For vehicles with battery-electric drive, the use of a range extender appears to be a suitable solution to ensure a range approaching that of a conventional vehicle. The Stuttgart specialist MAHLE Powertrain has developed their own range extender, with flexible mounting position, in just 10 months without the support of an OEM.
Range extender and powertrain characteristics
The 900 cm³ two-cylinder 4-stroke engine with a 0° / 180° firing order makes 30 kW with a maximum torque of 72 Nm at 2000 to 4000 rpm. The synchronous electrical machine delivers 55 kW (100 kW peak). The torque is conveyed to the wheels via a two-speed reduction gearbox with a maximum of 120 km/h on the road. With dimensions comparable to those of a hand luggage, the entire unit is much more compact than the 1.2-litre, in-line, four-cylinder engine from the base vehicle Audi A1. This leads to a significant reduction in NVH levels in the interior, as all major drive components, apart from the high-voltage battery, could be placed in the front of the car. The battery has a 350 V nominal voltage and over 14 kWh storage capacity and is located under the boot floor in the spare wheel well, although built-in the center tunnel and behind the rear seats would also be conceivable. An integrated management system has been used to ensure optimized thermal management.
The small battery package reduces cost and weight, so that the total vehicle only weighs about 200 kg more than the donor car. The range extender engine from MAHLE starts in response to both the applied load and the state of charge, but the battery in the prototype uses only around 80% of its capacity. Naturally, the range extender starts earlier on, especially at low temperatures, as for example, the vehicle must be heated, but even in winter – provided sufficient battery charge is available – a pure electric drive away is possible. Cold starts should be largely avoided by catalyst pre-heating before the engine starts.
Performances of the MAHLE demonstrator
MAHLE set the operating strategy at minimum fuel consumption in compliance with the limits for emissions, noise and vibration levels. The fuel tank has been reduced by almost half to 25 liters. In real traffic, an electric-only range of 70 km should be possible with an additional 430 km available using the range extender, with emissions of less than 45 g CO2/km. Fuel consumption would thus be around 2 l/100km when starting with a fully charged battery. This is achieved by the high efficiency of the MAHLE-drive engine, which at its best point is 31%. Of course, the fuel consumption is measured on the NEDC, whereas in this vehicle, the real world usage would further benefit from the regenerative braking to potentially achieve better values. But in order to be as close as possible to reality, the MAHLE engineers have optimized the vehicle for best results on the Artemis cycle. This is based on real world driving profiles which are currently being verified during the vehicle development. First results show that even emissions of about 40 g CO2/km are in the range of possibility.
In addition to the driving range, the market acceptance will also depend on the NVH levels. There is still potential for big improvements in this area – especially through optimization of the exhaust system. However, the reduced speed of the prototype in the test drive had a very positive impact on NVH reduction.
Of course, the cost of the range extender depends on the production volume, and the cost of the electric motor in particular is more critical than that of the internal combustion engine. MAHLE, however, could build and supply the range extender for several customers so higher volume would be possible, which would have a positive effect on the costs. The price for a vehicle with a range extender could therefore be more competitive than a series hybrid and thus become commercially viable. Plus a series hybrid is probably already more attractive than a pure BEV.
Romain Nicolas opinion:
According to me, range extender is a series hybrid vehicle with an optimized downsized thermal engine. This is a low cost solution to increase range of a battery electric vehicle. However, it is not well spread in the street because very small engines used for range extenders are not on the shelves at OEMs R&D center. They either need to outsource the engine, either start a new conception cycle for this purpose which takes time. Do you think OEMs will be interested in outsourcing range extender engines at Mahle for example?