The investigation has been divided into two parts. In part one, numerical investigations of the effect of humid air with different levels of humidity on gaseous emissions of a non-premixed combustion have been investigated. This part of the investigation was a feasibility study, focused on how different levels of humidity in the intake air affects the exhaust NO emission. Part two of the investigation was verification of the numerical results with a naturally aspirated engine with natural gas as the fuel. Here, we also investigated the impact of humid air intake on engine’s particulate matter (PM) emission.
For the numerical investigations, the non-premixed combustion in a single cylinder was simulated using the presumed probability density function combustion model. Simulations were performed for dry as well as humid intake air for 0%, 15%, and 30% relative humidity (RH). Numerical results have shown 65% reduction in NO emission at 15% RH, when compared with NO emission of dry intake air. With 30% RH, NO emission was reduced by 93%.
For the experimental investigations, a General Motors inline 4 cylinders, naturally aspirated engine with a maximum rated horsepower (HP) of 50.8 for natural gas fuel was used. The engine was connected to a water-cycled dynamometer. NOx emission was measured by a Horiba portable emission analyzer model 250 and exhaust PM was measured using a dilution tunnel in conjunction with a cyclone with teflo filters.. The experiments were carried out at four different horse powers (HP) of approximately 5, 12.5, 25. And 37.5, and three RH of ambient (30%), 45%, and 60%. Results show for each additional 15% increase in relative humidity, there was a nearly 10% reduction in NOx emission.
The PM emission increases with the addition of relative humidity, especially at low HPs. With increased HP, the PM augmentation is reduced significantly and at 37.5 HP , the ratios of PM emitted at 45% and 60% relative humidity, to the corresponding ambient baseline values ( at 30% RH), were near 2.0.