Development of Improved Loaded-Mode Test Procedures for Inspection and Maintenance Programs

The California Bureau of Automotive Repair (BAR) and Sierra Research, Inc. (Sierra) have evaluated a large number of alternative emissions test procedures and developed new procedures that could substantially increase the ability to identify defective vehicles under vehicle inspection and maintenance (I/M) programs. The primary focus of the study was to determine whether an economical “loaded mode” (dynamometer-based) test procedure could be developed that would accurately identify vehicles with emissions in excess of the applicable standards using the Federal Test Procedure (FTP).

The results of the BAR/Sierra study show that, using a new “Acceleration Simulation Mode” (ASM) test, 90% of all excess oxides of nitrogen (NOx) emissions can be identified without the use of transient testing or mass emission measurement. The optimal steady-state test for NOx emissions involves operating the vehicle on a chassis dynamometer at 15 mph with the dynamometer set to absorb about 8-20 horsepower, depending on the weight of the vehicle. The recommended emissions standards for the new test mode are a function of vehicle weight.

The BAR/Sierra study also indicates that loaded mode testing is not as effective for identifying excess emissions of carbon monoxide (CO) and hydrocarbon (HC). Excessive CO emissions are best identified by a 2500 rpm (no-load) test with standards that are a function of either vehicle weight or engine displacement. With such standards, the 2500 rpm test can also predict excessive HC emissions reasonably well. In contrast, the idle test mode has been shown to be poorly correlated with the FTP for all three pollutants.