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Simulation of the Mixture Preparation for an SI Engine using Multi-Component
Fuels
ICE Workshop, STAR Global Conference 2012 March 19-21 2012, Amsterdam
Michael Heiss, Thomas Lauer
Simulation of the Mixture Preparation for an SI Engine using Multi-Component Fuels Michael Heiss | Sheet 2
Content
Introduction & Motivation
Mesh Description
Definition and Implementation of Multi-Component Fuels
Verification of Spray Dynamics
Analysis of Piston Cooling due to Wall Wetting
Wall Film Formation for Single- and Multi-Component Fuels
Conclusion
Summary and Outlook
Simulation of the Mixture Preparation for an SI Engine using Multi-Component Fuels Michael Heiss | Sheet 3
Introduction
In order to reduce soot emissions and oil dilution of future SI-engines with direct injection a fundamental knowledge about wall film formation and its prediction is crucial.
In the presented work, a turbocharged SI engine with direct injection was used to analyze the influence of different injection timings on the wall film formation.
Numerical investigations and accompanying experiments at the engine test bench have been carried out to show the benefit of the multi-component fuel approach.
Simulation of the Mixture Preparation for an SI Engine using Multi-Component Fuels Michael Heiss | Sheet 4
Motivation
abrupt increase of soot formation
The measured soot increased abruptly after a critical threshold of the injection start timing (SOI) was exceeded.
Soot is an indicator for a diffusive combustion of wall film Analysis of the differences in wall film formation depending on SOI
high
low
20°CA
early late
Simulation of the Mixture Preparation for an SI Engine using Multi-Component Fuels Michael Heiss | Sheet 5
Mesh Description
6-hole injector
• Mapped meshing in es-ice V4.16
• detailed modelling of the spark plug geometry
• thin boundary layers
Simulation of the Mixture Preparation for an SI Engine using Multi-Component Fuels Michael Heiss | Sheet 7
Fuel Definition RON95 I
7 component approach having the same distillation curve as gasoline reference fuel
Batteh, J. J.; Curtis, E. W.: Modeling Transient Fuel Effects with Alternative Fuels, SAE Paper 2005-01-1127
high boiling temperature
low-boiling temperature
Simulation of the Mixture Preparation for an SI Engine using Multi-Component Fuels Michael Heiss | Sheet 8
Fuel Definition RON95 II
Simulation of the Mixture Preparation for an SI Engine using Multi-Component Fuels Michael Heiss | Sheet 9
Fuel Definition RON95 III
Implementation in user subroutine dropro.f
Bulk Properties:
Calculated for the 7 component mixture (density, viscosity, surface tension coefficient, thermal conductivity)
Component properties:
Vapour pressure for each component as temperature dependant functions having the form of the Clausius Clapeyron equation: y = y0 + C1 e
T C2
Simulation of the Mixture Preparation for an SI Engine using Multi-Component Fuels Michael Heiss | Sheet 10
Spray Dynamics – Verification with Optical Measurements
50 mm
10°CA after injection start
25 mm
1°CA after injection start
Droplet spectrum definition according to PDA spray measurements
Good correlation of spray penetration
Simulation of the Mixture Preparation for an SI Engine using Multi-Component Fuels Michael Heiss | Sheet 11
Analysis of Piston Crown Cooling due to Wall Film Wetting
Solid Piston Cells (Aluminium)
Constant Ambient Temperature
Calculation of the piston temperature considering evaporative cooling and heat conduction
Fluid Cells
Static Mesh @ 450°CA Motivation: Checking if a constant piston wall temperature is acceptable A moving mesh with solid cells is not possible yet. Therefore, the analysis was performed on a static mesh.
Simulation of the Mixture Preparation for an SI Engine using Multi-Component Fuels Michael Heiss | Sheet 12
Liquid Film Thickness [µm]
Analysis of Piston Crown Cooling due to Wall Film Wetting
Formation of Liquid Film
Δ Temperature [K]
0
-5
A maximum cooling of ΔT ~5 K was calculated.
Using a constant piston crown wall temperature is justified.
Simulation of the Mixture Preparation for an SI Engine using Multi-Component Fuels Michael Heiss | Sheet 13
Investigated Injection Timings
“late injection”
“early injection”
high
20°CA
low
Simulation of the Mixture Preparation for an SI Engine using Multi-Component Fuels Michael Heiss | Sheet 14
Formation of Liquid Film, “late injection”
1-Component Fuel Multicomponent Fuel
Liquid Film Thickness [µm]
Simulation of the Mixture Preparation for an SI Engine using Multi-Component Fuels Michael Heiss | Sheet 15
Investigated Injection Timings
“late injection”
“early injection”
high
low 20°CA
Simulation of the Mixture Preparation for an SI Engine using Multi-Component Fuels Michael Heiss | Sheet 16
Formation of Liquid Film, “early injection“
1-Component Fuel Multicomponent Fuel
Liquid Film Thickness [µm]
Simulation of the Mixture Preparation for an SI Engine using Multi-Component Fuels Michael Heiss | Sheet 17
Comparison with Smokemeter Measurements
The remaining wall film mass correlates with the measured smoke number
high
low
late injection early injection
Simulation of the Mixture Preparation for an SI Engine using Multi-Component Fuels Michael Heiss | Sheet 18
Multi-Component Liquid Film Composition, “early injection“
high boiling temperature
low-boiling temperature
injection
Components with lowest mass fractions in the fuel definition
Simulation of the Mixture Preparation for an SI Engine using Multi-Component Fuels Michael Heiss | Sheet 19
Vapour Sources, “early injection“
50°CA
Simulation of the Mixture Preparation for an SI Engine using Multi-Component Fuels Michael Heiss | Sheet 20
Mixture Preparation @ Spark Timing 717°CA, “early injection“
1-Component Fuel Multi-Component Fuel
Lambda [-]
4% higher global lambda
12% higher lambda close to the spark plug
Simulation of the Mixture Preparation for an SI Engine using Multi-Component Fuels Michael Heiss | Sheet 21
Mixture Preparation @ Spark Timing 717°CA, “late injection”
1-Component Fuel Multi-Component Fuel
Lambda [-]
global lambda is equal
lambda distribution is similar
Simulation of the Mixture Preparation for an SI Engine using Multi-Component Fuels Michael Heiss | Sheet 22
Conclusion
The CFD simulation for an early injection timing showed significant differences between the single- and multi-component fuel approach.
The intensified wall wetting and the remaining wall film mass after compression for early injection timings could only be represented correctly with the multi-component fuel definition.
The fast wall film vaporisation of the single-component fuel led to an overestimation of the charge homogenisation and the fuel vapour fraction especially close to the spark plug.
For operating points where only low wall film masses are to be expected the single-component approach is a reasonable simplification.
Simulation of the Mixture Preparation for an SI Engine using Multi-Component Fuels Michael Heiss | Sheet 23
Summary and Outlook
At the test bench a sharp increase of soot emissions was measured when the start of injection exceeded a critical limit.
With a 7 component fuel approach it was possible to calculate a remaining wall film mass after compression that correlated with the measured soot number.
In contrast, the single-component fuel led to considerable differences in wall film mass and lambda distribution for an early injection.
Therefore, the multi-component fuel approach has a big potential especially for operating points with intensified wall wetting e.g. cold start conditions and for a following combustion calculation where lambda needs to be as accurate as possible.
For alternative fuels with a higher heat of vaporisation like ethanol, a moving mesh simulation with solid piston cells would be necessary to account for the intensified wall cooling.
