ThrustSSC

I was talking to a friend the other day about the MAF sensor problem discussed on this Forum, and he was most interested - he needs to measure mass flow in another automotive application - this time a steam-powered land speed record car he is designing (yes, this is straight up, it's not the start of April...).

Can anyone offer thoughts on how the MAF sensor in a Scooby works? He's most interested in understanding the principles behind it and potential sources of such items.

Thanks in advance for all help! But sorry, no, you can't have a drive when it's finished I can recommend a visit to the car museum in Coventry, though.

dnb

In the older scoobs, it's a hot wire. The current required to keep it at a set temperature is measured. Air blowing over the sensor tends to cool it down - thus more current is required to keep it up to temperature.

Sorry - not a very good explanation - the link is a bit better - more general than just MAF sensors though

thermal anemometry

IN THE STICKS

If it's anything like my old MAF ....it doesn't work at all !!

MrShades

May be completely wrong, but as far as I understand there are three types:

- Early Imprezas used a hot-wire MAF

- Later ones changed to hot foil

- MY02 onwards, I think, use indirect hot foil


All operate in much the same way:

Take the first one as the easiest example. A piece of basically bare wire is extended across the air-intake within the MAF sensor and a voltage is applied across it. This causes the wire to heat up considerably. Due to the increase in temperature, the resistance of the wire reduces - which can be, and is measured (indirectly) by measuring the current in the circuit.

As the air-flow increases across the wire, the cooling affect cools the wire, increasing the resistance and increasing the current.

The current in the hot-wire circuit is therefore directly proportional to the air-flow across it.

Hot wires are prone to false reading through contamination - especially when oiled air-filers are used, as the oil hits the hot wire and sticks - changing the cooling characteristics.

Hot film works the same way, though bigger surface area - and I guess the foil is perpendicular to air-flow (and hence risk of contamination is reduced).

MY02 onwards use a (damn, can't remember the name for it) - anyway another pipe mounted perpendicular to the airflow, which houses the hot foil airflow sensor. The xxxxx effect causes a proportional airflow in this perpendicular tube, which the hot foil measures by virtue of current etc. As the foil is in now way in the primary air-flow it is very well protected from foreign object contamination.

May have got my increases and decreases the wrong way round in places - but hopefully you get the idea, and hopefully it's close to being correct

Shades

ThrustSSC

Brilliant, thanks, guys! I'll pass that info on.

Slightly off-topic, thinking about record-breaking cars and my Coventry museum comment, how about a Scooby gathering there in the New Year? I'm willing to organise, and could get us a better tour than the usual walk around. If I get a few 'interesteds' on this thread, I'll post another and get something sorted.

midlife-crisis-scooby

Mr Shades

is it the Venturi efect ??

Midlife.....