SAE International Comparison of Fuel Efficiency and Traction Performances of 6 × 4 and 6 × 2 Class 8 Tractors 2014-01-2358

The objective of this project was to compare the fuel consumption and traction performances of 6 × 2 and 6 × 4 Class 8 tractors. Two approaches have been considered: evaluation of 6 × 2 tractors, modified from 6 × 4 tractors, and evaluation of OEM 6 × 2 tractors. Compared to the 6 × 4 tractors, which are equipped with a rear tandem with both drive axles, the 6 × 2 tractors have a rear tandem axle with one drive axle, and one non-drive axle, also called dead axle. The 6 × 2 tractor configurations are available from the majority of Class 8 tractor manufacturers. The SAE Fuel Consumption Test Procedures Type II (J1321) and Type III (J1526) were used for fuel consumption track test evaluations. Traction performances were assessed using pull sled tests to compare pulling distance, maximum speed, and acceleration when pulling the same set sled on similar surface. Fuel consumption tests showed that 6 × 2 tractors consume up to 3.5% less than the similar 6 × 4 tractors, whilst pull sled tests showed shorter distance, lower maximum speed, and lower acceleration for the 6 × 2 tractors, when compared to similar 6 × 4 tractors. These differences are not critical or even not observable in normal operating conditions of on-highway transport but in specific conditions characteristic of particular northern operations, these differences could impact the performance of the tractor. However, systems are available for transferring the load from the dead axle to the drive axle in special conditions, for increasing the traction. With 6 × 2 axles, traction tires can be installed only on the drive axle, while the dead axle can be equipped with more fuel-efficient tires, which can bring more fuel savings and tire cost savings. Another advantage is the weight reduction: it was shown that the 6 × 2 tractors were lighter than the 6 × 4 tractors, and reducing a vehicle's tare weight can translate into an increase in payload, which would enhance freight transportation efficiency. At no additional cost, the 6 × 2 tractors replacing 6 × 4 Class 8 tractors could reduce annually the GHG emissions up to 1.93 tonnes per vehicle.