• Energy Research

[EN] This paper deals with the thermodynamic analysis of an absorption refrigeration cycle used to cool down the temperature of the intake air in an Internal Combustion Engine using as a heat source the exhaust gas of the engine. The solution of ammonia-water has been selected due to the stability for a wide range of operating temperatures and pressures and the low freezing point. The effects of operating temperatures, pressures, concentrations of strong and weak solutions in the absorption refrigeration cycle were examined to achieve proper heat rejection to the ambient. Potential of increasing Internal Combustion Engine efficiency and reduce pollutant emissions was estimated by means of theoretical models and experimental tests. In order to provide boundary conditions for the absorption refrigeration cycle and to simulate its effect on engine performance, a OD thermodynamic model was used to reproduce the engine performance when the intake air is cooled. Furthermore, a detailed experimental work was carried out to validate the results in real engine operation. Theoretical results show how the absorption refrigeration system decreases the intake air flow temperature down to a temperature around 5 degrees C and even lower by using the bottoming waste heat energy available in the exhaust gases in a wide range of engine operating conditions. In addition, the theoretical analysis estimates the potential of the strategy for increasing the engine indicated efficiency in levels up to 4% also at the operating conditions under evaluation. Finally, this predicted benefit in engine indicated efficiency has been experimentally confirmed by direct testing. (C) 2016 Elsevier Ltd. All rights reserved. Authors want to acknowledge the "Apoyo para la investigacion y Desarrollo (PAID)" grant for doctoral studies (FPI S2 2015 1067).

The scope of this study is the analysis of the influence of boost pressure and injection pressure on combustion process and pollutant emissions. The influence of these parameters is investigated for different engine speeds. Fuel mass was kept constant for all the tests in order to avoid its influence on the analysis. A single cylinder research diesel engine, equipped with a common rail injection system capable of operating up to a maximum pressure of 150 MPa was used. Special attention was paid to NOx, smoke (which are the most important pollutants for legislation) and brake specific fuel consumption.

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.

An experimental investigation has been performed on the potential of the Atkinson cycle and reducing intake oxygen concentration for pollutant control in a heavy-duty diesel engine. In this study the Atkinson cycle has been reproduced advancing the intake valve closing angle towards the intake stroke. In addition, the intake oxygen concentration has been reduced introducing exhaust gas recirculation. This research has been carried out at low engine load (25%), where the Atkinson cycle is known to improve the efficiency of the spark-ignition engines. The main interest of this investigation has been the comparison between the Atkinson cycle and the conventional diesel cycle at the same oxygen concentration in the intake gas. This analysis has been focused on in-cylinder gas thermodynamic conditions, combustion process, exhaust emissions and engine efficiency. In compression ignition engines, the Atkinson cycle basically promotes the premixed combustion, but in the range of these tests, a complete premixed combustion was not attained. Regarding exhaust emissions, the Atkinson cycle reduces notably the nitrous oxides but increases soot emissions. Finally, better global results have been found reducing intake oxygen concentration by the recirculation of exhaust gas than by the operation of an Atkinson cycle.

Abstract Among the possible electric powerplants currently driving low-payload UAVs (up to around 10 kg of payload), batteries offer certain clear benefits, but for medium-payload operation such as aerotaxis and heavy-cargo transportation UAVs, battery capacity requirements restrict their usage due to high weight and volume. In light of this situation, fuel cell (FC) systems (FCS) offer clear benefits over batteries for the medium-payload UAV segment (> 50 kg). Nevertheless, studies regarding the application of FCS powerplants to this UAV segment are limited and the in-flight performance has not been clearly analysed. In order to address this knowledge gap, a feasibility analysis of these particular applications powered by FCS is performed in this study. A validated FC stack model (40 kW of maximum power) was integrated into a balance of plant to conform an FCS. As a novelty, the management of the FCS was optimized to maximize the FCS efficiency at different altitudes up to 12500 ft, so that the operation always implies the lowest H2 consumption regardless of the altitude. In parallel, an UAV numerical model was developed based on the ATLANTE vehicle and characterized by calculating the aerodynamic coefficients through CFD simulations. Then, both models were integrated into a 0D-1D modelling platform together with an energy management strategy optimizer algorithm and a suitable propeller model. With the preliminary results obtained from the FCS and UAV models, it was possible to ascertain the range and endurance of the vehicle. As a result, it was concluded that the combination of both technologies could offer a range over 600 km and an endurance over 5 h. Finally, with the integrated UAV-FCS model, a flight profile describing a medium altitude, medium endurance mission was designed and used to analyse the viability of FC-powered UAV. The results showed how UAVs powered by FCS are viable for the considered aircraft segment, providing competitive values of specific range and endurance.

[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.

The newly designed Partially Premixed Combustion (PPC) concept operating with high octane fuels like gasoline has confirmed the possibility to combine low NOx and soot emissions keeping high indicated efficiencies, while offering a control over combustion profile and phasing through the injection settings. The potential of this PPC concept regarding pollutant control was experimentally evaluated using a commercial gasoline with Research Octane Number (RON) of 95 in a newly-designed 2-Stroke poppet valves Compression Ignition (Cl) engine for automotive applications. Previous experimental results confirmed how the wide control of the cylinder gas temperature provided by the air management settings brings the possibility to achieve stable gasoline PPC combustion at low and medium speed conditions (1250-2000 rpm) for the whole load range (3.1-10.4 bar IMEP) with good combustion stability (Coefficient of Variation (Coy) of IMEP below 3%), high combustiOn efficiency (over 97%), and low NOx/soot levels. In this context, present research focuses on the two main specific drawbacks of this concept. Firstly, the high Brake Specific Fuel Consumption (BSFC) due to the work required by the mechanical supercharger since the turbocharging system does not provide the suitable pressure ratio at low speeds. Secondly, the high level of noise generated by the combustion process, especially at high loads. Therefore, a dedicated analysis has been carried out to fully exploit the benefits of the gasoline PPC concept combined with the innovative 2-Stroke engine architecture with the aim of identify and break the most relevant trade-offs. The authors kindly recognize the technical support provided by Mr. Pascal Tribotte from RENAULT SAS in the frame of the DREAM-DELTA-68530-13-3205 Project.