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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.

wiseco_pistons_arent_round_002.jpg.0c0665e8c5811346e5de09fa22fbdb4c.jpg wiseco_sema_new_products_009.jpg.1388a7ab1c3296ddd25538e50387a743.jpg

(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.

wiseco_pistons_arent_round_009.png.321e1c057059d65e64dc9d3e8f4b88eb.png

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.

wiseco_pistons_arent_round_008.png.82fb1c36906b2696d366f750adbecd3d.png

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.

Diagram.PNG.4a91cb47ecc4c54b5372023ece749525.PNG

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.

wiseco_pistons_arent_round_006.jpg.6a4264e8f150a19e901899220e5d4034.jpgwiseco_pistons_arent_round_005.jpg.eeb30feb0ff1257eb67f2533924bafd0.jpg 

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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Thanks for this article Kevin. I have mentioned this to others in my tight circle and showed them the arc and hump, hump, hump. It is not a mind game (visual trick) I was playing. :smirk: 

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I have been building engines for over fifty years. These new reasons (to me) describing the attributes and needs of piston design have confused me. I thought I had solid reasons for my modifications to engines, pistons in this conversation, which refused to cooperate with the demand of more compression power and of course HEAT.

My solutions seem to be purely coincidental to success rather than meet your reasons for the taper and ovality of pistons. I speak from familiarity with two-strokes, four-strokes, hot air and of course the steam engine. I will re read the article tonight. Maybe it will make sense to me then. Thanks for the read.  

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What I learned in Diesel school is that pistons are also part oval because the metal expands as it heats up and the mass next to the wrist pin expands a little more because it gets hotter. And if you measure the bottom of a cylinder sleeve with calipers it will be wider by a small amount than the top of the cylinder sleeve. That is for diesel engines at least I don't know about all engines.

Edited by Tchaikovsky

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Yeah Thcai,

That was what made sense to me when I was taught and have applied. As I understand it the ovality is needed to compensate for the expansion  for the extra aluminum in the wrist pin areas. 

The concept of piston rock due to thrust forces requiring extra clearance 90 degree opposite the force is a new one for me. 

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even the two sides of the piston are different as the exhaust side gets hotter again...

choice of material and alloy becomes important.

same in fourstrokes in most regards, bit more clearance due to higher temps, more scuffing, and deeper skirts due to less support in the cylinder... and having to keep ports closed. just compare a crf to a cr piston... the crf also gets to have a shorter rod as it can tuck in closer to the shaft. higher revs... but more side thrust... no need for skirts under the bosses though!

oh. and the oil control rings...

they take some serious abuse!

nasty when they pop their top...

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I did know this actually.  I worked for a company that made machines that machined pistons for Briggs  and Stratton.   We had to check them on a CMM for the proper ovality.   I can’t remember what the tolerances were but they were tight.  

I never knew the reasoning though.  Good article.  

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The piston does not have to be round ... here square piston ... Cylinder in General does not wear out.The sealing can be made of teflon ... then it works without oil

halfrotate400.gif

Edited by Feliks
add tezt

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The new Ford diesel has steel pistons.  They say it holds up better to engine mods and they are able to have more oil passages within the piston.

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I raced in the seventies with a buddy who was a machinist.  He kept seizing his piston so he being a machinist took care of the out of round piston he discovered.  Sure didn't take care of his repeated piston seizures.  He later found a bent rod was the culprit but he learned something.

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Well, there is still a lot of learning in front of you ... :) 

Here is a 3 piston engine, with 125 ccm ,  I can do 50 Hp, at 10,000 rpm

And at one time I did not believe it was possible ... :) 

Andrew .. halfrotate400fazy.gif.d87ec88e4215dc211da8712a25df13f9.gif731536866_NE8E6F1.GIF.6035c35961b5f9e20de0782c6c5f3af3.GIF

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19 minutes ago, Feliks said:

Well, there is still a lot of learning in front of you ... :) 

Here is a 3 piston engine, with 125 ccm ,  I can do 50 Hp, at 10,000 rpm

And at one time I did not believe it was possible ... :) 

Andrew .. halfrotate400fazy.gif.d87ec88e4215dc211da8712a25df13f9.gif731536866_NE8E6F1.GIF.6035c35961b5f9e20de0782c6c5f3af3.GIF

yay... more animations of proposed engines with ridiculous claims on power that fall sadly short when actually BUILT.

the round (ish) piston, cylinder and rings all work together to assist with sealing. sealing has been the issue since way back... trevethick, palpin, watt... even that hero guy... all had issues with sealing.

wankel? blows seals...

the piston valve looks great on paper, but when you MAKE one... you suddenly start to see all these failings in the design.

rings still catch on ports... still get scuffing. lubrication.

and most importantly... rapid opening with a large aperture, that can remain closed for 3/4 of the pistons stroke... and fit into the space of a head...(its a fourstroke... think about it...)

the sleeve valve is superior and actually did make it into production. unlike these types of "inventions".

 

the rocking paddle (joined via crank and conrod so it produces secondary vibration thats nearly impossible to cancel out rather than true sinusoidal motion...) you will find very familiar if you dig through actual BOOKS from the 1850s to about 1930s... you dont find that stuff online. which is why people continue to recreate them every few years.

and michel, of thrust bearing fame, was making three cylinder opposed piston engines a century ago...

plenty of ideas but only two have survived... pistons and turbines. the rotor held on from sheer tenacity but sorry... its gone now. forget about it.

everything else is merely a pipe dream. square pistons defy the laws of combustion anyway. only morons want combustion chambers with corners.

try MAKING an engine. from scratch. on a lathe. these fancy ideas simply dont work.

 

 

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1513191064_feliksandnew4stroke1985.thumb.jpg.15f306aaed9b7f94d9f9e10005c547d7.jpgWell, I'm very happy Well, I'm very happy that you are skeptical .. because actually all these "inventions" actually do not suit anything .. And I have bad luck, because new ones have such opinions, more right ... BUT NOT MY :)
I write, b and I have already made two such engines on this lathe and the latter has 250 hp at 10,000 with 600ccm .. Well, the ports in two, are poorly lubricated, but I'm good, because it's 4 strokes .. I completely understand your aversion to these novelties ... But this time think about it and believe me, because you can not even count their jumping volume, and the rest of me can surprise you and this is unending in a minute .. Sometimes it could be in plus, but very rare .. and this all together and more than you can see is in plus ..

Edited by Feliks

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Well this ended up rather off topic...

Wiseco, thank you for the detail and explanation!  I knew they were tapered and oval but not to what detail.  Thanks for the details and reasoning.

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pffft. topic. who wants to stay on topic? i wanna bag out ideas on alternate engine designs! (feel free to move this and feliks contribution elsewhere-moderators)

such as...

seeing as the current pics are in the 60-70 era, lets go for ralph sarichs...

slwa_b3096480_3.jpg

 

ah yes, the orbital engine, touted as the next big thing... oops. did it actually ever manage to run under its own power? didnt he eventually stick pistons and cylinders on it as well? his only real advancement was on the fuel injection system he devised...

another australian... i like bagging out my own countrymen...

the beare head!

Exhaust+stroke+ends,+intake+begins.+rota

 

so, he gets a twostroke case and bolts it on in place of the traditional fourstroker head on his... bike.

i LOVED this idea when i saw it, but i could see the flaws...so many flaws...

like needing all these extra disc valves etc because sadly, a two stroke cylinder and piston with a direct 1:1 or even 2:1 gearing with a fourstroke piston and cylinder DOESNT WORK OUT.

 

i admit, i figured it out. theres a really simple method, and well... feliks, your three cylinder animation is so very close... yet so very far off. i dare say it could be much better if the animation is a good illustration of what you are doing... and it obscures the problem by having the little conrods change length... not a great animation.

its all to do with the stroke of the piston "valves".

 

you need them to cover the ports for 3/4 of their stroke... yet provide sufficient opening time in 1/4 of their stroke. whilst keeping dead space to a minimum. and this is extremely easy to do...if you think outside the box a bit.

yes, once you remove the valve train losses, and also add some extra POWER via the valve train instead...you can increase your power output. in that regard the "opposed piston" valve train is quite smart. still, you cant beat physics, carnots cycle, or thermodynamics (though i have my doubts...)

thing is... i aint about to disclose MY secrets on-line!

the commer knocker and napier delta and other opposed piston engines also tend to prove that the valve train losses arent that high anyway...

 

another one... hossack engine. i admit, you do find this used in air compressors at times. little toy air motors...

hossack-engine-2.jpg

 

its sorta clever as it has no conrod and the piston rocks instead. no little end lubrication issues, either... direct injection with wet sump anyone?

shame the "rings" dont seal as well as traditional round ones... and we still have that nasty combustion chamber with square corners to promote detonation...

 

a whole range of engines and pumps came from ...lamplough? lamplaugh? again... turn of the century, 1890-1910 era, when everyone was having a go at new fancy designs, they get the closest to successful.

yet we still only have the piston and the turbine...

the halt (hult?) engine? one of the first rotaries. twin door/shutter. single door/shutter. they always ended up with reciprocating bits and sealing issues and no torque and bam...back at the start again. its no longer rotary when it reciprocates.

occasionally you see "new designs" on youtube that are really just attempts at previous designs... they seem to pop up roughly every 3 decades if my collection of old mechanics illustrated serves me right... people spend a lot of time and money but fail to buy some old obsolete books on the subject first and get caught out with patent violations and the like.

and of course, these days...it isnt if an engine makes power... or how many CC it takes to do so...

its all about how many litres of fuel it takes to produce that power.

thats why the rotor is dead. we could handle its quirks, except that one. it drinks fuel and spits it out the exhaust, raw...

no new engine will ever be successful unless it drastically increases power to fuel consumption ratio, closer to something approaching the theoretical maximum rather than the current 25-35% at best...

 

jamming more fuel in achieves nothing. getting more bang for your buck does.

it also has to be cheap, and easily built. reliable.

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new4strokegreek2.gif.6fedb23aa1c531a7638a8e8f36cceafd.gif

Well, I see that there are many specialties from new things ... Even something about absurdities is saying .. And that's the way to learn about the new one at the very beginning ... But how someone thinks that such powers will be achieved after viewing the animation and the picture of the prototype, unfortunately he is in a big mistake .. Unfortunately he will have to exchange 75% of his knowledge about engines, then he will be able to enjoy the benefits of a new engine .. But I will not do another lecture .. you can do some research ... before this blessing of absurd will flow for critics .. because I started in 1979 and last confirmed in 1985.
It's good to learn what, you only need a little, not watching nonsense ..

http://forum.engine-research.com/index.php?PHPSESSID=tkuj76i7d40blvi8rqmgljnuq6&board=4.0

Edited by Feliks

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14 godzin temu Chris Russell powiedział:

yay ... więcej animacji proponowanych silników z absurdalnymi twierdzeniami o mocy, które niestety są krótkie, kiedy ZBUDOWANE.

wankel? wieje uszczelki ...

Wygląda świetnie na papier, ale kiedy WYKONUJESZ jeden ... nagle zaczynasz dostrzegać wszystkie błędy w projekcie.

pierścienie wciąż łapią porty ... wciąż się zacierają. smarowanie.

 

Przeczytaj o Commer T 4 tutaj zobacz, ile silników może przejść z portami i pierścieniami

https://en.wikipedia.org/wiki/Commer_TS3

widziałeś mój projekt commer?

16tlokow4.jpg.35a3c623bf9109f01225970bb50c283d.jpg

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55 minutes ago, Feliks said:

new4strokegreek2.gif.6fedb23aa1c531a7638a8e8f36cceafd.gif

Well, I see that there are many specialties from new things ... Even something about absurdities is saying .. And that's the way to learn about the new one at the very beginning ... But how someone thinks that such powers will be achieved after viewing the animation and the picture of the prototype, unfortunately he is in a big mistake .. Unfortunately he will have to exchange 75% of his knowledge about engines, then he will be able to enjoy the benefits of a new engine .. But I will not do another lecture .. you can do some research ... before this blessing of absurd will flow for critics .. because I started in 1979 and last confirmed in 1985.
It's good to learn what, you only need a little, not watching nonsense ..

http://forum.engine-research.com/index.php?PHPSESSID=tkuj76i7d40blvi8rqmgljnuq6&board=4.0

much better animation... the dwell issue is still there... rings will just love ports like that... lot of dead area on the exhaust stroke...unswept volume...so very close to the ideal but still...no. havent cracked it.

 

and how to balance?

 

dokładnie która strona jest odpowiedzialna za to tłumaczenie? to jest okropne! dość. nie mogę kontynuować w ten sposób. przeciwstawna koncepcja zaworu tłokowego to dźwięk. wykonalny. rozważałem tę samą koncepcję od lat. czuję, że brakuje jednej rzeczy. ekscentryczność. Pomyśl o tym. to wszystko. oprócz, jest to niewłaściwy obszar forum forum dla tej rozmowy.

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Occasionally I see mechanical discussions on forums get hijacked by “dreamers”.  Most of these dreamers fail to spend any time reading existing literature on designs that have already been conceived, designed, built and tested.  They are unfamiliar with current restrictions and goals that the real designers inside the great engine houses (Honda, Mercedes, BMW, VW, Renault, Ferrari, Kawasaki, Yamaha, KTM, GE, Rolls-Royce, etc) face.  Reality 1:  the types of fuels and lubricants available are governed by economies of scale for mass production.

Reality 2:  the materials, tools and methods available for construction are also restricted to those that are suited for mass production 

Reality 3:  serviceability on a consumer scale (if it needs service who can fix it and how can they get the parts)

Reality 4: does a design change 1) reduce weight, 2) improve power per unit of energy source, 3) decrease cost 4) improve reliability 

I joke that the goal of performance design is smaller, faster, cheaper, bigger, and exactly the same, as moving the capability in one dimension negatively affecting others.

Most exotic Engine designs are creative mechanical linkages that sacrifice some dimension of performance to compensate for a known deficiency.

The most recent drive train improvements to reach consumer are the double clutch automatic-shift transmissions that enable 8,9,10 and more gear ratios to keep the engine in the optimal power-torque range, electric hybrid engines that allow for higher efficiency Atkinson cycle engines and all-electric drive.  There is a common theme to these: none of which are bad seals or complicated linkages, or 90% reduction in reliability.

 

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Oh yes, the problem with the dwell  is probably that it is extremely small for a 4 stroke engine. there is no problem with ports, unless you think that 1,200,000 miles are too little ... Read more ...
if there is no excentricity, then look at such an engine, because the top gives the most torque increase .. and why? do not see the animations ...
ad 1. - first high-speed diesel .. very easy production in "flat version"
2. only cast iron and aluminum .. + bearing
3. no servicing ...
4.does a design change 1) reduce weight, 2) improve power per unit of energy source, 3) reduce cost 4) improve reliability
"higher efficiency Atkinson cycle" - If you learn how to calculate the volume of this engine, you will find that it is a completely new cycle .. But first you need to know V.
Well, if you know V already, then you'll find that the motor has a variable compression ratio, which you can not deduce from the animation ... Just change the angle between the shafts ... in a very large range ..

blockspark1.jpg.f7bbca0caf715db0dad58585616800e2.jpg6cilinder.jpg.ca1cef518746045ebdd90d3354257a06.jpgdolnozaworowy.jpg.193ff83b8e33b410e8096089a7c2b9e8.jpgOh yes, the 

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On 6/5/2019 at 5:44 PM, Feliks said:

Well, there is still a lot of learning in front of you ... :) 

Here is a 3 piston engine, with 125 ccm ,  I can do 50 Hp, at 10,000 rpm

And at one time I did not believe it was possible ... :) 

Andrew .. halfrotate400fazy.gif.d87ec88e4215dc211da8712a25df13f9.gif731536866_NE8E6F1.GIF.6035c35961b5f9e20de0782c6c5f3af3.GIF

You are packed so full of bullshit, you should be more interested in a methane powered engine....

  • Haha 2

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2 hours ago, mixxer said:

You are packed so full of bullshit, you should be more interested in a methane powered engine....

Well, I see that I would like to learn again 75% of what you know about engines, continue with your opinion and do not know how the engine that you use is inadequate ... and then even methane can be ... finally you have something to learn ... and do not think that you have already achieved everything ...

And here you also have to repeat the geometry from school to have this capacity to jump and look at the size of my hand ... I'm really not a giant ...

1820467928_halfrotate600ccm.JPG.cc935a6159e58e38c55dddb32a6a0a22.JPG

 

piston1.jpg.77f291a34ac7022db2cb4e5136d07714.jpg

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So...if you accidentally put the pistons in sideways, the oval shape can cause it to seize while running, from being tighter. Is that correct?

 

?

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5 minutes ago, Keebler750 said:

So...if you accidentally put the pistons in sideways, the oval shape can cause it to seize while running, from being tighter. Is that correct?

 

?

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

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