IN MEDIEVAL times, people thought sulfur had mystical powers. Today, in diesel and gasoline, it gives carmakers severe headaches.
Sulfur causes indirect problems. It doesn't add to the pollution of exhaust fumes. Instead, it does irreversible damage to exhaust catalysts required to meet new emissions standards.
Time adds to the carmakers' problem. New emissions standards will soon be in effect: EU III (in 2000) and EU IV (in 2005).
The frustration was recently highlighted by BMW Chairman Bernd Pischetsrieder. In a speech, he demanded that gasoline producers reduce sulfur levels to 30ppm in gasoline and 50ppm in diesel by 2005. Existing sulfur levels in European gasoline are around 200-300ppm.
Many reformulated, high-octane, unleaded gasolines have sulfur levels of 60ppm and below.
In the last 20 years, automotive emissions have been reduced by 90 percent. Oil companies brought no-lead fuel to the market to permit the first round of automotive catalysts to operate. Even so, most carmakers think oil companies have not contributed their share to the public's demand for clean air.
EU III levels are proving difficult and no existing engine meets them fully. EU IV will represent some of the most stringent emissions regulations yet seen: Levels of NOx and hydrocarbons in gasoline passenger cars plunging from a combined figure of 0.5grams/km to a proposed 0.18grams/km.
At the same time, a reduction in CO2 to 120grams/km has been requested by the EC as well as the 8 percent across-the-board reduction agreed at last year's Kyoto Summit.
The issue is further clouded by the structure of emissions standards.
Many catalytic converters need to attain high operating temperatures of 350C-400C. Currently, the testing procedure for emissions in the EU calls for a 40-second delay between ignition and measurements. This allows converters to heat up. EU III will end that, and measurements will start at ignition, which demands new strategies for fast heating catalysts.
With sulfur-rich fuel, it is difficult to reduce CO2 and NOx gases at the same time.
Gasoline direct injection (GDI) engines, which give good fuel economy, burn fuel in the presence of excess oxygen. This controls fuel consumption. The diesel versions also result in fewer particulate emissions. But direct injection causes more NOx.
GDI engines act as two-in-one engines by having dual operating stages.
In one mode, fuel is injected at the last moment as the piston nears the top of the compression stroke, creating a local stratified charge around the spark plug. This results in a lean air-fuel mixture of 40: 1. The other mode is to inject fuel earlier during the induction stroke, producing a more conventional mixture of around 14: 1.
Alternating both techniques lets the engine run lean to conserve fuel and develop high power when it is needed.
But when running in stratified charge mode, GDI engines generate high levels of NOx. Some is removed by exhaust gas recirculation, but the rest must be dealt with by either a lean NOx catalytic converter or a NOx trap.
Lean NOx catalytic converters convert around 40 percent of NOx.
They require a rich supply of carbon monoxide to react with the NOx. This means they must be placed ahead of a standard three-way catalyst, which removes CO. But this is a problem, because lean NOx catalytic converters are destroyed by heat and cannot be placed near the engine.
NOx traps are rendered virtually ineffective by sulfur.
The alkali metal in the trap meant to absorb the NOx instead absorbs the sulfur.
This cuts the NOx trap's efficiency in half within 8,000km. When the sulfur takes over, the trap is dead, and NOx reaches the atmosphere. Only in Japan, which adopted a low-sulfur policy in the 1960s, can cars be fitted with NOx traps.
The oil industry feels confident that car manufacturers will overcome the engineering difficulties by 2005.
Big oil companies are reluctant to spend the massive sums required to convert refineries until a proven minimum tolerance to sulfur for NOx converters has been set.
Said Mike Frend, director general of the UK Petroleum Industry Association: 'We've said, as an industry, that we don't want to make the investment if someone says later on that it wasn't necessary. Given the early stage of the technology, it could be possible to cope with current levels of sulfur.'
He said that changing refineries to cut down on sulfur will add CO2 to the atmosphere - 2.3 million tonnes annually in Germany alone.
Johnson Matthey, Degussa and other catalyst producers are investing heavily in developing lean NOx converters and particularly NOx traps.
But some engineers believe that for NOx traps to be viable, sulfur levels need to be reduced to 30ppm or less. Many think it impossible.
Pelham Hawker, of Johnson Matthey Catalytic Systems Division, said that despite our ability to shrink technology we would need 'a miniature chemical factory upstream of the trap to absorb the sulfur.'
Heidi Hautala, the Finnish Green Member of the European Parliament, believes there is sufficient information for legislative decisions to be taken.
Whatever the outcome, the position on CO2 looks bleak.
'Look at the profile of the fleets across the USA or the UK,' said Hawker. 'NOx has gone down but CO2 levels have increased. Vehicles are getting cleaner but the problem is getting bigger.'