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Recently, several theories have been offered as possible explanations for claimed increases in diesel engine heat transfer when combustion chamber surface temperatures are raised through insulation. A multi-dimensional computational fluid dynamics (CFD) analysis, using a recently developed near wall turbulent heat transfer model, has been employed to investigate the validity of two of these theories. The proposed mechanisms for increased heat transfer in the presence of high wall temperatures are: 1 piston-induced compression heating of the near wall gas which increases the near wall temperature gradient when wall temperatures are high; 2 increased penetration of hot, burned gases into the near wall flow during combustion through reduction of the flame quench distance.

A new model for corrective in-cylinder heat transfer has been developed which calculates heat transfer coefficients based on a description of the in-cylinder flow field. The combustion chamber volume is divided into three regions in which differential equations for angular momentum, turbulent kinetic energy and turbulent dissipation are solved. The resultant heat transfer coefficients are strongly spatially non-uniform, unlike those calculated from standard correlations, which assume spatial uniformity. When spatially averaged, the heat transfer coefficient is much more peaked near TDC of the compression stroke as compared to that predicted by standard correlations. This is due to the model's dependence on gas velocity and turbulence, both of which are amplified near TDC. The new model allows a more accurate calculation of the spatial distribution of the heat fluxes. This capability is essential for calculation of heat transfer and of component thermal loading and temperatures.

A set of methods is described to calculate boundary conditions for thermal and mechanical finite element (FE) analyses and to assess and present the results of those analyses in a clear and understandable way. The approach utilizes a combination of engine simulation programs and an empirical database of engine measurements developed over many years. The methodology relies on the use of specialized FE pre- and post-processors dedicated to the analyses of engine components. Gas-side thermal boundary conditions for combustion chamber components are calculated using an engine simulation code for standalone FE analyses or for FE analyses directly coupled to the engine simulation code itself. Coolant side boundary conditions are calculated using multidimensional flow analysis (computational fluid dynamics). Boundary conditions in intake and exhaust manifolds are calculated using a one-dimensional gas dynamics code.

An important aspect of calculation of engine combustion chamber heat transfer with a multi-dimensional flow code is the modeling of the near wall flow. Conventional treatments of the wall layer flow employ the use of wall functions which impose the wall boundary conditions on the solution grid points adjacent to solid boundaries. However, the use of wall functions for calculating complex flows such as those which exist in engines has numerous weaknesses, including dependence on grid resolution. An alternative wall modeling approach has been developed which overcomes the limitations of the wall functions and is applicable to the calculation of in-cylinder engine flows. In this approach the wall layer flow is solved dynamically on a grid spanning a very thin boundary layer region adjacent to solid boundaries which is separate from the global grid used to solve the outer flow.

The heat flux from the gases to the walls of I.C. engines is highly transient, producing temperature transients in thin layers of the walls adjacent to the combustion chamber. The resulting surface temperature swings affect engine performance, and also increase the maximum temperature of the engine components. To analyze these effects, a one-dimensional, time-dependent heat conduction model was developed, with the capability to handle layered or laminated walls and temperature-dependent material properties. The model is driven by a thermodynamic cycle code coupled to a steady-state heat conduction model of the engine structure. A parametric study was carried out in which boundary conditions representing a heavy duty diesel engine were applied to materials with a wide range of thermal properties.

An analysis was made of the effect of insulation strategy on diesel engine heat transfer, performance and structure temperatures. The analysis was made using a thermodynamic cycle code with a new heat transfer correlation which takes into account the gas velocity and turbulent intensity and provides a spatially and time resolved description of the heat transfer process. The cycle code is directly coupled to a steady state heat conduction code representing the engine structure, and a transient heat conduction code tracking wall temperature swings along the combustion chamber surfaces. The study concentrated on the effects of different insulation strategies and insulating materials placed at various locations within the combustion chamber. Among the outputs of these analyses were the thermodynamic efficiency, peak firing pressure, exhaust gas temperature, component temperatures (time-average and maximum), volumetric efficiency, major heat paths and fuel energy balance.

A new model for radiation heat transfer in DI diesel engines has been developed. The model calculates the heat transfer rates as a function of the instantaneous values of the radiation zone size, radiation temperature, and of the absorption coefficient of the soot-laden gas. The soot concentration levels are calculated from kinetic expressions for soot formation and burnup. The spatial distribution of the radiant heat flux along the combustion chamber walls is calculated by a zonal model. The model has been applied to a conventional heavy duty highway DI diesel engine to generate predictions over a range of engine speeds and loads. These predictions indicated a wide variation in the ratio of radiation to the total heat transfer, ranging from less than ten percent to more than thirty percent, depending on the speed and load.

Detailed heat transfer measurements were made in the combustion chamber of a Cummins single cylinder NH-engine in two configurations: cooled metal and ceramic-coated. The first configuration served as the baseline for a study of the effects of insulation and wall temperature on heat transfer. The second configuration had several in-cylinder components coated with 1.25 mm (0.050″) layer of zirconia plasma spray -- in particular, piston top, head firedeck and valves. The engine was operated over a matrix of operating points at four engine speeds and several load levels at each speed. The heat flux was measured by thin film thermocouple probes. The data showed that increasing the wall temperature by insulation reduced the heat flux. This reduction was seen both in the peak heat flux value as well as in the time-averaged heat flux. These trends were seen at all of the engine operating conditions.