Accel enrichment

[Joe Boucher]

BINGO!!!  I think you said the words that finally made the connection in my
head.

Let's go back to my example.  Steady state 20% throttle is 19 inches of
mercury.  40 % throttle is 16 inches.   Snapping the throttle from 20 to 40 %
results in an increase of manifold pressure to 10 inches.  Add that the mass
flow rate at 20 % is x lbs/min and at 40 % is y lbs/min.  y is greater than
x.

The reason the manifold pressure is less than atmospheric is the throttle
restricts the engine from pulling it's complete displacement through the
throttle.  Less molecules in the manifold means lower pressure.  When the
throttle is snapped open, the engine is temporarily the restriction.  The
throttle is positioned to flow at y lbs/min, but the engine is flowing at x
lbs/min and must catch up.  (That's not exactly true, if the pressure in the
manifold increases, the fow rate will increase some, but not to the y
amount.)  The tens of thousands of feet column of air now pushes more air
past the larger throttle opening, but it has nowhere to go, so the molecules
backup in the manifold, raising the pressure.  More oxygen requires more
fuel.  An enrichment shot is required and engine acceleration would benefit
from a richer mixture, therefore fuel enrichment is required till the engine
catches up with the throttle opening.  Now has the engine catches up to the
throttle opening, the flow rate approaches y lbs/min, absorbing the extra
molecules in the manifold, till the engine settles at the new steady state
rpm andbthe manifold pressure hits 16 inches.

Whether the explanation is understandable or not, I have satisfied myself.
This has been bugging me for days.  True happiness would be if I could
describe that mathematically.

Joe Boucher
'70 RS/SS Camaro  '81 TBI Suburban

Raymond C Drouillard wrote:

> On Mon, 23 Mar 1998 21:49:27 -0600 Joe Boucher <jboucher at ctelcom.net>
> writes:
>
> <snip>
>
> >>When you open the throttle, a whole bunch of air rushes in.  As the
> >>pressure equalizes, the air is still moving.  The inertia of the fast
> >>moving column of air "rams" more air in.  This effect is used to
> increase
> >>low-end torque in an engine.  An engine designed for low-end torque
> will
> >>have narrow passages in the intanke and exhaust manifolds.  This
> >>increases the velocity of the gas.
> >
> >I agree that has some effect, but the increase in pressure over the new
> >steady state value lasts for as long as the vehicle accelerates.  The
> >rushing in effect is almost immediate.  The only thing that comes to
> mind
> >is possibly the accelerating piston speed in some way causes an increase
> in
> >the flow rate.
>
> When I give an automobile engine partial throttle, the manifold pressure
> goes up.  As the engine speed increases, the pressure starts to drop
> because more gas is being pumped out of the manifold.  If I hold that
> throttle position long enough, equalibrium will be reached.  The engine
> will be producing exactly the amount of power needed to keep it moving at
> that speed.  In that case, the pressure rises suddenly, then slowly
> decreases until equalibrium is reached.
>
> If I reduce the opening before equalibrium us reached (I am going as fast
> as I want [dare] to go), the manifold pressure will suddenly rise when I
> increase the throttle opening, slowly decrease, then decrease suddenly
> when I partially close the throttle in order to maintain speed.
>
> >Joe Boucher
> >'70 RS/SS Camaro  '81 TBI Suburban
>
> Ray Drouillard
>
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