A radiator test system was developed to test the heat transfer performance of automobile radiators for various engine coolants. The system was made up from a circulation pump, coolant reservoir, axial fan, electric heaters, PLC controlled drivers and instruments for various mechanical measurements along with the tested radiators. Two different radiators, namely round and flat tube ones, and four different engine coolants, namely water, ethylene glycol, their 50/50 mixture and a commercial heat transfer oil, were tested. The experimental heat dissipation rates of the radiators were evaluated under a broad range of operating conditions. The air speed was changed between 2 and 4 m s-1 , the coolant flow rate was varied between 0.1 and 0.3 l s-1 , and the air temperature at the inlets of the radiators was changed between 25 and 35 °C, while the coolant temperature was fixed at 90 °C in all tests. The flat tube radiator dissipated on average 4.8% more heat than the circular tube one for water coolant, while it rejected on average 66.4% more heat than the circular tube one for ethylene glycol. Furthermore, when the heat transfer oil was used as coolant, the flat tube radiator dissipated on average 101.6% more heat than the circular tube one.

This study aims at evaluating and comparing the thermal performance of five different engine coolants employed in an experimental engine cooling system with a flat-tube louvered-fin automobile radiator. For this purpose, a PLC-controlled test system was set up. The system could maintain the temperatures of the air and coolant at the radiator inlet, the speed of the air and flow rate of the coolant at the required values during the tests. The tested coolants were pure wa-ter, 30/70 ethylene glycol (EG)/water, 30/70 propylene glycol (PG)/water, 50/50 EG/water and 50/50 PG/water mixtures. In all tests, the coolant temperature at the radiator inlet was kept at 90°C, while the coolant flow rate was varied between 0.10–0.25 l/s with 0.05 l/s increments. Furthermore, the air temperature at the ra-diator inlet was kept at 25, 30 and 35°C, and the air speed passing over the radia-tor was varied between 1–4 m/s with 1 m/s increments. The thermal performance of the radiator was evaluated by locating the measured coolant flow rate and coolant inlet/outlet temperatures into the conservation of energy equation. It was found that water yielded the highest radiator heat rejection rates. Compared with water, 30/70 EG/water, 50/50 EG/water, 30/70 PG/water, 50/50 PG/water mix-tures yielded on average 3.50%, 7.89%, 8.28%, 11.46% lower radiator heat rejec-tion rates, respectively. Since PG has some advantages over EG such as lower cost and toxicity, PG mixtures can be employed as antifreeze instead of EG mix-tures in expense of a slight decrease in the thermal performance.