Research Papers: Multiphase Flows

Multidimensional Modeling of Natural Gas Jet and Mixture Formation in Direct Injection Spark Ignition Engines—Development and Validation of a Virtual Injector Model

[+] Author and Article Information
Mirko Baratta1

Andrea E. Catania

Francesco C. Pesce

IC Engines Advanced Laboratory,  Dipartimento di Energetica, Politecnico di Torino, Corso Duca degli Abruzzi 24, Turin, 10129, Italyfrancesco.pesce@polito.it

Here the locution ‘Mach disk’ is not appropriate, since the shock has not a circular shape.


Corresponding author.

J. Fluids Eng 133(4), 041304 (May 12, 2011) (14 pages) doi:10.1115/1.4003877 History: Received July 29, 2010; Revised March 04, 2011; Published May 12, 2011

During the last few years, the integration of CFD tools in the internal combustion (IC) engine design process has continually increased, allowing time and cost savings as the need for experimental prototypes has diminished. Numerical analyses of IC engine flows are rather complex from both the conceptual and operational sides. In fact, these flows involve a variety of unsteady phenomena and the right balance between numerical solution accuracy and computational cost should always be reached. The present paper is focused on computational modeling of natural gas (NG) direct injection (DI) processes from a poppet-valve injector into a bowl-shaped combustion chamber. At high injection pressures, the gas efflux from the injector and the mixture formation processes include turbulent and compressible flow features, such as rarefaction waves and shock formation, which are difficult to accurately capture with numerical simulations, particularly when the combustion chamber geometry is complex and the piston and intake/exhaust valve grids are moving. In this paper, a three-dimensional moving grid model of the combustion engine chamber, originally developed by the authors to include simulation of the actual needle lift, has been enhanced by increasing the accuracy in the proximity of the sonic section of the critical valve-seat nozzle, in order to precisely capture the expansion dynamics the methane undergoes inside the injector and immediately downstream from it. The enhanced numerical model was then validated by comparing the numerical results to Schlieren experimental images for gas injection into a constant-volume bomb. Numerical studies were carried out in order to characterize the fuel-jet properties and the evolution of mixture formation for a centrally mounted injector configuration in the case of a pancake-shaped test chamber and the real engine chamber. Finally, the fluid properties calculated by the model in the throat section of the critical nozzle were taken as reference data for developing a new effective virtual injector model, which allows the designer to remove the whole computational domain upstream from the sonic section of the nozzle, keeping the flow properties virtually unchanged there. The virtual injector model outcomes were shown to be in very good agreement with the results of the enhanced complete injector model, substantiating the reliability of the proposed novel approach.

Copyright © 2011 by American Society of Mechanical Engineers
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Figure 1

Schematic representation of the NICE single-cylinder engine combustion chamber: Upper row, cylinder head; lower row, piston

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Figure 2

Effect of the nozzle pressure ratio on the Mach-disk location

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Figure 3

Mach-number profiles along the jet centerline for pressure ratios (prail /pchamber ) of 18.6, 46.5, and 81.4

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Figure 4

Influence of flow initialization strategy, t = 2 ms ASI. Left: strategy 1; center: strategy 2, T = T0 ; right: strategy 2, T = 1.25 T0 . Left side of pictures: velocity fields; right side: methane concentration isolines

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Figure 5

Pressure at injector inlet versus time for different flow initialization strategies: Left, strategy 1; right, strategy 2

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Figure 6

Normalized mass-flow rates at the injector inlet for different flow initialization strategies

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Figure 7

Normalized mass-flow rates at the injector inlet for different injector lengths (Initialization strategy 2, T = T0 )

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Figure 8

Normalized mass-flow rate and relative error versus GRP. Mass-flow rate is normalized by its final value

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Figure 9

Methane concentration isolines obtained with the original and with the high-accuracy model, at the indicated crank angles. Operating conditions: n = 3000 rpm, MAP = 1000 mbar, RAFR = 1.1

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Figure 10

Comparison between initial (left side) and final (right side) grid configurations

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Figure 11

Numerical model validation for injection in a constant-volume bomb, at the indicated operating conditions. Red lines bound the jet in the actual case; ref. case refers to Fig. 1a. Time: 1.2 ms ASI, Injected fluid: Nitrogen

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Figure 12

Influence of the upper-wall position on fuel concentration (a, b) and pressure (c, d) contours. Variable needle lift, t = 1 ms ASI. (a, c): slightly over the needle tip, (b, d): at same needle-tip level. Injected fluid: Methane

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Figure 13

Normalized static pressure along the jet centerline for different pressure ratios. The distance is normalized by the maximum needle lift. Tramp is the opening ramp duration value

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Figure 14

Computational grid used for SCE simulation. Left side: top view, center: bottom view, right side: section through the injector axis

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Figure 15

Influence of injection timing on RAFR distribution and fm value at 30° BTDC, for the SCE combustion chamber. Square symbols indicate the spark plug location in each picture

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Figure 16

Comparison between the complete numerical model with fixed needle lift (column 1, 3, 5) and the virtual injector one (column 2, 4, 6) in terms of methane concentration distribution, for different downstream pressures. Upper row: t/Tinj  = 0.5 ASI, lower row: t/Tinj  = 1 ASI

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Figure 17

Schematic of the procedure for virtual injector definition, in the case of variable needle lift

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Figure 18

Ramp slope dependence on chamber pressure

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Figure 19

Source cell inputs for the virtual injector model implementation (pchamber /prail  = 0.075)

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Figure 20

Comparison between the complete numerical model with variable needle lift (column 1, 3, 5) and the virtual injector one (column 2, 4, 6) in terms of methane concentration distribution, for different downstream pressures. Upper row: t/Tinj  = 0.25 ASI, lower row: t/Tinj  = 0.75 ASI

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Figure 21

Comparison between the complete numerical model with variable needle lift and the virtual injector one in terms of methane concentration distribution, for injection in the SCE combustion chamber at the indicated operating conditions



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