[Diy_efi] Crank trigger resolution

[Phil Lamovie]

I think you need to consider the problem from the
desired outcome backwards.

First determine the maximum spark timing error that you
are willing to accept.

Now the engineering decision becomes somewhat easier.

The TPU is the key. If you can't resolve the time to 1 microsecond
then all the teeth in the world won't give you an accuracy better
than + or - 3 degrees during hard acceleration. (street car)

You would think that this is a dawdle given the cpu speeds
available but real time driven interrupts still need to be serviced
and code bloat is a killer.

So now you need to decide HOW to determine the maximum
acceptable error or MAE.

Is this a ULEV vehicle or an INDY car? What is the performance
std you are trying to meet?

Now the cycle to cycle variations of air flow and fuel injection are
fairly chaotic so you might find that your MAE is more like 5 degrees.

Ford are fairly happy to dump the timing back to index on some of
their 32 bit 20 mHz systems under severe acceleration as they
know it has no effect on the drive cycle emissions or fuel
consumption.

Now why do you want to modify your system ? Which aspect do
you feel is not meeting it's target ?

HTH

phil





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