Cylinder shape

Cylinders are the most efficient shape to give maximum capacity within a confined area.

Rectangles and ovals would have sealing issues within the bore as well expansion issues

Engine bloks would have inherent stress points were the piston runs

Ever noticed on board a ship. hatches, windows doors etc within the superstructure are either round or have rounded corners , prevents stress cracking

Very hard to get cylinder sealing if you use anything other than round, not to mention rather awkward to machine. There have been oval piston engines made for racing (honda NR), but there the increased costs are irrelevant, as is longevity and practicality. The pistons were wide along the length of the crank with two conrods per piston and 8 valves. Going wide across the crank would give a lot of skirt drag and wear.

Square piston engines exist (eg hossack)

Its not exactly a square piston

Can you picture the issues getting the corners of piston rings to seal?

I agree that the corners would need to be rounded. That's why I mentioned oval....

Google Honda NR750

Honda made oval ones briefly (NR engine)

A false oval cylinder would be easier to produce technically. I'm not sure what the real name is for that, but think of e.g. a running track with parallel straights and 180 deg circular ends.

Obround

Looks like Honda NR750 indeed has done it that way.

But the ring sealing would be harder than with a true ellipse.

On 09/09/2013 12:01, Duncan Wood wrote: > On Sun, 08 Sep 2013 18:22:27 +0100, Adrian wrote: > >> On Sun, 08 Sep 2013 14:12:25 +0100, GB wrote: >> >>> Apart from the obvious reason that it's easier to make round holes, is >>> there any reason why engine cylinders are circular in cross-section, >>> rather than oval or even rectangular? Oval would allow more capacity for >>> a given length of engine, which is obviously important for transverse >>> engines, where the engine size is quite restricted.

That difficulty of manufacture translates into a huge cost penalty.

Can't hone the bore as hone only does round things.

NR was oval in axis of the crank. GB seems to be suggesting oval in the other direction. That wouldn't easily allow more valves 6,8 unless a more complex valve train was used and would give problems getting the ports to middle valves. The other alternative is return to 2 valves but that will restrict cylinder size and require a lot of cylinders, 2 for each single 4 valve cylinder we have now, that would put the cost sky high for little benefit.

NR was really a V8. 2 rods, 2 sets of 4 valves, with 2 inlet/exhaust, 2 injectors, with a spark plug set in the middle of each set of 4 valves on each "cylinder". They needed a V8 4 stroke to compete with V/square 4 cylinder 2 stokes. Rules only allowed a V4 so they made a V8 with adjacent cylinders combined.