This is not a reworked Briggs engine as the old units were. Probably the most significant change with the new units is the fact that Cummins is now manufacturing their own 999cc air-cooled engine. This is a Cummins made unit from the ground up. They use the same 999cc engine in all the air-cooled units. Because of this, all three air-cooled sizes fit into the same enclosure. The 13kW uses a smaller alternator than the 17kW and 20kW, but all three have the same footprint.