PUPPY TWO-STROKE 
I/C ENGINE
By Jan Ridders

After I designed my simple two-stroke engine back in 2006, it proved popular with many builders around the world. When recently helping a builder to sort out some problems he had in getting his engine to run, I realized that  the design could be further simplified and improved to produce a more reliable runner. I also thought that the original was not the most attractive of models and, although the engine runs well enough, it can be erratic as balancing a single cylinder engine 100% is impossible.

All this led me to make a new and significantly simpler and smaller version of this engine with improved running. Because of the much smaller dimensions I called it ‘2-stroke Puppy’. It will be a very suitable model engine for modellers with small machinery.


Design

  1. 1.The cylinder block is made of grey cast iron (GG25). Apart from the fact that the construction is relatively simple, this block also behaves as a good heat sink so that the temperature of the cylinder remains well below acceptable values. 


2. The piston is also from grey cast iron without a piston ring. With this combination no lubricating or cooling facilities are required.
3. The spark is made by the Mini Blokker circuit that is triggered by a very small reed switch and a Neodymium magnet in the flywheel or with a micro switch below a start-up/cam disc on the crank shaft. This spark circuit is built into the wooden base together with the 1.5 volt penlite battery.
The spark plug itself consists only of a Teflon insulator with a high-voltage electrode of spring steel wire soldered in a 3mm thread-end. TheTeflon insulator is screwed into the cylinder head. These thread connections in and on the Teflon insulator are also automatically 100% airtight.
4. The Petrol vapour carburettor is an elongated glass variant of my standard brass version that would be too big for this small model Puppy engine.

5 The dimensions and weight of the  Puppy are considerably smaller than those of the Simple 2-stroke Debbie. To give you an idea: five to six Puppy's will fit in a normal shoe box while one Debbie model is just a little too big for such a box. The Puppy weighs about 500 grams which is over 10 times less than its big predecessor.
The only thing that is exactly the same is the 2-stroke process itself.


Construction

  1. 1.The cylinder bore
    The bore must be cylindrical and pretty smooth. Honing is the best method, but it can also be done with an adjustable reamer if as one does not have honing equipment available. Proceed as follows:
    Drill the cylinder hole and then bore it to some 0.1 mm undersized diameter. Then ream the bore manually with an adjustable reamer, the reamer will locate itself in the cylinder bore. Clamp the reamer in the vice with the flat end of the cone. Turn the cylinder manually over the the reamer and not the other way around. Add plenty of oil while reaming. Turn the cylinder several times until the cylinder slides smoothly over the reamer. Then repeat this procedure with the reamer set a fraction bigger. Repeat these operations until there is actually no diameter difference measurable over the entire length of the bore; then the bore surface will also be nicely smooth.
    The bore diameter does not need to be exactly equal to that of the drawing because the piston is made nicely fit; see point 3.


    2. The guide bush for the piston rod
    The piston rod must be substantially fit leak-proof in the guide bush in the lower cylinder flange moving up and down with a relatively low friction. The alignment of a guide bush in this cylinder flange has to exactly coincide with the line of the cylinder bore in order to ensure that the piston does not twisting in the cylinder. Therefore, the cylinder flange has a raised edge which fits exactly into the cylinder bore. By clamping the cylinder flange with this upright edge in the lathe the bore for the guide bush can thus be made nicely centrical with the centre line of the cylinder bore.
    Often guide bushes are made of bronze and it is recommended to use standard ones together with a standard silver steel rod for the piston because they fit very well and smoothly together. It is possible to make them yourself, but difficult to make them with the same accurate fitting.
    However, I here made an alternative Teflon guide bush. The advantage of Teflon is that the piston-axis can run in it with a very narrow fitting on the one hand and with very low friction on the other. It is important to make the hole in the still solid Teflon bush again with the raised edge of the cylinder flange after it is pressed in the flange hole eventually, secured with some Loctite 603. This because, otherwise, the hole in the Teflon bush would be reduced by pressing-in if this this hole should have been made previously. The hole in the Teflon must be finished with a sharp H6 reamer with some white spirit or turpentine. If the piston rod is not running freely in the Teflon bush one must grind and polish the piston rod in the lathe with with fine emery cloth.



3. The assembly of the piston and its axis
In order to ensure alignment of the piston it may be best to proceed as follows:
- Turn the piston with a 1 to 2 mm diameter excess and make the central screw hole in it for the piston axis;
- Make the piston axis with the threads on both sides according to the plan and screw it in the piston with some Loctite 603. Preferably, cure the Loctite at 50 to 60 degrees Celsius for one hour creating a very strong and air-tight connection. Hardening at room temperature can also be done but that takes some hours for the same result;
- Turn back the diameter of the piston in small steps with the piston axis in the chuck of the lathe until the piston fits into the cylinder bore with the smallest possible clearance but in any case not exceeding 0.03mm.

4. The spark system
Because of the small dimensions of this Puppy it is not possible to build-in a standard automobile or motorcycle coil in the wooden base. Such coils are almost as big as the whole engine itself - that is a complete mismatch.
Therefore, I have again applied the mini circuit of a Blokker gas lighter (ED: details next time) The picture below shows the circuit in the wooden base with the 1.5 volt AA battery and the on/off switch.

On the page of this Blokker system you find three alternatives for the modification of this circuit. The alternative 1 is the most simple in which the thyristor is simply replaced by a micro switch that can be mounted under the start-up/cam disc pulley; see CAD figures at the top of this page (the micro switch is not shown there). Because with this alternative 1 the contacts of the micro switch eventually will burn-in on the long run I later made the alternative 2 with what the spark triggering is done with a reed switch with very low control current that drives the switching thyristor. One can use here also use a micro switch instead of the reed switch if wanted.
The spark has to come at the exact moment of maximum compression, so at the piston TDP position; this is rather critical.


Starting the engine

Fill the carburetor half with normal auto car petrol or Coleman Fuel. Especially for the first few times it is useful to start-up the engine using a hand drill machine comprising a nozzle with two 3mm pins that fit into the two holes of the start-up pulley of the engine. First turn the gas mixture regulator on the carburettor fully open and slide it gradually over the slot during start-up until you hear the engine is taking over. Around that setting the engine will run well and you can vary the speed a bit by moving the regulator somewhat over the slot in the lead. Once the engine has run it can also be started with some firm manually pushes to the flywheel.


Drawings

I have a CAD drawing available for the Puppy 2-stroke engine; click here to request a copy.