Turbo MAP & MAT interaction

[Webmaster]

Thanks for the reply Scott. I can't pretend to understand everything, I'm 
on a very steep learning curve at the moment. I'll take it home at the 
weekend and have a good think about what you say.

The sensor looks a bit like a spark plug, but instead of a spark tip there 
is a metal cage surrounding what looks like a thermistor (A two legged 5mm 
diameter round device). It appears to be coated in a film of petrol/oil, 
but this isn't too surprising coz the engine has done 92,000 miles. The 
thermistor looks like it is supposed to thermally isolated from the body of 
the sensor - which is hat makes me think it should be measuring air temp, 
not manifold metal temp.

I can believe that the MAT temp will vary between -40C and +200C, depending 
on the amount of boost on the turbo. This temperature will vary rapidly - 
full boost to vacumn in way less than a second when the waste gate opens 
for example. The actual manifold temp  (the metal casing) however will 
probably be at about the same temp as tha block 80-90C, and this won't 
change at all in response to boost pressure - in the short term at least.

But my basic question is still regarding the ALDL/Tech1 MAT(IAT) 
temperature reading. If I unplug the sensor's cable, the value goes to 00 
almost instantly. If I short the sensor out, the value goes to 0xFF almost 
instantly. So, assuming my sensor is good, it looks to me that the TECH 1 
is displaying the raw A/D converter value coming from the sensor. If this 
is the case, should the MAT reading be varying rapidly in response to boost 
pressure, or only very very slowly in response to falls and rises in the 
manifold metal temp ?

Thanks,
Malcolm Robb, Needing a new MAT sensor ?!

-----Original Message-----
From:	Scot Sealander [SMTP:Sealand at clarityconnect.com]
Sent:	Friday, April 07, 2000 2:22 AM
To:	gmecm at diy-efi.org
Subject:	Re: Turbo MAP & MAT interaction

Webmaster wrote:

> I suppose it could be measuring the inlet manifold temp (i.e. the temp of
> metal manifold, not the air itself) and this could account for the slow
> rate of change, but I'm not sure why the ECU would want to know this 
value.
> There is already a separate Coolant Temperature Sensor (CTS), so I would
> have thought the ECU would use that to work out how hot the engine is.
> There is no ambiant air temp sensor (i.e. before the turbo).

In any EFI system the ECM needs to know the amount of air mass that
is in the cylinder.  With a MAF sensor, this is pretty easy.  With
a speed density system, it is a liitle harder to know what is in the
cylinder, but it can be calculated.  The ECM needs to know 4 things:

1) The swept volume of the cylinder               (Displacement/#cyl)
2) The temp of air in the cylinder                (IAT, ECT or combo)
3) The pressure in the cylinder                   (MAP)
3) How well the cylinder got filled last cycle.   (VolEff)

For most of these requirements we cannot measure them *in* the
cylinder.  So we use sensors close to the cylinder to get a reasonable
number for needed values.

The first item is a known.  The swept volume can easily be calculated.

The temp of the air in the cylinder is tougher.  The IAT sensor in
your case (providing it is good) is the only temp sensor used to
get the contribution to air density due to temp.  I have reservations
about mounting the IAT in the intake manifold.  In all the speed
density turbo apps I have seen that use an intake mounted air temp
sensor, the air density table usually has very little variation from
cold to hot air temps.  My position is that this small variation
confirms that the manifold mounted sensor may be a comprimise.

The IAT in this case is capable of measuring about -40 deg C to
200 deg C.  Dry air density (at standard pressure) varies from
about 1.5 g/L at -40 deg C to about 0.75 g/L at 200 deg C.
Don't shoot me if these numbers are wrong, they are off the top
of my head...

There is a 2 to 1 variation in air density due to temp there.
In your case, the air temp density table has a value of 1.8 at
the -40 deg C, and 1.3 g/L at 200 degree C.  I did not plot it,
so I don't know if it is linear.  I my case, the Pontiac
Turbo GP, the -40 value (stock) is about 1.8 and the 200 deg
C value is 1.3....  Hmmm.  The TGP table is not at all linear
from cold to hot.

The SyTy is 1.5 g/L at -40 deg C, 1.2 g/L at 200 deg C.  I assume
this is done to be "safe".

The cylinder pressure is taken from the MAP sensor, and finally,
the VE comes from a lookup table based on MAP and RPM.

Put all this into an equation, and you can determine how much
air mass is in the cylinder.  Divide by AFR, and you have fuel
mass needed.  Multiply fuel mass by the injector constant, and
you have injector opening time needed to deliver the required
fuel.

The air temp is a hard one to get right.  The air is heated
depending on how long it is in the intake & cylinder.  There
are some schemes to try and calculate this heating, but it
is not done in your case, or my case (TGP) or the SyTy.

I have corrected this in my car, and the results look good.
Very little correction ever needed by the closed loop
feedback system!

Scot Sealander

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