Transient Fuel: Tips, Tricks, and Info. A how-to guide
This would be my first real how-to, so feel free to correct me or ask questions.
I have spent the last couple months tuning my truck with an intake style that never came on this engine. The injector angle is different, the runners are nowhere near the same, and to top it off, I cammed it. When I first put it together you couldn't rev it without it spiking so lean it would pop through the intake. And any quick tip out throttle movements produced huge rich spikes. After verifying my injector data was 100% correct many times over, and steady state tuning my VE table and MAF table, the only thing left was transient fueling.
I read the big long thread on here about it, but its full of too much "just add to the spot it goes lean" and not enough reason behind it. There is some great info in there don't get me wrong, but no real answers. Except the post Chris made describing transient fuel. He puts it in a very easy to understand way while throwing a bit of why and how in the mix.
Quote:
Originally Posted by
Chris@HPTuners
There are 2 main parts to this.
1. The Fuel Evaporation part
This is a model of how fast the fuel evaporates off the intake surfaces. Smaller numbers mean the fuel evaporates faster. What this means is that more of the fuel you inject from the injector that hits the intake surfaces makes it into the cylinder faster and the effects of any transients are shorter. This is pressure and temperature dependant process that follows an exponential decay law. Fuel boils off faster under vaccuum and at higher temperatures. There is also an airflow based gain that modifies the evaoration rate based on airflow.
2. The Extra fuel hitting the walls on each injector pulse
On each injector pulse a certain amount of fuel hits the intake surfaces and adds to the amount of fuel already there that is boiling off. Higher numbers mean more fuel hits the walls (hence less directly in the cylinder), meaning the PCM has to inject more fuel than it thinks. You'll notice that the numbers get smaller as the intake gets hotter and also bigger as manifold vacuum drops as you would expect. There is also a gain component that multiplies this value with airflow, as higher airflows mean less fuel will impact the walls etc.
You can think of this whole impact/evaporation process as a "leaky bucket", you have a bucket (the intake surfaces) that has an amount of water in it (an amount of fuel on the surfaces) that is leaking out (evaporating off the surfaces). You also have a hose that is filling the bucket at a certain rate (the fuel injected that hits the intake surfaces).
When you put it all together you have this:
1. Injector fires with the intent to inject a certain amount of fuel into the cylinder (Fdesired)
2. some of the fuel goes right into the cylinder (Fdirect)
3. some of the fuel hits the intake surfaces (Fimpact) and adds to the amount of fuel already there from previous cycles (Fwall)
4. some of the fuel that was already on the intake surfaces evaporated during the last cycle so it went into the cylinder on this cycle (Fevap), along with the fuel that already went directly in.
So the total amount of fuel that went into the cylinder (Fcyl) was:
Fcyl = Fdirect + Fevap
and
Fevap = Fwall(e-boiling time) ... an exponential decay
Fimpact = Fdesired * impact factor * gain
Fwall(new) = Fwall(old) + Fimpact - Fevap
From this you can hopefully see (if your head hasn't exploded yet) that there are 3 possible states this can be in:
steady state condition
Fevap = Fimpact then the actual fuel amount (Fcyl) equals the desired injected amount (Fdesired) so the PCM doesn't inject any extra fuel to compensate (no need to increase Fdesired)
positive MAP transient, you hit the gas
Fevap < Fimpact then Fcyl is less than Fdesired, because more fuel is adding to the fuel on the intake surfaces instead of going into the cylinder, so Fdesired must be increased by the difference between Fimpact and Fevap for a short period. This means the PCM needs to inject more fuel than it initially thinks otherwise a lean spike will occur.
negative MAP transient, you take your foot off the gas
Fevap > Fimpact then Fcyl is greater than Fdesired, because fuel is still evaporating from the intake surfaces instead and going into the cylinder, so Fdesired must be decreased by the difference between Fimpact and Fevap for a short period. This means the PCM needs to inject less fuel than it initially thinks otherwise a rich dip will occur.
The main things to mess with are the Impact factor and its gain and the boiling time constant and its gain. eg. you can see that in the stock tables as MAP increases (you hit the gas) the boiling time increases (fuel evaporates slower) and also the impact factor increases (more fuel hits the intake surfaces), both of these increases result in the PCM adding more fuel during the transient. You can also see that as MAP decreases the numbers get smaller, resulting in the PCM subtracting fuel for the transient.
Hope that helps,
Chris...
This is a great....correction..... AMAZING post. Before you try to tackle what you think is a transient fuel problem, I suggest trying to understand the post above the best you can.
Next, how to determine if it really is a transient problem, and what you must do first.