Purpose

The Exhaust Gas Recirculation (EGR) system is used in automotive engines to decrease the emission levels of oxides of nitrogen (NOx). NOx defines a group of chemical compounds containing nitrogen and varying amounts of oxygen that can have harmful environmental effects in large quantities.

NOx forms during the combustion process in amounts that are dependent on the concentration of oxygen in the combustion chamber and the duration that the combustion process temperatures exceed 1500°F. Decreased NOx levels are accomplished by reducing the peak combustion temperatures through dilution of the incoming fuel/air charge with exhaust gas. When combusted, exhaust gas (largely non-reactive carbon dioxide and water vapor) acts to absorb a portion of the combustion energy, resulting in lower temperatures throughout the combustion process and yielding lower amounts of NOx.

Desired amounts of EGR depend upon the geometry of the combustion chamber and the operating condition of the engine. Extensive laboratory and vehicle tests are used to determine optimal EGR rates for all operating conditions. Too little EGR can yield high NOx, while too much EGR can disrupt combustion events. This engine uses a computer controlled valve to precisely regulate the amount of EGR delivered to the engine for all operating conditions.