Fuel Injection or Carburation - Performance v. Reliability?

[airguy]

FI will handle autofuel just fine - you just need to make sure ALL parts of your powertrain from fuel tanks to pistons can handle autofuel. That means compression no higher than 8.5:1 (some will say 8:1, but Superior says their 8.5:1 is good for 93UL), no rubber parts in the fuel system (hoses need to be Viton or Teflon core, seals should be Viton), no rubber compounds in the tanks or fuel level sender, fuel pump should be automotive quality or capable of handling autofuel per the manufacturer. It's my understanding that capacitive senders will need to be recalibrated for 93UL vs 100LL, but I don't know that for a fact. Stay away from ethanol blends, you get into vapor pressure problems. Having dual electric fuel pumps in the wing root will help, but all the engine manufacturers that say their engines are good for 93UL specifically forbid running ethanol blends. I've been advised by several people whose opinions I trust that I should alodine all aluminum fuel-exposed surfaces (tanks, ribs, baffles, pickups, and any aluminum fuel tubing) to prevent possible corrosion from water trapped in ethanol blends in the event I should ever have some in the tanks.

[24Golf]

Do FI setups require a fuel return line to the tank/tanks?? I am in the process of constructing tanks and am considering fuel injection.

[Mike S]

Some do.

Some dont.

Check with the manufacturer of your chosen system, if you have already chosen one that is.

Pretty easy to put in a fitting, and just cap it if not needed.

[airguy]

And something to consider along those lines - a return line allows you to run a "purge" valve, which will assist considerably with hot starts (and to be fair, also gives another point of failure to keep an eye on). The return line from the FI should be (in my opinion) routed all the way to the tanks instead of Tee'd into the feed line to the pump, so the fuel coming back from the firewall (which has picked up some heat from the engine environment) does not continue to recirculate and get hotter and hotter until vapor lock occurs - this situation would be most aggravated during long periods of low power (low fuel usage, but still circulating and building heat), such as descent into an airport - and then when you need power for a go-around or pattern work, you may be vapor locked at the worst possible time - low and slow. Returning the warm fuel to the tank (preferably one or more bays away from the pickup) keeps cool fuel flowing through all your FWF lines to minimize this source of vapor lock.

[penguin]

Originally Posted by 24Golf

Do FI setups require a fuel return line to the tank/tanks?? I am in the process of constructing tanks and am considering fuel injection.

Only FI systems based on the Continental design require a return line - of the systems available for RVs, that is the ECi system.
Bendix/Precision systems have no facility for a return line.
The AFP system uses a purge system to circulate fuel to avoid hot start problems only, it is not used airborne.

I disagree with the previous post, the re-circ can T in after the fuel selector & before the electric pump - it does not have to go back to a tank as the heat that is of most concern is from the engine block heat soaking the mechanical pump - firesleeveing all FWF fuel lines and minimizing fuel volume FWF will help a lot. Also you would never use the re-circ airborne - it is only used pre-start or to shut down the engine. I have never experienced the vapor lock scenario described.

Re-circ fuel valves are very expensive and probably not required for AFP.

Hot starting has been discussed here many times before, with some very good explanations of what to do. It is all in your technique. It is possible to get in trouble, but after owing an injected airplane for a short while you will learn what to do.

[airguy]

I think you may have misunderstood what I was trying to say with regard to the recirc of the fuel - I was not implying that any sort of valve be used at all (aside from the purge valve supplied by AFP if that's your vendor of choice). My intent was the return fuel line not be routed back into the intake of the pump, but rather all the way back into the tank. Routing it to the intake of the pump will work, and there are certainly hundreds flying like that, but in certain conditions you will experience high fuel temps in the FWF section. Normally this is not too much of a problem for either 100LL or 93UL, but if you should (accidentally or intentionally) have a tank of 93UL ethanol blend, the higher vapor pressure of the ethanol blend combined with the higher temperatures significantly decreases your vapor lock safety margin. Putting the warm fuel back into the tank instead of the pump intake eliminates this possibility. It's entirely possible that by firesleeving the FWF lines and returning the warm fuel to the intake of a wing-root electric pump you dissipate enough heat just in the return line that it's not an issue, I don't know.

For a 100LL-only aircraft, it's not an issue. For a 93UL non-ethanol-blend only aircraft, it's not an issue. It MAY be an issue for an aircraft running ethanol-blend MOGAS. I intend to run non-ethanol MOGAS, but I cannot absolutely 100% guarantee that I won't ever accidentally get a batch of ethanol-blend fuel, so being that I haven't built my fuel system yet, I am planning it out and will build it to solve that (currently nonexistent) problem before it becomes a problem.

Penguin is absolutely correct in that the heat soaking from the engine block to the mechanical pump is the prime culprit - if it can heat the fuel passing through it enough to boil it, you're in trouble. If you have warm fuel in the pump to begin with, you're in trouble faster. The cooler the fuel is that runs through that engine-driven pump, the better off you are. It's not likely to ever be a problem - but the consequences are severe enough that I intend to eliminate even that small possibility. I would not advocate tearing apart and rebuilding the fuel system of a running aircraft - but since mine is still in cardboard boxes, it will be built as described.