The use of locally available non-edible type SVO (Straight Vegetable Oil) in existing diesel engine without any modification becomes an attractive option in rural and remote areas in decentralized power generation for irrigation and electrification, where grid power is not available, if performance is comparable to diesel fuel. The purpose of the present work is to determine how the commercially available additive influences the injection, combustion and emission characteristics of CI (Compression Ignition) engine fuelled with SVO at elevated temperature. In the present paper attention is concentrated to elucidate how IR (Injection Rate) diagram focuses on the injection characteristics which in turn affects the combustion and emission characteristics of DI (Direct Injection) diesel engine. This paper shows a conventional but novel technique to plot IR diagram by using modified fuel test bench equipped with mechanical stroboscope, which does not require costly pressure sensor in injection system, to measure flow rate and thus it is a cost effective method. A comparative study using diesel fuel and Karanj oil with and without additive at room temperature and at 65°C respectively was carried out on a Cummins two cylinders DI diesel engine by varying brake load. A detailed structural and chemical analysis of additive was done to understand the mechanism of fuel adulteration and interpret the test results. Physical properties of test fuels were also measured at different temperature to justify the engine characteristics. The experimentally obtained results reveal that Karanj oil exhibits advance injection timing, longer injection duration and higher injection rate as compared to diesel at room temperature which in turn increases ignition delay and thus leads to higher NOx (Oxides of Nitrogen) and lower smoke emissions. However, pre-heating retards injection timing which in turn reduces ignition delay, cylinder pressure and HRR (Heat Release Rate) and thus suppression of NOx emission. The use of additive at 65°C further improves the injection characteristics and thus simultaneous reduction in NOx and smoke emissions for Karanj oil is achieved.