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Port sizes

Posted by addiator 
Port sizes
October 19, 2015 07:47AM
First let me introduce myself and my problem. I am a Mechanical Engineering student, who has endeavored to build a Uniflow engine as my final project. I am trying to look at the matter thermodynamically and scientifically in general, with the focus being using the engine for co -generation, however a lack of literature available is making it all a tad difficult (I do have Stumpfs book BTW).

One thing that I have been trying to figure out is how to determine the port area for the inlet. I actually decided to go with piston valves, as it is to be a small engine - but aerodynamically designed ones to prevent excessive loss. I imagine that I could figure out the engine speed from the ports - slow enough for the friction loss not to be excessive, with the losses being possible to be determined for any assumed port area by some basic equations of fluid dynamics. Yet I wonder what would be a better way to go about it.

Another thing I wonder, is how to determine how much steam will escape by the exhaust ports and at what speed. Now, if they were to be perfect, it is easy - enough for the pressure in the cylinder and condenser to equalize. But how do I find out if there is enough time for that to happen? And finally, how can one predict the shape of the 'dip' in the end of the indicatior diagram for when the ports are uncovered?

I know that there are some people here who dealt with similar issues, so I am looking forward to your input. I have actually been told that I am over-thinking this a bit.
Re: Port sizes
October 19, 2015 08:26AM
Doble used the formula Port area = Piston area times piston speed divided by 10,000

Herreshoff used on D valves engines the height of the port = the bore diameter divided by 10 And the width = diameter times .75 to .88
You won’t find this in any books that I know of, only on his original engine drawings at the Hart nautical museum at MIT. And not on all the drawings.

Re: Port sizes
October 19, 2015 08:41AM
Lots of information on a real uniflow engine project and the forum discussion at [www.steamautomobile.com]

Lohring Miller
Re: Port sizes
October 19, 2015 07:17PM
I also built a steam engine for my engineering college senior project. I ended up finding a rule of thumb formula for port sizing in an old engineering reference handbook from the ~1900s that my professor lent me.

The formula is:

(Mean piston speed ft/min) = (Valve passage area in^2) * [(Constant ft/min) / (Piston area in^2)]

The constant is 9,000 to 15,000 for inlet, 6,000 to 7,000 for exhaust. Use exhaust constant if using single passage for both.

You can take the mean piston speed and convert it to RPM. You can also shift the equation around and find the minimum required valve passage area for a given max RPM.

For my project I did the full (ideal) thermodynamic analysis of the steam engine on a big excel sheet with macro and everything. I'll attach it for your perusal. Obviously you probably shouldn't use it verbatim in your report, and I don't guarantee to doesn't have any errors, but it might make for useful reference material.
open | download - XSteam_Excel_v2.6.xls (676.5 KB)
Re: Port sizes
October 20, 2015 06:58AM
Don’t get confused between port size and valve opening.
In the old books the port is the hole in the casting between the valve area and the piston area. Not the valve opening area.

In a double acting slide valve or piston valve engine eccentric operated valves, the valve opening is never as much as full port size only at the exhaust opening is it fully opened.

On a two valve engine such as a single acting uniflow where the inlet valve/port only lets the steam in the port would be smaller then the exhaust port. This would also apply to any four valve cylinder as on four valve mill engines.

Most of these formula’s from the early books were derived from double acting engines.
On a single acting uniflow the exhaust port can be huge. You’re primarily concerned with the inlet port and valve opening area.

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File Name File Size   Posted by Date  
XSteam_Excel_v2.6.xls 676.5 KB open | download zimirken 10/19/2015 Read message