Use of Carb Heat for fuel savings

[Robert Eilers]

I have a friend who flies a carburated 182. He tells me he pulls carb heat on about one quarter of an inch at cruise and then leans. He says the heated air improves the fuel air mixture and allows him to burn less gas per hour. Anyone doing something similar - first I have heard of this technique.

[GAHorn]

It's largely in their imgagination, is my take on it. If you pull carb heat, you do a couple things:

1- You bypass the air filter which, if restricted/dirty, may increase manifold pressure. (Solution, clean/replace your air filter and inspect for condition/security/cleanliness of the induction and air box.)

2- The basic theory behind the carb-heat/lean method is that increased vaporization of fuel will occur (presumbeably due to hot air enhancing evaporation of fuel), and that will enrichen the mixture, therefore if one leans the carb it will return the mixture to the correct one (approx seven to one) and that a total increase of thermal efficiency occurs due to the increased consumption of fuel.

Here's the problem with that theory: Heating the air creates a "high-density altitude" condition and therefore a net LOSS of available power. The reason the mixture must now be leaned is because too much fuel is being provided by the carb relative to the high-density altitude. Additionally, any ram-effect of the induction system is lost.

The BEST situation is high density air (as opposed to the low density condition called "high density altitude") which can better absorb more fuel. If the ideal ratio is 7/1 (seven parts air to one part fuel) then the more air molecules inhaled the more fuel molecules which can be vaporized and carried with that air. Decreasing the air molecules by heating them so that they expand to fill the fixed amount of cubic-inches, will result in a net loss of efficiency.

One of the nice coincidences of flight is that as altitude increases, temperature decreases. This helps make up for the net loss of density altitude due to actual altitude.

If the theory actually had validity... then why would mfr's/engineers go to all the trouble to supercharge/turbocharge engines.... yet COOL that air back down with intercoolers before fuel is injected? It's because hot air loses density and the availability to utilize fuel.

[Robert Eilers]

Thank you George

[hilltop170]

Robert-
I mostly agree with what George just said.

However, being curious, I have tried experimenting with carb heat on many different flights in the C-170 with an O-300-D engine. It has a six-position engine monitor so all six cylinder EGTs can be observed at the same time. What I have observed is it sometimes helps even out EGTs and sometimes it doesn't. In any case there is no set amount of EGT that will produce the desired results on every flight. And without some way to see all EGTs at the same time there is no other way to tell what is going on and if any improvement is being made.

I have found the carb heat does more to even out EGTs when the OAT is colder, say less than 50°F. With warmer OAT, carb heat does not offer an improvement and actually increases the EGT spread between cylinders.