Two-Zone Combustion Models

A two-zone combustion model consisting of burned and unburned masses was constructed to simulate an engine during combustion. The study of thick flame in turbulent flow in engines revealed there are microvolumes of unburned gases behind the flame front that progressively engulf the unburned gases at a rate determined by the turbulence intensity which in turn is a function of the inlet air velocity or the engine speed, where extra turbulence by squish at TDC is not generated. These small volumes are consumed at the rate of laminar burning velocity from their surfaces. Their typical size is found to scale with the dimensions of the combustion chamber and the turbulence Reynolds number. Two types of models exist. (a) A model which considers the delay and main combustion as a continuous phenomenon. Here, the rate of burning is directly proportional to the mass of unburned gases behind the flame front and inversely to a characteristic time. The time constant is again defined as the ratio of the characteristic size of the microvolumes or eddies to the laminar burning velocity, thus bringing the two important aspects of flow turbulence and chemistry into consideration. The size of the eddies is related to the inlet valve lift to account for the intake-generated turbulence. Later, it is argued that the size could be the Taylor microscale. (b) A model which describes two distinct phases namely the ignition delay period and main combustion duration. Here, the size of the eddies is considered of Taylor macro scale and the course of heat release itself is described using the familiar Wiebe or similar function that needs the durations of the two phases of combustion. The two-zone models with two engine-specific constants that are invariant with operating conditions successfully describe the rate of heat release and emissions of nitric oxides and hydrocarbons. They can also be used to predict knocking.