Energy & Fuels, Vol.33, No.1, 16-30, 2019
Target-Oriented Fuel Design for the Homogeneous Charge Autoignition Combustion Mode: A Case Study of a n-Heptane-PODE3-Ethanol Mixture. 1. A Pathway To Increase the Combustion Efficiency and Reduce Pollutant Emissions
This work offers a pathway to increase the combustion efficiency and reduce the pollutant emissions for the homogeneous charge autoignition (HCAI) mode of combustion. The combustion and emission characteristics of n-heptane-PODE3-ethanol mixtures were measured under constant volume conditions at a pressure of 2.703 MPa and equivalence ratio of 0.4-1.0. The fuel reactivity was varied by changing the blending ratio of high cetane number fuel and high octane number fuel. The main results are summarized below: First, the fuel reactivity should be adjusted according to the equivalence ratio to improve the combustion load adaptability. At fuel-lean conditions, the proportion of high cetane number fuel should increase to strengthen the ignition and combustion stability. As the load approaches the stoichiometric condition, the proportion of the high octane number fuel should increase to form a homogeneous charge to elevate the combustion efficiency. Pure n-heptane achieves the maximal combustion efficiency up to 81.06% at phi = 0.4, while that of 20% n-heptane-80% ethanol achieves maximum (98.87%) at phi = 1.0. Second, both the 40% n-heptane-60% ethanol and 60% n-heptane-20% PODE3-20% ethanol can break the NOx-PM (PM = particulate matter), NOx-HC/CO trade-off simultaneously at phi = 0.6. Fuel-lean conditions lead to low NOx emission (<2 ppm), while the high oxygen mass content (40% n-heptane-60% ethanol >14.17%, 60% n-heptane-20% PODE3-20% ethanol >16.79%) in the fuel and typical functional group (which are free of C-C bonds) are attributed to the low PM emissions. n-Heptane plays an important role in enhancing the PM, HC, and CO oxidation. Third, high autoignition temperatures (397.11-460.26 degrees C at phi = 1.0-0.4) and the high autoignition resistance (RON = 108.6) of pure ethanol make it a poor candidate for HCAI combustion. Introducing high reactivity fuel to act as the chemical ignition source can enhance the combustion efficiency and PM oxidation. Fourth, PODE3 is a promising PM inhibitor, even better than ethanol. Its particle mass and number emissions at (I) = 0.4 are 94.23% and 2.32% lower than the pure ethanol. But PODE3 should not act as the sole chemical ignition source due to the relatively high autoignition temperature (342.59-360.12 degrees C at phi = 1.0-0.4) compared to n-heptane (around 283.86 degrees C). Introducing n-heptane into a PODE3 ethanol mixture can improve the combustion efficiency; therein, the combustion efficiency of 60% n-heptane-20% PODE3-20% ethanol reaches 93.57-98.20% at phi = 0.6-1.0.