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Ford’s version of speed density is quite clever, ignoring volumetric efficiency and getting straight to the point, which is air mass....
Ford's method is based on the observation that for a given RPM, a plot of manifold pressure vs cylinder air mass lies on a straight line.
whittlebeast wrote:It just happens that I was working on this concept today. The big issue is that it requires a division by RPM that painful to do in code. Here is how the logic flows.
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Multiply MAP * RPM to get a Scaled function of Mass Air Flow.
Pass thru the MAP*RPM lookup curve to get Base Duty Cycle
Divide by RPM to get Base Pulse Width
robs wrote:Thanks for pointing out the article, it was fun and informative. Not really the same as the method here.
Ford's approach still uses a 2-dimensional table of RPM vs MAP it's just that it gives you the air mass directly rather than VE. Any VE table could be converted into an air mass table (and vice versa) with no loss of information.
The method experimented with here uses a 1-dimensional table looked up by the product RPMxMAP (a later tweak added a constant n x RPM slope adjustment) to give the VE. It can only represent a very restricted subset of possible VE tables. The hope was that this subset would include the optimum and simplify tuning. I think it was easier to tune, but didn't get very close to optimal.
I don't think Ford's approach would be any easier to tune than the usual VE table -- it might have some benefits (or drawbacks) when it comes time to working out the injector pulse width.
Have fun,
Rob.
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