Verification of diesel spray ignition phenomenon in dual-fuel diesel-piloted premixed natural gas engine

Abstract

Dual-fuel (DF) engines, in which premixed natural gas and air in an open-type combustion chamber is ignited by diesel-fuel pilot sprays, have been more popular for marine use than pre-chamber spark ignition (PCSI) engines because of their superior durability. However, control of ignition and combustion in DF engines is more difficult than in PCSI engines. In this context, this study focuses on the ignition stability of n-heptane pilot-fuel jets injected into a compressed premixed charge of natural gas and air at low-load conditions. To aid understanding of the experimental data, chemical-kinetics simulations were carried out in a simplified engine-environment that provided insight into the chemical effects of methane (CH4) on pilot-fuel ignition. The simulations reveal that CH4 has an effect on both stages of n-heptane autoignition: the small, first-stage, cool-flame-type, low-temperature ignition (LTI) and the larger, second-stage, high-temperature ignition (HTI). As the ratio of pilot-fuel to CH4 entrained into the spray decreases, the initial oxidization of CH4 consumes the OH radicals produced by pilot-fuel decomposition during LTI, thereby inhibiting its progression to HTI. Using imaging diagnostics, the spatial and temporal progression of LTI and HTI in DF combustion are measured in a heavy-duty optical engine, and the imaging data are analyzed to understand the cause of severe fluctuations in ignition timing and combustion completeness at low-load conditions. Images of cool-flame and hydroxyl radical (OH*) chemiluminescence serve as indicators of LTI and HTI, respectively. The cycle-to-cycle and spatial variation in ignition extracted from the imaging data are used as key metrics of comparison. The imaging data indicate that the local concentration of the pilot-fuel and the richness of the surrounding natural-gas air mixture are important for LTI and HTI, but in different ways. In particular, higher injection pressures and shorter injection durations increase the mixing rate, leading to lower concentrations of pilot-fuel more quickly, which can inhibit HTI even as LTI remains relatively robust. Decreasing the injection pressure from 80 MPa to 40 MPa and increasing the injection duration from 500 µs to 760 µs maintained constant pilot-fuel mass, while promoting robust transition from LTI to HTI by effectively slowing the mixing rate. This allows enough residence time for the OH radicals, produced by the two-stage ignition chemistry of the pilot-fuel, to accelerate the transition from LTI to HTI before being consumed by CH4 oxidation. Thus from a practical perspective, for a premixed natural gas fuel–air equivalence-ratio, it is possible to improve the “stability” of the combustion process by solely manipulating the pilot-fuel injection parameters while maintaining constant mass of injected pilot-fuel. This allows for tailoring mixing trajectories to offset changes in fuel ignition chemistry, so as to promote a robust transition from LTI to HTI by changing the balance between the local concentration of the pilot-fuel and richness of the premixed natural gas and air. This could prove to be a valuable tool for combustion design to improve fuel efficiency or reduce noise or perhaps even reduce heat-transfer losses by locating early combustion away from in-cylinder walls.

Spatio-Temporal Progression of Two-Stage Autoignition for Diesel Sprays in a Low-Reactivity Ambient: n-Heptane Pilot-Ignited Premixed Natural Gas.

“…In general, irrespective of the pilot-fuel injection parameters, the increasing NG concentration increases the ignition delay of the pilotfuel quite significantly (up to 55% increase for ϕNG = 0.5). The ignition delay increase is nearly linear with ϕNG, which is in good agreement with existing literature [1,3,4,7,14,15]. The observed increase in ignition delay is predominantly attributed to the inhibition of low-30/10/2020 temperature pilot-fuel ignition chemistry by NG, and to lesser degrees, changes in the compressed-gas temperature at SOI resulting from changes in the mixture specific-heat ratio and lower oxygen concentration due to dilution of air by NG [13][14][15].…”

“…The fumigation of NG into the intake stream causes variations in the physical and transport properties of the mixture, such as the specific heat ratio and to a lesser extent the heat transfer parameters [12]. Furthermore, the reduced oxygen intake partial pressure (due to the displacement of air by NG), the inhibitive effect of NG on the preignition (cool-flame) reactivity of the pilot-fuel, along with changes to the associated heat release and change in the residual gas composition (significant only for continuous engine operation, not relevant for skipfire operation employed in the study) can cause significant changes in the progress of pre-ignition processes of the pilot-fuel thereby resulting in longer ignition delay period [13][14][15].…”

Influence of H2 enrichment for improving low load combustion stability of a Dual Fuel lightduty Diesel engine

“…Several authors, such as Papagiannakis et al 9 and Lounici et al, 10 carried out investigations demonstrating the beneficial effects of using NG in DF operations, in particular for the reduction of NO x and soot emissions and the enhancement of BTE. Detailed studies have also been carried out to better understand the patterns of DF combustion: Niki et al 11 investigated the effects of methane on pilot-fuel ignition by means of chemical-kinetics simulations and imaging diagnostics; Rochussen and Kirchen 12 characterized the reaction zone growth rate through the premixed natural gas using high-speed imaging of OH*-chemiluminescence; Rochussen et al 13 were able to identify five distinct DF combustion phases thanks to combined heat release rate and OH*-chemiluminescence reaction zone analysis. Furthermore, investigations have been conducted in order to analyze the influence of fundamentals parameters affecting DF combustion, such as: NG composition, 14,15 Exhaust Gas Recirculation (EGR), 16 diesel injection strategy (number of shots, 17 injection timing, 18 injection duration, 19 post injection strategy 20 ), injector spray included angle, 21 diesel injector nozzle hole size, 22 swirl ratio, 23 combustion chamber shape 24 ).…”

Mapping the combustion modes of a dual-fuel compression ignition engine

“…While diesel is the standard HRF, the typical LRF's studied in the literature vary by the type of engine being studied. Light-duty land vehicle engines typically use gasoline/ethanol as the LRF, [7][8][9] while marine engines typically usually use methane/natural gas, 10,11 and heavy-duty land vehicle engines have been used to study both of these primary fuel types, gasoline/ethanol, 12,13 and methane/natural gas. [14][15][16][17][18] These are general only general trends; a wide variety of novel LRF's have been studied in a wide variety of engine types, such as a gaseous dimethyl ether (DME)/propane mixture in a light-duty engine.…”

“…[14][15][16][17][18] These are general only general trends; a wide variety of novel LRF's have been studied in a wide variety of engine types, such as a gaseous dimethyl ether (DME)/propane mixture in a light-duty engine. 19 In addition to ITE and NO x /soot benefits, recent topics of RCCI research in just this journal include compositional effects of premixed LRF, 14 its effects on diesel spray ignition 10 and cyclic variability, 15 and the ability to operate these engines with constrained stochastic control 16 and sensor-based health management. 11 This paper intends to build on the existing state of the art by bridging the advanced mode of RCCI to the conventional mode of CDC via the intermediate mode of PDFC in an optimized multi-mode strategy.…”