Despite some recent assertions, the battery-electric car will not make a dent in our carbon emissions. Even in New Zealand, with our large share of hydro electricity, the marginal unit of electricity always comes from a fossil fuel power station.

Counting the losses of the power station into the equation, an electric car, despite its own efficiency, does nothing to mitigate carbon emissions. It merely shifts the carbon output from the tailpipe to the smoke stack.

Generating the electricity from renewable sources, such as photovoltaic cells, is still forbiddingly expensive. Even if it was cheaper, it would be more sensible to feed the electricity directly into the grid, rather than into the battery of an electric car.

The electric car is no more than a tax avoidance scheme because its owner avoids paying the fuel tax.

Are hybrid cars, then, the answer? A conventional car with a combustion engine is wasting fuel in two major ways: when coasting or idling, and when turning kinetic (motion) energy into heat at the brake pads.

A hybrid, that is a combination of combustion engine and electric motor, minimises these losses.

While idling or coasting, the storage battery is being charged (or the car is running temporarily on electricity alone, which means there is no idling at all).

The car is braked by switching the electric motor to generation mode, thus saving the energy for a new acceleration process.

From that, it is immediately clear that a hybrid car is most effective in city traffic. On long highway stretches, a hybrid is pretty much a wasted effort.

Hybrids come in many variations, each having different characteristics.

On one side is the full (or in-line) hybrid. In its extreme form, the combustion engine is a very simple turbocharged diesel.

It is directly linked to a generator which doubles as a starter. The diesel runs at a set speed, therefore avoiding costly engine management systems such as variable valve and injection timing.

Four electric drive motors are integrated into the wheel rims. When braking, the drive motors switch to generation mode. The recovered brake energy is stored in a battery which also stores any surplus electricity generated by the diesel.

Once the battery is charged, the diesel engine stops until required to recharge the battery. Motor management, flywheel, clutch, gearbox, drive train, differential, stub axles - all gone.