A state of the combustion process in the cycle is calculated using the models described in the following section.
3. CALCULATION MODELS FOR A BURNING PATH
3-1. Single Zone Model
In the cycle calculation in the engine cylinder, the working gas during combustion process usually is treated as a homogeneous mixture having a uniform state [4][9]. When the specific heat of the homogeneous mixture is evaluated as a function of temperature and excess air factor, the cycle simulator for evaluating the cycle efficiency, and the output power can well predict the performance, and the results agree well with the actual performance of the practical engine. The cycle simulator has been put to practical use.
According to our experience, the maximum temperature of the cycle calculated by the above scheme exhibits a certain correlation with NOx formation in the combustion process. For the quantitative calculation of NOx formation, however, the single zone model is insufficient. In recent year, the analysis by dual zone model, assuming the burnt and the unburned zone in the combustion chamber. The state of each zone is assumed to change independently in the combustion process. Prediction of NOx emission by this scheme to approach the calculation of the actual emission level, has been examined [10][11][12][13].
We are going to extend this idea to the new approach using multi zone model. A further precise estimation of the respective zone temperature may give a NOx emission approaching the actual level [14].
3-2. 3-Zone Model
3-zone model is an extension of the dual zone model. The burning zone is newly defined in addition to the dual zone. The temperature of the zone is calculated as that in the thermodynamic equilibrium state.
The burning zone of each calculation step refer to the burnt mass calculated from the given heat release rate during 1｡?A. The zone temperature is calculated assuming the adiabatic and constant pressure combustion during the calculating step. In the following calculation step, the zone mixes with the preceding burnt zone to give a new burnt zone.
The state of the burnt zone in the next step is calculated assuming the homogeneous burnt mixture described above. A schematic illustration of the 3-zone model is shown in Fig.1.
Fig.2 shows a comparison of temperature paths between single zone and 3-zone model. The results from 3-zone model give a distinct difference of the temperature in the respective zone. The temperature path in the combustion process for the single zone model follows between temperatures of the burnt and the unburned zone.
As for the maximum temperature of the cycle, a tendency to NOx emission can be reasonably evaluated to some extent by using the single zone model. However, the difference between the crank angles of two models, where the maximum temperature appears, is considerable. In this respect, the single zone model is insufficient for estimating the NOx emission.
3-3. Multi-Zone Model
A burnt gas in the cylinder is concentrated in a single zone when 3-zone model is applied. As seen from the observation of flame behaviors, however, the burnt zones distribute in layers. And even in the case of apparently merged zone, it is hard to exist a well-stirred single zone.
In the present model, the burnt zone in each calculation step forms a separate zone, and individually changes the state. The thermodynamic equilibrium temperature is assigned to the temperature of the reacting zone in each calculating step. The zone experiences a reversible adiabatic change. A schematic illustration of the multi-zone model is shown in Fig.3.
