The simple two-stroke engine

An ideal beginners’ I/C engine

By Jan Ridders Model Engineer

Model engineers with limited experience and/or modest machinery often imagine that building a model I/C engine is too difficult. Things like a difficult crankshaft, valves with their complex driving mechanism, distribution system, castings, crankcase, carburettor, forced cooling and lubricating, piston rings, accurate grinding work, etc will put off all but experienced specialists. My engines are designed to encourage newcomers to engine building.
Having no mechanical background, only modest mechanical skills and machinery, I always try to make my engine designs as simple as possible so that they can be made by the simple turning and milling of basic materials. I always strive for simple, nice and reliable running engines, rather than for high performance types.
After I made two rather uncomplicated 4-stroke models the ‘Atkinson’ and the ‘Otto’ I had the idea to make the ultimate simple design for an I/C engine. I decided first that it should be a 2-stroke engine because unlike a 4-stroke a 2-stroke doesn’t have valves with a driving mechanism and distribution system, so eliminating some highly 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 that is even simpler, based on experience with my later I/C engines. The result is a very straightforward and rather good-looking little engine that runs nicely, is very reliable, and has a simplicity that will be hard to beat. if I may say so, it is the beginners’ engine par excellence.

Principle of the 2-stroke I/C engine
As with every I/C engine there are four processes: intake of the fresh gas/air 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 crankcase. At the moment the piston reaches the exhaust port in the cylinder wall the burned gases 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 gases through the exhaust opening and is compressed when the piston moves upwards again due to the flywheel inertia, ready to be combusted in this continuous process.
It is difficult to make this flushing process so that it drives all combusted gasses out and not to let unburned fresh gas mix escape directly through the exhaust opening at the same time. That’s why 2-stroke engines are less efficient than 4-stroke engines. But the uncomplicated design has made the 2-stroke engine very popular for small vehicles and other small machines.

Characteristics of this design
In order to make the engine as simple as possible I implemented the following simplifications compared to conventional 2-stroke engines:
1. No crankcase.
Most crankcases are castings with air-tight feed-troughs for the crankshaft, difficult to make for amateur model builders.  To eliminate such a crankcase I made an outside crankshaft 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 guide bearing in it for the rigid piston rod. This also made it possible to employ 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 to avoid the fresh gas mix below the piston to be pressed back in the carburettor. This eliminates the complexity of a driven valve and its adjustment.
3. Expansion vessel.
Except for housing the crankshaft the crankcase with a normal 2-stroke engine also has the function to avoid too high, and 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 cylindrical expansion vessel between the cylinder and the one-way ball valve on the carburettor. Experimentally, I determined the optimal volume of this vessel at about 12cc.
4. No piston rings.
Piston rings are omitted here. The engine's performance is much 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 lapping the very lightly oversized piston manually in the cylinder bore with a very fine paste. Clean thoroughly afterwards.
5. Petrol vapor carburettor.
I applied my ‘Petrol Vapour Carburettor’ instead of a classic one. This is a very simple arrangement in the fuel tank itself. The in-streaming air bubbles through, or strikes over, the liquid fuel. In this way it takes 100% molecular petrol vapour with it, instead of  mixing fine petrol droplets with air as this is done with the classic carburetor. This carburetor is much easier to make and has an excellent performance without the risk of carbon soot deposits on the spark plug and/or flooding the engine. The engine runs on normal petrol (gasoline) or, preferably, ‘Coleman Lantern Fuel’ which is available from camping supply shops.  The way of working with this carburettor is described on the last page of the drawing plan for this engine and on the page and the drawings for the carburettor which will be he subject of another article.   
6. No forced cooling and lubricating system.
This engine is not meant for  heavy duty tasks, a 10 to 15 minute runtime is mostly 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-greasing due to the relative 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-resistant and has no tendency to dig into, 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 withstands the combustion temperature easily. This isolator is threaded with what it is fixed in the housing, making it perfectly gas-tight at the same time. If desired one can use any other suitable spark plug.
8.The flywheel.
The flywheel must have a fair-sized mass weight to run this engine with its 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 base of the engine.
The wooden base has a cavity in which the electrical circuit for the spark ignition can be mounted. I use a classic circuit with a high tension coil and an external 6 or 12 volt DC battery supply. The contact breaker is mounted below and driven by the cam disc on the crankshaft. A piezo ignition was tried on this engine, but it appeared not to be reliable enough for a 2-stroke engine as, in my experience, it requires a more powerful spark than a 4-stroke engine.
High tension coils as used in classic cars or motor bikes are most suitable. Don’t use the small coils as used nowadays for scooters, mopeds, lawn mowers, etc because they generally are made for other supply voltages made by a generator on the crank shaft. Any other ignition circuit is applicable as long as it produces the necessary good and powerful spark.
Engine development
As stated this engine is a re-design of the first version I made some five years ago with some significant  simplification and improvements.
The most important changes are:

•The cylinder is strongly simplified by replacing the six rather complex 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 ‘surgical work’ of the six ports in the cylinder wall.

•The cylinder support is more solid now so that the cylinder is fixed firmly on the mounting plate.

•The piston rings are omitted.

•The diameter of the flywheel 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 as the original flywheel.

•The original carburettor with the two independent air adjusters is replaced by the latest version with a three-way throttle valve and short in-stream pipe on the tank for easier and less sensitive adjustments.

•In the one-way ball valve there is a steel ball instead of a neoprene one. This valve is a compact assembly together with the horizontal positioned expansion vessel.

 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 4:1.

•The engine runs on standard petrol (gasoline) for motorcars. Speeds can gradually be adjusted between about 300 and 1500 rpm with the three-way throttle valve on the back of the carburettor. A highly suitable alternative fuel is Coleman Fuel, a ‘super refined’ product with fewer heavy residues and noxious fumes. This fuel is widely used in camping equipment.

•The engine can be started with a loose belt around the pulley on the crankshaft and around a similar pulley in the head of your hand drill. But in good condition the engine will start easily by only pushing the flywheel once by hand, as you can see on the video. While starting-up, adjust the regulator on the carburetor 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.

1. •The engine in the photos and on the video differs somewhat from the CAD drawing because while rebuilding my old engine I re-used some parts such as the flywheel.
   

2. •MODEL ENGINEERING WEBSITE BY MODEL ENGINEERS FOR MODEL ENGINEERS not connected with Model Engineer or Model Engineers’ Workshop magazines. Just the Model Engineering web based weekly magazine
   

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