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We drive hybrid drive . . . GM Alliso Hybrid D This diesel-hybrid electric system can improve bus fuel economy by up to 40% and reduce exhaust emissions by as much as 90%. With over 400 GM Allison hybrid buses already in service, the system is proving successful in the marketplace. We drove a GMAllison bus recently brought to Australia for evaluation by state governments. By JULIAN EDGAR W hen talk turns to improving fuel vehicle economy, two technologies are likely to enter the discussion: hybrid petrol/electric drivelines like that used in the Toyota Prius and high pressure common rail diesels fitted to vehicles from makers like Audi, Peugeot, and Mercedes. European manufacturers have long championed diesels, while Japanese company Toyota has an apparently unassailable lead in hybrids. Despite German automotive electronics powerhouse Bosch being a prime mover in the development of car electrics and despite Toyota building and selling diesel passenger cars, the obvious step of combining frugal diesel power with low emissions hybrid technology hasn’t yet occurred. Or has it? Coming in from left field is a completely new player – 6  Silicon Chip GM Allison. GM currently sells some ‘soft’ hybrids but it is their heavy vehicle transmission arm Allison that is the dark horse in hybrid technology development. Not only has Allison developed a high efficiency, patented, two-mode hybrid transmission but it is a selfcontained unit that can be bolted to a variety of different engines – including conventional diesels. Rather than being controlled by the engine management system, the Allison EP system controls the engine via a standard communications interface, allowing it to work with a range of engines. The Allison EP system is currently available in two configurations, both primarily suited to heavy vehicles that work in a stop/start environment such as urban buses and garbage trucks. However, GM and partners DaimlerChrysler and BMW will soon incorporate the technology siliconchip.com.au on’s Drive Bus in hybrid passenger cars, potentially providing some real competition for Toyota and Honda. So what do the heavy vehicle systems consist of and why has the technology implications for fuel-efficient passenger cars? System overview Two hybrid Allison drives are available. The EV40 has a rated input power from the engine of 209kW and 1235Nm of torque and a total short-term output power of 261kW. The EV50 can accept 246kW and 1420Nm and has a shortterm output of 298kW. Each system uses a transmission that combines three planetary gear-trains and two electric motor/generators. The drive system has a mass of 428kg and is 810mm long, 442mm wide and 312mmm high. In appearance it is very similar to Allison’s B400R transmission. The AC inducsiliconchip.com.au tion motor/generators mounted within the drive unit are each rated at 75kW. Synthetic transmission fluid is used to lubricate and cool the system. The battery pack uses NiMH cells and is designed and manufactured by Panasonic, the same company that makes the Prius high-voltage battery. However, Allison suggests that in this application, the robustness of the cells had to be increased so that they would cope with the almost continuous use of a commercial vehicle. In the Allison system the nominal battery pack voltage is 600V but system voltage can vary from 430-900V. The battery pack comprises six fan-cooled modules. In addition, when the bus air-conditioning system is running, a refrigerant feed can be drawn from it to cool an evaporator specific to the battery pack. The battery pack has a mass of 408kg and in bus applications, is mounted within a roof pod. June 2006  7 Fig.1: in a series hybrid system, the combustion engine drives a generator which charges the battery and/or drives the electric motor. [Allison] Fig.2: a parallel hybrid system differs from a series system in that either the engine or the battery/electric system can drive the wheels. [Allison] Fig.3: a series/parallel system has elements of both series and parallel systems. This diagram shows the schematic layout of an Electrically Variable Transmission (EVT) series/parallel hybrid. [Allison] The large dual power inverter module is built by General Motors. It contains two inverters that use IGBTs (Insulated Gate Bipolar Transistors) to convert the input/output of the motor/generators from DC to 160kW continuous 3-phase AC. The inverter has a mass of 91kg and is normally mounted at the rear of the bus adjacent to the engine. It shares its oil cooling with the transmission. Two Electronic Control Units are used. They are the same control unit used in Allison’s 1000/2000/2400 Series transmissions but with software optimised for their hybrid role. They have self-diagnostics and can be reprogrammed in service. The controllers each have a mass of 2.3kg. The controllers communicate with the diesel engine management system via the standard SAE J1939 protocol used in most diesel engine management systems. The primary information sent to the engine management system comprises torque and speed commands. Fig.4: this diagram shows the relationship between input, output, electric motor/generator and road speeds of the Allison hybrid drive system. Note that from 16 – 105km/h, the diesel motor’s speed doesn’t change and that over the full speed range of the vehicle, each motor/generator (ie, units A and B) stops rotating twice. [Allison] Fig.5: the highest mechanical efficiency of the transmission occurs at road speeds where either motor/generator is stationary (indicated here by stars). As can be seen, these occur at typical urban and highway bus speeds. Also note how the mode change allows the electric motor/generators to be “re-used” up and down in speed. [Allison] 8  Silicon Chip The drive system The breakthrough in the GM Allison hybrid system is siliconchip.com.au Fig.6: this diagram shows how pure mechanical drive occurs at 40-56km/h and 97-11km/h, the two most common speed ranges for buses working in urban and open road environments. Note also the high proportion of electric power used to accelerate from a standstill, the situation in which an electric motor produces maximum torque. [Allison] Fig.7: the components of the hybrid drive system are distributed around the vehicle. The compound split parallel drive replaces the conventional transmission and is located in front of the rear-mounted engine. The battery pack is placed under a pod on the roof, the dual power inverter module is placed next to the engine while one electronic control module is located near the front and one at the rear. [Allison] the compound split parallel drive unit. Like the Prius transmission, the Allison drive unit combines both series and parallel hybrid approaches. So what are series and parallel hybrid systems, then? In a series hybrid system, the combustion engine drives a generator that charges the battery and/or drives the electric motor. There is therefore no direct mechanical connection between the internal combustion engine and the wheels. Fig.1 shows this approach. A parallel hybrid system differs in that either the engine or the battery/electric system can drive the wheels (see Fig.2). As the name suggests, a series/parallel system has elements of both systems. This approach is characterised by the requirement to combine engine and electric power in a varying manner, depending on driving conditions. Fig.3 shows the schematic layout of an Electrically Variable Transmission (EVT) series/parallel hybrid. Allison sees the major benefits of the EVT series/parallel The drive system has a mass of 428kg and is 810mm long, 442mm wide and 312mmm high. In appearance it is very similar to Allison’s B400R transmission. The AC induction motor/generators mounted within the drive unit are each rated at 75kW. Synthetic transmission fluid is used to lubricate and cool the system. [Allison] siliconchip.com.au June 2006  9 Fig.8: a schematic cross-section of the hybrid drive system. It uses two AC induction motor/ generators, three planetary gear trains and two friction clutches. [Allison] drive system as: • Series Mode • Continuously variable transmission. • Very strong acceleration off the line because of the availability of a large amount of electric torque. • Parallel Mode • Lower cost as electric motor/generators and inverters are smaller. • Higher transmission efficiency. In addition, an EVT allows straightforward implementation of regeneration braking, gives strong acceleration assist and can be programmed for transient response. But all of these are also characteristics of the Prius Power Split Device, so what are the advantages of GM Allison’s patented drive system? The internal mechanical complexities of the Allison transmission will not be covered here; suffice to say that a torque damper input device works with three planetary gear-trains arranged so that various elements can be driven, braked or held still by the two electric motor/generators. (If you want to see how the internals work, see US patent 5931757, available from the search page at www.uspto.gov/patft/index.html). However, it is the relationship between inputs, output, electric motor/generator and road speeds which is the key to understanding the driveline benefits. Referring to Fig.4, the light blue line shows engine rpm, the red line the speed of the first motor/generator (Unit A), the green line the speed of the second motor/generator (Unit B), and the dark blue line shows the output shaft drive speed of the electric drive. All speeds are plotted versus road speed. Two aspects are immediately clear: first, that from 16 – 105km/h, the diesel motor’s speed doesn’t change and second, there is the expected fixed relationship between output shaft speed and road speed. However, over the full speed range of the vehicle, each motor generator stops rotating twice. An analysis of how the drive unit works can be divided into two operational modes. Mode 1 extends from zero up to about 40km/h. At speeds greater than this, the transmis10  Silicon Chip sion works in Mode 2. In Mode 1 the motor/generator B operates as a motor. Motor/generator A acts as a generator until about 25km/h and thereafter operates as a motor for the remainder of Mode 1. The change from acting as a generator to acting as a motor is seamlessly achieved by the relationship of the number of teeth on the various planetary gear subsets, which cause the speeds of the two motor/generators to reverse at various road speeds. Mode 1 can also be called ‘electric launch’, which is perhaps a more descriptive term! Motor/generator A, acting as a generator, is used to feed electric power to motor/generator B which can also call upon battery power. The change to Mode 2, is caused by the action of hydraulic clutches within the drive unit which simultaneously release certain planetary elements and clamp others. In Mode 2, motor/generator A continues to operate as a generator, a state it achieved late in Mode 1. However, by a road speed of about 50km/h, it reverts to acting again as a motor. In Mode 2 motor/generator B initially operates as a motor but when road speed passes 55km/h, it becomes a generator until road speed reaches about 100km/h, whereupon its speed has decreased to zero. Reverse gear is achieved by reversing the direction of motor/generator B. The battery pack uses NiMH cells, designed and manufactured by Panasonic. It has a nominal voltage of 600V, a mass of 408kg and in bus applications is mounted inside a roof pod. [Allison] siliconchip.com.au Driving the Bus We were able to drive the demonstration bus equipped with the EV40 system. The bus, a New Flyer built in Canada, was 12.2 metres long and had a mass of 17.7 tonnes gross vehicle weight. It used a Cummins ISL diesel with a maximum power output of 209kW. The drive was undertaken on a closed ‘county road’ circuit at the driver training facility at Mt Cotton, near Brisbane. From a passenger seat the bus felt largely like a welldriven conventional bus. Take-off from a standstill was smooth and torquey and the normal noises of a diesel bus could be heard. However, from behind the large steering wheel, the sensation was quite different. ‘Drive’ is selected via a pushbutton pad and with the air brakes released, the bus can be driven off. The torque provided by the low-speed mode of the transmission and the electric motors was immense. From the driver’s seat it could be more clearly felt that there was little torque converter flare – as would be experienced with a conventional auto transmission – and that only a small throttle movement was needed to get the large vehicle smoothly moving. But it was the regenerative braking that was the most impressive. When the throttle was released at 60 km/h, the bus smoothly but strongly decelerated, coming to almost a standstill before the regen switched itself off. In urban conditions, the conventional brakes would almost never need to be used. If required, the bus can decelerate at an astonishing 0.48G on regen alone. Apart from adapting to the fact that the driver need only lift his/her foot to heavily decelerate, little driver adaptation is needed. There are no ancillary dashboard gauges and so the driver is unaware of the power flows occurring within the system and the state of the HV battery charge. In fact, the demonstration bus didn’t even have a fuel gauge, a request made by US municipal authorities to prevent drivers coming back to the depot early with a perceived low fuel status. With the greater involvement of driving rather than being a passenger, some noises from the drive train could be heard – especially on regen, the sound of the motor/generators changed in pitch as their speed was constantly altered to provide the strong but smooth braking. A pitch change could also be occasionally heard when the transmission switched modes, although this was certainly nothing like the audible gear-change of a conventional automatic transmission bus. In short, the demonstration bus was extremely impressive to drive – powerful and smooth in both acceleration and braking. Julian Edgar has driven a lot of high-performance vehicles in his time but here it was a hybrid-powered bus at a closed country road circuit. It was smooth and powerful in off-the-line acceleration and had very strong regenerative braking. siliconchip.com.au June 2006  11 In this view, one of the two induction motor/generators can be seen at left. The transmission also incorporates multiple planetary gear trains. [GM] The road speed at which either of the motor/generators is stationary is termed a ‘mechanical point’ – at these road speeds the maximum mechanical efficiency occurs. As can be seen, the highest mechanical efficiencies in the drive system occur at typical urban and highway bus speeds. Fig.5 shows these four mechanical points of highest drive efficiency and also how the mode change allows the electric motor/generators to be “re-used” up and down in speed. Note that this is quite a simplified analysis. Allison engineers state that the system has 57 different operating modes. Results Buses equipped with the Allison hybrid drive system are currently being used in 25 US cities and have covered nearly 23 million kilometres. Allison claims reductions in emissions of particulates, hydrocarbons and carbon monoxide of up to 90% and oxides of nitrogen by 50%. The reduction in emissions is particularly successful in acceleration from a standstill, especially with a cold engine. The company also claims fuel economy improvements of up to 60% but admits that the improvement of buses actually in service ranges from 20 – 40%. In addition to the reductions in fuel consumption and emissions, brake pad wear is vastly reduced. Performance comparisons 12  Silicon Chip of two buses with similar mass and diesel engine power show that 0-97km/h (60 mph) times drop from about 67 seconds to 31 seconds. The cost of the drive system, including transmission, battery pack, inverter and control system, is about US$160,000. Buses using the system are able to be software-configured for bias towards performance or fuel economy. An electriconly mode can also be enabled, giving the buses a range of about 2km. However, even in this mode, the diesel engine continues to run to provide air conditioning, etc. Conclusion The patented compound split parallel drive unit has the potential to boast a greater efficiency than other hybrid transmissions and is already proving itself in bus applications. GM is to launch an SUV in late 2007 using a downsized version of the two-mode system and DaimlerChrysler is expected to follow suit with a hybrid luxury car. However, the very nature of stop-start urban bus duties lends itself particularly well to hybrid electric/diesel drivelines – expect to see the technology spreading worldwide. It’s not for nothing that GM Allison chose to send a full-size bus and engineering staff on a world trip… SC siliconchip.com.au