Your EV questions answered: How do plug-in hybrids work in practice and do they make sense for your pocket?

No, a PHEVbasically drives like a regular hybrid vehicle so there’s no need to intervene. But perhaps it would help to have a grounding in just how a hybrid system actually works.
A hybrid vehicle generally speaking has a detuned petrol engine, which has been set up to deliver less power than it otherwise might, in the interests of saving fuel. Many hybrid petrol engines run on the Atkinson Combustion cycle, which holds the inlet valves (the bits of the engine that let fuel and air into the cylinder) open fractionally longer than normal. That means more air than fuel in the cylinder, which is great for economy but lousy for power.
To make up the deficit in power, an electric motor is fitted, which covers up the gaps in the engine’s power delivery and – initially as a bonus for early hybrids, and more as the primary design feature on more recent hybrid models – allows for short bursts of electric-only driving.
Early hybrids – notably the Toyota Prius – were great at providing spectacular fuel economy when driving around town, but would generally lose the plot when taken on long motorway journeys. That was because at motorway speeds, there would be no chance to charge up their batteries from regenerative braking, meaning that the hybrid battery would become, effectively, a lump of deadweight to be hauled around and you’d be maintaining motorway speeds on an underpowered Atkinson Cycle engine.
Steady improvements meant that hybrids became more all-round usable, and could take long journeys without annihilating their fuel economy.
Batteries and electric motors also became more efficient, so that hybrids could – at low speeds – start to spend longer running on electric power alone, especially in city driving. A UCD study has shown a regular hybrid could spend as much as 70 per cent of its about-town driving with the petrol engine switched off.
Needless to say, this led some bright sparks to realise that if you have a hybrid with a bigger battery that’s fitted with a socket by which it could be charged up from mains electricity, you’d have a car capable of much longer stints on electric power alone, but still with the flexibility of a petrol engine on board for longer journeys.
Toyota was first into the frame again with this idea, with a plug-in hybrid Prius, which launched in limited numbers in 2012, and which had an electric range of just 18km.
Opel and Chevrolet worked together on another type of plug-in hybrid – the Ampera and Volt – in which the on-board battery and electric motor were solely responsible for driving the car, but there was a 1.4-litre petrol engine and a fuel tank on board which kept you mobile on longer journeys. The Ampera was a noble idea, but it arrived too early and at too high a price, and in commercial terms sank without trace.
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Mitsubishi took up the baton with the original Outlander PHEV, which sold in pretty big numbers, especially once business users worked out that there were tax cuts to be had when buying a plug-in hybrid. While those tax advantages have diminished, the plug-in hybrid is now enjoying a renaissance, with new models ranging from the ultra-sleek latest-generation Toyota Prius, with its 77km electric-only range, all the way up to a full-size Range Rover P440e, which can go for more than 120km on electric power.
In all of these plug-in hybrids, the basic functionality remains the same. You charge up at home or at work from a slow AC charging point (although a growing number of PHEVs can now also quick-charge from DC rapid chargers, but be prepared for abuse from any electric car driver who catches you using one …) and, in theory, do your daily short drives – to the shops, to school, to work – on the battery.
Most PHEVs will give you a claimed range of 50km and a realistic range of 30-35km at least, which is usually enough to cover a large proportion of commutes, although the latest long-range PHEV models – such as the latest Skoda Superb or the Mercedes C-Class – can manage a claimed 125km on electric power on a full charge.
Once the big battery has been depleted, you’re back on to petrol power (there are a handful of diesel PHEVs too …) and no, you don’t have to tell the car to do that. The electronic management systems take care of all that, and will also fire up the engine if you need to suddenly accelerate hard, or if the air conditioning or heating is putting too much strain on the battery.
Many PHEVs can now also be ‘geo-locked’, an innovation pioneered by BMW. Enter a destination into the sat-nav, and if your journey takes you through an area with an enforced emissions limit, the car’s systems will work out how much electric power you need to keep for driving in and through that area, and will automatically save enough battery juice to do so.
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What you can manually control is how the battery is used as you drive. For the most part, once the main battery has been depleted, a PHEV will drive along just like a regular hybrid, juggling electric and petrol power as needed. If you’re cruising along, you can manually tell the car to charge up the battery, using engine and braking power as it goes. Some will only let you charge the battery to a preset maximum of, say, 50 per cent or so, but some others will let you charge all the way to full if your journey is long enough.
The penalty, of course, is increased fuel consumption – a problem for plug-in hybrids in general on longer journeys, as they tend to be heavier and less efficient when operating with flat batteries than a conventional hybrid, or a plain old petrol or diesel car. Put it this way, once the big battery is flat, a plug-in hybrid will generally hover at somewhere between 6.5 and 8.0-litres per 100km on a motorway run, compared to the 5.0-6.0-litres per 100km you’d get from a regular hybrid.
Not all are like this, of course. The Toyota Rav4 PHEV is arguably still – in spite of its advancing years – the best of the plug-in hybrid breed. No matter how we drove it, nor how many hours we spent on the motorway, we couldn’t get a Rav4 PHEV to deliver worse than 5.3-litres per 100km (53mpg).