Understanding Gasoline Engine Combustion and Emission Control Systems
Gasoline Engine Combustion and Emission Control Systems
Antiknock Index and Fuel Additives
The antiknock index measures a fuel’s resistance to detonation, indicating its ability to withstand compression without premature ignition. Gasoline additives serve several purposes, including protection against aging, corrosion, freezing, and intake system cleaning.
Challenges in Achieving Perfect Combustion
Modern engines face challenges in achieving perfect combustion due to varying demands and operating conditions. The air-fuel ratio needs adjustments, sometimes resulting in oxygen deficiency or excess. High engine speeds reduce the time for chemical reactions, leading to the formation of nitrogen oxides and incomplete combustion.
Types of Air-Fuel Mixtures
- Stoichiometric: 15 parts air to 1 part gasoline (ideal ratio)
- Rich: 12-13 parts air to 1 part gasoline (excess fuel)
- Lean: 15-16 parts air to 1 part gasoline (excess air)
Combustion Products and Gas Analyzers
Combustion produces both toxic and non-toxic gases. Toxic gases include carbon monoxide, nitrogen oxides, hydrocarbons, and soot particles. Non-toxic gases include sulfur dioxide, nitrogen, oxygen, water, and carbon dioxide. Gas analyzers measure these gases to assess engine performance and emissions.
Opacity Meter
An opacity meter measures the opacity or blackness of exhaust fumes, indicating the presence of particulate matter.
Engine Modifications for Improved Combustion
- Combustion chamber design: Irregular surfaces and high turbulence promote hydrocarbon emissions, while compact chambers with smooth surfaces reduce octane requirements.
- Heated intake manifolds: Reduce warm-up time and improve fuel vaporization.
- Variable valve timing: Optimizes valve opening and closing times for better efficiency and lower emissions.
- Variable intake manifolds: Adjust intake runner length based on engine speed for optimal airflow.
Exhaust Gas Recirculation (EGR) System
The EGR system recirculates a portion of exhaust gases back into the intake manifold to reduce nitrogen oxide emissions. The system considers engine speed, fuel flow, air flow, temperature, and pressure.
Pneumatic vs. Electric EGR
Pneumatic EGR systems use vacuum to control the EGR valve, while electric EGR systems use a solenoid valve controlled by the engine control unit (ECU).
Secondary Air Injection System
This system injects fresh air into the exhaust stream to promote further oxidation of pollutants, especially during cold starts when the catalytic converter is not yet at operating temperature.
Lambda Sensors
Zirconia vs. Titanium Lambda Sensors
Zirconia sensors only vary their internal resistance, while titanium sensors generate a voltage signal. Titanium sensors offer advantages such as faster response, higher resistance to lead poisoning, and quicker warm-up times.
Broadband Lambda Sensor Components
A broadband lambda sensor consists of a measuring cell and sensor electronics.
Broadband Lambda Sensor Operation with Rich Mixture
When the mixture is rich, the measuring cell voltage tends to decrease due to low oxygen content. To maintain a stable voltage, the sensor pumps oxygen ions into the measuring chamber, resulting in a current consumption that the ECU interprets as a specific lambda value.
Catalytic Converters
- Ceramic: Consists of a ceramic monolith with numerous channels for exhaust gases to pass through. Prone to breakage and backpressure issues.
- Metallic: Constructed from coiled sheets forming a spiral. Contains precious metals (palladium, platinum) for catalytic reactions. Also susceptible to backpressure problems.
- Metallic Radial Flow: Wedge-shaped design based on the Venturi effect to promote exhaust gas flow and reduce backpressure.
- Modular Metallic Substrate: Improved version of radial flow design with modular wedge-shaped sections for better performance and lower backpressure.
Catalytic Reactions
- Reduction: Removal of oxygen from exhaust gas components.
- Oxidation: Addition of oxygen to exhaust gas components.
Canister
The canister absorbs gasoline vapors from the fuel tank to prevent their release into the atmosphere.
Particulate Filter
The particulate filter traps soot particles from the exhaust gases. Components include a differential pressure sensor, fuel additive device, engine control unit, and gas temperature sensor.
Particulate Filter Load Levels
- Perforated filter
- Filter regeneration
- Intermediate zone
- Loaded filter
- Overloaded filter
- Clogged filter
Particulate Filter Cleaning and Maintenance
Particulate filters undergo automatic regeneration periodically during normal vehicle operation, typically every 400 to 1000 km, to burn off accumulated soot particles.
EOBD and Fault Codes
The European On-Board Diagnostics (EOBD) system, similar to OBD-II, monitors engine and emission control systems for malfunctions. Fault codes are generated when issues are detected, with each code providing information about the affected system and component.