speed-density and TPS

James Boughton jpb01 at ibm.net
Wed Oct 29 23:53:14 GMT 1997


Joe, I'm sure you know more about this than I do, but here's my thoughts.
That's the technical definition of the mass of air -- and it makes sense
but we don't want to "define" it, we want to "measure" it and I don't
see how you can get there from that formula.

The equation I listed is how the mass of air entering the engine is measu=
red in
a speed density system.


As I understand it, VE changes
dynamically with rpm and statically with engine design and aging.

Yes, VE changes with rpm and MAP which is why you must calibrate the syst=
em.
There is actually one system that uses the above equation as its basis an=
d you
calibrate
the VE in a surface of VE vs. (you guessed it) MAP and RPM.  Another syst=
em I have
worked
with actually simplifies the whole matter by just using a surface of PW v=
s. MAP and
RPM.  As
far as aging goes this is why there is an oxygen sensor.

  But that
puts us back to what I originally said, doesn't it?

No, this puts you back to what you originally said.  I understand what I =
am talking
about.

  We want to know the mass
of the air entering the engine -- and you can't get it from the above
equation since you don't know VE.  You *can* get it by

 1 - measuring VE and setting the controls of an alpha-n or SD system for
it (or tune by the seat-of-the-pants like I do -- actually, a tach, vac g=
age,
and stop watch).

This is called calibration.  By the way, I like the way you use MAP (vac =
gage) to
calibrate
your throttle based systemJ

 2 - directly measuring it by MAF -- but most maf gages measure reversion
pulses the same as incoming air so they have a deficiency with race cams
I'm told -- and it makes sense to me ??

>Where R is the gas constant for air.  This can be found on page 296 for =
the highly
motivated=20
>who would like to look this up themselves.  So now you know the mass of =
air
entering the engine
>and you have no need for throttle position.   Again, if the ECU could pr=
operly
determine the MAP
>under transients you could eliminate a need for throttle position altoge=
ther
(unless you use WOT
>to determine baro while the engine is running).=20
>
>Your comment about RPM is incorrect, because volumetric efficiency is a =
function of
RPM.=20

once again, I don't see how the reference you cite is of much use since
you VE is dynamic and also changes with modifications and aging.

Again, you calibrate to compensate for the dynamic nature of VE and the o=
xygen
sensor
accounts for aging

  rpm can
only tell you mass if you know VE and temperature (density of the air)??

You measure rpm, intake temp, and MAP, then you calibrate for the VE.

If TPS is not important, then why does every system I've seen use it?

I did not say TPS was not important.  I said it was not used in the basic
calculation
for engine fueling that is done in a speed density system.

>>- alpha-n is a system where the system uses rpm and tps only (think
>>that's right).  That's what the analog Holley ProJection system does.
>>It works fairly well, but there's a "dead" spot during part accel when
>>rpm hasn't caught up to the new tps setting and the mixture is lean
>>(this is only a problem for slow accel as there's a "Power" setting
>>that enrichens the mixture for WOT and an "Accelerator Pump" setting
>>that uses delta-TPS to richen the mixture).  The mass of the air is
>>deduced from pre-set values of TPS and RPM.  I'd say the main weakness
>>of this system is the change in air temp and barometric pressure /
>>altitude.
>
>I did not know the Holley ProJection was a throttle based system.  You l=
earn
something
>new every day:).  The problem you have with your system sounds like cali=
bration,
not the
>system.  Although, I am not sure what you mean by "the rpm hasn't caught=
 up to the
new

 ... could be

>> - speed density uses TPS and MAP.  knowing the size of the opening
>>into the plenum (intake manifold) and the pressure diff across the
>>opening allows calculating / deducing the air mass entering the engine.
>>Adding IAT will allow it to be more correct.
>
>Speed density uses MAP and RPM.  "...a speed-density system, where engin=
e speed and
>manifold pressure and air temperature are used to calculate the engine a=
ir flow."=20
Heywood
>again.

I can buy that .... except the VE term bites again.

No, that is what you are calibrating for.


  The use of delta-P
across the throttle "hole" and its area will tell you the volume quite
well -- and adding air temp will allow one to get a very good handle on
the actual mass flow.

The fact that at a given throttle opening and rpm the delta-P across the =
throttle
is a constant value is why an alpha =96n system works.


I've read a little highly technical info on fi

not enough

 -- but more on its practical
application -- and can not remember ever hearing or reading where the use
of throttle position wasn't important.

I didn't say it wasn't important, I said it was not used in the basic fue=
ling
algorithms
of a speed-density system.  If I did say it wasn't important I apologize,=
 because it

does play an important part of any system.  Remember a fuel injection sys=
tem must
work beyond the conditions of the basic fueling.

  Heck, it's even used on MAF
systems isn't it?  That's really puzzling to me .... why?

The throttle is used for things besides the basic fueling algorithms.  If=
 you think
the only
algorithm in a fuel injection system is the basic fueling then I could un=
derstand
why you
would be confused.  As I said before the TPS is used in the acceleration =
enrichment
algorithms, as well as several other places.

  I understand
what you quoted -- it just sounds more like a reference work than a desig=
n
manual to me.

This same information was used to design the basis for the fuel injection=
 system of
a major automaker.  If that isn't using for a design manual then I don't =
know what
is.

Tom Cloud

 Clothes make the man ....  Naked people have little or no influence on s=
ociety.

If you have specific questions please ask.  I do not wish to explain the =
entire
software
of production code in a piece of e-mail.

Jim Boughton
jpb01 at ibm.net




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