I am somewhat amazed by the latest attacks on diesel vehicles, particularly cars, as being (singly!) responsible for the alleged high levels of "pollution" found in our city centres. For the moment I will not discuss the dangers from the alleged pollutants, but simply the reasons for the levels found in London. In studying the report by the Mayor of London I see that about one third of the NOx levels is attributed to vehicles, and that this is worst in the areas where traffic is often stationary or slow moving. This result is hardly suprising as the measures taken to minimise such emissions are not very effective when there is not a reasonable exhaust temperature to operate the catalysts! It would seem to my simple engineering mind that the solution is to make sure that traffic is travelling at a reasonable speed at all times, perhaps 30mph, where there is at least a good chance to make the catalyst etc. operate properly. There is a fairly small window of operating temperature where this happens, and the maximum has to correspond with the engine at maximum power for obvious reasons.


There is a current feeling in the Department of Transport that reducing traffic speeds as much as possible is a "good" idea, being given the usual reason that "it improves road safety". There is very little if any evidence that the 20mph speed limits actually reduce accidents, and whilst they may reduce the severity of injuries in some cases the number of such reductions is unknown. If as is claimed thousands of people a year die from traffic pollution (a figure which I find difficult to believe), it is clearly not a good idea to slow traffic at all, in fact it should be allowed to travel faster as the lesser of two evils!


If diesel fuel were largely unused as a transport fuel, and were replaced with an even larger quantity of petrol, there would be a severe problem for the oil refineries, who would want the large gas oil fraction? The suggestion that we move to electric vehicles is commonly made, but this ignores the huge cost of both new vehicles and the provision of about 50% more generating capacity which is permanantly availabe for vehicle charging, along with the question of the fuel source. The loss of tax to the exchequer would also be very significant, and undoubtedly the overall economy would suffer too as increased costs worked through the system, again can no one see that this requires joined up thinking at the highest levels, and some significant input from Engineers and much less from politicians!