There is a growing demand for higher power density in diesel engines, making thermal management increasingly critical. 3D-CFD (Three-Dimensional Computational Fluid Dynamics) simulations play an important role in the thermal analysis of heavy-duty diesel engine pistons due to the complex heat transfer phenomena associated with sprays in diesel combustion. 3D CFD simulations rely on wall heat transfer sub-models to estimate heat flux at the combustion chamber surfaces, which are then used as boundary conditions to perform 3D finite element thermal analysis. Equilibrium wall heat transfer models are widely adopted alongside RANS and LES turbulence modeling approaches to minimize computational cost. In this study, fast-response surface thermocouples were used to validate local heat-flux predictions on the piston surface of a heavy-duty diesel engine. Comparisons between CFD-predicted heat fluxes using existing equilibrium models and experimental data revealed notable discrepancies and inconsistencies across various piston locations and operating conditions. A comprehensive theoretical and numerical analysis of the existing steady and unsteady heat transfer formulations was conducted to identify the root causes of their inconsistencies. These inconsistencies were primarily attributed to strong spatial variations in the near-wall flow field generated by spray plumes. Existing formulations generally use a two-layer approach consisting of the viscous sub-layer and the mixed/turbulent layer, which limits their ability to represent diverse local turbulence conditions. To address these limitations, an improved equilibrium heat transfer model is proposed. The model adopts a three-layer formulation in which the turbulent region is further discretized into two sub-layers, as shown in the equation below. This added resolution enables the model to better account for the spatial heterogeneity of combustion under varying engine loads. The proposed three-layer approach showed significant improvements in prediction accuracy and consistency compared with existing models across diverse local flow conditions. y(+ )<13.2 rho c(p)u(tau)/q(w )(T-g-T-w) = Pr.y(+) y(+)> 13.2 rho c(p)u(tau)/q(w ).ln(T-g/T-w) = 2.1ln(y(+))+ 13.2Pr(m) - 5.34 y(+)>200 rho c(p)u(tau)/q(w )(T-g/T-w) = 2.1 ln(y(+)) + 13.2Pr(m) - 5.34

Heat-resistant steels with high chromium additions (≥5 weight percent) are critical for many high temperature energy and manufacturing applications, including heat exchangers, pistons for engines, and dies for metal working and casting. However, while high chromium additions increase oxidation resistance at elevated temperatures, they also compromise thermal conductivity, resulting in a metallurgical trade-off between these two important properties. Here we show that a microstructure with both higher thermal conductivity and improved oxidation resistance at elevated temperatures is achieved in a unique steel with only 1 weight percent chromium, thereby overcoming the long-standing metallurgical trade-off. This is accomplished through a tailored thermal treatment that produces a tempered martensitic matrix with low solute content and a fine dispersion of copper precipitates and molybdenum enriched carbides. A further discovery is that the resultant thermally grown oxide includes an iron-copper-manganese-enriched outer layer that provides high-temperature oxidation protection equivalent to heat-resistant steels with five times the chromium content and 25% lower thermal conductivity.

3D-CFD (Three-Dimensional Computational Fluid Dynamics) simulations play a critical role in the thermal analysis of heavy-duty diesel engine pistons due to the complex heat transfer phenomena associated with sprays in diesel combustion. 3D CFD simulations rely on wall heat transfer sub-models to estimate heat flux at the combustion chamber surfaces, which are then used as boundary conditions to perform 3D finite element thermal analysis.,Equilibrium wall heat transfer models are widely adopted alongside RANS and LES turbulence modeling approaches to minimize computational cost. In this study, fast-response surface thermocouple measurements were used to validate local heat flux predictions on the piston surface of a heavy-duty diesel engine. Comparisons between CFD-predicted heat fluxes using existing equilibrium models and experimental data revealed notable discrepancies and inconsistencies across various piston locations and operating conditions.,A comprehensive theoretical and numerical analysis of the existing heat transfer formulations (steady and unsteady) was conducted to identify the root causes of these inconsistencies, which were primarily attributed to strong spatial variations in the near-wall flow field generated by the spray plumes. To address these limitations, a new improved equilibrium heat transfer model is proposed that incorporates a 3-layer approach to account for the spatial heterogeneity of the combustion process under varying engine loads

The study investigated how post-weld heat treatment (PWHT) temperature affects the microstructure and localized deformation/fracture during bend testing of rotary inertia friction welds (RIFW) between AISI 422 stainless steel and AISI 4140 steel. RIFW produced a fully martensitic interface with approximately 550 HV hardness in both the thermo-mechanically affected (TMAZ) and heat-affected zones (HAZ). Due to differences in temper resistance, the 4140 TMAZ/HAZ softened progressively under PWHT temperatures from 525 degrees C to 700 degrees C, while the 422 TMAZ unexpectedly maintained about 550 HV up to 600 degrees C before significantly softening at temperatures >= 625 degrees C. This asymmetric softening generated steep hardness gradients across the interface at temperatures <= 600 degrees C. Furthermore, carbon migration across the interface was minimal up to 600 degrees C, moderate at 625 degrees C, and by 700 degrees C produced a carbide-rich eutectoid layer in the 422 TMAZ alongside a carbon-depleted soft ferrite layer in the 4140 TMAZ. Strain during bending was PWHT-dependent, concentrating on the 4140 side; in as-welded joints, the high hardness led to deformation and crack initiation in the base metals, whereas in PWHT samples, cracking initiated in the softened 4140 TMAZ near the interface. The intermediate PWHT temperature of 625 degrees C offered the best balance of limited carbon diffusion across the interface, relatively low peak weld hardness and minimized hardness gradients across the interface, more homogenous deformation, and good bend test performance.

Mechanical failure of pistons due to heating is a concern in high-power density internal combustion engines for military and heavy-duty commercial applications. Injection pressure and nozzle diameter are investigated here as diesel combustion modifiers to reduce thermal loads on a heavy-duty diesel engine piston. A 3-D sector mesh CFD (computational fluid dynamics) model was developed based on a validated single-cylinder model, coupled with the piston finite element model to study the transient interaction between combustion and temperature distribution on the piston surface. A 1/8th sector of the cylinder was used due to the use of an 8-hole injector. Modern diesel engines do not have perfect axis-symmetry because of the complex geometry. Therefore, the implications of plume-to-plume variations when using a sector mesh are investigated by initializing the sector mesh several different ways to understand the consequences of the pre-combustion flow field. A box sweep with 900, 1200, 1700 bar injection pressures and 167, 190, 230 mu m nozzle holes was performed by holding the combustion phasing constant at the C100 operating condition (i.e., 1625 rpm, 17 bar IMEP). The largest nozzle hole with the lowest injection pressure was predicted to reduce peak piston temperatures by similar to 50 degrees C compared to the smallest nozzle hole at the highest injection pressure. Fuel droplet sizes and the injection duration played a major role in dictating combustion and heat transfer. The interaction between the piston and the modified combustion plumes induced local temperature differences up to similar to 100 degrees C with different configurations of injection pressures and nozzle hole sizes. However, efficiency and emissions are generally worse for the cases with reduced piston thermal load through combustion modification.

The heat transfer processes occurring in a compression ignition engine are complex, especially considering flame-wall interaction on the piston crown from impinging jets. To study the heat flux occurring on the piston in a heavy-duty diesel engine, a piston was instrumented with fifteen thermocouples and a wireless telemetry system. Eight of the thermocouples are high speed surface thermocouples placed primarily in regions with significant flame-wall interaction, providing crank-resolved surface temperature data. This work presents the first experimental datasets collected with this instrumented piston, describing in detail the thermocouple location selection process as well as data processing and uncertainty quantification for the high-speed surface thermocouples with a particular emphasis on cyclic variability and sensor-to-sensor variability. With this methodology established, data from this piston can be used for modeling and simulation studies as well as for studying the impact of operating conditions on heat flux and flame-wall interaction. The analysis showed that there were significant differences in observed cyclic variability of transient heat flux among the different surface thermocouples that did not appear physical. The sensors did appear able to capture phenomenological aspects of the heat flux process of mixing controlled combustion though the magnitude of transient heat flux appeared higher than expected and further work, including repeatability tests with additional instrumented pistons, is required to form stronger conclusions.

AISI 422 martensitic stainless steel with superior hightemperature performance (oxidation resistance and strength) is under evaluation for replacing current heavy-duty piston crown materials, AISI 4140 martensitic steel and microalloyed steel (MAS) 38MnSiVS5, to fabricate a multimaterial piston (Refs. 1, 2). This multimaterial piston concept further improved power density and fuel economy by allowing heavyduty diesel engines to operate at higher temperatures and pressures (Ref. 3). Joining AISI 422 steel piston crowns with AISI 4140 steel piston skirts is a key manufacturing step for this multimaterial piston. However, the significant differences in strength, elevated temperature flow stress, alloy chemistry, and temper resistance between these two martensitic steels cause some weldability issues (cracking) and metallurgical challenges (alloying element migration/segregation) when using conventional fusion-based welding processes (Refs. 4–6). Rotary inertia friction welding (RIFW), a solid-state welding process, has been the preferred method to join 4140 crowns to 4140 skirts (and MAS crowns to MAS skirts) in high-volume production of current heavy-duty diesel engine pistons. It has been used to join these two materials with relatively comparable alloy chemistry to fabricate pistons with MAS skirts and 4140 crowns. Meanwhile, RIFW has also been a preferred method of dissimilar metal welding (Refs. 7, 8). However, RIFW of dissimilar high-strength martensitic steels has yet to be widely pursued. The interfacial microstructure complexities created by the thermomechanical process and highly nonequilibrium phase transformations during RIFW are a significant challenge for understanding and predicting their joining behavior and have not been reported in detail. In this work, defect-free AISI 422 steel-AISI 4140 multimaterial pistons were successfully fabricated using the RIFW process. The interfacial microstructure and mechanical properties of dissimilar 422/4140 steel RIFW in the as-welded condition were experimentally studied in detail. The results provide critical baseline information for understanding RIFW mechanisms and guiding subsequent postweld heat treatment (PWHT) practice.

Design of internal combustion (IC) engine pistons is dependent on accurate prediction of the temperature field in the component. Experimental temperature measurements can be taken but are costly and typically limited to a few select locations. High-fidelity computer simulations can be used to predict the temperature at any number of locations within the model, but the models must be calibrated for the predictions to be accurate. The largest barrier to calibration of piston thermal models is estimating the backside boundary conditions, as there is not much literature available for these boundary conditions. Bayesian model calibration is a common choice for model calibration in literature, but little research is available applying this method to piston thermal models. Neural networks have been shown in literature to be effective for calibration of piston thermal models. In this work, Bayesian model calibration will be compared to two neural network-based calibration methodologies for piston thermal models. The models were compared for both computation time and error across three different data densities. Each data set represents an increasing density of steady-state temperature measurement locations. The results show that the error between the methods is largely consistent across the different data densities, with each model having similar error to the others at each calibration case. On the other hand, computation time highlights the advantage of the neural network methodologies over the Bayesian methodology. At the lowest data density, the Bayesian model calibration methodology had the fastest computation time but only by a few minutes. As the data density increased, the Bayesian model calibration method became hours slower than the Neural network methods, up to 4673.3% slower at the highest data density. Both neural networks-based approaches and the Bayesian model calibration methodology are effective at calibrating at low data densities but for higher data densities, the Bayesian model calibration becomes too computationally expensive.

A major concern for a high-power density, heavy-duty engine is the durability of its components, which are subjected to high thermal loads from combustion. The thermal loads from combustion are unsteady and exhibit strong spatial gradients. Experimental techniques to characterize these thermal loads at high load conditions on a moving component such as the piston are challenging and expensive due to mechanical limitations. High performance computing has improved the capability of numerical techniques to predict these thermal loads with considerable accuracy. High-fidelity simulation techniques such as three-dimensional computational fluid dynamics and finite element thermal analysis were coupled offline and iterated by exchanging boundary conditions to predict the crank angle-resolved convective heat flux and surface temperature distribution on the piston of a heavy-duty diesel engine. A Bayesian calibration method was used to arrive at heat transfer coefficients for some of the impactful piston cooling surfaces. This work provides insights about the potential use of injection parameters to manage piston thermal loads and maximize thermal efficiency at high load conditions. The influence of key injection parameters such as injection timing (start of injection) and pressure on the transient heat flux and surface temperature distribution on the piston surface were analyzed using the above-mentioned numerical technique. The results show that later injection timings result in lower piston surface temperatures, but also lower efficiencies. A reduced injection pressure at the earliest combustion phasing reduced the piston surface temperature with a slight drop in thermal efficiency. Significant spatial variations of heat flux and surface temperature were observed and quantified for the changes in injection parameters.

The effects of advanced fuel injection strategies on the combustion behavior of an unblended low-cetane synthetic jet fuel (Sasol isoparaffinic kerosene, POSF 7629, derived cetane number 31) were investigated in a single-cylinder research engine (SCRE) at several speeds and loads. The most significant finding of the current work is that the introduction of a small pulse of fuel prior to the main fuel injection event, termed a close-coupled pilot (CCP) injection, effectively mitigates the relatively longer ignition delay time of the DCN 31 fuel. Therefore, a potential technical solution exists that would permit the use of low-cetane jet fuels in military ground vehicles if the operational scenario required it. Citation: M. Tess, E. Gingrich, S. Stoll, “Combustion Strategies for Low-Cetane Fuels”, In Proceedings of the Ground Vehicle Systems Engineering and Technology Symposium (GVSETS), NDIA, Novi, MI, Aug. 13-15, 2019.

Simultaneous measurements of global engine heat transfer and subsurface piston temperatures were utilized to evaluate the potential of an extended duration combustion event to minimize heat rejection in a diesel engine. The combustion duration was parametrically varied by changing the injector orifice diameter (0.167, 0.196, and 0.230 mm) and fuel injection pressure (110–200 MPa) while exploring three high-output operating conditions. This investigation was limited to a single-pulse injection strategy while holding engine load constant to represent operating points at rated power and peak torque. For a constant combustion phasing, the results suggest the duration of the spray and combustion processes did not greatly influence gross indicated thermal efficiency (ITEg) or global heat transfer from the engine, except for extremely long combustion durations of approximately 50 crank-angle degrees or longer. At the longest combustion durations, there was a negative impact on ITEg and global heat transfer. Analysis of the steady-state piston temperature measurements at constant CA50 revealed two interesting, and opposite, trends: as the combustion duration was increased through a smaller orifice diameter injector, the piston temperatures increased by up to 60°C locally on the bowl rim; in contrast, as the combustion duration was increased through lower injection pressures, the piston temperatures decreased by up to 80°C locally on the bowl rim. CFD simulations coupled with a conjugate heat transfer model of the piston predicted the piston heat flux at the highest load operating condition. The CFD results qualitatively agreed with the piston temperature measurements in supporting the conclusion that orifice diameter had a stronger effect on piston heat flux than injection pressure. Overall, this research provides directional trends to engine designers and calibrators for optimizing the injection parameters for decreased engine heat transfer and managing piston temperatures.

In military applications, diesel engines are required to achieve high power outputs and therefore must operate at high loads. This high load operation leads to high piston component temperatures and heat rejection rates limiting the packaged power density of the powertrain. To help predict and understand these constraints, as well as their effects on performance, a thermodynamic engine model coupled to a finite element heat conduction solver is proposed and validated in this work. The finite element solver is used to calculate crank angle resolved, spatially averaged piston temperatures from in-cylinder heat transfer calculations. The calculated piston temperatures refine the heat transfer predictions as well requiring iteration between the thermodynamic model and finite element solver. Both the thermodynamics and the piston temperature predictions are validated against experimental data obtained from a heavy-duty single cylinder research engine equipped with a wireless telemetry system and piston surface thermocouples to measure piston surface temperatures. The piston backside conditions are critical to the performance of the temperature solver, therefore the tuning of piston backside conditions to match experimental data is considered and assessed. The validated model is then used to analyze the performance of the heat transfer correlations developed by Woschni and Hohenberg. The piston temperatures predicted by each of the correlations are compared to those measured in the experiment both in terms of the piston temperature swing and its sensitivity to injection timing. Finally, the capabilities of the coupled model are demonstrated by analyzing the effects of engine geometry on engine performance relative to critical limitations for military engines.

In this work, dissimilar rotary inertia friction welds between 422 martensitic stainless steel and 4140 martensitic low-alloy steel were made to fabricate prototype heavy-duty diesel engine pistons. The influence of the inertia friction welding process and post weld heat treatment (PWHT) temperature on the interfacial microstructure evolutions and corresponding effects on mechanical properties of the 422/4140 welds were evaluated in detail. Carbon diffused from the 4140 side to the 422 side during PWHT at 650 & DEG;C for 1.5 h, causing the formation of a hard carbide-rich layer on the 422 side, and a softer but discontinuous C-depleted layer the 4140 side. PWHT at 700 & DEG;C for 1.5 h greatly accelerated C diffusion across the interface relative to 650 & DEG;C, resulting in a thicker hard carbide-rich layer and a relatively thick and continuous layer of coarse C-depleted grains (ferrite) on the 4140 side. In addition, the PWHT temperature greatly influenced the tensile properties and fracture behavior of the welds, with the 650 & DEG;C PWHT-ed samples failing predominately in a ductile manner in the 4140 heat affected zone during tensile testing. Conversely, the 700 & DEG;C PWHT specimens exhibited a strength reduction compared with the 650 & DEG;C PWHT specimens because of additional coarsening of the interfacial ferrite layer and softening of the base materials during PWHT, with brittle fracture between the hard and soft layers the predominate failure mechanism. Based on the findings, a reduced PWHT temperature and/or time, minimizing the hardness differential of the base metals, and pre-heating the 422 steel prior to welding are the potential pathways to achieve a more optimal balance between desirable tempering and stress relief of the weld microstructure and undesirable C migration across the weld interface, and to reduce the strength mismatch across the weld.

Fatigue analysis of pistons is reliant on an accurate representation of the high temperatures to which they are exposed. It can be difficult to represent this accurately, because instrumented tests to validate piston thermal models typically include only measurements near the piston crown and there are many unknown backside heat transfer coefficients (HTCs). Previously, a methodology was proposed to aid in the estimation of HTCs for backside convection boundary conditions of a stratified charge compression ignition (SCCI) piston. This methodology relies on Bayesian inference of backside HTC using a co-simulation between computational fluid dynamics (CFD) and finite element analysis (FEA) solvers. Although this methodology primarily utilizes the more computationally efficient FEA model for the iterations in the calibration, this can still be a computationally expensive process. In this paper, several data reduction methods, such as principal component analysis, data clustering and resampling, sensor reduction, and uniform bin sampling are investigated to improve computation time while minimizing reduction in accuracy of the inference results. Each data reduction method is compared to a control case to determine change in accuracy and improvement in run time. Results indicate that most reduction methods were no more effective than using a smaller Latin hypercube design to inform the Gaussian process within the Bayesian inference code. Reduced error was observed for the structured sensor reduction method, indicating that further studies on the value of individual sensor locations to the overall calibration might be a viable path to reduce the computation time of the calibration methodology without compromising accuracy.

High-output diesel engine heat transfer measurements are presented in this paper, which is the first of a two-part series of papers. Local piston heat transfer, based on fast-response piston surface temperature data, is compared to global engine heat transfer based on thermodynamic data. A single-cylinder research engine was operated at multiple conditions, including very high-output cases – 30 bar IMEPg and 250 bar in-cylinder pressure. A wireless telemetry system was used to acquire fast-response piston surface temperature data, from which heat flux was calculated. An interpolation and averaging procedure was developed and a method to recover the steady-state portion of the heat flux based on the in-cylinder thermodynamic state was applied. The local measurements were spatially integrated to find total heat transfer, which was found to agree well with the global thermodynamic measurements. A delayed onset of the rise of spatially averaged heat flux was observed for later start of injection timings. The dataset is internally consistent, for example, the local measurements match the global values, which makes it well suited for heat transfer correlation development; this development is pursued in the second part of this paper.

The use of nickel- and iron-based alloy coatings containing Cr, Al, and Y (NiCrAlY and FeCrAlY) were evaluated as potential oxidation barriers for low-alloy steels for the next generation of high-output diesel engines. Rapid oxidation at temperatures above 500 °C currently limits the use of the 4140 steels used to manufacture pistons crowns in this application. Isothermal furnace testing, a novel highly transient combustion-based laboratory test, and direct high temperature engine exposure were used to assess the efficacy of the coating-substrate systems. Both NiCrAlY and FeCrAlY coatings appear to protect the substrate from oxidation at isothermal temperatures up to 677 °C. On direct exposure to combustion, through Impulse Cyclic Heating Tests, the NiCrAlY coatings themselves also appear resistant to degradation at very high temperatures, while the FeCrAlY coatings degrade more rapidly for equivalent thermal loading. Engine tests further validated the efficacy of NiCrAlY coatings, with neither coatings nor 4140 steel pistons showing degradation at temperatures estimated to be more than 500 °C. From this work NiCrAlY coatings are estimated to have a potential upper limit of 677 °C, at which interdiffusion between the coating and substrate will likely complicate their use.

Five different commercially available high-temperature martensitic steels were evaluated for use in a heavy-duty diesel engine piston application and compared to existing piston alloys 4140 and microalloyed steel 38MnSiVS5 (MAS). Finite element analyses (FEA) were performed to predict the temperature and stress distributions for severe engine operating conditions of interest, and thus aid in the selection of the candidate steels. Complementary material testing was conducted to evaluate the properties relevant to the material performance in a piston. The elevated temperature strength, strength evolution during thermal aging, and thermal property data were used as inputs into the FEA piston models. Additionally, the long-term oxidation performance was assessed relative to the predicted maximum operating temperature for each material using coupon samples in a controlled-atmosphere cyclic-oxidation test rig. A current commercial steel piston alloy, quenched and tempered martensitic steel 4140, was tested in a single-cylinder research engine for a baseline oxidation and mechanical performance assessment using an abbreviated (50h) durability test plan. The predicted suitability of a candidate piston material in an engine is primarily based on its elevated temperature strength, oxidation resistance, and the complex influence of thermal conductivity, the latter of which is substantially lower for the candidate materials considered in this research relative to the traditional alloys. Although the lower thermal conductivity causes the candidate alloys to operate in higher temperature ranges under identical engine operating conditions and piston geometries, increasing the likelihood of partially or completely negating their strength and oxidation resistance advantages relative to 4140 and MAS steels, this evaluation indicates that several of the candidate piston alloys are predicted to enable improved oxidation resistance under more severe engine operating conditions relative to the current piston materials. However, further evaluation is required to determine if the elevated temperature fatigue strength and durability of these alloys are suitable for more severe engine conditions.

The thermal and mechanical properties of martensitic stainless steel 422 were evaluated for suitability as a drop-in replacement for 4140 steel in next generation heavy-duty diesel engine (HDDE) pistons. The time and temperature of the austenitization and tempering steps were studied to achieve optimum materials performance in piston applications, including the balance of thermal and mechanical properties and resistance to long-term thermal aging. Reducing the tempering temperature from 700 to 600 °C caused a substantial increase in elevated temperature strength from 25 to 600 °C, but had no significant influence on thermal conductivity, suggesting that thermal conductivity in 422 is dominated largely by composition and distribution of alloying elements and mostly independent of the sub-grain structure size and precipitate size. Compared to the current HDDE piston alloy 4140, 422 exhibits substantially higher elevated temperature strength and lower thermal conductivity, the latter which will cause 422 to operate at higher temperatures in pistons, possibly requiring a piston redesign to take advantage of the improved high temperature strength of 422. Piston material selection and alloy design strategies with potential to mitigate some of the shortcomings of martensitic stainless steels, such as 422, as drop-in replacements are discussed.

A new temporally resolved spatially averaged heat transfer correlation was developed using the local piston heat flux data presented in the first part of this paper. The new correlation extends previous correlations that relate the Nusselt and Reynolds numbers through a power law by adding a dimensionless chemical energy release rate term. The new term, which arises from dimensional analysis, should enable similitude for diesel engines. Additionally, the characteristic velocity used in the Reynolds number was modified to include the integrated fuel mass injection rate. The new correlation was calibrated to the experimental data by minimizing the least squares error, and compared to existing correlations from the literature. On average, the new formulation was found to match the experimental data better than the existing models even when the existing models' constants were adjusted to best fit the measured data.