Temperature, EFI, and yes.... Carbs

[Caveman]

Okay here's one that will twist ya:

I'll admit that EFI is a better way in this case, but I want to know WHY...

Old aircraft with carburators had to warm up for awhile to avoid carb icing at altitude, where its COLD.
I presume the icing is caused by moisture in the air freezing, but was it inside or outside the carb?

Modern piston-engined aircraft have fuel-injection. No icing. WHY?

And here's another caveat: how cold could it get before a fuel-injected engine had problems with fuel vaporization?

[dieselgeek]

Re: Temperature, EFI, and yes.... Carbs

I'm an EFI nerd, and a Pilot. Not a carb guru, but it's worth pointing out that piston engien aircraft do NOT have "EFI" - they have mechanical fuel injection. The electronics are deemed "too complicated and unreliable" which some of us argue is BS. Lots of "experimental / homebuilt" aircraft, fly with EFI.

That being said, what happens in a carb is that the venturi effect, used to draw fuel into the intake air, causes both a cooling and a reduced pressure effect at the point where fuel enters the airstream. Certain barometric conditions (moisture, just the right dew point, etc) can allow moisture in that air charge to separate and form into ice, blocking the fuel/air passage into the engine. Thus, "carburetor icing"

Carb'd aircraft have a feature called "carb heat" that puts warm air which has been heated by the exhaust manifolds, into the intake air just above the carb. The reason an airplane must warm up before takeoff, is so that the exhaust is warm enough to heat this air when the "carb heat" is activated...

What's cool is, when you warm up the aircraft, one of the procedures is to check the carb heat by activating it. Engine RPMs should drop by 50rpm or so. This is because the warmer air is less dense, makes a little less power. It's a way to observe intake air temps at work. Pretty cool.


Hope that clears it up,

-scott

[dieselgeek]

Re: Temperature, EFI, and yes.... Carbs

forgot to add, the fuel injected aircraft have no carb icing because they have no venturi or carb to cause the icing. ;)

[Caveman]

Re: Temperature, EFI, and yes.... Carbs

I made a presumption about piston-engined a/c there... oops. I know a little about jets, but zip about piston flyers.

You're a pilot... sweet. Then you can help me understand this too:

Why does an aircraft engine (piston or jet) make more power at higher altitude? I know the higher you go, the colder it is; but the higher you go, the thinner the air (and oxygen) is...

So does the cold temp advantage outweigh the disadvantage of thin air?

And would an aircraft with EFI require 'carb heat' at very high altitude? (not a pun... just curious)

Tony

[dieselgeek]

Re: Temperature, EFI, and yes.... Carbs

Neither turbines, nor piston engine aircraft, make more power at altitude. Both experience power loss as altitude increases. However, they achieve higher "ground speed" because the thinner air presents less drag. A way to confirm this is to check into the climb rates of both jets and piston engine craft; they all climb faster at lower altitudes.

THis part, I'm not 100% sure about. I think Dynoroom on here might have better info. But to my knowledge, the only "high altitude" piston engines out there, use turbochargers, which keep the incoming air warm enough for good fuel atomization.

The cold temps do NOT offset the loss of oxygen at altitude, this I know for sure.

Some crazy ass questions for a car forum!

[CTX-SLPR]

Re: Temperature, EFI, and yes.... Carbs

Turbos and Superchargers keep the air density up as well as keeping it warmed up, that is where the power boost over a NA piston engine comes from. For a Turbine you'd need a variable inlet to create ram compression effect to feed higher density air to the compressor, if the pressure on the inlet side of the compressor gets to low you get combustion out both ends in what is known as a flame out.

[jays67]

Re: Temperature, EFI, and yes.... Carbs

you can have a carb on a car ice my truck when i used to drive it in the winter with the tri power the center carb {always in use} would ice inside the airhorn in the mornings when i would leave for work you would have to hold the rpm's up the guys are right around the vintrei would actualy get frosted over from my guess would be the air speed passing there fast as it could and the carb being cold the air mositure would freeze to the carb
prolly did not help it hnever had a choke to help the engine worm up lol
and the guys are also right piston aircraft run MFI kinda like me on my cars sometimes lol
but i do have to say a die hard carb guy i love my 92 chevy with the EFIit is so nice in the cold mornings lol

oh and my idea on carb iceing may be wrong as to why but it is how i look at it lol

[Caveman]

Re: Temperature, EFI, and yes.... Carbs

The cold temps do NOT offset the loss of oxygen at altitude, this I know for sure.

Some crazy ass questions for a car forum!

Bonneville is at some altitude, that's why the crazy ass questions... ;D ... put together all my question threads and ya may peek inside the cave 8) 8)

Basically, I wanna know how cold you can get air before it turns fuel into globs that won't burn.

[dieselgeek]

Re: Temperature, EFI, and yes.... Carbs

The cold temps do NOT offset the loss of oxygen at altitude, this I know for sure.

Some crazy ass questions for a car forum!

Bonneville is at some altitude, that's why the crazy ass questions... ;D ... put together all my question threads and ya may peek inside the cave 8) 8)

Basically, I wanna know how cold you can get air before it turns fuel into globs that won't burn.

it totally depends on the chemistry of the fuel. Too bad Rick Gold's not on here - he could talk about this at some length.

On the huber mustang, we found that our fuel choice (C16) wasn't happy with intake temps below 50 degrees F.

[Caveman]

Re: Temperature, EFI, and yes.... Carbs

Assuming B'ville/Maxton/Loring race gas. Aka, non 'fuel' classes. No alky.

On the Huber car, what was the fuel pressure at the fuel rail? Would that have an impact on vaporization at low temps?

[dieselgeek]

Re: Temperature, EFI, and yes.... Carbs

Assuming B'ville/Maxton/Loring race gas. Aka, non 'fuel' classes. No alky.

On the Huber car, what was the fuel pressure at the fuel rail? Would that have an impact on vaporization at low temps?

He runs about 37psi base pressure with the vacuum reference working. Add the 40psi of boost, and you get 77psi fuel pressure at dinnertime

not sure what the exact issue is, we were just playing with block ice in the intercooler and seeing how cold we could get the intake air temps. Car would misfire at just below 50 degrees F. FActors that changed that point were plug gap and reach... but we were locked into a certain plug heat range to support max power...

[Caveman]

Re: Temperature, EFI, and yes.... Carbs

How far was the swing when you changed that point? I.e., if you changed plug gap/reach, what was the result? If you had changed the plug heat range in that instance, what would've happened? Just trying to wrap my Neanderthal lil' brain around this.

[dieselgeek]

Re: Temperature, EFI, and yes.... Carbs

How far was the swing when you changed that point? I.e., if you changed plug gap/reach, what was the result? If you had changed the plug heat range in that instance, what would've happened? Just trying to wrap my Neanderthal lil' brain around this.

we didn't mess with it much. From bonneville experience, you're not going to have this issue anyways ;)

[JeffMcKC]

Re: Temperature, EFI, and yes.... Carbs

They also dont ice because of the fuel blend, there is a lot of things in AV-Gas that are not there as fuel which is one reason its not a good race fuel

[Caveman]

Re: Temperature, EFI, and yes.... Carbs

Any way to get Rick Gold into this site? What forums does he hang out on?