A Simple 2-stroke engine

Introduction.
For most model builders with limited experience and modest machinery an IC model engine is not that easy to make. Also beginners will surely think that this is not achievable for them at all. Things like a difficult crank shaft, valves with their complex driving mechanism, distribution system, casting moulds, crank case, carburettor,  forced cooling and lubrication, piston rings, accurate grinding work etc., seems to be the exclusive privilege for only few and very experienced specialists.
Being a model builder myself with no mechanical background and having disposal of only moderate mechanical skills and machinery I always try to make my engine designs as simple as possible so they can be made with a normal lathe and mil using standard materials. I always strive for a straightforward reliable engine rather thn aim for high performance.
After I made two rather uncomplicated 4-stroke models (“Atkinson” and “Otto”) I conceived  the idea to make an optimally simple design for an IC engine. I knew on beforehand that it should be a 2-stroke engine because in contrast to a 4-stroke a 2-stroke doesn’t have valves with a driving mechanism and distribution system so that should eliminate a large chunk of rather complex work.
I made the first version of this 2-stroke engine in 2004 and, after five years, I recently (November 2009) produced a redesign with significant further simplifications using some of my experiences with my later IC engines. I daresay that the result is a very straightforward and rather good-looking little IC engine that runs smoothly and very reliable with a simplicity that can hardly be surpassed in my humble opinion. I therefore feel that it would make an ideal model for beginners.

The principle of a 2-stroke IC engine
As with every IC engine there always are four process stages: intake of the fresh air/fuel mix from the carburettor, compression, combustion and exhaust. With 2-stroke engines these processes are divided over both sides of the piston. That’s why the whole process cycle can be done within two strokes instead of four as with 4-stroke engines. When the piston is moving upwards the fresh gas mix from the carburettor is sucked-in below the piston while the gas mix from the previous cycle is compressed above the piston. When the piston is moving downwards due to the combustion of the compressed gas mix above the cylinder (power stroke) the fresh gas mix below the piston is compressed in the crank case. At the moment the piston reaches the exhaust port in the cylinder wall the burned gasses escape and the fresh gas mix is injected at about the same time above the piston through the inlet port opposite to the exhaust port. The fresh gas mix drives the remaining burned combustion gasses through the exhaust opening and is compressed when the piston moves upwards again by the fly wheel inertia to be combusted in this repeating process.
This process is illustrated in the below animation. It is still based on my first design with 6 inlet ports in the cylinder wall from what only one is shown in the animation.


It is difficult to make this flushing process 100% efficient so that it drives all combusted gasses out but does not let unburned fresh gas mix escape directly through the exhaust opening at the same time. That’s why 2-stroke engine are less efficient than 4-stroke engines. But the uncomplicated design has made the 2-stroke engine very popular for small vehicles and other little machines.

The characteristics of this simple 2-stroke design.
In order to make the engine as simple as possible I implemented the following simplifications compared to usual 2-stroke engines:
1. No crankcase.
Most of the time crankcases are casting moulds with air-tight feed-trough’s for the crank shaft, difficult to make for amateur model builders.  To eliminate such a crankcase I made an outside crank shaft with a driving rod as is usual for steam engines. On the bottom of the cylinder there is a plain cover plate with a Teflon glide bearing in it for the rigid piston rod. This also made it possible to use a simple one-sided crankshaft on which the driving rod can be mounted easily.   
2. One way ball valve.
An easy-to-make one-way ball valve on the outlet of the carburettor opens and closes automatically at the right moment so that the fresh gas mix below the piston is not drivev back to the carburettor. This eliminates a driven valve and the need to adjust it.
3. Expansion vessel.
Except for housing the crankshaft, the crankcase volume of a normal 2-stroke engine also has the function to control the counter acting compression of the fresh gas mix below the piston. Because of the absence of a crankcase in this design this function is taken over by a small (12cc) cylindrical expansion vessel between the cylinder and the one-way ball valve on the carburettor (not drawn in the animation).
4. No piston rings.
Piston rings are omitted here. The engine's performance is fairly the same as with piston rings if the clearance of the piston in the cylinder is made <= 0.03mm which is easy to obtain by polishing the very lightly oversized piston manually in the cylinder bore with a very fine grinding paste.
 5. Petrol vapor carburettor.
I applied my “Petrol Vapor Carburetor” instead of a classic one. In fact it is a very simple arrangement in the fuel tank. The air streaming in is bubbling through, or striking over, the liquid fuel taking 100% molecular patrol vapor with it instead of  mixing fine petrol droplets with air as this is done with the classic carburettor. This carburetor is much easier to make and has an excellent performance without the risk of carbon soot on the spark plug and/or flooding the engine. The engine runs on normal auto car fuel or, preferably, “Coleman Lantern Fuel”.  The way of working with this carburettor is described on the last page of the drawing plan for this engine and on the page: "Petrol Vapor Carburetor" .   
6. No forced cooling and lubricating system.
This engine is not intended for  heavy duty tasks; a 10 to 15 minute run time is usually more than enough for a successful demonstration. The cylinder temperature doesn’t exceed 110 degrees Celsius without forced cooling. The cylinder and the piston are both made from pearlitic grey cast iron (GG25). This material is more or less self-lubricating due to the relatively high carbon content and the thermal expansion of this material is very low and equal for the piston and cylinder. Together with the fact that this material is very wear-free and has no tendency to seize, jamming of the piston in the cylinder never occurs despite the absence of  a forced lubrication system. Dosing some oil droplets in the cylinder from time to time is sufficient to keep the surfaces of the cylinder and the piston in good condition.
7. Spark plug
The spark plug is self made. It contains a Teflon isolator that easily withstands the combustion temperature. This isolator is threaded into the body, making it perfectly gas-tight at the same time. If desired, you can also use other suitable spark plugs which are becoming more affordable for model aero applications.
8.The fly wheel.
This must have a fair-sized mass weight to run this engine with a 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 is made from steel with a 110mm diameter and a width of 25mm. The mass weight is 1,2 kg and the dynamic energy is about 3 Nm at 500 rpm.
9. The wooden base of the engine.
This has a cavity in which the electrical circuit for the spark ignition can be mounted. I mostly use a classic circuit with a high tension coil and an extern 6 or 12 volt DC battery supply. The contact breaker is mounted below and driven by the cam disc on the crank shaft. The animation shows a piezo ignition but I substituted that later with a classic circuit with high tension coil because the piezo ignition appeared not to be reliable enough for a 2-stroke engine that require a more powerful spark than a 4-stroke engine according to my experience.
High tension coils as used for classic auto cars or motorbikes are most suitable. Don’t use the small coils as used nowadays for scooters, mopeds, lawn mowers, etc because they are generally made for other supply voltages made by a generator on the crankshaft. Any other ignition circuit is applicable as long as it produces a good and powerful spark which is necessary for any 2-stroke engine.

Changes compared to the first version
As alreadystated this engine is a re-design of the first version I made some 5 years ago. I implemented some significant further simplifications and improvements compared to the first version.
The most important changes are:
1. The cylinder is extensively simplified by replacing the rather complex six intake ports in the cylinder wall by a little by-pass system on the outside of the cylinder. In fact this was the main reason for the redesign to eliminate the "surgery work" of the 6 ports in the cylinder wall.
2. The cylinder support is more solid now so the cylinder is fixed firmly on the mounting plate.
3. The piston rings are omitted.
4. The diameter of the fly wheel is somewhat smaller to keep it just above the mounting plate. The width is made somewhat greater to make the mass inertia about the same.
5. The original carburettor with the two independent air adjusters is replaced by the latest version with the three way throttle valve and short in stream pipe on the tank for easier and less sensitive adjustments.
6. In the one way ball valve there is an common bicycle steel ball instead of the neoprene one. This  valve is a single compact assembly together with the horizontal positioned expansion vessel.

 

CAD picture of the revised engine.

Miscellany

-Both the diameter and the stroke of the piston is 24 mm. The working volume is also about 12cc. The compression ratio is about 1 to 4.

-The engine runs on standard petrol for motorcars with speeds that can be gradually adjusted between about 300 and 1500 rpm with the three-way throttle valve on the back of the carburetor. A very suitable alternative is Coleman Lantern Fuel, a "super refined" fuel with less heavy components and hardly any bad smell. This fuel is mostly used for camping stoves and can be obtained in every shore for camping stuff.

- The engine can be started with a loose belt around the pulley on the crankshaft and around a similar pulley in the chuck of your hand drill. But in good condition the engine will start easily by only pushing the fly wheel once by hand which you can see on the video. While starting-up adjust the regulator on the carburettor to a point where you hear the engine take over. With a half filled the tank the engine will run for 15 minutes or more.

- The engine on the photographic pictures and on the video on the right of this page differs somewhat from the CAD picture above because while rebuilding my old engine I re-used some parts such as the bigger fly wheel.

- Request for the CAD drawings plan can be done via the page "Questions a /o Comments".

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Trouble shoot directives


1. Check if the free running behavior and the compression are fairly the same with what the demo's are demonstrating at the end of the video of this engine.

2. The spark for a 2-stroke engine must be considerable strong. Don't use the (small) high tension coils as is used nowadays for mopeds, scooters, sawing-machines etc. but a high tension coil that is (was) used for classic auto cars or motor bikes mend to be connected to a 6 or 12 volt DC battery. The supply voltage for this kind of small (scooter) coils is mostly made by a generator on the crankshaft and some electronic circuit in between. Connected to a 6 or 12 volt battery they get very hot and produce very weak spark or not at all.

3. The spark must occur just at the highest piston position (Top Dead Centre; TDC) with the maximum compression. However some tolerance of +/- 5 degrees is allowed here and will not have big impact.

4. If the piston is at his lowest position (Bottom Dead Centre; BDC) the exhaust hole in the cylinder must be completely open. The upper rim of the piston must be at the lower rim of the exhaust hole. Significant deviation can influence the flush process above the piston in a negative way.

5. It is very important that the ball valve is working well. Mostly the reason for malfunctioning of the engine will be found here. The ball must seal well on its seat and may not float or rebound with the result that it is falling back on its seat too late. Therefore the seat must be made well smooth and the free stroke of the ball may not exceed about 0,5mm. This all will be the case if the valve is made according to the drawing plan, but it can happen that the valve is polluted internally. So take care that the valve is and stays clean.
Testing the good operation of this ball valve can best be done as follows:
Lengthen the air intake tube on the tank with some thin rubber hose so that it immerges the petrol in the tank. Drive the engine with your hand drilling machine with some significant high rpm. The in streaming air will now bubble through the petrol clearly visible. This will be practically always the case unless the ball should stay on its seat. There is only a very little chance for that: only when the dowel above the ball is too long or the ball is "gluing" on its seat due to some pollution. In this case check the length of the dowel and, with that, the free stroke of the ball and/or clean the ball and its seat thoroughly.
More often it occurs that you see some petrol being pushed out the tank through the in stream tube while the engine is turning around. In that case the ball isn't closing well or it falls back on its seat too late with the result that some gas mix that was sucked in below the piston will be pushed back into the tank. The temporary over pressure in the tank will push some petrol through the intake tube into the open. When this happens the engine will not run regular or not at all. Then check the seat for irregular surface and/or pollution. It is also possible that the free stroke of the ball is too big in which case the dowel must be extended so that the free stroke of the ball is about 0,5mm.
After this teat remove the rubber hose on the in stream tube.

6. All (screw) connections in the fuel system must be made air tight, for instance with Teflon tape or some few Loctite. Sealing the piston rod in its Teflon glide bush cannot be done like this of course. The clearance between the rod and the bush must be as small as possible but not that small that the engine turns heavily. A significant air leakage here is reasonable cause for bad running or not running at all. So it is a good thing to test this seal when the engine persistently refuses to run poorly or not at all. Connect some pressed air to the bottom of the piston for instance via the ball valve while the piston is set over the exhaust hole in the cylinder to close that. Then drop some few oil over the piston rod where it enters the Teflon glide bush. Mostly you will see tiny bubbles indicating that there is a acceptable small leakage. But if the oil is blowing away heavily you must make a new Teflon bush with smaller clearance.

7. Use normal fresh gasoline for auto cars or (preferably) Coleman Fuel (see the page for this Petrol Vapor Carburator). Before starting up adjust the intake for the extra air on the outlet of the carburetor completely open. Then drive the engine with your hand drilling machine and very gradually close the regulator untill you hear the engine takes over. Anywhere around this adjustment point the engine will start up and run best and you can regulate the speed by adjusting some more or less extra air.
The inner diameter of the in stream tube on the tank of the carburetor must be 4mm instead of 3mm as it was given in a former drawing plan for the carburetor. The performance with 4mm is somewhat better than with 3mm. I changed that some time ago on the drawing plan but it is possible that you don't have this latest version.

8. Put an oil droplet on the piston through the exhaust opening in the cylinder. Not really necessary to avoid jamming but "to keep the surfaces in good condition". Do this always when you store the engine for a longer time.

If you take these 8 remarks into account the engine must/will run nicely and very reliable.

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This engine made by students
WICO campus Belgie
(see ca 1,5 min after start video)