The new WLTP (Worldwide harmonised Light vehicle Test Procedure) for vehicle emissions testing, scheduled for 2017, will at a single stroke sweep away many of the techniques that powertrain engineers employ to ensure their vehicles satisfy the requirements of increasingly tough emissions regulations. In parallel, the US$300m penalties recently levied on a vehicle manufacturer for being optimistic in its fuel economy claims show that there is growing pressure from Governments and customers to ensure that the stated test results reflect what drivers can realistically achieve.
This means that the technology choices for the next generation of powertrains could be the most important ever made. Vehicle manufacturers have to find a way not just to hit the next generation of emissions and FE targets, but to hit them while also closing the gap between the results achieved using outmoded test cycles and how people drive their cars in the real-world.
The current New European Driving Cycle (NEDC) test cycle has been widely criticised because of the growing disparity between the ‘official’ figures it generates and real-world achievements for the same vehicle. One of the most significant areas in which its age is showing is that the test cycle includes only modest levels of acceleration, reflecting the substantially lower performance of almost every vehicle at the time of its development. This limited performance envelope, and other historical flaws, such as motorway driving not being represented, has led to the development of ‘cycle-beating’ calibrations that, according to the ICCT (International Council on Clean Transportation), show progressively improving Type Approval figures for vehicle CO2 emissions, without a corresponding improvement in the real world.
The ICCT report is in fact quite damning. Of 15 Euro 6 diesel car types tested using what appears to be robust protocols, only one met the Euro 6 NOx limits ‘on the road’ and none even came close to meeting the Euro 6 CO2 limits. The worst performing vehicle emitted an astonishing average of seven times the type approval NOx figure. The greatest disparities were seen with premium vehicles and, interestingly, today’s electric hybrids.
Replacing the NEDC procedure with the WLTP drive cycle will force vehicle manufacturers to close this gap, equivalent to mandating a significant cut in emissions. But it will also come with new, dramatically lower numerical targets. This means that the actual gap to which powertrain engineers must find a solution is in reality substantially wider than the figures suggest. Incremental improvements will no longer be enough; new technical approaches must be found.
The optimum powertrain for real-world emissions reduction
Real-world driving includes three main operating regimes: acceleration, which demands high power levels for short durations; steady cruising for prolonged periods, which may require as little as 20bhp even in a large sedan; and deceleration, where the vehicle’s hard-earned forward momentum is traditionally dissipated as heat in the friction brakes. Hybrids take a significant step forward by addressing the recovery of energy in the third state, but the ideal powertrain should be optimised for all three.
Current hybrids typically consist of a powerful combustion engine, producing over 100 kW, coupled with a much smaller alternative energy source of perhaps 25 kW. To address each of the three operating regimes with a hybrid powertrain strategy, this needs turning around so that efficiencies can be increased and more energy can be captured and re-used. Reaching future targets will be significantly simpler with something like a 30 kW internal combustion engine to cater for cruise conditions and a 100 kW alternative energy source providing acceleration and energy capture.
The challenge is cost. Even complex IC engines, with a host of downsizing and emissions equipment – expensive units in historical terms – are affordable when compared with the cost of high power electric energy recovery and storage systems. Add the investments required at dealers to allow them to handle high voltages safely, together with battery end-of-life issues, and you have a challenging picture.
It is no surprise, then, that research by the University of Michigan Transportation Research Institute (UMTRI) showed that hybrid penetration in North America is still only at 3.8%. This conflicts with UMTRI’s survey of buyer preferences in which 31% of current non-hybrid owners stated they would purchase a hybrid for their next vehicle. The report concludes that the barrier is purchase price, a fact confirmed by other independent researchers. This is despite vehicle manufacturers often accepting lower margins on these vehicles (Deutsche Bank, reported in Automotive World, 10 October 2014) to mitigate their high manufacturing, engineering and materials cost. Clearly, hybrids are the way forward and hybrid strategies can and must develop substantially, but today’s high-voltage electric hybrids are not be the whole answer.
Alternative approaches
Vehicle manufacturers are therefore searching for new ways to address this issue, leading to a greater appetite for innovation. Front-end 48 V systems, for example, enable some relatively low-cost hybridisation, currently predicted to offer up to 10 kW to supplement the IC engine. But as we have seen, although this is a clever and worthwhile system, on the technology strategy road map it is a move in the wrong direction if we are to take a substantial step forward.
At Torotrak, we believe a purely mechanical solution to the cost and complexity challenges of hybridisation offers great promise. By eliminating the high-voltage electrical systems, our Flybrid technology is substantially more affordable than conventional hybrids – around one quarter the cost – and at around 60 kg for a road car system, it is also around a third the weight. The key is to keep the recovered energy in its mechanical / kinetic form, storing it in a carbon composite flywheel that spins at up to 60,000 rpm. With low system weight, and output power typically in the 60-120kW range for a road car application, the system is very power dense, and is therefore well suited to supporting acceleration. Because it eliminates batteries, the flywheel system also provides full depth of discharge throughout the vehicle’s life, independent of temperature or drive cycle, and has no recycling issues.
Another benefit of eliminating batteries is that more energy can be recaptured more quickly and more cheaply. One of the reasons that battery packs are so costly is that they don’t like fast energy flows, so they have to be substantially over specified to ensure durability. This also means they can require heavy and costly thermal management systems to keep the large battery mass within its optimum temperature range. Because of the relationship between energy and speed, one stop from highway velocities can recover as much energy as many stops from urban speeds, if the system can capture it. Our tests show that contrary to popular myth, hybrids that can recapture kinetic energy at a high power rating produce substantial gains even outside an urban drive cycle.
Several manufacturers are at an advanced stage of prototype evaluation with flywheel hybrid vehicles and, even when a combustion engine remains the primary power source, some significant gains have been identified; Volvo, for example, has demonstrated a 25% fuel efficiency improvement in real world driving, compared to an equivalent IC engine.
The quantum leap in emissions reduction will come when a larger flywheel hybridisation system is integrated into a vehicle with a much smaller engine, acting as a ‘load leveller’ to deal with peaks in acceleration demand. We will then also see some interesting developments in combustion strategies and significant reductions in their complexity as they will no longer have to rely on costly technologies to provide a wide spread of power and torque from a small unit.
The majority of our current vehicle manufacturer evaluation programmes install the unit to drive the rear wheels as a first, easily implemented improvement to an existing front-wheel drive platform. We also have a programme that integrates the flywheel unit with the transmission, which provides further significant cost and packaging benefits. This could be the first steps towards a new type of integrated, FWD powertrain that costs no more than today’s complex, downsized engines yet makes a significant contribution to solving the emissions and fuel economy issues that are now challenging vehicle engineers.
Ironically, a mechanical flywheel hybridisation system could also be the enabler that helps EVs break free from the cost and range issues that are holding back their market acceptance. Using a flywheel KERS for very fast capture of kinetic energy allows a smaller, cheaper battery pack to be specified, giving the vehicle manufacturer new choices in how to balance cost, performance and range. Even electric vehicles can be better as hybrids!
So although WLTP and consumer and Government pressures on fuel economy and emissions present substantial challenges for the industry, it could be that the search for a step-change solution delivers a new, lower-cost starting point on which to build. The future doesn’t always have to be more complex. The best engineering solutions are always the simplest solutions – and that couldn’t be more true in an industry where cost, weight and durability are so critical.
