Effects of Charge Dilution on Nitric Oxide Emission from a Single-Cylinder Engine

The effects of charge dilution on the exhaust emission of nitric oxide (NO) from a single-cylinder engine were evaluated over a range of engine design and operating parameters.

Nitric oxide emission decreased as much as 70% as charge dilution fraction (volume fraction of product gases in the combustion chamber prior to ignition) was increased from 0.065 to 0.164 due to increased valve overlap, external exhaust recirculation, and reduced compression ratio. With these three variables, NO emission was strongly dependent on charge dilution fraction, but was independent of the specific method used to change charge dilution. Other variables such as valve overlap position, spark timing, and exhaust pressure also affected charge dilution and NO emission, but the relationship between charge dilution fraction and NO emission for these variables was not consistent.