Differences in the operation of steam engines with real steam and compressed air

[duncan webster 1Participant @duncanwebster1]

The original question related to conventional engines with pistons. The piston neither knows nor cares about heat content, all it knows about is pressure. During the admission phase air and steam will have the same pressure,  so same work output. During expansion air will lose pressure somewhat more quickly, so slightly less work output, but not massively. Then during compression, the air pressure will rise slightly more, so more work absorbed. If it wasn’t so cold in my dining room where the computer is lurking I’d put numbers to it. Stuart Turner engines built to drawing usually run at long cut off, so no difference air to steam.

For the same cut off and speed the volumetric flow will be similar, but as air is a lot more dense than steam the mass flow rate will be a lot higher. A second order effect of the increased density of air will be higher pressure loss in ports, but unless the engine is running fast I doubt this is significant. As I said before it is possible that at short cut off on air the ports will ice up, but I’ve never seen that

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[duncan webster 1Participant @duncanwebster1]

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[cedric 1Participant @cedric]

A steam engine is a machine for converting heat energy into mechanical energy. The specific heat of steam is about double the specific heat of compressed air. So steam can do more work. Ditto the specific enthalpy, which takes latent heat of water to steam phase change into account.

It is this extra heat that causes steam to expand and do further work after the steam inlet port is closed, moreso than air.

[duncan webster 1Participant @duncanwebster1]

The power output per stroke of a reciprocating energy is the area of the indicator diagram, less mechanical losses. Making the assumption of no wire drawing at inlet, adiabatic expansion and compression, blowdown from exhaust opening to atmospheric pressure taking 10 % of stroke the following indicator diagrams can be constructed by calculation, based on P*V^gamma being a constant (one of the gas laws). The old engine builders use 1 as the expansion index (gamma) of saturated steam. Nothing to do with initial temperature or specific heat (apart from gamma being the ratio of specific heats at constant pressure and volume). If anyone wants a copy of the calc let me know. I do know that steam is not an ideal gas, but it’s not far wrong.

Clearance volume set at 10%, which is typical of full size locos. Cutoff set at 25% of stroke, using single slide/piston valve with zero exhaust lap, the exhaust will open at 75% stroke and close again 25% before centre. You might expect 4:1 volumetric expansion with 25% but due to a combination of clearance volume and early exhaust opening you only get 85%/35% = 2.43:1. Note the area of the air diagram (blue) is smaller than the steam diagram (red), but not ‘massively’. I could go on to work it out if anyone was really interested

[Werner SchleidtParticipant @wernerschleidt45161]

Hi Duncan,

thank you for your calculations! As you mentioned the difference is not so big, but I think subjectivly noticeable.

My experience showed me by my first measurement trials with my indication setup, made with a small loco converted from 3,5 inch parts to a 5 inch locomotive with walchaerts gear, that with air pressure I was able to reduce the gear to the middle position some amount. Driving in full gear there was no difference with steam or air pressure, but with steam I was able to reduce the gear much more as with air pressure. As a side effect caused by the lower steam consumptions the boiler pressure go up and the engine increased in speed by the same throttle setting. I think this is one of the reasons that the observation came up “with steam it runs better”.

[cedric 1Participant @cedric]

Interesting, but quite technical, paper  HERE about why steam contains more energy than air, by a factor of 1.4, despite specific heat being more than double. Gets down into molecular structure and comparative compressibility, so all a bit above my pay grade . It is however based on superheated steam so perhaps more relevant to model locos so equipped more than wet steam traction engines. Still, an interesting discussion.

[duncan webster 1Participant @duncanwebster1]

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