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Experiments were conducted on a four-cylinder, four-stroke, direct-injected diesel engine to study the ffects of engine coolant temperature on both performance parameters and exhaust emissions. The energy balance is discussed on the bases of first-law analysis and second-law analysis. The range of speed investigated was 1000-2000 RPM for the torque range of 25-152 N.m. The coolant emperature was varied from 50 to 95 °C. The present study shows that the coolant temperature has a significant effect on the volumetric efficiency. It also shows that increasing coolant temperature decreases the mass flow rate of fuel consumption and the cooling losses. As a result, the brake specific fuel consumption decreases and the brake thermal efficiency increases. A chart was developed for showing the relationship between the coolant emperature, equivalence ratio, brake torque, and brake specific fuel consumption. The study shows that the coolant emperature has a significant effect on NOx emissions and minor effects on the volumetric percentages of oxygen, carbon dioxide, and carbon monoxide. The unburned hydrocarbons show insignificant variation. This work also shows that increasing coolant temperature slightly increases the availability of the coolant and decreases the total av ilability losses