Reed valve admission size ? Venturi ?

[KTy]

Alloha ! ;)

I am working on smallframe casing to fit a Polini 133, and I'm just wondering if you guys use some formulas or "common knowledge" for the size of the admission. I'm going to fit a big motocross reed valve membran (Gerhard did I think), but I am wondering if I should try to maximize the output surface or if I should weld some sort of a venturi inside the casing. Here is a picture that will explain better (maybe) :)

So is there a "perfect" ration c/b ? And will a venturi be usefull ?

Thanks guys ;)

KTy

P.S: Plz reply in english if you can... ;)

Bearbeitet 10. August 2004 von KTy

[sepp]

i fitted a malossi valve without what you call "venturi"!

by the end of next week i can tell you how it works!

cheers!

[KTy]

Actually, I discussed this on another forum and it appears that the venturi is useless as what really matters is the *quantity* of fluid that I want to maximize, not its speed (that's what a venturi does).

So in the end, to minimize "lost of load" we thought that the best solution is to have b = c !

Feel free to comment ;)

Bearbeitet 10. August 2004 von KTy

[gertax]

best thing imho would be C-C and to let the petals open in the flywheel to clutch direction. In this case the space between the crank webs is utterly made use of. Also try to get the petals as close to the crank as possible. This makes for better reed reaction. For safety reasons it is advisable not to undergo a distance of 5mm between petals and crank web

Can you post the link to the other forum? I like reading such stuff

[nop]

sure you have to maximize the *amount* of mixture and therefore it makes absolutely no sense to me to obstruct the gas' way by such a narrow point.

what reason should there be to accelerate the mixture even more before it enters the crankcase?

the only reason i can see at the moment for decreasing the inlet diameter after the reed would be minimizing the crankcase volume and thus increasing the primary compression...

[amazombi]

just to get that straight: does the reed valve you are intending to use come with some sort of reed stops, i.e. some curved sheet metal things which limit the amplitude of the reeds' oszilation? if so the venturi you are talking about will not be an obstruction in the gas flow at all, at least not if its area is not smaller than the area measured between the stops (minus the middle bridge(s) of the reed cage). if i were you i'd try to keep the volume of the crankcase on the low side, and since you are intending to weld on some material i believe it would be a good idea to angle the whole reed so as make it point at an area just above the big end bearing with a flow direction of about 90° to the cylinder angle. this flow direction appears to be rather crucial to the power output of the engine.

Bearbeitet 10. August 2004 von amazombi

[gertax]

with a flow direction of about 90° to the cylinder angle. t

yeah, but then your reed inlet where the manifold shall be attached points directly to the ground

[amazombi]

yeah, but then your reed inlet where the manifold shall be attached points directly to the ground

point taken. i do believe that should be a solvable problem with an angled manifold though. on the small autos the reed points in exactly the direction i have indicated, and they use an angled manifold (some 70° i would guess) to fit the carb at an usable angle, and that does not seem to be disatvantageous in terms of gas flow. and you still have a decent, read "modern", angle of the reed relative to the barrel.

[gertax]

on autos, gas flows towards ("upwards") the combustion chamber with the angeled reed position, which I think is a good thing. But on a non-reversed smallframe flow would be pointing away from the combustion chamber, downwards to the crankcase. If the reed flange is wanted to be perpendicular to the ground 45 degrees of deviation compared to 90 degrees to the barrel is the way to go. And that is 65° of total deviation compared to the autos.

Apart from that, all you can fit cross to the rod without moving cylinder studs is about 40-45mm of maximum width, that is not much bigger than a crappy malossi reed valve. Oh yeah, and there's a stud in the way, too. But who cares about studs anyway when it comes to power

Anolgside with the rod some seriously massive reed valves can be made to fit using some brains, and even the upper stud can be kept.

gotta go for now, I got an appointment with the TIG welding apparatus

Bearbeitet 10. August 2004 von Gerhard

[Rakete]

Äähmmm..."this is the Clark family"...

Absolutly Offtopic!

[kan_Kuma]

@Kty

2 things I think are crucial in my humble opinion.

1. Area and Flow: To keep the volumetric flow as high as possible, the area of the inlet after passing the reed petals should match the exit area of the valve in open position, if possible. It may be a little narrower, without any negative effect on power, yet the direction of the flow becomes even more crucial then. A side effect of this would be increased crankcase compression (which may or may not be desirable)

2. Direction: Considering feasibility of the solutions, the airflow should be aimed upwards towards the piston bottom, or at least into the general direction. Any direction pointing at a wall or in this case the crankshaft should be avoided. Amazombi's idea of pointing the flow towards the big end bearing of the crank in TDC position would be a good compromise, yet the feasability of this solution is questionable.

Cheers

Chris

[gertax]

these are both inlet ports of production racer engines. Position does not seem to matter as long as there's enough area for the incoming gases to get to the transfers (circeled red)

enjoy :

by the way, there's no venturi there, just space to enter as far as one can see.

Bearbeitet 10. August 2004 von Gerhard

[KTy]

@Gerhard: Sorry, you can't access the forum as you need to register and besides that, it's all in french ! (Anyway, here is the link http://forum.vespalambretta.org/index.php?showtopic=10247). But now, the discussion is way more interesting here on GSF !

@kan_Kuma: Very good point about "To keep the volumetric flow as high as possible, the area of the inlet after passing the reed petals should match the exit area of the valve in open position". I didn't really thought about that, but since I don't use (of course) full circle crank, it's indeed a good idea not to enlarge too much the inlet.

Thank you to all of you for your advice, I will try to make a little schematic of the best "theorical" inlet admission to see if we understood each other ;)

... @Gerhard: What did you weld this time ? ;) Thanks for the pics...

[KTy]

I was thinking about the airflow direction... On a smallframe with non-reversed barrel and case admission, You have maybe to take into account the crankshaft direction, i.e, the engine turns clockwise, so I think if you aimed upward toward the bottom of the piston, the airflow is going to be "disturbed" by the crank movement...

What do you think ?

I guess that's why the most powerfull engine have "reversed-barrel" and admission the from the other side so that everything "match"... That's also why you can get more power from a largeframe engine as the carb seat over the crank...

Here is the picture that shows what I meant... To me, actually, I think it would better to aimed the airflow downard the crankcase considering the crank rotation direction...

Discuss !!

Bearbeitet 11. August 2004 von KTy

[kan_Kuma]

The fact that "some" reversed barrel engines have a higher power output has nothing to do with the direction of crankshaft rotation, IF the crankshaft is a full material one, not one for a rotary inlet.

On modern 2 stroke engines, with nicely finshed crankshaft surfaces, gasflow is totally unaware of the rotation direction of the crankshaft.

So, in short: All depends on what type of crankshaft are you going to use

Bearbeitet 11. August 2004 von kan_Kuma

[Polinizei]

That's also why you can get more power from a largeframe engine as the carb seat over the crank... - That's a wrong assertion!

[KTy]

The point is, when you DON'T have full circle crank (like me, because I'm working on a Polini cylinder with case admission !), then to me, it seems it's not a good choice to aimed the airflow "against" (as shown in the picture) the rotation crankshaft... Is this assertion also wrong ??

I agree that with perfect surface full circle crankshaft, the airflow is not going to be (too) much disturbed.

@Polinizei: I know your(?) scooter, it's impressive, but your admission is directly in the cylinder and in the "good" direction according to the crankshaft rotation.

Bearbeitet 11. August 2004 von KTy

[kan_Kuma]

@KTy

I am sure Gerhard could help you with that. He is quite capable in modifying the crankshaft in such way as to allow for maximum airflow. His crankshaft is exremely lightened and "streamlined", so to speak.

If you leave the crankshaft original then it may be that the crankshaft would behave like a rotational pump, swirling the air as it enters the crankcase, heavily disturbing airflow.

So: yes, the rotating direction would be important here.

I'd suggest playing around with an old crank and see how much material you can take away, without weakening the crank too much. Don't forget to rebalance the crankshaft before riding :)

I haven't had much experience on crankcase inlets, since I've been concentrating on the Malossi136 which sports a direct inlet, but Gerhard would really be the guy to talk to, regarding crankcase inlets.

Cheers

Chris

Bearbeitet 11. August 2004 von kan_Kuma

[Soul Sucks]

have a look at the conrod while the piston is moving up (case admission)...the inlet area is free...the gasflow can pass through the crankcase....

the largeframe inlet will be closed by the conrod while moving up the piston....

so the smallframe construction seems to be more effectiv?!

[KTy]

Hi guys, little update just to have some feedback ;)

link to pictures;

http://ktylife.free.fr/vespa/KTy133/CrazyIntake1.jpg

http://ktylife.free.fr/vespa/KTy133/CrazyIntake2.jpg

http://ktylife.free.fr/vespa/KTy133/CrazyIntake3.jpg

http://ktylife.free.fr/vespa/KTy133/CrazyIntake4.jpg

http://ktylife.free.fr/vespa/KTy133/CrazyIntake5.jpg

http://ktylife.free.fr/vespa/KTy133/CrazyIntake6.jpg

and

http://ktylife.free.fr/vespa/KTy133/CrazyIntakeProject.jpg

I will remove the few epoxy that are on my casing, then I will make the reed cage (in 2 parts I think) from aluminum and ask a colleague of mine to weld it to the case. Then I will probably finish the intake with some smooooooth epoxy !

The reedvalve will be exactly centered on the crankshaft, so maybe a full-circle one or worb5 style one will do it better than a regular cutted one like mazuchelli.

The reed valve is from yamaha TZR 125, 6 reeds.

Cheers,

KTy

Bearbeitet 22. August 2004 von KTy

[KTy]

More Updates for the pple who cares about my project :)

Admission

With TZR reed valve

Engine in the frame

I'm waiting for the HP4 fans before I first kick the engine...

Polini133 based, 5mm alu spacer, exhaust: 185°, admission: 133°, zirri silent, 2.54 primary, 28mm carb (to start with :) )

[100 spezial]

Whats going on with your engine??

Are you just running??

[KTy]

Whats going on with your engine??

Are you just running??

<{POST_SNAPBACK}>

Hey !!

Yes, I'm in the process of running in... I already drove a few kilometers, it feels great and powerful. As expected the powerband is quite high, but the engine revs amazingly well ! I have to calm down to finish a proper running in !

Right now, I'm also working on a simlple variable ignition timing, but for running in purpose I set the ignition timing to a modest 16-17°... Actually at first I've made a mistake ! I set the timing to about 20-22°... Crazy how the engine was happy-revying ! But it was running hot; about 110°C at the cylinder head.

I also need to jet properly the carb, right now i'm running 110 main jet, I suppose it won't be enough, but because I'm driving not at full throtlle / full speed, i'm not in a hurry.

Finally, here is my new air-box, for street use, because else, it's way too loud !!

I'll keep you informed when the engine brakes ;) Actually, I believe my modified crank won't last too much, but whatever, I did what I wanted to do :) And maybe before this tragedy I will try to find a honnest shop to do a dyno, but I'm thinking of buying PowerDyn (google it if needed).

And the rest of the picture of KyT 1.0beta can be found here.

Cheers,