Bio-fuels are gaining public and scientific attention due to fuel price hikes, energy security, and greenhouse gas emissions from fossil fuels. In the present research work, karanja and kusum oils are used as fuel in the diesel engine in preheated mode. The engine test is conducted using preheated oil with the help of a heat exchanger at different loads. The heat exchanger was added to the engine set-up for saving waste exhaust energy and preheating the vegetable oil. The acquired engine data using a heat exchanger has been analyzed for emissions, such as CO, CO2, HC, and NOx.