Transient enrichment

Tue Nov 30 22:19:33 GMT 1999

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I've spend a while now trying to get to the bottom of the need for 
transient enrichment on MAP systems, especially that involved with 
transitioning out of a low rpm idle.

For those not in the know, snapping the throttle open at a low rpm 
idle causes a 1/2 second lean-out stumble. This is worse the
lower rpm you go, and worse the more cam overlap you run.

I run across bazillions of different theories, all of which I've
decided are crap (feel free to correct me though). 

e.g.

1) The flow change lags behind the pressure change.

Bollocks. That would make it run rich, and besides the
pressure change is CAUSED by the flow.

2) The intake runners have columns of air that need to be
accelerated to the new conditions.

Bollocks. That would also make it run rich, and besides the
flow always tops between intake cycles anyway.

3) There is a delay getting the fuel in the port to the cylinder.

Bollocks. Its squirted directly on the back of the valve, at low rpm 
it'll just run in under the influence of gravity, when the valve is 
opened.

4) Manifold wall wetting effects

Bollocks. This is port injection, the walls don't ever see fuel.

5) The vapourised fuel in the port condenses because of the pressure
increase.

Maybe, but it seems that this would not cause a major change in fuelling,
as its hard to see how the fuel in the cylinder will not be burnt. 

6) A richer mixture (i.e. more powerful) is needed to accelerate the engine.

Bollocks. Running a seriously rich mixture at say 1300RPM/0" vac, does not
help
the problem. Neither does serious amounts of dMap/dt. dTPS/dt helps a
little,
but not much. 

7) Ram Air effect of the flow coming into the manfold.

Perhaps, but hard to see how this will have more than about 10% percent,
running
that much richer steady state doesn't seem to help.

----------------------

Having stated what I think its not, here's what I think it is:

1) Fueling delays. The ECU not responding to changes in MAP as
quickly as the engine. The engine will respond on a cycle by cycle
basis, whereas an ECU and MAP sensor most likely will not.

2) The fact that the fuel is injected before the IVO (intake valve open) 
event, but the engine "samples" MAP at IVO. Therefore it may not be
predictable what the fuel will need to be, when the fuel is squirted in, 
because the MAP can change significantly then and IVO.

A carb will also experience delays in fueling, mainly due to distance
from the port, so hence the need for the throttle pump. Q. How do TBI system
deal with this?

Weber DCOEs throttle pumps will squirt seriously far and fast, and these
carbs are known for
being able to acheive exceptionally good throttle response. 

SU carbs combat this by momentarily restricting the airflow, thereby
bypassing
the problem. Plus the excellent vapourisation at low flows, will make the
engine more resistant to lean misfire.

I've heard rumours of some OEM ECUs, doing multiple injections per
revolution
to effect their enrichment, based upon dTPS/dt, this would certainly help.

Obviously the other idea that may help, it calculating the fuelling and
injecting at 
late as possible in the cycle, i.e. to coincide with the closing of the
valve.

Open to the floor for comments/flames.....

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