Spray characteristics of a gasoline-diesel blend (ULG75) using high-speed imaging techniques

Abstract

Partially-premixed compression-ignition (PPCI) is an advanced combustion mode that simultaneously reduces particulate and NOx emissions. Fuels with properties intermediate to that of gasoline and diesel are preferred for PPCI. This paper is a follow-on to a recent work published in Fuel from the authors’ research group on the topic of PPCI combustion with ULG75 (75 vol% gasoline/25 vol% diesel). That paper reported the opportunity of using ULG75 and hot external exhaust-gas-recirculation (EGR) for solving the low-load combustion-stability issue caused by the low reactivity of ULG75. That paper also concluded that ‘It was not possible to obtain stable ULG75 PPCI combustion without using low fuel injection pressure’. Additionally, from the point of ULG75’s volatility and viscosity, it does not require as high injection pressure as diesel. This paper assesses the spray characteristics (spray morphology, penetration length, cone angle and droplet size) of ULG75 at low injection pressures (Pinj = 35/75 MPa). In this work, high-speed macroscopic imaging (0.25 million fps) was carried out at backpressure (BP) of 0.1/4.0 MPa, and ambient temperature (Ta) of 25/125 °C. In addition, ultra-high speed microscopic imaging (1 million fps) was carried out at BP = 4 MPa and Ta = 25 °C. The same tests were conducted for diesel, and the results were used for comparisons. Under preheated ambient condition (Ta = 125 °C), ULG75 showed a noticeable improvement in the spray quality as compared to the non-evaporative condition (Ta = 25 °C), indicated by a shorter spray penetration length, larger spray cone angle and spray area. However, no such changes were observed for diesel spray upon increasing Ta from 25 °C to 125 °C. Both Pinj and BP strongly affected the spray penetration length evolution. However, they showed limited impacts on spray cone angle and spray area. In addition to the experimental study, empirical estimations were conducted to predict the spray penetration length, air entrainment and spray droplet size. The calculations predicted smaller SMD for ULG75 than diesel at all conditions and also indicated ambient temperature as a key factor affecting the spray quality, especially at low injection pressures.