Questionning Jennings! - Slanted Ports VS Straight-In Ports!

Ok, Scott and I started this discussion in private, after I questioned a post he made on a thread on another site. I wanted explanations and he offered his. Myself being involved in process dynamics most of my professional life, I had a hard time figuring out the reasoning behind the explanation Scott gave me. Scott has worked many years with small engines and made reference to Jenning's writings (known to many as one of the best 2-stroke references ever). I have several writings from Jennings and other on 2-stroke theory so I went back and read it again and actually found where he mentions this about slanted ports.

You will see in the first picture below, the subject I was questioning. Jennings offers that a port at an angle has less effective area as a "straight-in" port. I think he failed to explain this enough and left way too much to interpretation. I know I'm nobody to question Jennings, but I must say I did not agree with the statement, and wanted to start this thread to get others thinking and discussing this subject. It is very interesting!

To better illustrate my own point of view on the matter I took a piece of 0.750" ID tubing and cut one end 90 degrees and the other at 45 degrees. See pictures. I then took some measurements for the surface calculations.

Consider this tube to be our intake tract. Measure the surface at the 90 degree end exit (pi x r2)which equals to: 3.1416 x 0.375 x 0.375 = 0.442 sq inches. Now the end of the tube cut at a 45 degree angle and measure the surface at exit (pixAxB) A= smallest radius, B= largest radius... this approximates to (I took live measurements off the actual piece I cut, so it can be off a bit, but still gives a close approximation: 3.1416 x 0.375 x 0.475 = 0.559 square inches. Evidently the area of the angled cut is greater...

So you can imagine at this point, I was really beginning to question Jennings (and Scott ha ha!!)... well I went back a 5th or 6th time to re-read the text below the picture and it finally hit me! Jennings states in the very beginning "Window areas here are identical"... these are the KEY words to understanding what Jennings was saying. He's not saying that 2 ports with same bore will flow differently if angled or not...he's saying that if we want to maintain the same window opening (area) when using a slanted port, the bore will indeed need to be smaller... this is actually proven in my example above. (it's just in reverse)...

So in short, I still feel he didn't explain it clearly enough...it really leads to misinterpretation when he says that "effective area of straight in ports are greater than those entering the cylinder at an angle". This is only true if you are matching window sizes!

Marc,
By no means is the bulk of my ranting on line from down in the trenches of self discovery, but more of applied working within the rank & file of known applied designs and text on said subjects.
While in no way a designer of the modern 2 stroke, 30+ years now working within the commercial world of mass produced small 2 cycles have had the pleasure of bearing witness to many a success and failure of designs brought to market.

That said ... not shared thus far within this thread are some other dynamics that have not been shared within a public venue that are more in the grey area of self discovery and connecting the dots if you will

Can an INTAKE port of a piston port engine increase or decrease it's flow dynamics when entry angle of intake tract is altered from the typical 90* to pistons controlling face to say an upward or downward sloping angle ?

Good one... I follow you Scott, would a downward angle increase flow? Seems worth trying. Perhaps it would create a cleaner pulse? Hmmmmm

I think the downward angle will help direct the flow in the base better, especially considering the position of the crank at that point in time during it's rotation. I don't think the flow dynamics of the actual port will change though if they are both of the same nominal area measured in the port (at its smallest point). I say this because if you look at the K factor of a pipe exit (full open pipe), it will always remain a K=1, regardless of the edges (as long as they are not creating a reduction in diameter)... For example, rounded edges, square edge, sharpened edge, slant cut edge (which would be the case here)...K is always 1 at exit. Where this changes considerably however is at entry of pipe... this means a LOT for the design and shape of the edge of the carb side of the intake tract though. A little bit of gasket protruding in the port for example, can have more negative effect than one might think, at least at a theoretical level.

We need to get some thinkers in on this and also look at intake angles relative to piston skirts controlling face other than vertical alterations but side to side center line that also alters entry angle and shape of intakes ports controlling "aperture"

What I am eluding too is wave energy and how it relates to the environment in which it is contained.
If one starts seeing and relating to say a tuned pipes cones and how the wave energy behaves when subjected to changes in cross sectional area or converging / diverging angles of flow path .... one can really wonder what flow dynamics we may be missing using somewhat these same control and manipulation techniques within intake tracts ?

I think having the port on an angle may possibly allow more efficient flow of gases through the opening created by the piston skirt...the closer it gets to being parallel with the piston skirt face (again within limits of course), the less restriction and turbulence should be created as the gases pass through the opening created by the piston opening the intake port...I guess a simple analogy could be comparing a head on collision with a brick wall, as opposed to hitting it at 45 degrees!

So in short, my thoughts are that the flow of a slanted verses straight intake tract, if they are the same nominal inside diameter and same length, should remain identical when the port is wide open, but when partially restricted by the piston I think it would be more efficient for the slanted port.

Thoughts?

I know we have some deep thinkers trolling the site reading this .... get on board and contribute guys !

Marc wrote: I think having the port on an angle may possibly allow more efficient flow of gases through the opening created by the piston skirt...the closer it gets to being parallel with the piston skirt face (again within limits of course), the less restriction and turbulence should be created as the gases pass through the opening created by the piston opening the intake port...I guess a simple analogy could be comparing a head on collision with a brick wall, as opposed to hitting it at 45 degrees!

So in short, my thoughts are that the flow of a slanted verses straight intake tract, if they are the same nominal inside diameter and same length, should remain identical when the port is wide open, but when partially restricted by the piston I think it would be more efficient for the slanted port.

Thoughts?

Tons of research has been done on port flow in two strokes and how it effects power. The first, and still relevant work, was done by Alfred Jante. His research is summarized in SAE paper 680468. He developed a method of measuring scavenging efficiency in real engines under operating conditions. Gordon Blair spent years furthering Jante's work with research sponsored by Yahama. He developed more accurate methods of measuring scavenging efficiency. His transfer port layouts are still used today in the best engines. The work is presented in a long series of SAE papers and is summarized in his book "Design and Simulation of Two Stroke engines". His empirical formula for pipe design is widely available in different forms on the internet and his simulation programs were the starting point for the simpler engine simulation programs like MOTA. Below are a few of the transfer layouts tested. YAM 12, similar to the porting in most model engines, is one of the worst. YAM 14 was the best of the test series.

Lohring Miller

Too bad the board won't display the image large enough to actually read it.

Lohring Miller

Fixed the image...you just have to keep them at a max width of 1000 pixels to display properly.