The gearbox being developed to help put performance in to electric vehicles

Hybrid and electric vehicles have been on the market for some time now. Indeed, it was Toyota to claim the first serious volume production with the Prius. But nearly 15 years on, and 5 million cars later, the roll out of next generation clean vehicles seems to have been as slow and subdued as the vehicles themselves. Electric and hybrid vehicles have a bit of an image problem. They are perceived as sluggish and many are concerned about practicality. Range anxiety is now a phase commonly used to describe this feeling of apprehension. Overall all, electric vehicles – and hybrids by association – have been labelled as somewhat dull. And there are good engineering reasons why. The power density from batteries, extra weight from motors, two powertrains (if it's a hybrid), slow charging, and transmission trade offs... there have been a host of technical hurdles that are still some way off being completely understood and overcome. However, 2014 was a redefining year for electric and hybrid cars, and any thought that driving one is strictly for the dullard has been overtaken with a new generation of sports cars. This year has seen a shift from it all being about saving the planet to aiding performance. Formula E began in earnest: the first high profile fully electric racing series to put a bit of glamour and excitement, along with speed, into the market. In addition, Formula One cars, the McLaren P1 supercar, and BMW's i8 are all hybrids released this year that put electric power in the fast lane. "People want more excitement from electric cars," said GKN Driveline's vice president of product technology eDrive systems, Theo Gassmann. "And this is especially going to be the case going forward. At the moment they tend to be smaller city cars but we will see more high performance electric cars coming to the market as many people do not want to compromise that driving experience." GKN has recently completed a project for BMW, specifically to enable its i8 to have the kind of performance associated with higher performance BMWs. It has developed a transmission for the electric powertrain of the vehicle that was essential in providing the necessary speed and acceleration. The BMW i8 uses a downsized combustion engine, a 1.5-litre 3-cylinder turbocharged unit, which puts out 220bhp and 300Nm of torque. While impressive, with a curb weight close to 1500kg, it will not deliver the performance expected of a BMW sports car. Making up for this is an electric powertrain, rated at around 125kW (170bhp). "A significant part of the power and performance is in the i8's electric system," said Gassmann. "Some 'axle-split' hybrid cars use a disconnect, so at higher speeds the electric motor is disconnected from the wheels. It means the gear ratio is set to offer acceleration at low speed. But it also means you have to disconnect the electric motor or you compromise your top speed." For BMW and GKN a single fixed gear ratio would mean a significant part of the acceleration or top speed was compromised. With the aim to get the car from 0-100kph in less than 4.5seconds and enable a top speed of 250kph, both the initial launch capability and top speed were essential in offering the impressive hybrid performance that would change the perception of what a hybrid car can be going forward. The resulting innovation was the development of a 2-speed eAxle gearbox. GKN claim this is a world-first as the gearbox is able to operate over the entire speed range of the car and provide continuous electric power to the wheels in two different ratios to meet the performance targets. "The combustion engine has a 6-speed automatic transmission in the rear and in the front is an electric system driving the front wheels," explained Gassmann. "The eAxle gearbox in the front acts as a gear reduction system as the electric motor runs up to 12,000rpm, so you need to bring it down to the necessary wheel speed, and then the differential splits the power to the wheels." Transmission innovation GKN Driveline does not have much experience in conventional 2WD transmission technologies for the automotive industry and did not have a part in the 6-speed automatic transmission development for the combustion engine of the car. It also had little to go on, as electric powertrain transmissions are predominantly in the prototype, motorsport and concept stages. GKN is producing eAxle gearboxes for electric AWD and axle – split hybrid vehicles for quite a while, but they are all with disconnect system. So it knew a fresh approach was needed. Weight targets and packaging requirements were also pretty tight, as was the development time of just two years from concept to production. The team opted to keep components commercially available where possible and used conventional materials. For example, the synchroniser system for shifting gears is a commercially off the shelf component, which also helped to keep the cost of development and subsequent unit costs to the level required. "Essentially, we had to start from a blank sheet of paper," said Gassmann. "The first generation of electric transmission has been single speed to minimise risk and complexity, but this next generation of hybrids and electric vehicles is likely to start using more 2-speed transmissions." Diminishing returns Gearboxes for combustion engines have steadily seen an increase in the number of gears available over the years from four to six in manual cars, and up to nine for some automatic cars. So while the development of the 2-speed gearbox is an achievement, is it likely that we could soon see a move to 3- or 4-speed? "A 2-speed doesn't sound that exciting, when we have 9-speed automatic gearboxes, " Gassmann admitted. "But the challenge and the exciting part is to do it with an electric system. It is a totally different compared to a combustion motor. For a start you have no clutch so the motor inertia is directly linked to the gearbox and it is a very stiff arrangement with no elasticity. Shifting such a high inertia is a big challenge." And more is not necessarily better, as extra gears introduce losses. For electric motors that have very smooth and gradual power curves, rather than the power bands of combustion engines, it means that actually the 2-speed transmission is a good solution to the problem. "There are discussions around 3- or 4-speed transmissions and how many speeds we actually need if you have a proper electric motor in the driveline," said Gassmann. "It is a trade off between complexity costs and what you would gain in terms of performance. Simulation has shown that going from a single speed to a 2-speed will give you up a 10% gain in system efficiency. But, going more than 3-speed doesn't give you as much. The 2-speed is a very good compromise in terms of overall complexity, cost, packaging, and efficiency." The eAxle is set to bring together driving dynamics and as well as improved CO2 emissions as it enables combustion engines to be downsized. The technology responds intelligently to driver inputs and delivers the high-torque all-wheel driving experience as well as the useful pure electric mode or day to day efficient parallel hybrid mode, to offer something for every driving need. All in all, it hopes that it will help the BMW i8 win hearts and minds and set the road map to a cleaner future without compromising the fun.