Why is there a need for accel enrich ?

[Pierre]

Hello,

I am fighting right now with my accel enrich setting (can get the car running as smoothly as the stock L-Jet ).
But after a lot of thinking, I don't see why an efi needs that feature??

For a carburated engine I think that the reason is the bigger inertia of fuel vs the one of air.
But on an efi, I don't understand, with the rigth input (map, rpm...), the ouput (inj pulse) should be rigth at any moment. On my car, if I depress slowly the throttle, the AFR are stable, and I goes throught the map, for the same RPM from low MAP to high MAP.
When I depress the throttle faster, the engine does the same, just faster, but this time it hesitates.
Why the Ms does not give the right amount of fuel????

The only reason I have in mind is delay for the pressure info to reach the MAP sensor. This problem does exist with the MAF of the stock ecu.

Thanks for your answers,

Pierre

[Enthalpy]

I have a fascinating writeup on my thoughts about AE in this thread:

http://www.msefi.com/viewtopic.php?t=13851&highlight=

It outlines the only reason I can think of for AE, outside of user-imposed pneumatic limitations on the MAP sensor's reaction times (read: long, thin MAP sensor vacuum hose with restrictions in it).

[Pierre]

Hello,

I have a fascinating writeup on my thoughts about AE in this thread:

http://www.msefi.com/viewtopic.php?t=13851&highlight=

It outlines the only reason I can think of for AE, outside of user-imposed pneumatic limitations on the MAP sensor's reaction times (read: long, thin MAP sensor vacuum hose with restrictions in it).

I thank for your answer, and the link thread. Your idea is really interesting, I have also noticed that the car hesitates less at high RPM than low.

But I don't thank you for giving me a new part of this forum to read.... You gave me efi101 a few days ago, I won't be in my garage anymore, spending all my time on my computer....

Pierre

[woh]

Pierre

The L-jetronic also has acceleration enrichment. It uses the inertia of the air meter for enrichment and on some cars there is a sensor that turns on enrichment anytime the MAP changes downward. This is what the sensor looks like.

http://home.comcast.net/~whaussmann/Bos ... hLJet5.htm

Werner

[Pierre]

Hello Werner,

Pierre

The L-jetronic also has acceleration enrichment. It uses the inertia of the air meter for enrichment and on some cars there is a sensor that turns on enrichment anytime the MAP changes downward. This is what the sensor looks like.

http://home.comcast.net/~whaussmann/Bos ... hLJet5.htm

Werner

So you say the maf use the inertia of the air meter to increase shortly the pluse width. This means that the delay of the MAP is not the reason for the need of AE, because the MAF by construction as no delay.

For the link you gave, I didn't know about those parts (not present on my car) and that too goes against the fact that map sensor delay is the reason, because it uses map...

Pierre

[woh]

Hi Pierre

I do not really undertand why it is necessary. The Bosch L-Jetronic manual only says "When changing from one operating condition to another, deviations in mixture occur which are corrected to improve driveability" I do think the key is the transition of the MAP value.

I believe it is the transition that makes the difference. My understanding is that there is additional air entering the manifold to achieve the new MAP, as well as additional air to fill the cylinder. The amount of air entering the cylinder is determined by a changing MAP value not a single value MAP as captured by the CPU.

werner

[arny528]

So you say the maf use the inertia of the air meter to increase shortly the pluse width. This means that the delay of the MAP is not the reason for the need of AE, because the MAF by construction as no delay.

Pierre

Hello Pierre,

L-Jet uses AFM, not the more modern MAF. Since AFM is a mechanical contraption with a metering flap, it does have inertia.

[efahl]

Bruce has recently clued me in to some SAE papers and I'm finally understanding the true mechanism involved here. Transient fuel compensation (TFC) is required due to the fact in our "traditional" EFI and carburetted systems the fuel is not injected directly into the cylinder. Some of the fuel delivered contributes to the "film mass," i.e., the fuel that coats the inside of the intake manifold and ports, the rest is delivered into the cylinder. The rate at which fuel transfers into and out of the film mass varies with conditions: temperature of the various surfaces, rate of gas flow, current film mass being the three major ones.

Anyhow, if you really want to know what is going on with modern OEM algorithms, they are typically derived from the ideas contained in a paper by C.F. Aquino, SAE 810494, 1981. Aquino describes a means for using sensor history to know how much fuel is currently present in the film mass and thus how much fuel needs to be supplied to get the proper amount into the cylinder.

Eric

[newtyres]

Accel enrich seems a difficult thing to master for some tuners including myself. A problem I see at the moment (and I may be way out of my depth here, which means I probably am) is that accel enrich is based upon a threshold speed of opening the throttle, so once you exceed that threshold, an accel enrichment will occur. Is it a silly idea to suggest mimicking the old-fashioned accelerator pump and simply choose a "volume of fuel in the accelerator pump chamber", and then as the throttle is opened MS injects a percentage of that volume according to how far the throttle is moved? A slowly opened throttle will deliver less enrichment over a longer time, and a snapped-open throttle will dump the lot immediately (in the time it takes to get the throttle fully open). Then if we could disable accel enrich over (choose your rpm) that would be nice.
Ian.

[whittlebeast]

I have heard that new AE tuning options for MS2 are in the works. There is a simple RPM based version of AE tuning in the late versions of Extra (Starting with about 025v2 or later)