Internal Combustion Engine Flows Research Articles

Analytical and numerical modeling, sensitivity analysis, and multi-objective optimization of the acoustic performance of the herschel-quincke tube

This study presents the assessments of the acoustic performance of a Herschel-Quincke (HQ) tube, which includes sensitivity analysis and multi-objective optimization of its design variables. A proper model introduced using the flow characteristics of the HQ tube. Input variables were frequency (f), temperature (T), the ratio of areas (R), and tube length (L). These variables were chosen to be varied in a specified range to best characterize the exhaust flow in internal combustion engines. Then a proper multi-objective genetic algorithm (MOGA) was developed for the acoustic performance and geometry optimization of a two-duct HQ tube; the objectives were maximizing the transmission loss (TL) and at the same time minimizing the backpressure (BP). Besides, the local sensitivity analysis using numerical derivative and variance-based global sensitivity analysis (GSA) using Monte Carlo sampling was performed for the analytical model of the HQ tube. The results showed that the length of the bypass tube is a crucial factor for the maximum sensitivity index (SI) of the TL, while the SI of the BP was maximum for the ratio of areas. The proposed analytical model was proved to be reliable for the TL, showing low values of the error of the sensitivity index (eSI) and less reliable for the BP parameter, showing higher eSI values. For the evaluation of the TL sensitivity, Monte Carlo sampling is relatively inaccurate in the small sample size of 200. It was also observed that the Uniform distribution had lower eSI in lower sample sizes; however, Sobol sampling showed better performance in higher sample sizes. The MOGA optimization proved to be successful in maximizing the TL for the frequencies between 105.84 and 1981.11 Hz, with the TL greater than 20 dB, and the BP less than 0.1. The most efficient solution among the Pareto set after a tradeoff was at 105.84 Hz, for which the TL and the BP were 36.47 dB and 1.50E−02, respectively. The model with the best goodness of fit to represent the Pareto front was the power model with two terms. Then, the acoustic performance of the optimized geometry was investigated using computational aeroacoustics (CAA) in the presence of mean flow in Fluent software. Using the CAA approach, the maximum TL value was obtained for 104 0.4 Hz as 12.14 dB. The simulated TL by CAA had more broadband behaviors with fewer peaks than the analytical approach. The results of this study demonstrated the remarkable potential of the MOGA optimization and sensitivity analysis for acoustic performance and topology optimization for possible applications in internal combustion engines.

Methodology for the large-eddy simulation and particle image velocimetry analysis of large-scale flow structures on TCC-III engine under motored condition

Large-eddy simulation has been increasingly applied to internal combustion engine flows because of their improved potential to capture the spatial and temporal evolution of turbulent flow structures compared with Reynolds-averaged Navier Stokes simulation. Furthermore, large-eddy simulation is universally recognized as capable of simulating highly unsteady and random phenomena, which drive cycle-to-cycle variability and cycle-resolved events such as knocks and misfires. To identify large-scale structure fluctuations, many methods have been proposed in the literature. This article describes the application of several analysis methods for the comparison between different datasets (experimental or numerical) and the identification of large-structure fluctuations. The reference engine is the well-known TCC-III single-cylinder optical unit from the University of Michigan and GM Global R&D center; the analyses were carried out under motored engine conditions. A deep analysis of in-cylinder gas dynamics and flow structure evolution was performed by comparing the experimental results (particle image velocimetry of the velocity fields) with a dataset of consecutive large-eddy simulation cycles on four different cutting planes at engine-relevant crank angle positions. Phase-dependent proper orthogonal decomposition was used to obtain further conclusions regarding the accuracy of the simulation results and to apply conditional averaging methods. A two-point correlation and an analysis of the tumble center are proposed. Finally, conclusions are drawn to be used as guidelines in future large-eddy simulation analyses of internal combustion engines.

Application of a zonal hybrid URANS/LES turbulence model to high and low-resolution grids for engine simulation

A zonal hybrid unsteady Reynolds-averaged Navier–Stokes/large eddy simulation (URANS-LES) Zonal detached-eddy simulation (ZDES) model is applied to internal combustion engine (ICE) simulation and comparisons of predicted flow morphology and variability are carried out against on the transparent combustion chamber (TCC-III) particle image velocimetry (PIV) data set for motored conditions. To this aim, a previously developed model derived from a standard seamless-detached eddy simulation (DES) formulation is adopted for two different grid resolutions. In particular, two zonalization choices are evaluated based on previous single-grid results, in order to assess the model outcomes based on the joint turbulence treatment/grid density: the seamless-DES mode is applied (1) only to the cylinder (TCC-Z1) and (2) to the cylinder and intake port (TCC-Z2). Multi-cycle simulations (50 samples) are carried out and the results are compared to experimental data in terms of PIV images using multiple quality indices on multiple planes ( Y = 0 and X = 0). Finally, comparison of predicted mean flow fields is extended to standard URANS mode. Results show that the use of a cylinder-only seamless-DES treatment on a relatively coarse grid results in a quantitative agreement between simulated and measured (PIV) flow fields, both in terms of average morphology and flow variability, whereas the extension of the DES mode to the intake port does not introduce relevant variations. Quality indicators seem to be moderately sensitive to the grid resolution, thus confirming the adaptive potential of a ZDES–like model and promoting the use of DES–type turbulence modelling even on relatively low-resolution grids. The analysis of average fields compared to URANS simulations highlights the benefit for both grids of a scale-resolving ZDES modelling when the same underlying turbulence model ( k-ε RNG) is used. This study reinforces the recommendation in the use of hybrid URANS-LES models to simulate ICE flows. The adopted ZDES formulation based on the two-equation k-ε RNG model shows that high-quality results can be obtained even on engineering-grade grids, both in terms of average and cycle-to-cycle variation. The numerical results obtained using the two grids with variable resolution are consistent, and this further promotes a wider adoption of this class of models to simulate engine flows in industrial applications.

Time-sequenced flow field prediction in an optical spark-ignition direct-injection engine using bidirectional recurrent neural network (bi-RNN) with long short-term memory

To further improve the energy conversion efficiency of internal combustion engine, the transient and complex air flow movement inside the cylinder needs to be better understood and controlled. Although the in-cylinder flow fields are highly stochastic with strong cycle-to-cycle fluctuations, machine learning can still provide an efficient way to learn and regress the complex flow movement process inside the cylinder. In this work, a bidirectional recurrent neural network (bi-RNN) model with long short-term memory was applied to predict the in-cylinder flow fields at different time steps using training data from multi-cycle particle image velocimetry (PIV) measurements. To evaluate the agreement between the true and predicted flow fields, structure and magnitude comparison indices are calculated both globally and locally. The comparison results show that the bi-RNN model can accurately predict the bulk flow and vortex motions from early intake stroke to compression stroke. This work demonstrates that the machine learning model has the potential to predict the underlying dynamics of the interaction between in-cylinder flows and provides a reliable way to improve temporal resolution in PIV flow data to better reveal transient in-cylinder flow features.

Large eddy simulation analysis of the turbulent flow in an optically accessible internal combustion engine using the overset mesh technique

Computational fluid dynamics has become a fundamental tool for the design and development of internal combustion engines. The meshing strategy plays a central role in the computational efficiency, in the management of the moving components of the engine and in the accuracy of results. The overset mesh approach, usually referred to also as chimera grid or composite grid, was rarely applied to the simulation of internal combustion engines, mainly because of the difficulty in adapting the technique to the specific complexities of internal combustion engine flows. The article demonstrates the feasibility and the effectiveness of the overset mesh technique application to internal combustion engines, thanks to a purposely designed meshing approach. In particular, the technique is used to analyze the cycle-to-cycle variability of internal combustion engine flows using large eddy simulation. Fifty large eddy simulation cycles are performed on the well-known TCC-III engine in motored condition. Results are analyzed in terms of tumble center trajectory and using proper orthogonal decomposition to objectively characterize the spatial and temporal evolution of turbulent flow field in internal combustion engines. In particular, an original decomposition method previously applied by the authors to the TCC-III measured flow fields is here extended to computational fluid dynamics results.

Impact of Grid Density on the Analysis of the In-Cylinder Flow of an Optical Engine

The evaluation of Internal Combustion Engine (ICE) flows by 3D-CFD strongly depends on a combination of mutually interacting factors, among which grid resolution, closure model, numerics. A careful choice should be made in order to limit the extremely high computational cost and numerical problems arising from the combination of refined grids, high-order numeric schemes and complex geometries typical of ICEs. The paper focuses on the comparison between different grid strategies: in particular, attention is focused firstly on near-wall grid through the comparison between multi-layer and single-layer grids, and secondly on core grid density. The performance of each grid strategy is assessed in terms of accuracy and computational efficiency. A detailed comparison is presented against PIV flow measurements of the Spray Guided Darmstadt Engine available at the Darmstadt University of Technology. As many research groups are simultaneously working on the Darmstadt engine using different CFD codes and meshing approaches, it constitutes a perfect environment for both method validation and scientific cooperation. A motored engine condition is chosen and the flow evolution throughout the engine cycle is evaluated on two different section planes. Pros and cons of each grid strategy are highlighted and motivated.

Validation of a zonal hybrid URANS/LES turbulence modeling method for multi-cycle engine flow simulation

A zonal hybridization of the RNG [Formula: see text]-[Formula: see text] URANS model is proposed for the simulation of turbulent flows in internal combustion engines. The hybrid formulation is able to act as URANS, DES or LES in different zones of the computational domain, which are explicitly set by the user. The resulting model has been implemented in a commercial computational fluid dynamics code and the LES branch of the modified RNG [Formula: see text]-[Formula: see text] closure has been initially calibrated on a standard homogeneous turbulence box case. Subsequently, the full zonal formulation has been tested on a fixed intake valve geometry, including comparisons with third-party experimental data. The core of the work is represented by a multi-cycle analysis of the TCC-III experimental engine configuration, which has been compared with the experiments and with prior full-LES computational studies. The applicability of the hybrid turbulence model to internal combustion engine flows is demonstrated, and PIV-like flow statistics quantitatively validate the model performance. This study shows a pioneering application of zonal hybrid models in engine-relevant simulation campaigns, emphasizing the relevance of hybrid models for turbulent engine flows.

Wall heat transfer of unsteady near-wall flow in internal combustion engines

Although the near-wall turbulence is not fully developed in the engine combustion chamber, wall heat transfer models based on flow characteristics in fully developed near-wall turbulence are typically employed in engine simulations to predict heat transfer. Only few studies reported the wall heat transfer mechanism in near-wall flow where the near-wall turbulence was not fully developed as expected in the engine combustion chamber. In this study, the velocity distribution and wall heat flux in such a near-wall flow were evaluated using a rapid compression and expansion machine. In addition to the experimental approach, a numerical simulation with highly resolved calculation mesh was applied in various flow fields expected in the engine combustion chamber. As a result, the turbulent Reynolds number that represents the relationship between turbulent production and dissipation varied in the wall boundary layer according to the near-wall flow condition. This behavior affects the wall heat transfer. Considering this finding, a new model was formulated by introducing a ratio of turbulent Reynolds number in an intended near-wall flow to that in fully developed near-wall turbulence. It was confirmed that the proposed model could improve the prediction accuracy of wall heat flux even in near-wall flow where the near-wall turbulence was not fully developed. By applying the proposed model in engine computational fluid dynamics, it was found that the proposed model could predict the wall heat flux in a homogeneous charge compression ignition gasoline engine with acceptable accuracy.

Investigation of the Boundary Layer Flow Under Engine-Like Conditions Using Particle Image Velocimetry

The turbulent boundary layer flow in internal combustion (IC) engines has a significant effect on the in-cylinder flow and the wall heat transfer. A detailed analysis of the in-cylinder near-wall flow was carried out on an optical steady flow test bench by using high-resolution particle image velocimetry (PIV) in order to characterize the in-cylinder boundary layer flow in this study. The difference between the in-cylinder boundary layer and the canonical turbulent boundary layer was analyzed. The experimental results show that small-scale vortices with a length scale of about 1–2 mm in the instantaneous flow fields appeared in the wall jet region due to the entrainment of the free jet in the outer region of the wall jet. The viscous sublayer thickness decreased from 0.5 mm to 0.3 mm as the valve lift increased from 2.32 mm to 7.975 mm and the pressure drop from 0.5 kPa to 1 kPa. The dimensionless velocity profile is in good agreement with the law of the wall in the viscous sublayer. However, no obvious logarithmic law distribution region was observed in the logarithmic layer. The distribution of the Reynolds stress and the turbulent kinetic energy is similar to that of the canonical turbulent boundary layer. But the Reynolds stress had a much larger magnitude because the turbulent velocity measured in this boundary layer included not only the turbulence generated by wall shear but also the large-scale turbulent vortices caused by the wall jet.

A Study of the Organized in-Cylinder Motion by a Dynamic Adaptive Scale-Resolving Turbulence Model

The feasibility of using a scale-adaptive turbulence model for the simulation of the unsteady flow in Internal Combustion (IC) Engines is investigated. The original theory of the Dynamic Length-Scale Resolution Model (DLRM), which includes an adaptive rescaling procedure for the modeled turbulent length and time scales, has been trimmed and applied to the simulation of the flow field of a geometrically simplified square-piston engine working under motored conditions. The flow field exhibits a strong tumbling motion, which is a major characteristic of modern turbo-charged, direct-injection gasoline engines. Multiple consecutive cycles have been computed and turbulent statistics, including the ensemble mean and the variance of the velocity field, have been compared with both experimental measurements and simulation results predicted by URANS and conventional LES with identical numerical setups.

In-Cylinder Flow Evolution Using Tomographic Particle Imaging Velocimetry in an Internal Combustion Engine

In-cylinder flows in internal combustion (IC) engines have always been a focus of study in order to gain better understanding of fuel–air mixing process and combustion optimization. Different conventional experimental techniques such as hot wire anemometry (HWA), laser Doppler anemometry (LDA), and numerical simulations have been grossly inadequate for complete understanding of the complex 3D flows inside the engine cylinder. In this experimental study, tomographic particle imaging velocimetry (PIV) was applied in a four-valve, single-cylinder optical research engine, with an objective of investigating the in-cylinder flow evolution during intake and compression strokes in an engine cycle. In-cylinder flow seeded with ultra-fine graphite particles was illuminated by a high energy, high frequency Nd:YLF laser. The motion of these tracer particles was captured using two cameras from different viewing angles. These two-directional projections of flowfield were used to reconstruct the 3D flowfield of the measurement volume (36 × 25 × 8 mm3), using multiplicative algebraic reconstruction technique (MART) algorithm. Captured images of 50 consecutive engine cycles were ensemble averaged to analyze the in-cylinder flow evolution. Results indicated that the in-cylinder flows are dependent on the piston position and spatial location inside the engine cylinder. The randomness of air-flow fields during the intake stroke was very high, which became more homogeneous during the compression stroke. The flows were found to be highly dependent on Z plane location inside the engine. During the intake stroke, flows were highly turbulent throughout the engine cylinder, and velocities vectors were observed in all directions. However, during the compression stroke, flow velocities were higher near the injector, and they reduced closer to the valves. Absolute velocity during compression stroke was mainly contributed by the out of plane velocity (Vz) component.

Improvement of the two-fluid momentum equation using a modified Reynolds stress model for horizontal turbulent bubbly flows

Two-fluid equations are widely used for practical applications involving multi-phase flows in chemical reactor, nuclear reactor, desalination systems, boilers, and internal combustion engines. The popular two-fluid equation for a gas-liquid two-phase flow is based on the assumption of interpenetrating continua. According to the experimental data of fully-developed turbulent bubbly flows in a horizontal pipe, the bubble phase velocity is close to or slightly smaller than the liquid phase velocity. The velocity profile is nearly symmetric along the vertical centerline of the horizontal pipe, or tends to be slightly skewed toward the bottom region of the pipe. However, numerical simulations using the momentum equation based on interpenetrating continua showed that, in contrast, the bubble phase was faster than the liquid phase. In addition, the velocity profile was predicted to be skewed toward the upper region of the pipe. These simulation results are not consistent with experimental observations. In the meantime, there are particle averaged momentum equations in which the continuous and disperse phase equations are developed from the equations of motions of fluid and particle, respectively. We considered two different particle averaged momentum equations. The form of one particle averaged momentum equation is similar to that of the momentum equation based on interpenetrating continua, except for the laminar viscosity term. Thus, for a turbulent bubbly flow, this particle averaged equation showed similar results as observed in the momentum equation based on interpenetrating continua. The other particle averaged equation differs from the momentum equation based on interpenetrating continua in both laminar and turbulent viscosity terms. This particle averaged equation showed good agreement with experimental observations.

Comparative study of turbulence models for scale-resolving simulations of internal combustion engine flows

Scale-resolving simulations (SRS) are becoming more and more important for internal combustion (IC) engine simulations and are gradually replacing first-generation unsteady Reynolds-averaged Navier-Stokes (URANS) based approaches. In addition to the substantial improvement in the prediction of the local flow and mixing processes and their interaction with the spray and the turbulent flame, SRS are essential for the quantification of cycle-to-cycle variation (CCV) of coherent structures and sporadically occurring phenomena such as misfire and knock. However, the choice of a suitable scale-resolving turbulence model for specific applications such as IC engine flows is still an unresolved issue even in the scientific community. Typically, turbulence models are developed and validated for well-defined test cases and it is not clear whether these findings are also valid for flows in complex geometries with moving boundaries, where local flow structures might be significantly different. The flow in IC engines is characterized by strongly varying Reynolds numbers and exhibits several flow phenomena such as wall boundary layers, stagnation points, free-stream shear layers and edge-induced flow separations. The current paper is a systematic comparative study of selected and well-established scale-resolving turbulence models in the context of an IC engine flow focussing especially on the intake stroke. Different classical LES models (Smagorinsky, WALE, Sigma), a hybrid model (DES-SST) and a second-generation URANS model with scale-resolving capabilities (SAS-SST) are compared against each other. The models’ accuracy and capability to capture fluctuations, local flow structures and CCV during the tumble formation in the intake phase are investigated for a well-established IC engine benchmark case with a single non-moving valve. Though this is a simplified setup, it provides the unique possibility to compare the results to reference data obtained from experiment and direct numerical simulations (DNS). In a first step, the predicted averaged velocity and the resolved fluctuations obtained with the different models are compared to each other and to the reference data. Afterwards, the local structure of the resolved fluctuations is investigated in terms of an invariant analysis of the anisotropy tensor. Finally, the capability to resolve CCV is investigated based on the averaged and instantaneous tumble center, a large-scale flow structure specifically important for engine flows.

Numerical Analysis of In-Cylinder Flow in Internal Combustion Engines by LES Method

In this research, Large Eddy Simulation of in-cylinder flow during suction and compression stroke in an axisymmetric engine is performed. A computer code using Smogorinsky subgrid model is developed to solve the governing equations of the flow. A proper understanding of flow during suction and compression strokes, gives better information for fuel/air mixture and combustion. The results show that the advantage of LES model is the ability of computing turbulence characteristics in various crank angles of engine cycle. This advantage of model is highlighted by calculating RMS values of axial velocity in comparison with experimental ones. The results show that axial velocity fluctuations during intake reaches to a higher level than in compression stroke because of the inlet jet to the cylinder and intensive gradient of variables. In this regard, the flow in 100 degree ATDC during intake stroke reaches the maximum level of turbulence intensity and then turbulence generated during intake stroke decays rapidly. During intake stroke, three main vorticities are generated inside the cylinder. In the compression stroke these three vorticities are merged together to establish a new vorticity with direction of rotation opposite to the intake flow. Some smaller recirculating regions are also generated at 90 degree BTDC.

Wall Heat Transfer of Undeveloped Turbulent Flow in Internal Combustion Engines

Wall Heat Transfer of Undeveloped Turbulent Flow in Internal Combustion Engines

Flying PIV measurements in a 4-valve IC engine water analogue to characterize the near-wall flow evolution

For a deeper understanding of the highly unsteady near-wall boundary layer flows in internal combustion (IC) engines, PIV-based flow field measurements close to the inner cylinder and piston walls within transparent engines are required. The herein described flying PIV method in combination with a scanning light-sheet provides time-resolved PIV measurements in a transparent IC engine water analogue in a radial plane 1.5 mm apart from the planar piston crown while the piston is moving. The light-sheet is parallel to the piston surface and moves with the piston thanks to the scanning technique that synchronizes the sheet motion with the non-linear piston motion. A compact high speed camera is positioned within the piston shaft below the transparent piston head and records the particle fields within the illuminated plane in time-resolved manner. The measurements are realized in a water-analogue of a 4-valve engine at 950 rpm engine speed in real situation. Instantaneous pictures are compared to phase-averaged velocity maps and allowed to localize regions of high cycle-to-cycle fluctuations.

Feed-forward control of purge flow in internal combustion engines

Presented is a feed-forward controller that coordinates engine fueling with a purge event to reduce air–fuel ratio excursions. The controller employs a linear parameter-varying model to estimate the necessary changes in fuel pulse-width based on a hydrocarbon sensor located in the purge line. Synchronization between the purge fuel vapor arrival at the intake manifold and the fueling command is realized using a transport delay model. The proposed controller is experimentally validated on a Ford 5.4-L port injected engine and a 3.6-L EcoBoost engine. Significant reductions in air–fuel ratio excursions are achieved during the steady-state engine operating conditions associated with highway conditions.

An Eulerian CFD model and X-ray radiography for coupled nozzle flow and spray in internal combustion engines

This paper implements a coupled approach to integrate the internal nozzle flow and the ensuing fuel spray using a Volume-of-Fluid (VOF) method in the finite-volume framework. A VOF method is used to model the internal nozzle two-phase flow with a cavitation description closed by the homogeneous relaxation model of Bilicki and Kestin (1990). An Eulerian single velocity field approach by Vallet et al. (2001) is implemented for near-nozzle spray modeling. This Eulerian approach considers the liquid and gas phases as a complex mixture with a highly variable density to describe near nozzle dense sprays. The liquid mass fraction is transported with a model for the turbulent liquid diffusion flux into the gas. Fully-coupled nozzle flow and spray simulations are performed in three dimensions and validated against the X-ray radiography measurements of Kastengren et al. (2014) for a diesel fuel surrogate. A standard k–∊ Reynolds Averaged Navier Stokes based turbulence model is used in this study and the influence of model constants is evaluated. First, the grid convergence study is performed. The effect of grid size is also evaluated by comparing the fuel distribution against experimental data. Finally, the fuel distribution predicted by the coupled Eulerian approach is compared against that by Lagrangian–Eulerian spray model along with experimental data. The coupled Eulerian approach provides a unique way of coupling the nozzle flow and sprays so that the effects of in-nozzle flow can be directly realized on the fuel spray. Both experiment and numerical simulations show non-cavitation occurring for this injector with convergent nozzle geometry. The study shows that the Eulerian approach has advantages over near-field dense spray distributions.

Simulation and evaluation of a CCHP system with exhaust gas deep-recovery and thermoelectric generator

Combined cooling, heating and power (CCHP) systems are thought to be highly efficient in energy utilization. But there are still potentials to further improve system performance. This work proposed a CCHP system based on internal combustion engine (ICE) for power generation, refrigeration and domestic hot water production. Thermoelectric generator (TEG) and condensing heat exchanger are applied to efficiently recover the exhaust gas waste heat of ICE. All the energy flows are designed based on energy cascading utilization principle. Basing on the test results of a 16kW ICE, CCHP system characteristics are investigated by simulation from idling condition to full load condition. Especially, the part load performance of TEG and absorption chiller are simulated and discussed. The feasible operating zone of ICE and feed water flow rate are figured out to keep the domestic hot water temperature within a certain range. Results show that the primary energy efficiency of system can reach 0.944, thanks to the condensing heat recovery from exhaust gas. The primary energy saving ratio and cost saving ratio can reach 0.304 and 0.417, respectively. Considering some more equipment is incorporated, the total investment increment is about 11.1%.

Investigation into the benefits of reduced oil flows in internal combustion engines

The engine lubrication system is a vital element for engine health but causes a parasitic load on the engine which increases the fuel consumption: this load can be reduced by matching the oil flow to lubricating requirements using a variable displacement oil pump. In a first stage, two variable displacement oil pumps were installed on a 2.4-L diesel engine; experiments over the New European Drive cycle showed reductions in fuel consumption of up to 3.4% and up to 5.8% over the urban phase of the cycle. A variable displacement oil pump was subsequently installed on an instrumented engine capturing over 100 metal and fluid temperatures within the engine structure. This showed that reducing oil flows resulted in lower oil temperature by up to 4 °C during cold-start New European Drive cycle but hotter cylinder liner temperatures by up to 6 °C. The higher cylinder wall temperatures caused an increase of 3% in oxides of nitrogen emissions but a reduction of 3%−5% in carbon monoxide and hydrocarbon emissions. Finally, an energy flow analysis showed that the variable displacement oil pump can reduce oil pump energy consumption by 160 kJ (32%) but that this led to a 400-kJ reduction in friction and accessory work. These findings highlight the need for a system-level rather than a component-level approach to engine lubrication design to capture key thermal interactions.