Use of carb heat in flight

[Mak]

What effect's other than melting ice and reducing power does adding carb heat have on the engine and plane?

The question arises because as far as I can understand, adding carb heat reduces power but by reducing pressure differential in the carb venturi it will also reduce fuel flow rate. So it's not clear to me how it will affect comsumption (per nm). Another point I came across is that adding heat will help to make combustion in each cylinder more uniform. From this should follow a more efficient burn and maybe less vibration and less engine wear.

Mak

[FlyingForFun]

Mak,

The fuel flow into the carb is dependant on the volume of air passing through it. The volume of air, for any given throttle setting and engine speed, will not change if you apply carb heat. Therefore, the fuel flow will be identical.*

What does change, though, is the density of the air. Warm air is less dense, therefore there is less mass of air going into the engine (or, to put it another way, fewer molecules). If there are fewer molecules of air, then you can't produce as much power, since you need the air for the fuel to burn.

As a side effect, because there are fewer molecules of air but the fuel flow is unchanged, the mixture will be richer. This is why it is possible that you will get a rich cut if you add full power suddenly with the carb heat on (e.g. in a go-around).

Hope that answers the question,

FFF
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* Note the slight simplification - fuel flow will actually only be identical if the RPM is unchanged. Because the RPM drops slightly, you will use very slightly less fuel, but this is pretty insignificant

[Say again s l o w l y]

FFF has got it pretty much spot on, but I don't understand how adding heat to the airflow would make the burn more uniform. This is more dependant on the ratio of the fuel to air rather than as a function of the charge temperature. The design of most light aircraft engines means that they are in no way optimised for either efficiency or power outputs when compared to modern engine designs. Low compression ratios, manual mixture control, air cooling, 2 valves per cylinder, push rods etc...........

If all we had to do was to increase the temperature of the mix to increase combustion efficiency then life would be very easy for engine designers!! You really want as much air as possible going into the cylinders, therefore the colder the better as it is more dense and maybe add a turbo/supercharger for effect aswell. If increased heat aids combustion, then why do we have intercoolers to cool the charge down and help increase power outputs?

Cold air=good power, hot air=less power.

Where did you come across this idea?

[Mak]

I forgot where I saw the article that put forward the argument about carb heat resulting in a more uniform burn. But the explanation given was, if I remember right, that the air/fuel mixture out of the carb was more uniform, and so each cylinder got pretty much the same mixture. The author could back the conclusion with FADEC temperature readings of each cylinder (more uniform when carb heat was applied.

On the comsumption issue I just though that a good way to find out exactly is to look up the engine performance tables vs. OAT at some given altitude.

[Say again s l o w l y]

Hi Mak,

There are many much easier ways of increasing the mixing of the fuel/air mixture, rather than decreasing the volumetric efficiency on one hand by heating the air and (potentially) increasing it by ensuring a better mixing.

The first would be to just fit fuel injection and the high pressure atomises the fuel very effectively.
Increase the turbulence of the flow to ensure better mixing, though this would also have the negative effect of reducing the overall airflow velocity.
There are a 1001 different carb designs and fuel injection systems to help cope with this, heating the air seems like a bit of a waste of time to be honest as the benefits would be pretty minimal compared to just putting a decent induction sytem in the engine in the first place!!