Review on the use of essential oils in compression ignition engines

Essential oils are obtained from the non-fatty parts of a plant, such as the roots, bark, leaves, stems and flowers. These oils are mainly used in the natural medicine sector due to claimed health benefits, as well as the flavouring and fragrance sector, and the market has experienced rapid growth in recent years. The high quality required of the products leads to a very significant low-value waste stream, which is available for use in the transport and agricultural sectors. The use of essential oils in the compression ignition (CI) engine is a concept that has not yet been explored thoroughly. This paper analyses the available literature on the effect of essential oils and their blends on the performance, combustion characteristics and emission parameters of the CI engine. Regarding their properties, essential oils have similar properties to neat diesel. Engine performance using several essential oils and their blends improve brake thermal efficiency (BTE) and reduce brake-specific fuel consumption. A significant reduction in particulate matter (PM) emissions along with reduced hydrocarbon and carbon monoxide emissions has been reported in the literature. Furthermore, essential oils and their blends increase peak cylinder pressure and heat release rate (HRR) compared to neat diesel and biodiesel. However, most of the literature reported increased emissions of nitrogen oxides attributed to the lower cetane number and higher oxygen content. Due to having a low cetane number, essential oils can be used in CI engines by blending with either diesel or biodiesel. The rapid growth of the essential oil sector increases the likelihood of their utilization in CI engines in the future.