I want to build a small (model, but not exactly miniature) steam engine, but the potential risks of dealing with high-pressure steam are sort of putting me off (especially in the context I want to use it - to power a boat).
So I was thinking about a Newcomen-type engine, where the cylinder is filled with low-pressure steam which is then condensed, creating a partial vacuum, and atmospheric pressure pushes the piston inwards. As I understand it, the engineering and construction of these machines can be simpler, which suits my skills, workshop and budget.
These were pretty much the first practical steam engines, and they worked well in context - providing a long-pull linear output, which would be directly connected to a pump and used to lift water out of mines, etc.
But is it possible to build an engine operating on the atmospheric principle, but with the output converted to a steady rotary motion? I’m thinking maybe not - Newcomen engines seem to have a very long stroke which doesn’t all happen at the same speed. Has anyone tried this?
Last year i saw a Newcomen model made of wood and running off a domestic vacuum cleaner as power. Yes it had a long and very hesitant stroke not ideal for conversion to continuous rotary motion. Each stroke was a collection of valves being banged open and closed so it was nosey too!
Highly impractical, the power density, lbs/hp, is very low. You’ll also have trouble keeping the engine from reaching a temperature equilibrium. That was the problem that Watt solved. You might want to consider a Stirling Cycle engine as an alternative. Not necessarily practical, but somewhat more reasonable to expect a working engine to result.
I was thinking it might be possible to overcome that by making the piston and block out of a good insulator such as solid Nylon - that way, only the gases inside would be cooled, however, draining the condensed water is also a problem, as there doesn’t actually seem to be any part of the cycle where the internal pressure is greater than atmosphere (even when it’s filling with steam, the piston is being pulled down under load)
Yeah, I think I need to rethink my plan.