An overview of technologies for emissions compliance

Nov. 1, 2008
Come 2010, Engine manufacturers will have to meet the US Environmental Protection Agency's more stringent diesel emission standards. These regulations

Come 2010, Engine manufacturers will have to meet the US Environmental Protection Agency's more stringent diesel emission standards. These regulations dramatically decrease discharges of particulate matter (soot and ash) and nitrogen oxide (NOx), virtually eliminating these emissions from on-road diesel engines.

The standards reduce emissions to 0.2 gram per brake horsepower hour (g/bhp-hr) for NOX and 0.01 g/bhp-hr for particulate.

Basically, particulate matter (PM) is formed by the incomplete combustions of fuel in diesel engines. NOx is formed in small amounts when fuel is burned at high temperatures and pressures during an engine's combustion process.

Engine manufacturers are meeting the 2010 emissions standards through a variety of means. They are making engine modifications for cleaner combustion and adding exhaust aftertreatment (EGA) devices. These remove pollutants from exhaust gases after they leave the engine's combustion chamber.

Meeting the reduced pollutants standards has required a change in diesel fuel to ultra-low sulfur diesel fuel (ULSD) and a reformulation of engine oils.


Since 1993, diesel truck engines have been using low-sulfur diesel fuel, which emits 500 parts per million (ppm) of sulfur. Ultra-low sulfur diesel, required by the EPA for 2007-emission compliant engines, began replacing low-sulfur fuel starting in 2006.

ULSD is a cleaner burning fuel that inherently produces less particulate emissions in both older and new engines. It emits only 15 ppm of sulfur, a 97% reduction in the sulfur content compared to low sulfur diesel.


Lubricating oils had to be developed for compatibility with the new low-emissions solutions. Designated CJ-4, these oils are formulated with lower sulfated ash, phosphorus, and sulfur levels to ensure protection of the aftertreatment devices and to provide better wear protection, higher oxidation protection, and better soot handling characteristics.

Beyond this, engine manufacturers are using a variety of means to comply with the 2010 diesel emissions regulations. Two particular technologies have evolved to control nitrogen oxide emissions: selective catalytic reduction (SCR) and exhaust gas recirculation (EGR). To handle particulate matter emissions, diesel particulate filter technology has been developed.


In essence, SCR is a system that works by a chemical reaction triggered by heat. A fine mist of diesel exhaust fluid (DEF) is injected into the downstream exhaust. The exhaust's high temperature converts nitrogen oxide levels into harmless levels of nitrogen and water vapor and eliminates the diesel smell.

Diesel exhaust fluid, also known as urea, is an organic compound that is harmless to the environment. Urea already is widely distributed for many industrial and agricultural needs.

Urea consumption varies with duty cycle and other factors but is not expected to exceed 5% of fuel consumption.

SCR systems require a separate container for urea, along with extra wiring, hoses, and sensors to manage the injection flow of urea into the truck's exhaust stream.


In simple terms, exhaust gas recirculation, also referred to as cooled EGR, captures a small proportion of exhaust gas and injects into the engine's combustion cycle, along with fresh air and fuel. This slows down the violence of the combustion itself, resulting in a lower level of nitrogen oxide emissions. Unlike SCR, EGR does not require an additive.

Both EGR and SCR are proven approaches to emissions reduction and are used in a wide range of applications for diesel-powered commercial vehicles around the world.

Diesel particulate filters (DPF), installed in place of mufflers, typically contain a porous substance to “strain” and “catch” the microscopic-sized particulate matter from the exhaust stream and prevent these particles from reaching the atmosphere.


Over time, these traps “fill up” and need to be periodically cleaned by means of a regeneration process. Otherwise, the filter can plug up and adversely affect the engine's performance and fuel economy.

This regeneration process is typically achieved by burning off the trapped particulate matter.

The two types of regeneration are passive and active. With passive regeneration, particulate matter is continually burned off while a vehicle is driven. Active regeneration refers to a periodic burning of particulate matter by adding a small amount of diesel fuel into the exhaust gas.

Active regenerations typically won't be necessary for those applications where truck engines work hard enough to generate the heat necessary to continually burn off the trapped particulate matter, as in highway applications. They may, however, be required for applications such as city or suburban operations where vehicles do a lot of stop-and-go operation or prolonged idling, and engines don't generate enough heat for regeneration.

Drivers will not notice passive regeneration, as the engine continues to operate normally. The only sign of the regeneration is an indicator light on the dashboard.


A major concern about the adoption of SCR is whether an infrastructure for urea will be in place by 2010.

Toward that end, a number of companies in the refining, packaging, and distribution segments of the nation's fuel market are preparing to provide diesel exhaust fluid availability.

Technology choices

Engine manufacturers are taking different approaches to meeting the more stringent 2010 US EPA-mandated diesel emission standards for their heavy-duty diesels.

Navistar is going with exhaust gas recirculation (EGR) technology for its MaxxForce engines.

Detroit Diesel, Mack, Paccar, and Volvo will be using selective catalytic reduction (SCR) technology.

Cummins will offer both EGR and SCR technologies.

Caterpillar, which recently announced it will exit the North American heavy-duty diesel truck engine market by 2010, will not be supplying 2010 compliant engines.

Clean air

On the federal level, the EPA has regulated on-highway diesel engine emissions standards through the Clean Air Act (CAA) since 1970. Emission regulations are phased in over a number of years, gradually reducing the legal levels of emissions for various sizes of diesel engines.

These phases are identified as Tiers. The higher the Tier level, the greater the reduction in emissions.

Tier 3 emission standards will take effect in 2012, followed by Tier 4 in 2015.

The EPA estimates that Tier 3 standards will reduce NOx emissions from diesel engines by about 1 million tons per year, an effect similar to taking 35 million passenger cars off the road. Tier 4 standards are even more rigorous.

In the European Union, emissions standards are identified as Stages or Euro standards. Euro 4 standards were implemented in 2006, setting PM emissions at 3.5 gram per kilowatt-hour (g/kWh) and NOx emissions at 0.02 g/kWh.

Euro 5, which goes into effect in 2009, calls for a nearly 43% reduction in NOx emissions to 2.0 g/kWh. PM remains unchanged.

Euro 6 requirements, which will come around 2013, are still being decided upon. It is expected, however, these regulations will decrease emissions of PM by 50% and NOx about 75% to 80%, compared with Euro 5 standards.

About the Author

David Kolman