One way to get high efficiency in a piston steam engine is to use a compounded piston steam engine with steam reheat and heat regeneration. Three cylinders or more is preferable. There are many ways to go about this, but all have the same basic goal: expand high pressure steam through the engine while reheating the steam before it expands into the next cylinder - this boosts temperature (and therefore pressure) to increase engine output - then regenerate the excess heat back into the system. Thermodynamically the ideal state of affairs is heating the steam as it moves through the engine such that the steam temperature is never allowed to drop below max, fully expanding the steam down to condenser pressure, then sending the very high temperature but low pressure steam through an air preheater that is able to cool the steam down to saturation, sending the heated air to the furnace, then sending the low pressure saturated steam to the condenser (note that in practice it may not be possible to reheat the steam fully to the prior peak temperatures as this would generate steam at the final exhaust that contains so much superheat that it all could not be regenerated - so the goal is to reheat up to that point). This is basically what modern steam power plants do to approach 50% net thermal efficiency. There is no physical reason why a piston engine cannot be devised to see similar performance, but it would be a nightmare to actually build something like this. A two cylinder compound engine can see a net efficiency equal to a conventional small wood gas engine system with steam at 500 psig and less than 600F if it were a good engine and made good use of this strategy. So, it would have all the benefits of a small wood gas engine system with all the advantages of steam power (quiet, wider range of fuel, cleaner combustion, slow moving and long lasting, all heat available at condenser for ease and efficiency in cogeneration). Still, this option is not practical without access to the necessary hardware. For this reason biomass gasification for fueling internal combustion engines is the practical alternative for making use of biomass for cogeneration.
NOTE: A strategy that has been used in piston steam engine systems of the past was to transfer the heat in the system to a refrigerant part way through the cycle. This boosts the average pressure in the engine. Below a certain steam pressure, the higher friction in a piston engine can lead to diminishing returns. This is why small piston steam engines generally do not expand the steam below a certain value (generally keeping steam pressure well above atmospheric while in the cylinder). Well, this corresponds to a temperature on the order of 250F or higher. What can be done (and has been done) is to use the steam condenser to heat and vaporize a refrigerant under pressure. This high pressure refrigerant can then be used to drive another piston (or two). This keeps cylinder pressures very high to minimize friction losses, and allows for extending engine operation to much lower temperatures. I am aware of this strategy applied to a large stationary piston steam engine power plant that increased overall efficiency by about 50%. Another strategy that was used on the Titanic was to put a low pressure turbine on the exhaust of their compounded piston steam engines. This also increased efficiency on the order of 50%. Unfortunately, small turbines are not generally efficient, especially at low pressure. So, a low power (small) system could use the former strategy to boost efficiency. Still, a better strategy for very small systems is to increase efficiency by making full use of the heat from the system for other applications.
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