DIY_EFI Digest V1 #222

[talltom]

> From: RABBITT_Andrew at mv8.orbeng.com.au
> Date: Tue, 6 Aug 1996 08:10:10 est
> Subject: re: back-flow
> 
> >>
> >> 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?
> 

Well I hope you'll indulge my ignorance, but why do you care about some
known Flow? Maybe I'm 9 clouds out of orbit, but it would seem to me 
that one could set up some sort of graph with the differential output
as one axis and fuel injector pulse as the other and set the pulse 
width on a laptop once you put it on whatever the application is.
One possible complication I can see is that pulsing across AFM could
cause radical sensor outputs, but as long as the output differential
was used It would have to be accurate. One may need to cook up some sort 
of 
filter to slow up pulses if idle gets erratic, but you know that more air 
has to go in than out unless it pukes, at which point the backfire will
have heated the outgoing air and read as incoming which will in effect 
richen the mixture. At that point the tendency to puke will have been 
reduced. Kinda automatically  screws up in the right direction! Even
a bunch of burned intake valves shouldn't keep this thing from working,
as still more air has to go in than come out.   

Alright, let the flames begin!