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One of the problems with modern dual overhead cam systems is that in order to increase compression, domed pistons must be used, which can obstruct the flame front. Not to mention that they tend to be heavier, more complex, and more expensive than two-valve engines.
Imagine a conventional overhead
valve engine with two valves per cylinder more or less in the middle of the combustion chamber and a flat quench area on the left, taking up about one third of the cylinder head (see link below). Now add another pair of valves in the quench area, with the valve stems oriented vertically. All the valves can now be run off of a single camshaft, located at the intersection of the valve stems' axes.
You will still have all of the intake and exhaust ports coming out of one side of the head as with all wedge-head engines, but you will now have a high-compression engine with good swirl of the fuel-air charge, more valve area, and a compact, unobstructed combustion chamber. The only problems I foresee are in keeping the valves from interfering with the rising piston. You may need to close the vertically-oriented valves sooner than the angled valves to prevent interference.
If my description is lacking, please let me know and I will post a sketch of the idea.
dual-overhead cam design
http://en.wikipedia...derkopf-Schnitt.jpg
wikipedia illustration [discontinuuity, Apr 26 2007]
Wedge-head illustration
http://www.imps4eve...ylinderhead1963.jpg
[discontinuuity, Apr 26 2007, last modified May 20 2008]
OHC conversions
http://www.aardemasohc.net/index.htm
[Ned_Ludd, May 02 2007]
Illustrations
http://i198.photobu...uity/LastRoll-1.jpg
Quick sketch I made today. Note that the spark plug in the first drawing should be closer to the intake valves instead. [discontinuuity, May 05 2007, last modified May 06 2007]
Toyota five-valve engine
http://www.billzilla.org/vvt2.gif
shows the three intake valves arranged much like the valves in my design [discontinuuity, May 09 2007]
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Would probably work, although variable valve timing would probably be basically impossible. Not enough room for multiple cam profiles. Port routing would be pretty convoluted too. |
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As for valves hitting the piston, many high-compression cars have dimples cut into the piston head for just this reason. I have heard that in a formula one engine, these dimples actually constitute essentially the entire volume of the cylinder when the piston is fully compressed--the piston is otherwise essentially flush with the cylinder head. |
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If you don't mind all the ports coming out of one side of the engine (like with some old four and six cylinder engines), then port routing would be no problem. Keeping the heat from the exhaust out of the intake manifold would be, though. |
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I don't think that port routing would even be too much of a problem if you had the intake and exhaust on opposite sides of the head; at least, it would be just as much a problem as with any other wedge-head engine. |
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I just think that you'd have to choke the engine a bit, since routing all 4 ports through basically the same space on the engine means the diameters would have to be decreased significantly. 2 ports would be okay, but 4 is a bit much. |
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It's not necessary to have all the ports on the same side: most traditional American V8s have wedge chambers and cross-flow breathing. The intakes dodge pushrods: not necessary with your OHC arrangement. |
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It somewhat, though quite indirectly, recalls Pete Aardema's aftermarket OHC conversions (link). |
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After thinking about this, it would probably be easier to make it five valves if you wanted a cross-flow head. See the sketch I've linked to. |
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Do you think this is a marketable idea? |
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Have you looked at swapping the valves? i.e. putting the exhaust valves in a smaller wedge? It'd be better for the plug position. |
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Also, how about closing the included angle a bit, and then using the sort of cam followers one finds a lot these days, with one end on the valve and the other on an adjustable fulcrum, and the cam acting in the middle? It allows for a rocker ratio, so the lobe height can be smaller for any desired lift. You might also find that it'll make the packaging easier. |
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I did think of swapping the valves. The only reason I put the three intake valves on the inclined part of the head was because all five valve engines have three intake and two exhaust valves, and because I think it would swirl the intake charge more. I'm not sure how swapping the valves would give a better plug position, though. |
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I didn't want to use rocker arms since they tend to produce more friction and limit the revs you can reach. The idea was to keep things simple and inexpensive by using a single camshaft, but retain all the advantages of a DOHC engine. |
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By the way, do you think that having most of the combustion chamber over to one side of the cylinder would create bending moment stresses in the piston and connecting rod? I suppose it all depends on how the flame front propogates and if pressure becomes uneven. |
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I meant it'd be better for the plug position you've got: it's best to have the flame front travelling from hotter to cooler. |
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I've also had the intuitive sense that offsetting the combustion chamber would induce a rocking tendency in the piston, but common practice seems to belie the impression. I suppose there might be some sort of gas-inertia-impact effects while combustion occurs and the piston is pretty much at tdc; thereafter, i.e. throughout the power stroke, Pascal takes over and the pressure is distributed evenly over the piston crown. |
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It's one of those things that allow us to think, when considering side-valve engines, "what fools the ancients were!" That is always a dangerous way to think. Given fuels with an octane rating around 55 the side-valve combustion chamber was a perfectly rational solution. It worked well under the circumstances, even though combustion was not offset in but completely off the side of the cylinder bore. |
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