Research Papers

Fundamental Study of Diesel-Piloted Natural Gas Direct Injection Under Different Operating Conditions

[+] Author and Article Information
Georg Fink

Lehrstuhl für Thermodynamik,
Technische Universität München,
Garching 85748, Germany
e-mail: fink@td.mw.tum.de

Michael Jud

Lehrstuhl für Thermodynamik,
Technische Universität München,
Garching 85748, Germany
e-mail: jud@td.mw.tum.de

Thomas Sattelmayer

Lehrstuhl für Thermodynamik,
Technische Universität München,
Garching 85748, Germany
e-mail: sattelmayer@td.mw.tum.de

Manuscript received March 19, 2019; final manuscript received April 26, 2019; published online June 3, 2019. Editor: Jerzy T. Sawicki.

J. Eng. Gas Turbines Power 141(9), 091006 (Jun 03, 2019) (8 pages) Paper No: GTP-19-1136; doi: 10.1115/1.4043643 History: Received March 19, 2019; Revised April 26, 2019

Natural gas as an alternative fuel in engine applications substantially reduces both pollutant and greenhouse gas emissions. High pressure dual fuel (HPDF) direct injection of natural gas and diesel pilot has the potential to minimize methane slip from gas engines and increase the fuel flexibility, while retaining the high efficiency of a diesel engine. Speed and load variations as well as various strategies for emission reduction entail a wide range of different operating conditions. The influence of these operating conditions on the ignition and combustion process is investigated on a rapid compression expansion machine (RCEM). By combining simultaneous shadowgraphy (SG) and OH* imaging with heat release rate analysis, an improved understanding of the ignition and combustion process is established. At high temperatures and pressures, the reduced pilot ignition delay and lift-off length minimize the effect of natural gas jet entrainment on pilot mixture formation. A simple geometrical constraint was found to reflect the susceptibility for misfiring. At the same time, natural gas ignition is delayed by the early pilot ignition close to the injector tip. The shape of heat release is only marginally affected by the operating conditions and mainly determined by the degree of premixing at the time of gas jet ignition. Luminescence from the sooting natural gas flame is generally only detected after the flame extends across the whole gas jet at peak heat release rate. Termination of gas injection at this time was confirmed to effectively suppress soot formation, while a strongly sooting pilot seems to intensify soot formation within the natural gas jet.

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Fig. 1

RCEM test rig with optical setup and injector arrangement: (1) driving air supply, (2) driving piston, (3) hydraulic oil, (4) orifice, (5) working piston, (6) bypass valve, (7) combustion chamber, (8) natural gas injector, (9) diesel pilot injector, (10) arc lamp, (11) UV-filter, (12) pinhole, (13) parabolic mirror, (14) surface mirror, (15) 50/50 beam-splitter, (16) focusing lens, (17) shadowgraphy camera, (18) UV beam-splitter, (19) OH*-filter, (20) image intensifier, and (21) OH* camera

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Fig. 2

Multizone thermodynamic model of the combustion chamber

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Fig. 3

Heat loss model performance at OP1 and OP4 for an unfired case

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Fig. 4

Pilot ignition delay τign,p for variable jet interaction at OP1 (top) and OP4 (bottom)

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Fig. 5

Angular dependence of the pilot ignition delay at a relative gas injection timing of tinj = −0.5 ms

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Fig. 6

Angular dependence of the pilot lift-off at a relative gas injection timing of tinj = −0.5 ms and corresponding images at the time of pilot ignition

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Fig. 7

Left: natural gas ignition delay for variable jet interaction at OP1 (top) and OP4 (bottom); right: image sequence related to the point marked by (×) in the interaction map

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Fig. 8

Time of natural gas ignition after its SOI as measure for the degree of premixing at OP1 (top) and OP4 (bottom)

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Fig. 9

Heat release rates at OP1 and OP4 for different degrees of premixing (top) and SG/OH* image sequence representing medium premixing at characteristic times (bottom)



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