Evaluating temperature and fuel stratification for heat-release rate control in a reactivity-controlled compression-ignition engine using optical diagnostics and chemical kinetics modeling

Abstract

a b s t r a c tThe combustion process in a dual-fuel, reactivity-controlled compression-ignition (RCCI) engine is investigated using a combination of optical diagnostics and chemical kinetics modeling to explain the role of equivalence ratio, temperature, and fuel reactivity stratification for heat-release rate control. An optically accessible engine is operated in the RCCI combustion mode using gasoline primary reference fuels (PRF). A well-mixed charge of iso-octane (PRF = 100) is created by injecting fuel into the engine cylinder during the intake stroke using a gasoline-type direct injector. Later in the cycle, n-heptane (PRF = 0) is delivered through a centrally mounted diesel-type common-rail injector. This injection strategy generates stratification in equivalence ratio, fuel blend, and temperature. The first part of this study uses a high-speed camera to image the injection events and record high-temperature combustion chemiluminescence. The chemiluminescence imaging showed that, at the operating condition studied in the present work, mixtures in the squish region ignite first, and the reaction zone proceeds inward toward the center of the combustion chamber. The second part of this study investigates the charge preparation of the RCCI strategy using planar laser-induced fluorescence (PLIF) of a fuel tracer under non-reacting conditions to quantify fuel concentration distributions prior to ignition. The fuel-tracer PLIF data show that the combustion event proceeds down gradients in the n-heptane distribution. The third part of the study uses chemical kinetics modeling over a range of mixtures spanning the distributions observed from the fuel-tracer fluorescence imaging to isolate the roles of temperature, equivalence ratio, and PRF number stratification. The simulations predict that PRF number stratification is the dominant factor controlling the ignition location and growth rate of the reaction zone. Equivalence ratio has a smaller, but still significant, influence. Temperature stratification had a negligible influence due to the NTC behavior of the PRF mixtures.

An experimental investigation on the influence of piston bowl geometry on RCCI performance and emissions in a heavy-duty engine

“…Thus, RCCI concept allows an effective control of in-cylinder equivalence ratio and reactivity stratification [25] [26] through the gasoline fraction and direct injection timing variation [27][28][29][30]. That flexibility allows ultra-low NOx and soot emission levels with improved fuel consumption than conventional diesel combustion (CDC) in a wide engine operating range [31].…”

Effects of low reactivity fuel characteristics and blending ratio on low load RCCI (reactivity controlled compression ignition) performance and emissions in a heavy-duty diesel engine

“…The consequent increase in temperature and pressure initiates a reaction zone, identified in literature as an auto-ignition or flame propagation depending on equivalence ratio conditions, which proceed gradually from high to low reactivity regions of the combustion chamber [39][40] [41]. Focusing on the RoHR profiles in Figure 4, it is clear that the SOC pattern of the high temperature heat release (HTHR) stages between the different fuels is the same independently on PER.…”

Cycle to Cycle Variation Study in a Dual Fuel Operated Engine

“…Various reasons for this include non-optimal performance of the diesel injector (variability in relative magnitude of mass injected, variability in start of diesel injection, variability in injection duration, fuel nozzle hole-to-hole variability in diesel injected) [3][4], variability in the amount of trapped NG, stratification of NG in the cylinder [5][6] and the variability in the operating and boundary conditions (pressure, temperature and internal residuals). All these potentially affect the start, extent (depending on trapped NG) and rate of the combustion of pre-mixed NG and thus lead to typically higher CCV than in diesel engines.…”