'Plug-in' electric/petrol hybrid

As you saw in Section 5.2, hybrid vehicles use a battery and electric engine to improve the fuel efficiency of their internal combustion engine. They only run on electric power alone for limited slow-speed manoeuvring. 'Plug-in hybrid electric vehicles' (PHEVs) use this hybrid configuration to operate more on electric power. They thus have a larger battery than ordinary hybrid cars, with some running for 50 kilometres (30 miles) or more before the ICE cuts in; the ICE then runs as an efficient generator to provide power for the car's electric motors.

A key advantage of this configuration is that it overcomes the range limitation of a BEV while still allowing the car to run entirely on electricity for shorter trips. A further advantage is that these cars have the potential to be even more efficient than conventional hybrids, because (when used) the ICE runs at closer to its maximum efficiency.

The term 'plug-in hybrid electric vehicle' can lead to confusion between such cars and ordinary hybrids, and the term 'extended-range electric vehicle' is now also used to describe this technology. You should now watch Video 3, which explains how a plug-in hybrid works. This features the Opel/Vauxhall Ampera (Figure 16) which, under its US branding as the Chevrolet Volt, was the first extended-range electric vehicle to market: it was launched in the USA in 2011. About 25 000 US sales are expected in 2012, and Vauxhall also anticipates selling up to 3000 Amperas in the UK during 2012, largely to fleet buyers.

Download this video clip.Video player: Video 3 How a plug-in hybrid works

Show transcript|Hide transcript

Transcript: Video 3 How a plug-in hybrid works

STEPHEN POTTER

A variety of new, cleaner vehicle technologies are now coming to market. One design is a battery-electric car that has the backup of a petrol engine for longer trips. The Ampera is an example of such a car, and in this programme, I meet with Ian Allen from Vauxhall to explore how it works.

Ian, this is the Ampera which I tend to think of as being a plug-in hybrid. But I think you use a different label for it. Why's that?

IAN ALLEN

Well, the label we use is Extended Range Electric Vehicle, because really, we want to try and differentiate it from, if you like, the conventional hybrids that are already on the road today. Yes, we have two methods of propulsion in that we have a plug-in battery, but we also have a combustion engine in there. But it really turns the hybrid concept on its head in terms of how the market understands hybrid at the moment in the way that the hybrids that are already on the market are combustion engine vehicles which are supported by batteries. Whereas, the Ampera is it true plug-in battery-electric vehicle which is supported, where necessary, by a combustion engine.

STEPHEN POTTER

What's the big advantage of that configuration?

IAN ALLEN

The big advantage is that you get the opportunity to drive on a daily basis on pure battery. And on a full charge, you can get up to 50 miles of battery, tailpipe emissions-free, driving. And if you never do more than 50 miles a day, then in theory, you may never burn an ounce of petrol. So if you do you 50 miles and get back home, plug it in, charge it overnight, which we expect the majority of people to do, hopefully on a low-rate tariff, then you're ready to go the next day with another 50 miles of battery driving.

But I guess one of the big drawbacks with pure battery vehicles is that uncertainty. If you need to go further than your daily driving, a weekend away, or even if you have emergency or detours to take, you need that reassurance that, actually, you don't need a charging post in the next street or in the next town or whatever because you may have to travel a hundred miles in one go. And in the Ampera, you can do that.

STEPHEN POTTER

And when the petrol engine kicks in after those 50 miles or so, how does the battery help you to get good fuel economy?

IAN ALLEN

Well at that point, the engine is there to sustain a minimum charge in the battery, and the engine is there to generate electricity for the electric motor. It's the electric motor which drives the front wheels of the vehicle. And if that electric motor isn't running, then the car won't go. So when the battery has reached its minimum 30% level, it almost becomes dormant in a way because the engine is then creating the electricity to sustain that charge which continues to drive the car forward electrically.

STEPHEN POTTER

So lets go and have a look at how this works in practise on a vehicle itself.

IAN ALLEN

Well if you take a look at the car itself, we're very happy with the design of the car. It's got a very dynamic looking low stance to the vehicle, so we're very happy with that. And in here is the electric charging cable. So basically, when you get home and you charge it then at home, or when you get to a public charging post, you use the cable that's supplied with the vehicle, which is then plugged in to the charging point here which is nice and conveniently on the near side of the car.

STEPHEN POTTER

Ian, when you switch the Ampera on, you get this amazing series of display screens. Can you just take me through what information this provides for the driver?

IAN ALLEN

Yeah sure. As you can see, there are two display screens that you've got here. The middle screen, you can see, gives you a lot of information about where the power is coming from to propel the vehicle.

So if you've got charge left is your battery, then it'll show you that's it's battery power. If it's coming from the engine, it will show you that it's the engine that's creating the electricity. And when you take your foot off the accelerator or press it on the brake, you'll see that you're getting some regenerative braking go back into the battery. It'll also tell you how long you need to leave to get a full charge from your vehicle and also the energy information in terms of vehicle MPG and also how efficiently you've been driving for your last few journeys.

But of course, that's a place where you would expect also to see your sat-nav and also, of course, all your radio. And if you move over to this screen, it'll show you how fast you're driving. I'll also show you how much fuel you've got left.

And actually, because we've got two methods of propulsion, we've actually got two different fuels in the car. We've got the battery, which is showing that you've got 29 miles of battery range left, but you've also got an additional 193-- now you can see that that's the petrol as. if you like, it's visually showing exactly what the engine is there for, ie As a backup, as and when you need it. So if I got home tonight, not having used my 29 miles, then I would just recharge and I wouldn't burn and ounce of petrol. And you can see there's a 222 mile range on there.

STEPHEN POTTER

I noticed earlier on-- it's not displayed at the moment-- there was a sort of eco-driving about your acceleration and braking. Could we have a look at that? That helps the driver to drive in the most energy efficient manner.

IAN ALLEN

Yeah, and It's in the shape of a nice green ball with some leaves on there. The idea, really, is to keep that little green ball right in the middle there, because the more aggressive you accelerate or the more aggressively you decelerate, the more energy you're using. So ie, you're using more battery power or petrol the faster you accelerate, and the more aggressively you brake, the more the disc brakes are engaged, which means you get less regenerative braking going back into the battery. So I guess the learnings here is really to try and plan your acceleration and plan your braking as much as possible.

STEPHEN POTTER

Extended range electric vehicles like the Ampera offer low-carbon motoring without a range constraint or the need for new fuel supply systems. However, they are expensive to buy, and we have yet to see if their low running costs can win them a substantial market share.

End transcript: Video 3 How a plug-in hybrid works

Download

Video 3 How a plug-in hybrid works

Figure 16 The Volt/Ampera

The Ampera can travel around 60 kilometres (possibly up to 80 km if driven carefully) on its lithium-ion battery pack before the ICE cuts in. At £30 000 the Ampera is an expensive car for its class, but it qualifies for the £5000 UK plug-in grant. As with all low-carbon vehicles, there is a price premium that is counterbalanced by lower fuel costs. However, comparable ICE cars cost around £20 000 (or less).

The Toyota Prius plug-in is to be launched in the UK in late 2012, with a UK price about the same as the Ampera. The Prius plug-in (Toyota prefer the term 'plug-in') also uses a high energy density lithium-ion battery, but one that is smaller than the battery used in the Volt/Ampera; thus its range on electricity alone is only 20 kilometres (12.5 miles).

The Prius plug-in has official test CO₂ emissions of 59 g km⁻¹ (compared to 89 g km⁻¹ for a regular hybrid Prius). The Ampera, with its longer battery range, is rated at 42 g km⁻¹ CO₂. However, the actual emissions will depend very much on the mix of electric and ICE driving undertaken.

Further plug-in/extended-range EVs are due for market launch soon, and are viewed as having both the potential for high market penetration and the technical potential to achieve very low carbon emissions. For example, Volkswagen's XL1 two-seater electric/ diesel PHEV [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] concept car has a combined test fuel consumption of 0.9 litre per 100 km and test CO₂ emissions of 24 g km⁻¹. This achievement by a concept vehicle design is impressive, but for the car fleet as a whole the emission rate would be higher, as this design would fall at the bottom end of the range of vehicle sizes. Were a range of plug-in/extended-range EVs to be available, and assuming a certain amount of overall downsizing, a CO₂ emissions fleet average of around 45 g km⁻¹ might be achievable.

This emission rate could be further improved if this technology were combined with low-carbon biofuels. This would overcome both the disadvantages of BEVs and the current limited supply of sustainable biofuels. Moreover, it would further lower CO₂ emissions as both the electricity and the biofuel could, potentially, be produced in a decarbonized form.