This is very unusual, to see a British Commer at work in the US. But it was during the great import boom, so I suppose some trucks made the trip across the pond too. These were referred to as “Commer Knockers” due to the sound the gear train driving to the blower made when it aged. The engine is extremely unusual, and it’s probably best if you just watch the video below that explains it better than I can in words.
Three horizontal cylinders with twin opposed pistons working rocker shafts and connecting rods to turn the crankshaft below it. Opposed cylinder diesels are not really that unusual; Fairbanks Morse was famous for theirs that powered subs, ships and locomotives. They still build them. But they have crankshafts on both ends.
Commer didn’t come up with this on their own. This design has a complicated history. Here’s a patent from 1934 by Ransom E. Olds. But he didn’t really invent it either.
Its origins go back to the illustrious Swiss firm of Sulzer, which developed the ZG9 engine prior to WW1. Apparently they licensed it to a Hill Diesel Engine Co. in Lansing, MI for marine use. In 1924, R.E. Olds bought Hill, and decided to…ah…take out a patent on it for a truck diesel engine, as diesels were seen as the next new thing for trucks, and of course his firm was REO, a builder of trucks. There were some differences though: the Sulzer used a large single piston (upper left “Kolben Kompressor” to scavenge the two cycle engine. Olds used an external blower, apparently not shown on his patent drawing.
The Hill engine was used in stationary applications, but the Olds design never made it into trucks.
But Commer (Rootes) was also inspired by the Sulzer, and its engine was used in large numbers of their trucks in the 50s and 60s. Here’s one starting up.