• Energy Research

Significant efforts are under way to develop innovative ignition systems for spark-ignition engines used in transportation. Within this context, passive pre-chamber technology has emerged as a promising alternative for passenger cars. However, several uncertainties remain regarding the operation of this concept at low engine loads and speeds, as well as the impact of specific design features on combustion stability. Previous investigations have indicated that the tangential angle of the pre-chamber holes can play a vital role in stabilizing the combustion process. Nonetheless, the underlying thermo-physical phenomena responsible for these results have not yet been thoroughly studied. To address these knowledge gaps, this paper presents a numerical study using a computational fluid dynamics model that has been validated with experimental results. An alternative modeling methodology was developed to conduct multi-cycle large-eddy simulations and investigate two different pre-chamber configurations, one with tangential holes and the other with radial holes. The results revealed an intriguing correlation between the combustion stability and the spatial distribution of the flame inside the pre-chamber. The cycle-to-cycle dispersion of pre-chamber flow variables was significantly higher when using radial holes compared to tangential holes, potentially explaining the unstable behavior of the former design. Additionally, the undesirable flow-field of the radial-hole pre-chamber caused the flame to evolve asymmetrically, resulting in substantial variations in the ejected jets. This asymmetry can significantly affect the morphology of the main chamber ignition in each cycle.

[EN] The pre-chamber ignition system has demonstrated to be a suitable technology for increasing burning rates while reducing the cycle-to-cycle variability in spark-ignition engines. This concept offers an improvement in thermal efficiency through an increase in ignition energy and flame surface, allowing it to overcome knocking combustion issues at high engine load/speeds. This fact makes this ignition concept well compatible with the use of dilution strategies to control emissions or to further improve efficiency. However, despite promoting faster combustion, knocking combustion is still a major limitation at low rotational speeds and high engine loads (low-end torque). In this investigation, the performance of the passive pre-chamber concept is evaluated in a single-cylinder turbocharged spark-ignition engine fueled with compressed natural gas in EGR-diluted conditions. Several experiments and numerical simulations are combined to analyze the basis of the pre-chamber operation, while seeking to improve the global performance of the engine. To this end, a new pre-chamber geometry is proposed that is able to achieve better features in the ejected jets, to enhance the performance of the concept in the whole engine map. The work has been partially supported by the Spanish Ministerio de Economia y Competitividad, Spain through grant number TRA2017-89139-C2-1-R. The authors also wish to thank Mr. Gabriel Alcantarilla for his inestimable assistance during the experimental campaign.

[EN] Hydrogen combustion engines are considered one of the leading solutions for decarbonizing road transport, mainly due to the possibility of adapting current engines for hydrogen operation with minor changes. This research extensively analyzes the effect of combined EGR (exhaust gas recirculation) and VVT (variable valve timing) strategies on the performance and emissions of a commercial turbocharged SI engine fueled with hydrogen. To this end, a 1D model of the said engine, widely validated for gasoline operation, was adapted to simulate the engine's behavior with hydrogen. This adaptation involved hardware changes and the implementation of a predictive hydrogen combustion submodel, previously calibrated using experimental data from a single-cylinder engine of similar geometry. Firstly, 400 hydrogen engine simulations without EGR were conducted to optimize the VVT system for fuel efficiency over a wide operating range. A detailed explanation of the causality of varying valve overlap on pumping losses, in-cylinder gas composition, and combustion is provided from these simulations. Then, a series of EGR sweeps were simulated to study its impact on performance and NOx at various degrees of load; concluding that diluting with EGR, rather than air, leads to reduced NOx emissions in exchange for slightly increased fuel consumption. Funding for open access charge: CRUE-Universitat Politecnica de Valencia.

[EN] The need to improve the thermal efficiency of gasoline engines used in hybrid vehicles, has led to explore new solutions for reducing engine heat losses. Hence, it is important for the car manufacturers to be able to predict the heat transfer in the engine components. Numerical methods such as CFD (Computational Fluid Dynamics) or CHT (Conjugate Heat Transfer) can be used to assess the heat losses through the combustion chamber walls, but they are long and costly. In this regard, it is particularly interesting for the industry to use simplified models, which may play a key role in the design stage. In this work a 1D model integrated with 3D finite elements based on a commercial software is used to calculate the heat losses in a single-cylinder gasoline direct injection engine. The model is first validated, then a detailed heat transfer analysis is performed, and its results compared to those of a full CFD-CHT simulation. Results demonstrate that this approach is suitable to predict in a short time the heat losses and the spatial temperature distribution in the solid regions of an internal combustion engine. The model also yields accurate values in terms of engine performance

[EN] Compressed Natural Gas (CNG) appears as a midterm solution to conventional fuels, such as gasoline and diesel. The low carbon content and the possibility of being obtained from renewable sources (animal or agriculture waste, landfills, waste of the industry food or aquatic biomass) make CNG an attractive option to reduce Greenhouse Gases (GHG) emission. Applying lean combustion strategies on CNG improves efficiency levels while reducing pollutant emissions. In these conditions, heat transfer losses are reduced, and the thermal efficiency increased, especially at partial loads where increasing air dilution is one of the main strategies to reduce pumping losses. Hydrogen (H2) addition helps to enhance combustion in these diluted conditions and to reduce the combustion instability. This combustion concept has been widely studied over the last years, however further research is still needed. This investigation focuses on how hydrogen substitution affects the performance and emissions (both CO2 and pollutant) of a port fuel injection (PFI) spark ignition (SI) engine fueled by CNG. Thus, the main objective of this investigation is to contribute to the existent knowledge about dual-fuel combustion strategies based on CNG and H2 blends. Results demonstrated that hydrogen substitution helps to reduce the CO2 emissions by two ways: improving the engine efficiency and substituting part of the main carbon-based fuel. Despite of this advantage, NOx emissions are not reduced, and they will require after-treatment systems to deal with current pollutant regulations. This research has been partially funded by FEDER, Spain and the Spanish Government through project RTI2018-102025-B-I00 (CLEANFUEL). M. Olcina-Girona is partly supported by an FPI contract (FPI S2-22-37816) of the Programa de Apoyo para la Investigacion y Desarrollo (PAID 01 22) of the Universitat Politecnica de Valencia.

[EN] In this paper, a computational study was performed using a combination of several numerical tools to better understand the limiting aspects of combustion in a passive pre-chamber ignition system when operating at lean conditions. A specific methodology was developed to analyze in detail the scavenging and combustion processes of this ignition concept. Results show how the scavenging of passive pre-chambers is primarily dependent on the force that the piston makes on the gas during the compression stroke, being independent of the pre-chamber geometry as along as the ratio between the total cross sectional area of the pre-chamber holes and the prechamber volume is kept within a suitable range. Moreover, a successful lean combustion, with an air-to-fuel ratio around 2, cannot be achieved as the burning rates inside the pre-chamber significantly decrease due to the low laminar flame speeds, that results in low quality jets. Further results show that increasing the flow temperature can help to recover competitive combustion rates when knocking combustion is not a limiting factor. The contribution of the heat losses through the pre-chamber walls to the overall energy balance of the pre-chamber has been estimated, showing that their impact is negligible (< 5%). Alternatives for increasing the laminar flame speed were proposed in order to improve combustion inside the pre-chamber. Although the pre-chamber combustion profile was successfully improved, none of the proposed solutions were able to completely burn the main chamber charge with the current pre-chamber design. The work has been partially supported by the Spanish Ministerio de Economia y Competitividad through Grant No. TRA2017-89139-C2-1-R. J. Gomez-Soriano is partially supported through the Programa de Apoyo para la Investigacion y Desarrollo (PAID) of Universitat Politecnica de Valencia [Grant No. FPI-S2-2018-17367].