Electric Traction – A Comparison Of Different Wheel-Hub Drives
Abstract: Electric Traction – A Comparison Of Different Wheel-Hub Drives
In this paper, a comparison concerning electric traction drives for passenger cars is given. Electric traction drives presently available on the market are analyzed and future developments are described. Two main classes of such drives are presented: centre drives (like presently known from hybrid cars) and wheel-hub drives (which are still in the research and development phase). The wheel-hub drives are investigated in detail. Two different concepts are regarded: High-speed drive with gear-box and low-speed direct drive. The advantages and disadvantages of both concepts are shown, resulting in the fact that most probably the low-speed direct drive performs better.
Introduction: Electric Traction – A Comparison Of Different Wheel-Hub Drives
The present discussion on the CO2 emissions of passenger cars gives a new stimulus to electric traction drives. At least for city travel the fuel consumption and consequently the CO2 emissions can be reduced by applying a concept containing electric traction.
Automobiles with electric traction – such as battery, hybrid, or fuel cell vehicles – need additional space for the energy storage, if an acceptable operational range of the vehicle is required. This comes from the fact that the energy density of batteries or hydrogen storage tanks is much lower than that of conventional fuel tanks. Therefore, integrating the electric motor into the wheel-hub offers additional space under the hood, which may be used for additional energy storage.
In general, there are two different possibilities of realizing an electric wheel-hub drive: low-speed direct drive or high-speed drive incorporating an additional gear-box. In this paper, these two alternatives are compared concerning weight, volume, and complexity for a typical mid-class passenger car. For this application, weight is one of the most important parameters, because the unsprung mass is increased with (at least in principle) negative effects on the driving performance of the car.
Starting with the requirements to such an electric traction drive, both alternatives (low-speed direct drive and high-speed drive incorporating an additional gear-box) are investigated in detail. The most important influencing factors will be emphasized and analyzed. Electromagnetic and thermal optimizations using finite element method (FEM) calculations lead to electric motor designs for both alternatives, that can be realized with today’s technology. Finally, upcoming technical and economic trends are judged concerning the impact on electric wheel-hub drives.