Analysis and Prediction of in Cylinder NOx Emissions for Lean Burn CNG/H2 Transit Bus Engines 2004-01-1994

In the immediate future the introduction of a wider variety of fuel types will play a significant role in reducing emissions and in solving the energy needs of the transportation industry. Both compressed natural gas, CNG, and hydrogen are expected to play significant roles, and the present paper shows that these fuels, when used together, can offer large benefits in NOx emissions. Significant reductions in NOx emissions will be required for CNG transit buses and heavy duty trucks, if they are to meet the future stringent emissions standards that come into effect in the year 2007.

In the present paper we have applied a newly developed engine model with detailed chemical reactions to predict the “in cylinder” production to NOx under realistic engine conditions. The model consists of the following four parts: (1) A simplified engine model to predict the “in cylinder” maximum pressure for CH4/H2 mixtures; (2) A semi-empirical model to predict the detailed pressures, temperatures, and fuel rate curves during an engine cycle, which is based on achieving MBT; (3) A detailed flame structure model to predict NOx formation during the engine cycle with the use of GRI chemical reaction mechanism; and (4) A detailed chemical model to predict the influence of “in cylinder” compression and expansion on the evolution of the different burned gas parcels in the engine. The model has been applied to the entire engine cycle from the inlet to the exhaust processes. The results include a wide range of lean equivalence ratio studies with excellent agreement with the limited experimental results, and the full paper will discuss the trade offs between NOx production, equivalence ratio, power, and inlet boost pressure.