A range-extender (known as a REx or REEV) is pleasingly simple: it’s an electric vehicle with a smaller battery that can be topped up by a petrol engine when the state of charge drops.
Traditionally, a REx is a type of series hybrid, which means the electric motor turns the road wheels and the engine works as a generator, charging the battery.
Modern REx engines are small, light and tuned for high efficiency, and they run at a constant speed when the battery needs charging.
A REx has something in common with a PHEV in that both can be plugged in to charge the battery and run in pure-electric mode.
The main difference is that with a PHEV, both the engine and the motor are used to turn the wheels.
Both PHEVs and traditional full hybrids like the Toyota Prius are described as ‘parallel hybrids’, because the engine and motor can drive the wheels in parallel, ie at the same time.
Like other hybrids, a REx has the disadvantage of being more complex than pure EVs in the sense that they incorporate a mixture of both combustion and electric drivetrains.
Another potential downside is that a REx could go for longer periods with the engine lying dormant – but most are generally started periodically by the car’s electronic systems for a ‘maintenance cycle’.
RExs are ideal in parts of the world where the EV public charging infrastructure is sparse, and they are popular with Chinese manufacturers, which is a powerful incentive to make the technology easily accessible.
A REx’s high-voltage battery is a lot smaller and cheaper than that of a pure EV, too, making a turnkey REx drivetrain a cost-effective solution.
Suppliers including Horse Powertrain are now beginning to offer modular REx units, complete with integrated power inverters, which any manufacturer can buy and incorporate into its vehicle range.
Horse, a collaboration between Renault and Geely, is producing a number of REx solutions, including its small, ‘suitcase-sized’ C15 unit, which can be installed at the front or rear of an EV platform.
ZF is also starting volume production of modular REx units this year. One example is the eRE+, which operates as a generator but can also be connected to the car’s wheels by means of a clutch to give four-wheel drive.