MAHLE range extender vehicle

Using a combustion engine as a range extender, battery-powered powertrains in passenger cars can achieve a range that is comparable to that of conventional powertrains. The range extender can therefore become a gateway for broader series applications in vehicles with electric drive systems. MAHLE is currently investigating this powertrain technology using a range extender combustion engine and a demonstrator vehicle, both developed in house.

As the basis for the demonstrator vehicle with electric drive and range extender (Range Extended Electric Vehicle—REEV), MAHLE selected a conventionally powered B-segment vehicle. The main reasons for this decision were the market estimates for range extender powertrains and a typical user profile that includes daily countryside and freeway trips in the range of 50 to 80 km, as well as the package-related challenges. The compact and flexible installation position of the MAHLE range extender engine is particularly useful here.

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. Thanks 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.

MAHLE has 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. Dynamic load control of the generator and appropriate selection of the operating points provide a very good baseline. As shown in extensive cold start tests, partial-load operating points at low speeds provide the best compromise between low exhaust gas emissions (approximately 30 percent 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 so-called plug-in hybrid vehicles in general), the purely electric range must be maximized while the range extender must be prevented from overcharging the battery. Assuming that electric power is derived from "regenerative" energy sources (wind, water, or solar), any energy remaining in the battery at the end of the trip would be counterproductive from a CO2 standpoint. 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 operating strategy enables the demonstrator vehicle to reach beyond the purely electric cruising range of 70 km, adding more than 400 km with a total of less than 45 g CO2/km. This amounts to a CO2 reduction of around two thirds compared with the good initial figure from the basic vehicle. The key here is the very high overall efficiency achieved by the MAHLE powertrain. Greater than 31 percent at the best point in the overall powertrain is an impressive value, particularly considering the physical disadvantages in efficiency for serial hybrid drive concepts with two additional energy conversions (from mechanical to electrical, and electrical back to mechanical). While the efficiency of a modern conventional powertrain, at about 35 percent (gasoline) and over 40 percent (diesel) cannot be quite matched by a REEV due to the nature of the system, the difference is astonishingly small with the MAHLE powertrain.

Owing to global efforts to further significantly reduce fuel consumption and emissions of vehicle powertrains, a wide spectrum of different powertrains will be used in the future, depending on the area of application. This will be largely contributed to by continuously optimizing the established combustion engine. In the next decade, purely electric powertrains could reach a global market share of about 5% in the urban environment with small and compact class cars. Thanks to the range extender, these vehicles will significantly increase their previously limited cruising range and reduce overall costs as a result of the use of batteries with less storage capacity. The range extender can thereby make the electric powertrain much more attractive. Based on in-house powertrain scenarios, MAHLE thus anticipates that the range extender has a realistic opportunity with purely electric vehicles in the next ten years, and is actively shaping the ongoing development of future mobility with this product.