By Jan Ridders

My original design for an Otto four-stroke engine was updated in 2009 to incorporate some improvements and simplification. The video and photos do not include these amendments as they are of my original engine. However, this description and the drawings do include the changes.

Drawings for the Otto engine are on the right. Click on a drawing to enlarge, click on ‘download’ for larger still and to print. Drawings are in A4 format. The remaining drawings will appear next week. See Otto Part 2.

The idea behind this design
The Atkinson four-stroke engine that I build earlier is on the one hand a very exciting engine, but on the other its behaviour is very tempestuous, due to the multiple reverse and abrupt motions in the nice but rather complex crank system; see the Atkinson page on this website. I had to make great efforts to domesticate this engine to an acceptable behaviour. At the same time I learned a lot about the principles and the peculiarities of combustion engines and adjusting the very different parameters.
With this experience and knowledge I considered myself able to design a new four-stroke engine. This time it was my explicit purpose to obtain an engine with an exemplary quiet and friendly behaviour. I also wanted:
1. A transparent and good looking model, as close as possible to the the primary principles of a combustion engine. It must be able to demonstrate the four-stroke engine as clear as possible, even to non-technical people;
2. No more parts than strictly necessary to run the engine;
3. A minimum of rotating, oscillating or other reverse movements;
4. Direct as possible driving of all moving parts;
5. All mechanisms well visible, open and bare;
6. No complex cooling and/or lubricating systems;
7. No difficult fabrication techniques like casting, molding, cogs or chain wheels, accurate grinding, etc. It must be possible for every ‘average’ model builder to make this engine with simple materials and  tools.
So, my goal was to design and make a honest, reliable, nice and quiet four-stroke combustion engine, rather than a complex, high performance machine.

General design
To meet my objectives I assumed that the Otto principle should be a good starting point. Almost all of the motorcar engines are based on this Otto principle. Typical for the Otto is that the piston is driven by a singular crank, which is a considerably more direct and simpler construction than the complex crank system of the Atkinson engine. This should be the key to better dynamic behaviour. Here the crankshaft must make two revolutions for the four piston strokes: the intake-, compression-, power- and exhaust-stroke. The shaft for the valve- and ignition-cams must rotate with half the speed of the crankshaft. This is realized with a 2:1 distribution mechanism between crankshaft and camshaft.
So, the big difference with the Atkinson is the very direct driving of the piston and the driving of the valves and ignition arrangements. For the rest the four-stroke principle is the same as with the Atkinson, so that I could copy an important part of that design.


Designing the engine

1. The cylinder / piston combination
For both the cylinder and the piston I used pearlitic grey cast iron. In this case this material is preferable, maybe even conditional. The thermal expansion of cast iron is very low and, in any case, equal for cylinder and piston. Together with the fact that it is more or less self-greasing due to the relative high carbon grade, it prevents jamming of the piston, even without forced lubrication!
Furthermore, cast iron is highly temperature resistant and working up is rather easy. If the surfaces of the piston and the cylinder bore are made accurately, and smooth, there is no need for a piston ring.
Although forced lubricating is not necessary it is advisable to put an oil droplet through the spark plug hole now and then to keep the piston and cylinder surfaces ‘in good condition’. This is especially needed before storing the engine for a longer time because cast iron is somewhat rust sensitive when the surface is completely grease free.

2. The camshaft and its drive
I choose an overhead cam shaft for the reason that the cams can drive the valves in the most direct way: no intermediate cam lifter arrangements with pushing rods and oscillating tumblers. Only short pins in gliding bushes between the cams and valve stems to avoid transverse forces on these valve stems. The lengths of these pins are made in such a way that there is some tenth of a millimeter left between the pins and the cams when the valves are closed. Maybe a hazardous construction, but it works very well!
The ignition cam is on the same camshaft. All three cams are fixed with screws on the shaft, so they can be adjusted in every position within the four-stroke cycle. The camshaft is driven by the crankshaft with a synthetic toothed belt. The wheel on the camshaft has 70 teeth, the wheel on the crankshaft has 35 teeth. This kind of small and flexible belts and wheels are available everywhere. Every other flexible tooth belt than this one is OK as long as its circumference is anywhere between 400 and 450mm. The same counts for the cog wheels as long as they have about similar diameters and the ratio of teeth is exactly 2 to 1.
With such a toothed belt it is easy to bridge the distance from crankshaft to camshaft. Furthermore it provides a very smooth and noiseless drive without the need of greasing it.

3. Process timing schedule.
The time cycle measured on the camshaft is as follows:
0º: Upper position of the piston (TDP), the intake valve is opening;
70º:The intake valve closes; so 20º before the lower piston position (BDP);
90º to 180º: compression of the gas mix;
180º: Upper position of the piston (TDP); ignition spark (or a fraction earlier); ignition gas mix and start power stroke;
250º: the exhaust valve opens; so 20º before bottom piston position (BDP);
270º: Piston in bottom position (BDP); start exhaust stroke;
355º: the exhaust valve closes; so 5º before TDP;
355º: Process repeats.
I found this time cycle to be optimal at relative low engine speeds, which I prefer for this engine.

4. 4.The spark ignition
   Because of good experience with the Atkinson engine I choose again to use a piezo crystal for making the ignition spark. This is a very simple and above all a compact arrangement, compared to the classic system with breaker points, high tension coil, capacitor and battery. I took this piezo crystal with another two parts out of a hand lighter for gas cookers. A pushing rod, driven by the ignition cam on the camshaft takes over the role of the hand that normally operates this gas lighter. Well adjusted this system delivers reliable sparks, even at high engine speed. In fact the piezo causes within every cycle a short spark-train, which is probably an advantage for a reliable ignition of the fuel mix.
   In case one cannot obtain a suitable piezo it is well possible to use a circuit with a high-tension ignition coil as (was) used for classic auto cars and motorbikes. This coil can be build in the wooden base on what there is an electrical plug to connect an external 6 or 12 volt DC power supply for what the rechargeable battery of a hand drilling machine is very suitable. The switch (points) for that ignition circuit must be mounted below the cam disc instead of the piezo.
   The spark plug is self-made; see drawing. It contains a Teflon isolator that withstands the combustion temperatures without any problem. This spark plug is easy-to-make and the threads on the Teflon is not only for mechanical fixing but also to give air-tightness at the same time.
   
   5. The carburettor
   First I copied the more or less classic carburettor as I used for the Atkinson engine. Provided it is made accurately this carburettor operates reasonably. But it remained the most critical part of the engine. The adjustment is rather sensitive, there is a constant risk of ‘flooding’ the engine and carbon soot is easily deposited on the spark plug due to incomplete combustion of the fuel mix.
   So I was not satisfied with this carburettor and insiders told me that is was hardly possible to implement substantial improvements for this basic carburettor design. That was the reason I designed a complete new alternative for making a mix of air and 100% molecular petrol vapor in stead of fluid petrol droplets with the optimal ratio of about 1 to 14 (petrol/air); see the page for "Petrol Vapor Carburettor". The performance of this carburettor is astonishingly good and has many advantages that you can read on the appropriate page. I now use this carburettor for all of my stationary IC engines including this Otto.
   
   6. The flywheel
   The flywheel must have a fair-sized mass weight because this engine design assumes relatively low revolution speed. The dynamic energy of a bicycle type flywheel is E=½mw²r² where m is the mass weight, w is the radial speed and r is the radius of the wheel. The flywheel of this engine is made of steel with a diameter of 150mm and a width of 20mm. The mass weight is 1.2 kg and the dynamic energy is about 2.6 Nm at 300 revolutions per minute. With this flywheel it is possible to run this engine with a speed as low as about 150 revolutions per minute.
   
   Some other characteristics
   - Both the diameter and the stroke of the piston are 24 mm. The working volume is also about 12cc.
   - I choose a relatively low compression ratio: about 4.5:1. At higher compression I assume the start-up would be more difficult and give higher cylinder temperatures. The engine power could be higher in that case but, as I said, this was not my intention.
   - The revolution speed can be adjusted between 350 and about 2200 rpm but I never exceed about 1000 rpm because I don't like the behaviour at very high speeds.
   -This engine runs on standard petrol Euro 95 or 98 for motorcars with 1 to 2% thin (household) oil addition.
   -The cylinder temperature doesn't exceed 110ºC after 15 minutes runtime.
   

Changes in the latest version
For builders of the original version, these are the main changes.

1. Using the latest version of the Petrol Vapour Carburettor with the three-way throttle valve and the short in stream tube on the tank.

2. One solid block against the cylinder head with combined connections to the carburettor and the muffler.
3. Omitting the piston ring.
4. An optional construction for mounting the piezo or an electrical switch for use with classic circuitry with a high-tension ignition coil. Because the dimensions for these parts are not universal I could not give details on the drawing plan. The builder must make his own choice for the ignition equipment and the way of mounting.
5. The tensioning wheel for the toothed belt now is on the front side of the engine improving accessibility for adjustment.

For the remainder of the drawings see Otto Part 2