2 stroke engine intake position

[SRAD]

I saw two Gilera 125 engines which are exactly the same except intake position design. On one model intake it is positioned on crankcase and on the other model it is on cyilnder( the models are the same year and have same carburators). Are there any benefits of each design ?

Thanks

Srad

 

[Fabrico]

Not an expert in that area, but from what I've seen, crankcase only is usually for high speed power only. Reed valves and ram air.

Piston port, with or without reeds, offers a broader powerband with less top end.

 

[SRAD]

Thanks on reply. I was thinking for both intake designs with reeds,(newer bikes like Aprilia rs 125, Yamaha yz 125, Cagiva Mito...)
So it looks that cylinder intake design for serial bikes is maybe used because of cheaper engine development and without sensibly lack of effectivity in direct gas flow when piston goes from BDC to TDC and closing function of reeds when piston compresses gas into crankcase moving from TDC to BDC. Reeds function better in crankcase than in cylinder because of bigger diference in preasures.

Thanks

 

[Fabrico]

I doubt it has much to do with cheapness or lack of development. Racing go-karts often use crankcase/reed induction but so do some back-pack blowers. Old and new MX bikes use piston/reed induction. I still think it has everything to do with powerband. My guess is that piston induction, with or without reeds, maximizes lower speed power and throttle response. Reeds are not very efficient at low speeds. Compared to a piston, they open late and close early. That's why there are 2 and 3 stage reed blocks on the market.

Reeds may arguably support more ultimate power feeding into a crankcase but there is nothing to support that they "work better" in that application.

 

[SRAD]

This text is interesting and it says that crankcase reed valve design has better bottom power and torque compared to piston induction, this were one of first reed valve engines.

http://images.google.hr/imgres?imgu...alve&start=20&ndsp=20&svnum=10&hl=hr&lr=&sa=N

Does piston induction engines pump less gas into crancase because intake port on cylinder is opened very short time before piston close it(intake port)moving from TDC to BDC. This could be reason that most of cylinder intake pistons have holes on intake side to pump more gas?
Where are bigger pressure-vacum effects to induct gas, on cranckcase intake design or cylinder?

Thanks

Srad

 

[ctmtwilliams]

This was not one of the first reed valve engines!! McCulloch had them in the 50's with their D30 chain saws.

It's real hard to say which will give the more bottom end torque unless you compaired engines with the exact same port timing shape and size.

 

[kcj]

Piston porting is used with or without reed valves, depending on design, but still usually mounted on rear of cylinder.

Reed valves only, no piston porting, is often mounted on crancase oppsote the cylinder, say old lawnmower engines.

You may be seeing rotary valve/disc valve inlet to the crankcase, usually on the crank end, side of engine. That has endependant control of opening and closing, and wide open low restriction flow.

kcj

kcj

 

[SRAD]

Thanks kcj on reply, I know all itake designs, I wanted to know main differences between cylinder and case reed intake on newer 2 stroke engines?

Srad

 

[SRAD]

Sorry,I mean the main advantages of each intake design.

Srad

 

[SRAD]

Newer cylinder reed valve intake engines have windows in the piston which leaves intake port open for a full 360 degrees of crank rotation, letting the reed valve dictate opening and closing periods. They are also using chamber on cylinder behind the reed valve to help scavenging, by connecting it to an extra scavenge port. As well as providing extra gas flow to the cylinder, this also relived the pressure on the back of the valve , making it open faster when the next cycle began.

 

[Fabrico]

Case/reed intake was perhaps most famously used on the last decade's worth of Honda CR250's. Even with other bells and whistles incorporated, it never did make better or more usable power than those from the early 90's.