Investigation on soot emissions from diesel-CNG dual-fuel

Abstract

The burning of diesel and compressed natural gas (CNG) is attractive compared to diesel fuel because of the reduction of CO2 emissions and particulate matter (PM) emissions. While soot emissions from the diesel-CNG combustion can be tested in a real-world single-cylinder engine, the soot formation characteristics cannot be tested in the same way. Therefore, to understand the mechanisms behind soot formation in diesel-CNG combustion, soot evolution must be investigated using a simulation model. In this study, the soot evolution is investigated under different CNG substitution ratios with single and split fuel injection. An AVL 5402 single-cylinder diesel engine was modified to run diesel/CNG dual-fuel to investigate the combustion and soot emissions. A new soot model using KIVA-3V R2 code and integrated with a reduced heptane/methane PAH (polycyclic aromatic hydrocarbons) mechanism was used to simulate soot behavior. For the combustion, the results show that the ignition delay gets extended, the combustion duration gets shorter and the peak pressure can be improved when CNG substitution ratio is increased both with single and split injection. Additionally, a slight increase of pressure is observed when the split injection is used. This is because the split injection is an effective strategy to change the distribution and vaporization of fuel, which results in an incremental increase in combustion efficiency and increase pressure. As the CNG substitution ratio is increased, soot emissions get drastically reduced. The reason is the equivalence ratio distribution of air-fuel becomes more homogenous and the local fuel-rich region shrinks with increasing of CNG substitution ratios. Pyrene is an important intermediate specie to generate soot particles. The results show that pyrene distribution decreases, leading to a reduced generation of soot precursors. As a result, the soot mass of CNG70 is less than the other two cases. The basic reason is the prolonged ignition delay allowed for more time for fuel−air mixing, which reduces soot mass formation.