Emission Control Description & Operation - Diesel GMC Savana I

Crankcase Ventilation System Description

The crankcase ventilation system is used on diesel engines and is designed to maintain a slightly negative (vacuum) crankcase pressure across the speed range. The system consists of a crankcase depression regulator (CDR) valve, located on the right valve cover and attaching vent hose/pipes to the engine inlet system. The CDR valve is used only to regulate the crankcase pressure between +.249-.996 kPa (1-4 in. of water) depression over the engine speed range. The CDR valve is NOT an oil separator or a crankcase effluent flow regulator. Hence, the CDR valve DOES NOT prevent oil droplets/mist from entering the intake system, nor does the CDR valve affect the engine oil consumption.

The intake manifold vacuum acts against a spring loaded diaphragm in order to control the flow of the crankcase gases. Higher intake vacuum or high intake restriction (e.g., plugged air filter) levels pull the diaphragm closer to the top of the outlet tube. This prevents the vacuum level from getting too high in the crankcase. As the intake vacuum decreases, the spring pushes the diaphragm away from the top of the outlet tube, preventing the crankcase pressure from going positive.

EXHAUST GAS RECIRCULATION

The EGR system limits formation of oxides of nitrogen (NOx) emissions by reducing peak combustion chamber temperatures in which NOx is formed. EGR system consists of an EGR valve, EGR vent/EGR solenoids and EGR fault detection. A vacuum pump is required to provide a vacuum source to operate the EGR system.

EGR Valve

EGR valve reintroduces a small amount of exhaust gas into combustion chamber, diluting air/fuel mixture and reducing combustion chamber peak temperatures, thereby reducing NOx formation.

EGR Vent/EGR Solenoids

EGR vent/EGR solenoids are mounted at rear of engine as a single assembly. Using input from engine speed sensor and Accelerator Pedal Position (APP) sensor, PCM controls EGR by controlling amount of "on" and "off" time of EGR solenoid. When EGR is not needed, PCM energizes EGR vent solenoid to vent vacuum. Vacuum is used to control EGR valve opening.

Adaptive Learn Matrix (ALM)

The Adaptive Learn Matrix (ALM) is a matrix of cells arranged by RPM and engine load. ALM is used to adjust EGR vacuum control based on Mass Air Flow (MAF). The ALM cells may change as a result of back pressure increases over the life of the engine or other engine system variations. Too much EGR will cause ALM cells to go high, not enough EGR will cause the ALM cells to go low as shown below

1. If backpressure is excessive (too much EGR), the ALM cells will be high (above 128). This will cause the PCM to reduce vacuum to the EGR valve. This in turn will reduce the amount of EGR going into the engine.
2. If the EGR is not opening completely (not enough EGR), ALM cells will be low (below 128). This will cause the PCM to increase the amount of EGR entering the engine.

As the engine operating conditions change, the PCM will switch from cell to cell to determine what factor to use. ALM is made up of sixteen cells numbered zero to fifteen in which each cell covers a range of engine speed (RPM) and load (mm3). A normal functioning EGR system ALM cells should display a value between 115 and 140 (the scaling of these cells are 0 to 256).

Resetting the ALM cells will allow the EGR system to quickly return to a normal function (reset ALM cells will display 128). If the ALM cells are not reset, the vehicle may experience black smoke and poor driveability complaints until the system is able to adjust (approximately 5 to 10 miles) or a DTC P0401 may set.

EGR Fault Detection

The PCM uses input from the MAP sensor to measure amount of absolute pressure in EGR vacuum line. If a minor variation between calculated EGR and actual EGR is monitored by PCM, necessary corrections are made by the PCM. If variation is too great for PCM to correct, an error is detected. The PCM then enters into a default mode and sets a related diagnostic trouble code in memory.

Vacuum Pump

Vacuum pump provides vacuum for operating emission control components (some "C", "G" and "K" series vehicles), cruise control and A/C-heater servos. The vacuum pump is driven by either a belt or gear.

The belt-driven vacuum pump is mounted on right front of engine. Except for the pulley, vacuum pump is replaced as an assembly.

The gear-driven pump is mounted on the top rear of engine and contains a permanently-mounted speed sensor. Pump is driven by a cam inside the drive assembly to which it mounts. On the lower end of the drive housing assembly is a drive gear which meshes with the camshaft gear in the engine. The drive gear causes the cam in the drive housing to rotate.

CRANKCASE DEPRESSION REGULATOR

The Crankcase Depression Regulator (CDR) valve, located on right-side valve cover, is used on all diesel engines. Valve prevents crankcase pressure from accumulating during idle by regulating (metering) crankcase pressure back into the engine. Intake manifold vacuum (only slight vacuum is present) acts against a spring-loaded diaphragm to control flow of crankcase gases. Higher intake manifold vacuum levels pull diaphragm closer to the top of the outlet tube, reducing amount of gases drawn from crankcase. As intake manifold vacuum drops, spring pressure pushes diaphragm away from top of outlet, allowing more gases to flow from crankcase into intake manifold.

Optimum pressure in crankcase is one inch of water (as measured with a manometer) at idle to 3-4 inches at full load. Too little vacuum causes oil leaks; too much vacuum pulls oil into the air crossover.

Starting in January 2004, a closed crankcase ventilation system is used to meet new diesel emissions requirements.

Located in both valve rocker arm covers are diaphragms to control the venting of the crankcase gases. As the pressure of the crankcase gases increase, they overcome the spring holding the diaphragm in the closed position. If a vacuum situation arises in the crankcase, the diaphragm closes the port in the valve rocker arm cover. Closing the port will prevent unfiltered air to enter the crankcase.

The crankcase gases travel from the valve rocker arm covers through hoses to a tee, where they enter the turbocharger inlet duct. Because of the use of a closed crankcase ventilation system, it is normal for oil residue to be found on the turbocharger compressor wheel and inside the charge air cooler, pipes, and hoses.

No routine maintenance is required to the crankcase ventilation system.

Exhaust Gas Recirculation (EGR) System Description

The Exhaust Gas Recirculation (EGR) System is used to reduce the amount of nitrogen oxide (NOx) emission levels caused by high combustion temperatures. At temperatures above 1 371°C (2,500°F), oxygen and nitrogen combine to form oxides of nitrogen (NOx). Introducing small amounts of exhaust gas back into the combustion chamber displaces the amount of oxygen entering the engine. With less oxygen in the air/fuel mixture, the combustion pressures are reduced, and as a result, combustion temperatures are decreased, restricting the formation of NOx.

The EGR valve motor is a direct current (DC) stepper motor utilizing a worm gear that extends from the motor to push on the EGR valve stem. The worm gear is not attached to the valve stem, and can only force the valve open. A return spring is used to force the valve closed.

The mass air flow (MAF) sensor signal is used by the engine control module (ECM) to detect the proper amount of EGR flow. One EGR flow test is performed per ignition cycle. The ECM will close the EGR valve for 5 seconds, then open the EGR valve to 100 percent for 5 seconds. The ECM will then calculate the MAF difference and determine if the proper EGR flow has been detected.

Scheme 34

Callouts For EGR Valve

The exhaust gas recirculation (EGR) valve is controlled by the engine control module (ECM) through the EGR motor high control and EGR motor low control circuits. The ECM supplies voltage that is near ignition voltage to the high and low control circuits at all times. This voltage is used by the ECM as a reference voltage during non EGR operation in order to detect circuit failures. The ECM will pulse width modulate (PWM) the low control circuit to ground and an increase in amperage on the high control circuit can be observed with a DMM when the EGR valve is commanded open. A lower pulse width will increase the open position of the valve. In order to close the EGR valve, the ECM will PWM the high control circuit to ground.

When the ignition is turned ON, the ECM will drive the EGR motor worm gear out with just enough force to touch the EGR valve stem. The ECM will do this 3 times in quick succession. This action determines the minimum closed position of the valve and only happens once per ignition cycle. If the valve is prevented from closing all of the way after the minimum closed position is learned, the scan tool EGR Position parameter will not reflect this position until the next ignition cycle. The EGR motor worm gear is not connected to the EGR valve stem and can only push the valve open. The valve is returned to the closed position by a return spring.

The ECM uses the EGR position sensor to determine the position of the EGR valve. The ECM sends a reference voltage through the 5-volt reference circuit to the EGR position sensor. The ECM provides a voltage return path for the sensor through the low reference circuit. A variable voltage signal, based on the EGR valve position, is sent from the sensor to the ECM through the EGR position sensor signal circuit.

EGR Valve Control Enabling Conditions

Exhaust gas recirculation (EGR) valve control will only be enabled during idle and cruising conditions while the following conditions are met

1. The intake air temperature (IAT) is more than 5.25°C (41.5°F). EGR valve control will remain enabled until the IAT is less than 0°C (32°F) and will not enable again until the IAT is more than 5.25°C (41.5°F).
2. The engine coolant temperature (ECT) is between 60-96.75°C (140-206.15°F). EGR valve control will remain enabled until the ECT is less than 57°C (134.6°F) or more than 99.75°C (211.55°F) and will not enable again until the ECT is between 60-96.75°C (140-206.15°F).
3. The barometric pressure (BARO) is more than 74 kPa. EGR valve control will remain enabled until the BARO is less than 72 kPa and will not enable again until 74 kPa.

Diesel Particulate Filter Layout. Scheme 35

Exhaust Particulate Filter

The exhaust particulate filter (EPF) captures diesel exhaust gas particulates, preventing their release into the atmosphere. This is accomplished by forcing particulate-laden exhaust (1) through a filter substrate of porous cells, which removes the particulates from the exhaust gas. The exhaust gas enters the filter, but because every other cell of the filter is capped at the opposite end, the exhaust particulates cannot exit the cell. Instead, the exhaust gas passes through the porous walls of the cell leaving the particulates trapped on the cell wall. The cleaned exhaust gas exits the filter through the adjacent cell. The EPF is capable of reducing more than 90 percent of particulate matter (PM).

Diesel Oxidation Catalyst

The diesel oxidation catalyst (DOC) (7) has two functions. One function is to reduce emissions of non methane hydro-carbons (NMHC) and carbon monoxide (CO), from the exhaust gases. The other function is to help start a regeneration event by converting the fuel-rich exhaust gases to heat. The engine control module (ECM) monitors the functionally of the DOC by determining if the exhaust gas temperature (EGT) sensor 1 (1) reaches a predetermined temperature during a regeneration event. The Diesel Particulate Filter (DPF) System Description DOC and the exhaust particulate filter (EPF) (6) are downstream of the turbocharger, and are two separate components under the vehicle.

Differential Pressure Sensor (DPS) and Pressure Lines

Difference between the inlet and outlet of the exhaust particulate filter (EPF). When pressure difference has increased above a calibrated threshold, a high particulate loading condition is indicated. The ECM will command a regeneration event in order to restore the filter. If the pressure differential continues to increase across the exhaust filter without a regeneration event, the ECM will illuminate an EPF lamp or send a message to the driver information center (DIC) referring the customer to clean the exhaust filter. To clean the exhaust filter the vehicle must be driven under the conditions necessary for a regeneration to take place. If these lamps and messages are ignored, the ECM will eventually illuminate the malfunction indicator lamp (MIL) and revert to Reduced Engine Power which will require the vehicle to be serviced. The DPS sensor provides a voltage signal to the ECM on a signal circuit relative to the pressure differential changes in the EPF. The ECM converts the signal voltage input to a pressure value. The DPS pressure lines (2) are connected before and after the EPF. To provide the pressure sensor with accurate back pressure measurements, the DPS pressure lines should have a continuous downward gradient without any sharp bends.

Exhaust Gas Temperature Sensors

The ECM uses two exhaust gas temperature (EGT) sensors to measure the temperature of the exhaust gases at the inlet and outlet of the exhaust particulate filter (EPF). The EGT sensors are variable resistors, when the EGT sensors are cold, the sensor resistance is low, and as the temperature increases, the sensor resistance increases. When sensor resistance is high, the ECM detects a high voltage on the signal circuit. When sensor resistance is low, the ECM detects a lower voltage on the signal circuit. Proper EGTs at the inlet and outlet of the EPF are crucial for proper operation and for initiating the regeneration process. A temperature that is too high in the EPF will cause the EPF substrate to melt or crack. The ECM monitors the temperatures at the EPF inlet and outlet to regulate EPF temperatures.

Normal Regeneration

Regeneration is the process of removing the captured particulates through incineration within the exhaust particulate filer (EPF). Elevated temperatures are created in the diesel oxidation catalyst (DOC) through a calibrated strategy in the engine control system. Regeneration occurs when the ECM calculates that the particulate level in the filter has reached a calibrated threshold using a number of different factors, including engine run time, distance traveled, fuel used since the last regeneration, and the exhaust differential pressure. In general, the vehicle will need to be operating continuously at speeds above 48 km/h (30 mph) for approximately 20-30 minutes for a full and effective regeneration to complete. During regeneration the exhaust gases reach temperatures above 550°C (1,022°F). The ECM monitors the EGT sensors during regeneration. If the sensors indicate that regeneration temperatures are exceeding a calibrated threshold, regeneration will be temporally suspended until the sensors return to a normal temperature. If EGT temperatures fall below a normal calibrated threshold, regeneration will be terminated and a corresponding DTC should set. If a regeneration event is interrupted for any reason, it will continue, including the next key cycle, when the conditions are met for regeneration enablement. Normal regeneration is transparent to the customer.

Service Regeneration

A scan tool is an essential tool that is required for service regeneration. Commanding a service regeneration is accomplished using the output control function. The vehicle will need to be parked outside the facility and away from nearby objects, such as other vehicles and buildings, due to the elevated exhaust gas temperature at the tail pipe during regeneration. The service regeneration can be terminated by applying the brake pedal, commanding service regeneration OFF using the scan tool, or disconnecting the scan tool from the vehicle.

Regeneration Process

A number of engine components are required to function together for the regeneration process to be performed. These components are the fuel injectors, turbocharger, IA valve, fuel pressure control, and the intake air heater (IAH). The regeneration process consists of several stages: Warming up the diesel oxidation catalyst (DOC) to 350°C (662°F) by performing the following

1. Reducing air flow with the intake air valve
2. Increasing or decreasing boost pressure with the turbocharger, depending on engine load
3. Elevating the engine speed
4. Reduce fuel rail pressure
5. Retard fuel injection timing
6. Add late fuel injection pulses. The added fuel is not combusted but is oxidized by the DOC and exhaust particulate filter (EPF) to create heat.

Ash Loading

Ash is a non-combustible by product from normal oil consumption. Low Ash content engine oil (CJ-4 API) is required for vehicles with the exhaust particulate filter (EPF) system. Ash accumulation in the EPF will eventually cause a restriction in particulate filter. Regeneration will not burn off the ash, only particulate matter is burned off. An ash loaded EPF will need to be removed from the vehicle and cleaned or replaced.