Biodiesel is an eco-friendly, renewable biofuel derived primarily from vegetable oils and is a potential carbon neutral alternative to fossil diesel for compression ignition (CI) engine applications. The biodiesels produced from different feedstocks vary significantly in their fatty acid methyl ester composition and physico-chemical properties and thereby, engine performance and emissions. In the present work, experimental investigations are done with four candidate biodiesel fuels, viz. sunflower, rice-bran, palm and coconut in two different engine configurations, viz. light duty naturally aspirated (NA) and heavy duty turbocharged (TC) to establish the effects of biodiesel composition variations as well as engine type variations on the engine characteristics. To establish biodiesel composition effects on engine characteristics, two new parameters, viz. straight chain saturation factor (SCSF) and modified degree of unsaturation (DUm) are developed in this work, which can be estimated directly from the measured biodiesel composition. The obtained results show some contradicting trends with biodiesel compared to diesel in the two engines. The ignition delay is lower for biodiesel by 0.9 degree in the turbocharged engine, while, it is higher by 1 degree in the naturally aspirated engine. The dynamic start of injection timings are advanced for biodiesel in both the engines but the advance is relatively higher in the turbocharged engine. The heat release rates with biodiesel are premixed and diffusion phase dominant in the naturally aspirated and turbocharged engines respectively. The peak cylinder pressures and nitric oxide emissions are generally higher for biodiesel in both the engines. The smoke emissions with biodiesel are higher and lower respectively in the naturally aspirated and turbocharged engines. Further, investigations on biodiesel composition effects on the engine characteristics revealed a strong correlation between DUm, SCSF and engine parameters. An increase in DUm of biodiesel is found to deteriorate combustion quality, resulting in poor engine performance and higher emissions. However, biodiesel having higher SCSF exhibit better engine performance along with lower nitric oxide and smoke emissions. Thus, an estimate of DUm and SCSF of candidate biodiesel fuels provides a first approximation of the extent of variations in engine parameters compared to diesel and thereby, helps in making a careful choice of biodiesel feedstock for automotive engine applications. Further, based on the results obtained with four different biodiesels in two different engine configurations, the present study reveals that the contradicting engine trends with biodiesel compared to diesel are more influenced by engine type variations rather than by biodiesel composition variations. (C) 2017 Elsevier Ltd. All rights reserved.