cylinder oreintation

[snapper]

I have noticed some saws have vertical cylinder, like my poulan, and some have a horizontal, like my shindaiwa. What are the pros and cons of the different cylinder positions??

Thanks guys!!

 

[TonyM]

I believe the advantage has more to do with the location of the exhaust and intake ports. Many of the horizontal cylinder saws have the intake and exhaust port at 90 degrees to each other(carb on top, muffler on side). It is easier on a vertical cylinder to arrange the intake and exhaust ports at 180 degrees (directly opposite each other). With this arrangment the flow from the transfers and scavenging of exhaust are more symmetrical and more efficient.

 

[eyolf]

McCulloch was the first major name to lay the cylinders down on their saws back in the late '50's. Before that most saws had more or less vertical cyl's., whether piston-ported or reed-valved. Homelite followed suit, using the scheme to good advantage with their compact, light Xl saws. European saws didn't follow this plan, learning to make their saws light and compact anyway, as well as staying with the piston-port theme.

A piston-ported engine uses fewer parts and will usually provide a higher specific output at a higher rpm than a reed-valve type. The old saws of the '60's that used reed valves were pretty effective at least partly because the reed valve engine is able to seal the crankcase and fill the cylinder better accross a wide rpm range. Unfortunately, those reeds become a flow obstruction at extreme speeds, impeding cylinder filling at high rpms.

I think it's for these reasons most saw makers have returned to the vertical cylinder, piston-port layout.

 

[bwalker]

It is easier on a vertical cylinder to arrange the intake and exhaust ports at 180 degrees (directly opposite each other). With this arrangment the flow from the transfers and scavenging of exhaust are more symmetrical and more efficient.

Thats non sense as the incoming charge has to make a right angle turn to reach the crank case and another right angle turn to go up into the combustion chamber via the transfer ports. Cylinder orientation is a packageing and balance thing .

 

[TonyM]

The transfer ports open to the crankcase and the cylinder. The intake port and the transfer port cannot be in the same place. In order for the transfer(s) to adequatley push out the exhaust and charge the cylinder, the port has to be located and shaped to push the gases out the exhaust port and fill the cylinder with a fresh charge. With the intake port at the side of the exhaust, there are more limitations to good transfer port shape, size, and location. When the intake port is opposite the exhaust, the transfers can be shaped correctly and symmetrically to perform their task. Why do think that on a Stihl 041 (with a horizontal cylinder, carb on top) the designers went way out of their way to bring the exaust out the bottom of the cylinder. Maybe you think it's nonsense, but the fact that port shape, size, timming, and location, control how an engine performs, makes perfect sense to me.

 

[bwalker]

My bad. I thought you where confused as to how ports function. FWIW the best way to have cylinder postioned would be to have a reed valve that entered the crankcase in line with the cylinder. BTW what I was trying to say in my above post was that the postion of the intake and exhaust doesnt really matter as long as the transfers are perpindicular to the exhaust. The intake could be positioned anywhere as long as the transfers are postioned like this.

 

[TonyM]

Sorry I got a little defensive. I suppose my initial post was just trying to keep it simple, but I should have explained it a little more thouroughly. It sucks how when you don't take the time to do a job right the first time, it always takes more time in the end to correct it.