Unit 4: Emission Control

A truck exhaust pipe. Image courtesy of Newsweek.
Navigation: ETAP 623 Fall 2017 Section 7619 | Akshar Shastri Mini-Course


As discussed in Unit 1, diesel engines are preferable to gasoline engines for heavy duty applications. Unfortunately, the combustion of diesel fuel causes greenhouse gases such as nitric oxide (NOx) to be emitted into the air. This unit will explore the Exhaust Gas Recirculation (EGR) and Selective Catalytic Reduction (SCR) systems for remediation of the toxic emissions.

Learning Objectives

1.Students will be able to state some of the dangerous particulates present in diesel exhaust.
2. Students will be able to explain the differences between EGR and SCR emission technology.

Exhaust Gas Recirculation (EGR)

As the combustion temperature of diesel fuel is increased, NOx emissions increase exponentially. Therefore, in order to limit NOx emissions, it is imperative to reduce the temperature of the combustion chambers in a diesel engine. Exhaust gas recirculation recycles the combustion gas from the engine back into the cylinder after it is cooled. This process leads to a lowering of the combustion chamber temperature, and thus emissions with a lower temperature. When the exhaust gas temperature is reduced, NOx emissions are decreased significantly (White, 2017).

An easy way to think about EGR is adding cool water to a hot beverage. When the two fluids are combined, the amount of steam emitted from the hot beverage is reduced since the overall temperature of the beverage decreases. Reducing the temperature of diesel exhaust by 100-200 degrees significantly lowers the NOx particles present in the gases produced by combustion. However, it is important to note that no fluids are present in an EGR system (White, 2017).

Selective Catalytic Reduction (SCR)

Selective Catalytic Reduction (SCR) differs from EGR as it uses a fluid to extract the NOx particles from the diesel exhaust. A fluid such as ammonia or urea is injected into the exhaust stream, and the NOx particles are reduced into nitrogen and water. SCR technology alters the chemical composition of the exhaust gas after combustion (“Air Pollution Control Technology,” n.d., p. 3).

SCR technology is fundamentally different from Exhaust Gas Recirculation implementations in two critical ways. First, it relies on a nitrogen based fluid such as ammonia or urea to be replenished by the vehicle operator in order to function properly. SCR technology also does not reduce the NOx particles produced by the combustion process. Instead, it merely neutralizes the particulate matter after combustion.

Review Quiz

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Air Pollution Control Technology Fact Sheet [PDF]. (n.d.). https://www3.epa.gov/ttncatc1/dir1/fscr.pdf

White, R. (2017). New lifetime solution for diesel emission measurement. Automotive Industries, 1.