Injector Short Pulse Width Adder Table

[LeeE]

I know the recommendation is to use the injector short pulse width adder table data if possible. Other than making the fuel table smoother (or lumpier in my case) are there any other benefits?

[Adamw]

It is to correct for non linear injector behavior.  The symptom you typically see when operating in the non linear region without any SPWA correction is it will be idling rich one day, lean the next, rich at cruise sometimes, lean at cruise sometimes etc.  You will also see odd behavior in the fuel table around the idle and cruise area, for example you will make a cell smaller but the mixture will get richer.  It is usually most apparent with large injectors (>~1200cc) using petrol.

[LeeE]

57 minutes ago, Adamw said:

The symptom you typically see when operating in the non linear region without any SPWA correction is it will be idling rich one day, lean the next, rich at cruise sometimes, lean at cruise sometimes etc. 

Thanks for that.

I assume this fluctuation is because the SPWA must be a constant value but effectively becomes a variable when tuning without SPWA because the compensation included for injector non-linearity is subject to adjustment by the ECU for different operating conditions such as MAP?

[Adamw]

The commanded PW will vary with MAP, load, RPM, temperature etc.  For all the compensations to work correctly the fuel volume must be directly proportional to PW.  So for example if the PW is increased by 10% then you want to get 10% extra fuel.  

Below is a picture of an injector test for determining SPWA.  The straight dotted line is what the injector should flow if it was perfectly linear.  The other 4 coloured lines are what the injector actually flows (f different injectors)  

You can see if I was operating at around 0.4ms PW my injector flow would be about 42cc/min average.  Now say the AC just turned off and the ecu calculates I need 20% less fuel, so it reduces the PW by 20% so Im now operating at 0.32ms, you will notice my average flow is still about 44cc/min.  So the ecu has just commanded a 20% reduction in fuel but it has actually got 5% more.  This means you are now about 25% too rich.  Now lets say you change down a gear and the ecu calculates the engine needs another 20% less fuel, so our PW is now 0.25ms, at this PW our average flow has dropped to about 17cc/min.  So this time the ECU has commanded 20% less fuel but actually got 60% less. 

The idea of SPWA is to straighten out this erratic PW to flow relationship.  So at 0.4ms commanded PW a SWPA of 0.05ms would bring the flow up to the desired 50cc/min.  At 0.3ms the SPWA would be about zero as the average injector flow is close to the desired already.  At 0.25ms PW a SPWA of about 0.035 would bring the flow up to somewhere around the desired.   

 

 

  

 

 

[LeeE]

Great reply - thanks again.

Looks like I'm stuck with using the SPWA table even though it makes some of my fuel table look like it was tuned by a 5 year old

 

[Adamw]

It should really only affect the top 1 or 2 rows of your fuel table when PW is below about 1.5ms.