Through mechanisation, that relationship has endured. Although modern trailers can be quite sophisticated – and sport their own ECUs to control braking performance on each axle, for example – they continue to be the inferior party, receiving power, data and air signals through Suzie connections over the fifth wheel.
This makes sense. Not only is the driver sitting in the tractor, so is the engine, the drivetrain control intelligence, as well as the steered and driven axles. That ability comes at a price; tractors may cost ten times as much as do the trailers holding the goods that they haul. For that reason, while tractors need to be in constant motion to repay the investment, trailers can be left parked up, compensating for the vagaries of the logistical supply chain. In this view, trailers are dumb, empty and replaceable.
But electric powertrains are challenging the status quo, in the form of trailer e-axles. These systems collect energy from regenerative braking and store it in a battery on the trailer, which can then be used to reduce parasitic loads such as powering a fridge. BPW and Thermo King’s system is called ePower; ZF’s is eBeam; SAF’s TRAKr.
That same energy could just as easily be used to provide traction power, through the trailer e-axle. Doing so would provide a way to reduce the power demand on the prime mover, which will be much greater than any parasitic load. It might also provide useful additional traction for vehicles working off-road in difficult ground conditions such as deep mud or snow.
However, this possibility threatens the existing dominance of tractor over trailer, which makes truck OEMs uncomfortable. If the tractor is not necessarily in charge, they worry, how could the possibility of tractor and trailer working against each other be prevented?
According to BPW engineering manager Roger Thorpe, solving that dilemma will be a key element in creating more sustainable transport using e-axle systems.