Correlation analysis of road load fuel economy variations by energy difference for gasoline direct injection and diesel-powered vehicles

Test flexibilities, such as the tire pressure, adjustment of brakes, vehicle preconditioning, test mass, and running-in period, are multiple factors that can confound vehicle emissions and fuel economy tests. The road load force is the most influential flexibility factor for a type-approval vehicle certification test. Because of these various factors, it was revealed that there was a substantial difference between the type-approval emissions and those from a real-driving emissions test. In this study, the test cycle road load energy using the road load coefficients determined by the TA coast-down procedure was the base condition. After calculating the test cycle road load power, the constant term of the road load coefficient was adjusted by the energy loss. The tolerance of the road load in domestic regulation is 15%, so the constant term of the road load coefficient was increased by 5% in the cycle road load energy loss. Then, the road load power and tractive power were calculated from the force and vehicle speed. The vehicle fuel efficiency, under the road load variation conditions, was determined for modern 2.4 L gasoline direct injection and lean NO_x trap-equipped diesel vehicles on a chassis dynamometer. To assess the impact of different road load values on the FE and carbon dioxide emissions, the test cycle was performed over the combined modes of the federal test procedure-75 and the highway fuel economy test. To investigate the road load variations of the vehicle fuel economy in correlation with the energy difference, the statistical approach of the one-way analysis of variance was applied. The results showed that the variations of the road load energy of the tested cycle were closely related in a trade-off relationship with the vehicle fuel economy.