Hi to all model IC engine builders, about 3 years ago I built a Hit and Miss IC engine . It has a bore of 20 mm and runs with Methanol ( modelengine fuel). For the piston ring I used an O-ring. It runs fine with this kind of fuel. But I want to run it in my workshop too, so I decided to try to convert the fuel supply from metanol to propane. I built the propane demand regulator from the tips section. Between the gas bottle and the demand regulator I attached an aditional equipment to be able to make some experiments with the input pressure of the demand regulator. I adjusted this input pressure to 0.8 bar ( 0.8 kg/cm² ) The results where very different: If I got the engine to run ( which could be very, very hard work sometimes) it runs sometimes like an devil. But in other cases it made only some ignitions. The engine ignition works with a glow plug. At every intake step I'm able to see a movement of the diaphragm of the regulator so I think that it works correctly. To increase the vacuum in the intake step I reduced the intake area of the carb to about 1.5 mm. I think another improvement can be achieved when I increase the intake pipe of the carb. At the moment it has an inner diameter of 1.5 mm. The compression ratio is the same like in the methanol version. But like I said before too the engine don't run continously. Is it nesessary to make some changes in the adjustment of the cam to change the timing of the strokes? I made a lot of experiments regarding the output pressure of the demand regulator, made sure that the engine has enough lubrication. It would be very nice if someone is able to give me any advices.
I think you are flooding the engine with propane and causing the mixture too rich to burn. 0.8 Bar (12PSI) seems high. When I ran my propane I ran at about 0.05Bar. When you burn propane don’t forget to add a lot of oil to everything because it is a dry clean fuel Let us know what you find. David
Hi David, thank you for your answer. You wrote that you use a pressure of 0.05 bar. Is this the inlet or outlet pressure of the demand regulator? I tried to use a lower input pressure of the regulator but I wasn't successful with this. The next I'll try to get it to run will be to increase the diameter of the outlet hose of the regulator. Another change will be the usage of another carb ( a carb with a bigger inlet pipe). After I've done this I'll write if I'm successful in running the engine.
The 0.05 Bar pressure is the input to the demand regulator. The demand regulator is really a vavle that sences the negative intake pressure and opens. There should be no output if the engine is stopped. The hose to the carberator needs to be short and small so the intake pressure drop is not lost. The carberator needs to have a jet 50% larger for propane than for liquid fuel David
Hi Didi, My last two engines have been propane fueled, but both are throttle governed. Both are 1" bore engines. One is single cylinder and the other is a 4 cylinder. They both run well now, but it was frustrating at first. Both use spark ignition so I am not sure just how much of the following will apply to a glow plug ignition. I would not think a hit & miss engine could keep a glow plug hot enough for reliable ignition, and I don't think it is hot enough for reliable ignition of gaseous fuels. I would add a spark ignition sytsem if the following does not help.
I now use a fixed pressure supply regulator set at 4" water column pressure going to the demand regulator (Jerry Howell design). My fuel line from the demand regulator to the carb is .125" ID, and it is adequate for either engine. Longest length fuel line I have used is about 12" from demand regulator to the carb. I increased my jet sizes only about 20% on one engine and 0% on the other. I saw no difference, just a little more open.
Make sure your demand regulator includes a fuel priming button, and that the regulator works correctly. Pressurize the demand regulator at operating pressure, and with your mouth, suck gently on the fuel outlet pipe. It should allow fuel to flow at the slightest suction draw, and stop flowing when suction is stopped. The carb needs a restriction of some sort to provide draw on the fuel line. It can be a venturi or a restrictor plate. The restrictor size is experimental so make it variable. My carb bore diameters are near .188" dia. and the restrictor is usually about 30-40% open. Start there. Then start cranking the engine with the needle about 1/2 turn open, and keep opening it slightly. Pressing the primer button momentarilly occassionally helps to determine when you are closing in on a run setting. Once you get the engine running, adjust the restriction opening first, then fine tune with the needle valve. Throttle governed engines are more difficult to adjust, but a hit & miss should start pretty easily. After you have the correct settings, it should always start with one or two spins.
Remember, you are setting the fuel air ratio with the restrictor, then fine tuning it with the needle valve. If you change engine speeds, your settings will also change. Again, I don't think a glow plug ignition is hot enough for propane. Good luck.
Hi Jeff, thank you for your advices. Regarding the glowplug problem : I heat it constantly with an external power supply to keep it glowing. Can you give me some further informations about the primer button. Is this a device like a choke in a car that is been used to start it when the engine is cold? Does the primer button give an extra load of gas to start the engine? Tomorrow I 'll finish a new carb with a bigger fuel pipe cause I read on the floridaame site that this pipe has to be enlarged when you run a engine with propane. The new carb consists of 2 jets: One for the propane and the other for the air and both jets are adjustable to get an ideal mixture.
Didi, Propane requires more energy to ignite than glow fuels and methonal mixes. A spark plug can deliver a much higher localized igniton temperature, and therefore should be a more reliable ignition source than a glow plug.
The primer button on a demand regulator is a small button on top of the regulator that pushes down directly on the diaphram allowing gas to flow. Momentarily pressing the primer button provides a small shot of propane to the carb. It works well in starting a propane fueled engine.
I suggest that you also add a variable restrictor plate to your new carb. It allows you to adjust the fuel line suction if the carb throat diameter is too large to creat some suction on the fuel line. Fuel line suction is needed to operate the demand regulator. Most of us make the carb throat diameter considerably larger than it needs to be.