Anyone who has built a table engine or grasshopper beam engine, will know that they slow down on the up stroke and accelerate on the down stroke, due to all the iron-mongery hanging on the crank pin. I have a suggestion to mitigate this. These engine should all have at least a boiler feed pump, and probably and air pump. If a small tank of fluid were installed somewhere out of sight, the feed pump could draw from this unimpeded on the up stroke, and then discharge back into the tank on the down stroke via an adjustable valve to provide a load. If using the air pump, then air might work. The adjustment would probably only be correct for a specific setting of the steam valve but this is not really a problem. It would have to be an open system, as, if you try to connect the suction to the delivery to "tidy it up", then it would provide the same load to each stroke, which is not what you want. Of course, all of the above would only really be suitable if you are intending to run your engine on air. What do you think?
Regards Maurice