AccEnr MaxIgnRetard

[fphil]

The MaxIgnRetard parameter from the AccEnr menu is said to be scaled with Acc enr .

From the Acceleration Retard/Enrichment" calc model, I see that when the parameter is 6 deg and "VT accel enricment without fading" is 0.3 ms then "VT Accel retard" = 0.2 deg, and that this retard stays constant during the fading out.
Is this understanding right?
Actually does the Accel retard  stays constant or fades out (as it should)?

[VEMS]

Accel retard is scaled with the applied accel enrichment (e.g. more applied enrichment gives more retard), but remains constant during the accel retard period -> not faded out.

Best regards, Dave

[fphil]

Dave, why does it not fade out?
If the logic of the retard is to be proportional to the applied enrichment it should fade out as the applied enrichment does. Is it not better?
What is the purpose of this retard? Is it not to smoothen the sudden increase of the torque following a step throttle? If so the retard should not extend over the full acceleration period.
In other word, why to retard the ignition phase during an acceleration?

[VEMS]

Hello Phil,

The purpose of the accel retard (as it is with the accel enrichments) is to front run the added load resulting from the extra airflow during the transition of higher inlet vacuum to less inlet vacuum.  Which has to be applied only shortly until the manifold pressure has stabilized; yes faded out retard would (theoretically) be slightly better, but in practice it does not matter much.

For reference, i copied a small section from some old GM  (1981 - 4,245,605) patent about AE describing the effect properly (for that time):

"During transient engine operating conditions where the absolute pressure in the intake manifold is increasing, such as during throttle opening maneuvers, lean
air/fuel ratio excursions in the mixture drawn into the cylinders will typically result if the fuel supply rate is not increased beyond the normal steady state running
fuel requirements. While this is the case with asynchronous and part fuel injection systems, it is particularly the case in a synchronous throttle body fuel injection
system where fuel is injected into the throttle body once for each cylinder intake event and then drawn into the  intake manifold before entering the cylinders to
undergo combustion.

One reason for the lean air/fuel ratio excursion is that some of the air mass entering the intake manifold during a throttle opening maneuver remains in the manifold to
bring the manifold pressure up to the new higher manifold pressure value resulting from the increased throttle opening The determined fuel injection period based
solely on the higher sensed manifold pressure value does not account for the mass of air entering the manifold to bring the manifold absolute pressure up to the
new value. This unaccounted and unfueled air mass results in a lean air/fuel ratio excursion that may result in degraded engine performance and emissions. The
same effect is present in port injection systems to a lesser degree, the unaccounted and unfueled mass air being the amount required to increase the cylinder pressure
to the new manifold pressure value. In addition to the foregoing, the air/fuel ratio is further leaned during throttle opening maneuvers as a result of increased fuel
wetting of the throttle body and intake manifold during increasing values of manifold absolute pressure."

Best regards, Dave

[fphil]

Alright let speak about the time the MAP takes to stabilise, that is the time dMAP/dt = dMAP takes to return to zero with respect to the period of time the flag isAcc on which equals the period of time ignition retard is on.
Only 2 samples  (quickly computed)

dTPS      dMAP(sec)    isAcc=ignRetard(sec)
12           0.625           1.300
16           0.425           0.950

As soon as the MAP stabilises, is there any reason to maintain the ignition retard?

[VEMS]

As soon as the MAP stabilises, is there any reason to maintain the ignition retard?

No, as soon as MAP has stabilized, the ignition advance from the ign map will be correct (assuming speed density is used) or rather the MAP represents the real engine load. The time the accel ignition retard is applied can in usually be shorter than dMap(sec) on a mildly tuned (or stock) engine there rarely is any need to use accel retard.

Best regards, Dave

[fphil]

on a mildly tuned (or stock) engine there rarely is any need to use accel retar

For a stock turbo engines with no ETC, on can get a large increase of boost when throttle opens (say) from 40% to 60% which induces a step transition of the engine torque and spinning wheels.
Accel retard would help to smoothen the transition. Don't you think so?