Why an alternative for the classic carburettor ?
The carburettor is one of the most important parts of An I/C engine. It determines the performance and the behaviour of the engine to a large extent. However, it is not easy to make a conventional carburettor. Also problems such as unreliable start-up, ‘flooding’ the engine, and carbon deposits on the spark plug are all too familiar. At least that is my opinion, and I am certainly not alone in this view.
Without claiming that it is impossible to make a reasonable operating classic simple carburettor, I dare to suggest that it is hard for most model builders to make one that gives really satisfying results.
That brought me to think about an alternative process that provides for a mix of 100% molecular petrol vapour (rather than petrol droplets) and air with an ideal ratio of about 1 to 14. The idea was for a design that would make short work of those carburettor problems encountered by most amateur model builders.
The thoughts behind this alternative design
The task for a carburetor is to make a mix of air and petrol for combustion in the cylinder of the engine. A good carburetor must mix about 1 part of petrol with 14 parts of air. The petrol droplets must be as small as possible; the smaller the droplets, the better the combustion will be. Big or unequal formed petrol droplets often cause flooding the engine and/or carbon soot on the sparkplug due to incomplete combustion and, as a result, bad or irregular engine behavior. Ideal would be that the air is mixed with molecular petrol vapor instead of fluid petrol droplets.
Self made carburetors for model engines usually keeps far from this ideal. They don't reach a higher ratio then 1 to 10 and it is very difficult to avoid incomplete combustion. Insiders told me that it is almost impossible to make significant improvements with this kind of classic carburetors with a venturi, air throttle valve and petrol jet with needle and choke valve.
The theory about the ideal carburetor brought me to the idea to make an entire different carburetor design. The basic thought was that if air is lead through or over liquid petrol it will take molecular petrol vapor with it instead of liquid droplets. The only question was if the amount of petrol vapor would be enough to create the ideal ratio of 1 to 14. The fortune was with the the fool! Simple experiments with some petrol in a glass test-tube, closed with a rubber stopper with a metal tube in it taught me that the chance for success should be more than likely. I even found that there was more petrol vapour in the mix then necessary for the ratio 1 to 14. And that means that the combustion properties of the fuel mix could be influenced by adding some extra air between the carburetor and the engine and, with that, the speed of the engine.
Less than some experimental days delivered me the conditions for a childish simple carburetor with amazing good performances !Design concept
The task for a carburettor is to mix air and petrol for combustion in the cylinder of the engine. A good carburettor must mix about 1 part of petrol with 14 parts of air. The petrol droplets must be as small as possible. The smaller the droplets, the better the combustion will be. Large or unequal droplets often cause flooding the engine and/or carbon soot on the spark plug due to incomplete combustion and, as a result, bad or irregular engine behaviour. I set about my design so that the air would be mixed with molecular petrol vapour instead of fluid petrol droplets.
Self-made carburettors for model engines are usually far from this ideal. They don't reach a higher ratio then 1 to 10 and it is very difficult to avoid incomplete combustion. I have been told that it is almost impossible to make significant improvements with this kind of classic carburettor with a venturi, air throttle valve and petrol jet with needle and choke valve.
The theory about the ideal carburettor brought me to the idea to make an entirely different carburettor design. The basic idea was that if air is lead through or over liquid petrol it will take molecular petrol vapour with it instead of liquid droplets. The only question was if the amount of petrol vapour would be enough to create the ideal ratio of 1 to 14. Fortune was with the fool! Simple experiments with petrol in a glass test-tube, closed with a rubber stopper, with a metal tube in it, showed me that there was every chance of success. I even found that there was more petrol vapour in the mix then necessary for the ratio 1 to 14. And that means that the combustion properties of the fuel mix could be influenced by adding some extra air between the carburettor and the engine and, with that, the speed of the engine.
Result: An almost childishly simple carburettor with an amazing performance!
The proto type and the working principle
Below a sketch of the proto type with a jam jar and two independent air adjusters. With this I did my first experiments with the Otto 4-stroke engine.
On the lid of the jam jar there is a pipe with an adjustor to regulate the main in streaming air. This pipe reaches to the bottom of the jar. so it plug in the petrol. A second pipe on the lid of the jam jar connects this carburettor to an adjuster for mixing extra air on the intake manifold of the engine with a rubber hose. The in streaming air bubbles through the petrol taking 100% molecular petrol vapor with it. The amount of air and with that the amount of available petrol vapor can be regulated by a restrictor on the main pipe. The homogeneous mix of air and petrol vapor is sucked in the engine cylinder and is combusting there for 100% during the power stroke.
Because the petrol vapor concentration of the primary mix in the jam jar is higher than required for the ideal ratio 1 to 14 extra air must be added to it via the restrictor on the engine intake manifold. In principle it could be possible to add this extra air in the jam jar already, but I discovered that it was better to do this between the carburetor and the intake manifold of the engine. The reason is that the extra air disturbs the ratio of the gas mix when it is streaming over the patrol in the jar because it l takes extra petrol vapor with.The first version with two independent air adjusters
The first version of the petrol vapor carburettor was based on the prototype as described above.
Except for the sight glass the whole carburettor is made out of brass, not only because it is not fragile but especially to avoid possible dangerous static electricity caused by dry air steaming through the non conductive petrol. The brass tank is connected to the mass of the engine so eventually static electricity will automatically disappear.
The figure above also shows a integrated back stroke check valve which is always necessary for 2-stroke engines to avoid the compressed gas mix below the piston to be pressed back in the carburetor. With a 4-stroke engine this is only a safety measure to avoid back stream in case of an unexpected back stroke of the engine casing some petrol pushed in the open through the main in stream pipe.
This carburetor did work very well for all my 4- and 2-stroke stationary IC model engines with a lot of significant advantages compared to a classic carburettor that I will enumerate at the end of this page.
The second version with a single three way throttle valve
Although the first version worked very well there was a single problem with it, at least an inconvenience. The right adjustment for starting up and speed regulation of the engine with the two independent air regulators was not that easy and/or required rather much experience.
So I made a single throttle valve at the outlet of the tank with what is was possible to vary the gas mix with only one regulator. Two brass discs with a certain pattern of holes and slots can be turned around each other with what every gas mix can be made. So there will be always one position where the ideal gas mix occurs for a staedy start-up of the engine.
Tweede versie met drieweg regelkraan
Second version with three-way throttleAll three connection are on the stationary disc: the central connection to the engine cylinder, the connection to the petrol tank and a hole to the outside air. The latter two connections end in relative short slots on the inside of the stationary disc. The rotary disc has only one 180° slot that is connected to the central connection to the intake manifold of the cylinder. This slot connects the two slots in the stationary disc in a variable way. Depending on the relative position of the two discs the gas mix will be gradually changed from rich to lean and vice versa. The figures below illustrate these adjustments:
Cross sections of the three-way throttle valve
The upper sketch shows the middle position of the rotary disc. The two figures blow that show the extreme positions: respectively the position with much extra air (lean gas mix) and the position with maximum primary gas mix from the tank (rich gas mix). Anywhere between these two extreme positions there will be always one position where the ideal gas mix occurs and with what the engine will start easily. Turning the disc gradually around this position will cause the engine to run faster or slower.
With this three way throttle valve the problem with the two independent regulators was solved.
The third version with the short in stream pipe on the tank
More or less accidentally I discovered that it is not necessary that the in streaming main air is bubbling through the petrol to take more than enough petrol vapor with it. Stronger, with a short pipe that ends above the petrol surface all my IC engine run perfectly, may be better with som fantasy. In general it appears that the adjustment of the carburetor is less sensitive with the short in stream pipe. It looks as if the composition of the gas mix is more stable without the bubbling. For my 2- stroke Hit&Miss engine this version of the carburetor is even a precondition! I explain the reason for that on the concerning page for this engine.
Third version with short in stream pipe
An additional advantage of the short pipe is the fact that no petrol can be pushed to the outside in case of an unexpected back stroke of a 4-stroke engine at starting up. In fact I only use the short pipe for all my engines now. I don't exclude that the bubbling pipe could be better in case of much bigger engines with greater gas mix consumption.
The bubbling may be more spectacular to see, but I prefer the advantages of the short pipe more that the optical bubbling phenomenon. Besides, the in streaming air makes a dimple in the petrol surface that is well visible through the sight glass with what one can check if the engine is sucking in the gas mix that is very helpful in case of starting-up problems.The fourth and final version with an ultimate simple adjuster for adding the extra air
Everything worked perfect with the third version and yet there was still something to improve, or better, to simplify. It concerned the three way throttle valve from what some people told me that it was difficult for them to make it not having a rotary table on their milling machine to make the radial slots in the discs. Because I always strive to ultimate simplicity I was encouraged by this critical remarks to look for a more easy solution in terms of manufacturability. I tried the very simple regulator for the extra air that I already applied 4 years ago with the prototype as you can see on the first figure on this page. A little bit to my surprise it worked as well as the more complicated three way disc throttle valve. I tested it on most of my IC model engines and could not find any significant difference. This regulator is not more than a simple threaded connection tube with a slot through what the extra air can pass. A nut on that pipe can cover tis slot more or less with what the amount of extra air can be varied. This regulator can be made by every model builder with standard lathing work I think so that cannot be any problem anymore. The figure below shows how this looks; see also the picture on the right of this page.
Fourth and final versio with symplyfied adjuster for extra air
Well considered this carburetor is not much more than a petrol tank with two simple arrangements with what the ideal molecular gas mix can be sucked in by the engine. Because this system performs excellent for at least stationary IC engine models it is a good example for a "hallmark of simplicity". The disadvantage is that it looks dull but that is the other side of simplicity most of the time.
The fuel
Fresh auto car petrol contains a highly volatile and combustible component that must be mixed with plenty of air. For that it is necessary at the start-up of the engine to adjust the regulator for the extra air so that the slot in the pipe is completely open. While starting up the engine, i.e. with a hand drilling machine, the nut must be screwed gradually over the slot until one hear the engine take over. Around this position the engine will start up reliable normally. One tends to turn the nut in the direction for a richer gas mix if the engine refuses to start. But that is quite wrong! So always start with maximum extra air and then turn the nut gradually in the direction of less extra air until the engine give sign of life.
Depending on the consumption of the engine this volatile component will disappear gradually, causing a slight slowing down of the engine. At that moment the extra air must be decreased until the engine runs at the desired speed again. From then the situation becomes more stable. So, starting up with fresh auto car petrol this volatile component requires a rather sensitive adjustment, but this becomes less so as this component disappears gradually after some minutes running, resulting in a more stable and less sensitive situation.
A very good alternative for auto car petrol is "Coleman Fuel" with what this separation effect of the fuel can be avoided. It is a super refined kind of petrol with the absence of very volatile and heavy carbon hydrogen components. The composition of this fuel is nearly constant so the adjustment of the carburetor is much less sensitive than with auto car petrol. Because there are no residues of heavy carbon hydrogen's this fuel can be used until the tank is empty. This Coleman Fuel is used for camping stoves, mainly because it is practically odourless, which also is an advantage for us. It can be obtained at every shop for camping attributes.
I perceived that a 1 to 2% oil added to normal auto car petrol has a positive effect on lubricating the piston. Apparently the less volatile oil vapour is taken sufficiently well with the gas mix, rather to my surprise to be honest. I use the common household handy oil for this, but there will be other light oil types suitable as well or maybe even better. In case oil is added to the fuel it is advisable to refresh the content of the tank after several engine runs.
It is advisable to fill the tank not more than half full. Model engines up to 15 cc and with a speed of about 1000 RPM can run for 15 minutes or longer on that, which is more than enough for a successful demonstration run. The size of the tank doesn't affect the functionality of the carburettor at all. So one can enlarge the tank if wanted.
The advantages of this carburetor design compared tothe classic one
I found the following advantages of this new carburettor design:
1. Always 100% molecular petrol vapour in the gas mix. Consequently never any fluid petrol droplets in the cylinder and, as a result, no carbon soot or wet spark-plug, due to incomplete combustion;
2. The ideal ratio petrol vapour/air of 1 to 14 is easy obtainable. This very homogeneous gas mix provides for perfect running of the engine;
3. The engine is provided immediately with the right gas mix, so starting-up is very reliable and fast without choking. Choking is actually not possible with this system;
4. In fact it is no longer possible to flood the engine; at least haven’t succeeded in doing that so far;
5. The speed of the engine can be well regulated with the regulator for adding extra air on the rear of the tank;
6. This carburettor design is very simple: no venturi, no petrol jet with needle, no accurate dimensions, no chance for false air- and/or petrol leaks;
7. This carburettor may look somewhat bigger than the classic one, but the contrary is true: in fact it is only a small arrangement, integrated in and on the petrol tank;
8. This carburettor cannot overheat because there is no heat conduction from the cylinder to it. The length of the connection tube to the intake manifold is not critical at all. I didn't notice any difference between 10 and 50cm tube length! The location for this carburettor is therefore free to choose;
9. Regulation of the petrol level is irrelevant;
10. No risk of stoppages; there are no narrow flood gates and possible contingent dust particles remain visible in the petrol tank. They disappear with draining the tank at the end of a demonstration;
11. Petrol consumption is minimal; not more than necessary for the energy to be created;
12. No chance for petrol leaks to the outside of the carburettor, and therefore safe and with no unpleasant smells.
Finally
It is really a joy to see how reliable and fast the engine starts on this carburettor and how smoothly it runs. No more problems with choking, flooding and sooty or wet spark plugs is truly a big relief!
I assume that this design relieves many builders of internal combustion engines of a ‘carburettor nightmare’, except perhaps some notorious experts in this field.
I dare to postulate that this version of the carburettor in fact is the most possible simple and reliable solution. It is a striking example of how simplicity can bring you the best results.
It is amazing that recently Chuck Fellows also made a very nice design of this carburetor. He used a jam jar as petrol tank and the same regulator for the extra air. You can find it with a nice drawing picture on the HMEM web site.
Final version
Adjuster extra air
