Developing our own poppet valve resistance model

Out of our experience with aero data, we did not want to be dependent on flow data, with all possible inaccuracies. We decided to develop a resistance calculation for the poppet valves, to overcome this problem and were surprised by the accuracy.

The resistance calculation is mainly based on the area the poppet valve opens, with small correction factors dependent on lift.

Additional we added a global correction value for the valves. We have always kept this at 100% with our research and advice you to do the same.

Valve resistance model works for all configurations

Example of valve layout in cylinder head

You can choose to have 1 to 3 intake valves. In addition you can select 1 or 2 exhaust valves.

The position of the valves is done automatically, dependent on the size of the valve, as well as the amount and size of the other valves.

In the future we would like to supply the option to alter the position of the valves, if desired.

Comparing configurations from a flow perspective

Example of the flow results out of the simulation based valve lay-out

The graph in LapSim shows you the flow results based on these calculation. It makes it very easy to estimate the influence of changing the construction like valve size and/or angle on the flow results and subsequently the engine output.

There is often a discussion between engine builders whether increased flow on a test bench, directly relates to more engine power. The argument is that more flow (with the same construction) could also mean less swirl and turbulence which is bad for a mixture.

With our approach, there is a turbulence correction which is dependent on the amount pressure differences over the intake valve.

The dynamics are based on primairy lengths

Example of cylinder fill rate versus engine rpm for short intake runners
Example of cylinder fill rate versus engine rpm for long intake runners

Both the intake as the exhaust is modelled like a spring mass system, with additional resistance dependent on tube size.

The spring as well as the masses which cause the dynamics are determined by the size of the primary runners.

In the two figures the influence of a variable intake length on the fill rate can be seen. This example is of a Porsche 993 VarioRam engine, where the intake length can be increased by 225 [mm].

Due to the longer intake lengths, the peak fill rate decreases from 5700 to 5000 [rpm].

Amount of backpressure depends on volume per second

To avoid making the model too complex, we neglected the influence of the construction before the intake tubes and after the primary exhaust runners.

However, you can specify the influence of a more restricted exhaust by increasing the exhaust backpressure. We assume 100% to be a full race open exhaust.