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Asynchronous Accel Enrichment - why does it work?

 
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scotty305



Joined: 28 Sep 2008
Posts: 2

PostPosted: Sun Sep 01, 2013 1:39 pm    Post subject: Asynchronous Accel Enrichment - why does it work? Reply with quote

Hello all, I've read some interesting discussions here and was hoping someone could help me find some more information about acceleration fuel enrichment.

I understand that some ECU's (OEM and standalone) will inject additional 'asynchronous' fuel pulses for acceleration fuel in addition to simply increasing the pulsewidth of the normally-scheduled fuel pulses that would usually occur. A few examples are the OEM ECU's from WRX's and EVO's, as well as some Haltech standalones. I've found plenty of anecdotal examples along the lines of 'I turned on the async accel fueling and the throttle response was improved' so I am already convinced that it works.

I'm sure there is a good reason for this that involves physics, but I've had trouble finding any SAE papers explaining this side of the situation. References to math-heavy engineering publications are also welcome. I'm hoping someone can help me understand the theory behind why asynchronous accel pulses are a good idea.

Thanks in advance.
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baldur



Joined: 12 Oct 2007
Posts: 623
Location: Reykjavik, Iceland

PostPosted: Mon Sep 02, 2013 4:11 am    Post subject: Reply with quote

Simple, it works because it aids evaporation.
For one, by injecting a bit of fuel early instead of waiting for the next injection event, you start evaporating the extra fuel early and compensate for the difference in port wetting at low loads vs high loads. (At each intake cycle, the engine sucks in some fuel that was left over in the port from the last cycle, in addition to fuel injected during the current cycle, some of which will remain in the port after the inlet valve closes)
For another, if you inject a lot of fuel at once, the rate of evaporation will peak once the port is saturated with fuel, injecting more fuel and making the fuel film thicker will not increase the rate of evaporation because evaporation mainly happens from the surface of the fuel film.
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Matt Cramer



Joined: 25 Sep 2007
Posts: 398

PostPosted: Wed Sep 04, 2013 11:25 am    Post subject: Reply with quote

There's a fairly in depth explanation of the theory behind it here:

http://www.megamanual.com/ms2/xtau.htm
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Roberto Arano



Joined: 22 Nov 2005
Posts: 503
Location: colorado

PostPosted: Wed Oct 23, 2013 9:36 am    Post subject: Reply with quote

deleted..due to assuming question was about something else.
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Last edited by Roberto Arano on Thu Oct 24, 2013 4:07 pm; edited 1 time in total
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joe90



Joined: 19 Mar 2013
Posts: 382
Location: under the car

PostPosted: Thu Oct 24, 2013 1:59 am    Post subject: Reply with quote

On my first car, the carby had an accelerator pump. It was adjustable, 3 different positions with a pin which adjusted the pump stroke.
I'ts no different, just an electronic version. It's the same principle.
On the other hand, If you've tuned a car with SU carbs, they don't have accelerator pumps. To get the extra fuel you need, you put the right grade oil into the dashpot to richen up the mixture when you floor it. If you take the oil out altogether, it'll lean out, even cut out altogether, that's when you floor it. The dashpot rises slowly with oil in it (it's a damper), with no oil, it rises quickly.

The theory behind it used to be that, at least with a carby ..........When you open the throttle you've got the total volume of the intake manifold, that's a vacuum reservoir, you're venting that suddenly through the carby, but the engine can't consume it all, some of the fuel falls out. The problem is always at it's worst when the engine is cold, because the intake manifold is cold too.
When the manifold is in vacuum, the fuel vapourises easily (vapour pressure), that also sucks the heat out of the manifold making it colder.

EFI tends to get around the problem by having port injection but it makes sense to still have some sort of compensation.

Likewise if you've owned / tuned an engine running on LPG or CNG (with regulator/mixer), it doesn't need an accelerator pump because the fuel is always gas, you'll never get a flat spot with sudden acceleration.

Back to EFI and injectors.
Based on the link above, if you've got injectors with a fan like spray pattern, you'll get more fuel hit the walls and drop out. If you've got injectors with a straight shot at the back of the intake valve and no fanning, you'll get less sticking to the walls. That seems to contradict what most people will tell you about "ideal" spray pattern.
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scotty305



Joined: 28 Sep 2008
Posts: 2

PostPosted: Sun Dec 01, 2013 1:12 am    Post subject: Reply with quote

Thanks for the info, everyone.
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