Combustion Chemistry

The gasoline-powered internal combustion engine takes air from the atmosphere and gasoline, a hydrocarbon fuel, and through the process of combustion releases the chemical energy stored in the fuel.

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Introduction to Combustion Chemistry
Of the total energy released by the combustion process, about 20% is used to propel the vehicle, the remaining 80% is lost to friction, aerodynamic drag, accessory operation, or simply wasted as heat transferred to the cooling system. Modern gasoline engines are very efficient compared to predecessors of the late \’60s and early \’70s when emissions control and fuel economy were first becoming a major concern of automotive engineers. Generally speaking, the more efficient an engine becomes, the lower the exhaust emissions from the tailpipe. However, as clean as engines operate today, exhaust emission standards continually tighten. The technology to achieve these ever-tightening emissions targets has led to the advanced closed loop engine control systems used on today\’s Toyota vehicles. With these advances in technology comes the increased emphasis on maintenance, and when the engine and emission control systems fail to operate as designed, diagnosis and repair. 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.
In essence, only harmless elements would remain and enter the atmosphere. Although modern engines are producing much lower emission levels than their predecessors, they still inherently produce some level of harmful emission output. The Four-Stroke Combustion Cycle During the Intake Stroke , air and fuel moves into the low pressure area created by the piston moving down inside the cylinder. The fuel injection system has calculated and delivered the precise amount of fuel to the cylinder to achieve a 14.7 to 1 ratio with the air entering the cylinder. As the piston moves upward during the Compression Stroke, a rapid pressure increase occurs inside the cylinder, causing the air/fuel mixture to superheat. During this time, the antiknock property or octane rating of the fuel is critical in preventing the fuel from igniting spontaneously (exploding). This precise superheated mixture is now prime for ignition as the piston approaches Top Dead Center.

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