An Experimental and Simulation Study of Early Flame Development in a Homogeneous-charge Spark-Ignition Engine
Oil & Gas Science and Technology - Rev. IFP Energies nouvelles
An Experimental and Simulation Study of Early Flame Development in a Homogeneous-Charge Spark-Ignition Engine
Y. Shekhawat 2 3
D.C. Haworth 2 3
A. d'Adamo 1 3
F. Berni 1 3
S. Fontanesi 1 3
P. Schiffmann 0 3
D.L. Reuss 0 3
V. Sick 0 3
0 Department of Mechanical Engineering, University of Michigan , Ann Arbor, MI - USA
1 Department of Engineering “Enzo Ferrari”, University of Modena and Reggio Emilia , Modena - Italy
2 Department of Mechanical & Nuclear Engineering, The Pennsylvania State University , University Park, PA - USA
3 Clearance @ TDC , cm
- An integrated experimental and Large-Eddy Simulation (LES) study is presented for homogeneous premixed combustion in a spark-ignition engine. The engine is a single-cylinder two-valve optical research engine with transparent liner and piston: the Transparent Combustion Chamber (TCC) engine. This is a relatively simple, open engine configuration that can be used for LES model development and validation by other research groups. Pressure-based combustion analysis, optical diagnostics and LES have been combined to generate new physical insight into the early stages of combustion. The emphasis has been on developing strategies for making quantitative comparisons between high-speed/high-resolution optical diagnostics and LES using common metrics for both the experiments and the simulations, and focusing on the important early flame development period. Results from two different LES turbulent combustion models are presented, using the same numerical methods and computational mesh. Both models yield Cycle-to-Cycle Variations (CCV) in combustion that are higher than what is observed in the experiments. The results reveal strengths and limitations of the experimental diagnostics and the LES models, and suggest directions for future diagnostic and simulation efforts. In particular, it has been observed that flame development between the times corresponding to the laminar-to-turbulent transition and 1% mass-burned fraction are especially important in establishing the subsequent combustion event for each cycle. This suggests a range of temporal and spatial scales over which future experimental and simulation efforts should focus.
INTRODUCTION
Cycle-to-Cycle Variations (CCV) of flow and combustion in
spark-ignition engines have been the subject of extensive
research over the last few decades [
1
].
High-speed/highresolution optical diagnostics and Large-Eddy Simulation
(LES) have been brought to bear to understand the root
causes of CCV, and significant progress has been made in
that regard. Multiple sources of variability have been
identified. These include local variations in the flow,
temperature and mixture composition, and in the spark
discharge, as well as global variations in equivalence ratio,
dilution and trapped mass [
2–5
]. While these contributing
factors have been recognized for some time, their
interactions and influences on combustion remained unclear
due to a lack of multi-diagnostic data and the difficulty of
performing accurate LES simulations over a sufficiently
large number of cycles. Lacour and Pera [5] and Baum et al.
[
6
] performed multi-diagnostic experiments that allow a
deeper understanding of the coupled physics in engines. And
recently these interactions have been investigated using
multi-parameter experimental [
6, 7
] and simulation [
6, 8
]
approaches.
Earlier experimental and simulation studies (e.g., [
9, 10
])
have shown that what happens during the earliest stages of
combustion can determine, to a large extent, the subsequent
combustion process for that cycle. Under realistic engine
operating conditions, the velocity magnitude [
7, 8
] and
velocity gradient parameters [
7
] in the vicinity of the spark
plug at the time of ignition have been found to correlate with
the Indicated Mean Effective Pressure (IMEP) for the cycle.
Motivated by the difficulties of making accurate
pressurebased combustion measurements during the early flame
development period, some LES studies have focused on
early-flame-kernel development, toward developing an
understanding of the governing factors that result in cyclic
variation of flame growth. Granet et al. [
11
] demonstrated
that initial flame convection within the spark plug gap is a
probable reason for incomplete combustion cycles due to
local quenching, while Goryntsev et al. [
12, 13
] emphasized
the superposition of flow variation and mixture quality for
direct-injection engines, whose combined effect on
combustion CCV was elucidated by LES.
For these reasons, the focus here is on the early burn:
ignition through fully developed turbulent flame. A specific
goal is to assess the predictive capability of two different
ignition and turbulent flame propagation models, using the
same CFD code and computational mesh. The approach is to
compare the simulated and measured combustion using a
single set of metrics that are ap (...truncated)