Got Stumped...Need Help

[FlyingNole]

Ok, so I finally got a question that I think I should have known, but got totally stumped on.

The new 172SPs are fuel injected, and therfore have no carb, right?
If so, is the intake/filter on the nose just an air filter where the air passes and goes directly to the cylinders? Or to where?

The questions I was asked, was: "Can a fuel-injected engine lose power from ice, like when ice builds up on a carburated engine?"

I wanted to say yes, but then I wondered how it'd be possible, and if it is, what would be the remedy since there is no carb heat?


Thanks for your help guys.

[Ralgha]

Originally Posted by FlyingNole

Ok, so I finally got a question that I think I should have known, but got totally stumped on.

The new 172SPs are fuel injected, and therfore have no carb, right?
If so, is the intake/filter on the nose just an air filter where the air passes and goes directly to the cylinders? Or to where?

The questions I was asked, was: "Can a fuel-injected engine lose power from ice, like when ice builds up on a carburated engine?"

I wanted to say yes, but then I wondered how it'd be possible, and if it is, what would be the remedy since there is no carb heat?


Thanks for your help guys.

Yep, the intake air is filtered, and then mixed with the fuel at the fuel nozzles in the cylinders.

It can lose power from ice if the ice builds up on the intake, thus restricting the intake and choking the engine. The remedy is to use the alternate air valve, which draws unfiltered air from somewhere inside the engine compartment.

[FlyingNole]

Thanks...I thought the Alternate Air was for the static port (the red knob below the mixture control)

[Ralgha]

Originally Posted by FlyingNole

Thanks...I thought the Alternate Air was for the static port (the red knob below the mixture control)

That would be the "alternate static source". Make sure you know which is which, they do very different things!

[FlyingNole]

Originally Posted by Ralgha

That would be the "alternate static source". Make sure you know which is which, they do very different things!

I knew about that, I just had no idea there was an alternate air source....Where would that be? (Its 2am, and I dont feel like going to my car to get the POH)

[Ralgha]

Originally Posted by FlyingNole

I knew about that, I just had no idea there was an alternate air source....Where would that be? (Its 2am, and I dont feel like going to my car to get the POH)

Sorry, you'll have to get the POH for that, I've never flown a 172SP.

[SteveC]

I'm not familiar with the 172SP, but it might be an automatic type. Spring loaded door, and if the pressure gets too low in the intake manifold (due to the air filter being plugged by ice and the throttle valve open) the higher air pressure outside overcomes the spring pressure, opening the door, allowing unfiltered air into the intake.

Just a guess....

[PacMan4x4]

NOLE....

SteveC is correct. The 172SP has an alternate air door on the air box. You can't see it unless the cowling is off....next time you see one without the cowling on you can take a gander at it. The alternate air door is hinged and spring loaded to the closed position and as the filter would become clogged, differential pressure would cause the door to open from the inside (behind the iced over inlet filter). Hence, there is no pilot control mechanism. The air used for this comes from what is inside the cowling from the ram air that is forced over the cylinders and drawn to the bottom of the cowling. Though I haven't had this happen to me with a filter getting iced over....you would think there would be a power reduction. You will no longer have the ram air effect being forced through the filter. Also, the air where the alt air door will be pulling from will be warmer as it has passed over the top of the hot cylinders and is enclosed in the cowling where it is warmer. Hope this helps out a lil' bit.

Pac Man

[dpilot83]

Every engine needs several things to produce power. Air, fuel, and spark are the main ones. Based on this, every single engine out there is susceptible to losing power due to ice, whether it's fuel injected or not. The above posters were correct in that both carb engines and fuel injected engines can have the primary air filter (in the front) ice up. This will result in either you pulling the alternate air valve, or it will automatically open, depending on what you're flying.

If that standby mechanism somehow fails, you've got a boat anchor instead of an engine (assuming the primary air filter is completely iced over instead of partially). If the standby mechanism works as designed, you will have slightly less power due to no ram air and warmer air being drawn from inside the engine compartment as previously stated.

That is not the only way ice can affect engines though.

Originally Posted by FlyingNole

The new 172SPs are fuel injected, and therfore have no carb, right?
If so, is the intake/filter on the nose just an air filter where the air passes and goes directly to the cylinders? Or to where?

The airflow in a fuel injected engine goes from the airfilter (assuming it's not iced up) into a throttle body. This throttle body is directly connected to your throttle control and is basically a valve, controlling the amount of air entering the engine. From the throttle body, the air is routed to each of the cylinders, hopefully evenly. Notice that no fuel has been added at this point, and the air has gotten all the way to the cylinders. Once it's in the cylinders, the fuel injectors inject the correct amount of fuel directly into the cylinder. This is supposed to create a more equalized distribution of fuel throughout the 6 cylinders. With this scenario it is difficult to create a blockage in the system somewhere down the line from ice. This is where icing is differnt for a carb engine.

The airflow in a carb engine goes from the airfilter to the carb. As you probably know, the carb is basically a tube the air flows through, with a valve in it, just like the throttle body has. The difference is, the tube in the carb gets a little more narrow in the middle. For the air to fit through the narrow part, it has to go faster. This results in the venture effect, which creates lower pressure, or suction. This suction is used to suck fuel into the airstream. Therefore, if you open the valve in the carb with your throttle control, more air flows through, causing more suction, which automatically makes more fuel come in. Pretty handy, except that the lower pressure also causes lower temperature, making water particles in the air more likely to condense and possibly freeze. Additionally, all the fuel is vaporized as it goes into the airstream. Adding enough heat to the fuel to vaporize it requires taking that engergy from somewhere else, namely the air. These two things combined can decrease the temprerature significantly. This is why a carb engine needs something to heat the air. If enough ice forms on that valve, it can shut the engine down completely, and if the carb is iced up, the only way to get air flowing through again is to get the ice off....no alternate option available here. That's why it's important to use the carb heat before it's too late.

[FlyingNole]

Thanks alot guys. I knew someone out there would have my answer. I feel kinda dumb though for not knowing something like that on an aircraft I have 75+hrs on

[PacMan4x4]

No prob bro....Don't feel dumb though. I only knew because I wrenched at a Flight School that had and 172SP and 172R. Unless you saw the cowling off or talked to a mechanic, you wouldn't necessarily know. But, look at it this way...you asked a question that probably helped someone else out. Now, when folks read the forum you started someone will be like...hmmm, didn't know that. :-) Never hesitate to ask a question if you aren't 100% sure.

Pac Man

[CFIse]

Originally Posted by PacMan4x4

No prob bro....Don't feel dumb though. I only knew because I wrenched at a Flight School that had and 172SP and 172R.

Well to be honest there is another way - you could read the POH. It states in there that there is an automatic alternate air source......

[PacMan4x4]

Haven't really had a need to read that POH too in depth since I worked on the aircraft. If the POH does state an automatic alternate air source....I stand corrected--there are two ways :-) I would venture to say that seeing it and seeing how it works would prove to be a lil' more beneficial than just reading about it. Again NOLE.....don't ever feel dumb for asking a question. Someone will gain insight from it besides you.

[CFIse]

Originally Posted by PacMan4x4

Haven't really had a need to read that POH too in depth since I worked on the aircraft. If the POH does state an automatic alternate air source....I stand corrected--there are two ways :-) I would venture to say that seeing it and seeing how it works would prove to be a lil' more beneficial than just reading about it. Again NOLE.....don't ever feel dumb for asking a question. Someone will gain insight from it besides you.

I don't disagree that hanging around dismantled planes is a great way to learn about them - but a pilot should not NEED to take that path in order to learn what they need to know to operate the plane.

As I point out any number of times, I fly 'em, I don't build 'em or fix 'em and I'm not trained to do either.

[PacMan4x4]

True....the POH is a good learning tool and pilots shouldn't NEED to spend time around dismantled planes. However, majority of pilots would be better off if they did--in my opinion. It's easy for us to regurgitate (sp?) numbers and systems that we read about and try to impress our friends without really understanding what is being read. A lot of insight can be gained from B.S.ing with your mechanic when they/you have some free time and cowling/panels are off an airplane during an inspection. But since I do fix em' and fly em', I have the luxury of being around dismantled aircraft :-) I gladly make time for a new pilot who is inquisitive about learning more than what they simply read about.