Thursday, October 13, 2011

MAT correction

I've been driving back and forth to work running VE Analyzer each way.  I've noticed that my table looks different after driving in the cold morning air compared to the corrections I get driving home in warmer air.  Ideally it would be the same no matter what the temperature is.  I need to work on my MAT correction!

The amount of fuel injected is influenced by the Ideal Gas Law which is proportional to temperature in degrees Kelvin.

So if 50F = 283 Kelvin and 80F= 299 Kelvin then Megasquirt would want to inject 283/299 or about 6 1/2% less gas at the warmer temps.

Problem is, What seems great in theory isn't exactly what's happening in the engine and Megasquirt seems to over correct and run too lean at warmer temps.

There are 3 tools to correct that:

MAT correction table applies a straight % correction based entirely on the temperature.  It adds a little gas back at warm temps were the ideal gas law takes it away.
MAT scaling determines how much of the ideal gas law calculation to use.  In the above example between 50F and and 80F, the full idea gas law correction is 6 1/2 percent.  With scaling set to 70% the correction is reduced to .7x 6.5%= 4.5%. 

MAP/CLT blend attempts to correct for the fact that at lower rpms the air is moving slower and has more time to be heated by the motor.  It ignores load.   In this chart at 1000rpm it is using 3% of the coolant value and 97% of the MAT value.  If CLT is 180F and MAT is 80F it would use a MAT value of 83F (.97x80)+(.03x180)

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