Wednesday, 22 December 2010

Dual plane manifold

In parallel to the manufacture and testing of the Megasquirt controller, I have been working on the CFD analysis of options for the intake manifold. Having established my cylinder head model had at least a passing resemblance to reality in this post, I have moved on to creating a simple engine model comprising the manifold, two cylinder heads and outlet pipes representing the cylinders:

This model was then run 8 times, each with a different inlet valve being open. These were run with a constant pressure drop representing a high RPM running condition and maximum valve lift. This is the condition where runner mass flow variation will be most obvious. Once the simulations were complete, I interrogated the results and normalised them against the average mass flow for all 8 cylinders:

The chart indicates that the variation in inlet runner geometry produces quite a variation in mass flow into the different cylinders (from -11% to +15% from the average). This is not an issue with a carburettor providing the fuel because the mixing is done at the throttle plate so each cylinder gets an appropriate air fuel ratio. However, the fuel injection system will work on the total mass air flow being split evenly between the cylinders and fuel accordingly resulting in overly rich and overly lean cylinders. This is not conducive to a happy engine, so an alternative manifold is required. Onto the single plane design then...

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