Truck History: Freightliner Power Dolly – A Helper Engine On A Second Semi Trailer


When you spend as much time as I have lately at old truck forums, you inevitably run into some new and unusual things, like this Freightliner Power Dolly, created to provide additional thrust on a second semi trailer when two or three trailers were a bit much for the tractor in the mountains in Oregon.

Here’s a closer look at it. It’s got a 130 hp Cummins V6 diesel pushing through a torque converter and then directly to the axle differential. That’s to eliminate a clutch and gears, for obvious reasons. The controls for it in the cab had only three positions: 1) full throttle for starting off, 2) 80% throttle for running when power from the dolly was desired, and 3) idle. Applying the brake automatically kicks it into idle. And then apparently it shuts off, presumably to start up again when needed.

Here’s another look at one installed.

And with its fairings in place.

As per a comment by a knowledgeable poster:

The Power Dolly was created at the request of Bend Portland Truck Lines who wanted to begin hauling Triples between Bend and Portland but didn’t have the traction over the passes. Testing was done not only by Bend Portland, but also CF and Garrett.

Here’s a picture of one of the CF test unit heading up one of the snowy Oregon passes. Looks familiar.

From the same commenter:

The information from the testing was that the combination was more stable in the snow since the tractor drive axles were not operating so near their slip limits, and with the power dolly never approached its slip limits.  Despite this, I have found no information that Freightliner ever sold any commercially.

I’ve read more recently about some ideas of having trailers with batteries and their own EV drive trains, but there’s some issues with that, as a lot of capital in the batteries and motors would often be not in use, as trailers are not infrequently tied up at various locations.