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Pistons Aren't Round: Profile and Ovality Explained


Profile and ovality are two main characteristics of piston design. Here we'll take a look at why pistons are designed to not be perfectly round.

When you look at a piston, it is easy to think that they are a perfectly round,  cylindrical shape. After all, they go into a round hole (the cylinder!) So why shouldn’t they also be round?

The fact is, the external shape of a piston is very sophisticated. An internal combustion engine is a hostile environment where combustion gasses can reach dangerous temperatures, and there could be port windows and surface undulations from uneven cylinder cooling. Designing a piston that is optimized for combustion chamber conditions is an important challenge.

Throughout the years, piston materials and design characteristics to compensate for expansion under heat have evolved. Forging pistons out of aluminum provides great strength and durability, but it must be used in the correct design to properly optimize the performance of the piston.

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(Left) These are an example of early piston design, using steel as the primary material. These would not be sufficient for the requirements of modern engines. Compare with the variety of modern forged aluminum pistons from Wiseco (right) featuring different coatings and designs.

Read more about the forging process here.

There are two major characteristics of piston shapes: profile and ovality. Wiseco's Product Manager and long time engineer Dave Sulecki commented on these piston characteristics: “Piston profile and ovality are one of the most important features of a piston, these really determine not only how the piston will wear over time, but also how well the piston can perform. When the engineer calculates the piston to cylinder clearance, this is only the beginning of a complex determination of the final piston geometry."

Profile

If you roll a piston across a flat surface, you'll notice it does not roll in a straight line. You are observing characteristic number one: profile. Because aluminum conducts so much heat, pistons are designed with a taper -- the top of the piston, near the crown, is a smaller diameter than the bottom of the piston, near the skirt. The skirt of the piston actually is designed with what is called a barrel shape, illustrated below. This is beacuase temperatures near the dome of the piston vary from the temperatures at the skirt of the piston, resulting in different levels of expansion. The tapered shape allows the piston to expand as heat is applied, so the piston does not bind in the cylinder bore. The higher the temperature, the more the piston will expand. The design challenge then becomes calculating the degree of taper. Too tight of clearance can induce scuffing or seizure from heat expansion, while too loose of clearance can introduce noise from piston rock.

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This illustration shows piston profile: the barrel shape and taper pistons have. Because of this, measuring diameter on the skirts yields a larger number than measruing near the dome.

"The piston profile is critical to how the piston will support itself as it reciprocates in the cylinder bore. For example, the piston profile must help hold the piston vertical in the bore during combustion; imagine any excess leaning of the piston would allow piston rings to become “unseated” and not seal properly against the cylinder wall," elaborates Sulecki.

Ovality

As you roll the piston across the table, you will also observe the piston rising and falling in a “hump-hump-hump” motion, much like a wheel that has a flat spot. This characteristic is called ovality, also known as camming. In the simplest terms, ovality means that the piston is smallest in line with the wrist pin bore.

As the engine begins its movement, the connecting rod is not moving only up and down, but due to Thrust_load_diagram.jpg.5dd0c337b1b85733eee7c2305faea9e5.jpgthe rotation aspect is simultaneously moving sideways. This action from the connecting rod and the motion of the crankshaft place load forces on the piston along the plane of the connecting rod inline with rotation (known as the “thrust axis”). To allow the piston to move freely with this sidelong force, the piston cannot be perfectly round, or it would bind in the round cylinder bore. By applying ovality to the piston, the piston is free to move up and down as needed. The challenge in design is applying the proper amount of ovality. Too little ovality can cause the piston to contact the cylinder wall nearest the end of the piston pin, while too much ovality can cause the piston to ride too heavily against the cylinder wall along this “thrust axis.” Too much load along the thrust axis can result in heavy scuffing or seizure, when the piston breaks the oil film barrier and contacts the cylinder wall directly.

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This illustration shows piston ovality. The solid-lined ellipse represents the diameter of the piston as if you're looking down onto the dome.

Dave Sulecki commented on ovality, "Ovality is an unknown thing, when most people look at a piston they think it is round, and to the naked eye this must be the case. However, take a new two stroke piston and roll it across the table and what happens? You will see the uneven “hump, hump, hump” as the piston rolls in a large arc…you are seeing both the profile (the “cone shape” of the piston”, in combination with the ovality as the piston rolls unevenly. Ovality is necessary for the piston to move up and down in the cylinder bore, as the crankshaft and connecting rod try to force the piston upward, and combustion forces the piston downward, ovality allows the piston to move without binding in the round cylinder bore."

Your bike's engine need a complete rebuild? Or maybe just a piston and valves? Check out our Garage Buddy line of rebuild kits.

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Another visual representation of piston profile and ovality.

Ovality is a key detail to remember when measuring piston size. The piston must be measured at the bottom of the skirt, 90 degrees from the wrist pin hole to reach an accurate measurement. 

When measuring piston diameter, be sure you’re using the proper tools. Do not use calipers to measure your piston(s), as you won’t get an accurate measurement. The most accurate tool to use is a set of outside diameter micrometers.

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Your piston should be measured at the bottom of the skirt, 90 degrees from the pin hole.

Please note: The measurements displayed here are for representational purposes only. Measure each of your own individual parts for accuracy.

Some Wiseco pistons feature proprietary skirt coatings such as ArmorGlide or ArmorFit, which are designed to reduce wear, provide smoother and quieter operation, and are applied to last for the life of the piston. With certain skirt coated pistons, piston-to-wall clearance measuring specs will change, so be sure to read the instructions that come with your piston(s).

Click here to find out more about Wiseco's different coatings.

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I tried the 90 degree rotation but couldn’t get the wrist pin to fit right, so I turned the rest of the engine 90 degrees too and it fit in perfectly; no problems with seizing.

  • Haha 1

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On 6/10/2019 at 10:59 AM, Kevin from Wiseco said:

The orientation of the connecting rod wouldn't allow you to put the piston in sideways.

Yes, Kevin. I'm glad you have a sense of humor!

:)

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hey, theres that picture of that turbo twostroke with the piston outside the cylinder. not sure how it breaks speed records out there...

 

the RCV model engine line. rotary cylinder valve. very interesting take on the sleeve valve idea. 

last conversation i had with them, they had sold virtually all rights to the military for the UAV engines... sleeve valves are notoriously quiet. they had a nice prototype honda cub type engine just before that. limit themselves to only the RC engines now. no more in house development.

the sleeve valve was popular back in the 30's. starts having issues when you crank the power levels up. bad heat transfer and lubrication issues.

again... losing the side thrust and the valve train losses still doesnt make an engine much more powerful. especially if other aspects are compromised at the same time.

hope the achilles is a success. i like opposed piston engines...

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11 hours ago, Keebler750 said:

Yes, Kevin. I'm glad you have a sense of humor!

:)

I might work if you have a big enough hammer, though. 😉

  • Haha 1

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On 6/12/2019 at 7:24 PM, sirthumpalot said:

There is a 4-stroke where the cylinder rotates to open and close ports that act as valves, I don't know if any big ones exist but the design is used in RC model engines.  Here is a guy assembling and running one, interesting design. 

 

However, motors for children are not good in a large dimension .. Why? Because the heat from the cylinder does not flow to the cooling fins ... and probably the cylinder also sometimes expands to heat, and it does not burn into the body, or blurs. As it has a large gap, it probably also gets a lot of nagar through these ports , which helps in rubbing ... Yes, that the engine is just for fun for children .. The guy looks like a big one, but it's still a child :)

Well, here's how you can piston, even a small cooler with water. Well, how the cylinder and piston are cooled water, it's higher temperature than 120 degrees C is nowhere inside the engine and the seals can be made of Teflon ... and the engine can work without oil, .. I wonder what Castrol says for this :)1576342500_elastichose.jpg.7414dce588a026d880dea1f51425732d.jpgwalki.jpg.032038dad57d559846e9714c19a33b04.jpg

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