Outlook 2016: Fuelling the development of hybrid electric vehicles

Reducing carbon emissions and improving fuel economy are, more than ever, of utmost importance to the future of the automotive industry. Increasing effort is being made to preserve our fossil fuel reserves and stringent legislation is in place across the world to control harmful emissions. Both of these trends add greater impetus to a move away from combustion engine centric vehicle designs and towards greater proliferation of hybrid and electric vehicles (HEVs).

Carbon emission levels have already, to some degree been reduced through use of better air management and thermal management mechanisms. It is clear, however, that the law of diminishing returns is coming into play and the big improvements are depleted. The next major step toward further curbing of CO₂ output and engine downsizing, while still maintaining driving pleasure, will be through greater electrification,lowering the continuous load on the engine (with on-demand systems) and hybridisation (introduction of an electric powertrain) enhancing the overall efficiency.

Increasing vehicle electrification has already enabled significant improvements to their efficiency. For instance, by replacing hydraulic power steering with electric power steering, it has been possible to cut CO₂ emissions by up to 5% in some models. But other pumps and fluid circuits are undergoing this transformation too. Greater proliferation of HEVs will allow us to go beyond this.

The popularity of HEVs is rising. Industry analyst firms have made healthy predictions about its future value. Technavio, for example, expects the HEV sector market to witness a compound annual growth rate (CAGR) of more than 15% between now and 2019. Although Japan currently leads the way in terms of HEV uptake (with one in every five new automobile sales being some form of hybrid), Europe is also making a considerable contribution to the long term promotion of HEVs.

There are various ways in which automotive manufacturers can downsize their vehicles’ engines and reach the emission reduction targets that are now being mandated.

Through incorporating turbos, they have been able to optimise conventional combustion engine efficiency levels – leading to lower CO₂/kWh figures of merit which embody the dynamics of lower carbon emissions.

Turbo lag is an issue

However, it should be noted that one major issue with turbos is the lag in their responsivity. The turbo will only be activated once the revolutions per minute (RPM) has exceeded a certain threshold enabling relevant compression of air intake. Variable geometry turbines can shift the lag to lower RPMs, but electric superchargers, conversely, can offer an attractive alternative or complement. With these devices, compression of the air intake is carried out through employment of an electric motor (mostly effective at lower RPMs), rather than high pressure exhaust gas. This greatly reduces the lag effect.

Although it is widely accepted that start/stop systems will be universal in cars in the not too distant future, this functionality will not be enough on its own to bring about the reductions that legislative roadmaps dictate. Hybridisation/electrification of vehicles needs to go much further.

An array of different HEV types is now on the market – including micro hybrids, mild hybrids, full hybrids, plug-in hybrids and electric vehicles. Full hybrids will have a relatively large electric motor that complements the vehicle’s combustion engine. In some cases, they can reduce by more than one third the emission levels that corresponding combustion engine vehicles would cause. As an all-electric mode (at least up to modest speeds of 50km/h and subject to available battery charge) is supported in full hybrids, the vehicle has to be equipped with a heavy and costly battery pack in order to provide the energy required to propel such vehicles.

Full hybrids and other HEV types have now been joined by mild hybrids. These vehicles come with an electrical powertrain that micro-hybrids (or so-called Stop/Start enabled vehicles) lack, but at the same time they do not require the high voltage batteries or high energy storage resources that define full hybrids.

Taking the ‘best of both worlds’, mild hybrids are an intermediate stage. Here, the electric motor is not at any point responsible for the vehicle’s propulsion, which is provided by the combustion engine. The electric motor simply offers additional torque at peak demands and a generator enables the kinetic energy recuperation during braking. This energy is typically stored in a 48V battery for use with the electrical powertrain, either driving high-power loads without loading the combustion engine or powering an electric supercharger, resulting in improved air management.

In this way, improvements in fuel efficiency of between 10 and 15% are possible. This means that vehicle manufacturers can still tackle the emission problem and specify smaller engines, while allowing performance to be kept at high levels and making these vehicles more commercially appealing. The mild hybrid subdivision is proving to be particularly successful in Europe and it is predicted that, in the next few years, Europe will represent close to 50% of sales of such hybrids.

Mild hybrids also bring benefits from a reduction in wire harness weight and cost reductions from not having to meet the far-reaching isolation standards of high voltage batteries. While mild hybrids may not be able to deliver the degrees of fuel efficiency comparable with full hybrids, they do, however, enable marked improvements on combustion engine vehicles and also offer consumers more attractive price points into the bargain.

More attractive pricing

Whereas full hybrid functionality will add somewhere in the region of £3000 to £6500 to the vehicle’s cost, mild hybrid functionality will not present that much of a premium – normally less than £1000. This is mainly due to that fact that these vehicles do not require as much battery capacity. For this reason, IHS Automotive predicts that, by 2020, mild hybrids will comprise around 15% of all HEVs being sold.

If the power goals that mild hybrid inverters set are to be reached with a 48V system, the associated peak currents will need to be more than 200A – a region where managing power losses becomes very important. The financial investment in inverter hardware is large and one of the dominant sources of cost in mild hybrids. Through the specification of smaller inverters with lower power losses, HEVs could be made less expensive to produce and therefore more attractive to consumers. It would, as a consequence, mean these inverters are subject to higher power densities and this would have knock-on effects for the current sensing technology involved. By working with the world’s leading car manufacturers, Melexis can offer current sensing technology that is appropriate for modern HEVs, as well as for the larger currents that will be seen in future models as they are developed.

In conclusion, the downsizing of inverters and sophisticated 48V hybrid systems presents the automotive industry with a highly effective way to decrease CO₂ emissions. The mild hybrids subdivision is allowing car manufacturers to circumvent the cost and weight problems that define full hybrid systems. Improvements in fuel efficiency can still be derived, but without the cost or performance drawbacks of the past.

Through more advanced supporting electronics, power densities can be raised and higher operating temperatures dealt with – permitting the miniaturisation of inverter designs that next generation HEVs will require and thus keeping the price tag low enough to entice car buyers.