ARTICLE REF – ART229
I am going to talk about “starting” and “fuel transfer” issues on the FT8 liquid fuel system.
Liquid fuel starting issues are common on the FT8. One of the main reasons for starting issues on liquid fuel, is the lack of atomizing air at the injector to disperse the liquid fuel into a vapour, for easy light-off. This lack of atomizing air means everything else associated with light-off, such as spark plug, ignition lead, high voltage coils, fuel filters etc have to be in near perfect condition, there is not much margin for issues.
With liquid fuel there is a 30 second time window for lighting off. If your EGT has not risen by then, the start is aborted. The fuel system is drained and the start sequence has to start all over again.
The solenoids SOV 1403 / 1404 / 1405 drain down the liquid fuel as stated above, but also when the unit shuts down norally. In cold locations such as the mid west in the US or in Russia, where these units operate, it is important that the fuel system is drained after shutting down, as liquid fuel with water (from water injection) will freeze in the piping, mixing block, flow divider etc.
There has been a long history with these drain solenoid valves, but that is for another day. These drain valves are energized closed, which means that if the coil fails or a fuse blows the drain valve will open and dump liquid fuel to the common drain.
If you are having liquid fuel starting issues this is one of the first things to check. Because liquid fuel is incompressible any leak will cause a drop of fuel pressure and prevent light-off.
Here are some reference pressures that may help you. These are pressures I noted at light-off, not while in operation.
Fuel pressure at the fuel pump output is approximately 300 PSI.
At the drain line upstream of SOV 1405, I was getting approx 5 PSI prior to light-off, rising quickly to 40 PSI at lit-off. A similar pressure was found on the Flow Divider feed and drain.
You might want to check the flow at the input to the flow divider. You can expect to get approximately 4.5 liters of fuel (no water) at light-off time.
There is a pressure relief inside the pump which has o-rings. If the o-rings are damaged the pressure leaving the pump can be much less. Damage to the pump may also supply less pressure.
This location shown on the FCV1001 has the “fuel control vlave” output pressure. You will have to make up a fitting. I got 245 PSI at this location at light-off.
The pressure on the SOV1002 was 35 PSI
Keep in mind that a loss of pressure may also prevent you from a transfer to liquid fuel when running on gas, as the liquid fuel must be at a higher pressure than combustor pressure, otherwise it will not be able to enter the combustor.
While there are many issues that can cause light-off or fuel transfer issues these drain solenoids are often the issue.