Pulling together all of the research and thinking I've done on the question of engine efficiency over the last couple of months, here is my conclusion...
If I were to write a design spec. for the power source for my "dream-team" agricultural robot harvester/tender it would specify a hybrid design consisting of a single-shaft gas-turbine generator combination driving a super-capacitor bank.
It would come in a drop-in package that could be changed in/out with a 1/2" socket set and 3/8" flare nut wrench. The engine-generator set, the fuel tank and the super-capacitor bank would each be mounted as separate units.
But such a design is not realizable yet. To date, there are no small 1-2 kW gas turbine engines commercially produced. Though, the work being done on micro gas turbines makes one optimistic that someday they will be.
For now, if I had the machine shop to do it in, what I would do, would be to mount one of the Honda Gas Engines (showcased in a previous post) on a testbed and try to run it continuously for a few months and see how it holds up. If it does, then it would make a good second option for an agricultural robot harvester/tender's power source.
If more information comes my way, I may yet change my mind on this subject. But for now, I've gone as far on this subject as I can. So it's time to put this aspect of robot design to bed for awhile and start on something new.