What is a plug-in hybrid?

The demand for usable electric-only driving range coupled with the convenience of a ‘normal’ petrol or diesel car has spawned plug-in hybrid vehicles.

Essentially a plug-in hybrid is the next evolution of the ‘conventional’ hybrid vehicle with bigger batteries and the ability to plug into the electricity grid to charge, instead of relying on harvesting on the go. As with Toyota's Prius (and other older-style hybrid models), the plug-in hybrid is powered by an electric motor, but the electric motor operates more frequently in the case of the plug-in hybrid electric vehicle (PHEV). The PHEV is basically an electric vehicle (EV) with an on-board combustion engine to restore battery power when the charge runs low.

Almost every car company has a plan to electrify their fleet with plug-in models. Volvo is one company making an early commitment and even Bentley has promised a plug-in hybrid for each of its gas-guzzling models by 2025.

A plug-in hybrid is a bridging solution, much cheaper and more practical than a fully electric vehicle but with far better electric-only range than a Prius-style (non-plug-in) hybrid.

Because the plug-in hybrid retains a combustion engine, it also sidesteps the range anxiety which is one of the biggest barriers to widespread adoption of plug-in electric vehicles, particularly in countries like Australia.

If plugged in and fully charged the PHEV can drive a short distance without the combustion engine operating at all. Running in electric mode the plug-in hybrid produces no emissions of CO2 or nitrogen oxides, emissions that are associated with global climate change and health issues, respectively.

The rub is in the electric distance available. Most current PHEVs yield only around 40-60km – not far but sufficient for many Australians’ commute. And in the event that the driver must travel more than 60km – and the combustion engine starts to restore charge to the battery – the fuel consumption remains lower than that of a conventional car. Only when the driver chooses to use the combustion engine all the time is the PHEV less efficient than a conventional equivalent.

Australia expects a flood of PHEVs over the next few years in a wide variety of shapes and sizes.

Currently, plug-in hybrid vehicles like the Hyundai Ioniq, Mitsubishi Outlander and Porsche’s Cayenne E-Hybrid are high-profile examples.

In the example of these three PHEVs, the combustion engine does supply some motive power, whereas taking the example of the range-extended BMW i3 REx, the plug-in hybrid is nearer an electric vehicle, with the petrol engine only operating to recharge the battery.

As time goes by, more PHEVs will enter the market in this type of configuration. Mazda is understood to be developing a small rotary engine that will be fitted to production plug-in electric vehicles solely to charge the battery of the EV. This type of powerplant will draw fuel from a small tank to run and will still offer a range of up to around 400km for the plug-in version of Mazda's light hatch, the Mazda2.

Just as the plug-in hybrid is anticipated to supplant older hybrids, such as the Prius, so too the PHEV will yield to electric vehicles without combustion engines. And beyond that, fuel cell EVs will likely hit the market, offering longer range than EVs. All of this will be driven by emission legislation.

Hybrid technology in all its various forms – including the plug-in type – will be fundamental to powertrain progress in coming years, ahead of large-scale EV sales for a future in which the internal-combustion engine plays little or no part.