speed-density and TPS

[Tom Cloud]

>>- knowing the mass of the air entering the engine is what it's all
>>about (IMO) -- so you can determine the correct amount of go-juice
>>to add to the mix.  I'd say, in that regard, that rpm is a totally
>>useless datapoint (that's not true, I don't think, but it is in the
>>context of my simplistic statement above).
>
>Okay, since my simplistic explanations are leaving a bit to be desired from others, here is
>a more thorough explanation.  From Heywood's "Internal Combustion Engine Fundamentals":
>	"For warm-engine operation, the mass of air per cylinder per cycle m[sub]a is give by
>
>	m[sub]a = (vol. eff * disp. vol. * int. pres.)/(R * int. temp.)"

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.  As I understand it, VE changes
dynamically with rpm and statically with engine design and aging.  But that
puts us back to what I originally said, doesn't it?  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 gage,
and stop watch).

 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 
>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 properly determine the MAP
>under transients you could eliminate a need for throttle position altogether (unless you use WOT
>to determine baro while the engine is running). 
>
>Your comment about RPM is incorrect, because volumetric efficiency is a function of RPM. 

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

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

>>- 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 learn something
>new every day:).  The problem you have with your system sounds like calibration, 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 engine speed and
>manifold pressure and air temperature are used to calculate the engine air flow."  Heywood
>again.

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

I've read a little highly technical info on fi -- but more on its practical
application -- and can not remember ever hearing or reading where the use
of throttle position wasn't important.  Heck, it's even used on MAF
systems isn't it?  That's really puzzling to me .... why?  I understand
what you quoted -- it just sounds more like a reference work than a design
manual to me.



Tom Cloud

 Clothes make the man ....  Naked people have little or no influence on society.