MAHLE selected a conventionally powered B-segment vehicle as the basis of the REEV demonstrator. The in-line two-cylinder design with integrated generator makes the range extender significantly smaller than the 1.2-liter in-line four-cylinder engine installed in the base vehicle.

Due to the extremely compact package of the 55 kW (100 kW peak) electric traction motor and of the two-speed reduction transmission, all of the major drive components (including the inverter and control units), with the exception of the high-voltage battery, were easily housed in the vehicle’s front end.

The direct proximity of the combustion engine to the electric drive components in the already tight installation space of the vehicle’s front end requires the integration of separate cooling circuits.

The typical coolant temperature of about 90°C in the combustion engine must be prevented from affecting the significantly lower temperature level in the cooling circuit of the electric motor (about 40°C) by means of insulation. The cooling strategy of the main radiator in the demonstrator vehicle is based on the requirements of each of the individual circuits. When additional cooling is required in one circuit, it receives top priority in the interaction of the individual circuits.

Fundamentally, the electric powertrain was designed to allow the demonstrator vehicle to meet or even exceed the driving performance of the base vehicle, with the exception of top speed. It was also important to demonstrate that the drive configuration does not require sacrificing loading volume. The high-voltage battery—with 14 kWh storage capacity—is installed below the floor in the spare wheel recess without affecting loading space or passenger compartment. In addition, the approximately 45-liter fuel tank from the base vehicle was reduced by almost half, to 25 liters.

Operating strategy. MAHLE optimized the operating strategy of the range extender for minimal fuel consumption in driving operation while complying with limits for exhaust, noise, and vibration emissions. To this end, variations in rotational speed within a combustion cycle were reduced as much as possible in the early stages of development, thus minimizing the application effort required for the engine.

Partial-load operating points at low speeds provide the best compromise between low exhaust gas emissions (approximately 30% of Euro 6 limits) and fuel consumption in the NEDC.

Based on the current applicable European exhaust gas legislation, in order to obtain the least CO₂ emissions values in electric vehicles with range extenders (and plug-in hybrid vehicles in general), the purely electric range must be maximized while the range extender must be prevented from overcharging the battery.

An operating strategy for REEV that optimizes fuel consumption only starts the range extender when a low battery charge level is reached (to maximize the purely electric cruising range) and then remains only marginally above the currently required drive power (e.g., +1 kW). In order to reduce noise emissions in actual driving operation to practically below the background noise level, one potential operating strategy has the range extender starting only in exceptional cases (such as very low battery charge levels) if the power demand is below 5 kW or vehicle speed is below 45 km/h. In the standard operating condition, the power output is adjusted proportionally to the speed of the vehicle.

The advanced knowledge of the route prior to the start of a journey provided by GPS enables the new software to calculate the battery’s available power throughout a trip.

This enables the software to pre-determine the optimum operating strategy for the gasoline engine resulting in the best charging efficiency. We’ve seen considerable benefits from the use of the new control software. Those benefits include improvements in fuel economy and a reduction in emissions.

—Bernie Porter