Air-fuel ratio calculation for a natural gas fuelled spark ignition engine

Abstract

Air-fuel ratio (AFR) is a crucial parameter for combustion controls in internal combustion engines. An incorrect AFR metering for reciprocating internal combustion engine causes high toxic gases emissions formulation, serious fuel consumption problems and unbearable combustion noise and combustion deterioration. Traditionally, the AFR is obtained by direct measurement of intake air and the fuel either injected into the combustion chamber or pre-mixed at the carburetor. However, the accurate AFR obtained from direct measurement is difficult due to measuring equipments resolution prone to errors. This paper describes a method for accurate determination of air-fuel ratio based on exhaust emission gas analysis as an additional tool used to be validated the conventional direct air fuel flow rates measurement. This method explains all the possible parameters that may affect the accuracy of air-fuel ratio measurement which includes the instrument error, ambient conditions, the assumed water-gas shift reaction constant, the humidity of the atmospheric air and the inclusion of nitrogen in the air-fuel ratio model. Results show that four essential exhaust gas emission concentrations, namely carbon monoxide (CO), carbon dioxide (CO2), oxygen (O2) and unburn hydrocarbons (HCs) are adequate for obtaining an accurate air-fuel ratio. The fuel type and the range of parameters that may affect the accuracy of air-fuel ratio are properly defined. This paper will also present experimental results of a bi-fuel natural gas spark ignition engine to be compared with computational results. The results of this investigation will be used to develop a new dedicated natural gas engine. Copyright © 2004 SAE International.