Why the Purge Valve. A bit of history
Let me give you a bit of history. Back in 1984 (when we started the company) we knew that there was a lot of negative press about hot starts with aircraft fitted with Bendix fuel injection. When we designed the FM-series system the main objective was to simplify the regulator design and to make it handle fuel flows for engines ranging from 50 HP to 1200 HP, with gasoline or alcohol. In the design because of manufacturing reasons the idle and mixture control used rotary valves.
Identifying the reason for hot starts was key in developing the idea for the purge valve. With the Bendix fuel injection system as well as Airflow Performance system, there is no fuel return in the system. After shut down the heat from the engine boils the fuel in the fuel system. On the re-start there is part fuel part vapor so the start may not happen or the engine will start then stop. There is no way to get rid of the vapor, as the system has no fuel return. The only way to clear the system is to run the engine up to clear out the vapor.
By identifying this problem, a solution was to have a device that allowed the fuel to circulate through the system but not flood the engine. Putting the purge valve at the flow divider allowed 95% of the volume of fuel in the fuel system to be circulated thus eliminating the vapor and replacing the hot fuel with cool fuel. With this device the hot start problem was eliminated.
Now remembering that the FM-series fuel injection systems used rotary valves for the mixture control the fuel leakage at ICO was around 2-4 PPH. This did not always reliability shut the engine off and under certain conditions engine run on. Also since most injected Lycomings used a diaphragm fuel pump, after the engine stopped one stroke of fuel was put through the fuel injection system. The result being fuel dripping down the intake pipes and out the air inlet or out the manifold drain. By using the purge valve on these systems the shut down problem was also eliminated since the purge valve diverted the fuel at the flow divider and dumped the inlet fuel pressure.
We had a lot of push back on this design too as it required an additional cable control in the cockpit and the return line to one of the fuel tank feed lines. Basically some people loved it and some wired the device open. To me it was pretty simple, start and stop the engine using the purge valve, leave the mixture control full rich all the time unless leaning in flight at altitude.
We now offer a system that does not require the use of the purge valve. The FM-150, FM-100A, FM-200A, and the FM-300B have a new design fuel regulator that does not require the purge valve.
Don