Multi-dimensional modeling of natural gas ignition, combustion and pollutant formation in direct injection engines.

Abstract

Models for describing ignition, combustion and pollutant formation in direct injection (DI) natural gas engines are developed in this work. These models are coupled with a multi-dimensional reactive flow code KIVA-3 to study the energy conversion process in DI engines. Emphasis is placed on gaining a fundamental understanding of each of these processes by studying them in detail individually. Ignition of natural gas injected under compression ignition conditions is simulated by using a detailed chemical kinetic mechanism for natural gas oxidation (consisting of 22 species and 104 elementary reactions) in conjunction with KIVA-3. The mechanism is chosen after a systematic study of its predictive capabilities under typical end-of-compression conditions in DI engines. It is shown that the choice of a suitable detailed kinetic mechanism is essential to account for all the thermodynamic and fuel composition related factors that affect the ignition process in DI natural gas engines. Calculations of ignition delay of different blends of natural gas in a combustion bomb compare very well with measurements. The coupled detailed kinetics-multi-dimensional flow model is also used to investigate the effect of parameters like chemical additives, injection rate and fuel temperature on ignition delay and ignition location. It is established that a specified mass of fuel burned is a more consistent criterion to detect the length of the ignition delay period than pressure rise while comparing different natural gas blends over a range of temperatures and pressures. After ignition occurs, combustion of injected fuel with air is described using a mixing controlled model. Fuel and air are assumed to burn in stoichiometric proportions at a rate proportional to the rate of mixing. Transition from detailed kinetics to the combustion model is assumed to occur when 4% of the total injected fuel has burned. Pollutant formation is modeled by implementing the extended Zeldovich kinetic mechanism for thermal nitric oxide formation in KIVA-3. Finally, calculations are done using the geometry of a medium size heavy-duty diesel engine with pure methane as fuel. It is shown that operating parameters like engine speed and load, coupled with different strategies for injection timing and level of boost, can have a significant impact on engine output. The tradeoffs between performance and emissions are discussed in terms of variation in engine operating conditions and injection strategies.