AFP Fuel Injection (LONG)

[Flyrod]

Dan,

Great posting! This is just the kind of information that makes us operators and not just button and knob pushers. Don, thanks again for more great insight to a great product.

[David-aviator]

This is a most interesting thread but I am missing something.

If the problem of hot start is due in part to no return line, where does the purge valve dump the fuel? I've flown with Bendix FI in the Aztec and Arrow and know of the hot start issues. The AFP purge valve is a very good idea.

The fuel rail on the Subaru will stay pressurized for half a day or longer so the vapor problem exists there also. But it was easily solved with a tiny metered by-pass leak to the return line that reduces pressure to zero in about 4 seconds. It is simple and works.

Fuel pumps in general work well with a fluid, but they will not move vapor. The pumps we use will suck fuel up about 30 feet, but they won't suck vapor ahead of fuel up a 2' rise into a pressurized rail.

It may or may not be an issue with the AFP FI system but something to think about - if a tank is burned dry and there is glob of air in the line ahead of the pump, it might be worth while to have the purge valve to relieve the rail pressure and get things flowing again. Seems to me there is an issue concerning this matter with certified FI systems.

[KPmarc]

On the AFP system, a dedicated purge line is run back from the purge valve and, generally, teed into a fuel-feed line before or at the fuel selector. In my Sportsman, the return line tees into the right tank's supply line before the fuel selector. When purging, I just use the left tank and allow the purged fuel/vapor to run back into the right tank.

[w1curtis]

Originally Posted by frankh

Back when the sun was shining and it was 100F out I watched my friend with his V tail gas guzzling Bonanza cranking, and cranking and cranking. He then yelled outside the window to the rest of us patiently waiting in line for $4.00 100LL..."Hot start"...It was then I had my opportunity...Crank, crank brmmmm...Purr....

I think these Bonanza's use Continental engines and their fuel injection system and hot start procedures are a lot different that Lycoming. A lot of folks that are adamant about the installation of these purge valves are using examples such as this as their justification. I fly behind a Lycoming angle valve IO-360 with a Bendix fuel injection and hot starts are a non-issue with no purge valve. If you have a Lycoming engine and have a "hot start" issue then the "technique" needs to be worked on. For me it is, no prime, mixture to ICO, throttle 1/4 and just crank the engine. Engine ALWAYS starts in 2-3 blades.

This is a classic case of the law of unintended consequences. In an effort to try to avoid one problem, an additional --more severe problem is introduced.