Emission System Overview

by Mark Davidson

The automobile engine provides the locomotive force for the vehicle. Gasoline and diesel powered engines used in automobiles and light trucks may vary in size and design, but they all share the same operating principle. All of these engines are four-stroke internal combustion engines. Four-stroke describes the operating cycle of the engine. Each cylinder of the engine will fire once every two rotations of the crankshaft. The first stage of the cycle is called the intake stroke. As the piston moves down in the cylinder the intake valve opens, drawing the air/fuel mixture into the cylinder. As the piston reaches the bottom of the cylinder, the intake valve closes and the rotation of the crankshaft moves the piston upward in the cylinder. Both the intake and exhaust valves are closed during thExhaust Systemis cycle and as the piston moves upwards, the air/ fuel mixture is compressed. This is called the compression stroke. The compressed air/fuel mixture is ignited as the piston nears the top of cylinder. The expanding gases that result from the ignition of the air/fuel mixture, forces the piston down in the cylinder. This is called the power or ignition stroke. This is the cycle that produces the force that moves the crankshaft and allows the engine to rotate. The piston moves down to the bottom of the cylinder and the rotation of the crankshaft moves the piston back upwards in the cylinder. As the piston moves upwards, the exhaust valve opens and the burned air/fuel mixture is pushed out of the combustion chamber. This is called the exhaust stroke. This is a simplified explanation of the basic operation of an engine. There are many components involved in keeping an engine operating at peak performance and thus avoiding unecessary and costly auto repairs.

An internal combustion engine produces three pollutants that are targeted for reduction by the emission controls system. They are hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx). In addition, vapors produced by gasoline stored in the fuel tank are also a source of HC emissions and are subject to emission controls.

The brown haze that is referred to as smog is created when NOx and HC combine and react with sunlight to create ozone. While ozone in the upper atmosphere helps to shield the Earth from ultraviolet radiation, ozone at ground levels can cause severe health problems. High ozone levels are the main force behind smog alerts in large cities. High ozone levels can cause eye and throat irritation, along with breathing difficulties. The health affects of ozone can be particularly more damaging to those who spend a great deal of time outdoors performing strenuous activities. Engine emissions inspections are required in many states and are performed at certified auto repair shops who have the emissions testing equipment as regulated by the state.

Gasoline is a blend of many different hydrocarbons in various chemical forms. Hydrocarbon emissions from a vehicle are essentially gasoline vapors. There are two major sources of fuel vapors from a vehicle, unburned fuel in the engine exhaust and evaporating gasoline from the fuel tank.

When the air fuel mixture is ignited in the combustion chamber, some fuel will remain unburned. Gasoline that has not been consumed during the burning process is carried out of the combustion chamber with the exhaust gases as hydrocarbons. During an emissions test, the amount of hydrocarbons in the exhaust is measured in parts per million (PPM). A typical hydrocarbon level in the exhaust of a modern vehicle is below 100 PPM. When an engine misfire occurs, large amounts of hydrocarbons will be emitted due to the air/fuel mixture exiting the combustion chamber without being burned. The hydrocarbon emission level when this occurs can exceed 2000 PPM.

Carbon monoxide is a colorless, odorless, poisonous gas produced from incomplete combustion. High levels of carbon monoxide in the air can cause serious health effects. When carbon monoxide is breathed in, it accumulates in the blood stream, blocking the flow of oxygen to vital organs.

Carbon monoxide emissions from a vehicle are mostly the result of incomplete burning of fuel. During emission testing (performed at many auto repair shops) the level of carbon monoxide is measured as a percentage of the total exhaust gases. An acceptable carbon monoxide level for a late model vehicle, in good running condition, with functioning emission controls, would be below 1.0%. The major source of high carbon monoxide emissions is a rich air/fuel mixture. A vehicle with a malfunctioning fuel delivery system that is running fuel rich, can produce exhaust with a carbon monoxide level that exceeds 10%.

NOx is created when oxygen combines with nitrogen. This can only occur at temperatures above 2500 F. In an engine, combustion chamber temperatures can easily exceed the level necessary for the formation of NOx. Control of NOx emissions is achieved by lowering combustion temperatures.

The earliest method used to lower combustion temperatures was retarding ignition timing and decreasing engine compression ratios. EGR systems were developed around the same time to control peak combustion temperatures by using an inert gas (exhaust) to dilute the air/fuel mixture. Later, special catalytic converters were introduced to remove NOx from the exhaust stream after combustion. An auto repair shop may recommend replacing the catalytic converter if the vehicle fails an emissions check. In this case, get a second opinion before proceeding with such an expensive job.

Exhaust emission systems have undergone a number of changes and advancements since they were first introduced. Sometimes efforts made to reduce levels of one pollutant has caused levels of other pollutants to rise. Systems would have to be designed to reduce the level of the affected pollutants. The result could be a cumbersome mix of emission systems stacked on an engine. Today, through the use of computerized fuel and emission controls, vehicles have lower emissions and in most cases, fewer emission controls than their predecessors.