Effect of transmission design on Electric Vehicle (EV) performance

The current level of interest in electric vehicles (EVs) could hardly be overstated as manufacturers and governments around the world appear to have increased interest at a staggering rate. The resurgence of current interest in the early part of the 21st century has been driven by both political and technological developments, namely a requirement to control global emissions and the emergence of new battery designs with improved specific energy, energy density and rechargability properties. One of the great advantages of the electric motor is its torque characteristic which provides maximum torque from zero up to low speeds, and then it is governed by the maximum power available as motor speed increase. This has two significant advantages over the typical torque-speed properties of the competing IC engine: (1) It is fundamentally a more desirable characteristic spread of torque over the speed range in contrast to the peakiness of an IC engine. (2) It removes the need for any additional transmission clutch or gears. However, one of the main conclusions to emerge from the plethora of research work into energy efficient vehicles is that it is necessary to pursue every possible avenue for minor efficiency gains. It is therefore of interest to investigate whether it is possible to manage the efficiency of the electric motor, so that by using an intermediate gearbox the motor is operated more often in its higher efficiency region. The aim of this paper is to develop a simple EV model and predict its energy consumption with a variable and fixed ratio gearbox over a standard driving cycle in order to understand whether this could offer significant efficiency gains. The emerging conclusions are that it is possible to improve overall energy consumption levels by around 5 to 12% with a variable ratio gearbox depending on the driving cycle used. However, there are many other practical considerations which must be weighed against this positive result and the paper discusses the impact of several of these such as, gearbox efficiency, additional weight, cost and complexity, effect on drivability and potential for motor downsizing.