[300cc] 300cc engine design questions

Hi,

I was wondering if anyone could help me out here...
I have been planning on designing (building) a 2 stroke engine.
It will be based on my kawasaki kx250 engine, which has a bore x stroke of 66.4 x  72mm.
The new engine would have a bore x stroke of 72 x 72mm so that it would be about 293cc.
The goal is to make it as reliable as possible, making about 45bhp.
It would be for a go kart so the powerband can be quite narrow.

To achieve that reliability I reckoned it would be best to aim at about 7500rpm for max power and about 10bar theoretical compression. That would imo be the most reliable setup.

Now for the port area's I was planning on using Jennings time are and angle area formulas. Are they still up to date and useable today? I have used them already for a lot of cylinders, but this project is off course a little more complicated.

Then last but not least I would like to know your opinions on lotus' engineering software, I have found that it is not always that accurate, is it even usable for 2stroke modelling? Is for instance MOTA much better?

Thanks a lot!
Senne

Hi Senne

I'm confused as to why you want to go to the trouble of boring and re-plating the cylinder to a larger size, sourcing and fitting a bigger, heavier piston (hence probably have to alter crank balance factor and definitely have to modify the combustion chamber), talk about port designs, etc. and then looking for such a low power output at very low rpm?

In fact the standard KX250 engine straight out of the bike makes more power than this and would be extremely reliable with very long service intervals if rpms are restricted to say 9,500 rpm.

Seems your talking a lot of trouble and expense to go slower!!

Just trying to understand the reasoning behind the project?

Best Regards

Ian 

theres also other options if your not obligated to the kawasaki. cr250 (2001 and earlier) lower end with cpi puma or sphynx cylinder would put you at 72x72. i imagine they would do 60hp without much trouble. i really like the ktm 300 which is 72x72 and theyre about 50hp right out of the box

you could also do a 68 borex72 stroke cr250 with sphynx cylinder

Ian,
It would just be my goal to enlarge the service interval as long as possible.
I believe that the standard intervals are about 50h for the top-end?
For go karting that might be even less because the engine is 90% of the time above 8000rpm.
I understand your point of view though.

It would maybe be possible to enlarge it to by lowering the compression with a new head or thicker gasket.
And then limiting it to, say, 8500 - 9000rpm so that I still have a powerband of about 1500-2000rpm.

But still, the engine's plating is almost at the limit and I would certainly have to replace it, so it would look like a challenge to me to make a complete new cylinder.

Brokedown,
That would also be an option indeed, but I already have the Kawa engine, and as I wrote above it's mostly about the challenge to me.

Senne

This is the portmap I had in mind.
Has anyone got remarks or something?
Take care that this is not made for max power and that it is more important that it is reliable.
I know it is hard to judge from just a picture though!


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Would giving 7transfers instead of 5 make ringlife longer? Noticeably?
Thanks already!

this engine seem to have a very small Exh area compared to the Tr area, and no blowdown

this engine won't breathe !
=>
you'll have to widen the upper part of the Exh duct

Yes it is quite narrow indeed. But bear in mind that this is aimed for only 7000rpm.
According to Blair's guidelines for time area this should have enough exhp ta and blowdown ta.
Why do you think it is too small?

as i said before, the areas should almost be equal
and this engine has a small blowdown

How about I make each auxiliary exhaustport about 3mm wider?

yes, and reduce the 13 mm radius to something like 10 mm

if you want a longer ringlife you can winden by 1 mm the place between tranferts (as you have a lot of transfert area)

Ok thanks a lot I really appreciate your help!

did you have a look at the RSW drawings yet ?

I did, yes. The transfer ports should be about the same.
I wanted to make the auxiliary exhaustports the same too but I don't think I need that much exhaust area/ blowdown area.

if you do make the aux windows bigger just be sure you have sufficient material around the tunells to make them of appropriate size also. one time i made the mistake of enlarging the windows without noticing the casting thickness of the tunells. otherwise you may end up with a large window and small tunell like me   

Hi! I would like to discuss some porting work on a Bimota V-due cylinder, but I just can`t upload the pictures... I would be greatful for any advice!!!

Hi

1. Register with [Vous devez être inscrit et connecté pour voir ce lien] which is an image hosting site.
2. Upload the pictures to the site. (This will then be stored in your "library").
3. When you left-click click on the image in your library it will display it with a selection of links alonside it.
4. Left-click on the link called "IMG" This link will flash up yellow and say "copied"
5. Right-click on your post in the message box that you are writing your post in on here and then left click "paste" in the pop-up window.

Job done !! . . . . . . Like this

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Thanks Ian! I will soon get on with it

Here comes some pictures of a Bimota 500 Vdue cylinder.

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As you see from the pictures, I have opened up the boost transfer port to allow for more flow. I am also considering removing the port splitter in the cylinder window. Is this safe? The port is quite narrow anyway..


At the pictures of the cylinder with piston inside, the piston is at BDC. As you see, the piston skirt restricts the flow into all transfer ports. I intend to match the opening in piston skirt with the opening in the cylinder skirt at main transfers, but I`m afraid I will reduce the strenght of the piston in the boost transfer port area if I do the same here. Maybe I could shorten the piston by app 10mm before I make a window for the boost transfer port.


Anyone`s got an opinion on this?

you posted the wrong links, I see no pictures.

Hi JanBros! Well, this is not easy :) I see the pictures on my laptop, but not on the Ipad..I added a link to the pictures. Should be possible to see now..

Hi Helmers

Host those images to Photobucket!  

Matching the piston as you suggest would probably produce no improvement.
You would have to check the exhaust sub ports would not be opened to the crankcase around TDC.
Shortening the piston will increase the possibility of nipping the ring due to piston rock as the exhaust port closes and may again open the main exhaust to the crankcase at around TDC.
Of course beware of weakening the piston too much!

Does the piston have 1 or 2 rings and where do the ring gaps run? If it/they run up the centre vane of the 'C'-Transfer, don't remove it! If they run either side, it would be possible but again may have an impact on ring wear.

Be careful with those exhaust/sub-port dividers, if you have a seizure, they will crack (again?), particularly the left-hand side.

Does look a nice cylinder.

Best Regards

Ian

Senne s a écrit:

This is the portmap I had in mind.
Has anyone got remarks or something?
Take care that this is not made for max power and that it is more important that it is reliable.
I know it is hard to judge from just a picture though!


[Vous devez être inscrit et connecté pour voir cette image]


Would giving 7transfers instead of 5 make ringlife longer? Noticeably?
Thanks already!

Senne, You have to much tranfer surface for 7500 rpm, make timing about 3mm lower than it is exact enough, see portmap in link.
Make auxilleries closer to head exhaust.
You might even choose no auxilleries at all, when you make the headport a little wider 65% and 2mm higher (195degr).

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Ian Harrison a écrit:

Hi Helmers

Host those images to Photobucket!  

Matching the piston as you suggest would probably produce no improvement.
You would have to check the exhaust sub ports would not be opened to the crankcase around TDC.
Shortening the piston will increase the possibility of nipping the ring due to piston rock as the exhaust port closes and may again open the main exhaust to the crankcase at around TDC.
Of course beware of weakening the piston too much!

Does the piston have 1 or 2 rings and where do the ring gaps run? If it/they run up the centre vane of the 'C'-Transfer, don't remove it! If they run either side, it would be possible but again may have an impact on ring wear.

Be careful with those exhaust/sub-port dividers, if you have a seizure, they will crack (again?), particularly the left-hand side.

Does look a nice cylinder.

Best Regards

Ian

Thanks Ian! Sounds like you could see the picture at last!

I`ll check the location of the ring gaps again. I think it is between B and C port.
The pistons are std dual ring. I might buy some single ring Woessner pistons from FRP in germany.
I`ll check that the  piston does not uncover the sub-exhaust ports at TDC. Sub-exhaust ports are untouched. Very narrow in std shape.

I`m a bit suprised that matching the piston side cut-out with the cylinder liner transfer port opening would not improve flow into the transfers.. Can you explain this better :)

Helmers

Hi Helmers

It is just from experience and observation. Trying something, run it on the dyno and see if you get a gain, a loss or no change.

If you have the facilities try it and see.

Ian:D

Hi Ian

Okay. The ring gap is between C and B port on each side. The cylinder diameter is 72mm and the c port will be 27,5mm wide if I remove the splitter.

Regarding the exhaust sub ports, the piston skirt covers the port, but only with 4mm clearance at TDC. That means the skirt cannot be modified too much in this area. Towards front, it teoretically can.

What I could to is make an opening in the piston skirt towards the boost port, or a cut-out like many pistons have.

I know you should not fiddle to much with the cylinder ports, but I have a question regarding the B port trailing directional angle. If you look at the picture of thye Vdue cylinder, you see this angle is directed almost to the center of the cylinder. (Show with the pin) I have also attached a picture of an aprilia RSW or RSA cylinder, where you can see this angle is completely different, almost paralell to the boost port.

Do you have an opinion on this? If I copy the design of the aprilia boost port and make the port flow more air, should I also change the trailing directional angle of the B port and make it look more like the RSW/RSA?

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Interesting to discuss this, and hope you don`t mind!

Henrik