Around one-fifth of the UK’s total CO2 emissions is produced by road vehicles, and roughly half of that – amounting to some 19 million tonnes of carbon dioxide – is calculated to come from private cars. Manufacturers have responded by developing various methods of minimising consumption of petrol, which has high CO2 emissions, whilst still retaining the petrol engine that so many motorists feel most comfortable with. One effective solution that is being adopted by an increasing number of car makers is the use of hybrid technology.
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What is Hybrid Power?
A hybrid power system combines power from two different energy sources. In cars, one energy source is a normal petrol or diesel engine and the other is an electric motor, which draws energy from its own separate battery.
The world’s first hybrid vehicle was revealed as a concept car at the Tokyo Motor Show in 1995, went on sale in Japan in 1997 and was launched in Europe in 2000. Despite initial scepticism it proved extremely popular. The number of car manufacturers developing hybrids is steadily increasing, and the hybrid form is still evolving.
Hybrid drive systems vary slightly according to manufacturer, but they all operate on the same principle.
How do Hybrid Vehicles Work?
The car has an engine, normally fuelled by petrol, and one or more electric motors. Depending on the speed and acceleration demanded by the driver, the power will be supplied to the wheels either by the engine, or by the motor, or by both. Each energy source is switched on and off as appropriate by computer; no technical knowledge is required of the driver, although some systems do provide an option to override certain functions. From the driver’s point of view, the most noticeable difference is that when the car is being powered by the electric motor, acceleration is silent.
The electric motor draws its power from a battery, often a nickel metal hydride battery. This is normally connected to the engine’s alternator, which also charges the normal car battery; it can sometimes also be charged from the mains; but a central feature of hybrids is their ability to charge the motor battery by regenerative braking. The faster a vehicle travels, the more kinetic energy it has. When the brakes are applied, the kinetic energy that the car loses is normally converted into thermal energy by the braking system (the brakes get hot) and dissipated into the environment as heat (the brakes cool down again). Regenerative braking captures this energy, which is then transformed into electrical energy and stored in the hybrid battery.
Advances in battery technology have been an important factor in the development of hybrid vehicles. The more energy stored in the battery, the more work the motor can do and the less is required of the petrol engine; consequently, the lower the vehicle’s emissions.
Different Hybrid Systems
Full-blown hybrids use the electric motor when setting off from stationary and driving at low speeds; this means they use no petrol in stop-start traffic, therefore produce no emissions. The petrol engine takes over when normal cruising speed is reached, and the electric motor cuts in again intermittently to provide bursts of power when extra acceleration is needed.
An alternative power sharing system is for the petrol engine to act as the primary power source whenever the car is in motion, even at low speeds, but switching off completely when the car is motionless so that fuel consumption and pollution fall to zero whilst stationary in traffic. The electric motor is used to boost acceleration throughout the speed range, and this means that a hybrid with a small, economical engine is able to match the performance of a non-hybrid with a much larger engine.
The Future of Hybrid Vehicles
Hybrid technology will continue to evolve. To date, only petrol-engined hybrids are available; a diesel hybrid has been developed but is not yet in production. A ‘series hybrid’, that will use motors to drive the car all the time and incorporate a small engine solely to recharge the battery, is also under development.
Early fears that batteries would not retain sufficient charge to represent an effective energy source been proved wrong. So have predictions that hybrids would be sluggish; harnessing two different energy sources gives scope for high performance – one hybrid does 0-60mph in around 7 seconds – and also produces greater torque than would be possible from a petrol engine alone.
Hybrid models currently on the road vary considerably in terms of sophistication, performance, economy, emissions, and also on-the-road price; but it is safe to say that all hybrids are more eco-friendly than their petrol equivalents, and that all hybrids deliver great environmental benefits in city centre traffic.
