Madam, - While I welcome Dick Ahlstrom's article ("Diesel or Petrol: Which is Greener?", October 29th), I believe that by concentrating on the possible pollutants emitted by the two types of engine, he has missed the main argument in favour of using diesel fuels. Furthermore, he has also omitted any mention of lean-burn engines despite the fact that these offer the same very significant advantages over conventional petrol engines.
The main concern driving all developments in the automobile world at present is that of improving fuel consumption and hence reducing the production of carbon dioxide, the predominant greenhouse gas.
Diesels and lean-burn engines have much better fuel efficiencies than do the equivalent petrol engines, and hence produce much less carbon dioxide. The introduction of significantly more such engines in Ireland would help us to meet our obligations under the Kyoto Treaty.
Dick Ahlstrom's main objection to diesel engines is that they produce particulates matter.
However, it is generally accepted that the particulate problem applies largely to heavy-duty engines - such as those in trucks - and not to the smaller highly efficient engines installed in modern passenger cars.
Mr Ahlstrom also discusses in some detail the various possible (trace) gaseous emissions from both types of engine but ignores in so doing the strategies which already exist to deal with these. Everyone is by now aware that all modern passenger vehicles are fitted with catalysts.
With the petrol engine, these deal with NOx (the nitrogen oxides associated with acid rain), carbon monoxide (resulting from incomplete combustion), and unburnt hydrocarbons (including the "nasties" discussed in the article).
With the diesel engine, because the engine works with excess air and combustion occurs at a lower temperatures, CO is not formed and NOx emission is below the currently permissible limits.
As a result, a simpler and more robust catalyst is included in the diesel exhaust to destroy any unburnt hydrocarbons (and probably any soot particulates formed). In passing, it should be mentioned that "catalytic filters" have been developed to deal with particulate emissions for heavy-duty vehicles.
The article mentions the emission of SO2 from diesel engines. Because of the steady reduction of sulphur levels in refined fuels (both petrol and diesel), the level of emission of this gas is well within acceptable limits (much less than 50 ppm). However, this low level of SO2 does cause problems of catalyst poisoning and this has led to research on methods of scavenging SO2 from the exhaust gases before they encounter the catalysts by the introduction of " SOx traps".
(One such EU-funded project currently includes work on such traps at the University of Limerick, the partners in the project including Volkswagen, the Queen's University of Belfast and Johnson Matthey, a major manufacturer of exhaust catalysts.)
Finally, the article dwells at some length on the evils of oxygenates such as MTBE in fuels. Many harmless oxygenates which can be used to replace MTBE exist and work on some of these, including bio-derived molecules, is either in progress or planned at the University of Limerick and in many other laboratories around the world.
I hope that the points which I have made above may have allayed some of the fears which might have been caused by Dick Ahlstrom's article and that the Government will be encouraged to introduce favourable taxation strategies to encourage the introduction of both diesel and lean-burn engines in order to help Ireland reduce its CO2 emissions.
I might add that I personally have been coinvinced by the arguments which I advance here and my diesel-powered car gives me a fuel consumption of better than 60 miles per gallon, a significant improvement over its petrol equivalent. -Yours, etc.,
JULIAN ROSS,
Professor of Industrial
Chemistry,
University of Limerick,
Limerick.