Despite the current concerns over high petrol and diesel prices, the miles per litre delivered from these fossil fuels has never been better as new engines gain the optimum performance using "lean-burn" technologies. These involve using a "lean" mixture of fuel and air, using more air than is required to burn the fuel. But while they cut down on carbon dioxide, nitrogen oxide emissions become a greater problem.
"The more excess oxygen in the combustion mixture leads to a higher efficiency for the engine and, as a consequence of this, carbon dioxide production will be reduced. But it makes it much more difficult to control the emissions of nitrogen oxides to the atmosphere," explained Prof Julian Ross, professor of industrial chemistry at the University of Limerick.
Last week, Prof Ross received an award for excellence in research from the university for work carried out by his research group at UL, which has secured more than £2 million in research funding over the past nine years.
Nitrogen oxides, formed mainly because of high temperatures during ignition, cause acid rain and damage the ozone layer.
The team at UL is involved in two EU-funded projects involving nitrogen oxide removal from lean-burn engines, and another project on a related industrial problem.
Prof Ross's focus is on environmental catalysis, and his work on car exhausts involves finding a selective catalyst that will reduce the nitrogen oxide pollutants before they are emitted into the atmosphere. He describes it as "a quiet science" but one which plays a vital role in reducing pollutants emitted from internal combustion engines.
One of the technical challenges relates to the sulphur content of fuels which "poisons" the silver metal in the catalysts his team has been using. This leads to the formation of silver sulphide, "in the same way as a silver spoon turns black if you put it into an egg", he said.
Although the presence of sulphur in fuels had been reduced from about 200 parts per million three years ago to about 50 parts today, getting rid of the final traces would be an overly costly exercise for some time to come. "It is political chemistry as well, depending on the pressure brought by governments," he said.
One UL project, which includes as partners the German car manufacturer, Volkswagen, involves the development of technology similar to that pioneered by Toyota. This method traps the nitrogen oxides under lean-burn conditions, and then changes momentarily to "fuel-rich" conditions to reduce the trapped nitrogen oxides.
For that period of about a second, there is no excess of oxygen, which provides a window of opportunity to reduce the pollutant gases. "The trapping system involves changing the air/fuel mixture briefly. Again that is susceptible to sulphur poison.
"If we can release the sulphur at the same time as we release the nitrogen oxide when we go into the fuel rich conditions, that would be fine. But it does not work at the present time," he said. Formerly he worked on power station emissions where the problem of nitrogen oxide emissions was easier to control. "You can put in fairly large plant that you cannot put in a car. In a power station you use ammonia as a reducing agent."
Other projects he hopes to look at include "bio-diesel", making a sulphur-free fuel from bio-materials such as wood chippings and employing catalysis to get rid of the nitrogen oxide emissions.
"If we can produce non-fossil fuel, we are not going to be taxed on it," he added. "In the future, it is inevitable that we are going to be faced with taxes on carbon dioxide emissions."