MAFs

[Axel Rietschin]

----- Original Message -----
From: "Chris Conlon" <synchris at speakeasy.org>

> >What comes in, comes out, no? Less temp, more density, less speed but
same
> >amount of _air_, or did I miss something?
>
> Yes, you missed a small point. The location of the MAF (before or after
> the compressor), in a system that has a significant volume of plumbing
> after the compressor *does* make a difference. The reason being the
> "air through MAF = air into engine" relationship is only true in steady
> state. When the pressure post-compressor is changing, up or down, the
> airflow through the MAF is not equal to flow into the engine. Some is
> being either pumped up into the plumbing, or delivered from the pumped
> up volume into the engine.

You mean you can get a faster response from the MAF (less lag) when it's
located closer to the engine because the plumbing volume introduces an
additional delay? I didn't think if it at first but I see no problem with
that.

> I've measured this effect on my supercharged MR2 and it is very real.
> It's a very small lag in terms of watching a boost gauge but in terms
> of throttle response it's very very noticable... if you have a
> differently-responding setup to compare it to anyway.

For the above reason, I easily believe it. It's just that the MAF-after-IC
for temp correctness or whathever still is a no-no (or a yes-yes, I mean
before or after the IC and/or compressor is exactly the same thing as far as
air mass in concerned)

On the other hand, faster response is very desirable and it this respect, a
MAP sensor (2ms response time) in the inlet manifold is hard to beat.

> Questions about turbulence are very valid. An OEM engineer has reported
> to me that they flowtest the entire intake system and meter as a unit
> to generate each ECU's MAF voltage to airflow lookup table.

It that respect, the steadiest, less turbulent flow is likely to be found
right after a large volume airbox.

>
>
> >to what extent? T63 constant of automotive air temp sensors I know of
(Bosch
> >& Marelli) is in the 20s range, meaning it takes about a full minute to
get
> >an accurate reading, which rule out most 'real-time' control strategies
> >based on that value.
>
> I'm guessing that those sensors are designed for measuring ambient temp,
> not post-compressor temps?

Air charge temp. They are usually found in the inlet manifold. So called
fast sensors are usually in the 10s range, which still gives us about half a
minute to get a true reading.

> One thing you can do to greatly sharpen up the response of temp sensors
> is to make a first order slope correction. Measure temp (call it C),
> calculate dC/dt, and add k * dC/dt to the raw value C. Use this sum as
> your "actual" temp. You'll have to do some futzing to get k, but a couple
> of datalogs plus a heat gun should get you close fast.

It may be that the OEM ECUs are doing this? Dunno. For NA applications, I
don't think they bother.

> This is lots easier if your temp sensor is a bandgap or t-couple that
> reads a voltage directly proportional to absolute temperature, eg. TMP3x
> series. If you're using a thermistor you want to linearize the result to
> actual degrees before doing the math. The more resolution you have, the
> better this works, and the faster/more accurately you can compensate the
> value.

Definitely. For info, I use a custom-made air temp sensor (using a
Thermometrics FP07 NTC sensing element) for my own engine. This baby has a
time constant of 100ms in still air (7ms in water), just to say I'm _very_
concerned with sensor response :)

> A couple of MAP sensors now come with integrated temp sensors. I doubt
> their tc is very fast but they might be useful for someone.
>
>
>    Chris C.

-- Axel



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