In this experimental work, an optically accessible rapid compression machine is used to study the ignition and combustion process under engine relevant operation conditions for five different air-natural gas equivalence ratios (λ) ignited with either 12.2 mg or 6.7 mg of pilot diesel injected at 1,600 bar.Initial temperature of the ambient mixture, walls and injector was 333 K. Additionally, for the short (6.7 mg) diesel injection, the variation in the ID (ignition delay) for two higher ambient temperatures (343 K and 353 K) was measured.Pressure and piston displacement are recorded while two high-speed cameras simultaneously capture signals in the visible range spectrum and at 305 nm wavelength for OH* chemiluminescence respectively.ID is measured both from OH* and pressure rise.From the recorded data, the heat release ratio is estimated and compared with the visual signals.This gives an insight of the temporal and spatial evolution of the flame, as well as a qualitative perception of the transition from spray ignition into a premixed flame in the ambient fuel-air mixture.It was found that increasing the methane concertation delays the ignition, reduces the natural flame luminosity and enhances the OH* chemiluminescence signal.
