Developing Emission Reduction Strategies

8.2 On-Road Mobile Sources

8.2.4 Engine Technology and Emission Control Technologies for 4-Wheeled Gasoline Vehicles
Modern gasoline engines use computer controlled intake port fuel injection with feedback control based on an oxygen sensor to meter precisely the quantity and timing of fuel delivered to the engine. Control of in-cylinder mixing and use of high-energy ignition promote nearly complete combustion. The three-way catalyst provides greater than 90% reduction of carbon monoxide, hydrocarbons, and oxides of nitrogen. Designs for rapid warm-up minimize cold-start emissions. On-board diagnostic (OBD) systems sense emissions systems performance and identify component failures. Durability in excess of 160,000 km, with minimal maintenance, is now common.

The use of catalyst exhaust treatment required the elimination of lead from gasoline. Other gasoline properties that can be adjusted to reduce emissions include, roughly in order of effectiveness, sulfur level, vapor pressure, distillation characteristics, light olefin content, and aromatic content. Of these, sulfur is the most important in terms of the impact on advanced pollution control technology so its impacts on different technologies will be summarized below. No Controls/Pre Catalyst Controls

The amount of sulfur in the fuel is directly related to SO2 emissions; some SO2 emissions are converted in the atmosphere to sulfate PM.

For gasoline fueled vehicles with no catalytic converters, reducing sulfur will have no effect on the principal pollutants of concern, CO, HC or NOx. While the amount of SO2 emitted is in direct proportion to the amount of sulfur in the fuel, gasoline vehicles are not usually a significant source of SO2. Since SO2 can be converted in the atmosphere to sulfates, however, these emissions will also contribute to ambient levels of particulate matter (PM10 and PM2.5) which is frequently a serious concern. As noted earlier, US EPA models predict that over 12% of the SO2 emitted in urban areas is converted in the atmosphere to sulfate PM. Catalyst Based Controls

All catalyst technology is adversely impacted by sulfur with resulting increases in CO, HC and NOx.

Worldwide, approximately 90% of new gasoline vehicles are equipped with a three-way catalyst (TWC), which simultaneously controls emissions of CO, HC, and NOx. Sulfur in fuel impacts TWC functioning in several ways:

Fuel sulfur reduces conversion efficiency for CO, HC and NOx. Sulfur competes with these gaseous emissions for reaction space on the catalyst. It is stored by the TWC during normal driving conditions and released as SO2 during periods of fuel-rich, high-temperature operation, such as high acceleration. Reductions in sulfur levels in gasoline—from highs of 200–600 ppm to lows of 18–50 ppm—have resulted in 9–55% reductions in HC and CO emissions and 8–77% reductions in NOx emissions, depending on vehicle technologies and driving conditions. Greater percentage reductions have been demonstrated for low emission vehicles and high-speed driving conditions.

Sulfur inhibition in catalysts is not completely reversible. Although conversion efficiency will always improve with return to reduced sulfur levels, the efficiency of the catalyst does not usually fully return to its original state after desulfurization. In tests using 60 ppm sulfur fuel followed by a single use of 930 ppm sulfur fuel, HC emissions tripled from 0.04 g/mile to 0.12 g/mile. With a return to low sulfur fuel, emissions dropped again to 0.07 g/mile but fuel-rich operation (resulting in high exhaust temperatures) was required to regenerate the catalyst fully and return to original emissions levels.

Sulfur content in fuel contributes to catalyst aging. Higher sulfur levels cause more serious degradation over time and, even with elevated exhaust temperatures, less complete recovery of catalyst functioning. The high temperatures necessary to remove sulfur from the catalyst also contribute to thermal aging of the catalyst. Sulfur raises the light-off temperature—the temperature at which catalytic conversion can take place—resulting in increased cold-start emissions.

Regeneration requirements add to overall emissions and reduce fuel efficiency. Fuel-rich operation, required to reach regeneration temperatures, results in significant increases in CO and HC emissions. And PM emissions under these circumstances can actually rival diesel emissions. In addition, fuel-rich combustion requires increased fuel use. Vehicles that tend to operate at low speed and low load will have lower exhaust temperatures and fewer opportunities for desulfurization and catalyst regeneration. More Advanced Catalyst Controls

All catalyst technology is adversely impacted by sulfur with resulting increases in CO, HC and NOx. Some advanced catalyst technologies such as NOx adsorbers which may enter the market later this decade are precluded by high levels of sulfur.

The percentage benefits of reducing sulfur levels in fuels increase as vehicles are designed to meet stricter standards. Increasingly strict emissions standards require extremely efficient catalysts over a long lifetime. Recent regulations in Europe and the U.S. require warmed-up catalysts to have over 98% HC control, even towards the end of the vehicle’s lifetime (100,000 km in Europe and over 100,000 miles in the U.S.).