Engine Control System - Theory & Operation: Overview Pontiac Vibe I

DESCRIPTION

The fuel system is comprised of two sub-systems. The fuel supply system and the fuel metering system. The correct amount of fuel can not be delivered to the engine cylinders without a constant supply of air from the air intake system. The air intake system provides measured clean air to each cylinder for proper mixture with the pressurized fuel. The Powertrain Control Module (PCM), along with the information sensors and switches, controls the activity between all three systems.

The fuel supply system is a returnless on-demand design, that reduces evaporative emissions. The fuel metering system is a Sequential Fuel Injection (SFI) design. The function of the fuel metering system is deliver the correct amount of fuel to the engine under all operating conditions. The main component of the vehicle's fuel metering system is the fuel injectors. There is one fuel injector for each cylinder. The fuel injectors are controlled sequentially by the PCM. The PCM bases the control of the fuel injectors on several important engine parameters. These engine parameters include the following

1. Mass Air Flow (MAF) sensor.
2. Intake Air Temperature (IAT) sensor.
3. Engine Coolant Temperature (ECT) sensor.
4. Throttle Position (TP) sensor.
5. Engine speed (RPM).
6. Heated Oxygen Sensor (HO2S).

The engine is designed to use only premium unleaded fuel. Unleaded fuel must be used for proper emission control system operation. Using unleaded fuel will also decrease spark plug fouling and extend engine oil life. Leaded fuel can damage the emission control system. The use of leaded fuel can result in the loss of emission warranty coverage. The vehicle fuel system is equipped with an evaporative emission control system that minimizes the release of fuel vapors to the atmosphere.

The vehicle fuel system is equipped with an evaporative emission control system that minimizes the release of fuel vapors to the atmosphere.

System Description

The supply of fuel is stored in the fuel tank. An electric fuel pump pressurizes the fuel through to the fuel filter and out to the fuel lines and the fuel rail. The electric fuel pump is suspended from the top of the fuel tank as part of the fuel sender assembly. The fuel pump is designed to provide fuel at a pressure above the regulated pressure needed by the fuel injectors. The fuel pressure regulator keeps the fuel supplied to the fuel injectors at the regulated pressure.

The fuel pressure regulator is located inside of the fuel tank and is part of the fuel sender assembly. Locating the fuel pressure regulator in the fuel tank eliminates the need for a return line from the fuel rail. A returnless system reduces the internal temperature of the fuel tank resulting in lower evaporative emissions.

The fuel system is also equipped with an enhanced evaporative emission system and an On-Board Refueling Vapor Recovery (ORVR) system. Together the 2 systems minimize the release of fuel vapors to the atmosphere.

Air/Fuel Ratio Feedback Compensation (Closed Loop Operation)

In order to obtain efficient performance of the 3-way catalytic converter and a high clarification rate of CO, HC and NOx in the exhaust gas stream, the air/fuel mixture must be kept as close to the theoretical air/fuel ratio of 14.7:1 as possible. In order to accomplish this the PCM first compares the input voltage from the Heated Oxygen Sensor 1 (HO2S 1) with a specified reference voltage. If the HO2S 1 input voltage is higher than the specified reference voltage, the PCM determines that the air/fuel ratio is richer than the theoretical air/fuel ratio and reduces the fuel. If the input voltage from the HO2S 1 is lower than the specified reference voltage, the PCM determines that the air/fuel ratio is leaner and increases the fuel. By repeating these operations, the PCM can adjust the air/fuel ratio in order to be closer to the theoretical air/fuel ratio. Control of the fuel delivery system as just described is known as closed loop operation. See CLOSED LOOP FUEL CONTROL OPERATION table.

CLOSED LOOP FUEL CONTROL OPERATION

The closed loop fuel control operation will not take place under any of the following conditions

1. At engine start-up.
2. When the fuel injection is increased just after engine start-up.
3. When the Engine Coolant Temperature (ECT) sensor is indicating a low coolant temperature.
4. When the engine is operating under a high demand, such as at Wide Open Throttle (WOT).
5. During fuel cutoff.
6. When the HO2S 1 is cold (Open Loop operation).

Control of the air supply that is mixed with the metered fuel is detailed in the description of the air intake system. See

AIR INDUCTION SYSTEMS

Fuel Injection Operation

The PCM controls the amount of fuel the fuel injector supplies to each cylinder by controlling the ON time, or length of pulse, of each individual injector. The delivery timing of the fuel into the cylinder head intake port by the fuel injector is controlled by the PCM. The timing and pulse of the fuel injectors is carefully calculated with inputs from the various sensors so that a suitable air/fuel mixture is supplied to the engine for every driving condition.

There are two types of injection timing. One is synchronous injection, when fuel injection is synchronous with the ignition signal or the signal from the Camshaft Position (CMP) sensor. The other is asynchronous injection, when fuel injection takes place independently of the ignition signal or the signal from the CMP sensor.

The PCM first calculates the correct timing of the fuel injectors by factoring the engine speed and the air volume together. Then the PCM applies certain compensations that are based on the information provided by various sensors which detect the state of the engine and the current driving conditions.

Synchronous Injection - When starting the engine, the fuel injectors inject the fuel simultaneously and synchronously at every Camshaft Position (CMP) sensor signal. When the engine is starting at a cold state, the amount of fuel is determined by the Engine Coolant Temperature (ECT) sensor and is divided and injected. Once the engine is running, the fuel injection occurs in a cylinder only when the cylinder is in the exhaust stroke. The PCM detects the compression stroke of cylinder 1 through the CMP sensor signal.

Asynchronous Injection - Whenever a change in the throttle valve opening exceeds a specified value, as determined by the PCM, additional fuel is injected simultaneously into the cylinders which are in the intake and exhaust strokes. This is in addition to the above synchronous injection and is not based on the ignition signal.

Modes of Operation

The Powertrain Control Module (PCM) uses reference pulses from the Crankshaft Position (CKP) sensor in order to determine the engine speed. The PCM cannot operate the ignition system or the fuel injectors without the engine speed signal from the CKP. The PCM controls the ignition timing by controlling the ignition coils.

Each ignition coil has a built-in ignition module that controls the current flow in the primary coil winding. When the current flow is interrupted, the electrical field around the primary coil collapses and a high voltage is induced in the secondary coil. The secondary coil voltage travels from the coil output terminal, through the spark plug boot, and across the spark plug gap to the engine block. As a fail-safe function the ignition control module sends an ignition confirmation signal back to the PCM whenever the primary field collapses.

The Camshaft Position (CMP) sensor input is used to detect an engine misfire. The PCM also uses the CMP sensor signal as an input for modifying the fuel injection timing and for modifying the ignition timing.

The PCM receives information on the engine status from various engine sensors and then selects the most appropriate ignition timing settings from within the PCM's programming. The following are the most important inputs for determining ignition timing requirements

1. Engine speed.
2. Throttle position.
3. Intake air volume.
4. Engine coolant temperature.
5. Knock Sensor (KS) input.

Ignition coil primary feedback circuit, monitors the primary circuitry of individual ignition coils for the successful completion of an ignition event. The ignition trigger signal and ignition fail-safe input signal provide crucial information in the control of ignition system timing and the activation of the fail-safe function. Based on the initial timing angle, calculated from the crankshaft and the camshaft position signals, the PCM calculates the ignition trigger signal. The ignition trigger signal is then sent to the ignition control circuitry located in each ignition coil. The ignition control circuitry initiates the ignition dwell period by commanding the power transistor ON. When the PCM determines the proper time to provide spark, the PCM turns OFF the ignition trigger signal. In response to the PCM signal, the ignition coil control circuitry turns OFF the power transistor, producing a high voltage discharge from the secondary coil windings. After the spark has occurred, the control circuitry sends an ignition fail-safe input signal back to the PCM, confirming that an ignition event has taken place. If the ignition fail-safe input signal is not received by the PCM, a cylinder specific DTC will set.

Anytime the fail-safe input signal is lost, engine operation will enter the fail-safe function. If engine load exceeds certain parameters during the fail-safe function, Fuel Cut-off mode is activated and fuel injector operation is ceased. The suspension of fuel injection in the Fuel Cut-off mode protects the catalytic converter from damage. Damage can occur when excess unburned fuel reaches the converter and causes the catalyst to overheat.

EVAP Purge Valve Operation

If the EVAP purge valve does not seal properly, fuel vapors could enter the engine at an undesired time, causing driveability concerns. The PCM tests for this by commanding the vent valve closed and the pressure switching valve open causing a vacuum, in the entire EVAP system. PCM continues to operate the purge valve until the vacuum is lowered to a specific point, at which time the purge valve is commanded closed. PCM monitors the fuel tank pressure (FTP) sensor. If the PCM does not detect a drop in vacuum, or the vacuum increased beyond the specified limit, for 2 consecutive trips, the MIL will illuminate, and DTC P0441 will set.

EVAP Vent Valve Operation

If the EVAP vent system is restricted the fuel vapors will not be properly purged. When the vapor pressure rises to a specified point, the PCM commands the vent valve OFF, OPEN, and monitors the Fuel Tank Pressure (FTP) sensor signal. Vacuum will decrease rapidly because of the air allowed into the system. No decrease in vacuum or a decrease below a specified rate indicates a restriction on the air inlet side. If the PCM does not detect a decrease in vacuum, or the vacuum level does not change for 2 consecutive trips, the MIL will illuminate, and DTC P0446 will set.

Pressure Switching Solenoid Operation

The EVAP pressure switching solenoid is OFF, closed during engine warm up. with the pressure switching solenoid closed, vacuum pressure is blocked from entering the fuel tank and pressure in the fuel tank will rise. The FTP sensor will see a significant decrease in pressure in the tank only when the pressure switching solenoid is ON, OPEN, during a purge event. If the pressure switching solenoid malfunctions during this portion of the EVAP system operation, DTC P0440 and DTC P0441 may set.

During the DTC P0446 portion of the EVAP system diagnostic the pressure switching solenoid is ON. This enables the FTP sensor to see a rapid decrease in vacuum, when the air from the EVAP vent solenoid enters the tank. When the PCM commands the pressure switching solenoid OFF, the FTP sensor should see the vacuum pressure stabilize and not continue to decrease. If the FTP sensor still sees a significant decrease in vacuum, the PCM concludes the solenoid did not close and sets DTC P0446.

Catalyst System Low Efficiency diagnostic monitors the operation of the exhaust system catalytic converter. In order to control exhaust emission of hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx), a three-way catalyst converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO in the exhaust gas, converting the HC and the CO into harmless water vapor and carbon dioxide. The catalyst also reduces NOx by converting the NOx into nitrogen. The Powertrain Control Module (PCM) has the capability to monitor this process using a heated oxygen sensor (HO2S) located in the exhaust stream past the three-way catalytic converter. The PCM compares the signal from the heated Oxygen Sensor 1 (HO2S 1), located before the catalyst with the Sensor 2 (HO2S 2) located after the catalyst in order to determine if the catalyst has deteriorated. If the catalyst is functioning correctly, the HO2S 2 signal will be far less active than the HO2S 1. Both HO2S signals changing at similar rates indicates that the catalyst is deteriorating.

Voltage variations between the sensors allow the PCM to determine the performance of the TWC catalyst. When the TWC catalyst becomes less effective in promoting chemical reactions, the catalyst's capacity to store and release oxygen is generally degraded. The OBD-II catalyst monitor diagnostic is based on a correlation between the conversion efficiency of the TWC catalyst and the oxygen storage capacity of the catalyst. A good catalyst, 95 percent hydrocarbon conversion efficiency, will show a relatively flat output voltage on the post-catalyst sensor, HO2S 2. A degraded catalyst, 65 percent hydrocarbon conversion, will show greatly increased activity in the output voltage from the post catalyst HO2S 2. (Схема №9)and (Схема №10).

The post-catalyst HO2S is used to measure the oxygen storage and release capacity of the catalyst in the TWC. A high oxygen storage capacity indicates a good catalyst. A low oxygen storage capacity indicates a failing catalyst. The TWC and the HO2S 2 must be at operating temperature in order to achieve the correct oxygen sensor voltages, like those shown in the post-catalyst HO2S Outputs graphic. The catalyst monitor diagnostic is sensitive to the following conditions

1. Exhaust leaks.
2. HO2S contamination.
3. Alternative fuels.

Exhaust system leaks may cause any of the following results

1. A false failure for a normally functioning, good catalyst.
2. Prevent a degraded catalyst from failing the catalyst monitor diagnostic.
3. Prevent the catalyst monitor diagnostic from running.

The presence of HO2S contaminants may prevent the catalyst monitor diagnostic from functioning properly.

Three-Way Catalyst Oxygen Storage Capacity

The TWC catalyst must be monitored for efficiency. In order to accomplish this, the control module monitors the pre-catalyst (HO2S 1) and post-catalyst (HO2S 2) oxygen sensors. When the TWC is operating properly, the post-catalyst oxygen sensor will have significantly less activity than the pre-catalyst oxygen sensor. The TWC stores and releases oxygen as needed during the normal reduction and oxidation process. The control module will calculate the oxygen storage capacity using the difference between the pre-catalyst and post-catalyst oxygen sensor voltage levels. If the activity of the post-catalyst oxygen sensor approaches that of the pre-catalyst oxygen sensor, the catalyst's efficiency is degraded.

Stepped or staged testing levels allow the PCM to statistically filter test information. This prevents falsely passing or falsely failing the catalyst monitor oxygen storage capacity test. The calculations performed by the on-board diagnostic system are very complex. Post-catalyst oxygen sensor activity should not be used to determine the oxygen storage capacity unless directed by the service manual.

A 2-stage test is used to monitor the catalyst efficiency. Failure of the first stage of the test will indicate that the catalyst requires further testing in order to determine the catalyst efficiency. The second stage test looks at the inputs from the pre-catalyst and post-catalyst HO2S sensors more closely in order to determine if the catalyst is actually degraded. This two stage test further increases the accuracy of the oxygen storage capacity monitor. Failing the first stage test does not indicate a failed catalyst. The catalyst may be marginal or the fuels sulfur content could be very high.

Aftermarket HO2S characteristics may be significantly different from the original equipment manufacturer HO2S. An inferior HO2S may lead to a false pass or a false fail of the catalyst monitor diagnostic. An aftermarket catalytic converter that does not contain the same amount of cerium as the original catalytic converter can cause a false DTC to set. An incorrect amount of cerium in the catalyst can alter the correlation between the oxygen storage and the conversion efficiency of the TWC.

Misfire Monitor Diagnostic Operation

The misfire monitor diagnostic is based on crankshaft rotational velocity, reference period, variations. The Powertrain Control Module (PCM) determines the crankshaft rotational velocity using the crankshaft position sensor and the camshaft position sensor. When a cylinder misfires, the crankshaft actually slows down momentarily. By monitoring the crankshaft and the camshaft position sensor signals, the PCM can calculate when a misfire occurs.

For a non-catalyst damaging misfire, the diagnostic will be required to report a misfire that is present within 1000-3200 engine revolutions.

For a catalyst damaging misfire, the diagnostic will respond to a misfire that is within 200 engine revolutions.

Rough roads may cause a false misfire detection. A rough road will cause torque to be applied to the drive wheels and the drive train. This torque can intermittently decrease the crankshaft rotational velocity and cause a false misfire detection.

On automatic transaxle equipped vehicles, the Torque Converter Clutch (TCC) will be disabled whenever a misfire is detected. Disabling the TCC isolates the engine from the rest of the drive line and minimizes the effect of the drive wheel inputs (torque) on the crankshaft rotation.

When the TCC has been disabled as a result of a misfire detection, the TCC will be re-enabled after approximately 3200 engine revolutions with no misfire is detected. The TCC will remain disabled whenever a misfire is detected. This allows the misfire diagnostic to evaluate the system.

Fuel Trim System Monitor Diagnostic Operation

In order to provide the best possible combination of driveability, fuel economy, and emission control, the Powertrain Control Module (PCM) uses a closed loop air/fuel metering system. The PCM monitors the heated oxygen sensor (HO2S) signal voltage and when in closed loop adjusts fuel delivery based on the HO2S signal voltage. Changes in fuel delivery will be indicated by the long term and the short term fuel trim values that are displayed on the scan tool. The ideal fuel trim value is around zero percent. The PCM will add fuel when the heated oxygen sensor signal is indicating a lean condition. Additional fuel is indicated by fuel trim values that are above zero percent. The PCM will reduce the amount of fuel delivered when a rich condition is indicated by the HO2S. Fuel trim values below zero percent indicate a reduction in fuel. A fuel trim DTC can be set when exhaust emissions reach excessive levels because of a lean or rich condition.

In order to meet OBD-II requirements, the control module uses weighted fuel trim cells in order to determine the need to set a fuel trim DTC. A fuel trim DTC can only be set if the fuel trim counts in the weighted fuel trim cells exceed the specifications. A vehicle that has a fuel trim problem that is causing a concern under certain conditions but operates fine under other conditions may not set a fuel trim DTC. For example an engine that is idling high due to a small vacuum leak or an engine that is running rough due to a large vacuum leak may set an idle speed DTC or an HO2S DTC but not a fuel trim DTC. A fuel trim DTC may be triggered by many different vehicle faults. Use all of the diagnostic information available when diagnosing a fuel trim fault. See FUEL TRIM OPERATION table.

FUEL TRIM OPERATION