In the last couple of decades, countries have enacted new laws concerning environmental pollution caused by heavy-duty commercial and passenger vehicles. This is done mainly in an effort to reduce smog and health impacts caused by the different pollutions. One of the legislated pollutions, among a wide range of regulated pollutions, is nitrogen oxides (commonly abbreviated as NOx). The SCR (Selective Catalytic Reduction) was introduced in the automotive industry to reduce NOx emissions leaving the vehicle. The basic idea is to inject a urea solution (AdBlue™) in the exhaust gas before the gas enters the catalyst. The optimal working temperature for the catalyst is somewhere in the range of 300 to 400 °C. For the reactions to occur without a catalyst, the gas temperature has to be at least 800 °C. These temperatures only occur in the engine cylinder itself, during and after the combustion.
In this paper a study is presented where a second injector is installed in a Scania D13 cylinder head for urea injection purposes. Additionally, a crank angle resolved in-cycle NOx formation model is utilized to calculate the urea injection duration for each individual injection event. A gas temperature model based on the cylinder pressure is also used to decide the timing of the urea injection, although always occurring after the combustion and during the expansion stroke. A parameter sweep is conducted to evaluate the concept where also an SCR-catalyst has been introduced to the engine system. The parameter sweep done in the study contains sweeps of engine speed, rail pressure, engine load, EGR levels and timing of the urea injection (by changing the gas temperature limit at which the injection of urea is initiated). The engine tested is a modified single cylinder Scania D13 engine, rebuilt from a six-cylinder engine.
The results suggest that it is possible to reduce NOx in-cycle, up to 30% at 4.3 bar and 7.7 bar IMEPnet, and 50% at an IMEPnet load of 11.8 bar, with direct injection of commercial AdBlue. This relates well to the earlier numerical evaluation studies, in the field of direct injection of mixed urea and water solutions.