The homogeneous charge compression ignition (HCCI) engine has attracted a lot of attention in the combustion community for its low NOx emissions and high thermal efficiency characteristics. A major means of delivering reduced NOx is obtained through the adoption of increased exhaust gas recirculation (EGR). However due to the lower combustion temperatures and reduced oxygen concentration, EGR reduces the combustion rate more than excess air does this can lead to increased emissions of HC and CO. In addition, active components within the exhaust gases such as NO can promote ignition.
THE CHALLENGE
To gain further insight into the combustion characteristics of natural gas in HCCI engine operation through the development of computational models. These models should predict the combustion characteristics and emissions from a CNG engine at various EGR rates. In particular, reliable prediction of CO emissions which is a combustion intermediate.
THE SOLUTION
The adoption of srm suite and a CNG fuel model to examine the impact of EGR.
THE RESULTS
Insight into the combustion characteristics of natural gas
With increase in the amount of the EGR, the compressed gas temperature reduces thereby slowing the rate of reactions and delaying the auto-ignition and reducing the peak pressure. The thermal influence (e.g. charge heating effect) of EGR also can be accounted using the srm suite, however in this study the external EGR was cooled down to the room temperature.
CO and uHC exhaust emissions
The CO emissions were found to be influenced by fluid-wall interactions, mixing of hot and cold air-fuel particles, and the wall temperature. Furthermore, the inhomogeneities persisting in the expansion stroke dictated the level of CO emissions obtained at EVO. The prediction of the rapid rise in CO and HC emissions and their sensitivity to the EGR rates agreed with the test cell observations.
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