Methane-diesel direct-injection combustion under engine-relevant conditions

This study investigates the effects of varying fuel injection sequence, dwell time, ambient temperature (780–1050 K) and oxygen ( O X 2 ) (21–10 vol.%) on ignition and development of interacting n -heptane (pilot fuel) and methane (main fuel) jet flames under engine-relevant conditions. The results show that under baseline conditions (890 K and 21 vol.% O X 2 ), when n -heptane is injected before methane (pilot-main injection sequence), with a longer dwell time, the burnt pilot-fuel products undergo mixing and cooling, necessitating prolonged jet–jet interaction before methane jet ignition. When n -heptane is injected after methane (main-pilot injection sequence), the amount of methane injected and mixed before pilot ignition affects the heat release, with a longer dwell time leading to a higher peak heat release. Regarding flame lift-off behavior, the entrainment of the pilot-fuel products into the methane jet leads to an upstream shift in the flame base. In the pilot-main injection case with the longest dwell time, the extended interaction period before main jet ignition allows the pilot-fuel products to interact with a larger portion of the methane jet. In some cases, this results in the formation of a detached ignition kernel upstream of the primary flame body downstream, impacting the flame stabilization and heat release characteristics. Under lower ambient temperature and reduced O X 2 conditions, the flame lift-off exhibits transient or no stabilization before the flame base shifts downstream. This occurs despite the continued entrainment of pilot-fuel products into the methane jet. • Optical parametric study of interacting n -heptane and methane jet flames under high-pressure high-temperature environment. • Detailed characterisation of flame development with the heat release rate evolution. • Different dwell times between n -heptane and methane injections affect the flame stabilization and heat release characteristics. • Reduced ambient temperature or oxygen concentration results in transient or no stabilization for the flame lift-off behaviour.