# Heat Release Rates of Straight Soybean and Diesel Oil Blends in a Compression Ignition Engine
## 1 Introduction  
Global climate change and worldwide dependency on fossil fuels have encouraged the study of alternative energy sources. Biofuels are an interesting alternative for energy production in countries with an agricultural basis, such as Brazil, who presents the availability of land for energy crops, favorable climatic conditions, and well-established infrastructure for crop production, transport, and processing. In this context, straight vegetable oils offer opportunities for the distributed electricity generation in the energy, agriculture, and agro-industrial sectors. They are the preferred choice for replacing diesel oil in isolated regions, where the perils of transporting and storing diesel oil are an environmental and social hazard [1, 2].  
Currently, vegetable oil is converted into biodiesel through the transesterification process and is used in defined proportions in drop-in fuels for transportation. The diesel oil used in Brazil carries 13% (vol.) of biodiesel in its formulation [3]. However, in distributed energy production, straight vegetable oil (SVO) can be used directly in compression ignition engines, thus reducing costs and providing fuel availability in remote areas, such as the Brazilian rainforest. Worldwide, different vegetable oils have been used, either directly or after conversion to biodiesel, as compression engine fuel. Examples include soybean oil in the USA, rapeseed and sunflower oils in Europe, and palm, jatropha, and coconut oils in Asia and Africa [4, 5, 6, 7]. In Brazil, soybean and palm oils have been widely used in the production of biodiesel.  
Most studies on the use of SVO in compression ignition engines have focused on dynamometer-based bench and thermodynamic analyses of compression ignition enginesoperating with different vegetable oils. The main technological challenge addressed is associated with the differences in the physicochemical properties of vegetable oils in comparison with diesel oil. The larger molecular structure, higher viscosity, and lower volatility of SVOs compared to diesel oil significantly influence the atomization and combustion processes [8]. Heating and blending are normally used to approximate the properties of SVOs to those of diesel oil [9; 10]. The comparable performance of diesel oil in relation to thermal efficiency and other performance parameters has been reported in the literature [7; 10; 11; 12; 13; 14; 15]. Additionally, studies based on pressure readings to calculate the heat release rate have increased in the last decade for several pure vegetable oils and blends [16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33]. Results regarding the performance, emissions, and combustion parameters are compared in relation to diesel oil.  
This work focuses on the heat release rate of straight soybean and diesel oil blends as fuels in compression ignition engines. Soybean oil was chosen for this study because of its availability in the Brazilian market. Tests were carried out measuring the dynamic pressure in the combustion chamber of a single-cylinder engine. The heat release rate was estimated from the transient pressure measurements using a well-known zero-dimensional approach [34; 35]. The main objective of this work was to understand the influence of the physicochemical fuel and in-cylinder gas properties on the combustion process in order to contribute to further studies concerning the development of a combustion model for the analysis and design of electricity generation systems using straight vegetable oils in compression ignition engines.