Diesel engines are the primary source of power in many types of transport applications, so many countries are currently working to reduce the vehicle exhaust emissions from diesel engines. Jatropha oil methyl ester was mixed in various amounts (20%, 40%, 60%, 80%, and 100%) during an experimental investigation to assess diesel engines' efficiency and emissions at different engine speeds, such as 500 rpm and steps increase with 250 till 1500 rpm. For various blending ratios at each speed, the engine was run at no load, quarter, half, three quarters, and full load. The brake thermal efficiency, the exhaust gas's temperature, and the brake specific fuel consumption (BSFC) are the performance characteristics that are assessed, whilst the exhaust emissions include the specific emissions of O₂, CO, CO₂, and NO_x. Combustion modeling for the fuel was done using ANSYS Fluent to predict the performance and emissions for diesel and biodiesel fuels. It was found from both the simulated and the experimental results that the use of biodiesel blends causes a 21% decrease in brake thermal efficiency and a 32% increase in brake-specific fuel consumption. The use of biodiesel fuel blends results in a rise in exhaust gas temperature. Increased biodiesel content in fuel blends results in a reduction in specific emission of CO₂. The results suggest that biodiesel made from oils that are not suitable for human consumption, such as Jatropha, may be a suitable replacement for diesel fuel in diesel engines.