back-flow

[RABBITT_Andrew at mv8.orbeng.com.au]

>> 
>> Also, for everyone's info, Bosch has recently started production on 
>> a hot-film meter capable of measuring back-flow (properly!)  A neat
>> piece of design using micro-machined silicon technology.
>> 
>
>
>Hang the hell on here! Are you saying what I think you're saying?
>This thing can measure air flow in either direction, relatively
>accurately? If this the case, did anybody else see all those 
>arguments about wild cams up and fly out the window like I just did?
>

I'm not kidding you here.  Take a look at SAE paper #950433.  For 
those who haven't got access to it, it works something like this.  A 
heater is embedded onto a piece of silicon (somehow) with two platinum 
temp probes, one upstream and one down stream.  At zero flow, they 
both sense the same temperature, but as the flow increases from zero, 
the boundary layer flow cools the upstream sensor somewhat, but the 
downstream sensor remains relatively constant due to the air passing 
over the heater first.  Thus the airflow rate is a function of the 
temperature differential between the two sensors.  Obviously the sign 
of the differential is related to the flow direction.  Neat eh!

Trick is now, how do you sample these devices given that the flow is 
likely to be oscillating, sometimes to the extreme, and the sensor 
output is non-linear?  I suggest sampling in the crank domain, however
you've still got to reference it to some known flow to work out where 
the correct sample angle is.  Any ideas?