I've been reading the RRSC "engineering manual" and also some old steam engine design books based on full scale practice.
RRSC manual states ports should be at least .4 * diameter of cylinder long, then backs into width. The .4 factor is based on making cores that will stay during a pour. Writer ends up with a port of .8" long by .162", and rounds to 7/8" x 3/16 wide for a 2" cylinder, so port area before rounding is .163". Yes, I understand piston speed fits into this, and all I can say is that the RRSC likes to run much faster than I would think needed. I've been figuring on 5 mph.
I note on the Allen Mogul the steam ports are .25" x 1" for a 2" bore, or .25 SI. Much larger.
Following the calcs in the old prototype design manuals, I see that longer, narrower ports are what comes out.
In my case, for a 1.75 bore, I get .125" x 1.3", for .1625 SI area. RRSC might have me use .7" x .17" for .157" SI.
Given that I am not using a casting, so no worries about cores, would the narrower/longer ports be an advantage or is it more trouble than it is worth?
I think I read somewhere that Allen is not coring ports anymore, so this question might apply to more folks than those who build up cylinder/chest assemblies.
Calculate several times, cut once.
thanks
steam/exhaust port sizing
Re: steam/exhaust port sizing
The reason long narrow ports were historically preferred were that they require less valve travel to open (or close) a given port area. That also means the valve gear can be made more compact as it doesn't have to create as much travel.
GWRdriver
Nashville TN
Nashville TN
Re: steam/exhaust port sizing
Hi Rudd,
What gwrdriver says is true. From the dimensions you are quoting it sounds as if this could be for a 3 1/2" gauge locomotive, or perhaps 4 3/4".
Historically, when the elongated ports didn't give enough steam flow to the cylinders, the next step was to go to piston valves which essentially wrapped the port all the way around the valve spool, thus giving greater port length while preserving the desired short valve travel. It also had the benefit of being "balanced" which eliminated the down force on the valve because of the steam pressure in the valve chest. This became even more important as boiler pressures increased.
The proper passage size between the port and the cylinder is always a trade off. Too small and there will be too much pressure drop between the port and the cylinder, but if it is too big, there will be wasted steam since while the passage doesn't contribute to the power that the cylinder delivers, volumetrically it is part of the cylinder. Ideally, it should be as short as is practical and as smooth as it can be made. If the cylinders are to be built up rather than cast, formed tubing might be a good choice.
Finally, 5 miles an hour is too conservative. Yes, it might be roughly to scale, but the two 3 1/2" gauge engines I have respectively run at a top speed of 9 and 10 mph. At those speeds, they rock and roll a bit, but they stay on the track. You don't have to run them at those speeds, but they should be capable of that because it is an indication that everything is pretty much the way it should be. Suspension is also very important.
Hope this helps a bit.
Richard Trounce.
What gwrdriver says is true. From the dimensions you are quoting it sounds as if this could be for a 3 1/2" gauge locomotive, or perhaps 4 3/4".
Historically, when the elongated ports didn't give enough steam flow to the cylinders, the next step was to go to piston valves which essentially wrapped the port all the way around the valve spool, thus giving greater port length while preserving the desired short valve travel. It also had the benefit of being "balanced" which eliminated the down force on the valve because of the steam pressure in the valve chest. This became even more important as boiler pressures increased.
The proper passage size between the port and the cylinder is always a trade off. Too small and there will be too much pressure drop between the port and the cylinder, but if it is too big, there will be wasted steam since while the passage doesn't contribute to the power that the cylinder delivers, volumetrically it is part of the cylinder. Ideally, it should be as short as is practical and as smooth as it can be made. If the cylinders are to be built up rather than cast, formed tubing might be a good choice.
Finally, 5 miles an hour is too conservative. Yes, it might be roughly to scale, but the two 3 1/2" gauge engines I have respectively run at a top speed of 9 and 10 mph. At those speeds, they rock and roll a bit, but they stay on the track. You don't have to run them at those speeds, but they should be capable of that because it is an indication that everything is pretty much the way it should be. Suspension is also very important.
Hope this helps a bit.
Richard Trounce.
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Cary Stewart
- Posts: 542
- Joined: Wed Jun 08, 2011 5:54 pm
Re: steam/exhaust port sizing
Just as an aside, the RRSC Engineering Notes came from Winton Brown's last catalog. Chet took them and made the little orange book.
Cary
Cary
Re: steam/exhaust port sizing
Yes, that is what I have. Did not know where it came from though.
This is for a smaller 7 1/2" gauge loke, cylinders and saddle are grey cast iron. Say larger than a Fitchburg but smaller than a standard Mogul. They'll be silver soldered together. The port areas I have in the drawings now proportion out to the Allen Mogul when one adjusts for piston diameter only, steam are .188 x 1 and exhaust is .375 x 1". I'd had them longer and skinnier as above - and still have that layout, but I was having second/third thoughts on the matter.
Thanks for the help.
This is for a smaller 7 1/2" gauge loke, cylinders and saddle are grey cast iron. Say larger than a Fitchburg but smaller than a standard Mogul. They'll be silver soldered together. The port areas I have in the drawings now proportion out to the Allen Mogul when one adjusts for piston diameter only, steam are .188 x 1 and exhaust is .375 x 1". I'd had them longer and skinnier as above - and still have that layout, but I was having second/third thoughts on the matter.
Thanks for the help.