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The turbulent flow field inside the cylinder plays a major role in spark ignition (SI) engines. Multiple phenomena that occur during the high pressure part of the engine cycle, such as early flame kernel development, flame propagation and gas-to-wall heat transfer, are influenced by in-cylinder turbulence. Turbulence inside the cylinder is primarily generated via high shear flows that occur during the intake process, via high velocity injection sprays and by the destruction of macro-scale motions produced by tumbling and/or swirling structures close to top dead center (TDC) . Understanding such complex flow phenomena typically requires detailed 3D-CFD simulations. Such calculations are computationally very expensive and are typically carried out for a limited number of operating conditions. On the other hand, quasi-dimensional simulations, which provide a limited description of the in-cylinder processes, are computationally inexpensive.

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.

There exists a strong interaction between the engine cylinder, intake and exhaust gas flow dynamics and the dynamics of mechanical and hydraulic components constituting the valvetrain system, which controls the engine gas flow. Technologies such as turbo-charging and Diesel particulate filtration (DPF) can significantly increase port gas pressure forces acting on the exhaust valve. When such systems are introduced or undergo design modifications, the operation of valvetrain system can be greatly affected and even compromised, which in turn may lead to degradation of performance of the internal combustion engine. Often, the valvetrain system needs to be retuned. Further, predictive analysis of design issues or evaluation of design changes requires highly coupled simulations, combining models of gas pressure forces and the dynamics of all mechanical and hydro-mechanical parts constituting the valvetrain.

The advantages of Variable Valve Actuation (VVA) in the aspects of improved engine performance, fuel economy and reduced emissions are well known in the industry. However, the design and optimization of such systems is complex and costly. The design process of VVA mechanisms can be greatly accelerated through the use of sophisticated simulation tools. Predictive numerical analysis of systems to address design issues and evaluate design changes can assure the required performance and durability. One notable requirement for the analysis and design of novel mechanically-actuated VVA systems is a general-purpose fast and easy-to-use planar mechanism kinematics analyzer with cam solution/design features, which can be applied to general mechanisms.

An integrated simulation tool has been developed, which is applicable to a wide range of design issues. A key feature introduced for the first time by this new tool is that it is truly a single code, with identical handling of engine, powertrain, vehicle, hydraulics, electrical, thermal and control elements. Further, it contains multiple levels of engine models, so that the user can select the appropriate level for the time scale of the problem (e.g. real-time operation). One possible example of such a combined simulation is the present study of engine block vibration in the mounts. The simulation involved a fully coupled model of performance, thermodynamics and combustion, with the dynamics of the cranktrain, engine block and the driveline. It demonstrated the effect of combustion irregularity on engine shaking in the mounts.

This paper describes a comprehensive model of piston skirt lubrication, developed for use in conjunction with piston secondary dynamic analysis, to accurately characterize the effects of the skirt-cylinder oil film on piston motions. The model represents both hydrodynamic and boundary lubrication modes and applies an asperity contact pressure when surfaces are in close proximity with each other. In addition to skirt dimensions and surface roughness properties, the circumferential extent of lubrication, an arbitrary skirt profile and bore distortion are specifiable inputs to the model. The model is also extended to represent the oil starvation at the cylinder end of the skirt by allowing the axial extent of lubrication on the skirt surface to vary circumferentially and with time to satisfy continuity of oil.

This paper describes a general model for the analysis of secondary motions in conventional and articulated piston assemblies. The model solves for the axial, lateral and rotational departures in positions and motions from the nominal kinematics, resulting from clearances within the piston assembly and also between the piston assembly components and the cylinder. The methodology allows the characterization of conventional and articulated piston secondary motions in the thrust plane of the cylinder. Motions of the piston, pin, rod and (for articulated pistons) skirt are separately calculated, by integrating equations of motion for individual components and dynamic degrees of freedom. Various configurations with respect to rigid attachment of the wristpin to other components can also be represented. In the equations of motions solved, all gas pressure, inertia, friction and oil or contact pressure forces are accounted for.

The duration of piston durability tests is often shortened by increasing mechanical and thermal engine loads and loading frequency. Such durability tests are useful for assessing the structural integrity of new piston designs, but test results are generally not indicative of the expected service life. Furthermore, improper test procedures can introduce illegitimate failure modes which are not present in actual service. Two aluminum pistons were analyzed to gain a fundamental understanding of piston response due to accelerated test loads. The influence of engine speed, fuel/air ratio, intake manifold pressure and crankcase oil temperature on piston fatigue life was studied. Guidelines were established to aid in developing a more effective durability test procedure for natural gas engine pistons.

An S.I. combustion model has been developed for application in phenomenological engine simulations. The model is based on a turbulent flame concept, linked to an in-cylinder flow and turbulence calculation. The flame front is assumed to spread from the spark plug and propagate through the cylinder, while interacting with the combustion chamber geometry. The model predictions were compared to combustion rate measurements made in a single cylinder four valve passenger car engine. The data spanned a wide range of operating conditions, from an idle timing sweep, to part load EGR and mixture ratio sweeps, to a wide open throttle speed sweep. The results of the comparisons showed a generally good agreement. Some difficulties were encountered at idle, where cycle-to-cycle variability makes modeling difficult especially at early timing settings.

Inter-ring gas pressures and blowby in a diesel engine were investigated analytically and compared to experimental data measured at three engine speeds. Coupled simulations of ring dynamics, ring lubrication and inter-ring gas dynamics were carried out using the RINGPAK software, a code for the integrated analysis of ring pack performance and tribology. Inter-ring pressures and ring dynamics are known to have an important effect on the “blowback” mechanism of in-cylinder oil consumption, i.e. that of oil-laden gas flow from the ring lands into the cylinder. Predicted land pressures matched the experimental results very well qualitatively as well as quantitatively. The coupling between ring motions and inter-ring gas pressures and blowby, a key feature of the methodology, was seen to be crucial in obtaining agreement with detailed features of the land pressure data.

An integrated simulation methodology for the analysis of piston tribology is presented. The methodology is comprised of coupled models of piston secondary dynamics, skirt oil film elastohydrodynamic lubrication and wristpin bearing hydrodynamics, developed earlier by the authors. Models have been further expanded to calculate distributions of cumulative wear load on the skirt and cylinder and to account for details of skirt crankcase end geometry. The skirt elasticity model has also been improved to account for the effects of piston crown and pin boss stiffness in conventional, one-piece pistons. The model predicts piston assembly secondary motions, piston (skirt) friction, skirt and wristpin oil film pressures, transient deformations, skirt-cylinder contact/impact pressures and skirt and cylinder wear loads.

A model for oil consumption due to in-cylinder evaporation of oil in reciprocating engines, has been developed. The model is based on conservation of mass and energy on the surface of the oil film left on the cylinder by a piston ring pack, at the oil/gas interface, and also conservation of energy within the oil film and cylinder/coolant interface. The model is sensitive to in-cylinder conditions and is part of an integrated model of ring pack performance, which provides the geometry of the oil film left by the ring pack on the cylinder. Preliminary simulation results indicate that a relatively small but not insignificant fraction (2-5%) of the total oil consumption may be due to evaporation losses for a heavy duty diesel at the rated condition. The evaporation rate was shown to be sensitive to oil grade and upper cylinder temperature. Much of these losses occur during the non-firing half of the cycle.

Changing and more stringent emissions norms and fuel economy requirements often call for modifications in the fuel injection system of a Diesel engine. There exists a strong interaction between the injection system hydraulics and the dynamics of mechanical components within the unit injector and the camshaft-driven mechanical system used to pressurize it. Hence, accurate predictive analysis of design issues or evaluation of design changes requires highly coupled and integrated hydro-mechanical simulations, combining analysis of fuel injection hydraulics and the dynamics of all mechanical parts, including the cam-drive system. This paper presents an application of such an integrated model to the study and alleviation of an observed increase in mechanical vibration and related noise levels associated with a proposed design change in unit injectors and valve-train of a 6-cylinder truck diesel engine.

A model of roller chain and sprocket dynamics was developed, aimed at analyses of dynamic effects of chain drive systems in automotive valvetrains. Each chain link is modeled as a rigid body with planar motion, with three degrees of freedom and connected to adjacent links by means of a springs and dampers. The kinematics of roller-sprocket contacts are modeled in full detail. Sprocket motions in the chain's plane, resulting from torsional and bending motions of attached camshafts are also taken into account. One or two-sided guides can be treated as well as stationary, sliding or pivoting tensioners operated mechanically or hydraulically. The model also takes into account the contact kinematics between chain link rollers and guides or tensioners, allowing for guides/tensioners of arbitrary shape, or simpler (flat and circular) geometries. The model is first applied to study the chain drive and valvetrain of a 1-cylinder motorcycle engine.

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.

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.

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.