Piston cooling
As specific power output increases in modern combustion engines, the pistons are subjected to increasing thermal loads. In order to ensure operating safety, therefore, they must be cooled as effectively as possible.
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Piston cooling
As specific power output increases in modern combustion engines, the pistons are subjected to increasing thermal loads. In order to ensure operating safety, therefore, they must be cooled as effectively as possible.
5.1
Thermal loads
The chemical energy stored in the fuel is converted into heat in the cylinder during combustion. The piston, as a moving wall of the combustion chamber, converts part of this heat into mechanical work and drives the crankshaft through the connecting rod. The heat that is not converted into mechanical work is partially dissipated with the exhaust gas. The remainder is transferred, through convection and radiation, to the parts of the engine that are adjacent to the combustion chamber. In addition to mechanical stresses, this leads to thermal loading of these components, which can reach very high levels locally.
5.2 Combustion and flame jets In gasoline engines, the spark plug normally initiates combustion of the fuel-air mixture. The flame front expands quite evenly in all directions, so that all the surfaces enclosing the combustion chamber are subjected to the gas temperature. This results in a temperature gradient in the piston from the combustion chamber side to the crankcase. For diesel engines with direct fuel injection, combustion starts in the area of the injector holes, because the combustible mixture is first present at that point. After the mixture in this area has autoignited, several flame fronts in the form of combustion lobes run along the injected streams of fuel toward the surfaces enclosing the combustion chamber; Figure 5.1. This generates a nonhomogeneous temperature field in the piston, particularly at the rim of the combustion chamber bowl.
MAHLE GmbH (Ed.), Pistons and engine testing, DOI 10.1007/978-3-658-09941-1_5, © Springer Fachmedien Wiesbaden 2016
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Figure 5.1: Flame fronts in the diesel process
5.3
Temperature profile at the bowl rim
As a result of the nonuniform introduction of heat through the “combustion lobes” described above, a quasi-stationary, uneven temperature profile is induced at the bowl rim of the diesel engine piston. The temperature profile in Figure 5.2 shows this temperature condition along the bowl rim for a six-hole injector. The temperature difference between the locations that are located directly in the center of a combustion lobe and the areas in between can exceed 40 K. The resulting thermal stresses, in conjunction with superimposed loads (mechanical stresses, temperature cycles, oxidation, etc.) can cause cracks in the bowl rim.
Figure 5.2: Uneven temperature profile at the bowl rim
5.4 Piston temperature profile
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Between the intake and exhaust sides, the temperature profile is shifted toward the exhaust side. Moreover, additional influences must be considered, such as the coolant flow path between the individual cylinders.
5.4
Piston temperature profile
Because of the various combustion processes and the required different piston geometries, diesel and gasoline engine pistons exhibi
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