European companies that are taking diesel-hybrid technology very seriously include Land Rover, PSA Peugeot Citroën, Volkswagen, and Mercedes-Benz. The applications span all-wheel drive, off-road, and sedan applications, with a conviction that the CO2 and fuel consumption advantages of diesel are so marked that, despite the higher unit cost of an engine and NVH challenges, the technology provides the efficiency figures that end users expect and future legislation requires.
Land Rover is adamant that the diesel hybrid is the right approach for its vehicle range and cites its LRX concept as making the point. The company believes that a production vehicle of the type would be able to return a CO2 figure of 120 g/km and an NEDC (New European Driving Cycle) combined fuel consumption of 4.7 L/100 km.
The company’s initial move into production diesel hybrids has already begun, with a stop-start system on the 2.2-L Freelander 2TD4_e, which it claims as the world’s first intelligent stop-start SUV. Compared with the conventional diesel Freelander, consumption is improved by 12% in the combined cycle. CO2 figures are down 8%, from 194 to 179 g/km.
Particular challenges were to reduce shake during shutdown. This was achieved by precise control of throttle opening and fuel rampdown, together with switching off the alternator to reduce engine load.
Other changes made to the regular mechanical systems of the Freelander include a new polyetheretheketone friction control plate for the dual-mass flywheel.
Land Rover says that it has very extensive diesel hybrid plans. “The next milestone on our route map is a belt-ISG (integrated starter generator) micro hybrid,” said Murray Dietsch, Director of Land Rover Programs. “It will be used in conjunction with supercapacitors.”
Land Rover is working with the U.K. Technology Strategy Board on several hybrid and other green technologies. These include the plug-in range-extended hybrid vehicle; the kinetic hybrid (with a flywheel energy-recovery system); and the series hybrid, powered full time by battery electric drive. The vehicle would have a small three-cylinder internal-combustion engine driving a generator to recharge the battery.
The target for the REHV is to reduce CO2 emissions on the NEDC (combined) cycle by better than 35% and produce a zero-emissions range of at least 12 mi (19 km). In reality, this would mean CO2 emissions below 140 g/km on a future Range Rover and less than 100 g/km on a Freelander-size vehicle.
A further objective is to develop a parallel hybrid drive compatible with a fully capable off-road 4x4 system. This is Land Rover’s diesel ERAD (electric rear axle drive) hybrid concept, facilitating running in electric mode only in all-wheel drive. It would be modular and scalable.
Of course, advances in battery technology and capability remain high on the “need to achieve” list of every manufacturer of hybrid products. While nickel metal-hydride batteries are competent by today’s standards, recharging is a lengthy business. Lithium-ion batteries are lighter, have a greater energy density, and offer benefits in terms of rapid absorption of the charge from regenerative braking. The power-to-weight ratio is high; Land Rover quotes 2500 W·h/kg compared with 1400 W·h/kg for a NiMH, with 2.5 times more useable energy per kg.