There are stringent legislative policies now in place across the EU to curb harmful emissions, ensuring future generations can enjoy cleaner air and preventing irrevocable damage to the environment. At the same time, increasing effort is being made to reduce dependency on the planet’s dwindling oil reserves.
These dynamics both point towards a migration away from automobiles based on combustion engines alone and heavier use of hybrid and electric vehicles (HEVs). However, a series of obstacles have prevented widespread HEV proliferation from taking place. Recent technological breakthroughs have demonstrated the challenges are surmountable.
Carbon emission levels have been addressed through better air management and thermal management systems, which has allowed a certain degree of engine downsizing. The next major step toward reducing CO2 output will be increased usage of electrification and hybridization. This reduces the load on the engine (through more on-demand systems), and optimizes the combustion engine efficiency.
Electric cars are becoming increasingly popular. They already form the basis of many established elements of modern automobile design. For example, the replacement of hydraulic steering mechanisms with electric power steering has led to significant reductions in CO2 emissions (as much as 5 percent in some models). Through uptake of HEVs, things can be taken much further.
Though up until quite recently, automobile manufacturers doubted their validity, commercial acceptance of HEVs is taking place around the world. Industry analyst firm Freedonia has predicted that worldwide hybrid and electric vehicle (HEV) sales will more than double between now and 2018.
Currently, Japan leads the way with regard to the endorsement of HEVs (representing over 20 percent of its annual vehicle sales). This is followed by North America, then Europe —though in both of these cases the percentages are far lower. Various types of different HEV now exist, including micro hybrids, mild hybrids, full hybrids, plug-in hybrids, and electric vehicles. The options available stand as a testament to the popularity and importance of the HEVs.
Improving Energy Efficiency
Vehicle manufacturers are looking at different ways by which they can downsize engines and reduce emissions. Turbos have long been able to optimize the efficiency of conventional combustion engines as part of their air management function (with lower CO2/kWh figures resulting). Turbos use high temperature compressed exhaust gas to drive a secondary ‘cold side’ circuit, compressing the air intake for the next combustion cycle. Combinations of exhaust gas can be fed into the secondary side air intake in exhaust gas recirculation (EGR) systems, allowing compliance with the strictest of emission standards—such as those concerning nitrogen oxides (NOx), the other pillar in the cleaner air mantra. One of the handicaps intrinsic to turbos is the response lag caused because the turbo only works once a certain RPM threshold has been reached. Even with optimizations, such as variable geometry turbines (VGTs), which maintain the optimal aspect ratio as a function of the RPM, the lag can never be eliminated. One solution to this problem could be electric superchargers because compression of the air intake is not achieved via the exhaust gas’s high pressure, but through an electric motor, which is effectively ‘on demand’, with no lag to worry about.
It is generally accepted that all cars will eventually, at the very least, have start/stop functionality. Nonetheless start/stop alone will not pave the full road to the 2020 targets being carved in legislation. The hybridization/electrification needs to be brought to the next level.
Full Hybrids
Full hybrids normally rely on a 40kW to 70kW electric motor, which works in tandem with the vehicle’s combustion engine. Some of the most prominent examples of full hybrid vehicles are able to reduce CO2 emissions compared to equivalent combustion engine models by as much as 35 percent. The electric motor should be in operation for the whole time that the vehicle’s engine is running, or at least up to a certain speed. This mandates the specification of a large, cumbersome, and expensive battery pack, to provide the high voltage required for high power electric motors that enable a fully electric drive. Additional on-board charger electronics are required for the plug-in hybrids and the associated energy storage necessary to cover an extensive mileage.
Mild Hybrids
Alongside full hybrids, plug-in hybrids and electric vehicles, a new category of HEV has started to appear—mild hybrids. Mild hybrids possess greater functionality than stop-start systems or micro-hybrids. In most respects, these vehicles have more in common with conventional combustion engine based vehicles than with HEVs. What differentiates a mild hybrid from a full hybrid is the electric motor is not responsible for propelling the vehicle on its own, with power ratings in the range of 5kW to 20kW. The vehicle’s combustion engine takes care of this, with the electric motor just there to provide additional support, the so-called torque-assist. This approach offers improvements between 10 to 15 percent in fuel efficiency, whereas full hybrids can easily double that. With mild hybrids, car makers still have the goal of further minimizing engine size and delivering emission reductions. The benefits of lower wire harness weight, wire harness cost reduction and no longer needing to comply with the high isolation standards of high voltage batteries are to be derived and at the same time ensuring strong levels of performance are maintained and driving experience is unaffected. To this end it is necessary to deliver an electrical boost along with kinetic energy recuperation function on a lower voltage net than full hybrids. Typically systems known as integrated belt starter generator (iBSG), integrated motor assist (IMA) or belt-assisted starter (BAS) provides instantaneous boost upon acceleration. The kinetic energy recovery system (KERS) further increases overall efficiency by recharging the battery and/or powering the electrical loads.
Mild hybrids may not be able to deliver degrees of fuel efficiency comparable to full hybrids —they do, however, enable marked improvements on conventional combustion engine vehicles and also offer consumers more attractive price points. While a full hybrid will add at least $4,000 to $6,000 to the cost of vehicle model in a given category, a mild hybrid will only add around $1,000 —as it does not require the same quantities of battery power capacity and power in general. For this reason IHS Automotive predicts that by 2020, around 15 percent of all HEV production will be for mild hybrids.
Mild hybrids (along with micro-hybrids) have seen most traction in Western Europe. It looks likely for the next few years that the European market will constitute nearly half of the total global sales for this particular HEV category. If the power goals being set by mild hybrid inverters can be reached with a 48V system, there is no question that currents will be pushed way beyond the 200A mark.
The considerable expense of inverter hardware has, to some degree, restricted HEV adoption, with government subsidies or other incentive schemes—like the no road tax on hybrid vehicles in Japan and free license plates in Shanghai, which are otherwise valued at several thousands of dollars—being offered to add appeal to car buyers. The use of smaller inverters with lower power losses would make it possible to significantly reduce the costs. This would impact the commercial success of HEV models by lowering price tags, as well as reducing overall weight. It would, in turn, mean that these inverters exhibit much higher power densities, which would have implications for the current sensing technology employed.
Advanced 48V hybrid systems, such as the ones discussed in this article, are destined to present modern society with a credible way by which to save fuel and lower vehicle emissions without requiring major financial investment by consumers. The relatively high cost of automotive inverters have traditionally limited wholesale HEV adoption. Consequently, there is heightened interest in finding ways in which inverters can be downsized. The emergence of the mild hybrid subdivision of the HEV sector is allowing car buyers to derive some of the benefits of hybridization without being exposed to the higher costs normally associated with it. Vehicle manufacturers can avoid the cost and weight penalties that come with full hybrid designs while still improving fuel economy. Through use of more compact electric motors and smaller less capacious batteries, these vehicles will be able to gain a competitive edge in the HEV market. The support of next generation component technology will allow the higher power densities and elevated temperatures that define such implementations to be attended to.
