Internal combustion engines are one of my favorite interests. They are used to power everything cool, like cars, motorcycles and airplanes. There are innumerable variations on the original Otto cycle piston-and-crankshaft internal combustion engine, but for our purposes they may be classified into two groups: flathead engines and overhead valve engines.
Flathead engines are simpler and more primitive. Overhead valve (OHV) engines are more complex, more sophisticated and more efficient. Flathead engines are still used for some lawn mowers and other small, powered machines. OHV engines are used for all high power applications, and are even making inroads into the smaller machines.
Here are a couple of pictures from http://auto.howstuffworks.com/hemi1.htm that show the differences between the two engine designs. The Hemi is an overhead valve design.
Flathead engines suffer from one malady that does not afflict OHV engines. That is the accumulation of carbon in the combustion chamber. Due to the shape of the combustion chamber, flathead engines tend to accumulate substantial carbon deposits directly opposite the valves. Periodically the cylinder heads must be taken off and the carbon deposits removed. Failure to do this in a timely manner will result in damage to the engine as the valves will start to impact the carbon deposits.
Apart from the unfortunate cylinder head configuration, a flathead engine is otherwise perfectly sound and, this is the important part, cheap. If there were only some way to fit an OHV arrangement to the top of an inexpensive flathead engine, you could have the benefits of both. It took me a while to come up with an idea on how this might be done without having to resort to casting and machining.
Take two single cylinder flat head engines. Remove the cylinder heads. Take a piece of plate metal and cut two oval holes in it. Each hole would be big enough to accommodate both the intake and exhaust valves. When the valves open they protrude into the combustion chamber. The plate would need to be thick enough to accommodate this motion. The plate would be set on top of one (headless) flathead engine, one hole over the valves, and the other over the piston. The second engine would be inverted over this and set on top. The valves of the second engine would be positioned within the oval that is over the piston of the lower engine, and the piston of the lower engine would be positioned above the valves of the lower engine.
In cross section the combined engines would look something like this (Please ignore the line across the middle of the drawing. My computer picture editing skills still need some work):
The crankshafts of the two engines would be connected with a chain to synchronize their rotation. We would now have a horizontally opposed, two cylinder, OHV engine!
There are numerous details that would have to be dealt with, but I think that it could be done. First would be spark plugs, or rather holes to put them in. I do not imagine our new plate would be thick enough to accommodate a spark plug hole drilled from the side, but it might. Some other device may be necessary. Head gaskets would need to be fabricated, but they could be cut from a sheet of copper. As the engines themselves would be blocking access to the former bolt holes for the head bolts, holding the whole assembly together might require running long bolts from the bottom of one engine to the bottom of the other. The chain would be rather long and might be subject to stretching. For a small engine like this it might be possible to use a toothed rubber belt. That would also eliminate the need for the a sealed chaincase. (A chain would need lubricating oil.) If horizontal shaft engines were used, internal engine lubrication would have to looked at. It would not be a problem for vertical shaft engines.
Update December 2016 replaced missing pictures.
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