Clean and efficient dual-fuel combustion using OMEx as high reactivity fuel: Comparison to diesel-gasoline calibration
Introduction
Recent reports estimated that the transportation sector is responsible for more than 20% of the total CO2 emitted to the atmosphere [1], [2]. Inside this sector, the medium- and heavy-duty applications are the second most important CO2 source after the light-duty vehicles. In particular, they represent around 28% of the total CO2 share in the current transportation scenario [3]. Moreover, these applications are also responsible for other emissions released to the environment as soot and nitrogen oxides (NOx) [4], [5]. To limit the pollutant levels emitted by the internal combustion engines to the atmosphere, strict regulations were created [6]. To achieve the emissions standards imposed by the normative, different devices are used [7]. In this sense, a conventional medium- and heavy-duty powertrain consists of the internal combustion engine (power unit) and a complex after treatment system (ATS) to reduce the impact of the exhaust gases on the environment [8], [9]. This system is composed of a diesel oxidation catalyst (DOC) that converts the unburned hydrocarbons to CO2 [10], [11], a diesel particle filter (DPF) that traps the soot particles, oxidizing them in a regeneration step [12], [13], and finally a selective catalytic reduction system (SCR) that converts the NOx formed during the combustion to inert nitrogen [14]. These devices represent an expensive solution for both manufacturers and consumers. It is estimated that the conventional ATS used in an EU VI compliant heavy-duty 10 L engine can increase the final price of the truck more than $8000, which is transferred to the consumer as a vehicle price increase [15]. In addition, the ATS have other inherent drawbacks as the maintenance costs, operation costs (urea consumption in the SCR [16] and diesel fuel in the DPF [17]) and the engine efficiency reduction due to the increase of the pumping losses. These drawbacks enhance the CO2 production, being contrary to what is searched in a short-term future. As stated, the future regulations aim to reduce the main hazardous pollutants (NOx, soot, particulates, HC, CO) while achieving significant reductions in the CO2 emitted by the vehicle. The targets for CO2 reduction are already set as 15% in 2025 (H2025) and 30% in 2030 (H2030) as compared to the current levels [18]. In this sense, alternatives able to simultaneously reduce the engine-out emissions as well as the CO2 are needed to be integrated together in the vehicle. The improvement of the powertrain to achieve this goal can be tackled by different means: improving the combustion process (conversion efficiency), reducing the exhaust emission (decrease the ATS dimensions and associated fluid consumptions) and using alternative fuels with a positive carbon balance.
The increase of the conversion efficiency and simultaneous reduction of NOx and soot emissions has been extensively investigated in the last years [19], [20]. Some of the major advancements were based on the application of low temperature combustion (LTC) techniques [21]. This type of combustion relies on achieving a high premixing degree and diluted environments, allowing to promote fast combustion processes and low in-cylinder temperatures [21]. As a consequence of the fast combustion process the heat transfer losses are reduced [22] and the fuel-to-work conversion efficiency is optimized [23]. Moreover, the use of premixed mixtures and high exhaust gas recirculation (EGR) levels allows to avoid the conventional NOx-soot trade-off. Different LTC concepts were developed in the last years, as the homogeneous charge compression ignition (HCCI) [24], partially premixed combustion (PPC) [25], [26] and reactivity controlled compression ignition (RCCI) [27]. Among them, it has been found that the RCCI combustion mode offers important advantages compared to HCCI and PPC [28]. In this sense, the use of two fuels with different reactivity (a low reactivity fuel (LRF) and a high reactivity fuel (HRF)) allows to mitigate the combustion control issues found with the single-fuel LTC concepts, as HCCI [29] and PPC [30], and extend the operating limits by modifying the mixture reactivity on demand [31], [32]. Despite of the improvements, it was found that it is not possible to apply RCCI combustion along the whole engine map due to either excessive pressure gradients at high load or excessive HC and CO emissions at low load [33], thus requiring the use of conventional combustion strategies to cover the critical parts of the map [34]. To overcome this issue, Benajes et al. proposed the dual-mode dual-fuel combustion (DMDF) concept [35]. This concept relies on modifying the injection and air management strategies compared to RCCI to mitigate the mechanical restrictions found with a fully premixed combustion [36]. Thus, while the injection strategy at low and medium load is set to promote a premixed RCCI combustion, as the engine load is increased, the high reactivity fuel injection is shifted towards the top dead center (fire) to promote a dual-fuel diffusive combustion with low pressure gradients. It should be remarked that even at high load conditions, the low reactivity fuel is still present in significant fractions (more than 30% of the total energy), having a fundamental role on the mixture preparation and combustion progress. Moreover, the DMDF concept has found to be flexi-fuel capable, allowing to use different fuels either as low reactivity [37], [38] or high reactivity fuel [39]. Recent studies demonstrated that this concept still has benefits in terms of emissions and performance compared to conventional diesel combustion [40]. Nonetheless, further improvements are required to achieve the CO2 targets for H2025 and H2030. One feasible path is the use of alternative fuels that promote a reduction of the CO2 footprint in their lifecycle. This can be accomplished by different means as using bio-based fuels as ethanol or introducing advanced fuels, generally called e-fuels that rely on using the CO2 as raw material during their production process [41]. This second path also enables the use of alternative energy sources as wind power and solar to produce the fuel. Among the different e-fuels reported, Oxymethylene ether (OMEx) appears as a good direct substitute of the diesel fuel to be used in compression ignition engines, or to be used blended with diesel, to provide benefits in soot and CO2 emissions [42]. Moreover, the studies performed by Deutz et al. reported that the use of OMEx also allows to minimize the NOx emissions, since the EGR levels can be modified without exceeding the soot limits imposed by the authors [43]. The use of diesel-OMEx blends can enhance the market penetration of this fuel since it is expected that no modifications are needed in the distribution system as well as in the hardware of the internal combustion engine, i.e., it can be considered a drop-in fuel [44]. Nonetheless, the benefits on the CO2 reduction will be decreased since OMEx will not replace totally the fossil fuel (diesel) [45].
The use of OMEx in its net form is not deeply addressed in the literature. Previous results from the authors demonstrated that the DMDF combustion concept operating with OMEx can realize EURO VI NOx with ultra-low soot levels at four different operating conditions, even in the case of full load operation [39]. Only small fuel consumption penalties were reported due to the larger combustion durations as a result of the low lower heating value (LHV) of OMEx. To expand these findings, the current work aims to evaluate the real potential of using OMEx as high reactivity fuel in all the engine map. To do so, a dedicated calibration operating in DMDF combustion with OMEx and gasoline as high and low reactivity fuels, respectively, is carried out in multi-cylinder engine (MCE) platform. The performance, combustion and emission results are compared to the DMDF diesel-gasoline calibration, also developed in this work, which is considered as the reference condition for comparison as it uses market fuels. Each one of the calibrations were obtained following a specific calibration methodology optimizing the brake thermal efficiency while maintaining the emissions values under pre-established limits. The studies about OMEx are emerging at this moment in the literature, and there are very few investigations of its usage in internal combustion engines, no one dealing with a complete calibration in a production engine operating under an advanced combustion mode. These kind of studies are necessary since the use of OMEx in dual-fuel combustion can contribute to solve the most significant challenges of the heavy-duty transportation sector (NOx, soot and CO2).
Section snippets
Materials and methods
This section describes the experimental facilities, fuels and the calibration methodology used during the experimental tests.
Results and discussion
This section describes the combustion strategies used to calibrate the DMDF concept with each pair of fuels (diesel-gasoline and OMEx-gasoline) given the challenges that were found during the engine mapping. In sequence, the performance and emissions results for both concepts are presented. For each parameter, the results of the diesel-gasoline and the OMEx-gasoline calibration are shown. Finally, the maps showing the difference between both calibrations are presented to highlight the
Conclusions
This work evaluated the use of the dual-mode dual-fuel combustion in a stock multi-cylinder engine platform as a pathway to fulfill the current and future emissions legislations in terms of NOx, soot and CO2. To do this, a full map calibration was carried out following a specific calibration methodology to obtain the best fuel consumption with lower NOx and soot as always as possible. First, the use of conventional fuel for both HRF (diesel) and LRF (gasoline) was investigated aiming to enhance
CRediT authorship contribution statement
Jesús Benajes: Conceptualization and funding acquisition. Antonio García: Design of the experiments and validation. Javier Monsalve-Serrano: Investigation, Writing - review and editing. Rafael Sari: Formal analysis and original draft preparation.
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 thanks ARAMCO Overseas Company and VOLVO Group Trucks Technology for supporting this research. The authors acknowledge FEDER and Spanish Ministerio de Economía y Competitividad for partially supporting this research through TRANCO project (TRA2017-87694-R). The authors also acknowledge the Universitat Politècnica de València for partially supporting this research through Convocatoria de ayudas a Primeros Proyectos de Investigación (SP20180148). The author R. Sari acknowledges the
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2023, Proceedings of the Combustion InstituteDetailed analysis of particulate emissions of a multi-cylinder dual-mode dual-fuel engine operating with diesel and gasoline
2022, FuelCitation Excerpt :The limits for avoiding mechanical issues in the engine were the same as for the EU VI calibration (in-cylinder Pmax 180 bar, PRR 17.5 bar/CAD). More details about the injection settings and other aspects of the calibration can be consulted in the work from Sari R. et al., [10]. Measuring conditions and gas thermodynamic state can highly affect the total particle number measured since the mechanisms of nucleation, accumulation, and aggregation that are part of the formation path of solid particles can be accelerated or halted depending on temperature, concentration, and other thermodynamic and compositional parameters [31].