Can carb heat lower eng. temp?

Raskolnikov

Raskolnikov

Well-Known Member
Okay so the carb heat: unfiltered air passes over the exhaust manifold and goes into the carb to melt/ prevent forming of ice. But it also enriches the mixture, leaving more unburned fuel after combustion that will absorb some heat. We know a rich mixture can help cool an engine. But how much of that will be offset by the increase temp of the air coming into the cylinder on the intake stroke due to carb heat?

I suppose it might be a function of how high the rpm is. But I really don't have a clue.

Anyone ever come across this?
 
When I took my checkride I explained to the DE I put on carb heat on when the little arrow is in the white arc and/or I think I may be encoountering carb ice....I passed, so as far as I'm concerned that's the right answer. It would be "interesting" if it had a function for the exact opposite purpose than what every primary student is trained for.
 
:yeahthat:

Haha thats a great answer. Thats all I really said as well during my checkride and for all intents and purposes, thats all you have to say. But for the more in depth answer honestly I have never thought of that. Never had to, but it could be useful information I guess, but for my PPL students the basic answer is good enough:)
 
Raskolnikov said:
Okay so the carb heat: unfiltered air passes over the exhaust manifold and goes into the carb to melt/ prevent forming of ice. But it also enriches the mixture, leaving more unburned fuel after combustion that will absorb some heat. We know a rich mixture can help cool an engine. But how much of that will be offset by the increase temp of the air coming into the cylinder on the intake stroke due to carb heat?

I suppose it might be a function of how high the rpm is. But I really don't have a clue.

Anyone ever come across this?
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I would say yes. However, this appears to be more of a concern with turbocharged engines. In my airplane, use of the boost pump and mixture control are part of your engine cooling procedures. The carb heat acts the same way.
 
Keep in mind though that most present day turbocharged engines are fuel-injected, that is to say no carburator.
 
Raskolnikov said:
leaving more unburned fuel after combustion that will absorb some heat.
Click to expand...
An overly rich mixture does not cool the engine by evaporative cooling. It cools the engine because combustion happens more slowly, and peak pressure occurs in the cylinder at a later time. It has the same effect as retarding the ignition timing.
 
ananoman said:
An overly rich mixture does not cool the engine by evaporative cooling. It cools the engine because combustion happens more slowly, and peak pressure occurs in the cylinder at a later time. It has the same effect as retarding the ignition timing.
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Anonaman speaketh the truth.

Playing around wiht the carb heat trying to help the engine cool is spliting hairs. If you engine is hot, richen the mixture with the and lower the nose to get some airspeed to cool it off. If it still runs hot get it checked out by a mechanic, you probably have leaky baffels/seals.
 
Thanks for all the replies. I don't own an airplane and don't have an overheating problem. I just was curious because I was reading about how enriching the mixture can help cool the engine. I thought of it in terms of simple physics. If you take a spoon and dip it in a bowl of hot chili, the spoon comes out hot. That heat was transferred from the chili to the spoon. So it would make sense that the left over fuel after combustion would take some heat with it like the spoon does. Granted a metal spoon is better at absorbing heat than fuel is, but it should take some with it.

I guess the only way to find out for sure would be to measure the temp on the cylinder head to see how carb heat affects temperature.

Again, I wasn't asking because I thought it would be a good idea to use carb heat to cool the engine. I'm just curious and maybe a little too analytical. :)

I looked this up on Google and found a lot of interesting articles on running lean of peak (LOP) vs rich of peak (ROP). Some people think running full throttle all the time and adjusting rpm (fixed pitch prop) with the mixture, therefore always running LOP, is the best way to run an engine and save fuel. Others highly disagree.

Interesting stuff.
 
Raskolnikov said:
I looked this up on Google and found a lot of interesting articles on running lean of peak (LOP) vs rich of peak (ROP). Some people think running full throttle all the time and adjusting rpm (fixed pitch prop) with the mixture, therefore always running LOP, is the best way to run an engine and save fuel. Others highly disagree.

Interesting stuff.
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The POH for the PA28 contains info about running at full throttle and using the mixture to set rpm, but they run like crap if you do it. I don't think most carbureted engines have good enough mixture distribution to do this and run smooth. Nothing wrong with it if it works in your engine though.
 
It's amazing. People will do all sorts of things to save half of a gallon per hour. Including potentially damange an engine!!

I hope you're investing that $2 that you save per flight hour - you're going to need 5 times that amount to replace the engine!!!
 
Ha! LOL! I'm NOT doing this. Just READING about it. I use the throttle to control RPM and in the 150, only lean once above 5000, just like my flight instructor told me to.

There's no harm in reading about how other people destroy their engines...
 
First off, LOP operations are perfectly harmless if done porperly. Millions of flight hours have proved this. All old radial engine airliners were run in this manner for decades. Your car engine runs LOP all the time.

However you need the proper equipment and instrumentation to run LOP porperly. Carburated flat engines won't run LOP smoothly because of poor airflow distibution. Injected engines with factory injectors won't either most of the time. Accurate EGT information on all cylinders is critical.

By running at partial throttle and ROP you are chokeing the engine and pumping extra fuel overboard. By running at wide open throttle and LOP you are starving the engine for fuel resulting in a lower poer setting.


However as I said, unless you have a engine with exactly equal mixture distribution for all cylinders (GAMIjectors) you will experiance moderate to heavy vibration.

Also, unless you can precisely controll your fuel flow and EGT you will either be operating either at redicusly reduced power settings or in the dangerous area 50 deg either side of peak EGT.

Look at this chart closely.
 
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