Mitsubishi Eclipse. Technical Information Manual (1994) — part 16
1-42
ENGINE <NON-TURBO> Control System
Fuel System Monitor
Background
To control the level of undesirable emissions, the
fuel system must be able to maintain strict control
of the air/fuel ratio. Stoichiometry is the optimum
air/fuel ratio, which is 14.7 to 1. At this point the
best balance between the production of
and
CO’s (which drop as the mixture becomes leaner)
and
(which increases as the air/fuel mix
comes leaner) can be found. This is also the point
where the catalytic converter is most efficient at
converting all three gases to less harmful
pounds. The goal of the PCM is to examine input
information and control outputs to produce a
constant stoichiometric ratio.
Air/fuel ratio
ENGINE <NON-TURBO>
- Control System
I-43
Operation
The Power-train Control Module (PCM) varies the
pulse width of the fuel injectors to provide precise
control of the air/fuel mixture. Wider pulse widths
increase the volume of fuel delivered to the cylinders.
The PCM uses the input from a number of sensors
in its attempt to reach and maintain this air/fuel
ratio. Manifold Absolute Pressure (MAP), and the
sensor have the greatest influence (authority)
over injector pulse width. Other inputs such as the
Throttle Position Sensor (TPS), engine RPM, Engine
Upstream
sensor
Coolant Temperature (ECT) Sensor, Intake Air Tem-
perature
Sensor. Vehicle Speed Sensor (VSS)
and battery voltage all have varying levels of influ-
ence on pulse width, depending on the circum-
stances.
As noted earlier, the system uses two oxygen sen-
sors both of which monitor the oxygen content of
the combustion byproducts on their way out of the
engine as exhaust. Only the upstream sensor has
authority over fuel injector pulse width.
Downstream
sensor
Exhaust gas
If a large amount of oxygen remains following the
combustion process, this sensor produces a low
voltage. This indicates a lean condition caused by
a ratio greater than stoichiometric. Little oxygen
in the exhaust allows the sensor to produce a higher
voltage, indicating a rich condition where the air/fuel
ratio is less than
Upstream
sensor feedback to the PCM is used
to fine tune injector pulse width to maintain
metric and meet emission standards. It can increase
or decrease injector pulse width by as much as
50%. Pulse width is calculated from the data
plied from the MAP, TPS, ECT,
upstream
sensor, battery voltage, RPM and VSS.
ENGINE <NON-TURBO> Control Svstem
To control air/fuel ratio feedback, the PCM uses
short term correction and long term memory. Before
the PCM can alter the programmed injector pulse
width, it must enter closed loop operation. The re-
quirements for closed loop operation are listed be-
low:
l
timers have timed out -following the START
to RUN transfer (The length of these timers
varies with engine temperature)
l
Engine temperature exceeds
l
sensor is in the
mode
41 sec.
35 sec.
22 sec.
13) are used only during idle. Each cell represents
11 sec.
a manifold pressure and rpm range and can be
Once in the closed loop control, the feedback sys-
tems begin to operate. Short term memory works
with the long term memory, which is broken down
accessed with the MUT-II diagnostic scan tool.
into 14 different cells. Two of these cells (12 and
Long Term Memory Cells
ABOVE
2048
RPM
BELOW
Map Voltage
1
3
5
Purge
Free*
0
2
Purge
Free*
4
7
9
11
10
1.38
2.0
2.64
3.26
3.9
13
Idle
Neutral
Manual
Transaxle
Purge
Free*
*PCM disables the canister purge function in these cells to gather data used for purge control and diagnostics.
For example:
If the PCM were in cell 3, the MAP voltage must
be between 1.38 and
and engine rpm must
be greater than 2048. This is the cell that the control
system would update.
ENGINE <NON-TURBO> Control System
If the oxygen sensor registers a rich or lean condition
while driving in this cell, the cell will require updating
to aid in fuel control. The short term correction is
used first. It starts increasing pulse width quickly
(kick), then ramps up slowly. Each control is in in-
verse relation to the signal sent from the
sensor.
For example:
The
sensor switches lean to rich. Short term
compensation kicks in lean, then ramps lean until
the
sensor switches lean. At this point short
term compensation reverses the process.
ratio
.
If the oxygen sensor shows lean, the short term
compensation goes rich and multiplies the pulse
width from long term memory in that cell by an
amount greater than 1. If the sensor shows rich,
the short term compensation drives the pulse width
narrower by multiplying by a number less than 1
(perhaps 0.97). The short term compensation can
multiply pulse width by as much as 1.25 or as little
as 0.75 to compensate for lean or rich conditions.
In this way, the short term compensation can in-
crease pulse width by up to 25% (by multiplying
by 1.25) or decrease pulse width by up to 25%
(by multiplying by 0.75).
Example:
Pulse width 0.05 x 1.25 0.0625 (increase of 25%)
Pulse width 0.05 x 0.75 0.0375 (decrease of 25%)
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