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HeliTorque :: View topic - Car(b) Icing?
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HeliTorque Forum Index » Flight Dynamics

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Intentionally Blank
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PostPosted: Fri Sep 16, 2005 8:22 am    Post subject: Car(b) Icing? Reply with quote

Now, bearing in mind there is no such thing as a stupid question (!) :

Why do road vehicles not have carburetor heat? I know that in this day and age a car engine is a much more advanced thing than your average GA piston engine. But my dad has had some OLD cars and they never had carb heat!

I have read all the gubbins on carb icing but still have not worked out why carb icing is prevelant in aircraft but has never been an issue for cars etc...

Please - what is the trick I am missing? Purely the fact that air density reduces with altitude and makes carb icing more of an issue (because of some science stuff I have failed to grasp)? But then if you lived and drove up in't mountains...

Someone put me out of my misery!

Ta.


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Bierbuikje
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PostPosted: Fri Sep 16, 2005 8:42 am    Post subject: Reply with quote

Good question. I???ve often wondered that my self.

I???ve never know a car engine stop through carb icing, maybe it's possible? But if the engine did stop in a car due to carb icing it probably wouldn't be a serious life threatening issue of course in an aircraft it is life threatening, so I would say this is why we have carb heat in aircraft and not in cars.
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PostPosted: Fri Sep 16, 2005 1:24 pm    Post subject: Reply with quote

Glad I'm not the only one!

Perhaps cars do have carb heat but the heat is/was applied continuously in some way and without any driver input (therefor not realising it!). If so I guess this constant application is deemed undesirable (power reasons?) in aircraft and subsequently a pilot is given control over the carb heat (hence "What d'you mean carb heat? I don't have that in me car...").

Just conjecture...

(Edited to make more sense. Hopefully!)


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PostPosted: Fri Sep 16, 2005 5:33 pm    Post subject: Reply with quote

A slightly more permanent heat source from a car engine to the carburettor makes sense - the reduction of power not being quite as significant. Certainly, on an old Polo I had (Actually, not hugely old, say about 20 years), one moved an air-intake pipe between seasons - in winter, point it to the engine, in Summer, point it to the front of the car

Carb heat is important for Robinson pilots, more so than other aircraft, because the derated lycoming engine doesn't typically operate at anywhere near wide-open throttle! A smallish build up of ice starts to have an effect. Coupled with the automatic throttle control from the governor, to mask that build up, makes for a tricky situation if you're not on top of it!
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PostPosted: Sat Sep 17, 2005 1:54 pm    Post subject: Reply with quote

I can add some further confusion to the debate in that certain motorbikes were infamous for suffering from carb icing. I remember one Kawasaki in particular that was notorious for it so I'm wondering if the fact that the engine was that much more exposed was something to do with it. Having said that, I have never owned a fuel-injected motorcycle and none of them have ever, to my knowledge, suffered from carb icing Confused
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PostPosted: Sat Sep 17, 2005 2:30 pm    Post subject: Reply with quote

Thanks for your added confusion DBChopper; someone else who doesn't really know! Comforting at least!

But what's the deal?

Someone must know!
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PostPosted: Sun Sep 18, 2005 11:40 am    Post subject: Reply with quote

I'm not sure about this one, but it may shed some light:

An aviation piston engine generally has more cubic capacity per unit of power. For example, an O-200 engine produces 100 HP from 3.2 liters - my car engine produces 110 HP from 1.6 liters - this is (probably) due to the fact that aircraft pistons were designed from the outset to operate at higher altitudes where the air density is lower, as well as producing a lot of power on "normal" gasoline, but with a significantly reduced number of RPMs.

With that big cubic capacity, more air is needed, as well as more fuel. More fuel means that more heat is needed in the carb sprayer to vaporize the fuel, which is to be taken from the air. Despite the increased volume of air flowing through the carb, its density is lower, sapping more heat from it and lowering its temp quite a lot - since there are fewer mollecules to take the heat from, and the lower density reduces the flow of heat between individual mollecules.

This does not occur in car engines, since they, more or less, operate in constant high density with lower fuel flow figures - plus, the vicinity of an aviation piston in flight is colder than the vicinity of a car piston while driving - and since the intake duct(s) pass very near the engine block of a car, some additional heating comes from that source.
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PostPosted: Sun Sep 18, 2005 11:59 am    Post subject: Reply with quote

Thanks DDD! Is that a theory of your own or an "official line"? Whatever it is still food for thought - an angle I hadn't considered... Cheers!

DBChopper - I just re-read my previous message and hope you didn't read it as sarcasm or anything. The thanks were genuine! Thumbs Up
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PostPosted: Sun Sep 18, 2005 12:32 pm    Post subject: Reply with quote

It's mostly a home-spun theory based on observation and inflight experiences. At least in fixed-wings, while descending it would be advised to pull the heater if you're descending with a low prop RPM since then the mass of air flowing through the carb is greately reduced, but the fuel is still pumped in, again draining a substantial amount of heat from the air. This is not confined to higher altitudes however, since even in the circuit, at 1500 ft AMSL we pull it before turning to base, since the O-360 was known to hick-up without the heater when near idle.
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PostPosted: Sun Oct 23, 2005 9:32 pm    Post subject: Reply with quote

TripleDelta is quite on with the summary that cars have less power (gas) per cubic inch so in an aviation engine there is more air and gas shooting past the throat, but also, most aviation engines are extremely light, with very little "thermal mass" and little metal to rapidly conduct heat to the throat.

Also, most light aviation piston engines are very simple, and do not have the costly and maintenance-hungry compensation mechanisms that more complex engines had. A typical WWII aircraft piston engine had auto leaning, auto carb heat and full altitude compensation.

Also, note that cars are operated differently, with few times when idle or low power is held for long, unlike aviation engines.
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PostPosted: Sun Oct 23, 2005 10:16 pm    Post subject: Reply with quote

Nick, things must be quiet at Gulfstream for you to find yet another forum to look at!

For those who have not heard of our new member, he was Sikorsky's top development test pilot for many years and brought many products to market like the S92, and others which almost made it like the Comanche.

A world of knowledge here, folks!
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PostPosted: Mon Oct 24, 2005 3:11 am    Post subject: Reply with quote

Thanks for the kind words, AscendCharlie, but I learn as much as anyone from these exchanges! I REALLY like the picture you chose for your signature!! Very Happy
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PostPosted: Mon Oct 24, 2005 5:52 am    Post subject: Reply with quote

The throttle bodies and/or manifold in virtually all fuel injected cars is heated by engine coolant. Also cars operate at very low throttle openings compared to aircraft, and the lower airflow requires far less heat to prevent freezing.
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PostPosted: Mon Oct 24, 2005 8:45 am    Post subject: Reply with quote

Cars do provide for carb heating in other ways - I used to have a Fiat, which had a rotatable inlet to the air filter. It could be out to the side, or turned down to be between the exhaust outlets.

Luckily, Oz usually doesn't get cold enough to need that.

And for Nick,the avatar is Serial No 319, just ticked over 3000 hrs last week. Not many hours for nearly 20 years!
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PostPosted: Mon Oct 24, 2005 11:13 am    Post subject: Reply with quote

Ascend_Charlie wrote:
Cars do provide for carb heating in other ways - I used to have a Fiat, which had a rotatable inlet to the air filter. It could be out to the side, or turned down to be between the exhaust outlets.


So that could be a fairly common solution then?

And welcome Nick! Good to see you here. Wink
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