Re: Reasonable to force users to run both MAT AND IAT always?
Posted: Tue Jun 03, 2008 9:27 pm
I'm a little confused here, so I'll ask some questions. Basically, everything till the last couple paragraphs has a big question mark after it.
MAT - Manifold Air Temperature
IAT - Intake Air Temperature
The MAT is the temp measured inside the manifold. This should be susceptible to heak soak, as the hot manifold will heat the sensor.
The IAT is the temp in the IC piping, but after the intercooler. This is the temp of the air as it inters the manifold.
The MAT, depending on where it's mounted, is really just there to get a feeling for the drift in the temperature away from what the IAT was reading. This will including heating due to pumping effects through the throttle plates, and heating due to heat absorbtion from the manifold. If your MAT is mounted right at the entrance to the manifold, and you have long or variable/complex runners, you would expect it to read low anyway (though this overlaps with the errors inherent in measuring MAP)
So, since MAT is being used with MAP to get the actual count of air molecules in the manifold, that is what you should be using most of the time, and the IAT is just there to show you how wrong you were due to heat soak.
Certainly, to my mind, multiplexing a few inputs is the way to go. This is not super-time-critical information, as it's a second out of date at all times anyway. So, this would be a great case for something like an external chip/circuit that either switched between them or whatever.
My best suggestion would be to use a thermocouple in the manifold. They are much much much faster. People avoid them since the readout circuitry is more complex. But heat soak isn't an issue, it would equilibrate in a very short time, radiative thermal coupling to the manifold notwithstanding.
Really, how long does it take to empty the manifold? Just fudge the numbers for the first few engine cycles (or, minutes?) to be a bit leaner, based on coolant temps, and be done with it. If the coolant temps are low, there's no heat soak, you can trust the sensor. If not, there's a fudge factor you can let for for a while.
The option for more sensors is nice, but I don't think it should be forced, I'm not sure I see the gains.
MAT - Manifold Air Temperature
IAT - Intake Air Temperature
The MAT is the temp measured inside the manifold. This should be susceptible to heak soak, as the hot manifold will heat the sensor.
The IAT is the temp in the IC piping, but after the intercooler. This is the temp of the air as it inters the manifold.
The MAT, depending on where it's mounted, is really just there to get a feeling for the drift in the temperature away from what the IAT was reading. This will including heating due to pumping effects through the throttle plates, and heating due to heat absorbtion from the manifold. If your MAT is mounted right at the entrance to the manifold, and you have long or variable/complex runners, you would expect it to read low anyway (though this overlaps with the errors inherent in measuring MAP)
So, since MAT is being used with MAP to get the actual count of air molecules in the manifold, that is what you should be using most of the time, and the IAT is just there to show you how wrong you were due to heat soak.
Certainly, to my mind, multiplexing a few inputs is the way to go. This is not super-time-critical information, as it's a second out of date at all times anyway. So, this would be a great case for something like an external chip/circuit that either switched between them or whatever.
My best suggestion would be to use a thermocouple in the manifold. They are much much much faster. People avoid them since the readout circuitry is more complex. But heat soak isn't an issue, it would equilibrate in a very short time, radiative thermal coupling to the manifold notwithstanding.
Really, how long does it take to empty the manifold? Just fudge the numbers for the first few engine cycles (or, minutes?) to be a bit leaner, based on coolant temps, and be done with it. If the coolant temps are low, there's no heat soak, you can trust the sensor. If not, there's a fudge factor you can let for for a while.
The option for more sensors is nice, but I don't think it should be forced, I'm not sure I see the gains.