Influence of engine load and fuel droplet size on performance of a CI engine fueled with cottonseed oil and its blends with diesel fuel
Introduction
Interest in the use of biofuels in diesel engines is not new. Nevertheless, in the current world energy context, challenges of biofuels use are numerous. This interest is related to their potential role as alternative to fossil fuels and their contribution to reduce greenhouse gas emissions. They also appear as a way to ensure energy security in a world context characterized by the decrease in world oil reserves and an increase in fossil fuels prices. Unfortunately, on a global scale, the so-called “first generation” biofuels account for a small part of world energy consumption in the transportation sector (2.5% of world final energy consumption in 2010 [1]). The availability of land to produce sufficient quantities of biofuels to meet the energy needs on a large-scale without competition with food is another handicap.
However at a local level, “first generation” biofuels can be a real factor of development. As such, the use of vegetable oils as alternative fuels to diesel fuel in diesel engines is interesting, particularly in developing countries. Services in the rural areas are covering different sectors: agriculture, irrigation, power generation, drinking water supply, post harvest transformations (dehusking, milling), etc. Except for electricity generation, the common installed capacity is in the range of 2–20 kW. Millions of small diesel engines of conventional design, using low-pressure injection systems, are present in these areas. Therefore, this paper does not apply to modern high injection pressure engines.
Many studies on the characteristics and the use of vegetable oils or animal fats and their derivatives in diesel engines have been conducted over the last 30 years [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. Most of these studies showed that their behavior is very close to that of diesel fuel. Moreover, they are renewable, biodegradable and do not contain sulfur. However, diesel fuel and each vegetable oil or animal fat have their own characteristics and specific behavior that distinguish it from another. This is related to their specific physical and chemical nature. In particular, the fatty acid composition, high viscosity and low volatility are key factors of differences in the behavior of vegetable oils [4]. This can lead to ignition problems as well as coking of the colder parts of the combustion chamber due to thermal decomposition and polymerization under certain conditions of temperature [22], [23], [24], [25].
The literature review points out two main ways to resolve problems related to ignition and polymerization of vegetable oils during operation in CI diesel engines. The first way involves a decreased viscosity and increased volatility (better atomization) achieved by either oil preheating, blending with diesel, using microemulsion formulations [8], [9], [10], [11], [12], [13], [14], [15], [16], [26], [27], [28]. This option takes place before fuel injection and influences the spray characteristics. The second way occurs downstream of the injector inside the combustion chamber and consists in the improvement of the engine operating conditions. This can be performed by modification of the engine (thermally insulated cylinder, dual fuel operation or exhaust gas recirculation) [16], [29]. In this study, in order to investigate the behavior of vegetable oils in a diesel engine, we have chosen the easiest way for a user to act upstream of the injector by blending cottonseed oil with diesel fuel to reduce droplets diameters. Usually, the studies focusing on the spray characteristics of biofuels consider cone angle, spray area and penetration of the jet [30], [31], [32], [33]. In order to make our results coherent with our previous studies on single droplet evaporation behavior [17], [34]; we chose to consider only the droplet size distribution. On this basis, the influence of droplet size reduction and thermal conditions on the combustion have been studied.
The objective of this work is to determine the droplet size distribution of cottonseed oil and its blends with diesel fuel on the performance of a CI engine. These forms of use (pure vegetable oils or blends with diesel fuel) are easier to implement than processes such as transesterification especially in rural areas.
This work is part of a research effort led by the authors to develop sustainable technologies for the use of vegetable oils in specific applications. This research program includes the study of physical and chemical processes occurring from the production of vegetable oils up to their final use in internal combustion engines or burners. Consequently, data such as physico-chemical characterization, atomization, droplet size distribution, evaporation [17], [34], polymerization and finally combustion of different fuels [35], [36] are currently under investigation.
Section snippets
Engine test bench
Tests were carried out at CIRAD (Centre de cooperation International en Recherche en Agronomique pour le Développement) in the Biomass Energy laboratory of Montpellier (France), with the collaboration of the ‘‘Laboratoire de Physique et de Chimie de l’Environnement” of Ouagadougou (Burkina Faso).
A Hatz diesel engine (model 1D80) was installed on a test bench. Engine characteristics are described in Table 1. The system consists of a DI diesel engine, an eddy current dynamometer, a fuel tank,
Droplet size distribution
Table 5 and Fig. 2 respectively give the values of characteristic diameters (DV10, DV50, Dv90, and SMD) and the droplet size distribution for the spray of the six investigated fuels.
The analysis of the droplet size distribution (Fig. 2, and Table 5) shows that for high percentage of cottonseed oil in diesel fuel (CSO80), the blend has a behavior close to that of the pure cottonseed oil for which the droplet size distribution is higher than the usual size observed for diesel engines [37]. The
Conclusion
This work allows establishing the droplet size distribution of cottonseed oil and its blends with diesel fuel using standard injection equipment. Then, it highlights the influence of thermal conditions (load), and fuel droplet size distribution on the performance of a DI diesel engine fueled with cottonseed oil and its blends with diesel fuel. The mains conclusions are:
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Thermal efficiency obtained for the different fuels are close for all loads and globally slightly better for CSO20–100. If the
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