Lincoln Aviator: Exhaust System - 3.0L EcoBoost / Description and Operation - Exhaust System - System Operation and Component Description

System Operation

Catalyst And Exhaust Systems

The catalytic converter and exhaust systems work together to control the release of harmful engine exhaust emissions into the atmosphere. The engine exhaust gas consists mainly of nitrogen (N), CO2 and water (H2O). However, it also contains CO, NOX, hydrogen (H), and various unburned HC. The major air pollutants of CO, NOX, and HCs, and their emission into the atmosphere must be controlled.

The exhaust system generally consists of an exhaust manifold, a front exhaust pipe, a universal HO2S, a rear exhaust pipe, a catalyst HO2S, a muffler, and an exhaust tailpipe. The catalytic converter is typically installed between the front and rear exhaust pipes. On some vehicle applications, more than one catalyst is used between the front and rear exhaust pipes. Catalytic converter efficiency is monitored by the OBD system strategy in the PCM. For additional information on the OBD catalyst monitor, refer to the description for the OBD catalyst monitor in this section.

Only 2 heated oxygen sensors are used in an exhaust stream. The universal HO2S is before the catalyst (universal HO2S11 or universal HO2S21) and used for primary fuel control while the rear HO2S is after the catalyst (HO2S12 or HO2S22) and used to monitor catalyst efficiency.

Catalytic Converter

A catalyst is a material that remains unchanged when it initiates and increases the speed of a chemical reaction. A catalyst also enables a chemical reaction to occur at a lower temperature. The catalytic converter assists in controlling the concentration of exhaust gas products released to the atmosphere. It contains a catalyst in the form of a specially treated ceramic honeycomb structure saturated with catalytically active precious metals. As the exhaust gases come in contact with the catalyst, they are changed into mostly harmless products. The catalyst initiates and speeds up heat producing chemical reactions of the exhaust gas components so they are used up as much as possible.

Light Off Catalyst

As the catalyst heats up, converter efficiency rises rapidly. The point at which conversion efficiency exceeds 50% is called catalyst light off. For most catalysts this point occurs between 246°C to 302°C (475°F to 575°F). A light off catalyst is a TWC converter that is located as close to the exhaust manifold as possible. Because the light off catalyst is located close to the exhaust manifold it achieves the required temperature faster and reduces emissions more quickly than the catalyst located under the body. Once the catalyst lights off, it quickly reaches the maximum conversion efficiency for that catalyst.

Three Way Catalytic (TWC) Converter Conversion Efficiency

A TWC convertor requires a stoichiometric air fuel ratio of 14.7 pounds of air to 1 pound of gasoline, or 14.7 to 1, for high conversion efficiency. To achieve these high efficiencies, the air to fuel ratio must be tightly controlled with a narrow window of stoichiometry. Deviations outside of this window greatly decrease the conversion efficiency. For example a rich mixture decreases the HC and CO conversion efficiency while a lean mixture decreases the NOX conversion efficiency.

For vehicles using E85 the required air to fuel ratio is 9.8 to 1. For vehicles using E100 the required air to fuel ratio is 9 to 1. Other gasoline/ethanol mixtures require a variable air to fuel ratio between 14.7 to 1 to 9.8 to 1 dependent on the percentage of ethanol content.

Exhaust System

The exhaust system conveys engine emissions from the exhaust manifold to the atmosphere. Engine exhaust emissions are directed from the engine exhaust manifold to the catalytic converter through the front exhaust pipe. A universal HO2S is mounted on the front exhaust pipe before the catalyst. The catalytic converter reduces the concentration of CO, unburned HCs, and NOX in the exhaust emissions to an acceptable level. The reduced exhaust emissions are directed from the catalytic converter past another HO2S mounted in the rear exhaust pipe and then on into the muffler. Finally, the exhaust emissions are directed to the atmosphere through an exhaust tailpipe.

Underbody Catalyst

The underbody catalyst is located after the light off catalyst. The underbody catalyst may be in line with the light off catalyst, or the underbody catalyst may be common to 2 light off catalysts, forming a Y pipe configuration.

Three Way Catalytic (TWC) Converter

The TWC converter contains either platinum (Pt) and rhodium (Rh) or palladium (Pd) and rhodium (Rh). The TWC converter catalyzes the oxidation reactions of unburned HCs and CO and the reduction reaction of NOX. The 3 way conversion can be best accomplished by always operating the engine air fuel ratio at or close to stoichiometry.

Exhaust Manifold Runners

The exhaust manifold runners collect exhaust gases from engine cylinders. The number of exhaust manifolds and exhaust manifold runners depends on the engine configuration and number of cylinders.

Exhaust Pipes

Exhaust pipes are usually treated during manufacturing with an anti corrosive coating agent to increase the life of the product. The pipes serve as guides for the flow of exhaust gases from the engine exhaust manifold through the catalytic converter and the muffler.

Heated Oxygen Sensor (HO2S)

The HO2S provides the PCM with information related to the oxygen content of the exhaust gas.

Muffler

Mufflers are usually treated during manufacturing with an anti corrosive coating agent to increase the life of the product. The muffler reduces the level of noise produced by the engine, and also reduces the noise produced by exhaust gases as they travel from the catalytic converter to the atmosphere.

Catalyst Efficiency Monitor

The catalyst efficiency monitor uses an oxygen sensor before and after the catalyst to infer the HC efficiency based on the oxygen storage capacity of the catalyst. Under normal closed loop fuel conditions, high efficiency catalysts have significant oxygen storage. This makes the switching frequency of the rear HO2S very slow and reduces the amplitude, which provides for a shorter signal length. As the catalyst efficiency deteriorates due to thermal and chemical deterioration, the catalyst ability to store oxygen declines. The post catalyst or downstream HO2S signal begins to switch more rapidly with increasing amplitude and signal length. The predominant failure mode for high mileage catalysts is chemical deterioration (phosphorus deposits on the front brick of the catalyst) and thermal deterioration.

The catalyst monitor calculates the rear HO2S signal lengths for 10 to 20 seconds during part throttle, closed loop fuel conditions after the engine is warmed up, the inferred catalyst temperature is within limits, and fuel tank vapor purge is disabled. The catalyst monitor is enabled for 10 to 20 seconds per drive cycle. When the catalyst monitor is active, the PCM commands a fixed fuel control routine. During monitor operation the rear HO2S signal lengths are continually calculated. The calculated rear HO2S signal length is then divided by a calibrated signal length, which has compensation for mass airflow. The calibrated signal length is based on the signal length of an HO2S placed after a catalyst without a washcoat. An index ratio near 0.0 indicates high oxygen storage capacity and high HC efficiency. An index ratio near 1.0 indicates low oxygen storage capacity and low HC efficiency. If the actual index ratio exceeds the threshold index ratio, the catalyst is considered failed.

Inputs from the engine CHT or ECT, IAT, MAF, CKP, TP, and vehicle speed are required to enable the catalyst efficiency monitor.

Typical Monitor Entry Conditions:

• Minimum 330 seconds since start up at 21°C (70°F)
• Engine coolant temperature is between 76.6°C - 110°C (170°F - 230°F)
• Intake air temperature is between -7°C - 82°C (20°F - 180°F)
• Time since entering closed loop is 30 seconds
• Inferred rear HO2S temperature of 482°C (900°F)
• Part throttle, maximum rate of change is 0.2 volts/0.050 sec
• Vehicle speed is between 8 and 112 km/h (5 and 70 mph)
• Fuel level is greater than 15%
• First Airflow Cell
  • Engine rpm 1,000 to 1,300 rpm
  • Engine load 15 to 35%
  • Inferred catalyst temperature 454°C - 649°C (850°F - 1,200°F)
  • Number of universal HO2S switches is 50
• Second Airflow Cell
  • Engine rpm 1,200 to 1,500 rpm
  • Engine load 20 to 35%
  • Inferred catalyst temperature 482°C - 677°C (900°F - 1,250°F )
  • Number of universal HO2S switches is 70
• Third Airflow Cell
  • Engine rpm 1,300 to 1,600 rpm
  • Engine load 20 to 40%
  • Inferred catalyst temperature 510°C - 704°C (950°F - 1,300°F)
  • Number of universal HO2S switches is 30

Six drive cycles may be required to illuminate the MIL during normal customer driving, because an exponentially weighted moving average algorithm is used to determine a concern. If the KAM is reset, a concern illuminates the MIL in 2 drive cycles.

General Catalyst Monitor Operation

The catalyst monitor duration is 12 to 30 seconds, once per drive cycle. If the catalyst monitor conditions are met, the catalyst monitor may run and complete after all of the upstream HO2S functional tests are complete and the EVAP system is functional, with no stored DTCs; however, the catalyst monitor may run and complete before the downstream HO2S deceleration fuel shut off test is complete. In this case, the catalyst monitor inspection maintenance (I/M) readiness flag may indicate complete before the O2S I/M readiness flag indicates complete. If the catalyst monitor does not complete during a particular driving cycle, the already accumulated switch/signal data is retained in the KAM and is used during the next driving cycle to allow the catalyst monitor a better opportunity to complete.

Some vehicles that are part of the low emission vehicle (LEV) catalyst monitor phase in, monitor less than 100% of the catalyst volume. Often this is the first catalyst brick of the catalyst system. Partial volume monitoring is done on LEV and ultra low emission vehicle (ULEV) vehicles in order to meet the 1.75 emission standard. The rationale for this strategy is the catalyst nearest the engine deteriorates first, allowing the catalyst monitor to be more sensitive and illuminate the MIL correctly at lower emission standards.

Some applications use partial volume monitoring, where the rear HO2S is located after the first light off catalyst can or after the second catalyst can in a three can per bank system (a few applications placed the HO2S in the middle of the catalyst can, between the first and second bricks).

Index ratios for ethanol (flex fuel) vehicles vary based on the changing concentration of alcohol in the fuel. The threshold to determine a concern typically increases as the percent of alcohol increases. For example, a threshold of 0.5 may be used at E10 (10% ethanol) and 0.9 may be used at E85 (85% ethanol). The thresholds are adjusted based on the percentage of alcohol in the fuel. Standard fuel may contain up to 10% ethanol.

The PCM calibration prevents the catalyst monitor from running on a new vehicle until 60 minutes of time has accumulated with the catalyst temperature greater than 426°C (800°F) or 483 km (300 miles) have accumulated. A replacement PCM or updated calibration does not prevent the catalyst monitor from running.

• The HO2S can be located in various configurations to monitor different kinds of exhaust systems. Inline engines and V engines are monitored by their individual bank. A rear HO2S is used along with the front, fuel control universal HO2S for each bank. Two sensors are used on an inline engine and 4 sensors are used on a V engine. Some V engines have exhaust banks that combine into a single underbody catalyst. These systems are referred to as Y pipe systems. They use only one rear HO2S along with the 2 front, fuel control universal HO2S. The Y pipe system uses 3 sensors in all. For Y piped systems, the 2 front universal HO2S signals are combined by the PCM software to infer what the exhaust oxygen content would have been in front of the monitored catalyst. The inferred front exhaust oxygen content and the rear HO2S signal is then used to calculate the index ratio.
• The MIL is activated after the first concern is detected. When a concern is detected after a KAM reset, the MIL is activated after 2 concecutive key cycles.

Integrated Air Fuel Catalyst Monitor

The integrated air fuel catalyst monitor is an on board strategy designed to monitor the oxygen storage capacity of the catalyst after a deceleration fuel shut off (DFSO) event. The monitor determines the amount of fuel needed to drive the catalyst to a rich condition when starting from an oxygen saturated, lean condition. The monitor is a measure of how much fuel is required to force the catalyst from a lean to a rich condition. The monitor runs during catalyst reactivation following a DFSO event. The monitor completes after approximately 3 DFSO monitoring events have occurred.

Particulate Filter Monitor

The PCM monitors the particulate filter for leaks in the filter substrate, as well as for a filter substrate that has been removed. The particulate filter requires preconditioning before the monitor is enabled. There are three tests that are carried out by the particulate filter monitor. The first test is a clog monitor comparing the restriction of the particulate filter to the expected restriction values, which are a function of exhaust flow. The second test is a severely clogged monitor that uses the same monitoring method as the clog monitor, but uses a higher restriction threshold. The third test is a missing substrate monitor that ensures the particulate filter has not been removed.

The particulate filter monitor is enabled and runs continuously when certain base engine conditions are met. The typical monitoring duration for this monitor is 10 seconds. Inputs from the CKP sensor, ECT sensor, exhaust gas temperature (EGT) sensor, MAF sensor (if equipped), and particulate filter pressure sensor is required to enable the monitor. The monitor entry conditions include:

• Exhaust flow between 300 - 850 m3/hour (14,126 - 40,023 ft3/hour) •
• No fuel injector concerns.

For the clog monitor test, the PCM determines a pressure threshold value for the amount of pressure that should be present in the filter for a calibrated exhaust flow rate. The PCM compares the measured pressure to the pressure threshold value. A fault filtering metric starts when the clog monitor begins to run. When the measured pressure is greater than the pressure threshold, the metric value increases. When the measured pressure is less than the pressure threshold, the metric value decreases. If the metric value at the end of the clog monitor exceeds a calibrated limit, a DTC sets, and the MIL illuminates.

The severely clogged monitor test works the same way same way as the clog monitor test, but uses a higher restriction threshold.

For the missing substrate monitor test, the PCM determines a pressure threshold for the amount of pressure that should be measured by the particulate filter pressure sensor, for a calibrated exhaust flow rate. The PCM compares the measured pressure value to the pressure threshold value. A fault filtering metric starts when the missing substrate monitor begins to run. When the measured pressure is less than the threshold value, the metric value increases. When the measured pressure is greater than the threshold value, the metric value decreases. If the metric value at the end of the missing substrate monitor exceeds a calibrated limit, a DTC sets, and the MIL illuminates.

Particulate Filter Regeneration

Particulates in the exhaust are trapped by the particulate filter. Regeneration is the process by which the exhaust gas temperatures are increased and the higher exhaust temperatures burn off the particulates in the filter. Under normal driving conditions, regeneration is an ongoing passive process. When necessary, the PCM may initiate regeneration by creating a lean condition along with retarding the spark advance, raising the exhaust temperature to regeneration conditions.

During normal vehicle operation, the PCM estimates the amount of particulates that accumulate in the particulate filter. The estimated amount of particulates is based on a number of different vehicle operating conditions, including vehicle speed, engine run time, and load. Additionally the PCM monitors the following:

• Battery voltage.
• Engine coolant temperature.
• Engine speed
• Exhaust gas temperature (EGT) sensors.
• Fuel level.
• Fuel temperature.
• Intake air temperature.
• Turbocharger condition.

Particulate filter regeneration may be initiated by the PCM or manually initiated using a scan tool.

When the appropriate conditions are met, the PCM initiates a particulate filter regeneration. Regeneration is carried out when the PCM calculates the particulate level in the filter has reached a level that requires cleaning.

The regeneration process initiates while the vehicle is driven and may continue for up to 5 minutes after the vehicle is stationary.

The PCM may continue to initiate the regeneration process until the regeneration process completes. After the regeneration process is completed the filter is sufficiently cleaned and continues to trap exhaust particulate matter.

The following conditions are considered normal while the vehicle is in regeneration. No repairs are necessary if they are present:

• Regeneration does not initiate until the engine coolant temperature is above 70°C (158°F).
• White smoke from the tail pipe during cold ambient temperatures.
• Engine responsiveness may be slightly different.
• Exhaust smell may be noticed during the initiation.
• Engine pitch may be different.
• Intake air system sound on deceleration and engine shut down may be noticed.
• Exhaust gas temperatures are elevated.

Component Description

Exhaust Gas Temperature (EGT) Sensor

The EGT sensor is a thermistor sensor. The EGT sensor is an input to the PCM and measures the temperature of the exhaust gas passing through the exhaust system. The electrical resistance of the sensor decreases as the temperature increases, and resistance increases as the temperature decreases. The varying resistance changes the voltage drop across the sensor terminals and provides electrical signals to the PCM corresponding to temperature. The PCM uses the input from the EGT sensor to monitor the particulate filter temperature. The EGT sensors are located in the exhaust system downstream of the particulate filter.

Particulate Filter

The particulate filter collects the soot and ash particles that are present in the exhaust gas. The particulate filter assembly typically consists of active precious metals deposited on a substrate filter. The exhaust gas is forced to flow through the walls of the porous substrate and exit through the adjoining channels. The particulates that are larger than the pore size of the walls are trapped for regeneration. During normal operation, particulate filter temperatures may be greater than 550°C (1,022°F). These conditions provide an opportunity for passive regeneration. During deceleration conditions, the vehicle will enter deceleration fuel shutoff and provide additional oxygen at temperatures that are sufficient to burn soot, allowing the particulate filter to regenerate passively. The precious metal washcoat promotes the regeneration of the trapped particulates through the heat generating reaction and catalyzes the untreated exhaust gas. The substrate filter is held in the metal shell by a ceramic fiber support system. The support system makes up the size differences that occur due to thermal expansion and maintains a uniform holding force on the substrate filter.

Particulate Filter Pressure Sensor

The particulate filter pressure sensor is an input to the PCM and measures the pressure before the particulate filter. The sensor is a gauge type sensor. The particulate filter pressure sensor is referenced to atmospheric pressure and is located at the exhaust system upstream of the particulate filter. At ignition ON, engine OFF the particulate filter pressure sensor pressure value reads 0 kPa (0 psi). The range of the sensor is 0-80 kPa (0-11.6 psi). The PCM calculates soot load based on the particulate filter pressure and initiates regeneration conditions when the soot load reaches a threshold.