Motor Vehicle Emissions Variability.

Area of Science:

• Environmental Science
• Automotive Engineering
• Emissions Testing

Background:

• Test-to-test variability is a known issue in vehicle emissions testing.
• Current methods like the Federal Test Procedure and IM240 dynamometer tests exhibit variability.
• This variability is primarily attributed to the vehicles themselves, not the testing procedures.

Purpose of the Study:

• To investigate the sources of variability in vehicle emissions testing.
• To determine the impact of vehicle-specific emissions variability on Inspection and Maintenance (I/M) programs.
• To identify the characteristics of vehicles contributing most to emissions variability.

Main Methods:

• Analysis of data from multiple testing methods including dynamometer tests and remote sensing.
• Statistical evaluation of test-to-test variability across a vehicle fleet.
• Identification of vehicle populations exhibiting high emissions variability.

Main Results:

• Vehicle-specific factors are the dominant cause of emissions variability.
• Emissions variability increases with higher average emissions, particularly in newer vehicles (1981+) with defective emission control systems.
• Variable emitters represent 2.2%–4.8% of vehicles but contribute 8.5%–22% of total carbon monoxide emissions.

Conclusions:

• Multiple tests are essential for reliably identifying vehicles with significant emissions variability.
• Existing I/M programs may fail to address these high-emitting vehicles, allowing excess emissions to persist.
• Targeted testing strategies are needed to effectively manage emissions from variable-emitting vehicles.