Talk:Carburetor heat

WHY CARB HEAT

The present explanation (as of 19 Oct 2007) is only partially correct, as it ascribes the cooling effect in the carburetor only to the venturi effect, yet it states that fuel-injected engines do not need CH - does nobody see the inconsistency in that reasoning? Both engines have butterfly valves. The answer, of course, is the tremendous heat absorbed by the liquid (gasoline) being evaporated at the carburetor. I have rewritten the article to point this out.Raymondwinn 07:07, 20 October 2007 (UTC)

Necessity of leaning

Reduced density implies reduced oxygen content, and hence over-rich conditions upon application of carb heat. The engine must be further leaned after carb heat is applied, or efficiency will suffer.

Thermodynamic efficiency

The article states that thermodynamic efficiency is lost because the Carnot efficiency calculation must be done with a higher cold-side temperature. This is false, as the cold-side temperature is that into which heat is rejected, not the temperature of what is input to the engine before combustion. This is stated at the "thermal efficiency" wikipedia page. In thermodynamic calculations the temperature of the cold-side is the exhaust temperature, and the hot-side is the compressed, combusted mixture.

Potential reduced pumping losses

Like EGR in automobiles, there is a potential efficiency gain from reduced pumping losses (this is lost if the carb heat apparatus is difficult to pump air through). This is because the desired reduction of oxygen supply (to reduce output power to that desired) is achieved by reducing its density thermally using waste heat, rather than by expanding it through an orifice (throttle) using mechanical work.

As a vaporization aid

Carb heat allows for more complete fuel vaporization, which enables a more even fuel-air mixture across cylinders that share a single carburetor. It follows that the temperature discrepancy across cylinders is reduced. This allows leaner operation before one of the cylinders starts misfiring.

As a contributor to knock (detonation)

The increased temperature of the air-fuel mixture increases the likelihood of knock. In many aircraft engines with low compression ratios designed for 80/87 fuel, this is no factor when operating on currently available fuels. It is worth considering that the pilot operating handbook of, e.g. the Cessna 172, recommends cruising with carb heat on when there is visible moisture.

When Carb heat

The article said that "Temperature drops of 20 °C (36°F) or more are often encountered within the carburetor," and "A warm (35-40 °C) day with high humidity is considered the most likely scenario for carburetor icing." Perhaps true, but not if the outside air temperature is 35-40, and the drop is 20 degrees C.

A 20 degree drop from 35-40 degrees is still above the freezing point. The general comment that warm air is more likely to cause carb ice is, of course, correct but ice only forms at 0 degrees C or below. I deleted the "(35-40 °C)" reference. —Preceding unsigned comment added by 67.177.85.116 (talk) 18:33, 20 May 2009 (UTC)

Alternative ways to avoid carburetor icing: heating fuel

The polish training jet airplane: PZL TS-11 'Iskra' ('Iskra' is: 'Spark', from the name of a newspaper participated by V I Lenin), had a system that heated the fuel before reaching the turbine. The cause of carburetor icing being the: 'vaporization heat', the heat fuel absorbs when changing from liquid to vapor, heating fuel is a way to have the needed heat provided before fuel enters the carburation or fuel injection system. With the same aim, SAE paper 885033, by S. Velnovic et al, from the then united Yugoslavia, proposed introducing exhausts into carburetor as a way to increase fuel economy, a hard-style EGR. The air-cooled Citroën car engines had a plate in the admission manifold/ducts, heated by the exhaust gasses, to heat the mix and obtain a better vaporization of mix before entering cylinders. Spanish Utility Model 0248478 U (open and free access in Espacenet), by Manuel Fernandez-Fernandez, is about a fuel economizer made as a serpentine inside a cylinder, washed by the engine's hot cooling liquid, where fuel enters in a crosscurrent path, finding hotter liquid as it goes from the tube in to the tube out, and going out of device at a temperature preventing carburetor icing, and facilitating better fuel/air mix, the possibility of: 'vapor lock' with this kind of approach was not described in the patent office document. Some airplanes, as one of the Arup Flying Wings, or the Heinkel 177, caught fire from fuel lines leaks into hot places. Thanks.--Jgrosay (talk) 13:49, 7 February 2015 (UTC)