I will have to look up the performance curve to find maximum torque speed... most likely around 80% of max speed I would guess. Maybe around 3000 rpm? Then do the math for converting either horsepower hours or kilowatt hours, and convert the resulting (either grams or pounds) into Liters. Lister always gives "specific fuel consumption" as the rating methodology - in this manner. Then you would have to scale back the figures for your 2100 rpm target figure.
I will try to remember to get out that reference manual, unless someone else has the figures handy and beats me to it. In a nutshell, I think the AC is going to burn slightly more fuel for the same output because it is a much smaller displacement engine achieving its power via much higher rotational speed. In general, the larger the displacement, the better specific fuel consumption can be achieved.
The AC1 has a total mass around 100lbs, the CS is about 700lbs or higher if using heavy flywheels. We all know which unit makes more sense for stationary power! Large mass, low-speed!
dieselgman