Having commented in the last issue on the economy potential of electric cars and hybrids in relation to the type of motoring involved, it was interesting to recently come across some relevant research results regarding plug-in electric car fuel efficiency variations between different drivers.
Six drivers were asked to tackle four different types of test circuit in a fleet of identical cars, and the resulting spread of results for their energy efficiency (in terms of measured battery drain) was analysed. The variations between drivers for power drawn from the battery during high-speed, urban, hilly route, and handling circuit were minimal, and yet the overall fuel efficiency recorded varied significantly. It transpired that it was the rate and amount of deceleration generated by the various drivers that was the key. During the deceleration phase, which in practice is a varying combination of electrical regeneration and friction braking at the wheels, the most efficient car recovered 93 per cent of the available recoverable energy, whilst the least efficient one recovered only just over 5 per cent of the potential energy savings. That was apparently because the hard-driven car was reaching significantly higher speeds at times, necessitating harder braking, and leading to significantly lower levels of recovered energy.
In the city test cycle, the same effects were smaller, but still significant, the economical driver recovering 28 per cent of the energy used, the hard driver only 13 per cent of the energy that they drew from the battery. This highlights that, as a spin-off from different driving styles, significantly more electrical energy can be recovered during relatively gentle deceleration, without employing much braking, as compared with that when using higher deceleration rates, where harder use of the brake pedal leads to significant heat energy losses.
So, whilst we said in our last issue that “much of the fuel-saving work is done for you when you’re driving a hybrid” (and much the same goes for pure plug-ins), it’s still very much in a driver’s hands how much of that potentially recoverable energy is actually recovered. There’s a maximum potential energy recovery rate achievable in these battery regeneration systems, and thus a maximum deceleration they can offer before the conventional brakes are necessarily brought into action, with consequent total energy loss in the form of heat. Some hybrid and electric cars do offer different settings for levels of power regeneration and, as regeneration systems are further refined, the proportion of energy recoverable will no doubt rise. At present though, high level energy recovery settings can give a somewhat unsettling driver feedback, almost as if the handbrake has been left on.
In the meantime, whilst all hybrid and electric vehicle owners can benefit from minimising use of hard braking, you will pay little penalty for using hard acceleration, providing that you lift off early enough when you need to slow down, and thus minimising any braking, and allow the energy recovery system to operate most effectively. Of equal, if not greater, importance, some wider monitoring of the same group of drivers produced a variation in the effective driving range of their electric cars of nearly 2:1, or from 35 miles to 65 miles. So drivers who actively drive conventional cars with fuel economy in mind are probably better suited to benefiting from the advantages of hybrids and pure electric cars than many other drivers. So they should be amongst the ranks of the early adopters who might benefit most from energy-saving electric and hybrid car technology – although with the reservations that we’ve pointed out regarding their suitability for your type of normal everyday motoring.