Hemi-spherical combustion chambers (which give the Dodge Hemi engines their name) are a nice feature because they create a much more even flame front and more complete combustion. The only problems are that they require the use of only two valves per cylinder, and they discourage higher compression ratios.
Instead, you could shape the combustion chamber like an ellipsiod, a three-dimentional expression of an ellipse, much like a stretched-out or flattened sphere.
The flattened sphere shape would probably work best, making a roughly M&M shaped combustion chamber. This would allow less inclination of the valves, possibly allowing four valves per cylinder or at least easier breathing. You could also place the spark plug(s) very near the focus of the ellipsoid, resulting in more even pressure along the inside walls.
A stretched-out sphere, with a pointy end jutting up into the head, would allow for even better placement of the spark plug, but would create problems for the valves.-- discontinuuity, Jul 15 2005 Laser Spark Plug http://www.newscien...article/dn8907.htmlUniversity of Colorado's idea to create ignition with pulses of laster light via special fiber-optics [discontinuuity, Apr 03 2006] Dodge Hemi engine http://auto.howstuffworks.com/hemi4.htmHowStuffWorks article about hemispherical combustion chambers [discontinuuity, Apr 03 2006] Since the focus of a flattened ellipsoid is really a circular ring floating inside the middle of the ellipsoid shape, it would be possible to use the laser ignition system in the link, along with special lenses and mirrors, to create ignition along this ring of the focus, or simply at the center of the combustion chamber. This would make the flame front propogate just as far and just as quickly in all directions, especially if the charge is very homogeneous.-- discontinuuity, Apr 03 2006 Im not sure there is anything really new here. Having disassembled many engines I have found that there is a dizzying array of different cylinder head and piston configurations out there ranging from your basic flat head engine to one that utilizes a little tower in the center of the piston as the electrode(anybody remember who developed this?).
What are the advantages of this system over any of the exisiting systems?-- jhomrighaus, Apr 03 2006 //create ignition with pulses of laster light// Are these longer lasing pulses?-- coprocephalous, Apr 03 2006 The advantage of the pulsed laser would be that you wouldn't have to replace old corroded spark plugs. If anything, I think it would be a shorter pulse to concentrate the energy.-- discontinuuity, Apr 03 2006 this may be an impractical idea:
you know thoes things you see in frankenstein movies that look like "rabbit ear" TV antanie that have the ark of electricity that moves up it? (does anybody know what these are called?) could you make your spark in the combustion chamber "move" in a similar way? im guessing you would want it to move "down" toward the piston as it goes through the power stroke. would this let you better control the flame propigation? either way, if this is possible you wouldent need to worry about plug placement in the head. i'm sure there is a very good reason why this wouldent work. maybe thoes things dont ark hot enough to ignite gas or something.-- auricom_mech, Aug 02 2006 The thing is called a jacobs ladder. The voltage needed to create a spark increases with the fuel air mixture. In reality though that is all a spark plug really is.
As to replacing spark plugs, many modern engine designs can go 100,000 miles between spark plug changes, so im not sure worn out Spark plugs is a really good impetus to change things.-- jhomrighaus, Aug 02 2006 The Hemi is more efficient than other cylinder heads because for a given head capacity it has the smallest surface area i.e. less heat is lost through the head.-- madness, Aug 03 2006 That's where the thinking started, but in practice the conventional Hemi has quite a poor surface-to-volume ratio. The combustion chamber is shaped not like a solid hemisphere but like a piece of orange peel. This thin shell shape results in quite a long flame path. The advantage of the Hemi is rather that it allows for two huge valves.
Four-valve hemi-heads are possible. Abarth had the Testa Radiale conversion for the old twin-cylinder Fiat 500, with four inclined valves whose axes converged in the region of the piston crown at tdc.
I've given thought to properly spherical or, more practically, ellipsoid combustion chambers, which seem to me most viable in high-boost-low-compression supercharged applications. More on an ideal arrangement next year...-- Ned_Ludd, Dec 22 2006 random, halfbakery