A Computational and Experimental Study of Combustion Chamber Deposit Effects on NOx Emissions

The oxides of nitrogen (NOx) produced during combustion in an automobile engine play a major role in atmospheric chemistry and therefore need to be reduced by modifying vehicle engine designs and fuels of tomorrow. In a combustion chamber of a spark ignited engine, NOx is formed as atmospheric nitrogen competes with fuel molecules to couple with oxygen in the extremely hot burned gases behind the proceeding flame front (Zeldovich type) and as reactions occur directly in the combustion flame zone (“prompt” type). Since little nitrogen is present in the fuel, the fuel contribution to the overall NOx emissions is minor. Certain combustion chamber deposits have been shown to increase NOx emissions by thermally insulating the combustion chamber and taking up chamber volume, thus slightly increasing the compression ratio of the engine and raising the combustion gas temperature. For the Zeldovich NO production mechanisms, the reaction rates increase rapidly as the combustion temperature increases and therefore NO production is significantly larger at higher temperatures. For example, increasing the bulk gas temperature by 2.5%, from 2000°K to 2050°K, increases the NO produced by over 44%. The dependence of prompt NO production on temperature is more complicated because of the large number of intermediate reactions involving hydrocarbon free radicals.

A perfectly stirred reactor (PSR) computer model and a thermodynamic engine model have been utilized to study the temperature dependence of NOx formation and deposit effects on temperature. The PSR model developed was run to investigate the oxidation mechanisms of toluene, isooctane and methane as neat hydrocarbon fuels with varying levels of beat transfer to the cylinder wall to simulate thermal insulation due to deposits. The results indicate that the prompt mechanisms play a significant role in the production of NOx, primarily at the lower end of the temperature range which is sufficiently high to produce NOx and that changes in fuel chemistry affect the NOx production rates. The modeled NOx emissions however, are primarily a function of the Zeldovich mechanisms and a given combustion temperature. Empirical results from a GM 2.0L dynamometer test stand show substantial increases in NOx levels for the test fuels pure isooctane and a 50/50 mix of isooctane and toluene at several test conditions when operating with combustion chamber deposits built with a deposit build-up cycle versus a clean combustion chamber. Using a combination of theoretical calculations, model predictions and empirical validations, a relationship between combustion chamber deposits and NOx emissions is demonstrated and investigated.