Please illustrate me, why carburettors?

kitepilot

kitepilot

New member
Well, for years, I have heard the blanket statement:
'fuel injection is better than carburettor'
And I certainly like better fuel injection...

Homebuilders, however, still crank up airplanes with carburetted engines.
Why?
What's the logic here?

I did read somewhere that the '470 flavor' of Continental engines come with either carburettor or fuel injection systems. Same exact engine, just the feeding changes. If that's the case, why is it so difficult to just replace the carburettor with fuel injection and leave the rest of the engine alone? Why is it so difficult to sell the idea to our beloved 'Friendly Aviation Association' (FAA)?

Now, please let me set a baseline for any explanation you may want to elaborate, because you will be talking to a computer geek that has difficulty differentiating a set of pliers from a drill bit, and whose maximum engineering achievement can be accomplished by means of a Lego set.

Can anybody water down this explanation to a level that I can digest and still hold it to a quality that makes sense?
 
This is a great question! I'll give it a shot.
I have a -3 with the IO-470, and a Skylane with an O-470-U (the higher compression version)
Really the biggest advantage FI has is that it delivers a more balanced fuel charge to each cylinder. Simply put, since all 6 are running with the same fuel charge, they all work equally. A Carb'd engine develops its fuel charge a greater distance away, which then has to travel through an intake system of different lengths and environments to get to the cylinder - the result is by the time it's in the cylinder, there's subtle changes to each along with subtle differences in the way each cylinder works.
But Carbs have been around a long time, are well known, and simpler than an FI system.
There's a variety of different flavors of FI in engines: mechanical, electronic, direct injection, etc.. but sticking to comparing the O-470 and the IO-470 in Bellancas... They're not really the same exact engine. Same displacement and such but not as identical as you might think.
Converting an O to an IO is possible but there's a lot to it. The Carb is pretty simple and well known - in a high-wing plane it doesnt even need a pump. The IO requires a different pump, a "servo" that controls fuel pressure to the injectors, a distribution splitter (usually called a "flow divider"), injectors, and an air control valve that's tied to the fuel servo through a linkage. More fuel is delivered from the pump to the servo than can be used so, there's a return line to take the excess back to the tank. Now that means there's twice the fuel lines and the fuel tank selector valve now has to switch both the supply AND return lines between each of the three tanks. There's also a 2-speed electric pump instead of a wobble.
As far as FAA approval goes, it's no harder than any other Major Alteration. All you have to do is adequately show that you did your homework and all the changes, and the *effect* of them are at least equal to the original installation. Remember now, you're talking to guys that were hired to enforce the regulations. They didn't create the regs and have no power to change or "creatively interpret" them so be nice. They're not engineers and often are not even pilots so, you really have to be on your game. Be ready to fully address ALL the changes like:
the engine is now heavier.. what's the effect on the airframe, balance, useful load, flight characteristics? Here's a hint: Just saying there's no or negligible difference isnt enough..
Full-Power fuel flow is now higher, can the new fuel system handle it? (show your work)
The way you start it is now different so, Did you modify the operation instructions? WHAT? it didnt have any to begin with??? OK, Did you make one? does it meet *current* standards? How do you know? (show your work)
What about the prop? is it still the right one for that engine? can you provide approved data to support that? No? OK, can you show that it's the right prop now? what about the spinner? governor?
What about the motor mount? Still strong enough?
you get the idea.. it's not hard work, just a lot of work. If you dont have the expertise then there are folks out there you can tap to help but, they dont work for free.
Once you've done all that homework, it'll get sent off to some secret place where there are FAA engineers enslaved to review and grade your papers once they've dealt with similar requests from Rolls Royce wanting to get approval to mount their latest mega turbine on a heavy to someone wanting to use over-sized tires on their piper cub. Get ready to wait and dont be surprised if they want additional information.

That carb doesnt look so bad now huh? :D

of course you can always go Experimental but, that's a whole different discussion...
btw.. I'm A&P/IA for 24 years now, and working on getting DER but that's just my avocation. my planes fly because I'm an engineer specializing in automation working for a medical device company. If you think dealing with the FAA is hard try dealing with the FDA
 
Vapor lock is actually pretty hard to get, at least on the 470 system. The pump powers the fuel through and cools the lines. Hot starting is a different procedure but once you learn it, it's a no-brainer.

it's true, no carb ice BUT it's a real pain when you go back and forth between an IO and an O.. It seems I'm either forgetting to pull the heat in the Skylane, or fumbling around looking for the handle to pull in the Cruisemaster....
 
I'm thinking that when you notice that there isn't the handle in the Cruisemaster, and remember why, that has to be a good feeling. :wink:
 
Fuel injection injects the fuel into the cylinder.
Carburetors mix air and fuel.. and then the engine has to suck the mix up tubes.

To assure the most distant cylinders arent running too lean... (weak mix)
the others have to run too rich .

So begins a long string of compromises with carburetors.

More compromises to get idle mixture correct so engine will start easily.

Many carb settings must be fixed:

idle mixture
low speed cross over ( via emulsion tubes)
basic jet sizes

an accelerator pump is often used to allow smooth throttle response...
but it is also a fuel waster attachement.

All these setting are COMPROMISES that work pragmatically, but are often at odds
with best economy, best mixture, and on an on.

Fuel injection.. especially modern computer controlled injection can instantly change almost
any parameter,, quantity, timing, etc... based on many varying engine conditions,
altitude, manifold pressure ( engine load), EGT, detonation, temp, on and on.

However, Certified aircraft engines are caught in a time warp that looks a lot like 1965.
"Fuel injection" in most airplanes means a mechanical metering pump not quite as sophisticated
as a 1980 diesel vw had.

So, it offers very few of the things a computerized system offers.. not the least of which is
economy of fuel use, and the self adjusting properties .

What carbs offer is simplicity, relatively high reliability, and the ability to be overhauled
on a motel coffee table.

Aircraft injection systems have cylinder heads designed for the injectors to screw in.. and inject the fuel in
exactly the right place.

The only practical retrofit ( faa aside) would be using what is called "single port " fuel injection... where the injector is located in the intake manifold.. just like a carb would be.. but is controlled by a computer. This is a dirt simple and highly reliable system... with almost all the advantages of a more complex computerized injection system with an injector at
each cylinder.

FAA simply doesnt allow such changes to Certified aircraft.

A nice improvement of carb design is the Constant Velocity Carb ( see rotax 912) which self adjust for altitude.

Larry
 
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