Principle: Recirculates a portion of the exhaust gases back into the
intake manifold.
Function:
Lowers combustion temperatures.
Reduces NOx formation.
Operation:
An EGR valve, controlled by the ECU, regulates the amount of
exhaust gas recirculated.
Recirculating inert exhaust gases reduces the amount of oxygen
available for combustion, lowering the peak combustion
temperatures.
Impact: Significantly reduces NOx emissions.
2. Lambda Sensor (Oxygen Sensor):
Principle: Measures the oxygen content in the exhaust gases.
Function:
Provides feedback to the ECU for precise air/fuel ratio control.
Ensures optimal catalytic converter operation.
Operation:
Generates a voltage signal that varies with the oxygen content
in the exhaust.
The ECU uses this signal to adjust the fuel injection, maintaining
the air/fuel ratio close to the stoichiometric point (approximately
14.7:1 for gasoline).
Impact:
Optimizes combustion efficiency.
Reduces CO, HC, and NOx emissions.
Allows the catalytic converter to operate at its most efficient
point.
3. Oxygen Sensor (Same as Lambda Sensor):
Terminology: "Lambda sensor" and "oxygen sensor" are generally
used interchangeably.
Types:
Zirconia sensors: Generate a voltage signal based on the
oxygen difference between the exhaust gas and ambient air.
Titania sensors: Change electrical resistance based on oxygen
content.
Wideband sensors: Provides a more accurate and linear
output over a wider range of air/fuel ratios. This is used in more
modern vehicles.
Location: Typically located in the exhaust manifold before and/or after
the catalytic converter.
Function:
The sensor before the catalytic converter provides feedback for
air/fuel ratio control.
The sensor after the catalytic converter monitors the catalytic
converter's efficiency.
Importance:
Essential for closed-loop fuel control.
Vital for minimizing emissions and ensuring optimal engine
performance.
Catalytic Converters
Work in conjunction with the oxygen sensors to convert harmful exhaust
gases into less harmful substances.
They use precious metals (platinum, palladium, rhodium) as catalysts.
They reduce CO, HC, and NOx emissions.
Overall Emissions Control Strategy
The ECU, using input from the oxygen sensor and other sensors, constantly
adjusts the air/fuel ratio to maintain optimal combustion and catalytic
converter operation.
The EGR system reduces NOx emissions.
The catalytic converter further reduces harmful emissions.
Catalytic Converters
Function:
Catalytic converters are devices within the exhaust system that convert
harmful pollutants into less harmful substances.
They use precious metals (platinum, palladium, and rhodium) as
catalysts to facilitate chemical reactions.
Process:
Oxidation:
Platinum and palladium catalyze the oxidation of carbon
monoxide (CO) to carbon dioxide (CO2) and hydrocarbons (HC)
to CO2 and water (H2O).
Reduction:
Rhodium catalyzes the reduction of nitrogen oxides (NOx) to
nitrogen (N2) and oxygen (O2).
Types:
Three-way catalytic converters: Used in gasoline engines, they
perform all three reactions (oxidation and reduction).
Two-way catalytic converters: older type, that only oxidized HC and
CO.
Importance:
Significantly reduce harmful emissions, helping to meet environmental
regulations.
They are a vital component of modern emission control systems.
Lambda Sensor (Oxygen Sensor)
Function:
The lambda sensor (also called an oxygen sensor) measures the
oxygen content in the exhaust gases.
It provides feedback to the engine's ECU (Electronic Control Unit) to
regulate the air/fuel mixture.
Operation:
Generates a voltage signal that varies with the oxygen concentration.
The ECU uses this signal to adjust the fuel injection, maintaining the
air/fuel ratio close to the stoichiometric point (approximately 14.7:1 for
gasoline).
Location:
Typically located in the exhaust manifold, before and/or after the
catalytic converter.
Importance:
Ensures optimal catalytic converter operation.
Maintains efficient combustion and reduces emissions.
The sensor placed before the cat, is used to control the air fuel ratio.
The sensor placed after the cat, is used to monitor the cats efficiency.
EGR (Exhaust Gas Recirculation)
Function:
The EGR system recirculates a portion of the exhaust gases back into
the intake manifold.
This reduces NOx emissions by lowering combustion temperatures.
Operation:
An EGR valve, controlled by the ECU, regulates the amount of exhaust
gas recirculated.
Reduces NOx emissions, a significant contributor to smog.
It is most active during steady state cruising.
Oxygen Sensor Interaction
Feedback Loop:
The lambda sensor provides real-time feedback to the ECU about the
oxygen content in the exhaust.
The ECU uses this information to adjust the fuel injection, ensuring the
air/fuel mixture is optimal for the catalytic converter.
Catalytic Converter Efficiency:
A second lambda sensor, located after the catalytic converter, monitors
the converter's efficiency.
It compares the oxygen content before and after the converter to
determine if it's functioning correctly.
EGR and Oxygen Sensors:
When the EGR valve opens, it changes the oxygen content of the
intake air.
The oxygen sensors detect these changes and the ECU adjusts the
fuel mixture accordingly.
The oxygen sensors help the ECU to maintain the correct air/fuel ratio,
even with EGR operation.
In Summary:
Catalytic converters, lambda sensors, and EGR work together to reduce
harmful emissions.
Lambda sensors provide crucial feedback for air/fuel ratio control.
EGR reduces NOx emissions by lowering combustion temperatures.
The catalytic converter then finishes the job of cleaning the remaining
gasses.
Computerized Diagnostics Operations
Common Faults/Codes (DTCs):
Diagnostic Trouble Codes (DTCs) are the language of modern vehicle
diagnostics. They are generated by the vehicle's onboard computer
system when it detects a malfunction.
Understanding these codes is essential for efficient troubleshooting.
Key Aspects:
OBD-II Standardization: The OBD-II (On-Board Diagnostics II)
standard has created a uniform system for accessing and
interpreting these codes, making it easier for technicians to
diagnose problems across different vehicle makes and models.
Code Structure: DTCs follow a specific format, which reveals
information about the affected vehicle system. For example:
The first character (letter) indicates the system (P for
Powertrain, B for Body, C for Chassis, U for Network).
The subsequent digits provide more specific details about
the fault.
Common Fault Areas:
Engine Misfires (P0300 series): Often related to
ignition, fuel, or compression issues.
Oxygen Sensor Problems (P01xx series): Indicate
issues with the oxygen sensors, which are crucial for
emissions control.
Mass Airflow (MAF) Sensor Issues (P0100 series):
Affect the engine's ability to measure incoming air.
Evaporative Emission Control System (EVAP) Leaks
(P04xx series): Indicate leaks in the system that controls
fuel vapors.
Importance:
DTCs provide a starting point for diagnosis, guiding technicians
toward the affected systems.
They help to reduce diagnostic time and improve accuracy.
OBD-II systems are designed to monitor
emissions-related systems and ensure compliance with regulations.
Internet:
The internet has revolutionized automotive diagnostics, providing
technicians with access to a wealth of information.
Uses:
Code Definitions and Troubleshooting: Online databases and
forums provide detailed information about DTCs, including their
possible causes and solutions.
Technical Service Bulletins (TSBs): Manufacturers release
TSBs to address common problems and provide repair
procedures. Technicians can access these documents online.
Wiring Diagrams and Repair Manuals: Online resources
provide access to wiring diagrams and repair manuals, which
are essential for troubleshooting electrical and mechanical
problems.
Online Forums and Communities: Technicians can connect
with other professionals online to share knowledge and seek
advice.
Software and Firmware Updates: Scan tool manufacturers
and vehicle manufacturers provide software and firmware
updates online, ensuring that diagnostic tools are up-to-date.
Impact:
The internet has made it easier for technicians to access the
information they need to diagnose and repair vehicles.
It has also facilitated the sharing of knowledge and best
practices within the automotive repair industry.
