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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.

Selective reinforcement of squeeze-cast aluminum pistons by fiber matrix inserts is a method of improving high temperature strength in piston zones subject to severe thermal and mechanical loads in highly loaded diesel engines. An investigation was carried out into the effects of selective fiber-matrix reinforcement on the thermal and stress state of an aluminum piston for a heavy-duty truck diesel engine application. Specifically, effects of geometry of the reinforced zone (fiber matrix), fiber density in the matrix, fiber orientation and piston combustion bowl shape were sought. Thermal and structural finite element analysis of the configurations were carried out. Thermal analyses were fully coupled to a simulation of a highly rated heavy-duty diesel.

The warmup characteristics of an engine have an important impact on a variety of design issues such as performance, emissions and durability. A computer simulation has been developed which permits a detailed transient simulation of the engine warmup period from initial ambient conditions to a fully warmed up state. The simulation combines a detailed crankangle-by-crankangle calculation of in-cylinder processes and of engine air flow, with finite element heat conduction calculations of heat transfer from the gases, through the structure to the coolant. The paper describes one particular application of the simulation to the warmup of a 2.5ℓ spark ignited engine from cold start to a fully warmed up state at several speeds ranging from 1600 to 5200 rpm and loads ranging from 25% to 100% at each speed. The response of structure temperatures, charge temperature at IVC and of the exhaust temperature has been calculated and is documented in terms of characteristic warmup times.

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.

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 set of heat flux data was obtained in a Cummins single cylinder NH-engine coated with zirconia plasma spray. Data were acquired at two locations on the head, at several speeds and several load levels, using a thin film Pt-Pt/Rh thermocouple plated onto the zirconia coating. Careful attention was given to the probe design and to data reduction to assure high accuracy of the measurements. The data showed that the peak heat flux was consistently reduced by insulation and by the increasing wall temperature. The mean heat flux was also reduced. The results agree well with a previously developed flow-based heat transfer model. This indicates that the nature of the heat transfer process was unchanged by the increased wall temperature. Based on these results, the conclusion is drawn that insulation and increasing wall temperatures lead to a decrease in heat transfer and thus contribute positively to thermal efficiency.

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.

A comprehensive diesel engine system model, representing in detail engine heat transfer processes, has been applied to a study of insulated diesel engines. The study involved a broad design analysis matrix covering a range of engine configurations with and without inter-cooling and exhaust heat recovery devices, three operating conditions and seven heat rejection packages. The main findig of this study is that the retained heat conversion efficiency (RHCE), of the in-cylinder heat retained by insulation to piston work, is 35-40 percent; these levels of RHCE are larger than those predicted by previous models. This means that a significant part of the retained heat is converted directly to piston work rather than being merely available in the exhaust stream, from which it would be recoverable with a much lower efficiency.

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.

A detailed research program has been initiated to study the modeling aspects of turbulent heat transfer in engine applications using multidimensional codes. The main concerns are the representation of turbulent transport in flow regions that differ significantly from constant-pressure boundary layers for which the currently used models have been developed and validated. Both the treatment of near-wall and bulk-flow transport are being investigated. Models are being tested on several representative test cases. A number of such test cases have been identified, which contain key flow features relevant to engine applications and for which a good experimental data base exists. Calculations of these test cases have been made using a standard k-ε model.

An experimental study was conducted to measure the heat transfer in a direct injection 2.3 ℓ single cylinder diesel engine. The engine was operated at speeds ranging from 1000 to 2100 RPM and at a variety of loads. The heat transfer was measured using a total heat flux probe, operating on the principle of a thin film thermocouple, sensitive to both the convective and radiative heat flux. The probe was located in the head at two locations: opposite the piston bowl and opposite the piston crown (squish region). The measurements showed about twice as large peak heat flux in the bowl location than in the crown location for fired conditions, while under motoring conditions the relationship was reversed and the peak heat flux was slightly higher in the crown position. The experimental profiles of total heat flux were compared to the predictions obtained using a detailed thermodynamic cycle code, which incorporates highly resolved models of convective and radiative heat transfer.

An integrated transient engine simulation methodology has been developed to allow the calculation of a thermal shock as it propagates in time through the engine structure. It links, in a fully consistent way, a very comprehensive thermodynamic model of in-cylinder processes, including a detailed gas-phase heat transfer representation, with a turbocharger/air flow/plenum model and a finite element model of the structure. The methodology tracks the turbocharger boost increase and the cycle-by-cycle build-up of in-cylinder heat transfer during engine load and speed changes, producing a transient thermal response in the structure, until new steady-state is reached. The presented results highlight the calculated transient engine performance response and the thermal and stress response of various metal and ceramic components during sudden speed and load changes in heavy duty diesel engines.