Engine Cooling
In turning to the subject of engine heat transfer it is instructive to begin with a detailed look at where all of the fuel energy goes. If the entire engine is treated as a thermodynamic system, energy enters the system with the fuel. Air enters the syste
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13.1 Tracking the Energy Transfers In turning to the subject of engine heat transfer it is instructive to begin with a detailed look at where all of the fuel energy goes. If the entire engine is treated as a thermodynamic system, energy enters the system with the fuel. Air enters the system at ambient conditions and therefore, by convention, with zero energy. Under steady-state conditions, and averaged over an operating cycle, no energy is stored, and energy exits the system at the same rate it enters. In other words, all of the energy entering the engine in the fuel exits the engine at the same rate. The energy exits the system as either work, heat transfer, or with the exhaust flow. At first look, one would think that studying engine heat transfer and cooling is simply a matter of quantifying the energy transfer out of the engine as heat transfer. In reality this simplified look carries very little engineering value. In managing engine temperatures and developing effective cooling systems it will be important to understand the engine heat transfer terms on a much more detailed level. Toward this objective, the energy transfers within a turbocharged and aftercooled engine with cooled, high-pressure exhaust gas recirculation (EGR) are depicted in Fig. 13.1. The terms enclosed in boxes are the transfers in and out of the engine system, while the remaining terms identify energy transfers within the system. Looking only at the boxed terms in Fig. 13.1, one can already see engine heat transfer becoming more complicated, as the heat rejection is now distributed among three boxes: heat rejection through the charge cooler; heat rejection through the radiator; and heat rejection from engine surfaces. Various thermodynamic sub-systems can be selected within the overall engine system. Looking first at a sub-system consisting solely of the gas within the combustion chamber, energy is transferred into the system with not only the fuel but also the warm compressed © Springer Vienna 2016 K. Hoag, B. Dondlinger, Vehicular Engine Design, Powertrain, DOI 10.1007/978-3-7091-1859-7_13
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13 Engine Cooling
Fig. 13.1 Detailed breakdown of the energy transfers within a turbocharged engine system, with cooled high-pressure EGR and air-to-air charge cooling
air. The air that had entered the turbocharger compressor at ambient conditions (zero energy) has been compressed, cooled, and then warmed by the intake port walls and intake valves. There are piston work transfers in and out of the cylinder; at full load the net work transfer might be on the order of 40 % of the fuel energy. The heat transfer is also shown into and out of the cylinder. The net heat transfer out of the cylinder is on the order of 10–15 % of the fuel energy at full load. Finally, there is the exhaust energy leaving the cylinder with exhaust mass flow. At this point in the system the exhaust flow at full load carries on the order of 45–50 % of the fuel energy. Each of these numbers vary a great deal with engine speed and load, and depending on the comb
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