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To meet the strict emission regulations for diesel engines, an advanced processing device such as a Urea-SCR (selective catalytic reduction) system is used to reduce NOx emissions. The Real Driving Emissions (RDE) test, which is implemented in the European Union, will expand the range of conditions under which the engine has to operate [1], which will lead to the construction of a Urea-SCR system capable of reducing NOx emissions at lower and higher temperature conditions, and at higher space velocity conditions than existing systems. Simulations are useful in improving the performance of the urea-SCR system. However, it is necessary to construct a reliable NOx reduction model to use for system design, which covers the expanded engine operation conditions. In the urea-SCR system, the mechanism of ammonia (NH3) formation from injected aqueous urea solution is not clear. Thus, it is important to clarify this mechanism to improve the NOx reduction model.

This paper proposes a new mixing-controlled, low temperature combustion (LTC) approach for high-speed direct injection (HSDI) diesel engines. The purpose of this approach is to avoid the excessively high pressure-rise rate (PRR) of premixed, kinetics-controlled LTC and to enable the low nitrogen oxides (NOx) combustion to operate over the wide speed and load range of the engine. To address the soot/noise trade-off at high load LTC operating conditions, the pressure modulated multiple-injection coupled with swirl control was applied. This injection strategy enables the injection of high pressure (HP) main spray into the local high temperature region of the already burning low pressure (LP) pilot spray injected from the neighboring injection hole. By employing this injection strategy, the equivalence ratio (φ) distribution of mixture is drastically varied during main combustion processes.

In the urea SCR system, urea solution is injected by injector installed in the front stage of the SCR catalyst, and NOx can be purified on the SCR catalyst by using NH3 generated by the chemical reaction of urea. NH3 is produced by thermolysis of urea and hydrolysis of isocyanic acid after evaporation of water in the urea solution. But, biuret and cyanuric acid which may cause deposit are sometimes generated by the chemical reactions without generating NH3. Spray behavior and chemical reaction of urea solution injected into the tail-pipe are complicated. The purpose of this study is to reveal the spray behavior and NH3 generation process in the tail-pipe, and to construct the model capable of predicting those accurately. In this report, the impingement spray behavior is clarified by scattered light method in high temperature flow field. Liquid film adhering to the wall and deposit generated after evaporation of water from the liquid film are photographed by the digital camera.