Why round pistons not oval????

[vtec]

Oval pistons mean that you need to have near perfect design and macining to get them to fit. With oval, you need to line up the conrod connection perfectly with the cylinder, so that you aren't putting twisting force on the oval piston causing strange wear on the piston and cylinder walls, whereas with a circular piston, you just have to make sure that the conrod is in the centre of the cylinder.

Also, with oval piston, there is much more cylinder wall area for less piston surface area, thus meaning that more heat energy will be absorbed into the cylinder walls, effectively absorbing more of the explosive energy of the combustion than a round piston engine. This effect may be minimal though.

Another point was that if it's not circular, then there is more piston circumference for the same piston surface area meaning more piston to cylinder wall friction, again, I think this is minimal. But to be quite honest, I don't see the need for any of this fancy technology. All it seems to do is make it harder.

I haven't even put much thought into the piston ring difficulties that would come with oval pistons.

So, what are the advantages? You can make a larger bore engine skinnier... We don't really need to, the current flock of inline 4 engines are seriously bitchin'. If you want to make it skinnier while being large bore, then just use a V configuration... V4 or V5 really kicks the Llamas ass.

Anybody think of any other advantages? Apart from bending racing rules of course.

[Fooman]

Oval pistons mean that you need to have near perfect design and macining to get them to fit. With oval, you need to line up the conrod connection perfectly with the cylinder, so that you aren't putting twisting force on the oval piston causing strange wear on the piston and cylinder walls, whereas with a circular piston, you just have to make sure that the conrod is in the centre of the cylinder.

The Honda engine had 2 conrods per piston - Also any small misalignment in the cylinder is normally accounted for in the rings, they are providing the actual sealing surface

I haven't even put much thought into the piston ring difficulties that would come with oval pistons.

Honda have a few patents relating to the oval rings. That may be why others haven't come up with oval pistons - Honda may control the technology to make them work.

Anybody think of any other advantages? Apart from bending racing rules of course.

As previously mentioned, the reason Honda used oval pistons was to effectively have V8 valve area in a (limited by the rules) V4 motor. For the same displacement, the higher the number of pistons, the larger the effective valve area. The more valve area, the more efficient the combustion (more energy out). And pistons are smaller, so they generally rev higher.

More valves = more efficient * higher revs = more power.

As many have also pointed out, the most likely reason why we use circular pistons is that it is trivial to machine something with a circular cross section, both internal and external. Oval pistons are a lot harder = a lot more expensive.

FM