This Bolt-On Power Unit Turns a Diesel Semi Into a Hybrid Without Touching the Engine

A San Francisco company called Revoy is betting that fleets would rather add electric power to the trucks they already own than buy new battery-electric rigs. Its product is a 4-meter, 10-tonne detachable module it calls a dolly, packed with a battery, an electric motor, and its own drive axle, which locks in between the tractor’s fifth wheel and the trailer. Coupling takes about as long as hitching a trailer, and the diesel powertrain stays exactly where it is.

That design separates Revoy from the self-powered trailer systems gaining ground in Europe, where the battery and e-axle are built permanently into the trailer itself. With Revoy, the unit is a removable link, and the depleted pack is exchanged for a fresh one at a roadside hub. Because a 575-kWh lithium iron phosphate battery is far too large to top up quickly without idling a driver for hours, swapping is central to the business model: Revoy owns and maintains the dollies and bills fleets per mile, so there is no upfront purchase.

Mechanically, the dolly is a load-bearing part of the rig. It absorbs acceleration torque so the diesel engine can stay in its efficient band, and under braking its motor becomes a generator, capturing energy that would otherwise be lost as heat. It also carries active safety software aimed at rollover and jackknifing. During a yearlong Miami pilot with Ryder System covering over 50,000 miles without a safety incident, Revoy says it cut diesel use by roughly 40 percent at the start and, after software updates, by as much as 95 percent by the end. The first commercial route, launching in Portland, Oregon, is expected to run at an 80 to 95 percent diesel reduction.

The catch is that swap hubs must exist before trucks can rely on the system over long distances, and the 10-tonne module eats into payload. Revoy has raised $41 million to date and plans a third station near Boise, Idaho, along Interstate 84, while developing a lighter, cheaper dolly. Whether that scales beyond regional corridors is the real question.