An Experimental and Theoretical Study of the Effects of Excess Air Ratio and Waste Gate Opening Pressure Threshold on Nox Emission and Performance in a Turbocharged CNG SI Engine

In this research, effects of excess air ratio and waste gate opening pressure threshold on NOx emission and performance in a turbocharged CNG SI engine are experimentally studied at 13-mode ECE-R49 test cycle. The engine power, boost ratio and charge air temperature are investigated experimentally at the cycle for different waste gate pressure thresholds. A code is developed in MATLAB environment for predicting engine performance and NOx and the results are validated with the research experiments. The effects of excess air ratio on the engine indicated power and specific fuel consumption as well as NOx emission are numerically investigated at WOT by the code. NOx emission of WOT is max at excess air ratio of 1.1. Simulation reveals that higher excess air ratio at a rate of 20% decreases maximum indicated power 9% and improves minimum ISFC 7%. Experiments indicate that brake power augments with increase of the pressure threshold especially at high loads and speeds due to higher boost ratio. It is also found that changing the threshold from 16 mmHg to 200 and 265mmHg decreases total bsNOx at rate of 6 and 12%, respectively. The threshold increase to 323mmHg augments total bsNOx. Therefore, the threshold of 265mmHg is optimum threshold among the four pressure thresholds experimented.