Purpose developed flexible burn model

The calculated amount of air/mixture captured by the cylinder is compressed, subsequently ignited and burned by a two zone burn model.

The speed at which the burning continues, is a variable and dependent on the momentary temperature of the air/fuel mixture. Because the speed at which the burning continues is a variable, the model is capable of calculating the influence of compression ratio, advancing or retarding ignition, cylinder size and shape.

Flexible in compression ratio, cylinder size and shape

Figure showing combustion chamber with a burn sequence progressing

The burn model within LapSim, consists of two areas. The unburned mixture and the burned gas.

Both have their own temperature as degree of freedom (Tair and Tburn). Pressure is the same in both areas (Pcyl). The areas are separated by a 'burn boundary'.

The variable which determines the burn process

Figure showing the temperature and pressure traces of 2 zone burn cylinder model

The speed at which the 'burn boundary' between burned and unburned mixture proceeds is dependent on the temperature of the unburned mixture (Tair = light grey line in figure), the burned volume (dark grey area) and the octane number of the fuel.

In addition there is a turbulence correction which is dependent on the amount pressure differences over the intake valve.

In the graph showing the burn sequence, the cylinder pressure is plotted (Pcyl = white line) as well as the temperature of the burned mixture (Tburn = orange line)

Ignition advance is a variable

Figure showing the relation of the ignition timing on the engine torque output

Because of the presence of the 2 zone burn model, with its own degrees of freedom, it is possible to change the ignition timing.

Hereby one gets a clear indication of the influence of ignition timing on the generated torque / efficiency. This can be seen in the graph on the left.

On the vertical axis is the engine torque, the horizontal axis the ignition timing before TDC.

The model advances the ignition timing in such a way, that it creates the highest possible torque without exceeding this 'knock' limit (= orange line).

Unburned mixture exceeds knock temperature

Figure showing the temperature and pressure traces if the ignition is too much advanced and knocking occurs

The model assumes that when the temperature of the unburned mixture exceeds a certain temperature, the burn sequence becomes uncontrolled so that 'knocking' appears (orange star in figure).

Comparing this figure with the figure above, one can see the reaction of the model to the advanced timing.

The burn speed is significantly faster, the cylinder pressure higher and the end temperature of the mixture is higher.