Performance and emissions of straight vegetable oils and its blends as a fuel in diesel engine: A review

A rising concern on the depletion of fossil fuel and its growing demand for global energy has promoted research on alternative renewable biofuel. Vegetable oil is marked to be a potential source of energy that can substitute fossil fuels because of its comparable properties to diesel fuel, it is renewable and it is readily available. However, the high viscosity of straight vegetable oil (SVO) limits its application in diesel engines. The high viscosity of SVO can lead to incomplete combustion and carbon deposit in the combustion chamber of an engine. Thus, the blending of SVO with other fuels is seen as an essential and feasible solution in mitigating the issue. In this review, the effects of SVO and its blends on engine performance and emissions are comprehensively studied. Research on the direct use of SVO and several blends such as vegetable oil-diesel blend, vegetable oil-alcohol blend and vegetable oil-alcohol-diesel blend on diesel engine over the last decade are discussed. From these studies, it is found that brake specific fuel consumption (BSFC) for all blends fall between that of SVO and diesel. SVO-diesel blends containing less than 50% SVO have comparable brake thermal efficiency (BTE) to that of diesel fuel. Generally, NOX emitted by SVO, SVO-diesel and SVO-alcohol blends are lower or comparable to that of diesel fuel. Meanwhile, ternary blends of SVO-diesel-alcohol are found to reduce smoke, especially at high load. It is found that CO and unburn HC emissions are vary depending on engine specification, dynamometer operating condition and types of vegetable oil used. Overall, SVO blends are found as potential sources of renewable energy to partially or fully replace diesel fuel.