Monday, October 3, 2011

Starting fuel

I spent the weekend with the stock ECU measuring the prime and cranking pulsewidths for different temperatures.  I started with a hot, running  engine and pulled the fuel pump fuse and until it stalled, then pulled all 6 COP plugs to keep it from starting.

I recorded the block temperature and scoped the injector signal for 8 seconds of cranking.  The raw data looks like this:

Raw PW of prime and cranking PW vs temperature.
Of course, the above figures need to be adjusted for the injector dead time at about 10 volts, so subtract 1 second for true pulsewidth.

My thought was that it would be easy to translate this into Megasquirt and I would have a car that starts just like stock.. set the cranking ign to 0 BTDC, prime and crank like OEM and Blammo! I would have an engine that starts like stock. 

The problem is that the OEM ECU doesn't inject a constant cranking PW like Megasquirt does.

Cranking fuel decreases with the cranking time.  Which to use for Megasquirt??
About the best I can hope to do with this is plot the data and look at the general shape of the curves.  Increase of fuel vs temperature is relatively linear down to 85 degrees, at colder temps it starts increasing quickly.  Tuning Megasquirt for start is still a matter of increasing cranking fuel until it consistently starts.

OEM fuel curves for start.  Prime, first injection event and the 5th injection.  This data has 1 second subtracted for injector latency.
My current cranking fuel.  I will certainly tweak it but it's Ok so far.

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