Electric cars use an on-board battery to store electrical energy, which is recharged by connecting it to an electricity supply. When required, energy is drawn from the battery and converted to motion by an electric motor. The following examines what defines a plug-in hybrid vs electric car. It also reflects on how environmentally friendly electric cars are in reality.
There are three main types of electric vehicle or ‘EV’: Battery electric vehicles (BEVs), Plug-in hybrid electric vehicles (PHEVs), and Extended range electric vehicles (EREVs).
Battery electric vehicles have no combustion engine, only an on-board electric battery which provides energy to an electric motor. BEVs are charged by connecting to an electricity supply (usually the ‘mains’). When required, energy is drawn from the electric-cells and converted to motive power by the use of one or more electric motors.
Plug-in hybrids are part battery electric and part conventional car in that they have a large capacity battery and a small on-board combustion engine. Like standard hybrids, the use of a battery enables the main engine to be operated at close to its maximum efficiency. Unlike conventional hybrids, plug-in hybrids also have the ability to be charged directly from an external electricity supply. Plug-in hybrids, therefore, can be charged and driven like an electric car, with the added advantage of having an on-board engine that can be used when the battery is depleted.
Extended range EVs are also plug-in hybrids but with a particular engine-battery configuration. Most plug-in hybrids are ‘parallel hybrids’, which means that either the engine of electric motor can power the wheels directly. In their purest form, REEVs are ‘series hybrids’ with only the motors being able to drive the wheels. In most respects the vehicle behaves like a BEV, with the battery being charged by an external supply. However, the on-board combustion engine is available to recharge the battery if required.
In all EV types, the latest generation of rechargeable batteries includes lithium-ion (Li-Ion) and lithium-polymer (Li-Poly) cells. These provide a significant improvement in performance and vehicle range and are now preferred by most electric vehicle manufacturers. An on-board battery also enables the use of regenerative braking which tops up the battery during braking, which reduces overall energy use by up to 20%. In this way, all plug-in vehicles (PIVs) provide improved fuel economy and either zero or reduced emissions.
DRIVING AN ELECTRIC CAR
Although driving an all-electric car is a different experience to using a conventional vehicle, they do share some similarities to automatics in that there is no clutch. On depressing the accelerator, an EV initially moves in almost total silence, which can be a little disconcerting at first (some BEVs are fitted with noise generators for slow speeds). As the speed picks up, the small amount of ‘engine’ noise that can be heard is drowned out by wind and tyre noise, which become more noticeable as the speed increases.
Most electric vehicles have excellent acceleration and high torque (especially at lower speeds) and are more than capable of holding their own in city-driving conditions. Although some models are designed as city cars and are speed limited to around 40-50 mph, most of the newer high quality models can be easily reach 60-70 mph on a motorway (and are required to do so to be eligible for a Plug-in Car or Van Grant). Electric cars can also be high performance vehicles – the Tesla roadster is capable of 130 mph and goes from 0 to 60 mph in 4 seconds!
Battery electrics have a range and performance that is more than adequate for most driving applications including: city driving, commuting, regular drive cycles (such as delivery routes), short range trips and where only zero or low emission vehicles are allowed access. In general, BEVs have a real-world driving range of around 80 miles, or more for some premium models. As a result, electric cars are most suited for use as private cars for city use, for commuting trips, in commercial fleets (for small loads), and as company ‘pool’ cars.
The driving performance of a plug-in hybrid vs electric only is very similar to conventional hybrids. Most PHEVs offer at least two driving modes including: ‘eco-mode’, where the car decides how to most efficiently use conventional and electric power; and ‘zero-emission-mode’, where the car runs purely on electricity. Plug-in hybrids can therefore be used as a conventional or pure electric vehicle – although the electric-only driving range will be less than for a fully electric car – current models offer 15-40 miles in EV-mode.
As all plug-in hybrid cars can use conventional petrol or diesel, they can be refuelled in exactly the same way as for conventional vehicles. However, plug-in hybrids can also be charged directly using any suitable source of electricity – most owners will rely most on home overnight-charging using a standard 13 Amp supply. Being dual fuel, the driving range of plug-in hybrids is greater than for conventional vehicles (500 miles or more on a tank of fuel and full battery). With the added option of being used as a zero-emission vehicle, PHEVs therefore possess great potential to become one of the future standard automotive technologies.
With respect to driving experience, extended range EVs are similar in most respects to BEVs. However, despite having a limited range in electric mode (typically 80-100 miles), the on-board engine provides power to extend the range, or a back-up when the battery is depleted. The only limiting factor is the size of the fuel tank which, being the secondary energy source, tends to be small. Typical driving ranges for EREVs are 200-300 miles.
HOW GREEN ARE ELECTRIC CARS?
Electric vehicles are zero-emission at point of use. However, emissions are produced during the generation of electricity, the amount depending on the method of generation. Therefore, the emissions need to be considered on a life cycle basis so as to include power station emissions.
For climate change gases (such as CO2), electric cars charged using average UK ‘mains’ electricity show a significant reduction in emissions – the figures suggest a reduction of around 40% compared to an average small petrol car (tailpipe 120 gCO2/km).
However, if an electric car is compared with a fuel-efficient diesel car (tailpipe 99 gCO2/km), the life cycle carbon benefit for an electric car using average ‘grid’ electricity is around 25% – a smaller but still significant reduction.
The reduction in carbon emissions is mainly due to the fact that electric cars are more energy efficient than conventional vehicles. So-called ‘regenerative braking’, which returns energy to the battery when the brakes are applied, also improves fuel efficiency by up to 20%.
Larger carbon reductions are likely as the UK grid continues to ‘decarbonise’. If renewable or ‘green tariff’ electricity is used, then lifecycle greenhouse gas emissions are effectively zero.
For local air pollutants, including nitrogen oxides (NOx) and particulates (PMs), electric cars using average ‘mains’ electricity are increased. However, as these are emitted from power-stations which are well away from urban areas, their overall impact tends to be much less than when emitted from the exhausts of petrol and diesel cars.
As is the case with greenhouse gas emissions, if renewable electricity is used, then lifecycle regulated emissions are also virtually eliminated.
While electric vehicles can provide significant climate change benefits, reduce noise pollution, and reduce use of fossil fuels, they can also increase levels of air pollutants (such as SO2) leading to higher rates of acidification, and may increase the potential impact on human health in areas where resources (such as lithium) are extracted for battery production. Indeed, the sourcing of lithium remains contentious relating to the level of reserves and the local impacts on human health where lithium is mined.
Taken overall, and given that current road transport is responsible for significant emissions of nitrogen oxides and particulate matter, the impact on human health is likely to be reduced within urban areas, well away from the centres of battery production, due to the fact that most ULCVs are zero-emission at the point of use.