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Deadtimes and a fuel map vs VE question


CamB

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As part of my endless stuffing around / learning... G4 Storm, 4 cylinder sequential, 8 valve BMW M10 adding a turbo, and still some time away from running.

1. I have older Bosch "363" injectors - a very low impedance (0.7ohm) ~650cc injector - with the recommended 2R2 ballast resistors. I want to estimate dead times and have the data from Motec base files. Presumably this data is for the injectors operating peak and hold, and my expectation (and a bit of googling) suggests this would change if operating as saturated with a resistor. Is anyone aware of any rule of thumb for adjusting the dead times for the addition of ballast resistors?

My alternative plan is to establish a baseline deadtime at typical running voltage using a method explained on the internet (deadtime should be accurate if you can - at low pulse widths) switch injector rate from once per cycle to twice per cycle without AFR changing, or if you can add a trim (eg in AFR overlay table) which is accurate). Then drop voltage and adjust deadtimes for lower voltage to maintain AFR... lots of stuffing around. Any reason to think the alternative wouldn't work fine?

2. I have been thinking about the traditional fuel map and how it relates to VE, and just wanted to check I understand:

- modelled fuel (on a G4+) takes the VE, MAP, [engine size, injector characteristics, fuel type and pressure] and for an AFR target works out the required injector pulse (taking into account some other adjustments)

- traditional fuel has a ms for master fuel that effectively bundles the items in "[  ]" (other than dead time and non-existent minimum effective pulse width), and if you use the AFR target table it then adjusts this for target AFR, plus adds the other adjustments. 

As a result, would I be right in saying if I have accurate injector characteristics and compensations (especially for IAT) in a G4 that:

- I could look at the fuel table as a VE table times say 50% (ie you would see 50% where you would ordinarily expect 100% VE). There's no magic in "50%" either - it could be 75%.

- a good rough idea for the master fuel would be the calculated ms from around the internet (which references pulse width for a given engine size, injector size and VE) divided by 50% (or 75%, if that's what was chosen). Eg I work out ~4.5ms, so should try 9ms if my 100% ve area of the fuel map has 50 (or 6ms if its 75). 

 

Thanks for assistance

Cam

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Hi Cam,

I'm not aware of any rule of thumb for adjusting deadtime when adding ballast resistors. I think your alternative plan sounds sound. Others who have done this may add their comments. 

Your idea for the fuel table also sounds like it would work. However it may be just quicker to go to an approx mid VE point, set the table value to 50, and then adjust the master fuel value to achieve the target AFR.

Scott

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