Commer knockers are two stroke diesel, with two crank shafts and the pistons fire against each other, bit like a flat four but with the pistons running in the opposite direction. Two stroke diesels need the super charger to run. They put a GM two stroke to shame when it comes to the "screaming" part lol! Our neighbour had one and you could hear it about 5km away when it was pulling a load out of the paddock.