Soil microbiology

Since 1975, autos have had a similar system for returning some gases emitted through the exhaust valve - the main path taken by combustion by-products - back t the intake manifold to be reburned. This system, exhaust gas recirculation, reduces nitrogen oxide emissions.

Researchers have also studied the possibility of using the exhaust manifold as an «afterburner» to combust the exhaust gases before they enter the atmosphere., The manifold would have to be larger and its temperature maintained well above 5000C (9320F) to achieve combustion. But cars have been getting smaller, leaving little room for a large, hot «afterburner».

Farther down the exhaust pipe there is one final opportunity for capturing pollutants before they enter the outside air. In 1975, to meet emissions standards mandated by the Clean Air Act, automakers began to equip their vehicles with a catalytic converter, a device containing pellets or a honeycomb type of structure coated with a blend of such metals as palladium, rhodium, and platinum. When exhaust gases pass through these metals, unburned hydrocarbons, carbon monoxide, and nitrogen oxides undergo a chemical reaction that changes them into less harmful compounds such as carbon dioxide and water vapor. Although catalytic converters significantly reduce pollutants, they lose their effectiveness over time.

 

 

Researchers have also tried to reduce emissions by improving the mixture of fuel and air by controlling more precisely the amount of fuel and air in the combustion chamber, they have increased the likelihood that the mixture wil1 burn more completely and leave fewer polluting residues.

The technology to produce such tailor-made mixtures was made possible by the development of inexpensive, powerful microprocessors (tiny electronic components silicon «chips») located in a central computer system under the car dashboard. Combined with sensors (devices that react to such variables as temperature, motion, and oxygen levels), microprocessors can determine how much fuel is needed for the amount of air being drawn in. Because drivers operate cars under numerous weather and road conditions - and demand that their vehicle start, stop, speed up, and slow down at the drop of a foot - all the monitoring and decision-making needed to achieve proper fuel-air mixtures must be done accurately and rapidly, in some cases many times per second. This difficult task is made to order for computers, which were introduced into automobiles in the 1980’s and are now standard operating equipment on almost all new car.

The air-mass flow sensor measures the amount of air entering the engine and alerts the computer, which is also receiving other information about the engine from other sensors and microprocessors. The computer calculates how much fuel is required and sends a command to the fuel injection system, which sprays the correct amount of fuel into the air flowing through the intake ports of the cylinders. The computer also tells the fuel injection system precisely when to release the fuel so that the fuel and air will be properly mixed as they enter the chamber.

Other sensors determine if the fuel-air mixture ignites too quickly, before the piston rod is in the correct position to be forced down by the combusted gases. The sensors then alert the computer to alter the timing of the fuel-air injection to correct t s situation.

The design of the combustion chamber itself can also help control the fuel-air mixture. For example, the stratified (layered) charge combustion chamber, pioneered by Texaco Incorporated in the 1960”s and developed by several automakers since then, is designed so that the fuel-air mixture is richest near the spark plug and then becomes leaner farther away. The richer mixture is always easier to ignite, so combustion is assured and less fuel is needed overall.

After combustion occurs, an exhaust valve opens to allow the gases produced by combustion to escape from the cylinder through an exhaust port and into the exhaust manifold, which leads to the exhaust pipe that carries the gases away from the engine. Researchers today are developing ways to control both the intake and exhaust valves more precisely to foster more complete combustion.

 

 

In addition to improving the various components of the engine and exhaust system, engineers have worked on changing the overall design of the engine. Almost all automobile engines operate on a four-stroke cycle, in which each cycle - fuel-air intake, compression, firing, and exhaust - requires one up or down stroke of the piston. The first two functions and the last two functions may be combined so that only two strokes are required.

Two-stroke engines are simpler in design and smaller and lighter than four-stroke engines. Thus, they produce more power