really random question

[teyber]

hey all

i went to see conan live today and had to stand in a hour line :down: but i started thinking of the 4 stroke internal combustion engine/reciprocating compressors and had a little idea. Now im sure this is stupid stupid already been done or already implemented but just thought id ask.

*third warning- i have not yet taken any physics classes so just say your wrong if im wrong lol*

Wouldn't more surface area on the piston be much more beneficial? (in auto's and in refrigeration) so why isn't the top of a piston conical and mates inside another conical shape where the valves are, instead of having it be flat? wouldn't it be much more efficient?

Sorry for the random idea just wanted to ask.

thanks guys :up:

[Postal Dude]

You do get engines of this type of design. You ever heard of really old american muscle cars referred to as 'Hemi's'? They are called this because the internals of the piston and cylinder are hemispherical rather than flat. It does yield a good deal of efficiency increase but i believe its not cost effective with all the other developments in engine tech that yield greater power, such as electronic ignition and such.

[GTSRboy]

So basically you just had the idea for a Hemi engine, but your like 50 years too late

[teyber]

lol got it. Thanks folks :)

[SoddemFX]

I could be wrong but i thought that the "hemi" (hemisphere) refered to the combustion chamber and not the piston head...? :confused:

From my understanding the piston is still flat. I can't see any advantage in a hemispherical piston surface as all it will do is produce the same downward force, just with additional vectors into the side walls or piston itself.

Having additional dead space (the hemisphere) will decrease the volumetric efficiency of a compressor but i'm not sure how this would effect an engine.

I don't know much about this so i could be wrong.

Tom

[gosmeyer]

I could be wrong but i thought that the "hemi" (hemisphere) refered to the combustion chamber and not the piston head...?

Tom

Your Not!

[imprez55]

http://static.howstuffworks.com/gif/hemi-diagram.gif

http://static.howstuffworks.com/gif/hemi-flathead.gif

In a Hemi, just the combustion chamber is hemispherical, not the piston. If the piston were hemi (the entire top of the head) I would think that would add a lot of extra pressure to the sealing o-rings and make it easier to have compressed gas leak past, however, I did find this where the sphere is reversed http://www.patentstorm.us/patents/5727535.html (here)

There are actual pistons like you are talking about there there is a sphere but not the entire top of the head, they are called dome pistons. The do in fact produce a higher compression ratio but decrease airflow. So the piston would have to be larger for the same cfm in a compressor, but it would be a higher psi. A nice idea for a slow working high pressure (~3000psi) compressor maybe.

[GTSRboy]

Your Not!
whats this??
http://image.moparmusclemagazine.com/f/9604915/mopp_0602_07z+hemi_engine+hemispherical.jpg

[Sgrios]

whats this??
http://image.moparmusclemagazine.com/f/9604915/mopp_0602_07z+hemi_engine+hemispherical.jpg

In the picture you linked, It's called a piston crown..

http://www.dansmc.com/xr250piston.JPG

'Hemi' refers to hemispherical head design, IE. combustion chamber. The whole point of a hemispherical head is to have a centrally located ignition source. Also, try a hundred years ago, not fifty. Why people think Chrysler was the first to come out with a hemispherical head design is beyond me. Alfa Romeo in 1915 or so..

[wdrzal]

Lets say we got a 350 chevy normaly asperated with flat top pistons,say 10 or 10.5 to 1 compression ratio,that was pretty much stock depending on the volume (CC of the head) ..........you use domed piston to raise the combustion ratio to 11 or 12.5 to 1 this usually required high octane gas to prevent detionation. Now if you want to either turbo-charge or use a blower you use concave pistons say 8 to 1 so there is more room for the fuel/air thats under pressure.