speed-density and TPS

[James Boughton]

>Jim ... I've not been in on this discussion, didn't even read *this*
>post in its entirety -- scanned it  8^)   and this is in reply to what
>I read as a statement that TPS is an unnecessary datapoint.  I'm no efi
>guru, but I am opinionated  ;-)

>someone on this list defined the different types of efi systems sometime
>ago -- but I forget a lot ...

>- 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.)"

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. 

>- 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
tps setting".  Since MAP is a function of throttle position it is possible to know MAP based on
tps.  This is why the alpha-N system works, but it is typically used for systems where it is
difficult to determine the MAP reading (such as a racing system), or in the case of the Holley
to save the cost of that extra sensor (the MAP sensor) when you are going to have to have
a TPS anyhow.

> - 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.

 >- mass air flow uses a sensor to directly measure the mass of the
>air entering the engine.  Probably the most precise method, it has
>drawbacks that actually made the speed density system preferable --
>if you have the facilities for calibration (not the least of which
>is a dyno  ;-)

You don't need a dyno to calibrate.  I've done it in a car many times.

 >- EGO can be added to any of these systems to more precisely
>control mixture -- if you really care (do you have a cat?).

>Remember the caveat -- it's Monday morn, I'm getting ready to teach
>a class, I don't know what I'm talking about .... etc, etc

What do you teach?  I hope it is not IC engines:)  Just kidding.  For those who have
bothered to read this whole message, I highly recommend getting yourself a copy of
Heywood's book.  It is considered standard issue for anyone who considers themselves
knowledgeable about internal combustion engines.  Many of the questions from this list
are addressed in this book.


>Tom Cloud

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

Jim Boughton
jpb01 at ibm.net