Internal Combustion Engines

Internal combustion engines are devices that generate work using the products of combustion as the working fluid rather than as a heat transfer medium. To produce work, the combustion is carried out in a manner that produces high-pressure combustion products that can be expanded through a turbine or piston. The engineering of these highpressure systems introduces a number of features that profoundly influence the formation of pollutants. There are three major types of internal combustion engines 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 the improvements in cycle efficiency make it advantageous 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 also is used for stationary power generation.

Each of these engines is an important source of atmospheric pollutants. Automobiles are major sources of carbon monoxide, unburned hydrocarbons, and nitrogen oxides. Probably more than any other combustion system, the design of automobile engines has been guided by the 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 emit. Gas turbines emit soot as well. These systems also release unburned hydrocarbons, carbon monoxide, and nitrogen oxides in large quantities. In this chapter we examine the air pollutant emissions from engines. To understand the emissions and the special problems in emission control, it is first necessary that we understand the operating principles of each engine type. We begin our discussion with a system that has been the subject of intense study and controversy-the spark ignition engine.