Effects of ambient conditions on the ignition and combustion characteristics of ignited natural gas under methane atmosphere

Utilizing a small amount of highly active fuel to ignite natural gas in compression ignition engines is an effective strategy. Research on the ignition and combustion characteristics of natural gas/air premixed gas is crucial yet visually understudied. This paper innovatively uses an externally heated constant volume combustion chamber (CVCC) to create high-temperature methane/air premixed gas and investigates its characteristics using high-activity fuel. Optical diagnostic tests with natural luminescence and schlieren methods were conducted under different ambient conditions. The study focused on the influence of energy substitution rates (ESRs) at various ambient pressures (Pambs) and temperatures (Tambs) on the ignition and combustion process. Results show that the flame luminescence undergoes three stages: diesel-dominated, diesel-CH4 mixed, and CH4-dominated combustion. A high ESR, achieved by increasing Pamb or Tamb, enhances cool flame and high-temperature reactions, increases ignited methane, shortens ignition delays (IDs), and extends flame durations. Pamb increases advance both second stage ignition (SSI) and ITs ((from flame & heat release rate (HRR)), while Tamb decreases have the opposite effect. A rise in Pamb also significantly increases the proportion of flame area relative to the high-temperature reaction zone.