IN 2008 I made a 4-stroke engine with a rotary disc valve for the intake of the fresh gas mix and exhaust of the burned gasses. This engine can run perfectly but adjusting the spring pressures to keep the two discs in correct contact was critical. On the one hand the springs must avoid the discs to be pushed away from each other during high compression, although the friction between the discs must be fairly low to avoid jamming and/or damaging the disc surfaces.

Due to the difficulty of adjustment of the pressure balance I decided to withdraw the drawings because it will probably cause frustration to anyone making this engine and might not be able to get it to run reliably. I did not want to have on my conscience.
In retrospect, a rather simple calculation makes it obvious that this is a fundamental problem with this design; it can work, but not without continuous re-adjustments and other uncertainties. So I decided to make a drastic re-design of the rotary valve system. The result is the ultimate simple and reliable 4-stroke model that I gave the name: ‘Ridders 4-stroke engine’.

Principle of the ‘Ridders 4-stroke engine’
I maintained the principle to regulate the intake as well as the exhaust process with only one rotary valve because that means that the classic and complex system of poppet valves, tumblers, pushers and cam discs can be eliminated completely. The fundamental difference with the former design is that the two valve parts are not turning against each other but in each other. This makes it impossible for the gas pressures to push them from each other even without any springing system!
The CAD pictures (right) illustrate this new and very simple construction.

Recently I experimented with such a system for my RoDuBelle 2-cylinder 4-stroke but then I didn't succeed in making a good working rotary valve. It  became clear to me why this design failed. The bottom line is that the surface of the rotary mandrel was far too big causing enormous radial forces from the high gas pressures with instantly jamming of the mandrel in its valve body. Furthermore, asymmetries in the valve design were another negative factor.
From these experiences I designed a new rotary valve.

The rotary valve
In fact, the rotary valve mandrel is a standard steel 8mm shaft, turning in a bronze body with a precise and smooth fit. This stationary bronze body has only one central connection to the cylinder though which both intake and exhaust gasses pass after each other.

The valve body also has one connection to the carburettor and one to the exhaust. The rotating steel mandrel has two separate holes on the circumference, both connecting to a central bore. The holes connect the openings in the valve body at the right moment for the intake of the fresh gas mix, and exhaust of the burned gasses. In between the valve is blind during the compression and combustion stroke. In this way the 4-stroke process is completely and implicit determined by the geometry of the valve.
The geometry is completely symmetrical causing an equal gas pressure over the whole circumference of the mandrel so a kind of air cushion bearing occurs. Furthermore, the surface of this mandrel is a lot smaller than that of the former design so the forces on it are a lot smaller, too.
The mandrel turns in two ball bearings and with good alignment there are hardly any, or no, forces on the bronze valve body. This valve turns around light as a feather even when connected to a firm air pressure from a compressor; a very big difference with the previous version where the mandrel jammed immediately in that situation.
The valve mandrel is driven by a flexible toothed belt at half the speed of the crankshaft, normal and mandatory for all 4-stroke engines.
On the mandrel there is also a cam disc that initiates the ignition spark at the moment of maximum compression. As with all my IC engines I used the Petrol Vapor Carburettor connected to the valve with a rubber hose.
In fact, this is the whole story and it can't be made any simpler in my humble opinion.

Adjusting the engine
Adjusting this engine is extremely simple. The only thing needed is to make the start of the intake exactly on the moment the piston is in its (first) highest position (TDC). After this is done the rest of the process follows because it is implicit, fixed by the hole pattern in the rotary valve.
Making this intake adjustment can be done as follows:
1. Loosen the upper cog wheel on the valve mandrel so it can turn freely on that and then put the piston on its highest position (TDP).
2. Remove the spark plug and put a rubber hose on the intake pipe of the valve which the carburettor is normally connected to.
3. Blow in this hose while turning the valve mandrel clockwise until you hear the air escape through the spark plug hole.
4. Fix the cog wheel in this position on the valve mandrel again.

The hole pattern in the rotary valve is made such that at the start of each four process strokes (intake, compression, combustion and exhaust) is exactly on the ultimate four piston positions. In many designs there will be some deviation from that for efficiency reasons but the effects are negligible for small IC models and, in fact, they are of no importance here. This straightforward setting is not only easy, but the engine runs very well and reliably with it. So this meets my everlasting goal to keep everything extremely simple combined with high reliability.
The ignition spark must occur exactly at the moment the piston is on its second highest position when the fresh gas mix is compressed to its maximum pressure. This is a matter of fixing the ignition cam disc in the right position on the valve mandrel related to the electrical switch for the ignition circuit.
See for the working instructions for the Petrol Vapor Carburettor, the last sheet of the drawings for that.

Starting up and engine performance
One can best start the engine by turning the fly wheel with a hand drill with the air regulator on the carburetor quite open. While turning, decrease the air gradually until you hear the engine take over. At around this carburetor adjustment, the engine will run best while the engine speed can be regulated somewhat by gently adding more or less air.
The speed of this engine can be regulated between about 600 and 1000 rpm. Relatively low compared to, for instance, my ‘Otto 4-stroke’ with the same cylinder/piston combination that easily can run with double speed or more! Originally I thought that the relative narrow channels and holes in the rotary valve was the cause, but after some experiments this seems not to be the main reason. I now think the ‘half moon’ effects from holes that move over each other introduce a relative high resistance mainly at the start of the exhaust stroke causing a braking effect to the piston movement. Classic poppet valves will open more instantly I believe.

This could be a reason to choose for a somewhat earlier exhaust moment in the process, but that means changes in the angle between the intake and exhaust holes in the mandrel of the rotary valve. I keep it this way because in fact I like low speeds better than high speeds for models because 1-cylinder engines are often erratic and it is hard or not possible to balance them exactly.

Fuel for this engine
This engine runs on normal auto car petrol but Coleman Fuel is preferable.
Because this engine has no forced cooling the cylinder temperature will be 100 to 110 degrees Celsius after some 5 minutes running. No problem because the cylinder and piston is made from pearlitic grey cast iron with no chance of jamming at these temperatures. But the aluminum mounting plate will heat up to about 50 degrees Celsius. No problem also, and it works like a kind of heat sink.

But the carburetor that is screwed on this mounting plate will warm up to about 40 degrees as measured. Again no problem as such but it will influence the vaporization of mainly the volatile components in the auto car petrol making the adjustment of the carburetor somewhat more sensitive. Coleman Fuel has much less and different volatile components and the carburettor behaviour is much more stable for this application.

I have made CAD drawings for this engine, and I will send them by e-mail to for anyone who would like to build it; click here for a request.

By Jan Ridders