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Autogrumble by 'GOM'

4. Hybrid Swings and Roundabouts

Hybrid cars are promoted as being "green" and qualify for VED and congestion charge concessions, but are they really more environmentally friendly? I think the jury is out on this one! Let's have a look at what they do.

You have a petrol engine and generator which convert chemical energy into mechanical and then electrical energy wasting some as heat at each stage. The electrical energy is used to drive electric motors (more losses) but may be stored in or retrieved from accumulators depending on the demand.

The hybrid system has some advantages. In slow moving or stop-go traffic you can drive on electrical power only. The control software starts the engine automatically when more power is needed or if the betteries become discharged. The engine can then run at an efficient speed independent of road speed until the batteries are recharged. In addition it's possible to have regenerative braking in which the motors act as generators. They convert kinetic energy back into electrical energy which can be stored until needed. Friction brakes only come into play at low speed or when heavy braking is required. Do hybrid cars actually do this?

On the other hand in addition to a petrol engine you have to lug around a set of heavy batteries, an alternator, electric motors and the associated wiring and control gear. You'd save a bit of mass by losing the mechanical transmission. The increased mass of the vehicle cancels out most of the gains you get by storing energy. In practice the fuel consumption you get from hybrid cars (by which we usually mean the Toyota Prius) on long journeys is worse than similar sized straight petrol or diesel cars.

So the Prius could reduce pollution in towns and cities, especially on short journeys if you could charge it up from the mains and run on electric power (not possible on the UK model), but the overall fuel economy is no better. However if the engine is used intermittently the catalytic converter will not be at working temperature most of the time and non-carbon emissions will increase.

Modern control systems can switch off the engine when you're stationary and automatically restart it as soon as want to move without the added complications of a hybrid configuration. Using mains electricity may be cleaner at the point of use but the energy losses in transmission are no better than transporting chemical fuel in road tankers.


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Last updated 5th April, 2013