FIGURE 1
FIGURE 2
FIGURE 3
ORIGINAL LONG IN-STREAM PIPE VERSION
NEW SHORT IN-STREAM PIPE VERSION
AS PROMISED WITH THE ARTICLE ON BUILDING A SIMPLE TWO-STROKE ENGINE, HERE IS THE IDEAL CARBURETTOR
A NOVEL IMPROVED CARBURETTOR
By Jan Ridders Model Engineer
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.
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!
First version
The first version of a carburettor based on this concept was the Petrol Foam Carburettor. Air streams in the tank through a to the bottom of the tank, so the in-streaming air bubbles through the petrol creating a kind of foam, hence its name. Instead of petrol mist or droplets as is the case with the classic carburettor here the air takes 100% molecular petrol vapour with it, and that results in complete combustion in the cylinder. Because the concentration of petrol vapour in this primary gas mix is more than is required for the ideal ratio of 1 to 14 extra, air must be added with a second adjustable intake between the tank and the engine's intake port.
This carburettor worked perfectly on all of my 2- and 4-stroke model engines However, there was a small problem, or rather, some inconvenience with this concept. The two regulators operate independently from each other, and so it takes some experience to adjust them correctly relative to each other for a prompt start-up and to regulate the speed of the engine. Incorrect adjustment of one of the two regulators when trying to change the speed of the engine sometimes causes the engine suddenly to stop.
New version
The cross section drawings illustrate the principle working of this carburettor system.
The engine sucks the gas mix from the tank via the throttle valve at the back of the tank with the result that air streams into the tank through the pipe on the tank. The two versions are identical except for the air in-stream pipes on the tank. With the first version (fig. 1) this pipe is inserted in the petrol so the in streaming air bubbles through the petrol. The pipe for the second version (fig. 2) ends about 1 cm above the petrol surface so the in- streaming air is strikes over the petrol surface. In both cases the in-streaming air takes more petrol vapour with it than necessary for a good combustion in the engine, so extra air must be added to this primary mix with the help of the three-way throttle valve at the back of the tank.
Both versions have the same three-way throttle valve at the back of the tank allowing an adjustable amount of extra air to be added to the primary gas mix coming from the tank. This throttle valve consists of a stationary and a rotary disc.
All three connections are on the stationary disc: a central one to the engine's cylinder, a connection to the carburettor tank and a hole to the outside air. The last two connections end in relatively short slots in the inside surface of the stationary disc. In the rotary disc there is a blind 180deg. slot. This slot connects both slots in the stationary disc with the central connection to the engine's cylinder.
Depending on the angular position of the rotary disc there will be more or less extra air added to the primary gas mix from the tank. The figures below illustrate these adjustments:
The upper sketch on figure 3 shows the middle position of the rotary disc with the three connections on the stationary disc. The two sketches below that show the two extreme positions of the rotary disc: a) the position for less gas from the tank and more added air (lean mix) and b) the position with minimal extra air added and maximum gas mix from the tank (rich mix). Turning the rotary disc between these two extreme positions will change the ratio petrol vapour/air and so the engine speed. There will be a certain position between these extreme positions giving the ideal ratio 1 to 14 for easy starting. 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
Using the carburettor
See also sheet 4 of the drawings.
1. The moving piston in the cylinder sucks the air/petrol mix from the carburettor, below the piston with a 2-stroke engine, and above the piston with a 4-stroke engine. In case of the ‘bubble’ in-stream pipe the air bubbles through the petrol, with the short pipe the air strikes over the petrol surface. In both cases the air will take 100% molecular petrol vapour with it, making a pure gas mix without petrol droplets. The primary gas mix in the tank contains more petrol vapour than necessary for the ideal ratio 1 part petrol and 14 parts of air, and extra air must be added in the three-way throttle valve on the exit of the carburettor tank. Changing the ratio petrol vapour/air will vary the speed of the engine. So this valve is used both to find the right adjustment for starting-up and to regulate the speed of the engine.
2. Fresh 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 valve so that the opening to the carburettor tank is almost closed while the extra air is about at maximum. This is the position as sketched in the middle of figure 3. It is convenient (necessary) to make benchmarks on the outside circumferences of both discs that are exactly opposite in this 0-position of the valve (see picture). Somewhere around this 0-position the engine will start well with fresh petrol.
3.One tends to turn the valve in the direction for a richer gas mix if the engine refuses to start. But that is quite wrong! Always start with the 0-position where hardly any gas mix from the tank is sucked-in and then turn the valve a fraction in the direction of a richer mix until the engine starts.
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 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.
4.After a hint from my American friend Ernie Weinberg I recently (October 2009) tried Coleman Fuel instead of normal car petrol. This type of fuel contains none of the additives in car petrol. All of my model engines run perfectly on it and, most important, the adjustment of the carburettor is less sensitive which was my reason for trying out this fuel.
Coleman fuel is used for camping cooking stoves, mainly because it is practically odourless, which also is an advantage for us.
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5.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. It is advisable to refresh the content of the tank after several engine runs.
6. A one-way check valve must be inserted between the three-way valve and the intake manifold of the engine. This is always necessary for 2-stroke engines to avoid the gas mix below the piston to be pushed back into the carburettor. For 4-stroke engines this is a safety precaution because over-pressure can occur in the tank with an unexpected back fire of the engine. In that case some petrol can be pushed to the open through the air intake on the tank in the case of the bubble in-stream pipe. With the short in-stream pipe this cannot happen.
A plan for such a simple and universal one-way valve can be found on the last sheet of the drawings.
7. Generally there is a low gas pressure between the cylinder and the carburettor so that they can be connected with simple rubber hose. The length of the hose is not critical.
8. 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.
9. Lubricating the two discs is not necessary; a little thin oil is OK but don't put a lot of thick grease on it because that can pile-up in the slots causing an unwanted change in the adjustment characteristics.
Which type?
All of my engines run perfectly with this carburettor with the long ‘bubble’ in-stream pipe as well as with the short pipe. The fact that the short pipe gives more than sufficient petrol vapour was a surprise for me, to be honest. Also, I noticed that starting-up and speed regulation is less sensitive than with the longer ‘bubble’ pipe. It looks as if the ratio petrol vapour/air is more stable when the air is striking over the petrol surface rather than bubbling through it.
An additional advantage of the short pipe is that no petrol can be pushed to the open if an unexpected over pressure occurs in the tank, i.e. at a back fire when starting-up a 4-stroke engine.
In fact I now use only the short in-stream pipe for all my models. But I don't exclude that the longer ‘bubble’ pipe is better, or even necessary, for much bigger engines and/or at very high speeds. The bubbling may look somewhat more spectacular but I prefer the advantages of the shorter in-stream pipe to this visual bubble phenomenon.
Advantages of the carburettor
I found the following advantages of the new carburettor:
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 three-way throttle valve 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
- The sight glass can be made from 2mm thick Perspex but glass is preferable because Perspex can be affected by petrol long term. You can adapt the hole in the side-wall for this glass according to the dimensions of a glass you have obtained from, possibly, a watch maker or an optician. I glue this glass with a well-known instant acrylate glue which is resistant to petrol.
- I use standard auto car petrol Euro 95 or 98 with 1 to 2% addition of light oil, even for my two-stroke engines.
- It was Martin Alewijn who gave me useful safety hints to avoid electric static charge in the carburettor. That's why the carburettor is made from brass and screwed to the mass of the engine. Thanks to Martin for his comments, background information about fuel composition and some other recommendations.
- 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.
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