This is some info on transient fuel my buddy at GM just sent me.
Code:
OK - first I'll give a brief reason as to why you need transient fuel, and then I'll go through how to go about utilizing it.
In a PFI engine the natural tendency of the equivalence ratio (EQR, which is the inverse of Lambda) is to go lean on positive
MAP change, and rich on negative MAP change, due to fuel being lost on the port/cylinder wall. This is affected greatly by intake
valve temperature, SOI (start of injection timing), fuel volatility and engine age (mainly due to carbon buildup),. Valve
temperature should be modeled by the software. This is hugely important, as this plays a big part in determining how much fuel
you put in/take out during a transient manouvre. If you do not have a valve temperature model in your software, coolant temp
should get you close enough. SOI is controlled by the ECU and should be completely calibratable. Generally the earlier you inject,
the more wall wetting you get and the more transient fuel compensation you need. Most engine control software has no way
of knowing what the fuel volatility is (it is very difficult to accurately gauge by the ECU), and so the majority ignore it. This is
unfortunate, as volatility plays a massive part in transient response. As far as I'm aware, no software currently compensates
for engine age either.
The easiest way to think of it is that fuel builds up on the port wall in high load, and then evaporates off when you return back
to low load, and usually follows a skewed sinusoidal shape . Therefore you need to add on the way up and subtract on the way
out in order to keep fuelling at around EQR 1.00.
If you do not calibrate transient fuel correctly, the car will go lean on tipins, resulting in a stretchy acceleration (sometimes called rubberbanding).
Richness on tipout isn't as big a deal as far as drivability goes - its more of an emissions issue. However, if you make your
decel compensation too agressive (by the cal in (c) below), you can go lean on tipout, which can make your decels feel very
snatchy and unpleasant to drive.
Before you do ANY transient fuel calibration, you need to ensure that your air measurement and open loop fuelling are top notch.
You don't want to be compensating for poor fuel or air control with transient fuel.
To cut a very long and complicated algorithm down into its bare bones, you should have the following basic functionality
in your transient fuel cals:
a) a table of fuel adjustment as a function of valve temperature vs delta MAP. This is your "bread and butter" transient fuel table.
This gives you the height of the sinusoidal wave.
b) a table which modifies the fuel adjustment to "bleed out" the transient fuel applied in (a). This gives you the width
(or to put it more accurately, the duration of transient compensation) of the sinusiodal shape and also effects maximum
height to some extent. It models the evaporation of the fuel puddle off the port wall. It should also be a table of valve temp vs delta MAP.
c) a scalar to change wall wetting compensation for decels. You generally find that you need to pull out less fuel on
decels than what you put in on accels. This should just be a scalar of compensation vs valve temp. The software will
work fine without this, but its a good functionality to have.
d) functionality to suspend transient fuel for x seconds after start. You do not want transient fuel pulling out fuel
just after start, when MAP falls from 100 to idle MAP.
As far as calibrating transient fuel goes, you generally only need a lot of transient fuel up to around 70 or 80 degrees
valve temperature (depending on capacity and airspeed over valves etc). After that point you shouldn't need too
much transient fuel compensation at all, as practically 100% of your injected fuel should be going into the cylinder.
Below 10 degrees you may find you need a lot. If you are having poor drivability cold, in particular if you are feeling
stretchy accels, this is probably why.
Hope this helps, let me know if you have any questions about this stuff. It's a lot to digest, and its very hard to
explain some of this stuff without diagrams etc.
Originally Posted by joecar
The Tremor at AIR
Posting Permissions
Reply With Quote