Could somebody explain voltage correction?

[bobc455]

Okay, my alternator just went on the fritz, while I had loaned my car to someone. Fortunately they recorded a few runs.

The thing I don't understand is that even though I had very little voltage correction, I had tons of black smoke out the tailpipe, horrible idle, and lots of O2 correction (13% - 15%) at WOT.

After I replaced the alternator, all returned to normal.

What is the impact of a low voltage (11.8 - 12.2 volts)? The fuel pressure doesn't drop, so is it correcting for a phenonmenon that I am not aware of? I guess the injectors would open a bit slower, but what else happens? Does the reading from the oxygen sensor become incorrect (causing extra enrichment and therefore the black smoke)? Injector DC was a bit higher than normal, even though the O2 sensor was trying to make the engine richer...

Does the injector opening speed (and therefore batt corr%)matter whether it has high vs. low impedance injectors?

-Bob Cunningham
bobc@gnttype.org

 

[bruce]

the battery voltage correction is often overlooked since it's not used very much. Only if you have an alternator or battery failure does it really matter.

On the DIY-EFI list some time ago, the procedure was explained and written out. Thou, to this date I don't know of anyone that's taken the time to do it.

If you wanted to get it right, you could just disable your alternator, and then record the O2 correction as the battery discharged, and then work out the correct PW corrections per voltage level.

 

[bobc455]

I'm not trying to replicate the algorithm, only to understand in general terms how a fuel injection system is affected by low voltage.

Is the WBO2 sensor reading affected?

Is the injector opening time dramatically slower?

Does it make a difference, high impedance vs. low impedance injectors?

Does an ECM (FAST or otherwise) assume that fuel pressure drops when voltage drops?

Thanks for any insight.

-Bob Cunningham
bobc@gnttype.org

 

[bruce]

Originally posted by bobc455

Is the WBO2 sensor reading affected?
Is the injector opening time dramatically slower?
Does it make a difference, high impedance vs. low impedance injectors?
Does an ECM (FAST or otherwise) assume that fuel pressure drops when voltage drops?

Yes, if in open loop
YES
Yes the battery correction is injector specific. While some are close, close is just close, not the same.

Some GM code has a fuel pump battery voltage correction.

 

[Craig Smith]

In my experience most of the problem with battery voltage fluctuation seems to be related to the injector opening time. I don't believe that the fuel pump voltage is nearly as much of a problem as the injector opening time. The reason I think this way is because at idle, the pump has to provide only a small amount of fuel. As long as the regulator maintains pressure, even a crappy pump can provide the minimal volume of fuel needed to idle an engine. And as bob noted, the idle was terrible when the voltage went away.

Some injectors are much more sensitive to voltage fluctuations than others. We have a set of injectors in our test motor on the dyno that seem quite sensitive. Just turning the battery charger from 10 amp mode to 40 amp mode sends the engine from quite rich to quite lean. The set of injectors that WAS in there before did not exhibit this problem.

The O2 sensor should be unaffected by battery voltage.

The FAST unit makes no assumptions about fuel pressure vs. battery voltage. Not all fuel pumps are electric. Wouldn't be a very fair assumption.

What, you say? A mechanical fuel pump?!? Who the heck uses one of THOSE??? Well, me... for one. Belt driven pumps rule in my opinion. No draw on the battery, all the volume you would ever need, much more reliable, and TOTALLY unaffected by fluctuations in voltage or current availability (The latter being a common and serious problem during extended WOT runs). Of course I am an alcohol-burning bracket racer who never drives his hot rod anywhere but down the 1/4 mile, but hey... just my opinion.

 

[Otto J]

hey craig, do you think it is beneficial t orun the power side of the fuel injector off a relay with battery power?
thanks Otto
PS WELCOME BACK

 

[Craig Smith]

Originally posted by KLHAMMETT
hey craig, do you think it is beneficial t orun the power side of the fuel injector off a relay with battery power?
thanks Otto
PS WELCOME BACK

That's how we do it!

Thanks...

 

[bobc455]

Craig-

What you say seems counter-intuitive.

With 10 amps, you ran rich, implying that the injector is open longer (because it opened faster?).

With 40 amps, you ran lean, implying that the injector is open for a shorter period of time.

Wouldn't higher voltage open the injector quicker, causing the opposite phenomenon?

Also, because different injectors are apparently affected differently by voltage fluctuations, would it be a good idea to put some correction in the software based on the individual's injectors (for future versions of FAST)?

Does anyone have any information on how much difference there is in opening time at different voltages (general terms are okay, everyone's injectors are different)? Does it amount to 2% of injector opening time? 50%?

-Bob Cunningham
bobc@gnttype.org

 

[Craig Smith]

This may be a selling point for the user-programmable battery correction curve. The examples you gave in your last post are with the assumption that the there is no correction curve being applied to compensate for the variations in voltages. Where I suspect the problem lies is the fact that these injectors don't respond well to the correction curve built into the system.

 

[vtec4gs]

Originally posted by Craig Smith
What, you say? A mechanical fuel pump?!? Who the heck uses one of THOSE??? Well, me... for one. Belt driven pumps rule in my opinion. No draw on the battery, all the volume you would ever need, much more reliable, and TOTALLY unaffected by fluctuations in voltage or current availability (The latter being a common and serious problem during extended WOT runs). Of course I am an alcohol-burning bracket racer who never drives his hot rod anywhere but down the 1/4 mile, but hey... just my opinion.

Craig, do you really believe the extra load on the crank from manually driving a fuel pump outweighs the battery load of running an electronic pump ? that interesting ... maybe on the V8's cause you guys have gobs of torque to give away we 4 cylinder guys would kill for that torque.

I tell you, we do it differently because we try to take as much load off of our crank as we can, I went with a mezeire water pump and paxton granatelli pump and just run the alternator with a regular 400amp battery, just to get that lil extra power.

The only other component that I think would outweigh running extra load on crank is a 3 stage dry sump system, but of course thats outlawed from NHRA's import class ... go figure

Greg

 

[Craig Smith]

I realize that a belt drive pump isn't practical for all applications. I'm just throwing the advantages out there for people to consider. I definitely recommend them in situations where they make sense, which is admittedly a small portion of this forum.

FWIW, belt drive pumps take less than 1 horsepower to turn.

 

[ijames]

You will have about the same loss in the belt drive to the alternator as to the fuel pump, so that's a wash (maybe 10% loss here). The alt has to turn mechanical energy into electrical energy, at an efficiency of maybe 90%, and then the electric motor in the fuel pump has to turn the electrical energy back into mechanical energy, again at 80-90% efficiency. Then the pump has to pump fuel, again with some efficiency less than 100% but probably comparable between the belt-driven pump and a similar-sized electric fuel pump (note I slid in a joker because most of the time the electric versions deliver less flow/pressure/whatever than the belt driven ones). Multiply that out and the electric version delivers 0.9*0.9 or only 81% of the output for the same horsepower drain on the crankshaft. For efficiency you can't beat direct drive accessories. The big advantage to the electric versions is controllability - you can turn the cooling fan on and off, for example, but at wot you want the fuel pump doing all it can so the belt-driven pump wins in terms of fuel delivered versus hp loss at the crank.

 

[S2V6RACER]

Bob, I have the same problem but not at idle. I lost MPH and a little E.T. with low voltage. I just try to keep the battery charged as much as possible.