Three-stage hybrid NSD/DC plasma assisted n-C5H12/O2/N2 ignition: Improved energy efficiency and reduced NOx/N2O emissions

Abstract

This work presents a three-stage hybrid nanosecond discharge (NSD) and direct current (DC) discharge assisted -CH/O/N ignition with lower discharge energy and higher energy efficiency. The reaction rate coefficients between O(aΔ) and -CH are updated by mass/Master equation simulation. The promotive ignition enhancement via reaction channel O(aΔ) + -CH → CH + HO in the hybrid NSD/DC discharge is demonstrated. The results show that the ignition delay time of the three-stage hybrid plasma assisted ignition is 1–2 orders of magnitude shorter than that of the two-stage hybrid discharge when the reduced electric field strength in the DC discharge stage is over 10 Td. Meanwhile, the plasma-enhanced energy efficiency of three-stage hybrid discharge increases. This improved efficiency on ignition enhancement is due to the kinetic enhancement by the electronically excited species and radicals (O(aΔ), O(D), N(B) and O). The kinetic and thermal effects by plasma are effectively intergated to enhance ignition. However, more discharge energy deposited in the vibrationally excited states N() in the two-stage hybrid discharge is less efficient on ignition enhancement due to the thermal contribution via vibrational-translational relaxation. In the aspect of NO/NO emissions, reduction efficiency of the three-stage hybrid discharge is two times higher than that of the two-stage hybrid discharge. By controlling the N/N(D) production pathway e + N → e + N + N(D), the three-stage hybrid discharge reduces NO and NO production by 61.1 % and 90 %, respectively. The results show that the ignition delay time is non-monotonically dependent on the discharge pulse number. There exist optimal pulse number, reduced electric field and effective threshold power density of DC discharge to achieve maximum ignition enhancement and low NO/NO emissions. This study provides valuable insights into the use of NSD/DC hybrid discharge technology in advanced engines to achieve energy-efficient ignition enhancement and reduce NO/NO emissions.