System comparison and decision timeline

[rmartingt]

I'm still a long way off from an engine purchase (almost to sealing my first fuel tank), but I've decided to use ethanol-compatible fuel injection, and am trying to gather information now so I make sure to build the tanks correctly.

1a. My understanding is than an EFI system has individual injectors on each intake, and the injector delivers a timed "pulse" of fuel basically coinciding with the intake valve being open. So, when the valve is closed, no fuel is being delivered. Right?

1b. With other injection systems, does the injector deliver a constant stream of fuel to each intake? I assume the fuel is metered at one point, with each injector balanced with restrictors, etc.?

2. When does the decision on which system to use need to be made? By engine purchase? After engine purchase but before FWF install?

3. Is there any downside to fitting the tanks for return lines and capping them if not needed?

4. If my eventual engine/injector combo doesn't need a full-time return line, is there a downside to running a purge to one side, using dual wing root pumps with check valves, and eliminating a selector valve? I'm figuring on probably dropping the mechanical pump anyway...

[TS Flightlines]

I'd start thinking about it, but if you dont completely close up the cabin, you can plumb return lines pretty easy. IF you are going to use a fuel with ethanol, I recommend teflon lined hoses, instead of any that have a rubber liner. Ethanol will eventually break down the liner and cause some leakage. You didnt say what RV you were building. OOPS---RV7---ok
Tom

[Mike S]

Originally Posted by rmartingt

1a. My understanding is than an EFI system has individual injectors on each intake, and the injector delivers a timed "pulse" of fuel basically coinciding with the intake valve being open. So, when the valve is closed, no fuel is being delivered. Right? Right.

1b. With other injection systems, does the injector deliver a constant stream of fuel to each intake? I assume the fuel is metered at one point, with each injector balanced with restrictors, etc.? Right.

2. When does the decision on which system to use need to be made? By engine purchase? After engine purchase but before FWF install? Depends, but at engine purchase is a good time if you are having someone like Barrett or Aerosport or Lycon build up the engine.

3. Is there any downside to fitting the tanks for return lines and capping them if not needed? Not IMHO

4. If my eventual engine/injector combo doesn't need a full-time return line, is there a downside to running a purge to one side, Not IMHO using dual wing root pumps with check valves, and eliminating a selector valve? I'm figuring on probably dropping the mechanical pump anyway... No opinion on this one.

Hope this helps.....

[rv8ch]

Plumbing return lines is easy, and won't hurt a thing if you don't end up using them. We are experimenters, so trying something different is of course up to us. One thing that much more experienced builders have reminded me many times is that fuel systems issues are a frequent cause of off field landings.

[rv9av8tr]

IMHO, it sounds like you are contemplating making your fuel system way more complicated than necessary. Boost pumps in each wing root? Really? How are you going to access them to replace when they break? How are you going to resolve a piece of crud stuck in a check valve that allows it to leak?

Getting rid of the engine driven mechanical pump... why? Loss of all electrical and you have no fuel pressure to drive a high pressure fuel system. That would only create a loss of redundancy.

Good engineering is about making complicated more simple and easy to maintain.

A LOT of engine failures from fuel starvation (besides running out of fuel) is caused from "experimenting" with fuel layouts different than the original design. As an engineer, I'm a strong proponent of "don't mess with success" and "don't fix something that works".
When it comes to fuel systems, KISS rules.

I strongly recommend the Precision Airmotive Silverhawk FI system. It's simple and works GREAT. It doesn't have a fuel purge line (another fuel management complication). Three years and 300 hrs and I've NEVER had a start or operating issue, hot or cold. Hot starts are solved 100% by technique.

Food for thought......

[rmartingt]

Originally Posted by rv9av8tr

IMHO, it sounds like you are contemplating making your fuel system way more complicated than necessary. Boost pumps in each wing root? Really? How are you going to access them to replace when they break? How are you going to resolve a piece of crud stuck in a check valve that allows it to leak?

Remove wing root fairing, replace pump, reinstall wing root fairing.

Getting rid of the engine driven mechanical pump... why? Loss of all electrical and you have no fuel pressure to drive a high pressure fuel system. That would only create a loss of redundancy.

I'm planning to be ethanol/mogas compatible and have vapor lock concerns. The wing root pumps help with that. If I'm already putting pumps in the wing root to help with vapor lock concerns, and I can find pumps with the required output, I could possibly make them the main engine supply pumps and get rid of the mechanical pump and the fuel selector valve. But it will ultimately depend on what pumps are available and whether my analysis shows such a setup to be reliable enough.

Redundant pumps on each side and a redundant electrical system might be less likely to fail than a mechanical diaphragm pump. Solid-state attitude indicators and EFISs, when provided with suitable backup power sources, have shown themselves more reliable than mechanical gyros.

Finally, as others have pointed out, an engine with dual electronic ignition is already electrically-dependent. How would this be any different?

Again, elimination of the mechanical pump will depend on the injection system I choose and a detailed fuel system analysis.

Good engineering is about making complicated more simple and easy to maintain.

A LOT of engine failures from fuel starvation (besides running out of fuel) is caused from "experimenting" with fuel layouts different than the original design. As an engineer, I'm a strong proponent of "don't mess with success" and "don't fix something that works".
When it comes to fuel systems, KISS rules.

I'm an engineer, too. I work with airplanes that have electrically-dependent fuel and ignition systems, avionics, and even (gasp!) flight controls. A little additional complexity during design and installation, when properly and thoroughly tested, is worth the effort when it simplifies operations and improves overall reliability.

If we never tried to improve anything that worked "good enough", we'd still be flying vacuum gyros behind radial engines and navigating with four-course ranges and NDBs.

I strongly recommend the Precision Airmotive Silverhawk FI system. It's simple and works GREAT. It doesn't have a fuel purge line (another fuel management complication). Three years and 300 hrs and I've NEVER had a start or operating issue, hot or cold. Hot starts are solved 100% by technique.

Do you run mogas (straight or ethanol)? If so, how does your system prevent vapor lock under heat-soaked conditions and guarantee fuel supply at high altitudes?

[rv9av8tr]

I don't and wouldn't run mogas because of all the compromises and concerns you mention. I'm risk averse and this is one area I choose to avoid all those pitfalls. 45 yrs of flying and not one bent airplane... I plan to keep it that way.

[rmartingt]

Originally Posted by rv9av8tr

I don't and wouldn't run mogas because of all the compromises and concerns you mention. I'm risk averse and this is one area I choose to avoid all those pitfalls. 45 yrs of flying and not one bent airplane... I plan to keep it that way.

To each their own. We each have our personal level of risk tolerance.

But I've run into those problems--low fuel pressure and stumbling at high altitude, stumbling after hot starts--as well as a busted pump and leaking switch valves on the bone-stock, throttle-body-injected, avgas-fed RV-6 I (sometimes) get to fly. We've rebuilt and later replaced the selector valve, and replaced the failing mechanical fuel pump in just 1000 hours. And the electric turn gyro crapped out after just 600.

[Paris]

Originally Posted by rmartingt

To each their own. We each have our personal level of risk tolerance.

But I've run into those problems--low fuel pressure and stumbling at high altitude, stumbling after hot starts--as well as a busted pump and leaking switch valves on the bone-stock, throttle-body-injected, avgas-fed RV-6 I (sometimes) get to fly. We've rebuilt and later replaced the selector valve, and replaced the failing mechanical fuel pump in just 1000 hours. And the electric turn gyro crapped out after just 600.

Anyone who's owned an aircraft for very long knows that there's just some things which have a high failure rate. Gyros fail often. Sometimes you get lucky and they last for years, but if the plane flies very much chances are you'll be replacing them far more often than you'd like. In the last 1000 hours, my plane has gone through at least 6 gyros that I can remember. EFIS extends the MTBF greatly, but after you pay to have one G1000 panel replaced you realize mechanical gyros really weren't that bad of a deal.

Some of this is due to the parts you get and sometimes it's due to the design of the aircraft or issues with your particular aircraft. On my certified aircraft the mags are required to be rebuilt or replaced every 500 hours, so there are ways to deal with such things if you just resign yourself to overhauling certain parts on a schedule rather than waiting until they fail.

[pierre smith]

The O.P. asked about "pulsed" injection, timed to the open intake valve.

I thought that most injection systems in our airplanes are constant flow, not pulsed.

When/which systems are timed to pulse this way?

Thanks,