Title: 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, here we focus 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 aremore » 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.« less

Authors:

Niki, Yoichi  ^[1];

• Search DOE PAGES for author "Niki, Yoichi"
• Search DOE PAGES for ORCID "0000-0001-6294-542X"

Rajasegar, Rajavasanth ^[2]; Li, Zheming ^[2]; Musculus, Mark PB ^[2]; Garcia Oliver, Jose Maria  ^[3];

• Search DOE PAGES for author "Garcia Oliver, Jose Maria"
• Search DOE PAGES for ORCID "0000-0002-2676-9681"

Takasaki, Koji ^[1]

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+ Show Author Affiliations

1. National Inst. of Maritime, Port and Aviation Technology, Tokyo (Japan)
2. Sandia National Lab. (SNL-CA), Livermore, CA (United States)
3. Polytechnic Univ. of Valencia (UPV) (Spain)

Publication Date:

Tue Dec 29 00:00:00 EST 2020

Research Org.:

Sandia National Lab. (SNL-CA), Livermore, CA (United States)

Sponsoring Org.:

USDOE Office of Energy Efficiency and Renewable Energy (EERE), Transportation Office. Vehicle Technologies Office; USDOE National Nuclear Security Administration (NNSA)

OSTI Identifier:

1760366

Report Number(s):

SAND-2020-13341J
Journal ID: ISSN 1468-0874; 692649

Grant/Contract Number:  

AC04-94AL85000; NA0003525

Resource Type:

Accepted Manuscript

Journal Name:

International Journal of Engine Research

Additional Journal Information:

Journal Volume: 23; Journal Issue: 2; Journal ID: ISSN 1468-0874

Publisher:

SAGE

Country of Publication:

United States

Language:

English

Subject:

42 ENGINEERING; dual-fuel engines; lean-premixed natural gas combustion; diesel; two-stage autoignition; cool-flame chemiluminescence; OH* chemiluminescence; ignition delay; chemical kinetics