BACKGROUND
Powertrain developers have suggested that slip at the vehicle tire and chassis dynamometer contact point for US06 emissions testing causes unmanageable variability. In order to counteract slip, some developers have been requesting their vehicles be strapped down tighter. Strapping a vehicle down tighter may lead to unrepresentatively low fuel economy and high emissions (many tests are run FTP/Hwy/US06 consecutively).
EXPERIMENT
A study was developed to investigate the effects of dynamometer roll surface roughness and vehicle restraint strap tension on fuel economy, emissions, and the amount of wheel slip. In addition, a correlation may be established between wheel slip and fuel economy and emissions.
A three factor, two-level, full factorial design with three replicates was planned. The factors were dynamometer surface roughness, vehicle, and strap tension.
DATA
The data collected included dynamometer speed, vehicle speed, fuel economy, and emissions (HC, CO, NOx, CO2) for FTP, Hwy, and US06 test cycles. The dynamometer roll surface roughness was measured for the two test cells used, and the strap tension was set at either a tight setting or 0 tension for each test (where tight represents the most a driver could reasonably tighten the straps by hand and 0 tension means no strap was used to secure the vehicle). The slip criterion was defined as the amount of time when the vehicle speed was more than 2mph different than the dynamometer speed.
RESULTS
Results from this experiment revealed the restraint condition was not well correlated to emissions, fuel economy, or the amount of slip. The dynamometer roll surface roughness had the largest impact on the amount of slip.
A follow-up experiment was conducted with two additional vehicles that were known to have slip issues. Restraint was eliminated as a factor in this experiment. Results from this set of tests confirmed results from the first experiment; the dynamometer with the rougher roll surface showed the least amount of slip.
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