High cylinder temperature and pressure which occur during the combustion process can cause nitrogen to react with oxygen to form Oxides of Nitrogen (NOx). Although there are various forms of nitrogen-based emissions that comprise Oxides of Nitrogen (NOx), nitric oxide (NO) makes up the majority, about 98% of all NOx emissions produced by the engine.
Generally speaking, the largest amount of NOx is produced during moderate to heavy load conditions when combustion pressures and temperatures are their highest. However, small amounts of NOx can also be produced during cruise and light load, light throttle operation. Common causes of excessive NOx include faulty EGR system operation, lean air/fuel mixture, high temperature intake air, overheated engine, excessive spark advance, etc.
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