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Home Model Engine Machinist > Building Them > General Engine Discussion > Sealing high speed superheated steam poppet valve stem

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Old 03-01-2013, 09:42 AM   #1
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Default Sealing high speed superheated steam poppet valve stem

As I am in pursuit of that extra mph on my flash steam tethered hydroplane I enquired on another thread about piston design and initial tests with a lightened piston motored on the lathe look promising.

I tried to run the engine on compressed air but need about 150 lbs squ ins to turn it over and would not risk altering the old homemade compressor safety valve.

I noticed there was a loss of air down the valve stem when the cam lifts poppet valve.

The valve is made from a Rover K series exhaust valve due to high loads and temperature (inlet steam line red hot) the material has to be good.

I have used Nimonic motorbike valves in the past but my supply dried up but have found the Rover type suitable.

The valve in competition opens for 54 degree's and lifts 11,000 times a minute the steam pressure can only be a guess say 1,000lbs.

There is no slop on the valve and it has labyrinth grooves on the 0.203" stem it is just over 2"long.

Considering the speed the valve is running at will the steam loss be negligible or can it be sealed better without putting resistance on the stem?

I did a test some years ago by heating the 0.315" valve head to red heat and doing a quick measurement of its length it lengthened 0.010" the stem not sure of.

The above affects the critical valve timing and tappet clearance has to be closely monitored so no cam base circle contact, the cam opens and shuts fast.

Should I leave well alone or are there a simple means to reduce valve stem leakage?

Comments welcome.

Paul


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Old 03-01-2013, 11:25 AM   #2
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Paul
I calculated that at 500 C your valve stem will grow 0.0011 inch using the tables I have for Low Expansion Steels, and that ties in with the longitudinal growth that you measured.
By using compressed air your valve stem etc will be close to ambient air temperature and with a running clearance and labyrinth grooves you can still expect some gas slippage. You dont say what the valve guide is made from. With valve guides its a case of looking at low co-efficient of expansion in conjunction with a low coefficient of friction to maintain a running clearance at temperature without excessive leakage.

Ralph


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Old 03-01-2013, 11:28 AM   #3
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I forgot to mention that the growth in of 0.0011 inches is on diameter and in reality the valve stem will not be running at any where near 500 C so I would assume that the growth is a lot less
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Old 03-01-2013, 01:49 PM   #4
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Forgot to mention the valve guide is cast iron.

Paul
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Old 03-01-2013, 03:28 PM   #5
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Something tells me that the only way to get you much faster would require a major redesign. Any chance of getting a look at one of theese engines broken down so I can get a better idea of what you have? Im interested in the ports and how they enter to the cylinder. Whats the bore and stroke of the engine? I understand if you do not want to disclose this information, after all it is racing...
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Old 03-01-2013, 11:51 PM   #6
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Paul, I found the picture of your engine in your album. Very cool!! Took me a minute to realize where the valve springs were. Have your read the article by Tom Stocker "The pheonix of steam" regaurding high pressure steam and low clearance volumes?
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Old 03-02-2013, 12:35 AM   #7
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Thank you for your comment with steam engines many things have been tried before Bob Kirtley put the best together for our side of the hobby but the steam generator is the key for the power we are getting.

With a hydroplane apart from BHP, propeller design, aerodynamics and other problems have to be resolved.

In my case it took a few seasons to get the propeller skeg angle correct then to stop it becoming an aircraft, which normally meant total destruction.

A bit of Dan Gurney technology solved that problem but increased drag.

A relative simple engine compared to some of the masterpieces members create on this forum but developement keeps the brain cells active.

I will have a look at that book.

Paul
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Old 03-02-2013, 12:46 AM   #8
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You think your engine is simple You are far too modest. The pheonix of steam was a article published in the saca's bullitin a couple of years ago. If you would like I can scan it for you and e mail it to you. I think you would find it interesting.
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Old 03-02-2013, 02:05 AM   #9
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Hi Paul
Cast iron for valve guides has a relatively low co efficient of expansion as the valve material. The Question is does it get to the same temperature as the valve stem. If not then the clearance between the stem and the valve guide bore will reduce and there will be less leakage.

Ralph


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