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air to the mass of a particular fuel. This ratio is approximately 14.7 to 1 for Avgas and about 14.5 to 1 for Jet A (jets vary significantly based on the specific engine). In the complete com- bustion of hydrocarbons (stoichiomet- ric), the products are carbon dioxide, water and unaffected nitrogen. Notice how the CO from the first combus- tion equation above is missing from the second stoichiometric equation. With the incomplete combustion of hydrocarbons in recips and jets (the first equation), the products include unburned hydrocarbons, nitrogen ox- ides, carbon monoxide (the boogey- man in our story), carbon dioxide and water. These types of emissions are responsible for not only contrails but carbon monoxide poisoning. And there you have our story’s first spoon- ful of plain yogurt.
Why Not Stoichiometric?
Most recips and jets operate at something other than the stoichiomet- ric mixture in order to achieve better economy, power or engine life. For example, an idling engine runs richer and colder than at cruise power, is even further from stoichiometric and produces a relatively high amount of CO. And if you operate LOP (Lean Of Peak) in your recip, you pass through the stoichiometric mixture on your way to LOP, and for a brief moment, you have an ideal fuel/air mixture and all CO is burned (at around 609 degrees Celsius). Once you get to LOP, combustion is once again cooler and the CO remains unburned. Running at the stoichiometric mixture is im- practical because the mixture would burn too hot for engine longevity and power would be diminished. So, we are burning fuel at a ratio such that CO is produced and not burned. Now, here is a big spoonful of plain yogurt.
The Porphyrin Class
Carbon monoxide is a very im- portant industrial compound used in refrigeration and cooling, as an inert gas in chemical processes, in the synthesis of ammonia, and in the storage of carbon powder in fire extinguishers. It burns in air with a bright blue, 2,121 degrees Celsius f lame, is only slightly soluble in
water, and its physical properties closely resemble those of nitrogen. Carbon monoxide is a colorless, odor- less and insipid (there’s that word again) gas and is the number one cause of accidental death from poisoning. The human respiratory system pro- vides oxygen to the tissues and elimi- nates the carbon dioxide produced by them. When inhaled, carbon monox- ide passes from our lungs into our bloodstream, where it attaches to the
hemoglobin molecules that normally carry oxygen. Most of the O2 trans- ported by the blood is through its reac- tion with hemoglobin molecules. He- moglobin is a red protein responsible for transporting oxygen in the blood of all critters possessing an internal spine (that’s us). Its molecule is com- prised of four subunits, each contain- ing an iron atom bound to a heme group. Heme is an iron-containing compound of the porphyrin class (see
My FO Mike demonstrates the response to CO, hypoxia and smoke/fumes in the cockpit.
September 2020 / TWIN & TURBINE • 31