Exergy loss characteristics of DME/air and ethanol/air mixtures with temperature and concentration fluctuations under HCCI/SCCI conditions: A DNS study
Introduction
Decades ago, the homogeneous-charge compression ignition (HCCI) concept [1], [2], [3] started a new stream of internal combustion (IC) research towards higher efficiency and lower emissions by allowing lean combustion at higher compression ratio. Due to its limited operational range [4], [5], however, the HCCI strategies have since been modified by introducing some level of mixture inhomogeneities in order to moderate the combustion duration and to mitigate the rapid pressure rise [6], [7], thus enhancing the combustion control at wider operating conditions [8], [9]. Examples include stratified-charge compression ignition (SCCI) and reactivity-controlled compression ignition (RCCI) [10], [11], [12] utilizing multiple injections and dual-fuel strategies. At high-load conditions, the temperature and composition inhomogeneities lead to a smooth combustion process with lower peak heat release rate which exhibits a mixed combustion mode [13], [14].
Since the efficiency is of primary concern, a systematic thermodynamic analysis has been conducted to assess the ideal and practical efficiency limits of different IC engine concepts and implementations. The first law of thermodynamics is commonly used and quantifies the energy flow into useful work conversion and various types of losses, such as brake work, exhaust, heat transfer, friction, unburned fuel, and so on [15], [16], [17]. However, the energy-based first-law analysis only considers the quantity of the energy conversion, not the quality. The latter is more properly analyzed by the fuel availability (exergy) based on the second law of thermodynamics, serving as an alternative metric for efficiency [18], [19], [20].
According to the second-law analysis, approximately a third of the fuel exergy is destroyed in a conventional combustion process due to the inherent irreversibility of chemical reactions [21]. The exergy losses reduce the maximum output power and the engine efficiencies [22], [23]. To assess its impact more broadly, efforts were made to evaluate the magnitude of the exergy destruction in IC engines under different combustion modes. Li et al. [24] numerically compared the second-law efficiencies of three combustion regimes, HCCI, RCCI, and conventional diesel combustion (CDC), and reported that the exergy destruction in CDC was significantly higher. Rangasamy et al. [25] also made similar conclusions in their parametric study of the exergy loss in a methanol/diesel or methanol/biodiesel dual-fuel engines in CDC and RCCI modes. Exergy loss is caused by all the irreversibilities during combustion processes that are much more than just the chemical bond energy releases associated with elementary reactions. As indicated by Nishida et al. [26], the sources of the total exergy loss include chemical reaction, heat conduction, mass diffusion and viscous dissipation. Therefore, to better understand the exergy destruction characteristics of different combustion regimes, the specific sources causing exergy destruction under HCCI, RCCI and CDC modes were investigated by Li et al. [27]. The results revealed that for all combustion regimes, chemical reaction is the dominant mechanism for the exergy destruction, followed by heat conduction and mass diffusion. The larger exergy loss in the CDC mode was attributed to the enhanced heat conduction and mass transfer due to the excessive gradients of in-cylinder temperature and fuel concentration during the mixing-controlled combustion process. On the other hand, the exergy destruction in RCCI was found to be lower than that in HCCI due to the slower heat release rate (HRR).
While these studies provided general understanding of exergy destruction mechanisms in IC engines as qualitative behavior, more fundamental investigations are needed to understand how the exergy losses due to all irreversible sources are affected by the cumulative outcome of the local fluctuations in temperature and composition in the mixture. Furthermore, a detailed chemical pathway analysis is needed to identify the dominant reaction steps contributing to the exergy losses at different thermodynamic conditions. For HCCI conditions, the dominant reactions contributing to the exergy loss were investigated in perfectly stirred reactors for different kinds of fuels, such as DME [28], methanol [29], -heptane [29], [30], and -octane [30], [31]. For HCCI combustion with the thermal and compositional uniformities, not only the reactions primarily contributing to heat release such as CO OH CO H are identified as the main source of exergy loss, the reactions of H O2 (M) HO (M) and HCO (M) H CO (M), which partially contribute to the heat release, are also identified as one of primary contributors to exergy loss. However, the dominant reaction steps contributing to the exergy loss under SCCI conditions with a high level of temperature and concentration inhomogeneities remain unclear.
Therefore, the objectives of the present study are (1) to investigate the effects of temperature and concentration fluctuations on the exergy destruction in combustion processes under a wide range of conditions relevant to HCCI/SCCI modes, (2) to identify and compare the primary sources causing exergy destruction under HCCI/SCCI conditions by chemical kinetic analysis, and (3) to examine the relationship between the relative change of exergy destruction and the levels of fluctuations. As a well-defined parametric study, two-dimensional (2-D) direct numerical simulation (DNS) data [32], [33] are used to explore different initial mean temperature and thermal and/or concentration fluctuation levels at high pressure as typically encountered in IC engines. For simplicity, a single fuel SCCI mode is considered, while dimethyl ether (DME) and ethanol, representing fuels with and without negative temperature coefficient (NTC) behavior, respectively, are adopted for additional insights into the complex fuel chemistry effects. By performing a quantitative analysis based on the second law of thermodynamics, the effects of different kinds of inhomogeneities on the exergy loss, as reported in the literature for HCCI, SCCI and RCCI, are generalized. The results show how much thermal and compositional inhomogeneities can decrease the exergy loss, especially by promoting combustion in the deflagration mode, and the chemical reactions are shown to be the main source of the exergy loss.
Section snippets
Exergy analysis
The entropy generation rates induced by different irreversible sources are obtained by solving the entropy transport equation [30], [34]. Neglecting the effects of body force and viscous dissipation [35], [36], [37], [38], the local entropy generation rates due to heat conduction, mass diffusion, and chemical reaction are respectively calculated as:
Overall characteristics of the exergy loss
Figure 3 shows the ELR of the DME/air mixture for Cases 1–12 with negatively-correlated and uncorrelated fluctuations at three different of 770 K, 900 K and 1045 K, which lie within the low-, intermediate- and high-temperature regimes, respectively (see Fig. 1 and Table 1). All ELR quantities are normalized by ELR the maximum ELR for the corresponding 0-D case. Specifically, for the cases with of 700 K, the ELR curve for the 0-D case in Fig. 3a shows a distinct two-stage ignition
Conclusions
The exergy loss characteristics of DME/air and ethanol/air mixtures were investigated by analyzing a 2-D DNS data set. Exergy loss analysis was carried out over a wide range of thermodynamic conditions under HCCI and SCCI modes with different levels of temperature and concentration fluctuations, and the main conclusions are summarized as follows:
- (1)
For SCCI combustion, it is found that the addition of and significantly decreases the peak magnitude of the ELR and prolongs the combustion
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors would like to thank Prof. Tianfeng Lu for providing the code to leverage between the reduced mechanism and the skeletal mechanism. This work was sponsored by the research funding from King Abdullah University of Science and Technology, and National Natural Science Foundation of China (Grant Nos. 51861135303 and 51776124). This research used the computational resources of the KAUST Supercomputing Laboratory (KSL).
References (92)
- et al.
Fuel design and management for the control of advanced compression-ignition combustion modes
Prog. Energy Combust. Sci.
(2011) - et al.
Characterization of knocking combustion in HCCI DME engine using wavelet packet transform
Appl. Energy
(2010) Advanced compression-ignition enginesunderstanding the in-cylinder processes
Proc. Combust. Inst.
(2009)- et al.
Review of high efficiency and clean reactivity controlled compression ignition (RCCI) combustion in internal combustion engines
Prog. Energy Combust. Sci.
(2015) - et al.
Controlled three-stage heat release of stratified charge compression ignition (SCCI) combustion with a two-stage primary reference fuel supply
Fuel
(2011) The thermodynamic characteristics of high efficiency, internal-combustion engines
Energy Convers. Manage.
(2012)- et al.
Second law analysis of a low temperature combustion diesel engine: effect of injection timing and exhaust gas recirculation
Energy
(2012) - et al.
Second-law analyses applied to internal combustion engines operation
Prog. Energy Combust. Sci.
(2006) - et al.
Thermodynamic energy and exergy analysis of three different engine combustion regimes
Appl. Energy
(2016) - et al.
A comprehensive parametric, energy and exergy analysis for oxygenated biofuels based dual-fuel combustion in an automotive light duty diesel engine
Fuel
(2020)
Analysis of entropy generation and exergy loss during combustion
Proc. Combust. Inst.
Comprehensive analysis of exergy destruction sources in different engine combustion regimes
Energy
Exergy losses in auto-ignition processes of DME and alcohol blends
Fuel
Molecular Theory of Gases and Liquids
Analysis of entropy generation in a hydrogen-enriched turbulent non-premixed flame
Int. J. Hydrog. Energy
Second-law thermodynamic analysis in premixed flames of ammonia and hydrogen binary fuels
J. Eng. Gas Turb. Power
Direct numerical simulations of reacting flows with shock waves and stiff chemistry using many-core/GPU acceleration
Comput. Fluids
Several new numerical methods for compressible shear-layer simulations
Appl. Numer. Math.
Cantera: An Object-Oriented Software Toolkit for Chemical Kinetics, Thermodynamics, and Transport Processes
Direct numerical simulations of HCCI/SACI with ethanol
Combust. flame
Ignition of a lean PRF/air mixture under RCCI/SCCI conditions: chemical aspects
Proc. Combust. Inst.
Numerical simulation of compressible homogeneous flows in the turbulent regime
J. Fluid Mech.
Direct numerical simulations of ignition of a lean -heptane/air mixture with temperature inhomogeneities at constant volume: Parametric study
Combust. Flame
A DNS study of ignition characteristics of a lean -octane/air mixture under and SACI conditions
Proc. Combust. Inst.
A regime diagram for autoignition of homogeneous reactant mixtures with turbulent velocity and temperature fluctuations
Combust. Sci. Technol.
Direct numerical simulations of ignition of a lean -heptane/air mixture with temperature and composition inhomogeneities relevant to HCCI and SCCI combustion
Combust. Flame
Advanced compression-ignition combustion for high efficiency and ultra-low NO and soot
Effects of temperature and equivalence ratio on the ignition of -heptane fuel spray in turbulent flow
Proc. Combust. Inst.
Evaluating temperature and fuel stratification for heat-release rate control in a reactivity-controlled compression-ignition engine using optical diagnostics and chemical kinetics modeling
Combust. Flame
Simultaneous measurement of natural flame luminosity and emission spectra in a RCCI engine under different fuel stratification degrees
SAE Int. J. Engines
Multiple optical diagnostics on effect of fuel stratification degree on reactivity controlled compression ignition
Fuel
Numerical investigation of spontaneous flame propagation under RCCI conditions
Combust. Flame
On the effect of injection timing on the ignition of lean PRF/air/EGR mixtures under direct dual fuel stratification conditions
Combust. Flame
Ignition characteristics of a temporally evolving -heptane jet in an -octane/air stream under RCCI combustion-relevant conditions,
Combust. Flame
Direct numerical simulation of autoignition in non-homogeneous hydrogen-air mixtures
Combust. Flame
Direct numerical simulation of ignition front propagation in a constant volume with temperature inhomogeneities: i. fundamental analysis and diagnostics
Combust. Flame
Direct numerical simulations of the ignition of lean primary reference fuel/air mixtures with temperature inhomogeneities
Combust. Flame
Direct numerical simulations of the ignition of a lean biodiesel/air mixture with temperature and composition inhomogeneities at high pressure and intermediate temperature
Combust. Flame
Ignition of a lean PRF/air mixture under RCCI/SCCI conditions: a comparative DNS study
Proc. Combust. Inst.
Identification of premixed flame propagation modes using chemical explosive mode analysis
Proc. Combust. Inst.
Autoignition and front propagation in low temperature combustion engine environments
Combust. Flame
Direct numerical simulations of autoignition in stratified dimethyl-ether (DME)/air turbulent mixtures
Combust. Flame
Numerical study on exergy losses of n-heptane constant-volume combustion by detailed chemical kinetics
Energy Fuels
Direct numerical simulation of lean hydrogen/air auto-ignition in a constant volume enclosure
Combust. Flame
Cited by (16)
The exergy analysis of low carbon or carbon free fuels: Methane, methanol, and hydrogen under engine like conditions
2023, Fuel Processing TechnologyEffects of temperature and pressure fluctuations on exergy loss characteristics of hydrogen auto-ignition processes
2023, International Journal of Hydrogen EnergyPrediction of knock intensity and validation in an optical SI engine
2023, Combustion and Flame