Predicted Nitric Oxide Emission from an Ethanol/Gasoline HCCI Engine

Abstract

The main aim of this study is to investigate theoretically the formation of nitric oxide (NO) in a homogeneous charge compression ignition (HCCI) engine fueled with blends of ethanol and synthetic gasoline (RON25, RON50, RON75). A detailed kinetic mechanism involving 1086 chemical species and 4754 elementary reactions taking place in a HCCI engine modeled as an adiabatic single-zone reactor was applied to simulate the emission of the investigated pollutant. A close agreement between experimental and calculated results of ignition delay, species concentrations in ideal reactors, and laminar flame velocities of pure ethanol, n-heptane and iso-octane confirmed the reliability of the kinetic model. An analogous comparison, but considering a set of data (NO formation, temperature, and pressure) obtained in a single-cylinder HCCI engine operated with different mixtures of primary reference fuels (n-heptane and iso-octane) over the equivalence ratio range from approximately 0.15 to 0.4, revealed the consistency of the adiabatic single-zone model. Despite the low to moderate content of NO typically observed in the exhausts from HCCI engines, a further decrease of this pollutant was still revealed at all the considered simulating conditions when the percentage of ethanol in the fuel (synthetic gasoline) was increased. However, the extent of NO reduction was more evident for blends involving up to approximately 20% of ethanol (<E20) and synthetic gasolines with low octane number (<RON50). From a practical point-of-view, this finding reveals that the injection of a small volume of ethanol to gasoline (<20%) could be an alternative procedure to reduce the rates of exhaust gas recirculation applied to control the HCCI operation, making it a promising strategy to have a conciliation between low NO and moderate UHC and CO emissions in gasoline HCCI.