Understanding the Combustion Process

To understand how to diagnose and repair the emissions control system, one must first have a working knowledge of the basic combustion chemistry which takes place within the engine. That is the purpose of this section of the program.

The gasoline burned in an engine contains many chemicals, however, it is primarily made up of hydrocarbons (also referred to as HC. Hydrocarbons are chemical compounds made up of hydrogen atoms which chemically bond with carbon atoms. There are many different types of hydrocarbon compounds found in gasoline, depending on the number of hydrogen and carbon atoms present, and the way that these atoms are bonded.

Inside an engine, the hydrocarbons in gasoline will not burn unless they are mixed with air. This is where the chemistry of combustion begins. Air is composed of approximately 21% oxygen (02), 78% nitrogen (N2), and minute amounts of other inert gasses.

The hydrocarbons in fuel normally react only with the oxygen during the combustion process to form water vapor (H2O) and carbon dioxide (CO2), creating the desirable effect of heat and pressure within the cylinder. Unfortunately, under certain engine operating conditions, the nitrogen also reacts with the oxygen to form nitrogen oxides (NOx), a criteria air pollutant.

The ratio of air to fuel plays an important role in the efficiency of the combustion process. The ideal air/fuel ratio for optimum emissions, fuel economy, and good engine performance is around 14.7 pounds of air for every one pound of fuel. This "ideal air/fuel ratio" is referred to as stoichiometry, and is the target that the feedback fuel control system constantly shoots for. At air/fuel ratios richer than stoichiometry, fuel economy and emissions will suffer. At air/fuel ratios leaner than stoichiometry, power, driveability and emissions will suffer.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

Don't pay hundreds of dollars to find out what is wrong with your car. This book is dedicated to helping the do it yourself home and independent technician understand and use OBD-II technology to diagnose and repair their own vehicles.

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