Internal combustion engines are ubiquitous in today’s society. Applications for these engines cover a broad spectrum, ranging from vehicles that transport passengers and move goods to specialized vehicles and equipment used in the construction and agriculture industries. Internal combustion engines are integral to today’s economy, however, they have also long been recognized as a significant source of pollutant emissions that contribute to poor air quality, negative human health impacts, and climate change. Efforts to mitigate the emissions impact of these sources, such as regulatory control programs, have played a key role in air quality management strategies around the world, and have helped to spur the development of advanced engine and emission control technologies.
Regulatory control programs have typically been developed first for engines used for on-road applications. Complementary programs for other mobile source types, including non-road vehicles and equipment, locomotives, marine vessels, and aircraft, have generally lagged behind those for on-road vehicles. As a result, the relative importance of non-road sources of air pollution increases as emissions from on-road vehicles are reduced. Many countries around the world have progressed in controlling emissions from on-road vehicles and are in the process of targeting other source categories through enhanced emission control programs.
As non-road engine emissions control programs are developed in a growing number of countries around the world, it is instructive to look at the development of programs in two of the regions that have progressed furthest in controlling emissions from non-road engines, the United States and European Union. By understanding the progression of engine emission standards and parallel developments in engine emission control technologies in these countries, pathways for the continued control of emissions from this important source category can be projected for other regions.
While significant advances have been made to control emissions from non-road diesel engines, this source category remains a significant source of air pollutant emissions. As of 2011, non-road vehicles and equipment were responsible for ~20-45% and ~15-20% of mobile source PM2.5 and NOx emissions, respectively, in the United States and European Union. The introduction of more stringent Tier 4f/Stage IV emission control requirements in each region during succeeding years should help to reduce emission rates from new non-road diesel engines. However, current non-road regulatory programs lag behind comparable programs for on-road diesel engines, and are not stringent enough to compel the use of the best available technologies for the control of PM and NOx emissions from diesel engines: DPF and SCR systems. This is especially true of particulate filters, whose use seems to actually have decreased with the transition from Tier 4i/Stage IIIB to Tier 4f/Stage IV regulatory programs. In the European Union, the implementation of proposed Stage V standards should lead to the universal application of DPFs for engines between 19 and 560 kW.