Jump to content

Pistons


Recommended Posts

May be silly to some but was a bit curious to this.

Why are pistons the shape they are?

Why not a ellipse/oval instead of a circle?

-I'm guessing its to evenly distrubt the force but what effects would a different piston shape have? Also I know you dont want coners/sharp edges and a circle hole is a lot easier to make then a oval.

What would happen if the rod was offset from the center?

Link to comment
Share on other sites

Two fairly good questions, really.

The cylinder bore is typically circular primarily because it's easier to make them round than any other shape. The piston is round at the ring lands, but below that, it actually is not a cylindrical shape. Viewed from the bottom, a full "circle" piston skirt is elliptical, being larger front and rear (90 degrees to the wrist pin) than across the sides. Viewed from the sides, the skirt appears barrel shaped, being largest at the height of the pin bore, or close to that. All of this was once referred to as the "cam grind" of the piston, because before digitally controlled machinery, it had to be done on pattern grinders. The intent is to minimize the contact the piston has with the bore in order to reduce drag.

Honda built several experimental racing engines that did in fact have elliptical pistons. They have not continued to use this process that I'm aware of, so they may have discovered that they are either problematic or without sufficient advantage to warrant the extra effort. Virtually every manufacturing or maintenance operation typically performed on a top end is made more complex by any diversion from a circular bore.

But what of the wrist pin offset? A number of things can be accomplished by moving the pin one way or other. As the piston is driven down the bore under power, the crank rotating forward, the piston is driven against the back wall of the cylinder by the geometry of the rod vs. the crank. If the pin is offset forward, this thrust can be reduced.

Piston pin offset can be used to reduce perceived engine braking, reduce thrust as just described, alter the manner and severity of the rocking motion that occurs as the piston runs past TDC (which can reduce piston "slap"), among other things.

One other very significant thing it can do is to alter the rod angle during the power stroke. This is a little complicated, and I'm going to try to keep it brief, so if I loose you someplace, say something. The point in the crank rotation at which the rod can apply force to the crank with the most benefit in terms of power out put is when the rod stands at 90 degrees to the crank radius (a line drawn from crankshaft to crank pin). With all other features of the engine unchanged, an engine with a longer rod will have a smaller maximum angle between the bore center and the vertical center line of the rod, and the 90 degree crank angle will be reached at a greater number of crank degrees from TDC than it will if using a shorter rod.

Since fuel burns at a practically constant rate regardless of most other factors (for the sake of this discussion, at least), engines operated at higher speeds benefit from longer rods/smaller rod angles because they extend the amount of time in crank degrees that combustion has within which to peak before reaching an optimum crank angle. Conversely, engines built for very low speed operation can effectively utilize shorter rods. Changing the length of the rod, should a tuner wish to, in an existing design may not be practical. But, he can accomplish the same thing by offsetting the wrist pin, within limits.

This is essentially what Yamaha did with their 2010 YZ450. In this case, though, they offset the cylinder center line froward of the crankshaft, which gave them the low angle of a long rod without having to use one. In so doing, they were able to keep the lightweight and low cylinder height of a short rod engine with the power efficiency of a long rod. They were also able to use a much greater offset than is possible by simply moving the pin forward in the piston, too.

  • Like 1
Link to comment
Share on other sites

Very nice, what was Honda thinking? Man; They have came so far.👍

When 2 strokes started ruling the roost it was an attempt by Honda to get around the four cylinder limit with a 4 stroke motor that breathed and revved (somewhat) like a v8 of the same capacity (to put it very simply). With hindsight it was a failure, but history has also proven that pretty much the only way a four stroke will beat a two stroke is if the rules are bent in the "diesel's" favour. 🙂

I've often thought that given improvements in manufacturing techniques over the last 30 years that the oval piston may one day make a comeback - perhaps for a "no holds barred" large capacity single. But we'll probably all be riding electric motos by then...🤣

Great link btw highmarker.

Link to comment
Share on other sites

Engineers today are on never ending quest to find the max HP to min weight ratio. This type of foward thinking has not changed since the early days. like the old 8 valve oval piston design, engineers are trying to flow as much air as possible to make max HP.

In a normal 4-stroke engine, the width of the bore is similar to the stroke.

In the new Thumper engines, however, the bore is much wider than the stroke. The wider bore creates additional design requirements, namely a shorter stroke and a unique piston design. To maintain the same displacement

as a standard sized bore, a shorter stroke is required to compensate, and therefore a wider and shorter piston is also needed. This new engine design is called “over square”. The Thumper’s wider bore design is accomplished by-“over squaring” which contributes to its great horsepower. Comparing two 4-stroke engines with the same displacement, the wider bore allows more room for larger valves which means more air flow, resulting in greater horsepower.

There is more power to your rear wheel because the engine is exerting less

energy pulling and pushing air.

As we said, the wider and shorter cylinder geometry of the over-squared engines requires a wider and shorter piston. These wider, shorter pistons came straight from the world of Formula One racing a wide but thin piston called a “slipper” piston. This thin, light weight piston enabled these engines to produce more power with each stroke, but the result came at a cost. Normal pistons have long “skirts” on each side to prevent the piston from rocking side to side as it moves up and down. Also, normal pistons have 2 sealing rings and a 3rd ring to control the amount of oil that lubricates the cylinder wall. The “slipper” piston has virtually no “skirts” on the sides and has room for only two rings.

Link to comment
Share on other sites

there are limits to an "oversquare" engine design..

as the area increases in relation to the stroke the amount of time to burn the fuel/air charge is reduced..and becomes problematic at higher RPM's

There are, of course, limits to everything. One of the ways to deal with the reduced time available for combustion, which occurs at higher revs anyway, is to extend it by using a longer rod or offsetting the bore center line forward of the crank to reduce the rod angle.

The only thing that has given the 4-stroke the new life that it now has is the move toward extreme high RPM operation, which radically oversquare engines are conducive to. Two strokes cannot operate at such stratospheric speeds without loosing an impractical amount of ridability because of the extremely narrow power band they end up with.

Link to comment
Share on other sites

Another thing that happens with oversquare designs is that the area in an ideally shaped combustion chamber for valves is increased, which is a good thing. As a matter of scale, the bore size of a YZ450 (95x63.5) is the same as a Chevrolet 265, or the later 305, neither of which is an impractically large engine in any sense.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

×
×
  • Create New...