[Shotimes] BARO sensor
Adam Parrott
Adam Parrott" <parrotta@usa.net
Sun, 18 Sep 2005 10:26:37 -0500
>> Well, We get to mess with the DPFE which is MUCH more failure prone and
complicated than the BAP setup!!! <<
No argument there. :)
>> Which brings up a bit of trivia that I have not figured out. Maybe someone
with a California MTX can help? If the car is a MTX with EGR does it have both
a DPFE and a BAP sensor? Or does it lose the BAP and use the DPFE like all the
ATX cars do? <<
While I am no wiser than Ian regarding which CA MTX cars may (or may not) have
received a BARO sensor, I can satisfy the question regarding how BARO readings
are inferred on non-BAP-equipped cars (such as the Gen 2 ATX). To do this, I
refer you to the factory EEC strategy documentation:
--
INFERRED BAROMETRIC PRESSURE STRATEGY
BACKGROUND:
Earlier EEC systems used two pressure sensors (manifold absolute and
barometric absolute pressure) to provide full altitude capability. Since both
sensors perform the same function (i.e. measure pressure), elimination of one
sensor, via time sharing or software inference was considered a significant
system cost reduction. Since MAP is a primary input for both spark and fuel
control and BAP is a secondary modifier, elimination of BAP sensor resulted in
cost reduction.
In the Inferred Barometric Pressure strategy, the BAP sensor is replaced by a
software algorithm which uses available inputs (i.e. MAP, RPM, Throttle
Position, ECT) to infer the Barometric Pressure. The EEC-IV Barometric
Pressure (BP) is saved in Keep Alive Memory (KAM) to bridge the power-down to
power-up sequence.
FUNCTIONAL DESCRIPTION:
The barometric pressure is inferred from other sensor inputs. With air meter
equipped engines redundant sensor and actuator information exists which allows
for the inference of barometric pressure. The basic equation is:
29.92 * air mass from airmeter
Inferred BP = --------------------------------------------------------
sqrt(560/(ACT+460)) * inferred air mass (29.92,100 ACT)
The model implied by the inferred BP equation is that the inferred air mass is
really an air volume flow calculation, converted to mass units at the standard
temperature and pressure. The air meter is a true air mass measurement
device. Thus, deviations in mass from the inferred calculation are attributed
first to inlet temperature, which is measured and then to a drop in inlet
pressure, which is the inferred BP.
In the EEC the air mass flows are converted to LOAD by the formula:
load = air mass / (N * ENGCYL * SARCHG).
The inferred BP formula in EEC terms is thus:
29.92 * LOAD from airmeter
IBAP = --------------------------
FN059(ACT) * TOTLDST
N is engine speed in RPM.
ENGCYL is the umber of PIPs per revolution (cylinder number/2).
SARCHG is the standard air charge for a single cylinder.
TOTLDST is the inferred air mass RAM value. It is the sum of the flow through
the throttle body.
TPLDST and the flow through the air bypass valve, BYLDST. TPLDST is a table
lookup versus N and TP_REL.
BYLDST is the load equivalent of BYMAST, the air mass flow through the air
bypass valve.
BYMAST is calculated from a table lookup of air bypass duty cycle.
ISCDTY and a calculation of percent load, PCT_LOAD.
For calibration purposes, the calculated inferred value of barometric
pressure, can be fixed by using the calibrateable value BAPFMM.
--
Hopefully that didn't bore too many people. ;)
Adam