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Nearly every form of propulsion today involves the application of energy derived from oxidizing fuels (combustion). The energy is applied to machin- ery in some manner to move objects. The downside is that combustion is never 100 percent efficient and so always produces some unwanted byproducts. According to the EPA, aircraft account for 2 percent of all nitrogen oxides (NOx) and carbon monoxide pro- duced by vehicles and can be as high as 4 percent in areas around airports. A recent study showed aircraft at London’s Heathrow airport contributed between 16 and 32 percent of ground-level NOx. Jet taking off. The fuels of choice—whether avgas (aviation gasoline) for small piston planes or kerosene for large passenger jets—are hydrocarbons, compounds com- posed of hydrogen and carbon. When they burn they combine with oxygen to produce water and carbon dioxide as shown here: 2 C8H18 + 25 O2 → 16 CO2 + 18 H2O The production of water is not generally consid- ered polluting, although there is some evidence that the increase of water vapor in the stratosphere where passenger jets fly (5 to 6 mi high) has caused an increase in cloud cover. The production of carbon dioxide, however, is much more of a current concern as many studies have associated its increase with a rise in global warming. Aircraft currently produce up to 4 percent of the annual global carbon dioxide emissions from fossil fuels, and it is projected that the amount of air traffic is going to increase over the next decade. Reducing CO2 production will require the development and use of alternative fuels such as hydrogen rather than fossil fuels. The previous equation indicates complete com- bustion of C8H18, where 100 percent of the reactants go to make products. In the real world, engines are not 100 percent efficient in burning fuels. When engines are not running near full power, such as when aircraft descend or taxi, the engine temperatures are lower, resulting in less efficiency and thus incomplete com- bustion. In incomplete combustion, not all of the car- bon atoms of the hydrocarbon molecules are oxidized to CO2. The result is the production of carbon mon- oxide, CO, which is partially oxidized carbon, and soot and smoke, which are particles of completely unburned carbon. This is seen in a candle flame, where the unburned carbon particles glow yellow and then are deposited as soot (carbon) on anything held above the flame. Both carbon monoxide and particulate mat- ter are related to breathing difficulties. A third problem with engine inefficiency is unburned hydrocarbons. Some of the fuel gets through the engine unburned. This results in what engineers call “volatile organic compounds” or VOCs being released into the atmosphere. These molecules react with sun- light and oxides of nitrogen to produce ground-level ozone, a pollutant gas. The solution is to run engines at higher temperature to react all the carbon. NASA Glenn is studying new materials that withstand higher tem- perature, which would allow engines to run hotter and cleaner under these conditions and yet not increase weight. The trade-off to higher temperature engines is the production of pollutant nitrogen oxides NOx. In nor- mal combustion the two major components of the air, nitrogen and oxygen, do not react with each other. However, in the high pressures and temperatures of both piston and turbine engines, they do react to form a group of nitrogen oxides such as NO, N2O, NO2, and N2O4 . These are usually referred to as a group des- ignated NOx (pronounced “nox”). They are a precur- sor to ozone formation, a factor in acid rain, and an irritant in breathing. Cars reduce these emissions by means of a catalytic converter that causes the NOx to split back into N2 and O2. On turbine-powered aircraft Pollution Pushing the Envelope: A NASA Guide to Engines 39 CHEMISTRYPDF Image | NASA Guide to Engines
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