Effect of Fuel Unsaturation on Emissions in Flames and Diesel Engines

Abstract

Due to the emergence of a new generation of renewable fuels and the need to accurately model the combustion chemistry of multi-component fuels, there is growing interest in examining the effect of fuel molecular structure on fuel reactivity. This book chapter provides an overview of research dealing with the effects of fuel unsaturation on the ignition, combustion, and emission characteristics. Results from both laboratory-scale configurations, such as shock tube, rapid compression machine, and laminar flames, as well as from high-pressure sprays in compression ignition engines are discussed. Experimental and kinetic modeling studies of homogeneous mixtures provide clear evidence that depending upon the number and position of C = C double bonds, and the reactivity of long-chain hydrocarbons is significantly affected by fuel unsaturation, especially at low to intermediate temperatures. Ignition data for saturated and unsaturated components indicate that the presence of double bond inhibits low-temperature reactivity, modifies the NTC behavior, and leads to reduction in CN number in diesel engines. This has important consequences regarding the effect of unsaturation on combustion and emission in practical systems. High-pressure spray simulations under diesel engine conditions indicate longer ignition delays for 1-heptene compared to those for n-heptane. In addition, the n-heptane spray flame contains two reaction zones, namely a rich premixed zone (RPZ) and a nonpremixed reaction zone (NPZ). In contrast, 1-heptene flame is characterized by three reaction zones, i.e., a lean premixed zone (LPZ) in addition to NPZ and RPZ. Simulations of laminar partially premixed flames (PPF) indicate higher amounts of NOx and soot precursor species (C2H2, C6H6, and C16H10) formed in 1-heptene flames than those in n-heptane flames. Consequently, the soot emission is higher in 1-heptene flames than that in n-heptane flames. Simulations of turbulent n-heptane and 1-heptene spray flames in diesel engines lead to similar conclusions, i.e., higher NOx and soot emissions in 1-heptene flames. The increased formation of PAH species can be attributed to the significantly higher amounts of 1,3-butadiene and allene formed due to β scission reactions resulting from the presence of double bond in 1-heptene.