[300cc] 300cc engine design questions

Helmers a écrit:

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.

Henrik, that drawing does not show an RSW or RSA cylinder; it is a drawing I made in 1978 of a Rotax 124 cylinder, so it's almost 40 years old!
There are drawings from Aprilia cylinders elswhere in this forum but I cannot tell you where exactly; you will have to do some searching of your own.

Hi Frits! Thanks for the info. I found it in my library of pictures from the RSA/RSW125 forum, so I thought it was the right stuff.. I belive this is à drawing of an RSW/RSA cylinder.

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The B port trailing directional angle is not so steep on the aprilia as on the Vdue, as you see on this drawing, but maybe I should let it be as is and only work on the C port.

I measured the exhaust / inlet duration as well:

Exhaust main: 188
Exhaust sub: 180
Inlet A / B: 122
Inlet C: 120
Cyl gasket= 0,5mm

(If I use a thicker cyl. gasket and adjust for CR/squish, I could increase exh/inl.duration..)

Feel free to comment, You`re kind of the expert :)

Regards Henrik

Hi Helmers

Looking at the cylinder base weight relief, I am wondering if that C-port divider is more structural than anything to do with port design. Width would be OK, but, remove at your peril. We all know about the straw that broke the camels back!

The piston window, may add a little cooling/lubrication to the little end and the underside of the piston crown. If you do this, check very regularly for cracks in the piston, until you are confident.

Have you seen Frits' diagram regarding the design of the transfer port angles? I think he likened it to the "Leaning Tower of Pisa". It is on this series of topics. You would have to search unless Frits could copy it here.

Best Regards

Ian

Hi Ian! I have thought about the strenght of the cylinder myself, but the rest of it is quite rigid.

Regarding Frits diagram and the understanding of the column of fresh charge rising up the cylinder and pushed towards the cylinder exhaust side by the flow from the c port, forcing the exhaust to vanish, yes I have read it. (Hope my memory and understanding is correct)

I belive this is one of the most important factors in making an effective combustion, but also very difficult to forsee without having a test facility. I have only two cylinders, and there is no way back if I make an error by removing to much material :)

I`m far from an expert, but to me it seems that the design of the Vdue cylinder and the cylinder ports, especially the steep trailing directional angle of the B port, might center the rising column too much. If my assumptions are correct, the column of fresh air might mix too much with the exhaust, before the exhaust vanish out the port.

If I make the B port trailing directional angle less steep, I might shift the rising column towards C port, and more flow from the C port could then lean the column towards the exhaust side of the cylinder and push the exhaust out more efficient.

I might be wrong, and I have only talked about the radial port angles, not the axial, which will make this even more complex..

Henrik

My memory is unfortunately not as good as I thought. Regarding the leaning tower of piza, Frits said this;

"What we want to achieve, is an axial column that clings to the rear of the cylinder, so it can wash away the spent gases with as little turbulence as possible. "

My assumption that the charge from the C port pushed the rising column of fresh charge against the exhaust side is therefor incorrect. By rear, he means opposite of exhaust. I apologize for this.

Henrik

Bridge in C-port is removed. Hope it prooves to be the right thing!

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Henrik

Hi Henrik

That looks good.

In 2002 Honda introduced the crankcase induction Honda CR250 and you can see the cylinder on the left. This had large weight saving "voids" either side of the C port throat. This cylinder used to crack along the line shown in the image below. In 2003 they reduced the size of that void drastically and the cylinders didn't crack anymore!!
I hope yours is OK.
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When the cylinder cracked it could cause catastrophic damage!

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Best Regards

Ian

Hi Ian! Thanks. The broken CR cylinder did not look good.. The Bimota cylinders have such voids, so maybe I will fill them with GB weld to increase strenght.

Another question for you. What CR, head volume and squish do you run When lifting the cylinders for more agressive duration, I need to re-work the heads, and I would very much appreciate you're input!!

Henrik

when tuning KR1 cylinders succesful (meaning you get more power out of them), they also crack at that spot and they do not have any void's there.

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Hi Henrik,

I don't think JB Weld would really help stop the cracking. So hopefully your cylinders are good quality strong castings.

In 2015 we were limited to 100 Octane pump unleaded fuel

With the 2002-2004 crankcase induction CR250 (250cc) we raise the cylinders by 2.0mm by increasing the gasket stack to 2.5mm thick (1.5mm aluminium plate plus a Std 0.5mm Honda base gasket top and bottom). This saves a lot of port cutting. We cut the bottom of the transfers level with the piston edge at BDC. This gives approximately 132 transfer timing and 196 exhaust timing, we then raise the exhaust port roof to give 199 duration (superkart racing). We also carry out some general port-work/reshaping.

This gives us an installed piston recession down the cylinder at TDC of 2.3mm (we can adjust with thin gaskets of 0.1, 0.15 and 0.25mm to achieve this figure). We machine the standard Honda head to convert it to an 'O'-ring sealing, 2-piece with a changeable insert. Our insert then has a 1mm step down into the cylinder to leave us with 1.3mm squish clearance.

With a flat-top piston we use a 40% squish band with a 2.5 degree ramp angle and (as noted above) 1.3mm squish clearance and a total combustion volume of around 23cc (to the top of the plug hole). We always measure our head inserts upside down and then calculate total combustion volume as we use a toroidal chamber shape, which will trap an annulus of air if you measure the installed volume and give a false reading!

Here's some of our 2-piece heads. You can of course purchase manufactured 2-piece heads (by VHM, etc.), but our regulations say we cannot alter the silhouette of the engine and must use O/E parts.
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I hope that is of interest

Best Regards

Ian

Hi Ian

This is very valuable information to me. Thanks a lot!! I will also raise the cylinder for better timing, but not as aggressive as the superkart engine. A 1,8mm cylinder gasket will give me a duration of 192/128 deg and piston top in line with transfer ports. I might go for a 2,0mm gasket for practical reasons. Transfers have been "cleaned" for better flow.

I have some experience with the 125cc top-end set-up, but not much with 250`s.. The Bimota piston is domed and I`m not sure of the squish band area percentage of the head. The head does not have a toroidal shape, but more like a bathtub shape. I will run 98oktane pump fuel so I might go for a bit more combustion volume than 20,5ccm (23-2,5) but I guess the squish can stay at 1,3mm. The stroke is short, 61,25mm and the rpm will not be as high as on the superkart!



I also need to lower the head into the cylinder. It has the O-ring sealing as std., but the slot will have to be re-machined after beeing lowered.

Best regards Henrik

Good luck with your project, Henrik,and please keep us informed. That is a very short stroke, which makes things a little more difficult.

As well as having used the Honda CR250 (66.4 x 72), we produce our own mono cylinder Superkart engine which is now in it's 3rd year. This is 68 x 68.8 (same as Honda NSR 500 V-Twin). For the last 2 years we have used a non-powervalve cylinder in the UK only. This year we are developing a powervalve cylinder. Both versions of the engine are now registerd with the CIK/FIA for Superkart use.

The engine sits in the chassis with the carb rubber level. with inlet on the front and exhaust out of the back.

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Best Regards

Ian

Thanks Ian! Yes, the stroke is very short. Don't know why they made the engine this oversquare. In the 125cc size, all racing engines seems to have app same bore as stroke. I guess the longer stroke improves angle area time and gives more torque..

The honda 500V engine is pure art, but I've heard the aprilia rsw500 was even more powerful.

The Viper engine looks very nice! I guess it delivers a lot of power to keep up with the two cylinder superkarts!

Hi Henrik

Yes you are correct, with longer stroke it is easier to achieve the necessary port time/areas, but of course longer stroke brings higher piston speeds and internal engine forces. Like all things 2-stroke there is a best compromise and for a race engine this seems to be the same sort of ratio. i.e. 54 x 54.5 for a 125cc and 68 x 68.8 for a 250cc

I'm not so sure the Aprilia RSW500 made more power (or certainly more area under the graph of the required rev range) than the Honda NSR500-V2. I know that Jan has commented about the Aprillia, that it was a bunch of compromises, ruled by available budget and it was also very short stroke. Still an exciting engine!

This is the first year that the mono cylinder 250's have been allowed into the European Championship (mainly because the dropping number of expensive to purchase and maintain twins has left half a grid to fill!). Of course they will not compete power for power with a twin, but I'm sure that we will still see the best of the singles (with the best of the chassis and the best of the drivers) in the top 10. They will also be racing for their own awards as well as overall position.

Best Regards

Ian