Lloyd and Cackette

The diesel engine is the most efficient prime mover commonly available today. Diesel engines move a large portion of the world’s goods, power much of the world’s equipment, and generate electricity more economically than any other device in their size range. But the diesel is one of the largest contributors to environmental pollution problems worldwide, and will remain so, with large increases expected in vehicle population and vehicle miles traveled (VMT) causing ever-increasing global emissions. Diesel emissions contribute to the development of cancer; cardiovascular and respiratory health effects; pollution of air, water, and soil; soiling; reductions in visibility; and global climate change. Where instituted, control programs have been effective in reducing diesel fleet emissions. Fuel changes, such as reduced sulfur and aromatics content, have resulted in immediate improvements across the entire diesel onand off-road fleet, and promise more improvements with future control. In the United States, for example, 49-state (non-California) off-road diesel fuel sulfur content is 10 times higher than that of national on-road diesel fuel. Significantly reducing this sulfur content would reduce secondary particulate matter (PM) formation and allow the use of control technologies that have proven effective in the on-road arena. The use of essentially zero-sulfur fuels, such as natural gas, in heavy-duty applications is also expected to continue. Technology changes, such as engine modifications, exhaust gas recirculation, and catalytic aftertreatment, take longer to fully implement, due to slow fleet turnover. However, they eventually result in significant emission reductions and will be continued on an ever-widening basis in the United States and worldwide. New technologies, such as hybrids and fuel cells, show significant promise in reducing emissions from sources currently dominated by diesel use. Lastly, the turnover of trucks and especially off-road equipment is slow; pollution control agencies need to address existing emissions with in-use programs, such as exhaust trap retrofits and smoke inspections. Such a program is underway in California. These and other steps that can be continued and improved will allow the use of the diesel engine, with its superior fuel consumption, to continue to benefit society while greatly reducing its negative environmental and health impacts. The next ten years can and must become the “Decade of Clean Diesel.” INTRODUCTION This review summarizes and evaluates published information on diesel emissions, their effects on the environment and human health, and ways to minimize emissions and effects. It finds that diesels are an important part of the public and private transportation sector and their use will continue and grow into the future. Although large emission reductions have been achieved through fuel reformulation, engine redesign, and exhaust treatment, additional reductions are needed. These reductions are essential to reducing health risks, improving visibility, and minimizing water and soil contamination. They are practical and cost-effective. In the long term, fossil-fueled transportation systems may be replaced by renewable energies, probably using fuel cells for energy production. The diesel engine is a major tool in the day-to-day life of modern society. It powers much of our land and sea transport, provides electrical power, and is used for many farming, construction, and industrial activities. The diesel does all this in a package that is robust and durable, with lower fuel consumption than any other prime mover. But diesel pollutes, and concern is growing over how much diesel pollution affects human health and well-being. However, technology advancements are now becoming available that may dramatically reduce diesel emissions and make 2001–2010 the “Decade of Clean Diesel.” Much of the history of the internal combustion engine involved a search for the best way to get more power for less fuel cost. For example, James Watt (1736–1819) successfully halved the coal consumption of Thomas Newcomen’s (1663–1729) steam engine by modifying the cycle through the addition of a separate condenser.1 Diesel Engines: Environmental Impact and Control Alan C. Lloyd and Thomas A. Cackette California Air Resources Board, Sacramento, California