Experimental and numerical study on different dual-fuel combustion modes fuelled with gasoline and diesel
Introduction
Nowadays, the development in technology of internal combustion engines faces both the challenges of more stringent emission legislation and CO2 (fuel economy) limitation [1]. Compression Ignition (CI) engines are widely used in various applications due to their high fuel-efficiency and reliability. However, the CI engine combustion process encompasses locally rich as well as high temperature regions; therefore, the soot and NOX emissions are high, it is difficult to control them simultaneously by conventional means. Many researchers have demonstrated advanced combustion concepts; for example, Homogenous Charge Compression Ignition (HCCI), Premixed Charge Compression Ignition (PCCI) and Low Temperature Combustion (LTC) are promising techniques to achieve substantial reduction of NOX and soot simultaneously without aftertreatment system [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12].
However, since the combustion process is determined by auto-ignition chemical kinetics, HCCI/PCCI engines suffer from difficulties in controlling combustion phasing and heat release rate; as a consequence, the engine operation range is limited [2], [3], [4], [5], [6], [7], [8], [9], [10]. Yao et al. [13], [14] have demonstrated the possibility of extending the HCCI operation range by the use of low or high octane number fuel (changing the ratio of iso-octane and n-heptane in primary reference fuel, PRF) respectively at low or high loads. Furthermore, there exists an optimum octane number that would achieve the best indicated thermal efficiency as a function of engine load. Under diesel LTC, simultaneous near-zero NOX and soot emissions are achieved by “heavy EGR”, while high levels of unburned HC and CO are produced, so the fuel economy is greatly diminished, namely, a “New trade-off” between soot and fuel efficiency emerges [15], [16]. Johansson et al. first proposed to inject gasoline in a CI engine [17]. Similar results have also been reported by Kalghatgi et al. [18], [19]. In term of this concept, auto-ignition can be made to occur after the fuel and air are better mixed due to the longer ignition delay by the high octane number; as a result, soot emissions decline significantly. NOX emissions can be reduced through lowing combustion temperature by either lean mixture or EGR (not as much as that in diesel LTC for ultra-low NOX and soot emissions) [19], [20], [21]. Even so, the unacceptable peak cylinder pressure and maximum pressure rise rate (MPRR) from such premixed combustion at high or full load conditions, as well as unstable combustion at low loads remain challenges [22], [23], [24]. The previous research identifies that, fuel properties have a significant effect on advanced combustion concepts, and it is difficult to achieve high-efficiency and clean combustion at both low and high load conditions using a single fuel.
Several dual-fuel combustion systems have been studied in the literature. Premixed Compression Ignition (PCI) combustion using iso-octane and diesel has been developed by Inagaki et al. to achieve drastically low NOX and soot emissions [25]. Combination of Dimethyl Ether (DME) of high Cetane Number (CN) and methanol of high octane number has been investigated experimentally and numerically by Yao et al. [26]. The results show the obvious effect of methanol direct-injection timing on the heat release process and NOX emission of the dual-fuel compound combustion. Reitz et al. from University of Wisconsin proposed a Reactivity Controlled Compression Ignition (RCCI) concept which had been demonstrated as a promising method to achieve high-efficiency and clean combustion [27], [28], [29]. Fuel reactivity stratification is formed by in-cylinder blending using port fuel injection of gasoline/natural gas at early cycle and direct injection of diesel fuel. By optimizing gasoline/natural gas to diesel ratio, EGR rate, direct injection strategies and Intake Valve Closing (IVC) timings, dual-fuel combustion process can be effectively controlled, consequently up to 1.35 MPa IMEP load and 53% of net indicated thermal efficiency can be reached, while raw NOX and soot emissions are below EPA 2010 heavy-duty regulation limits.
To achieve high-efficiency and clean combustion over an extensive load range, the authors proposed a dual fuel approach with separated admission of gasoline and diesel into the engine [30]. Because of the high gasoline volatility, the gasoline air mixture is, to a large degree, homogeneous. The injected diesel fuel, because of its poor volatility, would form a stratified fuel vapor and air mixture. The ratio of gasoline and diesel fuel quantity and the diesel injection timing could be adjusted based on operation condition. To accomplish dual-fuel HCCI at low and medium loads, the diesel fuel is injected early in the compression process. The diesel fuel vapor would have sufficient time to coarsely mix with the charge to form a globally homogeneous but locally stratified mixture. The compression-ignited diesel vapor then becomes the ignition source for the (locally) homogeneously mixed gasoline/air mixture at multi-points; the combustion, however, is homogeneous in nature without a flame front. At high load, late injection of diesel is used to achieve smooth combustion with a significant global stratification of the diesel vapor. Then, the combustion process is a compound combustion mode: mixed flameless and diffusion combustion of the diesel vapor and the homogeneous combustion of the premixed gasoline/air mixture. Since highly premixed gasoline mixtures are featured in both cases, they are collectively referred to as Highly Premixed Charge Combustion, HPCC. And the early/late-diesel-injection HPCC are called simply E-HPCC and L-HPCC respectively.
In Ref. [30], the results for a dual-fuel engine operating on premixed gasoline plus an early or a late in-cylinder diesel injection (E-HPPC or L-HPPC) are reported at an intermediate load where both modes could operate successfully. For comparison, operation using a single fuel blend of gasoline and diesel at the same gasoline to diesel mass ratio (80%) is also included. The latter mode of operation is referred to as the blended-fuel LTC mode. In this research, for the E-HPCC mode, a small low temperature heat release (LTHR) peak is found in the combustion process, followed by a main heat release process typical of HCCI combustion; this mode delivers the lowest NOX and soot emissions, which are insensitive to EGR rate. In the case of L-HPCC mode, a prominent dual-peak heat release is observed; therefore, the smoothest combustion (lowest MPRR) is attained. The heat release schedule for the LTC mode is similar with that of E-HPCC, but the main heat release rate is larger; the shortest combustion duration leads to the highest MPRR. Ultra-low soot emissions are also achieved in the latter two cases, but the NOX emissions depend strongly on EGR rate.
Based on the statements above, the motivation of the current study is to explore the fuel–air mixing and combustion reaction process of the three dual-fuel combustion modes in more detail for further clarifying the combustion and emissions characteristics by Computational Fluid Dynamics (CFD) simulation and experiments (with various gasoline to diesel ratios).
Section snippets
Engine
The experiments were all performed on a heavy-duty six-cylinder CI engine which was modified to run as a single cylinder engine with separated intake and exhaust system to eliminate the effect of inter-cylinder interaction. The dynamometer was also driven by the other five cylinders which operated in the normal diesel mode. The specifications of the engine are listed in Table 1. An external air compressor supplied the air to simulate boosted condition. A back pressure valve was used to control
Computational meshes and models
In an effort to achieve better understanding of the combustion reaction and emissions formation mechanisms of the three combustion modes, numerical simulations were performed using KIVA-3vr2 CFD code coupled with the Chemkin solver for detailed chemistry calculations. The chemical properties of gasoline and diesel fuels were represented by those of iso-octane and n-heptane, respectively. A reduced PRF reaction mechanism made up of 45 species and 142 reactions described the combined oxidation of
Results and discussion
The previous research [30] was performed only with the gasoline mass ratio of 80%, it can therefore be expected that combustion and emission characteristics would be changed by various gasoline ratios. Thus, in the current research, the gasoline ratio of 80%, 70% and 60% are applied in E-HPCC, since the excessive one would reduce the combustion stability; four sets of proportions of 90%, 80%, 70% and 60% are used in the L-HPCC mode, and 90%, 80%, 70% for LTC. The combustion phasing indicated by
Conclusions
The combustion and emissions characteristics of the three dual-fuel combustion modes had been investigated experimentally and numerically, and the main conclusions are drawn as following:
- 1.
Most of the mixture in E-HPCC is uniform in both mixture concentration and reactivity, and to various degrees there are mixture stratification in L-HPCC and LTC.
- 2.
Due to the differences in in-cylinder charge distributions, the combustion occurs in the very center area of combustion chamber and the area closer to
Acknowledgments
The research is sponsored by Natural Science Foundation of China through the Project of Outstanding Young Scholarship Award (Grant No. 51125026), Natural Science Foundation of China through its project (Grant No. 51176140) and Ministry of Science and Technology through the project of National International Technology Cooperation (Grant No. 2010DFA74530).
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