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Not Just Low Pollution: No Pollution! Not long after the turn of the century, many vehicles will be required to emit not just low emissions but zero emissions. Battery power seems the way to go but currently the technology simply doesn’t exist to make it happen, especially for heavy vehicles. However, two European companies, ABB & Volvo, might have the answer with their new hybrid drive system. 14  Silicon Chip Hybrid Heavy Power For Vehicles January 1997  15 T HE MAJORITY of man-made emissions responsible for polluting our cities come from cars, trucks and other road vehicles. These offer greater flexibility in the urban transportation sector than the other major land-based transport system, the railways, most of which are electrified today in cities and are therefore less polluting. As a result, the emphasis around the world is to make road vehicles less polluting. Significant advances have been made in recent times but in many areas, not enough: new legislation in California, for example, will require 10% of all cars entering the market from 2003 to have zero emissions. Most large vehicles on the road today run on diesel fuel. Recently clean-air legislators have started calling for heavy vehicles to be more environmentally compatible. However, that is not simply a matter of lowering air pollution levels through the reduced emission of nitrogen oxides, hydrocarbons and suspended matter. Other factors such as the choice of materials, recycling potential and noise emissions have to be considered. Unless some breakthrough is made in the next few years, zero emissions Fig.1: block diagram of the drive and control system used in the concept vehicles Estop (at least as far as the vehicle itself is concerned) translates to battery powered vehicles. While battery power might become practical for cars and small vehicles, at the moment that is not the case, nor is it even on the horizon for larger vehicles carrying freight or passengers – trucks and buses, for example. In the past, a large-scale shift to electric drives has failed mainly due to suitable rechargeable batteries being unavailable. For electric vehicles to travel acceptable distances without having to be charged too often, their batteries would have to be so large that they would seriously reduce the payload space. The best solution, at least in the foreseeable future, is a hybrid vehicle, one which can operate from battery power in areas where zero emissions are required (eg in central cities) but switch to conventional or non-conventional motorised propulsion (albeit of low pollution) outside those areas. Conventional internal combustion engines (diesel or petrol) are not really a proposition because even the best designs cannot, at least currently, achieve low enough pollution levels. One proposal by ABB and Volvo is for a high performance hybrid drive Main control unit ( MCU ) Mode selector Ignition key Vehicle management unit ( VMU ) G Gas turbine Overvoltage protection ( OVP ) Box Y1 Rectifier 16  Silicon Chip To be commercially acceptable, hybrid vehicles have to perform as well as any modern, conventional road vehicle. Therefore a hybrid bus must be capable of about the same performance as a 'normal' city bus. The concept vehicles were designed for a speed of 100km/h on the level and 80km/h on a 2% gradient (1 in 50). This meant that the drive needed a continuous output of 100kW and a maximum output of 150kW. The same maximum output, although only for a short time, is also required when the vehicle is run off the battery alone. In a hybrid drive vehicle, the gas turbine can be shut down and the vehicle run from the battery alone; ie, with zero emissions. The battery-only range specified for the hybrid truck was 25km, with a minimum of 5km Brake pedal Estop CAB M/41 motor Inverter S7 Inverter S7 Inverter S7 Box Y2 GT starter inverter Battery management system (BMS) Acc. pedal Development goals Motor controller ( MPS ) Maincharger HSG module ‘Gear switch’ consisting of a gas turbine, a high speed generator and a battery. This new drive is designed to meet the stricter requirements of future clean air legislation. ABB & Volvo have produced two 15-tonne concept vehicles using such drives. The Environmental Concept Truck and Bus (ECT and ECB) were both designed especially for use in urban areas. DC/DC converter Battery Battery Auxiliary power supply Auxiliary Battery systems Transmission and axle Fig.2: low-emission concept bus and truck, each with a hybrid drive developed especially for urban service. The hybrid drive used in each case is an in-line unit consisting of three batteries, a gas turbine and high-speed generator mounted on the same shaft, and the electric rear-axle transmission. Hybrid drives reduce pollutant emissions and allow vehicles to be run on just batteries, for example in designated zero-emission zones. for the hybrid bus. Such a bus could start its journey in the centre of a city on battery power alone, with the gas turbine only coming on line outside the central business district. Parallel or series drive? Hybrid drives can have either a parallel or series (in-line) configuration. With a parallel unit, either (or both) the electric motor and combustion engine can power the vehicle, the driver (or a computer) switching between each as required. In an in-line configuration the vehicle is always powered by the electric motor, the combustion motor either supplying the motor current or keeping the battery charged, or both, or neither (where battery power alone is used). The hybrid drives installed in the concept vehicles employ an in-line arrangement and were developed jointly by Volvo Aero Turbines and ABB Hybrid Systems in Sweden. The hybrid drive consists of a gas turbine and high-speed generator. Batteries form the second energy source. The drive was developed and designed on the basis of experience with the Volvo's 1992 ECC (Environmental Concept Car) with gas turbine drive. The power plant’s gas turbine consists of the turbine itself, a compressor, a combustion chamber and a heat exchanger. Besides recovering heat from the exhaust gases, the heat exchanger also acts as a noise suppressor. Gas turbine A gas turbine engine burns fuel more completely than an internal combustion en- gine, resulting in lower emissions. In principle, a gas turbine can be run on virtually any type of liquid or gas­-eous fuel. Ethanol (ethyl alcohol) was chosen for the concept vehicles. Ethanol is a biofuel, obtainable from vegetable matter and is a natural, renewable and abundantly available resource. Unlike fossil fuels, it is environmentally neutral in terms of its CO2 emissions and therefore does not contribute to global warming. In addition, NOx emissions are one tenth of those of modern diesel engines. Suspended particle emissions are also marginal. The vehicle management com­ puter determines the actual power requirement which, since it depends on the Fig.3: the High Speed generator (HSG) power module for hybrid vehicles consists of a gas turbine and a high speed generator mounted on the same shaft. Ethanol is used as fuel. January 1997  17 The displays shows the drive mode (battery or hybrid), fuel consumption, outside temperature, etc. The dashboard consists of a main display and two ‘satellite’ units mounted either side of the steering column. traffic situation and the driving style, can vary greatly, particularly in a city. The rotational speed of the turbine can vary between 50,000 and 70,000 rpm, corresponding to a generator output of 30-110kW. High-speed generator As the turbine and generator are on the same shaft, the output of the generator can be easily regulated by varying the rotational speed of the turbine as the two are directly proportional. At 70,000 rpm, the line-to-line voltage is 450V. Excitation is by a permanent magnet and with an air-gap wound stator. Because of the high rotational speed, (circumferential speed is approximately 230m/s) the magnetic, electrical and mechanical stresses at the periphery are very high. However the design takes care of this. The high-speed rotor has a cylindrical, diametrically magnetised twopole permanent magnet encapsulated in a high-strength cylinder made of austenite steel. NdFeB with a specific energy density of 310 kJ/m3 is used as the magnetic material. The choice of cylindrical magnet and magnetic circuit allows an operating point which lies close to the maximum energy density. Since the compressor and turbine are also mounted on the same shaft, the encapsulation of the magnet improves the rigidity of the rotor. The water-cooled high-frequency stator has a three-phase ring wind18  Silicon Chip ing consisting of litz-wire stranded conductors with 3,780 insulated filaments. Punched magnetic sheet steel laminations, 0.2mm thick, make up the stator core. This is heat-treated in a special way to ensure very low hysteresis losses. The wound stator is encased in epoxy resin with boron nitride added to increase its thermal conductivity and strength. Low losses in the rotor and the low core losses in the stator result in the generator having an efficiency of about 96 percent. Although the high frequency of 1,170Hz causes additional losses in the stator, these can easily be dissipated. A filter limits the harmonic losses in the rotor. The generator also acts as a starting motor during run-up of the gas turbine. It is fed with AC power at an increasing frequency and amplitude until the gas turbine is able to continue under its own power. NiMH battery Nickel-metal hydride (NiMH) batteries developed by Varta Batterie AG are fitted to the concept vehicles. These are only half the size of conventional lead-acid batteries and have considerably less impact on the environment than either lead-acid or NiCd batteries, a fact which also applies to their recycling. Since NiMH batteries of the size and capacity required for heavy vehicles are still not yet available, three units were connected in parallel. Instead of an ignition key, a personal magnetic card is inserted to start the hybrid truck. During hybrid operation, the batteries are charged at a relatively fast rate – from 20% to 80% in just 20 minutes. The batteries can also be charged from the mains which means that a bus could start its day fully charged using low cost (off peak) electricity. Transmission Because the transmission is electric and the electric motor acts directly on the rear axle, a gearbox is not necessary. During braking, the electric motor functions as a generator. Instead of the braking energy being lost as heat, it can be fed back to the battery. In addition braking is smoother and the brake linings are subjected to less wear. Slight pressure applied to the brake pedal will at first cause the vehicle to be braked electrically; normal braking takes place only when stronger pressure is applied. The drive motor can brake with the same force as it can accelerate; only a small portion of the energy is lost during charging and discharging of the battery. Two drive modes An in-line hybrid vehicle is always driven by electrical energy, whichever of the two possible modes – hybrid or just battery – is chosen. In the hybrid mode, the vehicle is propelled by the electric motor powered primarily by the high speed generator. When only a small amount of power is required there will be a surplus of energy, which will be stored in the batteries. When the power level required is higher than can be supplied by the HSG (approx. 110kW), the batteries provide additional energy. The combined maximum output of the HSG module and batteries is 142kW. The driver can choose between automatic (ie, with the turbine switched on and off as a function of the battery charge status) and continuous turbine operation. In the latter case, if the batteries are fully charged the turbine runs at no load. The vehicle body Electric transmission makes it possible for the driver’s cab to be positioned just 60cm above road level, allowing eye contact between the driver and other road users as well as pedestrians. The transmission components are mounted in the roof of the bus. This enables its length to be reduced by 1.5m compared with conventional diesel-powered buses with the same number of seats. Instrumentation is simplified compared to a normal heavy vehicle. It consists of a main unit in the middle of the dashboard and two ‘satellite’ units, one on each side of the steering column and fixed permanently to it. Since these two units move with each new setting of the steering wheel, they remain at the correct distance from the driver. Other instruments show the power consumption, battery charge, fuel consumption and the remaining distance that can be travelled The headlights bear special mention. They consist of gas discharge and special UV lamps which allow the driver to see twice as far in the dark. Light-emitting diodes, which turn on much faster than ordinary filament lamps, are used for the turn indicators, side-marker lamps, rear and brake lamps. Drivers of vehicles following at a safe distance will therefore gain an extra five metres in which to respond if the hybrid vehicle driver has to brake sharply. Driving the vehicle is unusual: instead of turning an ignition key, the driver inserts his personal magnetic card into the card reader. A symbol (a Concept Truck & Bus Specifications HSG module Output Efficiency at full load Emissions NOx Suspended particles NiMH batteries Nom. energy storage capability Rated voltage Rated capacity Electric rear-axle drive Continuous rating Maximum rating Maximum torque Vehicle Efficiency at full load Total efficiency at full load Top speed on 1:50 gradient Range in zero-emissions mode 25km Weight (approx values) HSG module (turbine + generator) Electric motor Power electronics and servo-drives Batteries Cable Cooling plant Total red truck) lights up on the dashboard to tell the driver that the starting procedure has begun. Once the batteries have been switched into circuit, a quick check is automatically made of the system components to ensure that they are functioning properly. When the operating voltage has risen to 600V, the auxiliaries are switched on. After a few seconds, the red ‘truck’ symbol changes colour to show that the hybrid vehicle is ready. The driver releases the handbrake, turns the selector switch to D for drive and presses the accelerator, upon which the bus starts to move smoothly and quietly. At this point, the turbine has still not started up. Next to the selector switch is a changeover switch for the different drive modes. In the hybrid mode, the turbine starts automatically. All that the driver hears is a humming sound at a pitch which stays the same regardless of vehicle speed. Truck Bus 110kW 110kW 32% 32% 0.5g/kWh 0.05g/kWh 0.5g/kWh 0.05g/kWh 72kWh 45kWh 400V 250V 3 x 60Ah 3 x 60Ah 94kW 94kW 142kW 142kW 2850Nm 2850Nm 85% 27% 80km/h >5km 85% 27% 80km/h 400kg 400kg 100kg 100kg 500kg 500kg 1800kg 1100kg 100kg 100kg 200kg 200kg 3100kg 2400kg Since only very few operations, involving just a small number of controls are necessary, the driver can concentrate on the traffic. This also gives the hybrid vehicle a safety edge over conventional vehicles. Hybrid drives help to reduce the environmental burden being imposed by increasing road traffic. Both of the concept vehicles have been used to test a whole series of innovations, including active suspension, allwheel power steering and new lighting techniques, some of which are found at present only in sports cars or in test vehicles. At the same time the project has given the industry a further opportunity to demonstrate what it has to offer today to the transSC portation sector. Acknowledgement: the photographs and much of the original text in this article appeared the June/July 1996 issue of ABB Review, published by Asea Brown Boveri Ltd. January 1997  19