Port Width Calculator

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Marty Davis

Marty Davis

Well-Known Member
Joined
Sep 25, 2004
Messages
2,445
I am working on the revision to the Engine Analysis Software and need some help with a calculation.

Imagine a liner that has an outer diameter of 1".

If I measure across the transfer port with a set of calipers the width is really wider than the measurement if the curve of the liner is taken into consideration. I want people to be able to measure the port width with calipers, put that into a simple spreadsheet and have it tell them the REAL port width. Here is a simple sheet that needs to have that formula put into it.

http://rcboat.com/PortWidthCalculator.xls

OR, if you want to just put the formula in a reply that would also be great.

Getting closer with the new version with Time/Angle/Area. Just cleaning up a few little tweaks and then write the instructions. This will be a huge advance for all of us to optimize our engines.

If you can help, I would appreciate it......
 
Last edited by a moderator:
that's what i was thinking. just a pita figuring how many degrees the port width is. must be an easier way.........
 
Tom Foley said:
Call me simple but how can a straight measurement be longer than a curved measurement from the same point A to point B ?
Click to expand...
Straight measurement is shorter. I want the long arc measurement calculated from the short measurement.
 
I think Marty is measuring straight across the port and wants to know what the segment length (part of a circle) of that port is, the "curved" distance at the radius of the cylinder.
 
Paul Traufler said:
I think Marty is measuring straight across the port and wants to know what the segment length (part of a circle) of that port is, the "curved" distance at the radius of the cylinder.
Click to expand...
It is a little trickier than that. With the port having thickness, the micrometer is touching the inside of the liner straight across the port and the true arc is the outside of the of the liner arc. Makes it a little more complex.
 
Last edited by a moderator:
Here ya go:

http://en.wikipedia.org/wiki/Circular_segment

Looks like you need the angle, cosine law will find it:

http://en.wikipedia.org/wiki/Law_of_cosines

So to get the angle (call it A):

Cord(width of exhaust)*2 = radius(1/2 bore)*2 + radius*2 - 2(radius)(radius)cosA

so:

angle A = inverse cosine of :(1/2bore*2 + 1/2 bore*2 - exhaust width*2) / 2 x 1/2bore*2

The arc length (what you're after)S = R (1/2 bore) x A(in radians)

or

S = pi x r(1/2bore) x angle in degrees / 180

or

S = 0.01745 x angle in deg x r(1/2 bore)

Just measured (roughly) a CMB 67, width across exhaust about 0.880, bore about 0.966:

A = inverse cos (0.233 + 0.233 - 0.774) / 2 x 0.233

angle A = 131.4*

Arc length = 0.01745 x 131.4 x 0.483

= 1.11"

Sounds reasonable. :rolleyes:
 
Last edited by a moderator:
After drawing the liner with port http://rcboat.com/arc_measurement.jpg it is obvious that the arc that is wanted is the inner arc as that is the effective opening of the port. The effective width of the port if flat like a piece of paper.

So, the measurement of the width of the port using calipers as shown will yield the length of the arc on ID, which is the arc created by the diameter of the bore.

This measurment is complex for me but should be simple for the engineers. :)
 
Last edited by a moderator:
I just got back a reply from Dr Brian Callahan that indicates that there is a heated debate in the two stroke world whether the effective port opening is the unrolled opening or the caliper type measured opening. He is bringing home an SAE Research Paper on the subject and will study it and report back to me.

I will let everyone know what he says on the matter. VERY INTERESTING....

Just thinking out loud, it kind of makes sense that the effective port opening might be something other than that arc and might be closer to the caliper measured opening.

This will be essential in getting time area calculations in the new Engine Analysis Software ACCURATE in terms of EFFECTIVE PORT AREA. B)

Thanks for the several replies, as is the case in much of our hobby, it is more complicated than originally thought.
 
Last edited by a moderator:
MartyDavis said:
I just got back a reply from Dr Brian Callahan that indicates that there is a heated debate in the two stroke world whether the effective port opening is the unrolled opening or the caliper type measured opening. He is bringing home an SAE Research Paper on the subject and will study it and report back to me.

I will let everyone know what he says on the matter. VERY INTERESTING....

Just thinking out loud, it kind of makes sense that the effective port opening might be something other than that arc and might be closer to the caliper measured opening.

This will be essential in getting time area calculations in the new Engine Analysis Software ACCURATE in terms of EFFECTIVE PORT AREA. B)

Thanks for the several replies, as is the case in much of our hobby, it is more complicated than originally thought.
Click to expand...
Marty putting a piece of paper on the out side of the sleeve and making a impression with a pencil. then lay the paper flat and take the measurements should give a very good result.

David
 
I am just a dumb hillbilly but what about finding an o-ring that will drop in thru the top of the sleeve without compression to just above the widest points of the port then take a sharp exacto knife and cut the o-ring on each side at the port width, lay that segment out flat and measure it?

For example I calipered Marty's drawing: point a to point b measuring 2.40 and then found an o-ring that was the same size as the inner circle and measured that to be 2.10 in a straight line.
 
Last edited:
Dr Brian Callahan replied to me with a paper that was written by a guy that he attended QUB with. Brian said that he would study this over the weekend and see what I should do.

He said a quck glance indicated that the roll out system was the correct way he thinks.

This might be of interest to you. http://rcboat.com/Port_Paper.pdf
 
MartyDavis said:
Dr Brian Callahan replied to me with a paper that was written by a guy that he attended QUB with. Brian said that he would study this over the weekend and see what I should do.

He said a quck glance indicated that the roll out system was the correct way he thinks.

This might be of interest to you. http://rcboat.com/Port_Paper.pdf
Click to expand...

Marty, Thanks

You have just given me enough food for thought to keep me going for many hours. As always you are the man.

David
 
However, when you are measuring an engine for a simulation program or a time area calculation, you need to use the method the program is based on. The program I use, EngineMod2T, uses the flow width. That can be different than either the arc width or the cordial width.

Lohring Miller
 
lohring said:
However, when you are measuring an engine for a simulation program or a time area calculation, you need to use the method the program is based on. The program I use, EngineMod2T, uses the flow width. That can be different than either the arc width or the cordial width.

Lohring Miller
Click to expand...
Lohring,

have you checked the coding behind the program?

if i was programming for engineers i would also make them input the straight measurement.

it is not trust worthy to let people measure the arc length as the might need to resolve to tools with large margins of error.

then if it is assumed the arc length is the value needed for the sim it is easy to calculate this in the background.

the user normally doesn't get to see the coding so he might not know better then the straight value is the one to look for.

depending on the amount of output i guess it should be not to hard to check what the program really uses if you wanted to.

@marty,

nice paper by the way i am studying it tonight.
 
lohring said:
However, when you are measuring an engine for a simulation program or a time area calculation, you need to use the method the program is based on. The program I use, EngineMod2T, uses the flow width. That can be different than either the arc width or the cordial width.

Lohring Miller
Click to expand...
Lohring. Is the calculation for flow width in EngineMod2T built in to the program as it is in MOTA or do you have to calculate the flow width as a separate entity?

Dave
 
EngineMod2T is mostly used by advanced two stroke tuners. it was developed by a student of Dr. Blair and has had a lot of input from experienced two stroke tuners. They mostly use it to refine the details of existing engines. Simulations really save a lot of time in pipe development. It's a lot easier to measure the straight width on real engines. Like you suggested, the program can convert that dimension to whatever length the programmer wants to use. Port up and down angles also effect the port area.

Lohring Miller
 
I am thinking that the velocity of the port and the shape of the runner will make the most difference on the cyl charge. Even if you set the balance of intake to ex port size it will not tell the hole story.

The length and shape of the runner will determine the amount of flow and velocity to the cylinder. If you change any of theses things the calculation of port balance will change.

I think that the shape and length of the runner will determine more in the way of cylinder charge. A shorter distance from the case volume to the port exit in to the cylinder with out the divergence of two flow paths has worked good for me so far.

David
 
Simple engine simulators don't deal with scavenging flow directly. That's a very tough problem. Simulators model the pulsating flow between the various chambers in an engine that are connected by passages like the transfer passages. All the port dimensions do is measure an area to use in flow calculations. I have no idea what actual width was used to get the flow coefficients the program uses. I just take the measurements in the way the manual instructs. MOTA needed some external calculation to get the width the program wanted. EngineMod2T doesn't.

Lohring Miller
 

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