calc'ing A/F with DS data ?

[boostcreep]

Is the A/F ratio reported by DS a calculated value or is this value gotten from the ECM? Has anyone put the DS data into a spreadsheet and calc'ed the A/F ratio using the injector pw, rpm,voltage, and mass air flow to see if things jive? Does anyone have a formula for doing this?

Nick

 

[BoostKillsStres]

The DS value is the ECM commanded A/F. I don't think there's any formula out there thats going to give you actual A/F other than a wideband O2.

 

[boostcreep]

So those 6,7,8,9 to 1 A/F ratios that I see on people's DS runs are for real

Nick

 

[Eric Stage I]

No, that's not the actual A/F. That is the A/F that the chip burner has specified, but you have to go "artificially" low in order to make up for the fact that the MAF sensor stopped at 255gr/sec. So even though it may say 6:1, that's not what you actually get.

 

[boostcreep]

OK, so why can't you calculate the actual A/F ratio from the data? What's missing?

Nick

 

[bruce]

Yes the math is out there, I thought I had it handy but I don't see it at the moment.

The error is when folks run injectors static.
The ecm will generate a PW of say 110% DC, and the commanded AFR is based on that, trouble is you can't have a 110% DC in reality.

With enough injector, yes you can get a true 9:1 AFR but there'll be black smoke out the pipe if that high for very long.

During some AE stuff you MIGHT see some high AFRs, but that's cause getting it CORRECT in the chip is very time consuming.

At the end limits of a WB they can be fooled some, so they aren't totally accurate under all circumstances. I wouldn't trust one based on the NTKs at under 10:1 nor would I expect anyone to need to run there on gasoline.

With enough data logging power you can see the injectors go to Asynch fueling and then the real heavy AFRs.

 

[Eric Stage I]

Whats missing is the actual airflow. You have most of the fuel part because you know the pulsewidth and injector size, but since the maf won't read higher than 255 (on the stock setup), you don't have an accurate airflow.

You could estimate airflow and calculate it that way, but you would need to know things like VE, intake air temp, etc.

Regards,
Eric

 

[JDEstill]

Originally posted by boostcreep
OK, so why can't you calculate the actual A/F ratio from the data? What's missing?

Nick

Nick - what is missing for the most part is the air flow rate! Unless you've got an extender chip, the air flow is pegged at 255 gm/sec. If you are really flowing 300 gm/sec, or 400 gm/sec, how would you know? Without the right air flow, figuring up the right a/f ratio isn't going to happen.

Another problem may be the non-linearity of the fuel injectors. In other words, does a 50 lb/hr injector at 50% duty cycle actually deliver 25 lb/hr? It might be 20 lb/hr, it might be 30 lb/hr, who knows? Well, I know some of the chip gurus out there know, but I don't.

There's other problems too, like the BLM. Here's an example to illustrate, with data from an old DS run of mine:

200 gm/s air flow as shown by DS, 40 lb/hr injectors, inj duty cycle shown by DS is 72.5%. BLM is pegged at 160. Commanded a/f ratio per DS is 11.1:1

40 lb/hr per injector x 6 injectors x 453.6 gm/lb / 3600 sec per hr = 30.24 gm of fuel per sec at 100% duty cycle. At the shown 72.5% duty cycle we get 30.24 * 0.725 = 21.924 gm/s fuel flow.

Match that to the 200 gm/s air:
200/21.924 = 9.1 a/f ratio.

It should match the commanded 11.1:1. Why doesn't it? Because the ecm is correcting the inj pw for the high BLM. 160/128 = 1.25, or 25%. The ecm is adding 25% more fuel because of that high BLM. You can take that into account in a number of ways... adjusting the MAF rate is one: 200 * 1.25 = 250 gm/sec, divided by 21.924 gm/s fuel = 11.4:1, pretty close to the commanded ratio.

So what a/f ratio did I actually have? Was it 9.1:1? Was it 11.1:1? Somewhere in between? I have no idea... And that is when the MAF isn't even pegged! Peg the MAF and the picture has become even more hazy.

John

 

[dennisL]

Great explanation JDEstill. (Even I could understand it.)

DS is simply displaying the data value that is one of many used by the ecm to calculate fuel injector pulse width. The most familiar number is the one you see during closed loop driving - 14.7:1 or 14.8 as DS shows it.

During engine warmup is when you saw the low numbers (9 - 12). This number is derived by adding two table values together. After that it will be fixed at the programmed value, like 14.8, unless you inadvertently mash down the gas pedal. Then the number will be what the ecm reads from the PE AFR table.

So, the number DS displays is just a value read from the data portion of the chip or in the case of warmup, it is two data values added together.

As for the what the actual AFR is . . . . Its what they said.

 

[Dan K]

Originally posted by dennisL
mash down the gas pedal. Then the number will be what the ecm reads from the PE AFR table.

So, the number DS displays is just a value read from the data portion of the chip or in the case of warmup, it is two data values added together.

As for the what the actual AFR is . . . . Its what they said.

So what's the formula using the PE AFR tables to get the ECM commanded AFR?

 

[dennisL]

The formula is:

Injector Constant ($436) * Ref PA (rpm) * MAF table value * Fuel Trim * AFR * MAF Constant ($437) * MAF Table Scaler * BLM * Integrator

The Fuel Trim during PE comes from the RPM table ($56E) and the TPS table ($576).

The AFR during PE comes from the AFR table ($565).
The BLM cell used is probably 15, but doesn't have to be.
The Integrator during PE = 128 (multiplier of 1)

* = multiplied by

Hope this helps.