• Energy Research

[EN] Reactivity controlled compression ignition (RCCI) combustion has demonstrated to be able to avoid the NOx-soot trade-off appearing during conventional diesel combustion (CDC), with similar or better thermal efficiency than CDC under a wide range of operating conditions. The high thermal efficiency of RCCI is explained by the combination of a short-duration and well-phased combustion process, which maximizes the fuel-to-work conversion efficiency, together with relatively low combustion temperatures, which increases the specific heat ratio during expansion and reduces thermal gradients for heat transfer losses. The objective of this work is to study the RCCI heat transfer characteristics and compare them to those of the CDC regime. To do this, a single-cylinder light-duty research engine instrumented with 25K-type thermocouples distributed among the cylinder head and cylinder liner is used. First, the influence of some engine settings on the RCCI heat transfer phenomenon is explored by means of parametric sweeps. Later, the RCCI heat transfer characteristics are compared for two different low reactivity fuels (LRF), gasoline and E85. Finally, the heat transfer characteristics of RCCI and CDC combustion regimes are compared at some representative operating points in matched load conditions. The results show that both LRF tested are suitable to be used in RCCI giving similar results in terms of energy usage. Moreover, the ability of RCCI combustion in exploiting the fuel energy to extract useful work is demonstrated, reducing by 13% the heat transfer versus CDC. The authors gratefully acknowledge General Motors Global Research & Development for providing the engine used in this investigation. The authors also acknowledge FEDER and Spanish Ministerio de Economia y Competitividad for partially supporting this research through TRANCO project (TRA2017-87694-R).

The main goal of this work is to validate an innovative experimental facility and to establish a methodology to evaluate the influence of some of the engine parameters on local engine heat transfer behaviour under motored steady-state conditions. Instantaneous temperature measurements have been performed in order to estimate heat fluxes on a modified Diesel single cylinder combustion chamber. This study was divided into two main parts. The first one was the design and setting on of an experimental bench to reproduce Diesel conditions and perform local-instantaneous temperature measurements along the walls of the combustion chamber by means of fast response thermocouples. The second one was the development of a procedure for temperature signal treatment and local heat flux calculation based on one-dimensional Fourier analysis. A thermodynamic diagnosis model has been employed to characterise the modified engine with the new designed chamber. As a result of the measured data coherent findings have been obtained in order to understand local behaviour of heat transfer in an internal combustion engine, and the influence of engine parameters on local instantaneous temperature and heat flux, have been analysed. © 2011 Elsevier Ltd. All rights reserved.

[EN] In modeling an Internal Combustion Engine, the combustion sub-model plays a critical role in the overall simulation of the engine as it provides the Mass Fraction Burned (MFB). Analytically, the Heat Release Rate (HRR) can be obtained using the Wiebe function, which is nothing more than a mathematical formulation of the MFB. The mentioned function depends on the following four parameters: efficiency parameter, shape factor, crankshaft angle, and duration of the combustion. In this way, the Wiebe function can be adjusted to experimentally measured values of the mass fraction burned at various operating points using a least-squares regression, and thus obtaining specific values for the unknown parameters. Nevertheless, the main drawback of this approach is the requirement of testing the engine at a given engine load/speed condition. Furthermore, the main objective of this study is to propose a predictive model of the Wiebe parameters for any operating point of the tested SI engine. For this purpose, an Artificial Neural Network (ANN) is developed from the experimental data. A criterion was defined to choose the best-trained network. Finally, the Wiebe parameters are estimated with the neural networks for different operating conditions. Moreover, the mass fractions burned generated from the Wiebe functions are compared with the respective experimental values from several operating points measured in the engine test bench. Small differences were found between the estimated and experimental mass fractions burned. Therefore, the effectiveness of the developed ANN model as a prediction tool for the engine MFB is verified.

In recent years, due to the increasing need to reduce consumption of reciprocating internal combustion engines, new researches on different subsystems have raised. Among them, the use of nanofluids as a coolant medium seems to be an interesting alternative. In this work, the potential benefits of using nanofluids in the cooling system using an engine lumped model are studied. The methodology of the study starts with a whole description and validation of the model in both steady and transient conditions by means of a comparison with experimental results. Then, the potential benefits that could be obtained with the use of nanofluids are studied in a theoretical way. After that, the model is used to estimate the behavior of the system using different nanofluids in both stationary and transient conditions. The main results show that the advantages of using these new refrigerants are limited.

The need to reduce fuel consumption and exhaust emissions in internal combustion engines has been drastically increased during last years. One of the most important processes affecting these parameters is heat transfer from the in-cylinder gas to the surrounding walls, as this mechanism has a direct influence on the combustion process. Regarding the different walls (liner, cylinder head and piston surfaces), heat flow to the piston is especially important, as it is essential to avoid excessively high temperatures that could result in material damage and/or oil cracking. With this purpose different cooling strategies are used, among which the improvement of the piston cooling system by using oil galleries is preferred. In this work, the heat flow through the oil gallery in a Diesel piston was investigated on a dedicated test bench. This bench consists of a controlled heat source and a piston oil cooling system in which different test conditions were evaluated in order to obtain a correlation for the film coefficient associated with piston oil cooling. These experimental results were then incorporated into a lumped model for engine heat transfer. Finally, in order to evaluate the accuracy of this model and the effects of the correlation for oil gallery coefficient on engine heat flows, results obtained on a conventional engine test bench equipped with a Diesel engine, in which two piston temperatures had been measured, were used. The results show an improvement in piston temperature predictions when compared with those obtained using a previously reported expression for the calculation of the oil film coefficient.

Combustion diagnosis based on in-cylinder pressure signals as well as 0D thermodynamic modelling, are widely used to study and optimize the combustion in reciprocating engines. Both approaches share some uncertainties regarding the sub-models and the experimental installation that, for the sake of accuracy, must be reduced as much as possible in order to obtain reliable results. A methodology, based on the sensitivity effect of such uncertainties on heat release and simulated pressure, is proposed to adjust their values. The methodology is capable of identifying the separate influence of each parameter and to provide a set of values thanks to the Multi-Variable linear regression (MLR) in motoring conditions. The method is flexible enough to deal with different number of uncertainties and can be applied to different engines and thermodynamic models. The final results of the adjustment are validated in combustion conditions, showing an improvement of the apparent combustion efficiency of about 7% with respect to the reference values. The support of the Generalitat Valenciana (BEST/2010/145) is greatly acknowledged.

The use of biodiesel fuels in diesel engines is gaining attention as a promising solution to control CO2 emissions. Great research efforts have been carried out to identify the impact of biodiesel physical and chemical properties on engine systems and processes. Most of these investigations were performed in warm conditions, but the suitability of biodiesel for starting the engine at under-zero ambient temperatures has not widely evaluated. The surface tension and the viscosity of biodiesel fuels are higher compared to those of standard diesel and, in cold conditions, these differences become critical since the injection fuel rate is largely affected and consequently the combustion process can be deteriorated. In order to improve its flow characteristics at cold temperatures and make them more suitable for low temperatures operation, additives are used in biodiesel fuels. In this paper the suitability of different biodiesel fuels, with and without additives, for cold starting of DI (direct injection) diesel engines has been evaluated. The results have shown that the engine start-ability with pure biodiesel fuels can be largely deteriorated. However, using diesel/biodiesel blends the start-ability of the engine can be recovered with the additional benefit of reducing the opacity peak of the exhaust gases

In the present work a comprehensive study of turbocharger heat transfer phenomena is discussed, showing their relevance compared to gas enthalpy variations through the turbomachinery. The study provides an experimental methodology to consider the different heat fluxes in the turbocharger and modeling them by means of a lumped capacitance heat transfer model (HTM). The input data required for the model are obtained experimentally by a proper combination of both steady and transient tests. These tests are performed in different test benches, in which incompressible fluids (oil) and compressible fluids (gas) are used in a given sequence. The experimental data allows developing heat transfer correlations for the different turbocharger elements. These correlations take into account all the possible heat fluxes, discriminating between internal and external heat transfer. In order to analyze the relative importance of heat transfer phenomena in the predictability of the turbocharger performance and the different related variables; model results, in hot and cold conditions, have been compared with those provided by the standard technique, consisting on using look up maps (LUM) of the turbocharger. The analysis of these results evidences the highly diabatic operative areas of the turbocharger and it provides clearly ground rules for using hot or cold turbocharger maps. In addition, paper discussion advises about using or not aHTM, depending on the turbocharger variables and the operative conditions that one desires to predict. Paper concludes that an accurate prediction of gas temperatures at turbine and compressor outlet and of fluid temperatures at water and oil ports outlet is not always possible without considering heat transfer phenomena in the turbocharger.