抄録
The growing employment of alternative fuels and the necessity for further reduction of in-cylinder NO formation require additional knowledge about the fuel, design and operating variable effects on internal combustion (IC) engine performance and emissions. To this end, with the present work, an enhanced combustion diagnostic tool is proposed for the analysis of both heat release and flame propagation parameters in homogeneous-charge spark-ignition (SI) engines. It includes specifically developed sub-models for evaluating thermal and prompt NO so as to rank the effects of NO formation routes and thermodynamic parameters on nitric oxide emissions. The new diagnostic method is based on a quasi-dimensional multizone combustion model that takes the non-uniform spatial distribution of in-cylinder burned gas thermodynamic and chemical properties into account. As far as temperature calculation is concerned, the energy conservation law is applied to each zone, which can be considered to be either homogeneous or composed of two distinct parts: an adiabatic core and a thermal boundary layer. Each zone is generated at user-defined crank-angle increments, so that virtually all the cases between 'fully mixed' and 'unmixed' limiting models can be considered for investigation. The multizone thermodynamic approach for mass burning rate analysis is coupled with a CAD procedure for the computation of burned-gas mean expansion speed and burning speed. A new procedure for determining the start of combustion was embedded in the model. The calculation uses detailed thermodynamic gas properties that are capable of taking fuel composition and intake air humidity into account. The diagnostic method also includes previously proposed refinements for: modelling the surface-averaged heat flux in order to take the unsteadiness of gas-wall temperature difference into account; evaluating the unburned-gas zone temperature; calibrating the mass-fraction burned at the end of the flame propagation process. The NO calculation is performed on the basis of a six-reaction thermal formation mechanism and includes the contribution from the flame-formed NO. The quasi-dimensional multizone heat-release, flame-propagation and NO-formation integrated model was shown to be an effective and reliable tool of combustion diagnostics for SI engines through its application to the analysis of accurately measured pressure time-histories in the cylinder of an upgraded multivalve engine fuelled by either natural gas or gasoline under a significant sample of operating conditions. The results obtained from two-zone and one-zone heat-release models are also reported and discussed. The predicted average NO concentrations were in good agreement with the measured values when 10 to 15 burned zones were used in the model. Finally, heat release, flame propagation parameters as well as NO concentrations obtained from mean cycle and cycle-by-cycle calculations were compared, generally showing good physical consistency. However, cyclic pressure variations led to significant differences in the heat-release rate, combustion speeds and levels of nitrogen oxide emissions. The results indicated that cycle-by-cycle calculation should be considered for NO evaluation.
