The heat from the exhaust gas of diesel engines can be an important heat source to provide additional power using a separate Rankine Cycle (RC) or an Organic Rankine Cycle (ORC). Water is the best working fluid for this type of applications in terms of efficiency of the RC system, availability and environmental friendliness. However, for small engines and also at part load operations, the exhaust gas temperature is not sufficient enough to heat the steam to be in superheated zone, which after expansion in the turbine needs to be in superheated zone. Ammonia was found to be an alternate working fluid for these types of applications which can run at low exhaust temperatures. Computer simulation was carried out with an optimized heat exchanger to estimate additional power with water and ammonia as the working fluids. ANSYS 14.0 CFX software was used for the simulation. It was found that at full load 23.7% and 10.9% additional power were achieved by using water and ammonia as the working fluids respectively. However, at 25% part load, ammonia with 11.2% additional power worked better than water which was able to generate only 2.6% additional power.