Nitrogen Compounds (NO, NO₂, N₂O, and NH₃) in NOₓ Emissions from Commercial EURO VI Type Heavy-Duty Diesel Engines with a Urea-Selective Catalytic Reduction System

Abstract

Selective catalytic reduction (SCR) systems have been widely used in heavy-duty (HD) diesel engines to meet the stringent emission standards for nitrogen oxides. The mobile SCR system has improved the considerable reduction of NOₓ emission from HD diesel engines. Many investigations have been performed to enhance the catalytic performance and optimize the SCR system within the diesel engines. The purpose of the present study was to investigate the types of nitrogen components (NO, NO₂, N₂O, and NH₃) in the NOₓ exhaust emissions prior to and after passing through an SCR device. A EURO VI type commercial heavy-duty diesel engine was equipped to an AC dynamometer. World harmonized stationary cycle and transient cycle were introduced to operate the test engine under road conditions. A new quantum cascade laser analyzer was employed to measure the nitrogen emission species in real time. The engine-out NOₓ emissions were strongly affected by the load conditions of the engine, which also determined the efficiency of SCR conversion. Total conversion rates of up to 96% were achieved for both test modes. Various concentrations of urea were used as a reducing agent in the SCR system. On the basis of the urea concentration, the conversion efficiency and composition of nitrogen oxides varied under the same engine conditions. The fraction of nitrogen compounds in NOₓ emissions changed during the catalytic processes within the SCR system. The results yielded accurate concentration values for nitrogen compounds in the commercial heavy-duty engine, warning of the possibility of a new greenhouse gas due to converted NOₓ emissions.