Ah. Good last post BrianH.
Now I know you are interested in actual personal usable IC engine shaft power and not just char making for the sake of it . . .
Your numbers stand up well to real in the world experiences.
IC piston engine are rule of thumb rated as 1/3 of the fuel heat converted into mechanical shaft energy; 1/3 of the fuel heat rejected out the engine cooling system; and the final 1/3rd rejected out the engine exhaust stream. Proportions will vary some with the engine design, the actual fuel used and Especially the engine load. Higher engine loads always means higher efficiencies. Important to realize that piston IC engine power is determined by the pressure/temperature differential rise in the cylinder. If you do not let out the un-to-mechanical-shaft converted heat energy the engine materials will overheat to materials failure points. If you do not let out most of the residual remaining unconverted heat and pressure energy you cannot next genrate that harvestable P/T differential on the next power cycle.
Once you step into raw tree wood fuels these “waste” gasifier and engine heats are no longer wasted as they are very much needed to dry, condition and pre-warm incoming wood fuel loads and input air. The dryer and hotter the actual gasifier incoming fuel whether raw wood or charcoal and the hotter the gasifer input air the more efficintly it will operate. It is by taking these non-ideal as individual 16% to 30% efficient processes and combining these into an intergrated system that you can then reach usable 70%+ full cycle efficiencies.
My very modern engineered made in Washington State and Washington State government certified raw wood heating stove is an actual 80% efficient at putting woodfuel heat energy to the space being heated. The other “wasted” 20% of woodfuel energy is quietly, cleanly, non-electrically and non-electronic needed draft drawing the woodstove; needed humidty and occupancy air change ventilating for a healthy house.
90%+ efficinciy systems ALL require expensive bought out refined spec grade fuels, lots of electrical pump, actuator and other motors, and electronic contols to achieve these 90%+ efficiency claimes. That last 10-15% efficincy is very expensive to obtain. Dimishing returns is very real and a finacial put into real world use system killer.
Just as you ran into local restictions contemplating using cardboard as a fuel stock, more and more places even here in the US of A if you are using wood based fuels in a CHP process it is now very resticted and directed to the point of being impractical and illegal in many places here already.
E.G. Here in Washington State ANY woodfueled HEATING device/system must pass a very specific set of lab tests using ONLY a specified predried amount of crib stacked stick form wood to be able to be “emissions” certified with a rivited on metal certification plate and be put into service. This has nothing to do with an actual closed cycle gasifier. But “der law ist der law”. I now know of actual two parties here had to cease all woodfuel based CHP work in my state ran afoul of these "local’ regulatory agencies.
So why you will not be hearing the word CHP cross my lips much anymore.
My Belgium brother-in-law living much as you do now has changed over his central oil burner and now happily space heats with commercial woodfuel pellets. He’s off that Dino fuel now. He will be expending into domestic hot water with his available woodfuel pellets next. He hopes to cut his propane denpandency then too.
Please do continue with your woodfuel engine pellet work.
He is now very interested in wood pellet fueled IC engine power for electricity as a cloudy weather supplement to his rooftop PV solar.
Ha! Ha! Operating IC piston engines on postage stamp lots with nieghbors is a whole developement project in and of itself!