# Why circular pistons in engines? **URL:** \ **Category:** Factual Questions\ **Created:** [August 17, 2005, 1:53am UTC](https://boards.straightdope.com/t/why-circular-pistons-in-engines/317370 "2005-08-17T01:53:46Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Absolute](https://avatars.discourse-cdn.com/v4/letter/a/b2d939/32.png) [@Absolute](https://boards.straightdope.com/u/Absolute)\ **Post date:** [August 17, 2005, 1:53am UTC](https://boards.straightdope.com/t/why-circular-pistons-in-engines/317370/1 "2005-08-17T01:53:46Z")
Why are circular pistons the only shape used in engines? It would seem that using square, rectangular, or triangular pistons would result in a smaller and/or lighter engine block, because they can be packed together more efficiently. You could increase the displacement of an engine without increasing it’s external size if you simply used square pistons instead of circular ones. Is it simply too difficult to manufacture differently-shaped pistons and combustion chambers? Are the stresses too unevenly distributed to make it feasible? The Wankel engine uses a quite irregularly-shaped combustion chamber and rotor, so I wouldn’t think these would be insurmountable difficulties. ---
**Author:** ![brianjedi](https://avatars.discourse-cdn.com/v4/letter/b/3be4f8/32.png) [@brianjedi](https://boards.straightdope.com/u/brianjedi)\ **Post date:** [August 17, 2005, 2:05am UTC](https://boards.straightdope.com/t/why-circular-pistons-in-engines/317370/2 "2005-08-17T02:05:55Z")
One word: corners. The edges and corners on a non-round piston would get worn smooth after only a few hundred miles. When that happens, you lose compression because you can’t maintain a tight seal in the combustion chamber. ---
**Author:** ![gazpacho](https://avatars.discourse-cdn.com/v4/letter/g/6f9a4e/32.png) [@gazpacho](https://boards.straightdope.com/u/gazpacho)\ **Post date:** [August 17, 2005, 2:20am UTC](https://boards.straightdope.com/t/why-circular-pistons-in-engines/317370/3 "2005-08-17T02:20:06Z")
Also the corners on the “cylinder” walls are a weak point and will propogate cracks. Round cyliners don’t have these weak points. ---
**Author:** ![Hoodoo\_Ulove](https://avatars.discourse-cdn.com/v4/letter/h/58956e/32.png) [@Hoodoo\_Ulove](https://boards.straightdope.com/u/Hoodoo_Ulove)\ **Post date:** [August 17, 2005, 2:48am UTC](https://boards.straightdope.com/t/why-circular-pistons-in-engines/317370/4 "2005-08-17T02:48:28Z")
It’s been [tried](http://int-trap-halt.razor.jp/photo/honda_collection_hall/pic/P7300379.JPG). This is some of the Honda NR500 gran prix racer from around 1980. I think the rules specified a 4 cylinder maximum, and this was their way to get more valves per cylinder (8) and better volumetric efficiency. It didn’t prove faster than the competition and the idea was abandoned. ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [August 17, 2005, 4:14am UTC](https://boards.straightdope.com/t/why-circular-pistons-in-engines/317370/5 "2005-08-17T04:14:14Z")
I remember reading (somewhere) that some high performance engines have oval shaped pistons. This would seem to be a reasonable compromise. ---
**Author:** ![Can\_Handle\_the\_Truth](https://avatars.discourse-cdn.com/v4/letter/c/6a8cbe/32.png) [@Can\_Handle\_the\_Truth](https://boards.straightdope.com/u/Can_Handle_the_Truth)\ **Post date:** [August 17, 2005, 4:22am UTC](https://boards.straightdope.com/t/why-circular-pistons-in-engines/317370/6 "2005-08-17T04:22:13Z")
Trust me - as a former manufacturing engineer - making pistons in any shape other than round would be a manufacturing nightmare. Just off the top of my head, I wouldn’t be surprised if making square pistons and chambers proved to be ten times more expensive than round. ---
**Author:** ![cornflakes](https://avatars.discourse-cdn.com/v4/letter/c/9d8465/32.png) [@cornflakes](https://boards.straightdope.com/u/cornflakes)\ **Post date:** [August 17, 2005, 4:49am UTC](https://boards.straightdope.com/t/why-circular-pistons-in-engines/317370/7 "2005-08-17T04:49:53Z")
> [@1920s Style "Death Ray"](#): > > I remember reading (somewhere) that some high performance engines have oval shaped pistons. This would seem to be a reasonable compromise. Look at **Hoodoo Ulove** ’s link. Honda tried using oval pistons in an attempt to get the performance that could be had with an eight cylinder 500cc bike. The last I heard of the project was a press release saying that they did not see any increase in engine efficiency until 20,000RPM. ---
**Author:** ![zagloba](https://avatars.discourse-cdn.com/v4/letter/z/977dab/32.png) [@zagloba](https://boards.straightdope.com/u/zagloba)\ **Post date:** [August 17, 2005, 6:05am UTC](https://boards.straightdope.com/t/why-circular-pistons-in-engines/317370/8 "2005-08-17T06:05:59Z")
Getting the maximum displacement per total engine volume is only one consideration. The head has to be designed so that valves and manifolds can be placed for maximum flow in and out of the combustion chamber. You want the air/fuel charge to mix properly in the combustion chamber. The propagation of the flame front at ignition needs to be controlled – you don’t want hot spots that can lead to predetonation. These things are harder to acheive with more angular shapes. ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [August 17, 2005, 6:13am UTC](https://boards.straightdope.com/t/why-circular-pistons-in-engines/317370/9 "2005-08-17T06:13:24Z")
> [@cornflakes](#): > > Look at **Hoodoo Ulove** ’s link. Honda tried using oval pistons in an attempt to get the performance that could be had with an eight cylinder 500cc bike. The last I heard of the project was a press release saying that they did not see any increase in engine efficiency until 20,000RPM. According to my engine rebuilder all pistons are round in the area of the rings and very slightly oval in the skirt area. The oval shape is obtained by cocking the part slightly as the skirt is machined on a lathe. ---
**Author:** ![Hail\_Ants](https://avatars.discourse-cdn.com/v4/letter/h/dc4da7/32.png) [@Hail\_Ants](https://boards.straightdope.com/u/Hail_Ants)\ **Post date:** [August 17, 2005, 6:40am UTC](https://boards.straightdope.com/t/why-circular-pistons-in-engines/317370/10 "2005-08-17T06:40:00Z")
> [@](#): > > The Wankel engine uses a quite irregularly-shaped combustion chamber and rotor, so I wouldn’t think these would be insurmountable difficulties. Yes, but getting the corners of the rotor to seal properly and not wear out quickly was/is a big problem with rotary engines. ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [August 17, 2005, 6:43am UTC](https://boards.straightdope.com/t/why-circular-pistons-in-engines/317370/11 "2005-08-17T06:43:51Z")
Yeah I saw the link. The one I read about (it must have been a MotoGP bike, that’s the only engine related stuff that I read) just had nice oval shaped pistons and was being used currently. ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [August 17, 2005, 7:26am UTC](https://boards.straightdope.com/t/why-circular-pistons-in-engines/317370/12 "2005-08-17T07:26:28Z")
I would think that a square cylinder would be less efficient than a round one for pretty much the same reason that a flat top cylinder is less efficient than a hemi. ---
**Author:** ![cornflakes](https://avatars.discourse-cdn.com/v4/letter/c/9d8465/32.png) [@cornflakes](https://boards.straightdope.com/u/cornflakes)\ **Post date:** [August 17, 2005, 2:06pm UTC](https://boards.straightdope.com/t/why-circular-pistons-in-engines/317370/13 "2005-08-17T14:06:12Z")
> [@David Simmons](#): > > According to my engine rebuilder all pistons are round in the area of the rings and very slightly oval in the skirt area. > > The oval shape is obtained by cocking the part slightly as the skirt is machined on a lathe. Right. The piston crown is smaller than the skirt and the area around the wrist pin is oval in cross section. All this is done to allow for thermal expansion. I’ve also heard of some pistons being ‘barrel-shaped’, where the bottom of the skirt tapers inwards. I believe that it’s done to help quiet piston slap. ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [August 17, 2005, 3:16pm UTC](https://boards.straightdope.com/t/why-circular-pistons-in-engines/317370/14 "2005-08-17T15:16:59Z")
Lathes inherantly produce nearly perfectly round parts , and round holes are also simple to drill, bore, and hone to good finish and exact dimension. Any other shape requires incredably more complicated machinery to produce and finish…and likely not possible to equal tolerance achieved on a simple lathe even given cubic dollars to devote to the task. Pistons seal to the cylinder using rings. A round ring produces equal force and therefore wear in all directions. Deisigning a non-round ring that would do such might be possible, but would probably be difficult if not impossible to install on the piston. > [@engineer\_comp\_geek](#): > > I would think that a square cylinder would be less efficient than a round one for pretty much the same reason that a flat top cylinder is less efficient than a hemi. A hemi is not a particularly efficient combustion chamber overall. Thermally it is good, but it is not conducive to most of the high swirl/tumble techniques used to improve efficiency in modern engines. A hemi head is a way to allow slightly larger valves in simple two valve/cylinder designs, and also can result in a straighter shot from intake/exhaust port to combustion chamber (depending on engine configuration). It also seems to be a very effective marketing device. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [August 18, 2005, 8:29am UTC](https://boards.straightdope.com/t/why-circular-pistons-in-engines/317370/15 "2005-08-18T08:29:33Z")
What about propagation of the burn from the spark in a space that has corners that are significantly further from the spark than other parts of the bore? ---
**Author:** ![bouv](https://avatars.discourse-cdn.com/v4/letter/b/bc8723/32.png) [@bouv](https://boards.straightdope.com/u/bouv)\ **Post date:** [August 18, 2005, 1:02pm UTC](https://boards.straightdope.com/t/why-circular-pistons-in-engines/317370/16 "2005-08-18T13:02:58Z")
> [@engineer\_comp\_geek](#): > > I would think that a square cylinder would be less efficient than a round one for pretty much the same reason that a flat top cylinder is less efficient than a hemi. What, exactly, is the difference between a regular type engine and a emi? (Since Dodga has started putting Hemi’s back in their cars, I have always wondered what ther big deal was.) ---
**Author:** ![BubbaDog](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bubbadog/32/333_2.png) [@BubbaDog](https://boards.straightdope.com/u/BubbaDog)\ **Post date:** [August 18, 2005, 1:19pm UTC](https://boards.straightdope.com/t/why-circular-pistons-in-engines/317370/17 "2005-08-18T13:19:37Z")
> [@1920s Style "Death Ray"](#): > > I remember reading (somewhere) that some high performance engines have oval shaped pistons. This would seem to be a reasonable compromise. I had a 1980 Nissan (Then Datsun) 200SX that had oval pistons. It wasn’t high performance but it was a new engine design for the model. I don’t know if Nissan continued to use the design in later year models. ---
**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\ **Post date:** [August 18, 2005, 2:15pm UTC](https://boards.straightdope.com/t/why-circular-pistons-in-engines/317370/18 "2005-08-18T14:15:32Z")
> [@Kevbo](#): > > A hemi is not a particularly efficient combustion chamber overall. Thermally it is good, but it is not conducive to most of the high swirl/tumble techniques used to improve efficiency in modern engines. A hemi head is a way to allow slightly larger valves in simple two valve/cylinder designs, and also can result in a straighter shot from intake/exhaust port to combustion chamber (depending on engine configuration). It also seems to be a very effective marketing device. Getting the plug smack dab in the middle of the combustion chamber is very valuable, and in larger displacement engine that inherently produces good torque at low RPMS --\*\*and in which torque at low RPMs the goal \*\* – there is less need/emphasis on creating the high swirl in the combustion chamber. A hemisperical combustion chamber and a large displacement engine intended to tow and pull at low RPMs is a match made in heaven. ---
**Author:** ![PBear42](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@PBear42](https://boards.straightdope.com/u/PBear42)\ **Post date:** [August 18, 2005, 3:43pm UTC](https://boards.straightdope.com/t/why-circular-pistons-in-engines/317370/19 "2005-08-18T15:43:41Z")
The [HowStuffWorks](http://auto.howstuffworks.com/hemi.htm) on hemis. ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [August 18, 2005, 4:14pm UTC](https://boards.straightdope.com/t/why-circular-pistons-in-engines/317370/20 "2005-08-18T16:14:58Z")
> [@Philster](#): > > Getting the plug smack dab in the middle of the combustion chamber is very valuable. Indeed, which is possible in multi-valve heads. The exhaust and intake valve seats nearly touch near the center of any high performance two valve engine. The sparkplug is therefore offset to one side.Since the driving design factor of a hemi head is to permit even larger valves, they are as large as possible, and so the center is not available as a spark plug location. Half the thread of a centered plug would be visible in this [cutaway photo](http://www.mrtruck.net/hemi.h2.jpg). Instead the spark plug is placed through the boss that crosses through the green painted coolant passage. If the valves are made small enough to utilize a centered plug, then there is little to be gained by the increased valve train complexity required in a hemi design. Two valve engines can gain the benifits of a centered plug (and more) by[dual plugging](http://www.cylinderheadservices.com/5.7%20hemi.jpg). [Next page](https://boards.straightdope.com/t/why-circular-pistons-in-engines/317370.md?page=2)