THE BOUNCING TWO-STROKE

By Jan Ridders

 

If you do not know what do next, there are several options: sit still; kill time with a completely different hobby, or design and make a crazy 2-stroke engine full of risks. I chose the latter because I happen to like playing with basic physics, and incorporating that in a working model.
I thought that it would be fun to make a combustion engine in which the compression is made purely by gravity, without flywheel and crankshaft. A weight on the piston axis must compress the the fresh gas mix below the piston and the 4.5 x greater combustion pressure must push the piston upwards again beyond the exhaust hole in the cylinder. With a pipe system with two one-way ball valves and an expansion vessel between the top and the bottom of the cylinder the fresh gas mix is sucked-in from the carburettor and compressed, so that at the right moment the fresh gas mix flushes the burnt gasses out of the cylinder enabling the process to repeat itself. This is, of course, the well known 2-stroke process.
I realized that there are at least three "bears on the road" making the project extremely risky and that chasing them away was actually my main challenge. Even if the engine would actually do what I had in mind it would not have any practical significance, but that goes for all my model engines. Some will understand my intent, others will probably raise their eyebrows at this strange project. It's like a joke: raising a smile, or a compassionate shake of the head.


The design

The compression pressure is created below the piston when the piston is pulled downwards by its weight and at the end closes the exhaust port in the cylinder. This compression is determined by the mass of the weight, but also by the kinetic energy (1/2mv2) of the weight.
The spark must occur at maximum compression pressure, controlled by a switch operated when the piston reaches its lowermost position. The approximate 4.5x higher combustion pressure pushes the piston back upwards again to well beyond the exhaust port in the cylinder. As the piston opens the exhaust port the cylinder will be flushed with the compressed fresh gas mix. Above the piston the process of sucking-in the fresh gas mix from the carburettor takes place and also the compression.
The process is completely pressure controlled by two automatic ball valves as I did earlier with my pressure controlled 2-stroke engine. The connections of the ball valve system on the cylinder are reversed on this new engine because the combustion chamber is below the piston is instead of above. The expansion vessel in this system ensures that the compression pressure of the fresh gas mixture is not too high to avoid slowing down the upward movement of the piston.

What were these three ‘bears’ that could obstruct the road to success?


Bear # 1: The compression and the piston stroke

There is a certain minimal compression required for a reliable gas mix ignition and/or with sufficient expansion of the ignited gas mix. Just what the minimum compression should be I was not sure, so it could be that for the right compression a very heavy weight would be needed.

The active piston surface in this design is about 2 square centimeters so, statically given, a weight of about 4 kg is needed for e.g. 2 bar compression pressure and that would actually be unacceptably high in my opinion. Now it is true that the speed that the weight falls down appeared to be a significant factor. In fact the gas mix appeared always to ignite, even at low pressures that arose when I made the spark occur shortly after the closing of the exhaust port in the cylinder. But I noticed that the upward stroke of the piston was clearly smaller than when I set it to spark later, with a lower piston position, and therefore making a higher compression and gas expansion. This upward stroke must have a certain minimum value to suck-in sufficient fresh gas mix above the piston to completely flush the cylinder. This quantity (PxV) must be slightly larger than the volume of the combustion chamber below the piston and that is another important condition for the process in addition to making sufficient compression.


I made numerous experiments with various weights, with and without tensile and/or compression springs. I found that adding springs not only makes the system very complex, but didn't contribute anything to solve the puzzle. The engine only showed signs of life with only a weight on the piston, which is understandable. Springs always cause forces in only one direction which increase proportionally with stretching or compression. A stiffer spring will increase compression but it also reduces the upward stroke of the piston and, with that, also reduces the amount of the fresh gas mix out of the carburettor.

This contradiction is less present with only a weight on the piston because the weight inherits kinetic energy in both directions: in the downward movement by gravity and in the upward movement by the gas explosion under the piston like a bullet out of the barrel of a gun. An extremely heavy weight will cause a small upward stroke and with an extremely light weight the compression and the expansion will be too small. Thus, there is a certain optimum for the mass of the weight with a maximum effect for the compression on the one hand and the upward stroke on the other hand.

Would the engine actually want to run with this optimal weight? Luck was with the stupid. Step-by-step I reduced an initially excessive weight until the engine showed some signs of life. In that situation I could investigate other sensitivities and eliminate or reduce them. Then I reduced the weight until the engine continued to run, at least for some time.

So bear # 1could be moved out of the way.


Bear # 2: The Start-up of the engine
Unlike an engine with flywheel, it is not possible to use, say, a hand drill to start the engine. The question was whether the engine would start with manual lifting and dropping the weight. Again, luck was with the stupid: a few of these drops are usually sufficient to start the engine, as long as the carburettor is properly adjusted. So I can be brief about this bear: he was not there.

Bear # 3: Engine shut down after an ignition miss
There is no flywheel effect to help the engine after a missed ignition. This engine would immediately shut down without a self-starting again. That is a serious risk due to the high demands on the reliability of the ignition which is not exactly the strongest feature of 2-stroke engines, which generally tend to skip strokes now and then.
This bear indeed is explicitly present and I have not been able to dislodge him 100% but I succeeded in sending him in a sort of soft sleep so the engine runs for an acceptable time by which I mean about two minutes, at least for the time being. An ignition miss can have several causes such as a weak spark, the spark being  too late or too early, a too rich or too poor gas mix or insufficient flushing of the cylinder resulting in an impure gas mix. It was a small miracle that this 2-stroke engine performed for so long without a single ignition miss. That result was was reached without struggle:

  1. I decided initially to use a classic motorcycle ignition coil built into the wooden base of the engine. Such a coil makes a pretty strong spark and I know that this is needed to prevent a 2-stroke engine from missing too easily. The micro switch which controls the spark is struck by a central pin attached to the weight and which is adjustable in height. With this pin I can adjust the timing of the spark and with that the actual compression where that happens. This setting is quite sensitive, but with acceptable margins.

  2. I found that the volume of the combustion chamber plays an important role. Not surprising since it partly determines the flushing process and, therefore, the purity of the gas filling in the cylinder. Luckily I had made this volume somewhat too big so I could simply reduce it in small steps sawing small pieces from the bottom of the glass cylinder. After each reduction I noticed an improvement in the running behaviour. I stopped reducing when I felt I had reached the optimum volume, although I'm not quite sure of that because I did not want to go into the situation: "too much sawed off and still too short".

  3. The sensitivity of the engine to carburettor adjustment, in other words the ratio air/fuel vapour, is remarkable. The margin is very small, but the correct setting for the controller of the additional air can be found with some care. I will have another good look for the reason why this is that sensitive and whether there is anything to be improved. Coleman Fuel instead of car gasoline doesn't seem to bring a noticeable improvement in this respect.

  4. The setting of the ball valves is not a problem and is similar to that for the "Pressure controlled 2-stroke engine".

Part two: an improved Mk2 and details of drawings