Optical investigation of methane, ethane, or propane and diesel at low load RCCI conditions

Abstract

Natural luminosity (NL) imaging diagnostics were employed to investigate the effects of natural gas (NG) components on reactivity-controlled combustion, using an ultra-low sulfur diesel fuel and three NG major components (methane, ethane, and propane). The experiments were performed in a single-cylinder heavy-duty optical compression-ignition (CI) engine at 500 bar diesel injection pressure, 1000 RPM, ∼ 6.6 bar IMEP, and ∼ 64% energy-based diesel substitution ratio. NL confirmed the premixed nature of the combustion, which started inside the squish area and propagated towards the center of the combustion chamber. The high intensity NL regions that appeared towards the end of the premixed combustion were the result of the diffusive combustion of end-of-injection fuel dribble or of the diesel fuel that condensed on the bowl window surface or accumulated in the bowl straight corners (typical of an optical engine piston). The time it took for the spatially integrated NL (SINL) to decrease from its maximum to its minimum was similar for all investigated cases, suggesting that the gas addition did not change the fraction of the diesel fuel that participated in the diffusion-controlled combustion. Differences in oxidation properties between NG components affected the RCCI combustion at these low load conditions. Propane RCCI was characterized by 1 crank angle (CA) to 3 CA more advanced combustion phasing (despite an increased ignition delay) and higher SINL compared to methane and ethane RCCI. However, as NL for the propane RCCI was lower at similar combustion phasing, it suggests that propane addition created a more stratified diesel-gas mixture, with a richer mixture inside the squish volume, supported by faster and more intense combustion compared to the other two RCCI cases. As a result, the flame speed slowed as it progressed from the squish volume towards the center of the combustion chamber, therefore the delay in capturing the premixed combustion in the NL images. The amount of fuel that burned after the premixed combustion completely engulfed the piston bowl was relatively low compared to fraction of the fuel that burned before it. Finally, the results suggest the importance of tailoring the low–high fuel reactivity ratio of the dual-fuel combustion based on the operating condition.