Quantifying the emission benefits of opacity testing and repair of heavy-duty diesel vehicles.

The objective of this study was to begin to quantify the benefits of a smoke opacity-based (SAE J1667 test) inspection and maintenance program. Twenty-six vehicles exhibiting visible smoke emissions were recruited: 14 pre-1991 vehicles and 12 1991 and later model year vehicles. Smoke opacity and regulated pollutant emissions via chassis dynamometer were measured, with testing conducted at 1609 m above sea level. Twenty of the vehicles were then repaired with the goal of lowering visible smoke emission, and the smoke opacity testing and pollutant emissions measurements were repeated. For the pre-1991 vehicles actually repaired, pre-repair smoke opacity averaged 39% and PM averaged 5.6 g/mi. NOx emissions averaged 22.1 g/mi. After repair, the average smoke opacity had declined to 26% and PM declined to 3.3 g/mi, while NOx emissions increased to 30.9 g/mi. For the 1991 and newer vehicles repaired, pre-repair smoke opacity averaged 59% and PM averaged 2.2 g/mi. NOx emissions averaged 12.1 g/mi. After repair, the average opacity had declined to 30% and PM declined to 1.3 g/mi, while NOx increased slightly to 14.4 g/mi. For vehicles failing the California opacity test at >55% for pre-1991 and >40% for 1991 and later model years, the changes in emissions exhibited a high degree of statistical significance. The average cost of repairs was 1088 dollars, and the average is very similar for both the pre-1991 and 1991+ model year groups. Smoke opacity was shown to be a relatively poor predictor of driving cycle PM emissions. Peak CO or peak CO and THC as measured during a snap-acceleration were much better predictors of driving cycle PM emissions.