Depletion of fossil energy and ecological deterioration is the major crises the world is now dealing with. In light of these improvements for the environment, biodiesel has recently gained popularity. Empirical work was done to evaluate a conventional diesel engine’s efficiency and emission characteristics; running on Karanja oil Biodiesel (KBD 100%) at various compression ratios (CR). The experimental engine is a 3.5 kW, single-cylinder, 4-stroke, water-cooled, direct-injection VCR diesel engine for this investigation. At typical injection timing of 23°bTDC, with the following loading conditions: 20%, 40%, 60%, 80%, and 100% i.e., brake power of 0.7 kW, 1.4 kW, 2.1 kW, 2.8 kW, and 3.5 kW, respectively; the experimental setup was tested for three distinct CRs of 17, 16.5, and 16. For the Karanja oil biodiesel mode at 100% load and compression ratios of 17, 16.5, and 16 correspondingly, the highest brake thermal efficiencies are observed to be 26.08%, 23.37%, and 21.4%. And the optimum thermal efficiency for diesel fuel mode at compression ratio of 17 is observed to be 26.95%. CO emissions for KBD mode at compression ratios 16, 16.5, and 17 are much lower than for diesel mode at full load. However, HC emissions as well as NOx emission trends are nearly identical at compression ratios of 16, 16.5, and 17 in every loading condition when comparing KBD mode to diesel mode. The purpose of this study is to compare emission and performance characteristics of diesel engines operating on biodiesel fueled by Karanja oil at compression ratios of 17, 16.5, and 16 with conventional diesel fuel.