PROFESSIONAL MOTORSPORT

SIMULATION SOFTWARE

Intake runners and primary exhaust

Figure showing the data traces of the intake dynamics of the LapSim Engine model

The model is a dynamic model, taking the inertia of intake and exhaust into account, dependent on their length and width. They are modelled like spring / mass system.

The intake dynamics can be seen in the figure. Grey area is piston travel. White line is the amount of air passing the intake valve, the light grey line represents the position of intake volume. Both relative to cylinder volume.

The orange line represents the cylinder pressure, the dark grey line the exhaust pressure just behind the exhaust valves.

Example of the dynamic system

Figure showing the movement of the intake air of the LapSim Engine model during the complete intake stroke

Out of the size of the intake and exhaust runners, a compression volume just in front of the valve is determined. Due to the dynamics, this volume will be compressed and expanded, acting like a spring.

The total volumes of the intake and exhaust are represented by two masses with its own inertia. The global behaviour can be seen in the figure, where you see the cylinder volume as well as intake volume.

It can clearly be seen that the intake volume has a phase lag over the cylinder volume, due to the inertia of the intake volume.

Determines the cylinder fill rate

Figure showing the movement of the intake air of the LapSim Engine model during the valve overlap at top dead centre

In the plot results one can distinguish a white and a light grey line. The white line is the air passing the intake valve. The grey line represents the position of the intake volume.

Especially where the intake stroke starts, the difference between the two can be seen. As soon as the cylinder pressure is below 1 [bar], the flow over the valve starts. The volume in the intake runner has a clear phase lag compared to this.

Achieving a fill rate above 100% cylinder volume

Figure showing the charge fill of the intake air of the LapSim Engine model at the end of the intake stroke due to dynamics

By keeping the intake valve open after the bottom dead centre, the inertia of the intake charge helps to fill the cylinder above 100%.

In the figure an example of this can be seen. When the piston is at bottom dead centre, the fill rate of the cylinder is just below 100%.

However, because the intake valves are still open in combination with the speed of the intake air, its inertia pushes more air into the cylinder, although the piston already moves upwards.

This results in a fill rate significantly above 100%.

High end power versus low rpm torque

Figure showing the reverse flow of the intake air of the LapSim Engine model at the end of the intake stroke

Using the charge fill of the intake only works at high enough revs, where the effect of the inertia is big enough.

This can be seen in the example where during the intake stroke, the intake volume follows the cylinder volume much closer.

Subsequently the long opening duration of the valve leads to a significant reverse flow of the intake air, when the piston moves upwards after bdc.