Exhaust emissions produced from internal combustion engines have an effect on the severe air pollution and it has become a very important issue in the world. Off-road diesel engine, such as agricultural machines and construction machines are especially considered as one of the primary sources of the air pollution. For that reason, Tier4 emission regulation which is in force from EPA(United States Environmental Protection Agency) for off-road diesel vehicles have been reinforced in the world. In one of the exhaust emission standards, NOx(Nitrogen Oxides) is a major issues in the emission regulation of off-road vehicles. Because it is difficult to meet the emission standards, pretreatment technology as well as aftertreatment technology for diesel engines are essential in the present situation. As a NOx reduction technology, Urea-SCR(Selective Catalyst Reduction) is one of the promising of NOx generated in the engine. High NOx reduction efficiency is a challenging problem due to deactivated SCR reaction and formation of urea deposit during cold start. Also, NH3 uniformity is a primary parameter to achieve the high DeNOx performance. In this study, off-road diesel engine with aftertreatment system composed DOC(Diesel Oxidation Catalyst), DPF(Diesel Particulate Filter) and Cu-zeolite SCR(Selective Catalytic Reduction) catalyst was used. As a Urea-SCR system, an Emitec commercial UWS(Urea Water Solution) dosing system from Germany was used. Not only the density of exhaust emissions but also HNCO which is byproduct of urea pyrolysis and N2O which can be generated in the SCR reaction were measured. Also to figure out of the affection of particle materials generated during the injection of UWS at low temperature, the weight of PM(Particulate Matters) and PN(Particle Number) were measured. In order to compare with the exhaust emissions on experimental parameters, the NRTC(Non Road Transient Cycle) which is measurement methods of transition drive mode in off-road vehicle, was used. An aim of our study is to meet NOx emission standard of Tier4 emission regulation. This study was performed on the effect of UWS dosing start temperature, ANR(Ammonia NOx ratio), AOC(Ammonia Oxidation Catalyst) and insulation.
First, we determined UWS dosing start temperature. When UWS is injected at the duration of cold start, urea deposit caused by incomplete pyrolysis is formed. The urea deposit develops the solid ammonium which has high molecular weight. These kind of solid ammonium can not be decomposed easily and remain in the pipe at general diesel condition. In case of formation, a great deal of solid ammonium can be affected by blocking the way of flow in the catalyst and pipe. For this reason, UWS is generally injected above 200℃. In this experiment, we determined NOx concentration and formation characteristics of solid ammonium when UWS is injected below 200℃ at NRTC.
Second, we determined NH3/NOx(Urea dosing ratio, α) ratio. It is defined that actual UWS flow rate/stoichiometric UWS flow rate. When UWS is overdosed, NH3 is excessively generated with an increase of NOx reduction rate, and so it causes NH3 slip. In order to determine NOx emissions characteristics, the experiment was performed at α=1.0, α=1.2, and UWS dosing start temperature was set 150℃ based on the First experiment.
Third, when UWS overdoses excessively, NH3 can be emitted, due to a lack of reaction with NOx in the SCR catalyst. In this case, to prevent NH3 slip, AOC(Ammonia Oxidation Catalyst) is applied by setting it up behind of SCR catalyst, and also NOx can be formed in one of the side reactions of AOC. In this experiment UWS dosing was set in 1.0, 1.2 and, NOx emissions and NH3 exhaust characteristics were compared and analyzed.
Fourth, we determined the effect of insulation. On the basis of the third experiment, NH3 slip was prevented below 10ppm, although NOx emission was not enough for Tier4 NOx emission standard due to NOx generation caused by the AOC side reaction. In this experiment, aftertreatment system was covered with insulation. The characteristics of SCR catalyst used in this study had a low DeNOx performance at a low temperature regime. To achieve high DeNOx performance in overall NOx emission at NRTC, the SCR catalyst needs to be operated in the high temperature regime. For that reason, to achieve high DeNOx performance over NRTC, insulation was applied to aftertreatment system and the experiment condition was determined on the basis of UWS dosing start temperature, ANR and AOC experiments. As a result, with all the parameters explained above, Tier4 NOx emission standard was met.