PETER Spinney (May 19) gives the correct wording of the scientific definition of efficiency but the wrong context. Useful work is not defined in the sense of being worthwhile, so whether a car's journey is useful or not doesn't enter into it. You can in the scientific sense calculate the useful work in raising a guillotine blade to the top of its frame. Not everyone would regard that as useful in the wider sense.

The appropriate mechanical figure of merit for a car is the proportion of the fuel energy that appears at the road wheels. Tests have shown that is about 20% whether you burn petrol in a car engine or burn fuel in a power station to charge batteries for an electric car.

That is not really surprising. The French physicist Sadi Carnot laid down the principles of the limiting efficiency of engines 170 years ago. There is little prospect of any significant improvement in petrol engine efficiency. The limits are fundamental and not technological.

Indeed, one recent development, the catalytic converter, has reduced the efficiency of internal combustion engines. An engine fitted with a catalytic converter will produce more carbon dioxide than a similar engine without one. In engineer's language, the converter is an obstruction in the exhaust system and that increases the back pressure. The loss in efficiency is readily calculable.

The converter was justified on the grounds that it reduced NOX and SOX gases which were more harmful that the carbon dioxide which it produced.

I am quite puzzled by Peter Spinney's statement that cars are six to eight times less polluting than 10 years ago. It seems devoid of scientific sense. There is no global index by which all forms of pollution can be combined into a single number, but there is one way in which he might get such a figure. Most of the cars that ran on leaded petrol have been phased out in that period so lead pollution is enormously reduced, but if that is what is meant the statement is quite misleading.

The only way to compare relative pollutions at two different times is to give pollution profiles for then and now. Anyone can pull a single figure out of thin air, and, with a little ingenuity, justify it.

Chris Parton,

40 Bellshill Road, Uddingston.

May 19.