Hybrid drives deliver E-Xceptional payback
Tom Shelley reports on developments in hybrid transmission systems for military vehicles and fast-moving construction plant
By adopting hybrid diesel electric power train designs in heavy-wheeled and tracked vehicles, significant benefits in fuel economy can be realised, as well as a reduction in vehicle sizes and weights.
The main aims of one particular project are to cut back military vehicle fuel consumption, thereby aiding logistics, while at the same time have large amounts of electric power on hand to meet the demands of increasingly sophisticated electrically powered systems. There is the added goal of being able to provide the means to export power, when stationary, to lessen the need for towed generators.
Since 1996, QinetiQ has been actively involved in the analysis, design, development and testing of hybrid electric vehicle components and systems. As part of an MOD research programme completed in April 2007, it built and tested a wheeled, 18-tonne 6x6 hybrid-electric demonstrator (HED) vehicle. As a result of a separate tracked vehicle programme, it also unveiled its patented E-X-Drive electro-mechanical transmission recently, showing it at the Defence Vehicle Dynamics Show.
The vehicle completed formal trials at the end of 2006. Leighton said that the project, “Investigated the military impact of using hybrid electric drives in vehicles ranging from Land Rovers, through to truck sized vehicles and medium tracked armoured vehicles.
The HED is powered by two conventional 2.5 litre Volkswagen diesel engines driving two 80kW generators. Power is stored in two 80kW Zebra sodium nickel chloride high temperature batteries. Each of the six hubs is driven by a 50kW continuously rated permanent magnet brushless motor, which can deliver a peak power of 100kW for short periods, such as for skid steering. Each hub motor is oil cooled and connected to an individual oil-water heat exchanger in the spine. So, as Rob Leighton, lead engineer on the HED project, points out, “if you lost a wheel, you wouldn’t lose the whole lot”.
Everything is controlled by a power management computer system. “The driver can select from a choice of operating modes,” he explain, “hybrid electric, diesel electric, electric only or transition to battery.” This latter mode ensures that the batteries are fully charged up, ready for battery-only operation with the engines stopped.
The HED is a Series hybrid, Leighton emphasises, where there is no direct mechanical connection between the engine and wheels, whereas the Toyota Prius is a Parallel hybrid, retaining a mechanical link to the wheels. The weight of each hub is much heavier than in a typical civilian hybrid design, because each hub includes a gear train, with provision to change between high and low ratios plus a heavy-duty mechanical braking system, as well as electrical braking. The brakes are segmented disk brakes, akin to the brakes on a train. Due to the size of the vehicle – 18 tonnes – it has hydraulic power steering, the fluid being delivered from an electric pump. “If the whole system dies,” adds Leighton, “the power generated by the hubs, by virtue of the fact that they are rotating, is sufficient to keep the power steering going.”
As well as military vehicles, Leighton suggests that the technology is particularly appropriate for use by aircraft crash tenders, because they need high levels of starting torque. The transition to battery mode is also relevant to civilian hybrid vehicles required to run solely on batteries in city centres, something that is both desirable and likely to be required in more than a few urban centres in the near future.
Trials of the HED, conducted alongside QinetiQ’s conventional powertrain 6x6 High Mobility Demonstrator, have validated hybrid-electrical and conventional powertrain models in HYSIM, the software environment based on Simulink that the institution has developed for this work.
Mike Parsons, capability group leader, Hybrid Electric Drives, points to how the hybrid approach reduces the mass and volume of the power train. “Compared to that in a conventional vehicle, it’s half the size. In a Warrior, this would mean we could free up space or reduce the length of the vehicle by one wheel station. With a lot of electrical kit going on vehicles these days, we also need to be able to cope with increased power consumption.” The motors in the E-X-Drive are up to 97% efficient. Fuel savings come from a combination of the high competence of the motors, reduced gear meshes, and the improved operation and cooling of the engine.
Although the transmission has not yet been put in a tracked vehicle, business group leader Ian Beaton describes it as a novel arrangement, based on very well proven motor technology and epicyclic gear trains. The development, he adds, is for a large US military programme being undertaken with BAE Systems and Honeywell. “This is a second generation prototype,” he says, “and is potentially useful for any fast-moving tracked vehicle.”
According to Mike Parsons, its main components are two permanent magnet brushless motors, linked by a controlling differential, which is the key to its compactness. On the outside are two smaller steering motors, which impose a speed difference through the differential and allow the mechanical transfer of power, or regeneration, from one track to the other during high-speed steering events. When the driver wants the vehicle to go in a straight line, the steering motors are held stationary, which effectively locks the two main traction motors together. Parsons says that, although two steering motors are fitted, an impressive fall back capability was available with only one motor operational. The other motor is there to provide redundancy. The transmission and its components have been extensively tested in the laboratory.
The alternative to the two main motors, differential and steering motor(s) would have been to have an independent drive for each track sprocket. This would have required the drives and associated power electronics to be rated such that they could regeneratively brake the inside sprocket and transmit this power electrically to the outside sprocket. In order to match the steering performance of the E-X-Drive, this would have required a two- to three-fold increase in the power ratings of the motors and power electronics. This is due to the combined engine power and regenerated inner track power all being transmitted through one motor at the outer track during a turn. The E-X-Drive, with its integrated cross-shaft arrangement between the two sprockets, transmits the regenerated power mechanically to reduce the demands on the electrical system. As a result, when combined with a two-speed range reduction, the drive system used only requires 25% of the cornering power rating, compared to an independent sprocket drive system. This reduces the stress on the components and also the technical risk of the solution.
While most tracked construction equipment moves relatively slowly - in which case this type of epicyclic differential drive would not be a practical solution – in some circumstances, it needs to be able to move around quite fast. Fuel economy is an issue for all types of mobile equipment, whether civilian or military. So the lessons being learned from developing this hybrid transmission for military tracked vehicles are highly relevant to designers of tracked construction equipment generally, although most will doubtless want simpler and lower cost versions of the technology.
Total mass of the transmission is about 400kg, volume 112 litres, nominal power 330kW, specific power 0.825 kW/kg and power density 2.9kW/litre
* Hybrid diesel electric transmissions in military vehicles offer major fuel consumption savings, reducing logistical problems.
* Software models have been developed and validated to the point that future transmissions can be expected to work as required, right first time
* A series hybrid transmission system for tracked vehicles is half the size of its conventional equivalent and also offers major fuel consumption advantages, as well as providing power for other electrical systems
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