Soil microbiology

If a mixture of gasoline and air were free of impurities, then, in theory, it could be burned completely. For example, if pure gasoline, composed solely of carbon and hydrogen atoms, were burned in pure air, consisting only of molecules made of two atoms of oxygen and two of nitrogen, then some oxygen a ms would attach to the gasoline’s hydrogen atoms to form water (H2O) and some would attach to the gasoline’s carbon atoms to form carbon dioxide (CO2). No free oxygen would be remain the nitrogen would be unaffected, and no other harmful by-products would be created (carbon dioxide, which is linked to global warming, the projected rise in Earth’s average temperature, is an unavoidable by-product of the combustion of gasoline or other types of fossil fuels.)

However, other factors, such as the ratio of fuel to air, affect combustion. For complete combustion to occur, about 6 per cent of the weight of the fuel-air mixture must be in the fuel. If the fuel makes up a larger percentage, the mixture is said to be «rich». If it composes a smaller percentage, the mixture is «lean». A minimum and maximum percentage of fuel to air must exist for combustion to occur at all.

When too much air is in the mixture, excess oxygen atoms from the air can link up with the nitrogen to produce nitrogen oxides, compounds that contain varying numbers of oxygen atoms. Nitrogen oxides can harm both the environment and human beings. If inhaled, nitrogen dioxide can cause inflammation of the lungs. Continued exposure may lead to long-term lung disease and increased risk for infections of the respiratory system. Nitrogen oxides can also mix with moisture in the atmosphere to form nitric acids that fall to Earth as precipitation, commonly referred to as acid rain, which can damage plants and wildlife.

 

 

If the fuel-air mixture contains too little air, the result is incomplete combustion. Not all the gasoline’s carbon and hydrogen atoms can hook up with oxygen atoms to form water or carbon dioxide. When a carbon atom hooks up with only one atom of oxygen, a molecule of carbon monoxide (CO) is formed. Carbon monoxide is a poisonous gas that can deprive the human body of oxygen. High CO concentrations are known to damage the heart and blood vessels, particularly in people with heart disease. Unburned carbon, or soot, appears as smoke or as black deposits on buildings and other surfaces and is another suit of incomplete gasoline combustion. Soot particles inhaled from the air may cause lung inflammation and breathing problems. And unburned hydrocarbons, some of which are thought to cause cancer, constitute another dangerous emission from automobiles.

Hydrocarbons and nitrogen oxides in the air can react in the presence of sunlight to produce photochemical smog. Smog contains ozone (molecules composed of three oxygen atoms, high levels of nitrogen dioxide, and a wide variety of hydrocarbon molecules that have partially combined with oxygen. Although ozone in the upper atmosphere helps shield the Earth from the sun’s ultraviolet rays, ozone in the lower atmosphere causes eye, nose, and throat irritation, as well as impaired breathing and coughing. These effects are most severe in the very young and the very old.

 

 

Obviously, combustion of even the purest gasoline and purest air can easily give rise to pollutants if the fuel and air are not in perfect proportion. The fuel and air must not only a properly balanced, they must also be thoroughly mixed. If they are not, some parts of the combustion chamber may contain lean mixtures, and others rich, even though the total amount of fuel and air is perfect.

In addition, the walls of the combustion chamber may further foster incomplete combustion of a perfect fuel-air mixture. A tiny burr or soot deposit protruding from the combustion chamber wall is likely to become hotter than other areas of the chamber and could cause the fuel-air mixture to ignite before it should. This can produce nitrogen oxides. By the same token, cool areas in the chamber can enhance the formation of carbon monoxide.

The fuel itself contains impurities that can create harmful compounds when burned. Even the air drawn into the engine contains impurities. And it is unlikely that either gasoline or air will ever be perfectly pure. In fact, to enhance automobile performance, gasoline contains additives, which may, promote pollution. Lead, for example, was once added to gasoline to help prevent preignition of the fuel-air mixture. Later, lead was generally