Optical Investigation of Ignition Timing and Equivalence Ratio in Dual-Fuel CNG/Diesel Combustion

Abstract

Dual-fuel engines are recognized as a short-medium term solution to reduce fuel consumption and pollutant emissions of CI engines, while maintaining high energy efficiency. Methane (CH 4) was chosen as it offers the best compromise between its heating value and H/C ratio. The high auto-ignition temperature of CH 4 requires auto-igniting a small quantity of liquid diesel before it initiates the combustion of the mixture. Therefore, new engine operations need to be specifically developed. This investigation explores the impact of time sequences of injection of the liquid fuel on the ignition of homogenous methane/air mixture. Experiments were performed on a Rapid Compression Expansion Machine (RCEM), to reproduce the operating and dynamic conditions encountered in a diesel engine cycle, allowing visualizations of fuel injection and combustion processes through a transparent piston. For the purpose of this work, the RCEM was modified to operate under dual-fuel conditions, while controlling the amount of diesel and methane-gas fueled. Experiments were performed for a wide range of equivalence ratios of the premixed charge. The study of the liquid fuel penetration and its consequence on igniting the homogenous charge was achieved using high-speed optical diagnostics. High-speed Schlieren technique (~22 kHz) was applied to characterize the diesel spray penetration, as well as the in-cylinder liquid fuel distribution. High-speed shadowgraphy and OH*-chemiluminescence techniques were used to determine ignitions delays. Moreover, the latest diagnostic was used to analyze the flame structure propagation and the heat release evolution.

Effect of methane on pilot-fuel auto-ignition in dual-fuel engines

“…Nonetheless, literature suggests a considerable increase of the pilot fuel ID in presence of premixed methane [5][6][7]. We systematically investigated the methane influence on pilot fuel auto-ignition.…”

“…Experiments are however reported for optical engines and Rapid Compression (Expansion) Machines, RC(E)M, e.g. [3][4][5][6][7] and references therein. These optical investigations of dual-fuel combustion mostly focused on the detection of ID [6,7], flame luminosity [3][4][5]7] and sooting propensity [4].…”

“…A common feature observed is the considerable prolongation of the pilot fuel ID in the presence of methane in the compressed charge [2,[5][6][7].…”

Impact of diesel pilot distribution on the ignition process of a dual fuel medium speed marine engine

“…This second stage, defined by HRF Ignition, depends on HRF fuel distribution and thermodynamic conditions (temperature, pressure and oxygen concentration) in the combustion chamber and this is where the ignition delay is determined [15]. This is usually characterized by ignition of the gas-mixture in the vicinity of the pilot spray [16]. Finally, LRF ignition and flame propagation take place in the remainder gas-air mixture and it highly depends on the reactivity of gas-mixture and the charge thermodynamic conditions, since the flammability of a natural gas mixture is highly affected by these parameters [17].…”

“…Pilot ignited dual fuel combustion is highly sensitive to mixture global equivalence ratio [16,21] [22]. Main engine specifications are presented The engine is instrumented with the necessary equipment required for its operation and control, as illustrated in Figure 2.…”