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Electric vehicle transnrlssion options Just as with motors, there are many options when it comes to selecting the transmission systems for electric vehicles. There ca.n be all sorts of belt and gear drive systems combined with one, two or more motors. By GERRY NOLAN We've come to the point in our story where we have to consider converting all of that perfectly controlled power from the electric motor(s) to motion of our electric vehicle. This is an area of vehicle development that lends itself to the greatest variety of innovations and choices. We have to consider whether to change gear or not, whether to use manual or automatic gears, and whether to use a geared transmission, direct drive, chain drive, geared belt drive or continuously variable drive . The number and type of wheels and tyres are also important considerations: three or four wheels, front or rear drive, rolling friction, tyre profiles and pressures. A great deal of time and money can be saved by mod- elling the drive train, taking into consideration all the variable parameters such as: vehicle design and structure, batteries, driveability, suspension and steering, weight, materials, and driver and passenger safety. Generally speaking, the transmission parameters for an EV are the same as for a conventional ICE vehicle with one major difference. If a petrol or diesel engine is used, you have no choice but to use a gear-changing mechanism of some kind. As we found out in the third article in this series (March 1991), this need not be the case with EVs as torque and speed can be controlled electronically. The type of transmission used will depend on the usual vehicle weight, size and cost parameters. Neverthe- Table 1: Drive Cycle Comparisons Cycle CVS HWY SAE J227 TAXI DELIVERY SAE J227a-D SAE J227a-C 6 SILICON CHIP Duration sec. 1372 865 150 1100 2500 122 80 Speed Max. Avg. km/h km/h Power Max. Avg. kW kW 91.2 96.4 72.4 53.9 52.5 72.4 48.3 37.0 31.0 26.8 32.4 24.0 29.7 20.1 31.4 77.7 38.8 11 .1 5.6 45.0 23.4 5.6 13.8 6.5 2.5 1.1 6.9 5.2 less, the purpose for which the vehicle is designed will be the major consideration and one of the most important aspects of this is the driving cycle. Driving cycles Component and vehicular energy efficiencies are obtained by adding up the energy use as a vehicle is driven through a particular series of driving operations known as a driving cycle. Typical examples are: city, rural, commuter, delivery, taxi and so on, all of which have their own pattern of idle, acceleration, cruise speed, coast and deceleration, and all of which will have a bearing on the type of transmission used. Some standardisation of driving cycles was obviously desirable from the start so that meaningful vehicle comparisons could be made. Several standards are in use today, the most common in the United States being the Federal Urban Driving Cycle, usually referred to as the CVS Cycle (constant volume sampling of emissions), and the Federal Urban Highway Cycle (HWY). Europe uses a composite of these two in its ECE Cycle. The first standard driving cycle for EVs was the SAE J227 (1972) EV Cycle, which was designed to give approximately the same road-load energy per kilometre as the CVS Cycle but with lower peak road-load power. Because it soon became apparent that many EVs already in existence couldn't achieve the road-load power levels required in the SAE J227 Cycle, it was re-issued as a set of four simplified cycles, designated SAE J227a -A, - B, - C and - D. The various driving cycles are summarised in Table 1. Changing gears It might seem that, because of the degree of motor control already mentioned, it would be unnecessary to motor speeds. And, as well as being one of the most reliable devices used in vehicle drivetrains today, manual transmissions generally have higher energy efficiencies than electronic controllers. In essence, what we are saying is that the motor required to perform a specific task will be smaller in size, weight and cost if gear changing is used than the motor required if no gear changing is used. The advantages of selectable gear ratios are illustrated in the graphs of Fig.I. Tractive Effort (N) 7000 6000 /1st 5000 4000 3000 2000 Manual or automatic 1000 0 0 20 40 60 80 Speed (km/h) Fig.I: this graph shows the performance through the gears of the Finnish ELCAT electric vehicle project. use gears. However, if high loads are expected, either because of steep terrain or heavy payloads, gears may be advisable to reduce excessive motor currents which can cause overheating. We should al('>o bear in mind that the power semiconductors used in the controllers must be selected on the basis of maximum armature current - even if it is only expected for a few moments of the driving cycle. This means that the maximum size, cost and weight of semiconductors must be carried at all times for a few moments of use. The same applies to battery requirements, although this may be minimised by controller design, but only with the aforementioned penalty, so we're back to where we started. In fact, reductions in the size and weight of the motor and its controller are the main advantages to be gained from using gears, provided of course the geartrain itself does riot outweigh the advantage gained. Reducing motor size by including a gearbox will nearly always result in an economic gain, simply because motors are generally constructed from costly materials, while transmissions are among the lowest cost devices around (on a $/kg basis) . With a transmission, the EV drivetrain can be operated at nearly optimum efficiency over the whole driving cycle, as the efficiency of the transmission may vary little with speed and torque. Using a gear-changing mechanism also greatly enhances regenerative braking over a much wider range of Assuming that the above discussion has convinced you that a gear changing transmission is the way to go, would you choose manual or automatic? As most readers will know, automatic transmissions are not as efficient as manual transmissions, mainly because of losses in the torque converter. This problem has been overcome in the Nissan Miera EV-2 prototype by the use of a one-way clutch for Ist gear and an electromagnetic clutch, which acts as a 'binary transmitter', sending either all or no power to the drivetrain, for second gear. Fig.2 illustrates this particular transmission scheme. At least one electric vehicle in Australia, a Suzuki locally built by Les Puklowski at his Huntington Electric Vehicle factory for a specific client, uses an automatic transmission. According to Les , the vehicle is very smooth to drive. On the other hand, The Fiat Panda Elettra uses a 4-speed manual gearbox and has automatic regenerative braking. JUNE 1991 7 2ndGEAR ELECTROMAGNETIC CLUTCH lstGEAR other types of transmissions. The drive for the Solar Star II is from the motor via a geared belt to a jack-shaft and lightweight differential with a 1:1 ratio, giving an overall ratio of 8:1 from motor to drive axles. In-wheel electric motors Fig.2: the 2-speed electric automatic transmission scheme used in the Nissan Miera EV-2. the Finnish EV-project, called ELCAT, a lightweight delivery van which has been converted to electric drive, has a 5-speed manual gearbox. Its performance through the gears is shown in Fig.1. Several vans, namely the Peugeot JS/Citroen C25 van and the Fiat 900 E/E2 electric van use manual gearboxes. The Fiat Panda Elettra (pictured) uses a 4-speed manual gearbox and has automatic regenerative braking. Les Puklowski, who has built over 50 electric vehicles, the latest being the Solar Star II, is firmly convinced that an 80V-120V DC motor with manual transmission is the best way to go for small EVs. Table 2 gives estimated weight and energy efficiencies for 2-speed, manual EV transmissions. Continuously variable transmissions Ideally, these maximise motor/controller/battery efficiency over all the vehicle speed and torque requirements. They improve acceleration and give automatic control comparable to all-electronic motor control, as well as automatic down-shifting during regenerative braking. 8 SILICON CHIP A great deal of work has been done on CVTs, particularly in America, but most types have proven to be too costly, noisy or inefficient for EV applications. Nevertheless, one of the most-likelyto-succeed CVTs for electric vehicles is the belt-type, a schematic of which is illustrated in Fig.3. Even as far back as 1982, the Van Doorne CVT, which uses a metal belt and variable-ratio conical pulleys, achieved a zero drivetrain loss at vehicle standstill - such as waiting for a stop light. This can result in a 10% fuel saving over the CVS cycle, a very desirable objective in any power train. Final drives Twenty-eight of the 33 solar electric vehicles which started in the 1990 World Solar Challenge, including the winning Spirit of Biel, used chain drives and five used toothed belt drives. Four of the chain-drive vehicles also used manual gears . If a 'solid' transmission such as mechanical gearing and a tailshaft is used, some type of differential is obviously required. The energy efficiencies of typical differentials in conventional cars are in the range of 92-95 % , which compares very favourably with Apart from the obvious disadvantages of high unsprung weight and running an electric motor in a hostile environment of heat, dust, mud, slush and vibration, the idea of building electric drive motors directly into the wheels of a vehicle has some merit. John Hill, currently national secretary of the Australian Electric Vehicle Association, built and successfully raced his Dart electric car in 1988 with an in-wheel motor of his own design and construction. More recently, a Japanese consortium built an electric vehicle with a motor in each of its four wheels and achieved a top speed of 110 km/h and a range of 240 kilometres. Mr S. Monji, of the Kyushu Electric Power Co. Inc, has also produced both 2-wheeler and 3-wheeler scooters using in-wheel motors. He claims considerable advantages in efficiency, as well as weight savings and more room for the batteries. At the other end of the scale is the CNR-IVECO Fiat dualmode, articulated bus with integrated wheel motors. These are powered by an on-trolley bus line system integrated with a diesel generator unit and a high power nickel-cadmium battery. How many wheels? Because the two main energy losses in an electric vehicle are caused by tyre rolling resistance (or friction) and wind resistance, any way in which these two factors can be reduced must be considered. For a vehicle in slow moving city traffic, tyre rolling resistance is the greatest loss. Readers who have seen solar electric vehicles or pictures of them will have noticed the prevalence of skinny wheels, often with streamlining discs or fairings, and bicycle tyres. This type of running gear keeps the rolling resistance to a minimum and is suitable for lightweight vehicles, but is fragile and susceptible to damage. More practical vehicles need to compromise with tyres that have greater load bearing capability. More research is being done with very low Table 2: estimated weights and energy efficiencies for twospeed manual EV transmissions Electric motor max. rpm Weight kg 6000 9000 12000 19.0 17.3 16.8 profile, high pressure tyres. For example, the General Motors Impact uses specially developed Goodyear G-22 tyres that operate at air pressures of 450kPa (65psi), or about twice normal tyre pressure. This, coupled with a narrower than usual, rib-like tread, which has small block elements and numerous 'sipes' (small slits in the tread) to improve grip, gives a rolling resistance that's about 55% less than conventional tyres . When deciding on the number of wheels, wheel profile and tyres, the preferences of the buying public will have to be taken into consideration. Rightly or wrongly, we more readily accept four wheels and wider tyres for aesthetic as well as perceived safety reasons. A good example of this is the Solar Star II, which looks instantly acceptable as a road vehicle . Front or rear drive? With EVs, the choice between front or rear-wheel drive is wide open. The field covers everything from directdrive in-wheel motors to electric motors driving the rear wheels by chains or geared belts and a conventional ICE engine driving the front wheels through a gearbox and differential, or vice versa. Efficiency % 97 96 .8 96.4 Table 3: EPA and optimum gear-change schedules for a four-speed transmission EPA Gear change km/h 1-2 2-3 3-4 Placing the batteries in the rear of the vehicle and using the electric motors to drive the front wheels, by one of the methods we've discussed above, would make for a , well balanced vehicle from a weight distribution point of view but then, so would mounting the batteries in the front as BMW has done on an experimental vehicle - and driving the rear wheels electrically. The choice will be determined by many factors and can be arrived at by experimentation or by the cheaper method of computer modelling. Computer modelling Making a computer model of your EV, taking into consideration all the variable parameters, can save a lot of time and avoid design problems. One such problem is weight compounding, where an increase in battery capacity to increase range (say), results in an increase in battery weight, which requires a stronger, heavier frame, which needs a larger motor to attain the same performance, which needs more battery capacity to reach the same range, and so on. As with any other system, the ap proach to the modelling system will depend a great deal on the desired end results. 24.0 40.2 64.4 Optimum km/h 15.6 30.0 38.6 Some definitions will help to clarify this: • Performance is used to describe vehicle acceleration - usually in the wide-open-throttle (WOT) or maximum power condition; • Fuel economy refers to the distance travelled per unit of energy and is the reciprocal of fuel consumption in kilowatt hours per kilometre; • Vehicle range is the distance a vehicle can travel per charge of input energy; all of which are measured for a specific driving cycle; • Inertia weight or test weight refers to the vehicle weight used in testing any of these parameters; Energy efficiency is the ratio of the road-load energy to input energy during a specific driving cycle. Whether we are modelling for range and performance predictions, dynamic or economic analysis, vehicle optimisation or component or vehicle design or size, it is most important to consider the following elements. Drive cycle This will almost certainly have been in mind from the very beginning. If an electric vehicle is being designed to win competitions, the drive cycle, and consequently the design, is obviously going to be completely different from Below: the Australian-designed "Solar Star II". ' t<:'.'\.' • <- , ,. * I COMPUTER PRINTERS/ JU NE 1991 9 Low-ratio used is equal to a reference charge. Gauges are becoming available that indicate the amount of charge remaining. One type indicates the· discharge in ampere-hours as a minus value and adds back to zero as the battery is recharged. A point to remember is that the available capacity usually decreases as the rate of discharge increases. Meters that indicate the vehicle range in kilometres at the current rate of energy use are being developed and will no doubt be readily available as soon as the demand justifies it. The future ofEVs in Australia High ratio Fig.3: basic scheme for a belt-type constant velocity transmission (CVT). A constant velocity transmission is just one of the many transmission options available to electric vehicle designers. that of an EV intended as a commuter vehicle. Weight considerations These will include structural and frame factors, which are in turn determined by the battery, its weight and location, heat transfer provisions, charging gas ventilation, crashworthiness, and battery maintenance requirements. Other factors will be the size and weight of the auxiliary power systems, driveability (including suspension and steering), weight compounding, materials selection and driver and passenger safety. The body design will need optimising for minimum drag, maximum strength to weight ratio, and stability. Th ese factors determine the materials us ed, taking into acco unt their strength, weight and shaping potential, and their availability. Drivetrain control strategy The drivetrain contro l strategy is arrived at after considering the type of motor and controller, and the points rais ed in the above discussion; ie, the number and type of wheels and tyres; the type of transmission; gearbox or 10 SILICON CIIIP electronic motor control or a combination of both; and how the regenerative braking is to be arranged (automatic on throttle release, brake pedal activated or a combination of both). Gear-changing strategy This is the fancy name given to the predetermined set of speeds at which you change gear. The most commonly used is the EPA schedule, which is used by the US Department of Energy for emissions and fuel economy testing of US passenger cars. However, as Table 3 indicates, the EPA schedule speeds are quite different from the optimum speeds for a 4-speed gearbox. The main criterion for selecting the optimum gear change speeds is that sufficient motor torque is available at the change speeds. Battery control strategy The most realistic cut-off point for a battery is that at which the battery can no longer meet the road-load . power required by the drive cycle. In other words, 'the battery's flat , Mum'. More convenient methods of indicating this are a specified minimum terminal voltage or when the charge After researching this series of articles, I have learnt that the technology to make practical, economical electric vehicles is available right now. General Motors are ready to go into production with their Impact and literally dozens of other major vehicle manufacturers around the world have working prototypes. Electric vehicles have been delivering milk in England and mail in America for many years. What's stopping the introduction of electric vehicles for passenger transport then? This can be summed up in one word: demand - or the lack of it. Demand will increase when EV prices drop, either through manufacturing economies or tax incentives, so that the cost of buying and running an EV is less than the cost of buying and running an equivalent ICE vehicle. Don't hold your breath waiting for tax incentives. The Australian Electric Vehicle Association wrote to the Federal Government in July 1989 asking it to consider removing sales tax from electric vehicles (for 5 years, say) to spur the development of such vehicles. The reply (dated 23 January, 1991) stated that the Government was "reluctant to add to the number of sales tax concessions". In short, the answer was 'No'. The large manufacturers are already well down the track towards producing acceptable EVs from both aesthetic and economic points of view. But does this mean that the small-time manufacturers and inventors have missed out? Not from this writer's point of view. Literally hundreds of opportunities exist for improvements and innovations to make EVs more practical and desirable. As the technology develops, there will be lots more. SC