Air vs Steam

[bikerbob]

I am new to the forum and steam engines. I have a question regarding design of model steam engines. Air does not expand like steam in the cylinder; therefore I would assume that it would lead to design changes (port areas; lead and lag timing etc.); yet all engines seem to be designed for steam yet run on compressed air. Would someone please attempt to explain.
Thank you
Bikerbob

 

[Blogwitch]

Most of the 'easier' steam or air engines will run quite happily on both, but when running on steam, the correct steam oil is required to be used, usually an easy to make displacement oiler in the steam line will do the trick, whereas a quick squirt of engine oil or similar into the inlet line of the pressurised air will keep things running nicely.
The main problem is that the hot steam will soon find any leaks or weak spots on the engine sealing.

There are more complex steam engines that won't run on air, such as a triple expansion engine, these feed the used steam from the high pressure cylinder to the medium pressure cylinder, then when power has been extracted there, it is then passed to the low pressure cylinder to extract the last bits of power out of the steam. These are about the most economical of steam engines to run, but are very complicated to build by the beginner.

This is a very basic explanation and there will most probably be lots of other engines that won't run on just air, requiring the expansion of steam to run.

Hope it helps

John

 

[Charles Lamont]

Air does not expand like steam ...

Well, not exactly like steam, but it does expand.

As Blogwitch suggests, multiple expansion engines work best when exhausting into the vacuum of a condenser, which is not easy to do with air, but they will generally run on air quite acceptably for demonstration purposes.

You can tell an engine running on air is expanding if the cylinders get cold. This can cause condensation, and I think it is therefore a good idea to use air-tool oil in the cylinders of engines running on air.

 

[bikerbob]

Thanks for the replies;; I gather that for most model engines I don't have to worry about steam or air as a source

 

[Mechanicboy]

With air as pressure the tripple expansion steam engine works as a freeze machine. See my picture..

 

[Gronk]

Engines with a lot of cutoff don't like running on air.

A full size Stanley steam car engine (40% cutoff in full gear) runs like ####! on air. If you want to get the engine to have an even beat at low speed (which every owner wants) I found early on that the valves had to be set with steam.

Gronk