Piston skirt below bore at BDC

[borna]

Hello all
When designing an engine, how much of piston skirt can go below bore at BDC?
Is there a disadvantage when piston skirt comes out of bore about 1/8” to ¼”?
Assuming the stroke is 1.5”, piston length is 1.125”

Thanks
Borna

 

[MachineTom]

If using full scale engines as a guide, 25-35% of the piston lenght comes out below the bore. Due to the short amount of running time in a model engine, you can push that, but the placement of the wrist pin will always be the limiting factor.

 

[borna]

Hi Tom,
Thanks for reply. But is there advatage to keep the skirt inside bore?
kind of want to know the design advatange or disadvantage.

Thanks
Borna

 

[cyclerider57]

There isn't really an advantage to letting the piston run out of the bottom of the cylinder unless you wanted to say that there was a reduced friction point near bdc. To make that argument you would need to also take into account that this is near a slow piston speed location of the stroke where friction is lower. Hydrodynamic lubrication is eliminated at bdc and tdc but the associated wear would be minimal. The real problem comes when you look at your rod length. With a 'short' the piston receives a lot of side load which drives the skirt into the cylinder wall. A longer rod will help eliminate this provlem but generally the piston skirt runs out of the bottom of the cylinder with long stroke engines... which force people to shorten the rod unless a design change is undertaken. There are also other things to consider such a piston ring stability but for a model engine discussion they can be overlooked without harm. As long as your wrist pin location is high enough in the cylinder at bdc and you still have a decent amount of skirt left, a model engine won't have any issues.
-Jordan

 

[borna]

Thanks, very interesting
you mentioned

"The real problem comes when you look at your rod length. With a 'short' the piston receives a lot of side load which drives the skirt into the cylinder wall. A longer rod will help eliminate this provlem but generally the piston skirt runs out of the bottom of the cylinder with long stroke engines..."

What length rod considered short in 1" bore x1.5" stroke?

Also is there a ratio between 1" bore and piston length?

I'm interested in a very slow running engine, between 500 to 1200 RPM. Is compression ration of 5:1 with 5.5" Dia. flywheel sounds good? Can I go with lower ratio like 4:1?

Thanks
Borna

 

[mu38&Bg#]

Having looked at many production model engines, some engines let the piston decend below the sleeve such that the entire wrist pin remains in the bore plus maybe 0.5-1mm high. Trying to design a compact engine myself, it's really an issue of how tight you want things to be. In a two stroke, space is required for transfer passages so the sleeve is kept high. A four stroke is tall due to valve gear so the design is made as compact as possible with a short rod. The rod angle is small near BDC.

 

[cyclerider57]

In the modern automotive world, a rod to stroke ratio of <1.8 is considered "short rod" and a ratio >1.8 is considered long rod. Motorcycles tend to run shorter rods because of their compactness. With a stroke of 1.5" a short rod would be considered a rod <2.7". A typical high revving motorcycle would have a rod/stroke ratio of ~1.65 which would = 2.475". The rod length can have multiple affects on cylinder filling, piston speeds, and piston rock but for a model engine it can all be neglected. Piston bore to skirt length varies greatly depending on the engine. Some of my race engines run bores over 96mm with a skirt length of only ~25mm and total piston height of 35mm. And yes the rod angle isn't very much near bdc but with a stroke of 1.5" and a short rod (~2.5"), the crank angle could be over 50 degrees after bdc before the piston has moved up enough to fully seat a skirt that was below the cylinder sleeve by .25"
-Jordan

 

[mu38&Bg#]

Those would be pretty extreme specs for a model engine. A short piston with short rod would definitely not want much piston travel below the sleeve. Model engines are nowhere near this configuration.

 

[MachineTom]

If you want a slow running engine, look at the full size engines that are slow running, long stroke, to bore ratio, long rod to stroke ratio, anything below a CR of 6:1 is going to make starting and running a bit rough. Valve timing will need to be designed to that low rpm range.

 

[cyclerider57]

I would agree that model engines wouldn't match that type of configuration (96mm bore with only 35mm height). That scaled down would be like running a dime as a piston. The 2.5" rod x 1.5" stroke with .25" of piston underhang at bdc I referred to are from specs in his other post.