Engine Cooling
Depending on its size, principle of operation and combustion system, a diesel engine converts up to 30–50% of the supplied fuel energy into effective brake work. Apart from conversion losses during combustion, the remaining percentage is released into the
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Engine Cooling Klaus Mollenhauer and Jochen Eitel
9.1
Internal Engine Cooling
9.1.1
The Function of Engine Cooling
9.1.1.1
Heat Balance and Heat Transport
Depending on its size, principle of operation and combustion system, a diesel engine converts up to 30–50% of the fuel energy supplied into effective brake work. Apart from conversion losses during combustion, the remaining percentage is released into the environment as heat (Fig. 9-1), predominantly with the exhaust and by the cooling system. Only a relatively small percentage reaches the environment by convection and radiation through the surface of the engine. In addition to the component heat transferred to the coolant, the heat dissipated by a cooling system also includes the heat dissipated in the lubricating oil cooler and intercooler. Utilizing the energy loss for heating purposes and the like (see Sect. 14) requires a detailed analysis of the enthalpy content of the individual kinds of heat as well as the engine’s use and type. The external cooling system (Sect. 9.2) also has to be incorporated in the analysis. Internal engine cooling essentially covers the wall heat losses that occur when energy is converted in the combustion chamber (see Sects. 1.3 and 7.2) and reaches the coolant by heat transmission. Other engine components, e.g. injection nozzles, exhaust gas turbochargers and exhaust manifolds, are often directly cooled too. Analyzed from the perspective of energy conversion alone, engine cooling appears to waste energy. This raises the question of whether an uncooled adiabatic engine might not represent a worthwhile goal of development. The belief that high temperature strength and heat insulating materials had been discovered in newly developed ceramic materials and an adiabatic engine, one of Rudolf Diesel’s basic ideas, was one step closer was widespread in the early 1980s. The rise of engine component temperatures to approximately 1,2008C when cooling is discontinued was already
K. Mollenhauer (*) Berlin, Germany e-mail: [email protected]
pointed out in 1970. Even today, this remains an uncontrollable temperature level for reciprocating piston engines [9-1, 9-2] and is compounded by the decrease of the cylinder charge and thus the specific power at such wall temperatures when the charge loss is not compensated by supercharging or increased boost pressure. Experimental tests on an engine with an insulated combustion chamber detected a noticeable deterioration of fuel consumption instead of the expected improvement in consumption [9-3]. A strong rise of the gas-side heat transfer coefficient in the first part of combustion, thus causing more rather than less heat to reach the coolant, was demonstrated to be the reason for this (see Sect. 7.2). Engine process simulations ultimately revealed [9-4] that effective engine cooling that prevents component temperatures from rising above the level common today is one of the basic prerequisites for low nitrogen oxide emission. Thus, one essential function of engine cooling is to lower the temperatu
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