The Extra Mile

Invaluable advice in keeping your running costs down and efficiency up

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Regenerative braking is now a commonly encountered technology, and increasingly so as plug-in hybrids (PHEV) and battery electric vehicles (BEV) significantly depend on it for their efficiency, and to achieve maximum battery range. But it’s also been used for quite some years on fossil fuel cars, by drawing on the available kinetic energy when a car is decelerating, it reduces the need for using engine power to keep the conventional 12-volt battery fully charged, thereby improving fuel economy, albeit by relatively small amounts. Putting aside the technology involved in this, and simply considering the benefits and efficiency of such energy recovery systems, we might ask how much real-world potential for fuel saving and battery range extension they actually offer? Tests on the full WLTP cycle show that, without any recovery system, around 25 per cent of the energy used over the cycle is dispersed by braking, in the form of heat. In a typical BEV, where around four miles are covered with one kWh of electricity, the potential exists to extend this to around five miles per kWh if all of that energy is recovered. That’s significant, although in practice at best 70 per cent of that is recoverable, reducing the potential range extension benefit to around 15 per cent, on the four-part WLTP cycle.

 

But there are big differences in the structure of the various WLTP sub-cycles – low, medium, high, and extra high speed. As much as 45 per cent of the energy used during the low speed test is potentially available for recovery, mostly due to the start/stop nature of the city motoring involved. That’s even more significant with the notable weight of the BEV battery taken into account. At motorways speeds though, the maximum recoverable energy figure drops to around 10 per cent, or even less in traffic-free conditions, when steady-speed cruising, with negligible braking, is possible. That raises another issue, where the technology of active cruise control systems can help reduce high speed fuel consumption, by pre-empting the need for any braking, and using the ability to maintain consistent inter-vehicle distances simply by early reductions in engine power.

 

Real life data derived from on-road research demonstrates that there’s actually a wide spread of energy recovery levels achieved by different drivers. Economical drivers actually stand to benefit somewhat less from the energy recovery systems than those who habitually drive with somewhat greater speed and aggression. That’s been confirmed in real life by journalist’s test driving to assess the energy-saving effects of the usual energy recovery settings available on most BEVs. More than a few of them have found that, with fairly serious economy-focused driving, they can achieve better energy usage figures, and push up the predicted battery range, just as well as, or better than, with the maximum energy recovery settings. If you drive in such a way as to largely avoid the necessity for braking, then there’s obviously less waste energy available to actually recover… and if you only recover at best 70 per cent of that, then skilful driving can actually “beat the system!” Also, dare we say it, the “electric” instant throttle response and massive torque of BEVs can easily become a fun feature which some drivers, including motoring journalists, may be tempted to use rather too often.

 

In conclusion then, eco drivers should appreciate that they can do quite as well as these hi-tech waste energy recovery systems – essentially by not creating that potentially wasted energy in the first place – and not dispersing it by braking when they do. Diesel, Petrol, Hybrid, and pure Electric cars, all have really significant levels of kinetic energy when accelerated up to their cruising speed, and few drivers probably realise just how much. On a flat road, a typical car can freewheel for well over a mile after lifting off the accelerator at 50mph. And a 300kg heavier BEV will coast a lot further still. The preservation of this energy, and avoidance of its wasteful use, is at the very heart of economy driving. When, for some reason, we might unnecessarily brake hard from an established cruising speed, do we really appreciate just how much potential free motoring mileage we have destroyed? Watch your trip computer mpg figures as you accelerate back up to cruising speed, and you’ll see just how much.

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