Rodoc
Active Member
I run EFI with an O2 sensor and mostly Mogas in my RV6. Here is some input.
Mogas:
Not really available at many airports west of the Rockies. East of the Rockies you are only going to be saving about $0.25/gal when purchased at the airport. However, if you are like most of us, 80% or so of your flying is going to be local in which case the best (cheapest) practice is to fill up a large container at the gas station and keep it in your hangar. The price difference for me will vary but right now I am saving $1.50/gal. Mine often sits in storage for up to 2 months without a detectabe loss of quality. I once kept some around for 2 years and put it in my car and did not notice any problems.
Using occasional 100LL with an O2 sensor is not a problem. I have 550 hrs on my O2 sensor and use 100LL whenever it is more convienient than Mogas. the O2 sensor still works fine.
Mogas has ethanol, so far I have not noticed incompatibility issues, though others have more experience/knowledge in that regard. Ethanol CAN attract water. It is a REAL concern. Just because it is rare, dosent make it OK. Fly in winter up to 15k and your gas will get very cold. It will stay cold after you land etc. Automotive style EFI will solve that problem. (more below)
Ethanol also increases volatility. That can be a non-issue or engine killing. Again, an automotive style EFI solves the problem completely.
EFI:
Has many advantages and some disadvantages (mainly dependance on electrical power unless you are using mechanical fuel injection).
It is important to note that there are 2 major types of fuel injection. The type Ross and I use - I am referring to as the automotive type (though many cars, especially newer ones, are using the other type). In this type there is fuel flowing under high pressure (30-100psi depeding on the system) and high flow (double the needs of the engine at full power - many times engine use while at idle - my system flows 50 gal/hr for example). A regulator at the end of the fuel rail sets the pressure (like a thumb over a hose) and returns excess fuel at low pressure back to the tank.
The other type is that commonly used in aircraft such as the IO-360. The pressue is set by the pump (usually just high enough for the needs of the engine) and there is usually a relief or purge valve at the end of the rail. There is no return to the tank.
Both system have the advantages over carbiration that include the ability to very finely, and easily, balance the flow to each cylinder; and indeference to gravity (that is what got you started on all this). Also, carb ice becomes much less possible. There is still a small temp drop across a throttle body, and given enough freezing rain anything can freeze over - but this only comes into play in know ice conditions. So switching to fuel injection does eliminate the possiblility of carb ice for most homebuilts.
The aircraft system has the further advantage of not being dependent on eletrical power (as most of them are actually Mechanical Fuel Fnjection). I guess the certification powers the be felt that lack of dependance on electrons was more important than the benifits of a return flow system. Those advantages include the virtuial elimination of issues with water contamination (unless it is a massive amount of water). In the return flow system, any gulp of water quickly runs past the injectors and back to the tank where it is properly mixed. With the one-way system the entire gulp of water must run through the injectors as if it were fuel. Purge valves work OK on preflight (if you have one) but do nothing for you if you get a gulp of water at rotation.
More importantly, the return flow system eliminates the possiblility of vapor lock. Take a winter blend mogas (more volatile) and run it at slow speed through a hot engine compartment at altitude (where the pressure is low) and have the fuel pump create a suction trying to get the fuel out of the tank, and you have a set-up for vapor lock. The low pressure on the warm fuel can cause it to create a vapor bubble. The one-way systems do not help with this issue. But with a return flow system the high pressue prevents the vapor, the high flow keeps the fuel cool also preventing the vapor, and if a small amount does form most of it is quickly run past the injectors an back into the tank. Note that it is important to keep the fuel pumps low, and as close to the tank as possible (or in them altogether) to prevent large amounts of vapor from appearing upstream of the pump (somethng that many installers of automotive fuel sustems fail to to - the pumps must not be in the engine compartment to attain this great advantage)
O2 sensor:
I agree with Ross on the closed loop issue.
It is possible to run closed loop in the aircraft but it is not worth while. While you can adjust the computer to run at whatever stoichiometric mix that the O2 sensor is capable of sensing (which could keep you at, say, appx. 100 ROP) the lack of frequent power changes in the aircraft and the desire for running different stoichiometry during different phases of flight make closed loop not worth the hassle, not to mention an additional failure point.
I find the O2 sensor most helpful for tuning the computer(s) and giving giving a quick look at where I am at on the mixture curve (it is much faster than EGT, but has less resolution). But I stlill use EGT for the final mixture adjustment in flight, after all that is what is important to me, keeping to turbo temps under control. You would use EGT to balance you cylinders not O2. It is nice to have, but given the choice between O2 and EGT I would take EGT.
All in all, EFI is a great addition, return flow type is better IMHO. Using mogas can save a ton of money at current prices. Etanol is not a problem IF you undertand and mitigate the issues. O2 sensors are nice and don't exclude the use of 100LL, but don't bring all that much to the table either.
