fuel intake

[DEEJACDESIGNS]

man ,i have built an almost complete engine but i cant find plans or an explaination for the fuel supply...from what i gather(please correct me if im wrong) there are two ways to go about the fuel intake...one sprays the fuel outside the (is it called venturi??)the cone shape in the front..and this will suck the fuel in to the combustion chamber...the second method is to have the fuel line go straight through the center of the reed valves and is deposited directly into the combustion chamber..am i correct about these two methods??my question is...is there a fuel pump that sends the fuel to the chamber??if so what kind of pump should i use..the second question is...how is the fuel atomized?? im sure the fuel cant go in as a liquid and it needs to be mixed with air..how is this done?? i wish someone could explain how i can go about building this and any links to diagrams would be great..iv heard of some people using fuel injectors from cars...but am unsure how to even adapt these to suit my needs..thanks in advance...oh yea...has anyone ever used jet fuel to power their pulse jets?? i hear people using gasoline mixes and propaine but have never heard of anyone using actual jet fuel...can it be used and is it available??..oh by the way...my engine is a 9lb thrust(well i hope is will be)similar to bailey(reedvalve type) a little over 3" diameter combustion chamber)i provided this info just in case the type of fuel system reccommended depends on the type of engine..if a pump is needed,and weight is an issue,couldnt you substitute the fuel pump,by storing the gas in a metal container that can be pumped up,therefor pressurising the gas,and this pressure will spray the fuel instead of a pump??thanks for the advice..im sorry if my questions are too elementery ,but i am just beginning to work with these engines and am having trouble locating any info on building them,with the exception of detailed drawings of the engines themselves,i have found little info for the fuel supply and electric(spark) info..thanks in advance.

[larry cottrill]

man ,i have built an almost complete engine but i cant find plans or an explaination for the fuel supply...from what i gather(please correct me if im wrong) there are two ways to go about the fuel intake...one sprays the fuel outside the (is it called venturi??)the cone shape in the front..and this will suck the fuel in to the combustion chamber...the second method is to have the fuel line go straight through the center of the reed valves and is deposited directly into the combustion chamber..am i correct about these two methods??my question is...is there a fuel pump that sends the fuel to the chamber??if so what kind of pump should i use..the second question is...how is the fuel atomized?? im sure the fuel cant go in as a liquid and it needs to be mixed with air..how is this done?? i wish someone could explain how i can go about building this and any links to diagrams would be great..iv heard of some people using fuel injectors from cars...but am unsure how to even adapt these to suit my needs..thanks in advance...oh yea...has anyone ever used jet fuel to power their pulse jets?? i hear people using gasoline mixes and propaine but have never heard of anyone using actual jet fuel...can it be used and is it available??..oh by the way...my engine is a 9lb thrust(well i hope is will be)similar to bailey(reedvalve type) a little over 3" diameter combustion chamber)i provided this info just in case the type of fuel system reccommended depends on the type of engine..if a pump is needed,and weight is an issue,couldnt you substitute the fuel pump,by storing the gas in a metal container that can be pumped up,therefor pressurising the gas,and this pressure will spray the fuel instead of a pump??thanks for the advice..im sorry if my questions are too elementery ,but i am just beginning to work with these engines and am having trouble locating any info on building them,with the exception of detailed drawings of the engines themselves,i have found little info for the fuel supply and electric(spark) info..thanks in advance.

Deejac -

I don't know anything to speak of about direct injection into the chamber, but I might be able to roughly explain venturi carburetion.

Looking at the longitudinal section of a venturi, it is fairly obvious that air traveling through it will have to speed up to get through that narrow throat in the middle. What happens is that the same number of air molecules pass through that narrow throat per second as enter the wide mouth per second, and that means the flow has to be faster. What is not so obvious is that the pressure of the air drops significantly as the air passes through the throat at that high speed. This drop in pressure can be significant -- in the Dynajet venturi, the suction is sufficient to lift fuel several inches against its own weight in the fuel line.

The lowest pressure will be at the very narrowest part of the throat, so that's where you locate your little fuel ports. Now, when the fuel is drawn by the pressure difference out to the edge of the port, it is immediately sheared off by the very fast air going past [the air flow is many times the speed of the fuel flow reaching the port face]. This shear causes a rapid breakup of the liquid into tiny droplets, a process called atomization. How good the atomization is depends on the fuel flow, air speed, design of the ports, etc. The idea of atomization is to get droplets so small that they will almost IMMEDIATELY evaporate -- that's especially important in something the size of the Dynajet, where the fuel is delivered just a few centimeters ahead of where we need vapor to burn. Atomization is also important for good mixing of the vapor with the airstream, so the fuel vapor isn't 'localized' but is spread out through all the air that gets into the combustion area.

So, the venturi is a way of delivering fuel, atomizing fuel, forming combustible vapor, and achieving good air/vapor mixing for combustion. There are actually many different designs for getting fuel well delivered and vaporized in venturi carburetors, but they all amount to introducing the fuel at the point of highest velocity and lowest air pressure to get the job done. All other details are just 'tuning'.

L Cottrill