Keith Holmes of Dunedin asks :-

Is it true that water can be electrolysed by a car battery into hydrogen and oxygen to be used as a fuel supplement to petrol?

Bob Lloyd, a physicist at Otago University, responded.

A car that can run on water is the motorist’s dream, especially since oil prices have been skyrocketing lately. Unfortunately, as they say, there is no such thing as a free lunch and a basic law of physics prevents us from getting energy from thin air, or even slightly impure water. The law involved is the first law of thermodynamics, which says that energy must always be conserved.

It is true that water can be electrolyzed to its atomic constituents to produce free hydrogen and oxygen gasses. Electrolysis in fact is the reverse process to burning hydrogen in oxygen to produce water.

The bad news is that the electrolysis reaction takes as much or more energy than is gained when the two gases are burnt to produce heat energy. Typical efficiencies of electrolysers are around 70 per cent, with the balance going into heating the water. Thus if you used your petrol to drive the motor at 20 per cent efficiency to charge your battery at 45 per cent efficiency, then use the battery to electrolyse water at 70 per cent efficiency, then burn the hydrogen to supplement your petrol at 20 per cent efficiency, the end result would be an overall efficiency (i.e. relative energy output to energy input) of around 1 per cent. Hmmm! it is clearly much better to use the original petrol to drive the car at 20 percent efficiency and clearly no mileage improvement would be obtained by the relatively complicated add on. In fact quite the contrary. You would lose mileage.

The volume of gas that could be obtained per litre of water is irrelevant as it is only the energy flows that matter. So I’m afraid the only way of cheating on the high price of petrol is by driving conservatively, or moreso, by cycling or walking.