The road to full trailer electrification will be a long one, but certain technologies that will improve overall efficiency are closer than we might think. Trailers equipped with electric axles and hub motors will be able to contribute to an engine’s tractive effort, reducing fuel consumption. Vehicle stability and braking performance can also be improved by using more advanced systems.
Wolfgang Hahn, systems innovation leader at ZF Group North America, told attendees of the Canadian Transportation Equipment Association’s (CTEA) annual meeting in Victoria that it’s all about recuperating otherwise unharnessed kinetic energy from vehicle motion.
“Only a small portion of the total energy content of a liter of diesel fuel actually contributes to the forward motion of the truck,” Hahn said, calling out internal engine friction, pumping losses, and electrical loads as significant energy drains.
“By far the biggest losses to overcome are driving resistance: aero drag, tire rolling resistance, resistance to acceleration, and driving uphill — in other words, overcoming inertia,” he said. “Loss might not be the right word. Energy is never lost. It can be converted to some other form, but it’s lost to our use.”
Energy sinks such as aero drag and rolling resistance can be mitigated, but never eliminated. On the other hand, it is possible to make up some of the losses by recovering some percentage of the kinetic energy lost to heat and metal wear during downhill braking or coasting.
Hahn says there are tremendous opportunities to recuperate energy, using electric generators in trailer axles or wheels to decelerate the vehicle rather than using traditional compression and friction braking. The recovered energy can be stored in batteries and later used to ease the load on the engine during acceleration and moments of peak power demand.
“Practically, this could allow us to use less engine power and thus operate with greater fuel efficiency,” Hahn said.
Engine compression brakes or exhaust brakes are useful in resisting downhill acceleration, especially on long grades in what Hahn calls endurance braking, but they contribute nothing to a recuperative effort. However, if the primary endurance braking strategy was the trailer’s e-axle with a motor/generator, a high percentage of that kinetic energy could be easily and cost-effectively recovered, Hahn argued. The engine brake would then become the second stage for speed control during endurance braking, saving the service brakes for last.
So, what can be done with all that “free” energy?
“Typically, we see this with conventional tractors combined with trailers equipped with e-axles and batteries to store the energy,” he said. “The energy is often used to replace the engine that drives the compressor on refrigeration.”
Over the past few years, ConMet eMobility has been working to develop such a system. Called eHub, ConMet has paired an in-wheel electric motor with its ConMet PreSet Plus hub assembly to capture kinetic energy and convert it to electricity. The electricity is stored in what the company describes as a “high-capacity, lightweight battery” that sits beneath a trailer. When shared with a transport refrigeration system, ConMet claims the recuperated energy is sufficient to replace the diesel engine formerly used to drive the reefer compressor.
There are units in service now in California, through a partnership with Carrier Transicold, Great Dane Trailers, and foodservice distribution company Sysco Corp.
Such configurations have been used in Europe for the past 20 years, Hahn said, acknowledging that North American deployment at scale will be some time off.
European and North American technology pathways diverged some years ago, with European Union countries embracing electronic controls and greater tractor-trailer integration. At the same time North American truck makers decided to develop enhanced sensing and networking capabilities for their tractors.
There has been a big push here since 2019 to develop a new tractor-trailer interface that will facilitate this level of inter-vehicle integration. It’s a slow process, but nearly everyone acknowledges that we must progress beyond the J-560 seven-pin connector as soon as it’s possible and practical.
How much energy can be recuperated? It depends on the application and the duty cycle. Obviously, applications with higher numbers of sustained deceleration events will produce greater returns. In 1999, the EU established a testing regimen designed to provide a standard measurement for energy consumption and emissions in various scenarios.
Using a regional distribution model with a predetermined 100-km course with various road gradients and start/stop cycles, the test truck consumed 42 liters of fuel while producing a calculated possible recuperated braking energy of 42.3 kWh.
The test with the trailer equipped with e-axles — but not fully integrated with the tractor — consumed five fewer liters of fuel (a 16% fuel savings) and generated 33.4 kWh of energy through recuperative braking — about 79% of the calculated potential energy.
“That was with controls on the trailer only,” Hahn added. “It was a prototype, not a fully optimized vehicle. With the trailer not integrated with the engine, you’re limited to recuperating only about 50-60% of the amount of energy possible when the two units are more closely integrated.”
Ideally, the tractor and trailer would be deeply and broadly connected so that tractor powertrain controllers could manage the trailer’s energy recuperation and drive capabilities. While we’re not even that far yet, Hahn is already hyping the benefits of the ultimate offshoot of deeper vehicle integration: electronic brake controls.
“Electronic brake control for trucks and trailers would be a major step forward in vehicle stability and braking performance,” he said.
Brake by wire
Simply put, electronic brake systems would use the CAN network to control brake application rather than the pneumatic control lines used today. The obvious benefits to electronic actuation control are more precise brake force modulation, faster and simultaneous response of all the vehicle’s brakes, and greater vehicle stability during braking maneuvers.
Because of the time required to build sufficient pressure in the control line to actuate the brakes — and further, for the pressure at the brake actuator to come up to the desired application pressure — Hahn said it can take up to two seconds for the brakes to reach full application pressure at the rear-most axle on a long-combination vehicle (LCV).
“With electronic control, the trailer’s e-axle can also be included in the braking capability, which can shorten overall stopping distances,” he noted.
Hahn did not address the question of electric brake actuators versus the current pneumatic actuators. Today’s pneumatic brakes could simply and easily be controlled by electronic signals at the modulator valves rather than air pressure. Converting the actuators themselves to electro-mechanical from pneumatic is a different story.
Brake experts are, of course, looking at the possibility and testing such systems. Looking forward to the day batter-electric vehicles become more common, replacing air brakes with electric brakes would eliminate the need for air compressors and a whole lot of plumbing.
Trucks will move in this direction eventually, and Hahn’s presentation at CTEA’s annual meeting in Victoria last week just touches on the full potential of trailer electrification.
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