The Passion of Mixture
An incensed reader, reacting to my parenthetical remark [“Making Range,” Technicalities, August 2012] that, contrary to widespread belief, a leaner-than-peak-EGT mixture reduces cylinder head temperatures, wrote:
Maybe I am missing something, but it is against the laws of physics that a leaner mixture can run cooler, as the more dense the mixture, the cooler the chamber is, thus EGT rises when lean’d, so how can this be “contrary to widespread belief”?
He adds an apocalyptic postscript:
P.S. We all know that lean mixtures can melt valves, crack seats, melt heads, destroy plugs, crack cylinders, burn valves, cause premature destruction of all metals the “hot spot” touches!! Now some of Peter’s readers will think lean is good — what about the idiot that overleans cuz Pete says is good cooler and “OK”!! Liability?
Since he doubts, I will call my impassioned correspondent Thomas. His views are those of a great many pilots, though his Shakespearean spelling (“lean’d”) endears him to me above most others.
The subject of mixture is not very complicated in principle; where it gets complicated is in practice. Let’s start — since dueling Thomas has chosen that heaviest of weapons, the Laws of Physics — with the physics.
In reciprocating engines, heat is generated by burning a fuel composed of molecules called hydrocarbons, which consist of hydrogen and carbon atoms in various combinations. What we call burning is a rapid chemical reaction in which oxygen molecules dismantle hydrocarbons and reassemble their components into carbon dioxide (one carbon and two oxygens) and water (one oxygen and two hydrogens). Energy, in the form of heat, is released in the process. The temperature of everything in the cylinder rises, including that of the other gases, like nitrogen, that compose nearly 80 percent of atmospheric air and do not take an active part in the combustion process. The gases expand, pushing the piston down, and off we go.
If you took high school chemistry, you may recall, if nothing else, that chemical reactions involve atoms matching up in certain definite numerical relationships, like dancers at a prom — at least the old type of one-male-to-one-female prom. If more oxygen atoms are present than there are hydrogen and carbon atoms for them to link up with, they remain, at the end, wallflowers. Likewise the hydrogens and the carbons. When the amounts of oxygen and hydrocarbon molecules present are perfectly matched, the mixture is called “stoichiometric” — stow-icky-oh-MET-rick. The stoichiometric ratio of air and avgas is about 15.2 pounds of air to one pound of avgas — which may be visualized as a 6-foot cube of air and a 4-inch cube of fuel.
That is the idealized picture. Reality is more complicated. Because both fuel and air contain a potpourri of components, a lot of other chemistry goes on in the high-temperature environment of the cylinder. Burning, furthermore, is not a pure and instantaneous process; it is affected by spark timing and flame propagation rate and quenching by the relatively cool cylinder walls and various other things. Nevertheless, a few general statements about mixture and engine health can be made. They are not the ones offered by Thomas.
We can’t weigh the air and fuel entering the cylinder while we’re flying, so we measure the mixture by the exhaust gas temperature (EGT) instead. You would expect that an excess of fuel or air would dilute the heat released by combustion and thereby lower the EGT. So it does. You might also assume that the highest power output and the highest cylinder head temperature (CHT) would coincide with the highest EGT, but there you would be wrong, because of some of the aforementioned complications of combustion. Different engines will have slightly different characteristics, but a typical Lycoming chart shows that power output peaks when EGT is about 125 degrees Fahrenheit on the rich side of peak (ROP). Cylinder temperature is highest at 25 degrees ROP. Efficiency — power produced per pound of fuel burned — is greatest at about 40 degrees on the lean side of peak (LOP). These figures vary somewhat with power setting and among different engines, but the general trends remain the same.