My ‘Otto’ engine already described in these pages is a reliable four-stroke engine that runs very nicely and is the result of much reverse engineering, successfully as far as I know (see description here). This bar stock design is relatively easy and all parts can be made by turning and milling from standard materials.
The only somewhat complex part is the cylinder head with the spark plug and the two spring-loaded poppet valves in it. The space for these parts is only just enough and one can say that making this cylinder head is kind of ‘surgeon work’. These valves are driven by tappets that run against cam disks on a overhead cam shaft.
Realizing this, the construction demands somewhat more knowledge and experience than for the remaining parts of the engine. Problems that occur mostly concern the cylinder head and sealing the valve seats, also keeping the the valve stems leak free. In addition, making the cam disks with the exact profiles that determine the valve timing is not daily work for most model builders. A practical problem is that cam disks and tappets can hammer-in to some degree in the long run, unless they are made from high tensile and hardenable steel.
One or two things gave me the idea to design an alternative system for this classic valve system that should eliminate these disadvantages/problems. Always trying to simplify things is my permanent and freaky objective.
During my working life I picked-up experience with disk valves that connect gas exchange processes, leak free, between rotating products (fluorescent lamps) and stationary pumping and gas flushing equipment. It is this experience that I used to replace the classic system with poppet valves and the associated drive mechanism.

The principle
Two disks with smooth flat surfaces and a keen hole and slot pattern can turn around against each other on a joint axis.
The stationary disk contains the connection to the outside world for the intake of the gas mix from the carburettor, for the exhaust of the burned gasses and for the connection to the cylinder head. There are also tiny holes and channels for lubricating the disk surfaces from outside with some oil.
The rotary disk is driven by means of a tooth belt with exactly half the rotary speed of the crank shaft that drives the piston. This 1 to 2 speed distribution is needed and is normal for every four-stroke engine. This rotary disk contains two separate blind slots that connect the lead to the cylinder head with the intake hole in the stationary disk for the gas mix and the exhaust hole respectively at the correct moments in the process cycle.
The picture illustrates these two disks with their hole and slot patterns.

A cam disk on the same axis pushes the piezo crystal to make the ignition spark when the piston is at DTP where the gas mix is compressed to the maximum. The whole set of disks is constantly pressed together with a set of cupped spring washers.
The figures below illustrate the overall construction.

The remaining part of this design is based on the proven ‘Otto’ engine and, from what I know, the exact timing of the gas exchanging processes. I made the hole and slot pattern in the disks so that that this process is copied exactly. As a result this process is implicit and distinct, fixed by the geometry of the hole and slot patterns in the disks. Practically it is a matter of ensuring the right radii and inscribed angles on the milling machine. Some little puzzle, once, only but not really difficult.

Advantages of this disk system

I am pretty sure that this system will have the following important advantages compared to the classic system with poppet valves and the driving mechanisms, at least for a single-cylinder model engine:

1. Except for the central positioned spark plug, the cylinder head only contains one and stationary lead connection for the intake of the gas mix as well as for the exhaust of the burned gasses. A simpler cylinder head is barely imaginable;
2. A leak free seal between the disk surfaces is considerably easier to make and stable than sealing the two poppet valves against conical seats. The valve stems and the seats must be exactly in line and the angle of the valves and their seats must be exactly equal too, so they must be lapped together. Damaging the valve seats by burning/erosion is eliminated here because the sealing surface of the disks is completely outside the cylinder;
3. Due to the absence of the poppet valves an a air leak along the valve stem and its bronze guide bearing can not occur here.The cylinder head of this design has 100% reliable air-tight sealing;
4. Making the hole and slot patterns in the disks is a matter of making the right adjustments of the disks under the head of the milling machine. Easier and much more accurate than making the cams for the poppet valves;
5. Wearing by hammering cam disks/valve pushers is absent here. The wearing of the relative big, flat and oiled disk surfaces is negligible or not present at all. For the material pearlitic cast iron seems to be a good choice, but probably almost every good quality steel, brass or bronze can be use for the two disks;
6. Adjusting valve springs and clearances are eliminated with this design. Once the two disks are fixed in their correct relative position nothing can drift away anymore;
7. The movement of the rotating disk is 100% continuous and as a result there are no shock loads on the crank shaft. This will improve the engine behaviour for sure.

Logbook

When I started the practical work I made a kind of log book so interested readers can follow my ups and downs during building the engine. To be honest I did not expect much stumbling because the engine is guided by the existing Otto engine and because the system with the disks contained few risks. However, it is all very well to be optimistic but problems can crop up.

November 28th 2008
I went to my good friend Jos to show him my first hand made sketches of my design. He is always critical in very positive sense. Together we checked all processes and constructions to make sure that there are no process and/or construction errors. As usual this resulted in some optimization and the mutual conviction that this design should work as forecast.

December 2nd 2008
The last few days I have been busy making the drawings for all the parts with 3D CAD and to assemble the engine on my computer screen with it. It is funny because you have the feeling that you are building the engine in your workshop without any machinery or materials. But get a nice idea of how the engine will look and if everything fits together.
My rough estimate is that anywhere in February 2009 the engine will be ready so that I can see then if my expectations become reality.

December 10th 2008
The two disks are ready and, in fact, it was a rather easy job. I used pearlitic grey cast iron and I think this is the best choice. Except for the fact that the machining is very easy I am convinced that this material contributes much to the fact that the disks turn very lightly against each other, even when I press them together firmly, The surfaces of this material never dig into each other and it is very wear-free. Because of the microscopic tiny holes in the material the surfaces can be lubricated with some oil, nd for that I made tiny oil channels in the stationary disk. I am pretty sure now that the friction between the disks will not cause any problem at all, even when I have to use rather high spring pressure to prevent the motor compression from blowing the disks apart. This, in fact, was my only concern over this design, but today I got the strong feeling that this risk does not exist.
I noticed that the friction decreases, the smoother the surfaces are made. Although it looks very good already after only turning I think it will be best to machine grinding the surfaces after all (with the help of a colleague of mine). I don't like abrading with sand paper because that can easy result in surfaces that are not flat.
I also worked-over my (existing) cog wheel. The rotary disc is butted against this cog wheel and I pressed a needle bearing in it.
It all looks perfect and it is going more prosperous than I thought it would. I consider the rest of the engine to be without risk as far as I can see now. Still some work to do of course but today I am even more than before convinced that it is worthwhile to do it.

December 13th 2008
Today I made the cylinder. I used my well-tried method again to make the cylinder bore exactly cylindrical and smooth. First I turned the bore to about 23,9mm diameter. Then reaming the cylinder bore manually with an adjustable reamer and with ample oil, turning around the cylinder several times with the same reamer adjustment until the reamer can pass easily through the bore. Then adjusting the reamer a fraction wider and repeating this treatment again and again until no more metal is removed. It takes some time, but it works very well, at least with the easy-to-machine (pearlitic) grey cast iron. This way of working can compete with honing and is a good alternative if you don't have honing equipment.
The result was fantastic: bore diameter 23,992 +/- 0,003 mm over the whole cylinder length! I measured this with a micrometer with reading accuracy of +/- 0,002mm and I could not measure any difference with that everywhere in the cylinder bore. So I can say that the taper is less than 0,003mm. Excellent, of course, the more so as I will not need piston rings for this engine.

December 17th 2008
Cylinder head, mounting plates, piezo construction and supports for disk-axis roughly ready. If I keep up this pace I estimate that the first start can be tried in the first week of January 2009.

December 21st 2008
The upper construction with the two valve disks, the ignition cam disk and the pressing mechanism with the thrust ball bearing and the pressure spring is ready.
I flattened the disk surfaces manually with fine sand paper on a glass plate and with some white spirit. It did take some time but the result was perfect: even with a spring pressure of 3 kg force the disks (with some oil in the oil channels) turn extremely lightly against each other. Fine grinding with a machine is probably the best thing to do but I don't have that equipment.
The spring pressure must be 3 kg force in this case according to the following reasoning:
The cold compression is about 4atm because the piston decreases its volume above with a factor 4 while it is moving upwards. When the gas mix ignites this pressure will be about 4 times higher; lets say 16atm. This high pressure will decrease fast when the piston is moving downwards to 4atm at his lowest position when the exhaust is opened.
The surface of the short slot in the stationary disk on what the lead to the cylinder head is connected is 0,25 square cm. At the moment of ignition this slot is positioned blind against the rotary disk.
The force on the rotary disk at the highest cold compression is 0.25x4=1kgf. At the ultimate moment of ignition the force on the rotary disk increases to 0.25x16=4kgf. As said this force decreases very fast because then the piston already is on its way down. The surface of the piston is 4.5 square cm so 18 times bigger than the surface of the slot in the stationary disk. So, at the moment of ignition the force on the piston will be 16x4.5=72kgf!
Theoretically it could be that with this 3kgf spring pressure the disks can be blown somewhat from each other during a fraction of a second but I think that will not have noticeable consequences. In worst case it could mean a slightly loss of engine power and I always can increase and I always can increase the spring pressure to 4kgf. This can not cause any problem looking at the ease with what the disks turn around now with a pressure of 3kgf. I even don't exclude that I can decrease the spring pressure without any noticeable consequences. I am not aiming making a hooligan out of the model engine. It is my only interest to make a 100% reliable and very smooth running engine with the most simple construction. The latter is a relative conception, of course, but nevertheless this engine can be made by just turning and milling with moderate mechanical craftsmanship. The only indispensable tool may be an indexing plate on your milling machine to be able to do the machining for the holes and slots in the valve disks at the right angles.
During half the engine cycle there are atmospheric pressure, namely during the intake of the gas mix and the exhaust of the burned gasses when the sealing requirements for the disks are minimal or none at all.
I will now start building the main structure of the engine plus the crank shaft, the piston and its driving rod. Then only the pressure roller for the tooth belt remains. After that I can make the first starting attempt, ‘loaning’ a Petrol Vapour Carburetor, a spark plug and a fly wheel from one of my other engines for the time being.

December 28st 2008
The base is also ready now, I only have to make the piston and the flywheel to attempt the first firing up. May be this can happen just before New Year.

January 3 rd 2009
The engine didn't run at the first attempts to start it, altough I did expect that to be honest. After some analyses and experiments I discovered that I have to change the system that press the two valves together somewhat. It is a long story to explain this in my very limited English but I will try to do that when I have solved the problem. I hope that this will be the case within some days from now.

January 5 th 2009
After some days of experiments and some debugging the engine is running! on the test bench. I temporary borrowed the carburetor, the flywheel and an ignition circuit with high tension coil from another engine of mine. It will take me some ten days to make the engine stand alone, to determine if the engine will run on the piezo ignition (which I assume it will) and to finish the drawing plan. Finally it will be given some cosmetic attention.

January 7th 2009
I tried to let the engine run on a piezo spark, but did not succeed. The engine will run on on the piezo but it is not reliable. This is not new for me: two of my eight I/C engines run perfectly on the piezo but the other six run poorly or not at all. I did numerous investigations and measurement in the past to find out why it is not reliable, but I never found the real reason. The energy of the piezo spark is much less that that of a high tension coil so it looks that the piezo spark is kind of ‘border-line case’ so that subtle differences can just cause an engine to fail. I won't give up but I reckon with the possibility that I have to use the classic circuit with the (motor car) high tension coil. Should be pity but not that bad because I can build-in the high tension coil in the wooden base of the engine which I did with the other engines.
I need some ten days to finish the engine, inclusive polishing and lacquering work, and to bring the drawing plan up-to-date.

January 11th 2009
I have decided not to use a piezo for the spark ignition. The engine runs on it, but it is very unreliable and I don't like that. I build-in a rather small high tension coil and the capacitor in the wooden base so this more bulky equipment is totally invisible. With the external 12 volt supply from my recharable battery for my hand drilling machine the spark is much more powerful and the engine runs 100% reliably on it.

January 15th 2009
I finished the engine by giving it the final cosmetic treatment. So the project is ready now and you can see from this from the log book that it took me two months from idea to end result, including making the drawing.
Tomorrow I will see if the small improvement that I made for the regulator on the petrol vapour carburetor will work as I think.

January 21th 2009
I encountered a little problem with the disks after all: after some engine runs the disks run more and more heavily and finally I cannot start the engine any more. After inspection I discovered that the surfaces of the disks have oxidized around the exhaust slot causing the too high friction. The rust propensity of pearlitic cast iron indeed is one of the few disadvantages of cast iron and apparently the warm exhaust gasses with a lot of water in it are oxidizing the cast iron rather quickly. I am looking now for another material and Teflon (PTFE) seems to be very suitable here after I searched again for its properties: very resistant for chemical corrosion, a very low friction coefficient and resistant to high temperatures (260 °C). It need not to be lubricated and, in fact, one is even dissuaded to do that.

January 27th 2009
The first results with the Teflon disks are very positive. More details after I have optimized this construction tomorrow or the day after.

January 29th 2009
I made the first disks from 10mm thick solid Teflon. My presumed advantages of Teflon came true: very low friction without any oil lubrication and no corrosion at all. But I was caught with a new surprise: during running the disks gradually split from each other and the engine stopped running after about 1 minute when the space between the disks became about 0.1mm. Apparently the disks were warping a little due to very small thermal expansion. Indeed the thermal expansion coefficient of Teflon is rather high and its thermal conductance is very low. So, I think that local warming-up near the exhaust slot due to the hot exhaust gasses caused this warping. I contemplated that this effect would be more or less proportional to the thickness of the Teflon. And that appeared to be right: after I made two new 9mm thick disks from Aluminium with a 1mm thick Telon sleeve glued on it. The problem was solved. Glueing Teflon to aluminium is perfectly possible with the well known acrylate instant glue after raising the surfaces with rough sand paper.
I can say now that the project is finished and very successfully. Tomorrow I will adapt the drawing plan according to the lastest improvements so I can send these again to everybody interested.