► We explain how range extenders work
► EREV? REEV? REx? Which is best?
► Can they get you more use out of your EV?

Welcome to the weird world of range extenders – a place of terminological nightmares and quite a lot of confusion around what the term even means. But with big-money luxury EVs selling like cold cakes, the concept is back in the spotlight.

Let’s start with the EV: one in which the entirety of the propulsion is provided electrically. The primary goal of fitting a range extender should be to extend the EV’s range by generating electrical power. Subject to a detail exception I’ll come back to, this means that in a range extended electric vehicle (or REEV, as I prefer, although to some it’s an EREV) there is no mechanical transmission of power from the range extender to the wheels.

Our next necessary definition is the series hybrid, where there is no mechanical link between an engine and the wheels; the engine is powering the electric motor. In the other main type of hybrid, the parallel hybrid, power can be transmitted either mechanically or electrically, giving two paths in parallel.

If you attach the electrical part in any hybrid to a ‘shunt’ battery you have the ability to smooth the power required from the engine by either taking extra energy from the battery or putting it in. This gives greater flexibility in operation, for instance by enabling recovery of kinetic energy through brake recuperation. By adding a larger capacity battery than is just required for driving around, you provide the ability to take energy aboard from outside the car – by plugging it into the grid, and thus achieving the plug-in (rather than ‘self-charging’) hybrid.

In an REEV, the aim is to cover nearly all of the vehicle mileage using battery energy. The range extender is there to alleviate range anxiety; if the battery is running low, you know the range extender (usually but not necessarily a combustion engine) is there to help out.

It’s a technology very close to my heart. We developed one of the first dedicated range extenders at Lotus in 2010, and this elicited commercial interest – a proposed production version started development, but in hindsight we were too far ahead of the market.

BMW separately launched the i3 range extender in 2013, using a modified engine from one of its maxiscooters. The thinking is that the range extender only needs to be powerful enough to propel the vehicle to a useful speed (not the maximum achievable when the battery is sufficiently charged) and thus to have some excess power over any lower steady-state cruise to charge the battery. A small range extender is sometimes termed an auxiliary power unit, or APU.

A range extender can be constrained to operate in a highly-restricted power range and thus be simplified versus a conventional engine (because it doesn’t need to cover the whole operating range, or react as quickly), while being optimised for efficiency. However, this is where philosophies diverge again.

For a truly optimised, practical daily REEV, the most desirable attribute of a range extender is low installed mass, because it’s carried all the time and so is always a draw on energy. Low mass is actually more important than outright efficiency – because the vast majority of driving should use stored electrical energy taken from the grid. We knew this at Lotus, and Mahle arrived at the same thinking, as did Austrian consultancy AVL, who proposed – and nearly got into production with Audi, in the form of the A1 e-Tron – a Wankel range extender.

However, people now seem hell-bent on developing very powerful REEVs, where a much greater proportion of the performance is covered by the engine; Audi’s Dakar-winning RS Q e-Tron takes this to extremes. These ‘power REEVs’ could be desirable for more luxurious applications. Lotus’s new Eletre X is an example. Because a much wider range of operation is now in play, the engines end up looking much more conventional. They are no longer APUs. Many Chinese manufacturers seem to favour range extenders with more power than is strictly necessary for everyday use.

That exception I mentioned earlier to the REEV definitions? Sometimes manufacturers mechanically connect the running range extender to the wheels. This is not really the same as a parallel hybrid because the electrical power is much greater than that of the engine, but it can be worthwhile – even over a range extender’s restricted operating range – because it reduces the losses involved in converting engine power to electricity and back to mechanical power at the wheels. The original Chevrolet Volt from 15 years ago was an example of such a ‘battery-dominant’ EV.

Now that engines will officially be with us for a while yet, customers are committing to PHEVs in ever greater numbers, while REEVs are attractive to brands who have sunk billions into potentially-stranded EV platforms that can relatively easily be modified to take range extenders but not conventional engines. The advent of the power REEV may even help to break down some of the reticence towards pure electric propulsion among high-end car buyers: I really hope Lotus succeeds in this.

When people realise how little the range extender is active that will be a useful step towards mass acceptance of full EVs. The opportunities are myriad.