Ciro Pabón wrote:Well, I'm not mechanical engineer, but let's see if my "barbarisms" attract somebody who knows: when you ask for
contributions, nobody appears, but when you ask for
contradictions, everybody is eager to post!
It seems to me that a shorter stroke (maintaining cilinder capacity) should make your life easier and the components lighter: a longer stroke engine would have pistons that move faster. I'm not sure about the influence on the speed of pistons during the "reversal" and the acceleration implied, but you can easily imagine how that influences the friction, heat and the resistance of the package under high rpms.
Another point: if I can extrapolate what I know from column design
, I would say that a longer rod will weight more, because it would buckle easier. We all civil engineers know that higher ceilings mean more weight on the columns and the difference is no small potatoes, at least for buildings. The slender (ratio of width to length) the column, the more fragile.
I have noticed that most diesel engines have longer strokes than their petrol or gasoline counterparts (for a given capacity). Long stroke engines have better torque, because you can have more leverage, if I imagine correctly the behaviour of an engine. Also, I know that the older the engine, the longer the stroke (think about how much time it took to have small-block engines). I guess this is because it is easier to machine a long cilynder than a short one and the tolerances don't have to be so small. Just think how hard would be to machine a cilynder that is much more larger than a standard one. Imagine an engine with a stroke of 1 millimeter: how large would be the piston? It wouldn't be an easy thing to manufacture and you would end with a huge block.
On the other hand, I'm pretty sure that piston pressure is higher at the beginning of compression, once most of the fuel has ignited, so a shorter stroke engine would give you more force on the rod when it's near to be tangent to the crank. The shorter stroke engine should be closer to tangent faster than the longer stroke one, if you follow my drift.
Anyway, if I were building this thing (heaven forbids!
), I would start by
finding the piston pressure curve and balancing that against the mechanical leverage of the rods to find the optimum point, trying the different g-forces (and the associated strength and weight of components) before deciding for a particular combination of bore and stroke.
I also "see" that the dwell time have to change with stroke length and I have no idea how would this affect the "breathing" and the valve configuration...
So, where did I make mistakes? I guess this is going to take a dozen posts to eliminate all of the errors and wild guesses I made...
