programming under boost questions

John Dammeyer johnd at autoartisans.com
Tue Feb 1 17:00:18 GMT 2000


>
>Date: Mon, 31 Jan 2000 15:59:36 -0800
>From: Dave Plummer <davepl at MICROSOFT.com>
>Subject: New member, programming under boost questions
>
[snip]
>My big question is about the WOT Enrichment vs. RPM table.
It's described
>in some places as specifying a desired AF ratio, in others
as a percentage
>of extra fuel to add during WOT.  Which is more accurate?
How does the O2
>sensor get figured into this blanket enrichment at WOT?


At WOT we disable O2 sensor feedback so the software isn't
trying to alter a deliberate enrichment.

As I see it,  too much fuel at WOT provides cooling reduces
power.  Too little fuel (below 14.7:1) and you end up with
EGT that is high and not enough energy to increase RPM.  In
other words our top end experiments on the dyno have shown
that there is a definite 'window' and there is a reduction
in power outside of that window.    When we use a load (the
Giant Computer Fan) that limits the RPM of the engine we
find at 5900RPM that maximum fuel required to accelerate up
to that speed is higher than the fuel required to stay
there.  ie:  If we hold the engine at 5600RPM with, say,
8.9ms PW on the injectors,  we need as much as 12ms to
accelerate but once at 5900RPM we can reduce PW to 9.3ms and
not have the RPM drop nor the EGT climb excessively high;
about 1400F.  Adding more fuel at 5900RPM just reduces the
EGT without increasing the RPM.

>
>My second question relates more specifically to
accommodating boost.  Since
>the VE tables are limited to 100%, what's the best way to
program for boost?
>I'm thinking this:
>
>1) In the RPM range where boost is present, in the 100kPa
column, set VE to
>100%
>2) Get the extra fuel to accommodate the true >100% VE by
upping the WOT
>enrichment table entries


What I've been able to find out about VE is that it
represents the amount of air that will flow through the
intake manifold at WOT at various engine RPM assuming
constant Atmospheric Pressure.  This is why the ratio of
MAP/Barometer is used to modify with the VE value;  Change
the atmospheric pressure and you change the velocity of the
air moving through the intake manifold.  At a lower RPM the
air mass has to stop and start and bounces against the end
of the plenum and the intake valves.  At higher RPM the air
doesn't have to stop as often or for as long a period of
time so you start getting more air into each cylinder while
the valve is open.

Think of it this way.  If you had air that had no mass,
then, when the intake valve opens the cylinder would
immediate reach ambient air pressure;  then the valve would
close.  At idle,  you'd then inject the correct amount of
fuel to match that air charge assuming 14.7:1.  Well,  with
that much air and fuel you wouldn't stay at idle for very
long because the power produced would accelerate the piston
to a higher speed rather than what maintain the value of
torque for idle.

The above is based on the assumption that the air has no
mass and accelerates instantly but in real life it doesn't
happen that way;  it takes time for the air to start and
stop moving.

Now if you load the engine so it cannot go faster than
1000RPM,  and you open the throttle wide, the amount of air
going into each cylinder is less than the cylinder can hold
at atmospheric pressure because the air isn't moving when
the intake valve opens.  It then has to start moving and
flow into the cylinder.  Before the cylinder is full,  the
intake already closes.  In other words,  once the intake
valve closes,  the air pressure inside the cylinder is less
than ambient because the piston moving down created an empty
space that wasn't completely filled during the time that the
intake valve was open.

Your Turbo Charger changes this which is why at any given
RPM the load required to hold the 1000 RPM constant at WOT
is larger.  The Turbo,  by raising the pressure at the inlet
of the manifold accelerates the air mass so that it flows
into the cylinder faster (over time more air is ingested)
resulting in more air available for combustion.

So the VE table represents how well the air flows into the
cylinders based on the RPM where 100% VE  implies that when
the intake valve closes the air pressure inside the cylinder
is at atmospheric pressure.  Longer runners verses shorter
runners move the torque around and the internal shape and
volume also changes engine performance.  The newer Honda
engines and even the Mercedes SUV modifies the intake
manifold by opening and closing a passageway inside the
plenum at certain RPM values.

My guess is that if your VE[RPM] table is correct for a
non-aspirated engine,  the MAP/Ambient ratio will compensate
for the boost because the ratio is now > 1 and therefore
VE[RPM] * MAP/Ambient will result in values greater than
100% VE.

IMO

>
>I can't really do #2 without understanding that table a
little better, hence
>the original question.  Any feedback on how I should go
about programming
>the LT1 controller for boost would be greatly appreciated!
>

Can't help you there.

>Thanks,
>Dave
>
Regards,

John Dammeyer



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