Fuel system is a thing that seems to cause problems for some people for all the Sonex line of aircraft. It seems most of these problems are caused by people using auto fuel but not using a fuel system that is designed for auto fuel. The problems seems to be engine "burps". These "burps" are caused by fuel boiling in the fuel lines (vapor is formed). Auto fuel is much more susceptible for this than Avgas. When the vapor reaches the carb, this causes engine misfire - "burps". The fuel is fed by gravity, and with the tank behind the firewall there is not much pressure in the fuel lines, evidently not enough to stop vapor from forming in hot conditions. The Aeroinjector has no float and float chamber, so all eventual vapor is fed directly to the needle, and this will exaggerate the vapor problem for these kinds of carburetors. This setup will not experience vapor lock in the correct sense, but the cause of the problem is the same.
For the Onex, the drawings show two different setups. One with a gascolater and one without.
The one without a gascolator has an inline filter and a drain point just before the carb. The idea with this setup is the continuous droop of the fuel line will cause the vapor to run back and out in the fuel tank. It seems like a simple and functional setup, not much can go wrong there. The one with the gascolator is similar, but the gascolator and the fuel line from the gascolator to the carb are inside the heated cowl. The gascolator must be the lowest point to collect water. Vapor created in the gascolator and the fuel line has no where to go but to the carb. Of these two systems, the first one is definitely the better one in my opinion; shortest lines, less bends and fittings, eventual vapor will just float back to the tank, and is the one I will use. The one with the gascolator is probably OK when using avgas only and when properly insulated/cooled.
(Update) From the comment below it is possible to duct ambient air to cool the gascolator. Further it is possible to shroud the gascolator in a cooling box. This should fix all burping issues.
Then I suddenly remembered I have seen this somewhere else. AirWard has a complete kit for this. Apparently this will fix all "burping" issues when using autofuel.
Some people have installed a "burp tube". This is a "vent line" just before the carb. The idea is to collect the vapor and vent it to the tank.
I guess the "burp tube" works, but when the fuel level is low (low fuel pressure) and the aircraft is at an angle, the carb could theoretically start to suck in air/vapor from the burp tube instead of fuel from the fuel line.
When (if) the Rotec TBI is to be installed, this simple system will not work satisfactory. The Rotec has a pressure regulator, so the Rotec requires higher min fuel pressure than the Aeroinjector. It will probably run without a pump, but it will be outside the specs (min 0.5 PSI) when the fuel level is less than full.
The simplest Rotec system will be using one pump. If the pump quits, the engine will still run (probably), but this has to be tested.
(Update) Rotec has just upgraded their TBIs to a Mk II. The mark II has the pressure regulator as in integrated part, making it much simpler to install and also more robust. Also, there is evidently no problem running the Rotec on pure gravity feed. Several people are running a setup with one pump, gascolator and the Rotec. The pump is an assurance for fuel pressure during take off and landing. The Rotec TBI Mk I/II, one pump and the fuel cooling system therefore seems to be a fairly simple and nice solution.
A "standard" fuel system seems like a better option with the Rotec. Rotec also recommend a fuel return line when using mogas.
Such a system will have to have two pumps for redundancy, and since both are electric, one of them preferably has to run from an aux battery. This is a much more complex system than the simple default system in the drawings, but with the exception of using two electric pumps, it is a system used in thousands of aircraft.
For an inverted fuel system, pumps are needed. The tank also has to be modified somehow. One method will be to use an inverted "header" tank with a flop tube mounted just downstream the main tank.
For the Onex, the drawings show two different setups. One with a gascolater and one without.
The one without a gascolator has an inline filter and a drain point just before the carb. The idea with this setup is the continuous droop of the fuel line will cause the vapor to run back and out in the fuel tank. It seems like a simple and functional setup, not much can go wrong there. The one with the gascolator is similar, but the gascolator and the fuel line from the gascolator to the carb are inside the heated cowl. The gascolator must be the lowest point to collect water. Vapor created in the gascolator and the fuel line has no where to go but to the carb. Of these two systems, the first one is definitely the better one in my opinion; shortest lines, less bends and fittings, eventual vapor will just float back to the tank, and is the one I will use. The one with the gascolator is probably OK when using avgas only and when properly insulated/cooled.
(Update) From the comment below it is possible to duct ambient air to cool the gascolator. Further it is possible to shroud the gascolator in a cooling box. This should fix all burping issues.
Then I suddenly remembered I have seen this somewhere else. AirWard has a complete kit for this. Apparently this will fix all "burping" issues when using autofuel.
Some people have installed a "burp tube". This is a "vent line" just before the carb. The idea is to collect the vapor and vent it to the tank.
I guess the "burp tube" works, but when the fuel level is low (low fuel pressure) and the aircraft is at an angle, the carb could theoretically start to suck in air/vapor from the burp tube instead of fuel from the fuel line.
When (if) the Rotec TBI is to be installed, this simple system will not work satisfactory. The Rotec has a pressure regulator, so the Rotec requires higher min fuel pressure than the Aeroinjector. It will probably run without a pump, but it will be outside the specs (min 0.5 PSI) when the fuel level is less than full.
The simplest Rotec system will be using one pump. If the pump quits, the engine will still run (probably), but this has to be tested.
(Update) Rotec has just upgraded their TBIs to a Mk II. The mark II has the pressure regulator as in integrated part, making it much simpler to install and also more robust. Also, there is evidently no problem running the Rotec on pure gravity feed. Several people are running a setup with one pump, gascolator and the Rotec. The pump is an assurance for fuel pressure during take off and landing. The Rotec TBI Mk I/II, one pump and the fuel cooling system therefore seems to be a fairly simple and nice solution.
A "standard" fuel system seems like a better option with the Rotec. Rotec also recommend a fuel return line when using mogas.
Such a system will have to have two pumps for redundancy, and since both are electric, one of them preferably has to run from an aux battery. This is a much more complex system than the simple default system in the drawings, but with the exception of using two electric pumps, it is a system used in thousands of aircraft.
For an inverted fuel system, pumps are needed. The tank also has to be modified somehow. One method will be to use an inverted "header" tank with a flop tube mounted just downstream the main tank.
1 comment:
Put the gascolator in a box or shroad an pipe ambient air to it and the burp problem is solved. I know because we did it on our sonex and it worked.
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