Introduction If you don't know what you now have to make again, there are several options: sit still, killing the 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 shape it into a working model.
I imagined that it would be funny to make a combustion engine in which the compression is made purely by gravity force, so 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 also, so that at the right moment the fresh gas mix flushes the burnt gasses out of the cylinder so the process will repeat itself; actually the well known 2-stroke process.
I realized that there are at least three "bears on the road" making the project extremely risky and chasing them away was actually my main challenge. Even if the engine would actually do what I had in mind it will 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: it is laughing or compassionately shaking with one's head.
The design The CAD pictures below illustrate the design:
The compression pressure is created below the piston when it is pulled downwards by the weight and at the moment the bottom of itcloses 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 that is pressed at the moment 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. At the moment 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 placeand also the compression of it.
The process is completely pressure controlled by two automatic ball valves as I did earlier with the "Pressure controlled 2-stroke Sabine", see the concerning page. The animation on that page demonstrates how this works, but the connections of the ball valve system on the cylinder are reversed here 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.
The"bears on the road"
As said I foresaw at least three "bears" who could obstruct the road to success:
Bear # 1: The compression and the piston stroke. There is a certain minimal compression required for a reliably gas mix ignition and/or with what there is sufficient expansion of the ignited gas mix. I was not sure how exactly great the minimum compression should be, so it could just be that for the right compression a very heavy weight would be needed. The active piston surface in this design is about 2 square centimetres so, statically given, a weight of about 4 kg is needed for eg 2 bar compression pressure and that would actually be unacceptably high in my opinion. Now it is true that the speed with what the weight falls down quadratically participate in the generation of the dynamic force on the piston, and that appeared to be a significant important factor. In fact the gas mix appeared to ignite always, even at low pressures that arise 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 clearly is smaller than when I set the the spark occurrence later with a lower piston position and therefore making a higher compression and gas expansion. This upward stroke namely 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 at least equal (or actually slightly larger) than the volume of the combustion chamber below the piston and this is another important condition for the process apart from making sufficient compression. I have done numerous experiments with various weights, whether or not combined with tensile and/or compression springs. I found that adding springs not only makes the system very complex, but also that this didn't contribute anything to solve the puzzle. The engine only showed signs of life with only a weight on the piston and that is understandable. Springs always cause forces in only one direction which increases proportionally with the stretching, resp. compression thereof. 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 much less present with only a weight to the piston because the weight inherits kinetic energy in both opposite directions: in the downward movement by the gravity force and in the upward movement by the gas explosion under the piston that pushes the piston upwards like a bullet out of the barrel of a gun. An extremely heavy weight will cause a small upward stroke also 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 what there is a maximum effect for the compression on the one hand and the upward stroke on the other hand. It was still the question whether the engine would actually wants to run with this optimal weight , but the luck was with the stupid. Step by step I turned gradually something of an initial excess weight until the engine showed some signs of life. In that situation I could investigate other sensitivities and eliminate or reduce them. Then I turned the weight that much back until the engine continued to run, at least for some time. So bear # 1 was present indeed, but in fact I could dislodge him away far enough.
Bear# 2: TheStart-up of the engine. Unlike anengine withflywheel, it is notpossible to use a hand drill to start the engine. Itwas therefore the question whether theenginewould startwith manualliftinganddroppingthe weight. Again, luck was withthestupid: some few ofthese movementsare usually sufficientforto starting the engineifthe carburettoris adjustedwell at least. So I canbe brief a bout this bear: he was not present.
Bear# 3: Engine shut down after anignitionmiss. Thereis noflywheeleffect here whereby theenginecan be helped over a deadtimeaftera missedignition ofthe gas mix. In suchcase,this enginewill immediatelyshut down without a self-restart. A seriousriskdue to the highdemands onthereliability ofthe ignition whichis not exactlythe strongestfeature of 2-strokeengineswho generally tend to skipstrokes now and then. This bear indeedis explicitlypresentand I havenot been able todislodgehimfor 100% but I succeeded in bringing him in a sort of soft sleep so the engine runs for an acceptable time with what I mean about 2 minutes in best case, at least for the time being.An ignition miss can have several causessuchas a weakspark, a toolateor tooearlyspark, atoo richortoo poorgas mix or insufficientflushing ofthe cylinderwith an impuregas mix as a result. It isalready a small miraclethat this 2-stroke engine performsforso longwithouta singleignitionmiss.But this result was was reached without any struggle: - I havedecided for the time being tomakethesparkwith a classic motorcycleignition coilthatI build in thewoodenbase of theengine. Such acoilmakes a prettystrong sparkand I knowthat this is neededto preventa 2-stroke engine making ignition misses easily. The micro switchwhichcontrolsthesparkisstruckby acentralpin attachedto the weight and which is adjustable in height. With thispin I canadjust the timing of thespark andwith that theactualcompression. This settingis quite sensitive, butwith acceptablemargins.
- I found thatthe volume of thecombustion chamberplays an important role. Not surprisingsince itpartly determinesthe flushing processandthereforethe purityofthegas filling in the cylinder. Luckily I hadmade this volume somewhat too bigso Icould simply reduce it in small steps sawing small pieces from the bottom oftheglass cylinder.After eachreductionI noticedan improvementinthe running behaviour. I stopped reducing when IfeltIhad reachedtheoptimum volume, although I'm not quite sureof that because I did notwant togointo the situation: "too much sawed off andstilltoo short".
-Very remarkably is the strong sensitivity ofthe engine for thecarburettor adjustment, in other wordstheratioair/fuelvapour. The marginisvery small, but thecorrectsetting forthe controlleroftheadditionalair can be found with some care.I will haveanother goodlook for the reason why this is that sensitive andwhetherthereisanythingtobe improved.
The improved version MK2 I've tried to replace the motorcycle ignition coil with the much smaller miniature "Blokker" circuit but as I already feared this was just a bridge too far. The spark energy of this mini circuit seems marginal or rather insufficient to keep this 2-stroke running reliable for a longer time. The engine did run indeed, but he stops much more often and sooner than with the stronger spark from the motorcycle ignition coil despite frantic attempts to avoid that. As mentioned, a single ignition miss is fatal immediately because here there is no flywheel effect that this helps the engine to overcome such a dead moment. Nevertheless, these experiments in more critical circumstances provided me some new insights. I found that the compression should be increased somewhat to let the engine run longer and more reliable. This compression increase can only be made with a heavier weight and I feared that the upward stroke of the piston would be smaller also with the heavier weight resulting in less intake of the fresh gas mix from the carburettor. And that appeared not to be the case: the stronger combustion also results in a greater expansion of the gas mix and the upward piston stroke certainly was not less, even larger. I finally increased the mass of the weight from about 150 grams to about 350 grams; considerably larger so, but the engine runs much better with it. A second change concerned the rubber O-ring that must seal the glass cylinder airtight from the outdoors. I now put this O-ring in the cylinder head instead of around the glass cylinder, see the sectional drawing at the top of the page. The 6mm high pressure washer for the O-ring around the cylinder could therefore be eliminated with the result that one can look now in the combustion chamber seeing the combustion fires. In fact only an optical improvement, but I thought it was a pity that the combustion fires were shielded by the former 6mm high washer. Now the glass cylinder is clamped against the O-ring with the black plastic pressure washer on top of the cylinder. Thus, together with some other minor changes such as the spark plug and the intake pipe on the carburettor, the improved MK2 version was created with quite optimal performance as far as I can judge now.
The picture below shows this final MK2 version; on the video you can see the remarkable performance of this tricky thing:
Preliminary resultsandconclusions All in allit was amatter of"trial and error", but withsomesystematics andrationalthinking to the extentthatit is given to me.PresumablyI havenot reached the realoptimum for all parameters involved because thesensitivitiesare somewhatcomplexand interrelatedandbecause someeffectsweredifficult to reverseastheturningofthe weight andcutting-off theglass cylinder.
I was amazed seeing the thingbouncingfor the first time at a rateofapproximately5 beatsper second,so about300 timesper minute. Quitefaster than theapproximately1 per secondthatI had expected, about what apilingmachineis doing normally. Iam therefore very satisfiedwith the resultfor the time being because the engineruns with a nicesteady paceandmuch longerthan Ieverthought was possible. The factthat I have to restart the thingafter 1or 2 minutes most of the time I consider as a relativelylimitedevilbecause Ihonestlytook intoaccount from the very start thatheshould not make not more than a few turns in a row hadat the best.
Drawing Plans I made CAD drawing plans that are available for every one interested; click here for a request.
However one warning: the engine is relatively easy to make (provided you have the proper glass for the cylinder and the graphite for the piston) but a series of hidden and interrelated sensitivities makes it a decent challenge to adjust the engine right. It's like with a dog lover: it will take a while before the boss understand the dog and vice versa. But with enough persistence and patience, they can become good friends sooner or later.
Practical use ?? I get a lot of suggestions to implement one or more magnets on the weight for generating electrical power by letting them move up and down through a coil or drive a water pump with it, etc. Apart from the fact that I never tend to make practical applications for my small models, I think the power of this model is hardly enough to let a single LED lamp flash and that is not very spectacular in my opinion and not really worthwhile to do. Whether or not an industrial embodiment of this design would be interesting for generating any kind of energy I leave to the judgement of professionals in this field, but I have my reasonable doubts because I think a normal 2-stroke generator with a flywheel will be more efficient and they exist and are plenty for sale.
Finally The majority ofall 2-stroke(model)engineshave the propertythat theyoccasionallymakeamisfire, oftendue to an accidental lessoptimumcylinderfilling.Because aflywheelis missing here that can help the processover such adeadlockthis engine willstop running then immediately.It already strikes methatthe thingactuallyis bouncing continuously for some 1 or 2 minutes, provided everythingis setoptimally. It is thereforeinevitable thatthis engineregularlymust be waked-up bylifting and dropping the weightagain.It is likethelittle dog thatoccasionally must be called to the order with atug onthe leash;itmust be accepted I fear but Idon't mind.
Alternative for the glass cylinder If one cannot obtain this very special glass (not unlikely) and/or graphite one can make the cylinder from bronze and the piston from cast iron. Well made (fairly leak free and with low friction) the engine will run also. Then the metal cylinder must be closed on the top in one way or another with a connection tube to the ball valve system. The only difference will be that the combustion fires will not be visible; a little bit pity of course but the rest of the engine keeps the same.