It's no secret that the market appeal of battery-electric vehicles (BEVs) is limited by their batteries. To achieve even moderate range and performance in a compact BEV weighing 1.4 t (1.5 ton)—for instance 80 km (50 mi) maximum range per full charge, a 160-km/h (99-mph) top speed, and 120-km/h (75-mph) cruising—the electric powertrain would require "something like a 55-kW traction machine with 100-kW peak, and a 46-A·h battery that can support 15 kW·h at 320 V”, said Dr. Bernd Mahr, CEO of Mahle Powertrain Ltd. in Northampton, U.K.
Extending this electric range by another 30 km (18.6 mi) would add roughly €3000 of additional cost, "which appears somewhat out of proportion,” Mahr noted.
Mahle is therefore offering a solution for expanding the BEV's appeal. At the Frankfurt Motor Show the company presented a new, specialized range-extender (RE) engine. According to Mahr, the RE is designed to add 550 km (340 mi) to the maximum range of a 80-km BEV, but at an incremental cost equivalent to adding 30-km (18.6-mi) additional battery range.
When the battery is low, the RE will start-up and re-charge it, which turns the BEV into a series-type hybrid whenever the RE is running.
The new twin-cylinder, port-injected gasoline combustion engine displaces 900 cm3. Rated output is 30 kW (40 hp). The engine drives a generator that is fully integrated into the cylinder block.
The ICE is designed to have two main operating points: If the battery needs to be re-charged, the engine is run to deliver 15 kW (20 hp) at the crankshaft. If the battery charge is to remain high despite a fast cruising speed, the RE is taken to full load at 4000 rpm to deliver its nominal 30 kW.
“The prototype has confirmed these performance targets on the engine test bed,” Mahr told AEI. Specific consumption is said to be 250 g/kW·h at 4000 rpm (0.41 lb/hp·h) and 240 g/kW·h (0.30 lb/hp·h) at 2000 rpm.
Fully dressed, the engine/generator unit weighs 65 kg (143 lb), with a 2.15 kg/kW power-to-weight ratio. When a full 40-L (10.6-gal) fuel tank plus controller are added, the RE's total weight (as installed in a vehicle) is 130 kg (287 lb).
A compact package
One of the challenges in RE vehicle design is integrating the combustion engine. Bearing this in mind, the Mahle RE prototype measures 481 x 416 x 327 mm (18.9 x 16.4 x 12.9 in). Overall, the unit is about the size of a small suitcase, enabled by an intake system that nestles closely to the crankcase.
“By using 1-D and 3-D simulation it was possible to sufficiently dampen resonant frequencies despite the short intake length," Mahr explained. "In total, the engine has an excellent level of NVH, which is more than a convenience in a serial hybrid. The RE must blend in harmoniously when active.”
Also the engine can be installed upright (0°), horizontally (90°) or at any angle depending on packaging requirements. As the ignition sequence was defined to be at 0° crank angle and at 180° of crank angle the engine does not require a balancing shaft.
An overhead camshaft indirectly actuates two valves per cylinder via roller finger followers. Tested within a numerical drive train model to different user profiles, this RE undercut the reference vehicle CO2 emission levels during all driving situations, including the worst-case scenario of an empty battery.
This is Mahle's second ICE to be engineered and built in-house. The RE probably benefited from the know-how the supplier accumulated during the development of its extremely downsized 1.2-L three-cylinder demonstrator engine. However, in contrast to that unit (which was intended only as a technology showcase), the range extender is a prototype that Mahle might well manufacture in numbers.
Said Mahr: “For vehicle manufacturers it makes sense to outsource this type of engine manufacturing as the initial numbers will be low. From our ongoing discussions with OEMs we think that some OEMs may not wish to invest in setting up a new engine line for such a niche product.”
The future market potential of RE technology in general is still undetermined. More than halfway through their first year of production, the battery-electric Nissan Leaf is outselling the range-extender Chevrolet Volt by a considerable margin.