EXHAUST GAS RE-CIRCULATION CONTROL
Exhaust Gas Re-circulation Control (EGR)
The exhaust gas re-circulation system provides a means to direct exhaust gases from the exhaust manifold into the intake manifold. This is accomplished using a vacuum diaphragm valve and an electronic vacuum regulator valve (EVRV). Ported engine vacuum is directed to the EVRV which processes a 128 Hz pulse width modulated signal from the ECM to determine the amount of regulated vacuum signal to be applied to the vacuum diaphragm value (EGR is active with vacuum applied The regulated vacuum signal is also electronically fed back to the ECM for use in EGR diagnostics (see ?.1I Section 8).
2.0 Special Electronic Vacuum Regulator Value (EVRV) Control Conditions
Under certain conditions, the E'IRV is controlled independently of the EGR control algorithm. These conditions involve high battery voltage engine not running, diagnostic/factory test, I~U active and reset.
2.1 High Battery Voltage
The EGR duty cycle is set to 0% (EGR off) if the battery voltage is greater than a nominal 16.9 volts for 200 msec.
2.2 Engine Not Running/Diagnostic Mode
The EGR duty cycle set to 100% (EGR on) if the engine is not running and the diagnostic mode is selected. The EGR duty cycle is set 0% (EGR off) if the engine is not running and diagnostic mode is not selected.
The EGR duty cycle is set to 100% (EGR on) when the ECM is reset.
2.4 Factory Test Mode
Duty cycle of the EGR output in factory test mode is dependent on which test mode function is enabled (see 3.2 Section 4). See the following chart:
Test Mode function EGR Duty Cycle
Mode 1 (All Off Mode) 0% (EGR off)
Mode 2 (I/O Check Mode) 50% @ 32 Hz. PWM rate
Mode 3 (Misc. Test Mode) 0% (EGR Off)
Mode 4 (I/O Check Mode) 50% @ 32 Hz. PWM rate
EGR duty cycle may be slewed if the HUD and I2U are active and *KAFOPTI*, bit 0 is zero (=0).
3.0 Control Algorithm
Provided that the engine is running and special control conditions are not present, the EVRV is controlled by the EGR control algorithm. In accordance with the control algorithm, the ECM will enable EGR when coolant temperature, throttle position, and engine vacuum criteria have been met. Conversely, if any of the above criteria are not met, EGR is not enabled.
3.1 Enable/Disable Criteria
3.1.1 Coolant Temperature Criteria
The coolant temperature criteria for EGR on is met when the coolant temperature equals or exceeds *KEGRTEMI*. The coolant temperature criteria shall always be met if *KEGRTEM1* is set equal to 0. Setting *KEGRTEMI* equal to 255 will result in the coolant temperature criteria never being met and EGR never enabled.
3.1.2 Throttle Position Criteria
When the EGR is disabled, the throttle position criteria for EGR on shall be met when throttle position is greater than calibration memory parameter *KF4TPSl*. When the EGR is enabled, it shall remain on if the throttle position remains greater than calibration memory parameter *KF4TPS2*. For proper system operation, *KF4TPS1* must equal or exceed *KF4TPS2*. Setting *KF4TPS1* and *KF4TPS2* equal to 255 will result in the throttle position criteria never being met which disables EGR
3.1.3. Engine Vacuum Criteria
When EGR is disabled, the engine vacuum criteria for EGR on shall be met when engine vacuum is less than or equal to the calibration memory parameter *KEGRVAC2*. When EGR is enabled, it shall remain enabled if engine vacuum remains less than or equal to the calibration memory parameter *KEGRVAC1* for proper system operation, *KEGRVAC1* must equal or exceed *KEGRAVAC2*. Setting *KEGRVAC1* and *KEGRVAC2* to 255 will result in the engine vacuum criteria always being met.
3.2.1 *F60* Table
The *F60* table provides the altitude factor for adjusting BPW and EGR versus barometric pressure and manifold air pressure.
The *F60* table is a two dimensional lookup table with barometric pressure and manifold air pressure as independent variables.
3.2.2 *F72* Table
The *F72* table provides the desired percent EGR versus manifold vacuum and engine RPM.
The *F72* table is a three dimensional lookup table with manifold vacuum and engine RPM as the independent variables.
Manifold vacuum is defined as Barometric pressure minus manifold absolute pressure.
3.2.3 *F73* Table
The *F73* table provides the EGR duty cycle versus the EGR valve effective flow area.
The *F73* table is a two dimensional lookup table with EGR valve effective flow area in units of grams per second per kPa as the independent variable.
3.2.4 *F75* Table.
The *F75* table provides EGR flow pressure. compensation versus pressure.
The *F75* table is a two dimensional lookup table with barometric pressure in kPa as the independent variable.
4.1 Engine Air Flow
The following computation shall be used to obtain the airflow (1 bit = 0.25 grams/sec.) through the engine:
Air Flow = BPWM*KAIRF~w*2l8*KNUMcyL*25*AIRFUEL*NTRPMx32
Where: BPWM = Base pulse width for EGR
KAIRFLOW = Air Flow multiplier.
KNUNCYL = Number of cylinders*32.
AIR FUEL = Engine running air fuel ratio*IO.
NTRPMX = Computed engine RPM scaled 25 RPM/bit.
4.2 Exhaust Back Pressure
The following computation shall be used to obtain the exhaust system back pressure (kPa gage):
Rack Pressure = KBP2*(AirFlow**2)+KBPI*AIRFL0W+KBP0
Where KBP0 = Back pressure multiplier in KPA.
KBP1 = Back pressure multiplier in KPA*sec/gram.
KBP2 = Back pressure multiplier in KPA*sec*sec/gram*gram.
The following computation shall be used to obtain the EGR value equivalent flow area for the desired percent EGR
EGRFLOW = EGRDESA*AIRFLOW*F75( BP+VAC)
Where EGRDESA = Desired Percent EGR (altitude compensated).
AIRFLOW = Computed engine air flow.
F75 Table = EGR Flow pressure compensation versus pressure.
4.4 EGR Solenoid Duty Cycle
The following computation shall be used to obtain the EGR solenoid duty cycle:
EGR Duty Cycle = F73(EGRFLOW)
Where: F73 Table = EGR duty cycle versus EGR valve Flow normalized to pressure across the valve (grams/sec)/F75(KPA).
5.0 Computation Rate
The EGR logic is performed once per 100 msec.