The subject of this study is the effect of in-cylinder selective non-catalytic reduction (SNCR) of NOx emissions in diesel exhaust gas by means of direct injection of aqueous urea ((NH2)2CO) into the combustion chamber. A single cylinder diesel test engine was modified to accept an electronically controlled secondary common rail injection system to deliver the aqueous urea directly into the cylinder during engine operation. Direct in-cylinder injection was chosen to ensure precise delivery of the aqueous urea without the risk of any premature reactions taking place. The injection strategy was four molar ratios, 4.0, 2.0, 1.0 and 0.5 with five varying injection timings of 60, 20, 10, 0, and -30 degrees after top dead center (ATDC). The main secondary injection fluid, aqueous urea, was mixed with glycerol (C3H8O3) in an 80-20 ratio, by mass, with the desire to function as a lubricant for the secondary injector. In addition to the base line and aqueous urea tests, neat water injection and an 80-20 ratio, by mass, water-glycerol solution tests were also conducted to compare the effects of said additives as well. The data collected from the engine tests showed that the aqueous urea-glycerol solution secondary had no effect on the reduction of NOx and even resulted in an increase of up to 5% in some tests. This was due to the low average in-cylinder temperature as well as increasing the required residence time, prohibiting the reduction reaction from taking place. The neat water and water-glycerol solution secondary injection was found to have a reduction effect of up to 59% on NOx production in the emissions due to the evaporative cooling effect and increased heat capacity of the water.