Reading light...................................................................................................... 1 Fan blower...................................................................................................... 29 Rear compartment fan blower........................................................................ 16 Airconditioning ................................................................................................. 3 Audio systems and telephone.......................................................................... 10 Wiper stage II.................................................................................................... 9 Auxiliary fan..................................................................................................... 25 Total Maximum Consumption.................................................................. 428 Fig. 1: the maximum current consumption of the BMW 750iL is a staggering 428 amps! The short-term load placed by the two catalytic converter heating elements makes up much of this. [BMW] Luxury cars like this BMW 7-series have huge electrical power loads. This requires the use of a water-cooled alternator and twin batteries. [BMW] battery services other loads. Two redundant temperature sensors monitor the temperature of the starter battery; if it falls below 0°C, the electric cat converters are not heated. (Incidentally, US authorities require the use of the two temperature sensors to reduce the possibility of malfunction.) ENGINE CONTROL UNIT ELECTRIC CATALYTIC CONVERTER CONTROL UNIT ELEC.CATALYTIC CONV. PROG TIME CONTROL POWER OUTPUT SWITCH TEMPERATURE CONTROL SECONDARY AIR PROGRAM CAN CAN RECORD. OF MEAS. VALUE DIAGNOSIS DIAGNOSIS SECONDARY AIR SECONDARY AIR PUMP VALVE SECONDARY AIR VALVE * BATTERY TEMPERATURE SENSORS * * BATTERY ISOLATOR SWITCH STARTER BATTERY ELECTRIC CATALYTIC CONVERTER 2 Fig.2: a schematic diagram of the BMW catalytic converter electric heating system. [BMW] h 0A 11 STARTER MOTOR ELECTRIC CATALYTIC CONVERTER 1 LIQUID COOLED ALTERNATOR VEHICLE CIRCUIT LOAD converter to reach operating temperature – in the meantime, emissions are high. Manufacturers strive to reduce cat converter warm-up time by placing the converter close to the engine and by using exhaust pipes prior to the cat converter that have little thermal mass, eg, double skin pipes. However, under high engine loads, these approaches can result in cat converter overheating, with the resultant destruction of the ceramic matrix within the converter. Electric heating of the cat converter results in reduced cold-start emissions, while still allowing it to be placed sufficiently far from the engine to ensure durability. In the BMW 750iL, two catalytic converters are used, both electrically heated for a maximum of 30 seconds. The heating starts once the engine speed exceeds 400 RPM (ie, as soon as the engine is started). A dedicated electric cat converter control unit is used, linked to the engine control unit via a CAN bus. The vehicle uses a two-battery electrical system – a 110Ah battery is used to start the vehicle and provide cat converter heating current, while a second smaller (55Ah) VEHICLE CIRCUIT BATTERY Ah 55 JULY 2000  5 A current car has a wiring loom that in some cases stretches for 2000 metres. Using a higher system operating voltage could reduce the mass of the loom significantly. [DaimlerChrysler] The electric cat converter control system has full self-diagnostic ability, with up to 14 error messages able to be recalled. Fig.2 shows a schematic diagram of the BMW system. While cat converter heating is one of the greatest of the new electrical loads, automotive technology being developed also involves substantially increased electrical demands. These include: • electromagnetic solenoid operation of the valvetrain; • electrically assisted power steering in large vehicles; • brake-by-wire; • ride control systems. In fact, US automotive supplier Delphi predicts that within 20 years, the electrical power consumption of a typical car will have reached 10kW without any form of electrical propulsion being employed and more than double this if the electrical power is used to aid vehicle motive power. Given that 10kW represents a current flow of 725A at 13.8V, it is not surprising that moves are afoot to raise the standard voltage of car electrical systems! 42V Systems About 40 years ago automotive electrical systems moved from a 6V standard to 12V. Now a change to 42V systems is being proposed. While called a 42V system, this uses a 36V battery and 42V alternator output, much like the current system uses a 12V battery but a 14V rail (actually 13.8V) and alternator output. 6  Silicon Chip Two multi-company committees are working on the new standard. At the Massachusetts Institute of Technology, the Consortium on Advanced Automotive Electrical/Electronic Components and Systems includes General Motors, Ford, Daimler-Chrysler, BMW, PSA-Peugeot/Citroen, Renault, Volvo and automotive electronics suppliers Delphi, Bosch and Siemens. In Europe, Sican – an organisation in Hanover, Germany – is working with major German carmakers and component suppliers to formulate the new 42V standard. The commitment to the new standard is high. For example, the French automotive component company Valeo has eight of its nine component divisions working on products using 42V technology. The commercial risks to a car manufacturer of embracing 42V technology and at the same time undertaking a major redesign of all the electronics in the car means that, initially, dual 12V/42V systems are likely to be introduced first. As Delphi state, “The increase in voltage means rethinking and possibly redesigning everything from light bulbs to major components”. Says Daimler-Chrysler: “We have decided to retain a 12V supply so that components in standard use today can remain operable.” The major benefit of the higher voltage is in the reduced current flows that are then possible for the same power consumption. Wiring bundles could be as much as 20% smaller, in turn reducing cable mass and so benefiting fuel consumption and emissions. One need only attempt to pick up the wiring loom of a modern car (which can stretch to a combined length of 2000 metres) to realise that the mass of copper used is considerable. DaimlerChrysler again: “We see the development of a 42V net not only as a technological necessity, but as a contribution to lessening the environmental burden.” A number of approaches to the introduction of 12V/42V architecture are proposed: • Single voltage generation and single voltage energy storage –     a 42V alternator charges a 36V battery which services 36V loads, with a DC/DC converter to charge a 12V battery that services 12V loads; • Single voltage generation and dual voltage energy storage –    a 42V alternator charges the 36V side of a dual 12V/36V battery, with a DC/DC converter to charge the 12V portion of the battery; • Dual voltage generation and single voltage energy storage –    where a dual 14V/42V alternator charges two separate systems, one 12V and the other 36V; • Dual voltage generation and dual voltage energy storage –    where a dual 14V/42V alternator charges a dual 12V/36V battery. Each of these approaches is shown in Fig.3. The current and future technologies which would benefit from the introduction of a 42V architecture are shown in Figs.4 & 5. New Alternator Designs The very high electrical power demand of modern cars is also resulting in the development of more efficient alternator designs. One approach is to water-cool the alternator, circulating engine coolant through passages cast in the alternator housing. In some cases, the alternator may be entirely surrounded by a water jacket. A liquid-cooled alternator design was first introduced (in very small numbers) in passenger cars in 1995. That design used two conventional Lundell-type alternators axially mounted on the one shaft and developed 14V/220A with low noise levels. Subsequently, BMW has introduced (on cars such as the 750iL cited above) a water-cooled alternator that uses a single Lundell brushless design developing 14V/150A. The BMW alternator uses liquid cooling for two major reasons: to reduce the alternator noise associated with normal fan-cooling by up to 3dB and to increase electrical performance. Other advantages of the design include: • rapid engine warm-up due to the utilisation of alternator waste heat; • packaging advantages due to the absence of an alternator aircooling duct; • a longer alternator life; • good fording ability for the car. DC-DC CONVERTER However, I am sure that the last advantage is of theoretical nature only – how many owners would drive a $272,000 BMW 750iL through a river!? When high temperatures are present in the engine bay, air-cooled alternators experience elevated temperatures through heat-soak. As a result, air-cooled alternators are normally significantly de-rated in capacity in order that their temperature rise is not excessive when there are combined high electrical loads and high heat-soak thermal loads. A water–cooled alternator is comparatively insulated from engine bay heat variations and so temperature increases from heat soak do not need to be taken into design consideration. This results in more power being obtainable from the same sized package. The durability of a water-cooled alternator is improved by the reduction in the degree of thermal cycling that the alternator undergoes. Air-cooled alternators experience a rapid increase in temperature immediately following start-up. As the alternator speed then increases, the forced-air cooling system becomes more effective and so the temperature decreases. With water cooling, marked temperature peaks no longer occur, especially in the 2500 – 4000 RPM alternator speed ranges most often used. The possibility of using short-term boosting of the alternator output DC-DC CONVERTER 14V 14V 12V BATTERY 12V LOADS 42V 12V/36V BATTERY 12V LOADS 42V ALTERNATOR ALTERNATOR STARTING MOTOR 36V BATTERY STARTING MOTOR 36V LOADS Single-voltage generation and single-voltage energy storage Single-voltage generation and dual-voltage energy storage 12V LOADS 12V BATTERY 14V 12V/36V BATTERY 14V ALTERNATOR 42V 36V LOADS 12V LOADS ALTERNATOR 42V 36V BATTERY STARTING MOTOR 36V LOADS Dual-voltage generation and single-voltage energy storage STARTING MOTOR 36V LOADS Dual-voltage generation and dual-voltage energy storage Fig. 3: the different approaches that could be taken to adopting 14/42 volt architecture in automotive applications. [Delphi] JULY 2000  7 Current Technology Benefits of 42V Architecture Electric power steering More power, improved fuel economy Electric brakes Redundant power supplies Power windows, power seats, Reduced size and mass of motors; more efficient operation power hatchback lifts Heated catalytic converter Lower emissions; quicker light-off time Heating, ventilation, airconditioning Greater efficiency; smaller/lighter units; flexible packaging blower motors and cooling fans Mobile multimedia More power available for video, phones, navigation systems, audio amps, fax machines Water pumps Improved efficiency; longer service life Selected engine management system Reduced size and mass; increased performance components (eg exhaust gas recirculation valves, ignition systems, control actuators) Fuel pumps Reduced size and mass Heated seats Faster heating, more efficient operation; increased power Fig.4: the benefits to current automotive electrical technology of adopting a 42V system. [Delphi] Future Technology Ride control systems Brake by wire Steer by wire Electromagnetic valve control Integrated starter/generator Benefits of 42V Architecture Improve ride, handling and vehicle stability Improved vehicle packaging and vehicle performance Enhanced performance; improved packaging; improved passive and active safety Lower emissions; optimal power; individual cylinder control; lower cost Faster starts; quicker charging; design flexibility; low noise & vibration; improved fuel economy Fig.5: the benefits to proposed automotive technology able to be realised with a 42V system. [Delphi] reveals another potential advantage of water-cooling. The output of an alternator can be increased by a number of means: • feeding the excitation circuit with an increased voltage supplied by a DC/DC converter; • using a higher amount of excitation by employing an excitation winding layout with lower resistance; • operating the alternator in self-excitation at higher terminal voltage. Since these methods are all limited by heat build-up considerations, the greater thermal mass of a water-cooled alternator has inherent advantages when any of these approaches is taken. 8  Silicon Chip Each of these short-term boost techniques is being actively considered for automotive use. Lundell alternators are also being supplemented by water-cooled hybrid permanent magnet/Lundell designs. Delphi produces one such design, capable of developing 13.5V/150A at 6000 RPM alternator speed. The alternator requires a minimum of two litres/ minute coolant flow at a maximum temperature of 130°C. Fig.6 shows a drawing of the Delphi design, with the coolant connection pipes prominent. Probably the most dramatic development in alternator technology, however, is the Integrated Starter Alternator Damper (ISAD) being developed by German company Continental ISAD Electronic Systems GmbH & Co. The ISAD combines the function of a starter motor and alternator into one assembly, located between the engine and gearbox. Fig.6: a drawing of a Delphi 13.5V/150A water-cooled hybrid permanent magnet/ Lundell alternator. Note the water pipe connections. [Delphi] ISAD is able to generate output voltages of 12, 24, or – significantly – 42V. The device eliminates: • the conventional starter motor and solenoid; the flywheel; • the conventional alternator; • the alternator pulley and belt drive • system; • and in some cases, the harmonic balancer. Both BMW and Citroen have shown vehicle prototypes using 42V ISAD systems. In a car equipped with a 42V ISAD system, each normally belt-driven device could be replaced with an electric motor. In some cases this would have significant advantages – the aircon-ditioning compressor could be located close to the cabin instead of at the front of the engine, for example. Conclusion Cars featuring water-cooled alternators or combined starter/alternators, 42V wiring and much higher electrical loads are likely to be appear in the next few years. No longer will “12V” be synonymous with cars… SC