Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: absolute pressure



Vickie Frohne wrote:

one can get aneroid barometers that don't use mercury.

At 01:52 PM 6/4/02, John D responded:

Yup. At the auto-parts store among other places.
http://www.nettally.com/silly34/mapbp.htm

..which raises the question:
'Why would an auto engine want to know manifold pressure
and barometric pressure?'

It's a bit like a pilot being provided with an altitude or height or
flight level by a barometric instrument: better than nothing - but
not the entire truth, if the ambient temperature is far from the pre
agreed height/temp profile decided by the ICAO as typical of
mid-latitudes.

What an engine computer would *really* like to know is the mass
flow rate of air, because from a potted insight into the stochiometric
proportion for the fuel, given some insight into the driver's desires
(Accelerate? steady-state?) via a throttle sensor pot., it could then
provide a jolt of gas thorough the injector or injectors that would
get the mix close enough for an oxygen sensor to trim the last
few percentage points of optimality of whatever kind desired
(on some autos, the switch options are performance or economy).

A swing door or a hot filament can do the mass flow sensor job,
it turns out.






Brian Whatcott
Altus OK Eureka!