MAHLE Compact Range Extender Engine

With legislation forcing OEMs to further reduce the tailpipe CO2 emissions from their vehicles and with spiralling fuel prices driving consumer demand, the growth in ‘zero emission’ electric vehicles (EVs) is gathering momentum. However, to achieve acceptable range and dynamic performance, pure electric vehicles require batteries which are expensive, heavy and bulky. This leads to compromises in vehicle design, performance, packaging and cost. In recent years, the automotive industry has become increasingly focused on developing alternative solutions and new technologies for hybrid and electric powertrains.

One feasible solution to overcome the limitations in current battery technology is the integration of an on-board ‘fuel converter’. Its function is to convert liquid fuel, such as gasoline, into electrical energy whilst driving, thus enabling the traction battery storage capacity to be reduced, whilst still maintaining an acceptable vehicle range. Based on this primary function, the on-board fuel converter has become known within the industry as a range extender. In simple terms, the range extender is an auxiliary power unit consisting of a small internal combustion engine coupled with an electric generator which is used to re-charge the battery pack.

MAHLE Powertrain recognised the potential benefits in the application of range extenders and set out to design and develop its own unit in-house.
From the outset, it was important to fully understand the functional requirements of the range extender engine by defining the appropriate boundary conditions. Based on an application for a typical C-class vehicle, specific criteria and targets were established:

  • Maximum continuous vehicle speed = 120 km/h
  • Maximum (peak) vehicle speed > 160 km/h
  • Electric only range > 65 km
  • Additional mass added to vehicle < 400 kg
  • Capable of launching on 20% grade
  • Comparable dynamic performance to base vehicle

A detailed theoretical assessment was conducted to establish the rating of the traction motors and the battery pack which would be required to achieve a realistic cruising speed, maximum speed, acceleration and gradeability (the weight of the additional hardware was also taken into account). The well established Artemis cycle was used as the baseline for drive cycle modelling to represent real world driving conditions. From this analysis, and considering an optimum charging cycle, a range extender output of 30kW was calculated as being required.

A broad spectrum of alternative engine layouts and configurations were analysed using a ranking matrix for the key attributes including cost, weight, package volume and NVH. This detailed comparison resulted in a reciprocating, 2 cylinder, in-line engine of 900 cc capacity being selected due to its optimum combination of features. To allow cost, weight and package flexibility to be optimised, the electric generator is fully integrated within the assembly.

MAHLE Compact Range Extender Engine [PDF; 727 KB]