Trucks with a 6×2 axle configuration are lighter in weight compared to their 6×4 counterparts and can achieve greater fuel efficiency (FE) due to reductions in frictional losses obtained by having only a single differential and the lack of an interconnecting driveshaft. The additional FE benefits range from 1.9 to 3.5%. Researchers at Daimler Trucks North America examined whether even greater FE benefits could be obtained by using the air spring suspension to shift load to a non-driven rear axle.
The researchers found that potential benefits in FE existed when transferring load from the drive axle to the non-driving axle if specific drive axle tires along with trailer tires on the non-driving axle were used. To fully understand and characterize the possible benefits, a study was conducted that used both theoretical calculations and data obtained from full vehicle track tests.
Figure 1 shows the loading of the non-driven axle (rearmost axle in a 6×2 axle configuration) to 19,600 lb, which is near the legal federal highway limit of 20,000 lb per axle. The resulting loads are on the driven (11,200 lb) and steer (12,600 lb) axles.
Observation of data, both theoretical and test track, revealed that transferring load from the driven to non-driven axle, with the proper tire configurations, leads to a FE benefit (see Figure 2). Details of system operation in the track environment indicate the real-world FE benefit for end users. Initial testing showed that for repeatable results, the tires must be warmed up consistently, as the rolling resistance of the tires changes with tire temperature. This consistent warm up is crucial for shorter track tests, but for long-haul operation, the tires should operate at a constant temperature for most of the drive.
The choice of tires for each axle is critical for good performance. The greater the difference in rolling resistance between the tires of axle(s) that the load is being transferred to and the tires of the other axles, the greater the FE delta of the system will be.
The condition of tires has a large effect on their rolling resistance. If new trailer tires are put on a dead axle on a vehicle whose driven axle tires are near the end of their lives, the well-worn tires could have a lower rolling resistance than the unworn trailer tires. In this case, transferring load to the dead axle would actually decrease fuel economy. In the future, for successful use of this road transfer technology, technicians should be informed to have the proper combination of tires on the driven and non-driven axles. Additionally, technology should be implemented that can inform the suspension control system of changes in tire conditions.
Another aspect that affected the test results was a phenomenon called longitudinal slip. This is the difference between the instantaneous tangential velocity of the tire at the road surface and the velocity of the vehicle. It is caused by the inability of the tire to have 100% traction. This factor did reduce the total fuel saved, but the loss proved to be minimal.
Daimler Trucks successfully tested a load transferring system and concept that can be implemented using an electronically controlled suspension (ECS). The system may improve the fuel economy of a vehicle with at least three axles (one driven and one non-driven) by more than 0.5% depending on the tire configuration and vehicle loading. This concept has the potential to save considerable fuel cost for commercial vehicle fleets and owners.
An ECS can already provide greater traction for 6×2 configured trucks and can maintain vehicle ride height, but the additional ability to transfer load to the non-driving axle should be a great value for fuel-conscious long-haul customers.
This article is based on SAE technical paper 2016-01-8031 authored by Nicholas Atanasov and Evan Chenowith of Daimler Trucks North America. The paper will be presented at the 2016 SAE Commercial Vehicle Engineering Congress.