An easy-to-make 4-stroke Otto model engine
May 2011 I made a revision based on my new CAD drawing plan. Now the pictures and the video on the right of this page are up-to-date.
The idea behind this design.
My first IC model was an Atkinson 4-stroke engine. A very exciting engine but its mechanical behavior is very tempestuous, due to the multiple reverse and abrupt motions of the spectacular but rather complex driving system for the piston. I had to make great efforts to domesticate this engine to an acceptable behavior, but at the same time this learned me 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 4-stroke stroke engine. This time it was my explicit purpose to obtain a model engine with an exemplary quiet and friendly behavior. In addition to this major purpose I had some other intentions:
1. A transparent and good looking model, as close as possible to the primary principles of a 4-stroke internal combustion engine. It must demonstrate the four engine strokes as clear as possible, even for not technically orientated spectators;
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 system for all moving parts;
5. All mechanisms well visible, open and bare;
6. No complex forced cooling and/or lubricating systems;
7. No difficult fabrication techniques like casting, molding, accurate grinding, cog- or chain wheels, etc. It must be possible for every "moderate" model builder like me to make this engine with simple standard lathing and milling work and from standard bar stock materials.
So, my goal was to design and make a honest, reliable, nice and quiet running 4-stroke internal combustion engine, other than a complex and high power machine.The basics of this design
To gain my object I assumed that the Otto principle should be a good starting point for this 4-stroke combustion engine. 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 considerable more direct and simpler construction than the complex crank system of the Atkinson engine. This should be the key for a much better dynamic behavior. Here the crankshaft must make two revolutions for the four piston strokes: the gas intake-, the compression-, the 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 to 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.
The working-out of this Otto engine design
Below the design sketch for this Otto engine.
1. The cylinder/piston combination.
For the cylinder as well as for the piston I used pearlitic grey cast iron. In this case this material is at least highly preferable, may be 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 oil greasing!
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 accurate 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 driving system.
I choose for 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 the valve stems. The lenghts of these pins are made in such a way that there is some tenths of a millimeter left between the pins and the cams when the valves are closed. May be a hazardous construction, but it works very well !
The ignition cam is on the same cam shaft. All three cams are fixed with screws on the shaft, so they can be adjusted in every position within the four stroke cycle. The cam shaft is driven by the crank shaft with a synthetic tooth belt. The wheel on the cam shaft 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 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 teeth 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 kind of model engines.4. The spark ignition.
Because of very good experiences 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 an hand lighter for gas cookers. A pushing rod, driven by the ignition cam on the cam shaft 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 is/was used for classic auto cars and motor bikes. 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 plan. 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 fixation but also for the air-tightness at the same time.5. The carburetor.
First I copied the more or less classic carburetor as I used for the Atkinson engine. Provided it is made accurately this carburetor operates reasonable. But it was the most critical part of the engine. The adjustment is rather sensitive, there is a constant risk of "flooding" the engine and carbon soot easy arises on the sparkplug due to incomplete combustion of the fuel mix.
So I was not satisfied with this carburetor and insiders told me that is was hardly possible to implement substantial improvements for this basic carburetor design. That was the reason I designed a complete new alternative for making a mix of air and 100% molecular petrol vapor instead of fluid petrol droplets with the optimal ratio of about 1 to 14 (petrol/air). The performance of this carburetor is astonishing well with multiple advantages (see also the concerning page):
- Always 100% molecular petrol vapor in the gas mix. Consequently never any fluid petrol droplets in the cylinder and, as a result, no carbon soot or wet spark-plug, due to incomplete combustion;
- The ideal ratio petrol vapor/air of 1 to 14 is easy obtainable by adjusting a simple regulator for adding extra air. This very homogeneous gas mix provides for a perfect running engine;
- The engine is provided immediately with the right gas mix, so starting-up is very reliable and fast without choking. Choking is actually not possible with this system;
- In fact it is no longer possible to flood the engine; at least haven’t succeeded in doing that so far;
- The speed of the engine can be well regulated with the regulator for adding extra air on the rear of the tank;
- This carburetor design is very simple: no venturi, no petrol jet with needle, no accurate dimensions, no chance for false air- and/or petrol leaks;
- This carburetor may look somewhat bigger than the classic one, but the contrary is true: in fact it is only a small arrangement, integrated in and on the petrol tank;
- This carburetor cannot be overheated because there is no heat conduction from the cylinder to it. The length of the (rubber) connection tube to the intake manifold is not critical at all. I didn't notice any difference between 10 and 50cm tube length! The location for this carburetor is therefore free to choose;
- Regulation of the petrol level is irrelevant;
- No risk of stoppages; there are no narrow flood gates and possible contingent dust particles remain visible in the petrol tank. They disappear while draining the tank at the end of a demonstration;
- Petrol consumption is minimal; not more than necessary for the energy to be created;
- No chance for petrol leaks to the outside of the carburetor, and therefore safe and without unpleasant smells.
I now use this carburetor for all my stationary IC engine, so also for 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 200 revolutions per minute.Some characteristics
- Both the diameter and the stroke of the piston are 24 mm. The cylinder volume is also about 12cc.
- I choose for a relatively low compression ratio: about 1 to 4. At higher compression I assume more difficult start-up and higher cylinder temperatures. The engine power could be higher in that case but, as I said, a high power was anything but my intention.
- The revolution speed can be adjusted between 200 and about 2000 RPM but I never exceed 1000 RPM because I don't like the behavior at very high speeds.
-This engine runs on standard petrol Euro 95 or 98 for motorcars with 1 to 2% thin (household) oil addition. One can "Coleman Fuel" also which is preferable because it contains less different hydogen carbon components and it has no bad smells.
- The engine can be started with a loose belt around the pulley on the crank shaft and around a similar pulley in the head of your hand drilling machine. Well adjusted the engine can be started also with a firm push to the fly wheel by hand.
- The cylinder temperature don't exceed 110º Celsius after 15 minutes runtime.
Beautiful replica from Jonathan van Geyt
and his colleagua students:
