Internal Combustion Engines

Internal combustion engines are devices that generate work from combustion reactions. Combustion products under high pressure produce work by expansion through a turbine or piston. The combustion reactions inside these engines are not necessarily neutralizing or complete and air pollutants are produced.

There are three major types of internal combustion engine in use today: 1) the spark ignition engine, which is used primarily in automobiles; 2) the diesel engine, which is used in large vehicles and industrial systems where cycle efficiency offers advantages over the more compact and lighter-weight spark ignition engine and; 3) the gas turbine, which is used in aircraft due to its high power/weight ratio and is also used for stationary power generation.

Each of these types of engine is an important source of atmospheric pollutants. Automobiles are the major source of carbon monoxide, unburned hydrocarbons, and nitrogen oxides. Probably more than any other combustion system, the design of automobile engines is now being guided by requirements to reduce emissions of these pollutants. While substantial progress has been made in emission reduction, automobiles remain important sources of air pollutants.

Diesel engines are notorious for the black smoke they can emit, and gas turbines because of soot emission. These systems can also release unburned hydrocarbons, carbon monoxide, and nitrogen oxides in large quantities.

Catalitic Convertor

From the chemical point of view, a catalyst is any substance able to accelerate a chemical reaction while maintaining its own structure. In the case of automobiles, the catalyst is a box, located between the gas collector of the motor and the muffler of exhaust tube. Exhaust gases are passed through the convertor where an almost total degradation of exhaust smoke can be achieved by catalytic reactions over a hyperactive area made of platinum and rhodium.

In spite of its modest volume, the active surface area inside a catalytic convertor would cover two football fields.

Inside the motor, a piston pushes residual hot gases from the combustion chamber to the exhaust valve, and when these gases pass through the active areas inside the ceramic cells of the convertor, two opposite chemical processes take place simultaneously: an oxidation reaction which converts hydrocarbons and carbon monoxide into carbon dioxide and water; and a reduction of nitrogen oxides to produce pure nitrogen.