Internal Combustion Engines

Spark-assisted Advanced Compression Ignition (SACI) Engines

Multi-Mode (MM) Combustion

Lean and dilute gasoline compression ignition (GCI) in spark ignition (SI) engines offers a promising pathway toward achieving high fuel efficiency while keeping NOx emissions low. However, this combustion strategy is generally constrained to low-load conditions because of the inherent difficulty in controlling autoignition. To address this limitation, multi-mode (MM) operation has been proposed, which applies advanced compression ignition (ACI) at low loads and conventional SI at higher loads. Yet, a major challenge lies in seamlessly switching between SI and ACI modes, since reliably sustaining these transitions (mixed-mode) under practical engine operating conditions remains difficult and has so far been demonstrated mostly under lab-scale studies. To characterize the mixed-mode operation, this study numerically explores a strategy of mode transition between SI and ACI combustion.

Argonne Experiment and CFD Simulations: DOE (Department of Energy) Co-Optima project

Argonne single cylinder engine test: The experimental activities were performed on a single cylinder GDI engine at Argonne National Laboratory (ANL). For ACI operation, the engine was equipped with a high compression ratio piston, but otherwise the standard GDI engine setup was retained.

CFD simulation test: A commercially available CFD tool package, CONVERGE (v2.4.20) was used to perform full open-cycle simulations of MM mode operation. Important dynamics of turbulent flame propagation was modeled by using newly developed hybrid combustion model (G-eqn.+SAGE) to successfully cover the transitional mode change.

New Chemical Kinetics Model for E30 Fuel (Supported by Prof. Tianfeng Lu's group at University of Connecticut)

The chemical kinetics behavior of BOB-E30 was modeled using a 211-species skeletal mechanism for Co-Optima core research fuels, including BOB-Alk, BOB-Aro, and BOB-E30, which is reduced from a 2878-species detailed mechanism for gasoline surrogates developed by the Lawrence Livermore National Laboratory. To this end, directed relation graph (DRG) was employed as the ﬁrst step to reduce the large mechanism with H radical selected as the starting species in DRG. Then the DRG aided sensitivity analysis (DRGASA) was applied to further eliminate unimportant species and reactions by specifying a worst-case relative error of 30% for ignition delay and extinction residence time in PSR. The obtained 211-species skeletal mechanism is then validated against the detailed mechanism in evaluating the ignition delay times as shown below:

Mixed-Mode Combustion: Ignition-Assisted Advanced Compression Ignition Combustion With Lean Mixture

This study created a virtual test condition for realizing mixed-mode while maintaining the same engine dimensions with the ACI and SI operations. The benchmarked engine was featured of direct injection spark ignition and explored to realize the spark ignition-assisted ACI (SACI) operation.

Figures on the left show the pressure and heat release rate traces obtained from a single simulation at the virtual test condition. Although no experimental data for direct comparison is available for validation purpose, the simulated condition apparently captures an evidence of ﬁnding mixed-mode combustion dynamics. Time sequence of ﬂame structures for two different combustion regimes is illustrated in the figures below; upper row with mixed-mode and lower row with SI only mode. These combustion regimes can be distinguished by ﬁnding the spontaneous ignition of unburnt gas. The mixed-mode combustion shown here, sequential event of SI initiated deﬂagration wave and subsequent end-gas autoignition, can also be distinguished by apparently fast ﬂame front progress rate indicated by the dark iso-contour surface. Therefore, it can be hypothesized that the fast burning ﬂame front pushes the end-gas mass toward cylinder wall. This may result in accelerating the end-gas heating and spontaneous combustion.

In the analysis of mixed-mode combustion, it is useful to utilize the end-gas reactivity as it pushes the spark-initiated flame propagation and leverages the ACI mode. In this context, pressure and temperature trajectory of the end-gas phase was captured after spark timing and displayed on the fuel-speciﬁc (E30 in this test) static ignition delay time. Symbols in the figure (left) mark sample points of CADs along the trajectory; hence one can see the progress rate of end-gas compression. From this analysis, it is found that the mixed-mode appeared to be promoted by the accelerated end-gas pressure rise by the ﬂame front, whereas the SI only mode failed to reach to a state of thermodynamic that can sufﬁciently initiate high-temperature kinetics.

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