cylinder design

[udontknowme]

i been trying to understand why the majority of 2t used the 4 port trans rather than 6. just not sure if i understand completely. if the total width and height was the same respectively wouldnt a 4 port always give more area than 6 ?  and if its true the dividers act as heat exchangers wouldnt it make sense to have as few dividers as possible ?

[Jerry Hall]

i been trying to understand why the majority of 2t used the 4 port trans rather than 6. just not sure if i understand completely. if the total width and height was the same respectively wouldnt a 4 port always give more area than 6 ? and if its true the dividers act as heat exchangers wouldnt it make sense to have as few dividers as possible ?

One port would give the most area with the space available around the circumference of the cylinder.

One round port of lets say 1 square inch of area will always flow more than the total flow through 2 round ports of ½ square inches each even though the total area is the same.

An efficient 2T engine must simultaneously have the highest airflow possible through the transfer ports and have the direction of the transfer ports optimized to produce the most efficient scavenging possible.

Adding a divider in a port is often necessary to gain directional control of the scavenging streams or it just may be necessary to prevent the piston or rings from snagging in the port as it passes over it. It is always a battle when designing ports as to which is most important, scavenging or airflow. Engines that have a high power average over a wide RPM range have usually found the best of both worlds for the designed/intended use.

Some company’s testing may show that they can get the best of both worlds with 4 transfers while other’s testing show 6 transfers are best for them. Some companies are known for a certain port design like Yamaha's 5 valve 4 stroke heads and may be reluctant to change even if testing show them that less valves may be better.

When Honda developed the TRX 250R engine they designed it to make about 33 HP to 34 HP. Why did they make a 33 to 34 HP engine when they already new how to make a stock CR 250 make about 40 HP? Because engineering was just filling an order placed by the marketing department.

[rsss396]

control of the flow is just as important as total flow, I thin my port dividers to try and make up for lost area and like jerry said they still willnot flow the same total cfm as 4 larger ports
But I feel the better control helps over come the flow advantage.

[ledperformance]

Direction of flow is more important than flow in two stroke. There are angles in the transfers that reduce air flow but increase hp at most rpms. That is part of reason we see power gains through out the rpm range with the Sphinx cylinders over the older designs with the same displacement. Savaging is everything!

[udontknowme]

you think 6port give wider power curve than 4port ?    really i cant think of another reason why calvin went with 6.

[Jerry Hall]

you think 6port give wider power curve than 4port ?    really i cant think of another reason why calvin went with 6.

If CP's testing had shown that 4 transfers produced better scavenging over the widest RPM range they would not have produced a cylinder with 6 ports.  Every engine manufacture has different ideas as to what is best.  The proof is in the power curve shape.  An engine that has a wide power band and high power numbers has to be doing something better than the company that has less power over a narrow RPM range.

[udontknowme]

i see what your sayin


thinking over the winter im gonna start on a new project.  got a spare cr500 sitting around so i might do a liger. another idea i had is ktm 380. its sqaure bore and stroke with case reed, 4 port trans, tripple exh, mechanical powervalve driven by centrifical timer. probly try to scale up the rsa125 specs and apply it to the 380. at a minimum i'd like to replicate the transfer ducts. whats the typical life span of epoxy inside the engine ? pretty sure ill need some if i go the 380 route

[udontknowme]

found some info regarding the original topic. still it begs the question, why are the single cylinders using the 7 port. i wonder if the 5 port was ever tested ?  be neat to hear calvins thoughts on this

Why was the 5 transfer port layout chosen for the RSA over the 7 port?
Every divider between two ducts acts as a heat exchanger: the mixture passing along them is heated by the heat in the metal. So what you would really want, is one large transfer port on each side of the cylinder. But then the ports would become too wide for the piston ring. And the directional stability of the transfer streams would suffer. Five ports is the best compromise.
Citation:
Will the same rules apply to my cylinder if the bore/stroke ratio is the same?
Yes.
Citation:
under what conditions do the 7 ports become preferable?
When the shape of the transfer ducts is inadequate for good directional flow stability, like in engines where the cylinder bores are too close together (overbored RD350/Banshee).

[rsss396]

You cannot compare a small bore 125cc motor to a large bore cylinder, it is much harder on a big bore to scavage the spent gasses after each power stroke. You cannot increase the port sizes and shapes in proportion to the increase of bore size and expect the same percentage of power increase.

[udontknowme]

i know the power doesnt increase directly proportionate with engine size. just trying to get a better understanding of why certain designs are used

from what i recall someone mentioned using a 78x78 case reed 5port engine and applying scaled up rsa125 specs 54x54.5 case rotary valve 5port. ill go back and read again but im sure frits said it should work well even though its 255cc difference . possibly because theres enough similarities between the engines.

sounds like with the banshee cylinders so close together the extra trans port works better and theyre over sqaure along with most puma and spynx over sqaure so maybe the 5port idea goes out the window

[rsss396]

I did not word that very well when I said power what I really should have said was results. the bigger bores IMO need more control of the airflow than the small bores, go look at 50cc motors, many of them are 2 or 3 transfers only

[Jerry Hall]

found some info regarding the original topic. still it begs the question, why are the single cylinders using the 7 port. i wonder if the 5 port was ever tested ?  be neat to hear calvins thoughts on this

Why was the 5 transfer port layout chosen for the RSA over the 7 port?
Every divider between two ducts acts as a heat exchanger: the mixture passing along them is heated by the heat in the metal. So what you would really want, is one large transfer port on each side of the cylinder. But then the ports would become too wide for the piston ring. And the directional stability of the transfer streams would suffer. Five ports is the best compromise.
Citation:
Will the same rules apply to my cylinder if the bore/stroke ratio is the same?
Yes.
Citation:
under what conditions do the 7 ports become preferable?
When the shape of the transfer ducts is inadequate for good directional flow stability, like in engines where the cylinder bores are too close together (overbored RD350/Banshee).

A 7 port scavenging system would be preferable if and only if 7 transfer ports can be arranged and angled so that that the cylinder SCAVENGES better and TRANSFERS MORE CFM SIMULTANEOUSLY from the crankcase to the volume above the piston than a cylinder with less transfer ports.

The maximum theoretical transfer area that a cylinder could have would be one transfer port around the whole circumference of the cylinder.  Port window area has a huge influence on the volume (CFM) that can flow through a port into the cylinder. Each time a divider is added to this one huge theoretical transfer port, the maximum possible flow area is reduced. When a divider is added to a port, the surface area of the port walls is also increased.  Additional surface (port wall) area reduces flow because of the frictional effects of the added surface area.  

Increasing the number of dividers does increase the heat exchange between the cylinder and the mixture flowing through the port.  Be careful in assuming that this area of the cylinder is always hotter than the air in the crankcase flowing through the ports.  The crankcase is an air compressor.  Heat is ALWAYS added to air being compressed due to the work it took to compress the air.   A supercharger or a turbo charger always adds heat to the intake air as it is being compressed.  Why do we use inter-coolers on the intake air on a forced induction engine?

We can observe the trends of two stroke engine development over the last 100 plus years just like we can observe how aircraft shapes have evolved to a point where most of the commercial aircraft look very similar regardless of the manufacturer.  I see a similar evolution in two stroke engine design where the state of the art engines are beginning to look alike. One company is not necessarily copying another company's design but independent testing by many companies are coming to similar conclusions.

As we gain experience in engine design we look back over the last 25 to 40 years of two-stroke engine development and is easier for us understand why many designs were not as good as others. We have to remember that the majority of us on this forum are working on engines where the designs are approximately 25 years old and are far from state of the art.  Some of these old engines can have there existing castings modified using some of the newer technology as well as applying much of the newer technology to the exhaust systems.

[udontknowme]

ya everytime i look at my bikes i know theyre ancient technology. got a couple cr500 and ktm500. feels like im polishing a turd tryin to make em go faster. last year i did buy a new ktm 250 and put the 300 jug on it and im real impressed with how it runs for being stock. bone stock except procircuit pipe. clearly though its more of a modern design engine. and they made better use of the available bore area for larger better designed ports. geez i should make a port map of this 250 and compare it to the 500. ill bet the 250 has dang near the same port area. i know the aux exh ports on this 500 werent any bigger than a pencil. 250 aux ports are alot bigger. its funny i took apart a stock ktm 500 and the trans ducts were black. no wonder because the darn blowdown was so small it was pushing exh back into the transfers. another thing about this 500 that stumps me is the primary compression is over 1.6:1. that cant be good when your trying to gain extra power

[jcs003]

this raises a question for me.

type of flow (i.e. laminar, turbulent and etc.) and its function of time when entering the combustion cycle are extremely important in engine efficiency.  my question is: is it unnecessary to texture the intake side of a cylinder to promote atomization?  I know its important to have a nice laminar flow through the intake tract.  my knowledge in fluid mechanics leads me to believe a textured flow in the intake tract only creates unnecessary friction and in-turn decelerates the fuel/air charge.  any thoughts on this?

john

[rsss396]

IMO a methanol bike may benefit most from a rougher surface such as finish left from a carbide tool. With methanol you are more likely to have raw fuel still entering the combustion chamber compared to gasoline.
But some may also argue a boundary layer is formed that promotes better flow even though it is rougher and if that is the case it would be a benefit for any type fuel.
Jerry can probably better expand on this but I will just say if its a difference its not much as I have had rough finished intakes and transfers, then smoothed them with a 80 grit and honestly did not feel a difference in the field.

I will just say I finish all my intake and transfers with 80 grit sanding rolls and 120 grit sanding rolls on my exhaust port.