Operation - Turbo
OPERATION
EVAP SYSTEM OPERATION: The Evaporative Emissions System is designed to manage fuel vapors. The Powertrain Control Module (PCM) monitors the performance of the Evaporative Emission System. The system uses the principle of Natural Vacuum Leak Detection (NVLD) to determine the integrity of the system. When fuel vapors build and exit the Fuel Tank they are routed and stored in the Charcoal Canister. Fuel vapors entering the Charcoal Canister are absorbed by the charcoal filter until they can be drawn into the Intake Manifold during purging and burned by the engine.
The two main areas being monitored are the integrity of the system against leaks and the ability of the system to purge fuel vapor from the canister to the Intake Manifold.
- The basic strategy used for leak testing is that in a sealed system , pressure will naturally increase or decrease in relation to temperature. The Evaporative System Integrity Monitor (ESIM) Switch is used for all leak fault detection. There is a vacuum actuated switch that closes when the vacuum reaches a calibrated threshold. If the switch closes it indicates that the system is not leaking.
- The basic strategy used for purge performance testing is that as flow through the system increases, so does the pressure drop in the system. The Fuel Tank Pressure (FTP) Sensor is used for purge fault detection. The PCM monitors the Fuel Tank Pressure Sensor and looks for increasing vacuum in the Fuel Tank with increasing purge flow. Conversely, it looks for decreasing vacuum in the Fuel Tank with decreasing purge flow.
PURGE FLOW OPERATION
While the leak detection principle is the same on the Turbocharged engine as it is on the Naturally Aspirated engines, the purge system operation and diagnostics are different. The Turbocharged engine requires the ability to draw fuel vapors from the Charcoal Canister during both Naturally Aspirated operation and during boost conditions. Typically engine vacuum is used to draw the fuel vapors from the canister. This is easily accomplished on a naturally aspirated engine that has sufficient vacuum in the Intake Manifold during most engine operating conditions. It is more difficult when a Turbocharged engine is in boost mode creating positive pressure in the Intake Manifold. This is accomplished using a different Purge Solenoid than is used on non-Turbo engines along with some additional hardware. The function of the purge system in each mode of operation is as follows:
- Naturally Aspirated: When the Purge Solenoid is energized the vacuum from the Intake Manifold pulls check valve 1 open. The vacuum created inside the Purge Solenoid pulls check valve 2 closed, sealing it off to the Ejector Tee. Fuel vapors are drawn from the canister, through the Purge Solenoid, and into the Intake Manifold.
- Boost: When the Purge Solenoid is energized the pressure in the Intake Manifold closes check valve 1 and opens check valve 3 allowing airflow to the Ejector Tee. The airflow through the Ejector Tee creates a venturi effect creating vacuum in the line between the Purge Solenoid and Ejector Tee, pulling check valve 2 open. Fuel vapors are drawn from the canister, through the Purge Solenoid, and into the Air Cleaner box where they travel with the intake air to the Intake Manifold.
Functional Operation: The normally closed Purge Solenoid is energized to open and allows the fuel vapors to be drawn from the Charcoal Canister into the engine. The PCM energizes the Purge Solenoid using a Pulse Width Modulated (PWM) High Side Driver (HSD) control circuit. The solenoid has one inlet port that connects to the canister and two outlet ports. The outlet ports connect to the Intake Manifold and Ejector Tee. Each outlet port has an internal check valve. The check valves allow flow through the line that has vacuum applied to it during purging.
- When vacuum is present in the Intake Manifold the check valve in the outlet port to the Intake Manifold is pulled open. The other check valve in the outlet port to the Ejector Tee is pulled closed by the vacuum in the cavity of the solenoid and the purge vapors are pulled into the Intake Manifold.
- During boost, pressure in the line from the Intake Manifold closes the check valve in the outlet to the manifold. Vacuum created in the venturi of the Ejector Tee creates vacuum on the line connected to the outlet port for the Ejector Tee, opening the check valve and allowing fuel vapors to be drawn into the Air Cleaner box.
The OBD port connects to the OBD Vent Valve. The normally closed OBD Vent Valve is energized to open when it is necessary to vent vacuum trapped in the EVAP system.
Diagnostic Overview: The Purge Solenoid functionality can be tested using a hand vacuum pump:
- With the Purge Solenoid de-energized, connect the hand vacuum pump to the outlet port for the Intake Manifold and apply approximately 10 - 15 in. Hg. The solenoid should hold the vacuum if the Purge Solenoid and check valve #2 are closed.
- With the Purge Solenoid de-energized, connect the hand vacuum pump to the outlet port for the Ejector Tee and apply approximately 10 - 15 in. Hg. The solenoid should hold the vacuum if the Purge Solenoid and check valve #1 are closed.
- To verify the Purge Solenoid is not stuck closed, energize the solenoid to open it with vacuum applied at either outlet port. The vacuum should disappear when the solenoid is energized.
Typical High Side Driver Function and Fault Detection: This type of driver circuit is generally used for relay control, controlling a solenoid or a similar type of driver device. The PCM provides 12 volts to operate the device when switched on. The PCM also provides fault detection for the device, wiring and internal driver. In the example below, the PCM uses an internal pull up diagnostic resistor and calibrated voltage reference (V-Ref) as a comparator for fault detection.
- Circuit Open and Circuit High Detection: The PCM monitors for an open circuit and short to voltage when the internal driver is switched off. A milliampere voltage is provided to the device through the internal pull up diagnostic resistor connected in series with the device. The diagnostic voltage is monitored between the pull up resistor and device and compared to V-Ref. If the resistance in the device or circuitry becomes too large (approaches open) the voltage supply will increase on the comparator circuit and become greater than V-Ref, and a fault is detected. A short to Battery voltage will have the same effect.
- Circuit Low Detection: The PCM monitors for a short to ground when the driver is turned on. When switched on, the voltage to the device should be close to the 12 volt supply voltage. A short to ground will pull the voltage at the comparator circuit below V-Ref and a circuit low fault is detected.
A maintenance free vapor canister is used on all vehicles. The Charcoal Canister is filled with granules of an activated carbon mixture. Fuel vapors entering the Charcoal Canister are absorbed by the charcoal filter until they can be drawn into the Intake Manifold during purging and burned by the engine.
The ESIM is configured with a normally open vacuum switch and normally closed vacuum relief and pressure relief valves (see illustration above). The PCM uses the ESIM for all system leak diagnostics (P0440, P0455 and P0456. The relief valves regulate the system pressure between approximately 250Pa and -500Pa.
- When the system vacuum reaches between (-187Pa) and (-250Pa) it acts on the diaphragm and closes the vacuum switch. When the switch closes it allows continuity between the terminals. This allows the ESIM Signal to pass through the switch, pulling the signal low to ground. The PCM monitors the signal to determine the state of the switch (open or closed). The ESIM Signal can be a 5.0 volt or 12.0 volt signal depending on the vehicle . When the PCM reads voltage on the signal circuit it determines the switch is open and when the signal is pulled low the PCM determines the switch is closed.
- The vacuum relief valve is intended to maintain the seal on the system during engine off events. However if the vacuum exceeds the regulation point the valve will pull off the seat to protect the system from excessive vacuum conditions. During purging, the vacuum relief valve inside, the ESIM is opened allowing fresh air to be drawn through the fresh air filter and Charcoal Canister to aid in drawing the fuel vapor from the canister.
- The pressure relief valve limits any pressure build up in the Fuel Tank during refueling and allows the tank pressure to normalize during increasing temperature events. This is additionally beneficial because it achieves a switch closed state (pass threshold) sooner than if the tank has to decay from a much higher pressure.
The Canister Fresh Air Filter is mounted at a high point of the vehicle next to the filler so that in the event the vehicle is used in deeper water the tank can still vent through the Charcoal Canister. A hose connects the Fresh Air Filter to the Evaporative System Integrity Monitor (ESIM). The Fresh Air Filter prevents dirt and contamination from entering the system during purging.
The FTP Sensor is located on the Recirculation Tube. It measures the pressure in the Evaporative Emissions System and converts it into a voltage. The PCM uses the FTP Sensor for the purge system performance diagnostics (P0441 and P1CEA). The Sensor is used in the following manner:
- When the Purge Solenoid is actuated and the PCM monitors the change in the FTP Sensor reading over time. The PCM expects to see the pressure/vacuum reading decrease more than a calibrated threshold.
- When the Purge Solenoid is then deactivated and the PCM monitors the change again over time. The PCM expects to see the pressure reading increase more than a calibrated threshold over a period of time.
Functional Operation: The normally closed OBD Vent Valve is used to provide open venting to the atmosphere when energized upon command by the Powertrain Control Module (PCM). The PCM energizes the OBD Vent Valve using a Low Side Driver (LSD) control circuit. The valve is used as part of the Evaporative System diagnostics on Turbocharged engines for two purposes:
- ESIM Switch - Stuck Closed Diagnostic: During small leak testing vacuum is often trapped in the EVAP system when the engine is turned off. The system is vented to atmosphere to relieve the vacuum and open the switch. Since the system is sealed to atmosphere through the check valves in the Proportional Purge Solenoid (PPS), the OBD Vent Valve is energized by the PCM to vent the system.
- Purge Performance Diagnostic: This diagnostic looks for the delta pressure change in the Fuel Tank Pressure (FTP) Sensor signal when the Purge Solenoid is energized. If vacuum is already trapped in the system it may be difficult to generate enough of a delta pressure change to obtain good separation between a passing and failing system. Because both sides of the system are tested during each drive cycle the OBD Vent Valve is used to eliminate the excess vacuum before the Purge Flow Monitor Diagnostic begins for each side.
Diagnostic Overview: The PCM performs open circuit and short circuit diagnostics on the control circuitry for the OBD Vent Valve. Detecting a valve that is mechanically stuck open or closed is caught by other EVAP system diagnostics. A stuck open valve would cause a system leak and would fail the small leak or large leak diagnostic. A valve that is stuck closed would cause one or both of the ESIM Switch Stuck Closed or Purge Performance diagnostics to fail.
Typical Low Side Driver Function and Fault Detection: This type of driver circuit is generally used for relay control, controlling a solenoid or a similar type of driver device. The PCM provides a ground to operate the device when switched on. The PCM also provides fault detection for the device, wiring and internal driver. For diagnostic purposes the PCM uses an internal pull down diagnostic resistor connected in series and a voltage reference (V-Ref) comparator for fault detection:
- Circuit Open and Circuit Low Detection: The PCM monitors for an open circuit and short to ground when the driver is switched off. When switched off, the available voltage passes through the device and the internal pull down resistor connected in series. The voltage at the comparator circuit should be close to Battery voltage since the majority of the voltage drop occurs through the diagnostic resistor. If the available voltage is less than the V-Ref, a fault is set. In this scenario the V-Ref would be slightly below Battery voltage.
- Circuit High Detection: The PCM monitors for a short to voltage when the driver is switched on. When the driver is switched on providing a path to ground through the transistor, the available voltage should be pulled low, near zero volts since the comparator circuit is monitoring the ground side of the device. If the voltage is greater than V-Ref, a fault is detected. In this scenario V-Ref would be slightly above zero volts.
A load that has a resistance that is below manufacturer specification, or a second load device shorted to the low side driver circuit can cause excessive current draw on the internal driver. The driver will be switched off to protect against overheating and damaging the driver. In this instance the Circuit High fault may be detected because the available voltage on the comparator circuit is above V-Ref.
Functional Operation: The Ejector Tee is mounted inside the Air Cleaner box. It is used to create vacuum to draw fuel vapors from the Charcoal Canister into the Intake Manifold when the engine is in boost. To achieve this the nozzle utilizes the venturi effect. The boost air enters the nozzle through the boost air fitting and passes through the nozzle across the end of the purge tube fitting creating a pressure drop and vacuum in the purge tube from the Purge Solenoid.
Diagnostic Overview: The Ejector Tee functionality is diagnosed as part of the Purge Flow Monitor diagnostic. If the nozzle is damaged or blocked it will not allow vacuum to be created and will fail the boost side Purge Flow diagnostic and set a DTC (P1CEA) .
There are three internal Tank Vent Valves on the Fuel Tank. During refueling the liquid fuel enters the Fuel Tank through the filler tube and displaces the vapor in the Fuel Tank. The Fuel Tank vapors escape through the Tank Vent Valves to the Charcoal Canister side of the system, including the Recirculation Tube, in a controlled approach to meet On-Board Refueling Vapor Recovery (ORVR) requirements.
- Tank Vent Valves: The fill limit vapor valve and liquid vapor separator drain valve are open when the fuel level is at lower levels. As the fuel level increases above 75% the float valves start to close. When the tank reaches it's fuel capacity the valves are fully closed preventing liquid fuel from entering the Charcoal Canister and Recirculation and Purge Tubes. The float valves are also designed to close during a roll over event.
- Grade Vent Valve: The Fuel Tank has a vapor space that is approximately 10% of the fuel volume to allow for expansion of the Fuel Tank vapors when the tank is full. The Grade Vent Valve is located in this portion of the Fuel Tank. The Grade Vent Valve is a float valve like the other valves. It is used to allow the vapors to escape into the canister side of the system when the Fuel Tank is full. The float valve will close preventing fuel from entering the canister side of the system if the Fuel Tank is significantly over-filled or during a roll over event.
- Fuel Filler Check Valve: The inlet check valve at the bottom of the Fuel Filler allows the fuel to enter the tank while also deterring vapor from coming back up the filler tube.