Characterization of reaction zone growth in an optically accessible heavy-duty diesel/methane dual-fuel engine

Self Cite

Natural gas is an attractive fuel for internal combustion engines in light of its potential for reduced greenhouse gas and particulate emissions, and significant reserves. To facilitate natural gas use in compression ignition engines, pilot-ignited direct-injection natural gas combustion uses a small pilot injection of diesel to ignite a more significant direct injection of natural gas. Compared to modern diesel combustion, this strategy is a promising technology for the reduction of CO2 emissions while retaining diesel-like efficiency without a significant CH4 emission penalty. To further develop this technology, investigation of in-cylinder combustion processes is needed to identify the primary fuel conversion processes. The objective of this work was to provide a framework of conceptual understanding by identifying key processes in a typical pilot-ignited direct-injection natural gas combustion event and characterizing their sensitivity to fuel injection parameters. A parametric sweep of injection pressure, natural gas injection duration, and relative timing of the diesel pilot and natural gas injections was performed in an optically accessible 2 L single-cylinder engine. Combined heat release rate and OH*-chemiluminescence reaction zone analysis was used to demarcate the transition from ignition reactions to primary natural gas heat release. Five distinct combustion processes were identified: (1) pilot auto-ignition; (2) natural gas ignition; (3) rapid, distributed partially premixed natural gas combustion; (4) non-premixed combustion; and (5) late-cycle oxidation. While natural gas ignition was found to be insensitive to injection pressure, it was strongly affected by the time between pilot and natural gas injections. Reducing the relative injection timing from +8° to −6° resulted in the primary natural gas heat release transitioning from non-premixed, to distributed partially premixed, to stratified premixed flame propagation as a result of increasing natural gas premixing. The presented measurements and analysis serve to refine an initial conceptual model of the combustion process and lay the groundwork for future, more focused studies of pilot-ignited, direct-injection natural gas combustion.

Section: Experimental Facility and Methodsmentioning

confidence: 99%

Section: Pilot and Ng Ignitionmentioning

confidence: 99%

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Parametric study of pilot-ignited direct-injection natural gas combustion in an optically accessible heavy-duty engine

Rochussen

McTaggart-Cowan²,

Kirchen

2019

Self Cite

“…The idea of DF combustion in CI engines is to burn one (or a combination of) low reactivity fuel(s) (LRF), such as methane, with the assistance of a high reactivity fuel (HRF), such as diesel. With relevance to this study, pilot HRF is injected to the lean mixture of premixed LRF/air to initiate ignition and burn interactively with LRF [1][2][3][6][7][8][9][10][11] . Typically, diesel is injected in small amounts to avoid high emissions, however, it should release sufficient energy to ignite the LRF under desired conditions.…”

Section: Introductionmentioning

confidence: 99%

Large eddy simulation of diesel spray–assisted dual-fuel ignition: A comparative study on two n-dodecane mechanisms at different ambient temperatures

Kannan

Gadalla

Tekgül

et al. 2020

In dual-fuel compression ignition engines, a high-reactivity fuel, such as diesel, is directly injected to the engine cylinder to ignite a mixture of low-reactivity fuel and air. This study targets improving the general understanding on the dual-fuel ignition phenomenon using zero-dimensional homogeneous reactor studies and three-dimensional large eddy simulation together with finite-rate chemistry. Using the large eddy simulation framework, n-dodecane liquid spray is injected into the lean ambient methane–air mixture at [Formula: see text]. The injection conditions have a close relevance to the Engine Combustion Network Spray A setup. Here, we assess the effect of two different chemical mechanisms on ignition characteristics: a skeletal mechanism with 54 species and 269 reaction steps (Yao mechanism) and a reduced mechanism with 96 species and 993 reaction steps (Polimi mechanism). Altogether three ambient temperatures are considered: 900, 950, and 1000 K. Longer ignition delay time is observed in three-dimensional large eddy simulation spray cases compared to zero-dimensional homogeneous reactors, due to the time needed for fuel mixing in three-dimensional large eddy simulation sprays. Although ignition is advanced with the higher ambient temperature using both chemical mechanisms, the ignition process is faster with the Polimi mechanism compared to the Yao mechanism. The reasons for differences in ignition timing with the two mechanisms are discussed using the zero-dimensional and three-dimensional large eddy simulation data. Finally, heat release modes are compared in three-dimensional large eddy simulation according to low- and high-temperature chemistry in dual-fuel combustion at different ambient temperatures. It is found that Yao mechanism overpredicts the first-stage ignition compared to Polimi mechanism, which leads to the delayed second-stage ignition in Yao cases compared to Polimi cases. However, the differences in dual-fuel ignition for Polimi and Yao mechanisms are relatively smaller at higher ambient temperatures.

“…To ignite such a mixture, high ignition energies are needed 7 which can typically be provided either by pilot injection of a high-reactivity fuel (e.g. diesel, 8 alternatives such as poly(oxymethylene) dimethyl ethers (OME) 9 or blends), 10 by turbulent jet ignition using a prechamber or by alternative ignition concepts such as those discussed in previous studies. 11,12 As pilot injection requires a second fuel in combination with a comparably expensive high-pressure injection system and as pilot injection has problems to provide stable ignition with low pilot quantities at low load, 13,14 the use of a prechamber seems to be more attractive for light-duty vehicle use.…”

Section: Introductionmentioning

confidence: 99%

Efficient light-duty engine using turbulent jet ignition of lean methane mixtures

Soltic

Hilfiker

Hänggi

2019

Diesel engines use diffusion-controlled combustion of a high-reactivity fuel and offer high efficiencies because they combine lean combustion with a high compression ratio. For low-reactivity fuels such as gasoline or natural gas, premixed combustion is used, which leads to lower efficiency levels as usually stoichiometric combustion is combined with lower compression ratios. Trying to apply diesel-like process parameters to low-reactivity fuels inevitably leads to problems with classical spark ignition systems as they are not able to establish robust flame propagation for such hard-to-ignite conditions. One possibility to enable fast combustion for diluted mixtures at high pressure levels is to establish ignition in a prechamber and ignite the charge of the main combustion chamber using the turbulent jets exiting the prechamber. In this study, the experimental results of a prechamber-equipped four-cylinder natural gas engine with 2 L displacement are discussed in detail. In the majority of the engine map, auxiliary fueling is used in the prechamber and a global air–fuel equivalence ratio λ is set to 1.7. At full load, a λ of 1.5 is applied without auxiliary prechamber fueling. The experiments show that such a setup is able to achieve brake efficiency levels of above 45% while maintaining peak brake mean effective pressure levels above 20 bar. At high load conditions, cylinder pressure levels at ignition timing achieve more than 80 bar and cylinder peak pressures of around 180 bars occur. The technology proved to enable robust and very fast combustion at comparably low NOx levels. A remaining challenge for the on-road use of such a technology is the reduction of the methane emissions at lean conditions.