Performance on Air vs. Steam

[Slipped Eccentric]

Hello all, I've run into something that I've never experienced before. After timing and running the engine on air, the engine has plenty of power and the speed/power output increases as the throttle is opened. But on steam, the engine runs freely for a very small amount of throttle opening with very little power. Opening the throttle any further just produces a lot of noise from the stack and a reduction of wheel speed.

All of this happens with the engine blocked up and the drivers in the air. I thought it might be blowby but on steam I can grab and hold the drivers and there is no leakage of steam out of the stack. Maybe a restriction in the exhaust lines? But if so I would expect a similar performance on air.

The engine has slide valves and o-ring piston rings.

Any thoughts or ideas would be appreciated.

[Highiron]

You say you have the engine "blocked up"....are the drivers hanging or do you have spacer blocks installed to keep the at a simulated ride height and alignment?..if they are just hanging this will effect the geometry of the valve motion

[Slipped Eccentric]

The drivers are just hanging in the air. The effects are the same either blocked up or on the rails, though. Blocked up (where it was timed) or on the rails, on air, the engine is pretty square with lots of power. Blocked up (in the position the engine was timed in) and on steam, the performance is very poor, and on rails the engine will barely drag itself along.

The engine itself is a gauge 1 (g scale) locomotive with very heavily sprung drivers that move very little if at all. I've worked on and run multiple engines with the same steam line, slide valve, port size, cylinder size and exhaust line sizes that have never exhibited this behavior before.

So far I've made a new steam line from the throttle, increased the depth of the pocket in the slide valve and run wires and compressed air through any any port or tube that I can find to remove any blockage with no change in performance.

[JR May]

Off hand, have you checked the lubrication side of things? What are you using for cylinder oil? Sounds like something binding from lack of oil.

J.R.

[cbrew]

I was going to say lub also,
But, It could also be the orings are swelling too much from the heat of the steam. I bet by now, you can pull the pistons and inspect the orings for damage.

[WJH]

My guess is that the added heat of steam is causing binding. Clearances to run on air may be too tight for steam depending on materials used.

[tetramachine]

Air temperature 70-80°F. Steam temp 300-400° F, piston expansion. oring getting soft?

[cbrew]

Air temperature 70-80°F. Steam temp 300-400° F, piston expansion. oring getting soft?

that's why i said what i did about the o rings.
I can see if the clearance are "right", the o rings expanding and has to go somewhere

[Slipped Eccentric]

Pulled the cylinder heads off last night and everything was bone dry. The o-rings have flats worn or squished into them. Gave it some fresh oil and another steam and things got worse. Over the run all of the oil in the displacment lubricator was used up and by then end the engine slowed to a crawl. Pulled the heads again and dey cylinders.

The engine does run wet, could the wet steam be washing out the oil? Maybe a combination of that and too-tight o-rings?

[tsph6500]

I have learned through experience and advice that o-rings should have almost no squeeze (compression). They create a lot of drag if oversized and get worse if they swell due to mis-application.

[Fred_V]

go here and look at the dimensions. reciprocating motion is very different from static.
http://www.marcorubber.com/glands_dynam ... cating.htm

[David Powell]

If your pistons actually fit properly,ie 1 -2 thous clearance max, then you will be far better off with proper steam engine packing than O rings. The packing will hold oil almost indefinitely and if the lubricator should fail then the oil in the packing helps prevent harm. The little engine I recently bought, ( A Juliet, built just post ww2)and presumably fitted with soft packings was a bit tight to turn, due to standing for years, a dose of thin oil in the steam line a few turns by hand followed by air at 20 lbs brought her back to life with a good exhaust beat and no blowby. I have had some success with O rings but much prefer soft packings and will now always recommend them for bronze or bress cylinders. Regards David Powell.