5 Gas Emission Analysis

Determining Engine Efficiency through Emissions

Analyzing exhaust gas composition provides valuable insight into engine combustion efficiency and emissions performance. Carbon dioxide (CO₂) is especially useful as a quick indicator of combustion quality. CO₂ levels below approximately 12% often suggest incomplete combustion or exhaust dilution, while strong CO₂ levels (12% or higher) indicate that the engine is capable of efficient combustion.

When CO₂ is strong but hydrocarbons (HC) and carbon monoxide (CO) remain elevated, the cause of the emissions failure is more likely reduced catalytic converter oxidation efficiency rather than a major base engine mechanical problem.

Components of Emissions

Hydrocarbons (HC): Unburned gasoline that results from incomplete combustion. HC combines with NOx and sunlight to create photochemical smog.
Carbon Monoxide (CO): Partly burned fuel, indicating a rich air-fuel mixture. CO is extremely toxic and reduces oxygen in the bloodstream.
Oxides of Nitrogen (NOx): Formed at high combustion temperatures (>2500 degrees) when oxygen mixes with nitrogen. Together with HC and sunlight, NOx contributes to the formation of ozone or smog.
Carbon Dioxide (CO2): A measure of combustion efficiency, formed during complete combustion.
Oxygen combines with HC and CO to produce CO2 and water. Oxygen (O2): Supports combustion and acts as a lean indicator.

Causes of HC Emissions

Various factors can lead to elevated HC emissions: Electrical misfires: The absence of a spark leads to unburned fuel in the exhaust. Advanced ignition timing: Premature ignition results in incomplete combustion and increased HC levels. Lean air-fuel mixtures: Insufficient fuel relative to oxygen leads to incomplete combustion. Rich running conditions: Excess fuel causes incomplete combustion, raising HC and CO levels. EGR valve open at idle: Inert exhaust gas at idle increases HC levels. Low idle speeds: Insufficient airflow reduces combustion efficiency, raising HC emissions. Additional causes include leaking piston rings, burnt valves, and malfunctioning components like air injection systems or catalytic converters.

Causes of CO Emissions

CO emissions arise from incomplete combustion caused by factors such as rich air-fuel mixtures or: - A clogged PCV valve limits oxygen supply, leading to incomplete combustion and increased CO levels. - A dirty air filter reduces intake manifold vacuum, triggering a rich fuel command and raising CO emissions. - Unmetered fuel in the crankcase drawn through the PCV system elevates CO levels. - A saturated EVAP canister allows uncontrolled vapor and fuel flow into the engine, resulting in a rich mixture. Other factors that can contribute to high CO emissions include malfunctioning air injection systems, catalytic converters, or computer controls.

Causes of NOx Emissions

High combustion temperatures lead to increased formation of NOx, which is exacerbated by factors such as: - Advanced timing: Premature ignition increases pressure and temperature, leading to higher NOx emissions. - EGR valve malfunction: Inadequate exhaust gas recirculation raises combustion temperatures, further increasing NOx levels. - High-performance parts: Enhanced engine efficiency can raise combustion temperatures and pressures, resulting in higher NOx production. - Cooling system problems: Inefficient heat dissipation can also contribute to higher NOx emissions. - Low octane fuel: Lower ignition temperatures can cause premature fuel ignition, leading to elevated NOx levels. Other factors that can contribute to increased NOx emissions include lean air-fuel mixtures, carbon build-up, and the use of turbochargers or superchargers.

Causes of CO2 Emissions

CO2 levels are a reflection of combustion efficiency, and deviations from the optimal levels may indicate combustion issues. Factors that can affect CO2 levels include incomplete combustion caused by misfires or component malfunctions.

Diagnostic Tip: If CO₂ is strong (often 12% or higher) and O₂ is low, but HC and CO remain elevated, the engine is generally producing efficient combustion and the emissions failure is more likely due to reduced catalytic converter oxidation efficiency (or a catalyst being overwhelmed by rich operation) rather than a major base engine mechanical problem. Confirm by checking fuel control, misfire data, and comparing pre-cat vs post-cat exhaust readings when possible.

Causes of O2 Emissions

O2 emissions indicate unburned oxygen and can act as a lean indicator. Reasons for high O2 levels include lean conditions, misfires, air injection issues, or exhaust leaks.

5 Gas Emission Analysis

Determining Engine Efficiency through Emissions

Components of Emissions

Causes of HC Emissions

Causes of CO Emissions

Causes of NOx Emissions

Causes of CO2 Emissions

Causes of O2 Emissions

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