Bulk Temperature Research Articles

Gear and bearing power losses: from dip to minimum quantity lubrication

PurposeThis paper aims to address the influence of lubrication methods on operational characteristics, power losses and temperature behavior of gears and bearings. It contributes to the improvement of resource and energy efficiency of geared transmissions.Design/methodology/approachExperimental investigations were performed at a gear and bearing power loss test rig. Thereby, dip lubrication, injection lubrication with injection volumes from 0.05 to 2.00 l/min and minimum quantity (MQ) lubrication with an injection volume as little as 28 ml/h were considered. Measurements were evaluated in terms of no-load and load-dependent power loss, bulk temperatures and mean gear coefficients of friction.FindingsResults show strongly reduced no-load gear and bearing losses for lubrication methods with low lubricant quantities. Load-dependent losses are similar to conventional lubrication methods and tend to be lower at high speed. This is related to higher bulk temperatures, as the heat dissipation of lubrication methods with low oil quantities is limited. Limited thermal load limits were shown to be extended by LowLoss gears.Originality/valueSystematic investigations were conducted to evaluate the influence of dip, injection and MQ lubrication on power loss and temperature behavior of gears and bearings. The results of this study support further research on needs-based lubrication methods for gearboxes.

Investigation of the thermohydraulic characteristics of vertical supercritical CO2 flows at cooling conditions

The thermohydraulic characteristics of supercritical CO2 flows in a vertical tube at cooling conditions are numerically investigated, and the influence of the heat-flux condition and of the flow direction are evaluated. Constant (i.e., uniform), linearly increasing and linearly decreasing heat-flux conditions are considered as three typical heat-flux distributions over the pipe length. The simulation results show that there exists a maximum heat transfer coefficient at all heat-flux conditions when the fluid bulk temperature is slightly higher than the pseudo-critical temperature, but also that the heat-flux condition has little effect on the peak value of the heat transfer coefficient. From the viewpoint of the second law of thermodynamics, the influence of the heat-flux condition on the local entropy generation can be attributed to the distributed matching between the heat flux and the difference between the wall temperature and the fluid bulk temperature, as a better matching is associated with a higher uniformity of the local entropy generation and reduced overall irreversibilities. Upward and downward flows are considered, along with flows without gravity as a baseline case for comparison purposes, with the field synergy principle employed to explain the different phenomena in these flows. The buoyancy effect laminarises the downward flows and raises the temperature gradient; hence, the heat transfer deteriorates and the irreversibility increases. In the upward flows, the buoyancy effect augments the turbulence and alleviates the variations in temperature and velocity in the core region, consequently reducing the irreversible loss and enhancing heat transfer. The present study provides insights into the mechanisms of supercritical CO2 heat transfer characteristics as well as practical guidance on the design and optimisation of relevant components.

Convective flow of a Williamson hybrid nanofluid in a porous medium through a cone and wedge with the effect of the shape of nanoparticles

AbstractThe performance of nanofluid in the heat transport phenomena showed satisfactory results, which have been considered by scientists and researchers. The dispersion of two nanoparticles to form a mixture which named as hybrid nanofluid. It has been proved experimentally that hybrid nanofluid has good thermal performance compared to unitary nanofluid. The present study inspected the convective flow of a Williamson hybrid nanofluid through a cone and wedge in a porous medium of different shapes of nanoparticles (cylindrical‐, spherical‐, blades‐, platelets‐, bricks‐) under the influence of a magnetic field. The partial differential equations were converted to an ordinary differential equation and the resulting equations were solved using the bvp4c MATLAB code. The effect of some important physical parameters on both the velocity and temperature profile was graphically plotted and the effect of the shape and size of nanoparticles was discussed, the numerical values of the friction coefficient and Nusselt number were obtained. As observed, the friction factor of Ag–TiO2–H2O hybrid nanofluid was more than that of TiO2–H2O nanofluid. A significant enhancement in bulk temperature is seen for the Ag–TiO2 hybrid and nanofluid formed by blade‐shaped nanoparticles followed by cylindrical, platelet, brick, and spherical nanoparticles. In addition, for a 5% volume fraction of using blade nanoparticles shape givean increase in skin friction factor about 4.8% compared tobrick hybrid nanoparticles. A comparative study of different forms for Ag‐TiO2was presented graphically.

Enhanced heating in plasma bulk due to electron cyclotron resonance in weakly magnetized capacitively coupled plasmas

An enhanced electron heating mechanism based on a resonance between the cyclotron motion of electrons and radio frequency (rf) electric field in the plasma bulk is reported in weakly magnetized capacitively coupled argon plasmas at low pressure. When the electron cyclotron frequency coincides with the applied power source frequency, the bulk electrons can continuously acquire energy from the background electric field within certain rf periods during the cyclotron motion, inducing overall distinct increase of excitation rate and electron temperature in the plasma bulk. This enhanced electron heating effect has been examined by a combination of kinetic particle simulations, experimental measurements, and an analytical model, and the dynamics of electrons are revealed at resonant conditions.

Low-temperature microwave-driven thermochemical generation of hydrogen from steam reforming of alcohols over magnetite

In a previous study, we reported that hydrogen generation can be achieved quite efficiently from seawater and contaminated waters at low temperatures using a microwave-driven hydrogen production (MDHP) process and waste activated carbon as the microwave absorption heating element (MAHE). However, the problem with this method is that activated carbon (a heat source from the microwaves) is used in the decomposition of water in which the quantity of activated carbon continues to decrease in parallel with the evolution of hydrogen. This problem has been resolved in this study by using magnetite as a novel MAHE component at low temperatures; the energy-saving thermochemical steam reforming reaction was performed with a mixed water/ethanol solution, for which results showed a maximum hydrogen generation yield somewhat greater than 80%. No hydrogen evolved in the thermochemical steam reforming process upon heating the magnetite at 350 °C in a conventional electric furnace, in contrast to the case where hydrogen was generated in yields greater than 40% by heating at 350 °C with microwaves. The rate-determining energy required for hydrogen generation is evidently heat, as evidenced by the formation of microwave-generated microscopic high-temperature fields (hot spots) at an estimated temperature of ca. 760 °C at the magnetite surface, a result of microwave heterogeneous microscopic thermal effects (MHMEs), and this even if the average temperature of the magnetite bulk were 350 °C.

Probing the Accuracy of Experimental Data on Nusselt Numbers Within Miniature Heat Sinks

Abstract Achieving accurate experimental data in conjugate heat transfer studies to calculate Nusselt number can be challenging due to its complex three-dimensional thermal hydraulics nature. This study is devoted to evaluating the accuracy and reliability of experimental approaches used to calculate the Nusselt number in miniature heat sinks. It is observed that three major parameters including (1) axial heat conduction within the solid substrate of heat sinks, (2) thermal contact resistance, and (3) assumed uniform wall temperature, as well as wall heat flux distributions, influence the reported experimental data in the literature. The results obtained from the developed analytical and computational models in this study revealed that the assumptions of local uniform wall temperature and heat flux distributions for small-scale heat sinks result in underestimated Nusselt numbers calculated from experiments. At lower Reynolds number (<200) flows in miniature heat sinks with a high solid to fluid thermal conductivity ratio (>> 1), it is shown that the fluid bulk temperature should be measured away from the heat sink inlet and outlet to minimize the effect of axial heat conduction within the solid substrate of the microscale heat sinks on calculated Nusselt numbers. As the third important parameter, the influence of thermal contact resistance on the Nusselt number calculation in a miniature heat sink is studied where thermal slip length is considered. Finally, the concurrent effects of thermal contact resistance and thermal developing region are considered to explicate the obtained trends in the experimental Nusselt numbers dataset.

A novel method of measuring microtube wall temperatures by thermocouples to investigate sCO2 flows

Experimental study on convective heat transfer through microtubes encounters many challenges such as measuring the wall temperature of the tube with a reasonable uncertainty, relatively high heat loss from the outer surface of the tube, and difficulty in measuring the fluid bulk temperature at the tube outlet. These issues become exacerbated with flow at low Reynolds numbers. The present study presents a new method to measure the tube wall temperature with a small solder cast where a thermocouple is embedded. The net heat flux to flows, determined with the heat loss on the outer surface and the axial heat conduction, is integrated to obtain the local mean enthalpy of fluid and then the fluid mean temperature along the tube. Thus, convection heat transfer coefficients are calculated without measuring the mean temperature at the outlet. The new method is successfully validated with laminar water flows through a horizontally configured microtube, 0.501 mm in inner diameter. All Nusselt numbers are found to lie between 4.36 and 4.36 + 10%. Finally, the method developed in this study is applied to supercritical CO2 flowing upward at 8.0 MPa through a microtube, 0.509 mm in inner diameter. For both high and low mass flow rates, a local maximum of the convection heat transfer coefficient is observed at a location where the film temperature is close to the pseudo-critical temperature. On the other hand, the second local maximum of the convection heat transfer coefficient appears only at low mass flow rates.

Modeling and CFD simulation of heat transfer process coupled with Unmixed Combustion for the application of generating superheated steam

• Unmixed combustion (UMC) process is investigated for generating supherheated steam. • A design of steam superheater integrated with UMC based reactor system is proposed. • A model of the proposed system is developed and CFD simulations are performed. • Superheated steam of desired temperature (1 atm) is obtained with sustained UMC operation. • Generation of LP and MP superheated steam using UMC was also found to be feasible. Superheated steam is produced in a super-heater using the energy of hot flue gases generated in the premixed combustion and it requires a larger surface area. Recently, unmixed combustion (UMC), an environmentally better process to premixed combustion, a variant of chemical looping combustion (CLC), has been investigated successfully for transferring heat to air, with a relatively compact system. Hence to address the limitations of the size of a super-heater and environmental aspects, it is decided to investigate the application of UMC for generating superheated steam. With this objective, the existing design of UMC based heat transfer system was modified for heating saturated steam to a desired temperature on a continuous basis. The modified system with Cu-based Oxygen Storage and Release Material (OSRM) was investigated using modeling and simulation study. A 2-D computational fluid dynamics (CFD) model was developed and simulated using COMSOL TM . Oxidation and reduction cycles of UMC were studied separately and the study was restricted to laminar steam flow conditions. Results of the representative cases showed that the superheated steam of desired bulk temperature of 650 – 720 K at atmospheric pressure and heat flux of 13,000 – 15000 W/m 2 at the interface can be obtained with sustained combustion (the bed temperatures between 723 and 873 K) in both the cycles. The effect of four important operating parameters viz. inlet steam velocity, inlet reactive gas velocity and concentration, metal loading, was also investigated and optimum ranges were suggested to achieve the maximum possible heat transfer without affecting the bed operation. The simulation study was then extended to generate low and medium pressure superheated steams and obtained results have indicated that it is still possible to generate superheated steam at desired temperatures using UMC.

Continuous no-till decreased soil nitrous oxide emissions during corn years after 48 and 50 years in a poorly-drained Alfisol

The soil quality benefits from switching from chisel-disk (CD) operations to continuous no-till (NT) in corn (Zea mays L.) and soybean (Glycine max L.) rotations have been proven over time; but to mitigate climate change, effects of continuous NT on nitrous oxide (N2O) emissions must be evaluated. The objectives of this study were to determine the influence of contrasting tillage practices (CD vs. NT) on soil N2O emissions, soil nitrogen (N) dynamics, corn grain yields, N removals and partial N balances, soil volumetric water content (VWC) and soil temperature following 48 and 50 years of tillage implementation in a long-term corn-soybean rotation experiment in a poorly-drained Alfisol. A four-time replicated randomized complete block design was conducted with tillage treatments [CD (grower's current practice) and NT] as main plots and fertility [a no-fertilizer control (CTR) and fertilizing corn N, P, and K (NPK)] as subplots. Corn grain yield, N removal, and partial N balances were greater in CD than NT in 2018 but not in 2020. Soil N2O-N was similar among tillage treatments in 2018 (3.2 kg N2O-N ha−1) but higher in CD (8.5 kg N2O-N ha−1) than in NT (6.2 kg N2O-N ha−1) in 2020. The CD treatment had higher two-yr cumulative N2O-N emissions (11.9 kg N2O-N ha−1) than NT (9.1 kg N2O-N ha−1), indicating that NT has a potential for reducing N2O-N in poorly-drained Alfisols. Grain yield-scaled N2O-N was lower in NT than CD in 2020 but not in 2018. Soil N2O emissions were positively associated with soil NO3-N, partial N balances, and corn grain yield and negatively associated with soil bulk density and temperature. We concluded that after 48 and 50 years, continuous NT can maintain corn grain yield and mitigate N2O-N emissions indicating to mitigate climate change and increase multi-sphere benefits, continuous NT practices should be implemented.

Re-entrant Spin Glass and Magnetocaloric Effect in Frustrated Double Perovskite Ho2CoMnO6 Flat Nanorod

In the present work, we have synthesized single-phase flat nanorods of double perovskite Ho2CoMnO6 (HCMO) by following a hydrothermal route. The sample crystallizes into a monoclinic structure with a P21/n space group. Expansion of MnO6 octahedra and hence increase in unit cell volume as compared to its bulk counterpart is observed. Unlike the single ferromagnetic transition in the bulk HCMO sample, here magnetization versus temperature measurements show three magnetic transitions corresponding to ferromagnetic (FM), antiferromagnetic (AFM) and re-entrant spin glass (SG) ordering at 182, 97 and 31 K respectively. The FM transition (Tc) which is much higher than the bulk is attributed to super-exchange interaction in Co2+- O - Mn4+ networks and the AFM transition is due to exchange interactions across Co2+- O – Co2+ and Mn4+- O - Mn4+ networks, as well as due to the presence of Co3+ and Mn3+ ions, which is proven by the XPS analysis. The glassy magnetic phase is confirmed by magnetic relaxation and aging effect studies. A low value of coercivity (Hc) and saturation magnetization (MS) is observed for HCMO flat nanorods (8.52 µB/f.u) as compared to that of bulk HCMO (14.9 µB/f.u). It may be due to the presence of higher oxidation states of ions as well as the introduction of AFM ordering and frustration in the system. A study of magnetocaloric effect gives a change in entropy value of 12.4 J/K.Kg at low temperature for 7 T.

Crude Oil Foulant Deposition Studies on a Heated Surface Using a Novel Batch Stirred Coupon Test Rig

The petroleum refining industry employs a wide variety of heat transfer-based equipment which tend to foul due to the complex nature of associated fluid streams and process conditions. Over the years, different test methods have been researched to understand fouling at the lab, pilot, and/or plant level. Several of these investigations have been limited to understanding fouling in static refinery streams or under non-practical operating conditions. The present study experimentally demonstrates the potential of a high-temperature batch stirred coupon test rig to characterize fouling under noncoking conditions for a specific refinery stream of interest. Experiments were conducted using a representative crude oil stream (obtained from a refinery) in a 2-liter batch autoclave system with a facility to immerse coupons attached to a rotating shaft into the stream during related test runs. The coupon Material of Construction (MOC) was chosen similar to the MOC of the tubes used in a refinery crude heat exchanger train. Experiments covered a range of bulk temperatures from 250°C to 300°C at 100 RPM. The foulant deposits obtained were found to have a strong (hard) and weakly (soft) adhering component. In the temperature range investigated, the hard and soft deposits were largely inorganic and organic in nature, respectively. The fluid bulk temperature was found to influence hard and soft deposit formation. The foulant deposits were characterized using TGA, CHNS, XRD, and SEM/EDAX, respectively. The iron and sulfur content in the hard deposit was more than 45% in total, indicating that the fouling mechanism was corrosion-based resulting from the formation of FeS on the surface. The thermal history of the soft deposit was distinct from the crude oil used. The results obtained confirm the potential of the coupon test rig for understanding fouling in refinery streams.

Hydrostatic pressure effect on melting temperature and lattice thermal conductivity of bulk and nanowires of indium arsenide

The Morelli–Callaway model was used to calculate the lattice thermal conductivity (LTC) of indium arsenide in both zinc blende and wurtzite phases of bulk and nanowire (NW) forms under applied hydrostatic pressures. Calculations were performed for NWs with diameters of 50, 63, 66, 100, and 148 nm in the temperature range of (0–400) K. The melting temperature and hydrostatic pressure phase diagram of the bulk and NW forms were predicted using the Clapeyron equation. A new method was developed to examine various related parameters, such as bulk modulus and mass density. The influence of pressure on melting temperature, melting enthalpy, melting entropy, surface energy, and stress. Results indicate that the calculated values of group velocity increased with the increase in NW size. The melting temperature dropped sharply with the rise in pressure. The pressure and temperature dependencies of the LTC were obtained, and they decreased with applied hydrostatic pressure.

Sliding wear response of ultrasonic-assisted stir-squeeze cast Al-Zn(-Mg) alloy/ZrO2p composite: wear mechanism and subsurface deformation

The character of hard ZrO2p in dry sliding wear of Al-Zn(-Mg) alloy was investigated in the cast and heat-treated conditions using a pin-on-disc wear tester at fixed sliding speed (3.5 m s−1) and varying applied pressure (0.5–2.125 MPa), giving special accentuation on the response parameters such as seizure resistance, wear rate, and bulk temperature rise. Hardness behavior was also studied for the same concerning materials in accordance with ASTM standards. The composites exhibited enhanced hardness, wear and seizure resistance, and greater temperature rise over the alloy. Indeed, the wear rate at preliminary stages increased with increasing applied pressure up to a transition limit beyond which it upsurged abruptly to greater value at the seizure pressure. Heat treatment also significantly improved the hardness, and resistance to wear and seizure. Amongst, heat-treated composite embedded with 4% ZrO2p yielded the highest hardness of 180 HV and seizure pressure of 2.125 MPa, respectively. The deformation in the subsurface was examined in terms of microhardness variation along the depth perpendicular to the worn surface. It was detected that the presence of ZrO2p not only enhanced the seizure resistance but also effectively hampered the extent of subsurface deformation at the seizure. The wear mechanism was prevalently governed through the establishment of the mechanically mixed layer (MML), oxide layer, cracking, and deformed subsurface.

General heat transfer correlations for supercritical carbon dioxide heated in vertical tubes for upward and downward flows

Supercritical CO2 is a promising working fluid for many industrial applications. To improve the performances of relevant components and systems, the prediction of the heat transfer of supercritical CO2 is an important research topic. General explicit heat transfer correlations of supercritical CO2 for upward and downward flows heated in circular tubes were established using the genetic programming (GP) method. A total of 12720 experimental data points from 22 publications were collected to develop the models. The data included hydraulic diameter from 0.0992 to 22 mm, bulk temperature from -6.0 to 134.5°C, pressure from 7.44 to 10.50 MPa, mass flux from 50 to 4834 kg•(m2•s)−1, heat flux from 2.9 to 748 kW•m−2 and wall temperature from 6.4 to 368.2°C. The database was divided into four parts according to the flow direction and the relationship between the bulk temperature and the pseudo-critical temperature. The developed correlations considered various non-dimensional parameters as the independent variables to reflect the effects of supercritical properties, flow acceleration and buoyancy on the heat transfer. The results showed that the proposed correlations had excellent accuracy with a mean absolute relative error (MARE) of 20.10% based on prediction with the iterated wall temperature. The developed correlations outperformed the existing correlations in the literature. Compared to other correlations, the trend analysis indicated that these newly developed correlations could appropriately present the physics sense when the condition parameters varied.

Numerical investigation on heat transfer and flow characteristics of supercritical methane in a horizontal tube

The main component of liquefied natural gas (LNG) is methane. The regasification process of LNG is usually under a supercritical condition. In this paper, a three-dimensional model is established to study the heat transfer and flow mechanism of supercritical methane in a heating process. The special physical properties of supercritical methane are compiled and imported through user-defined database. The standard k–ε model with the standard wall function is selected for simulation. The results show that deterioration of heat transfer occurs on the upper surface of the channel, which is most evident when the bulk temperature approaches the pseudocritical temperature. And the heat transfer coefficient increases first and then decreases along the channel with the bulk temperature rising. The peak value appears near the pseudocritical temperature. The pressure also has a great influence on heat transfer and flow. Besides, the buoyancy effect is strong near the pseudocritical point, leading to nonuniform distribution of temperature and density on the cross section, and causing significant radial circulation. The heat transfer deterioration can be effectively reduced by using special structures such as fins to hinder the buoyancy.

Reference temperature correction and dimensionless number analysis for heat transfer of supercritical CO2 in horizontal tubes

Researchers have extensively studied convective heat transfer of variable property fluids in tubes because of their different heat transfer characteristics to constant property fluids. However, the heat transfer correlations are often limited to small or large tubes or other limited conditions when the properties of fluids change dramatically, which may be due to inaccuracy definition of the reference temperature in average heat transfer coefficient (HTC) and incorrect understanding of dimensionless parameters in Nusselt correlation.As a typical case, this study numerically investigates the cooling heat transfer of sCO2 in horizontal tubes near the pseudo-critical point. Eight numerical models are compared with experimental results in the literature, and the Realizable k–ε model with enhanced wall treatment is best for predicting cooling HTC. A new definition of reference temperature for average HTC is proposed, which is more accurate than the traditional arithmetical average of inlet and outlet bulk temperature. Sensitivity analysis of HTC is carried out for heat transfer mechanisms. In addition, the definitions of dimensionless numbers in Nusselt correlation are analyzed, and more accurate definitions are proposed. Based on the conclusions, a newly developed Nusselt correlation with concise form shows excellent results with a Root Mean Square (RMS) error of 2.9% for 6.22 and 9.40 mm tubes. It can still maintain high accuracy when it is extrapolated to 4.15 and 14 mm tubes. The numerically derived correlation agrees well with the experimental results for 2–6 mm tubes in literature and can also be extrapolated to constant property cases.

Experimental investigation on heat transfer performance based on average thermal-resistance ratio for supercritical carbon dioxide in asymmetric airfoil-fin printed circuit heat exchanger

The discontinuous airfoil-fin printed circuit heat exchanger is a leading candidate due to its high compactness, high efficiency, and ability to endure extreme conditions. In this paper, an unusual asymmetric airfoil-fin heat exchanger is experimentally investigated using the supercritical carbon dioxide and water experimental system. The effects of working parameters and flow direction on the heat transfer performance are analyzed based on the average thermal-resistance ratio. Meanwhile, the effects of flow directions on the comprehensive performance are quantitatively evaluated. Compared with the overall thermal resistance, the average thermal-resistance ratio is more sensitive in evaluating the heat transfer performance. The reverse flow hardly improves the heat transfer, while significantly amplifies the pressure drop. Compared with the reverse flow, when the bulk temperature of CO2 is 38 °C, 41 °C and 47 °C, the comprehensive performance of forward flow increases by 6.1%, 4% and 1.8%, respectively. Finally, the unified heat transfer correlation for two opposite flow directions is proposed within applicable ranges.

Generation of localized reactor point kinetics parameters using coupled neutronic and thermal fluid models for pebble-bed reactor transient analysis

The systems analysis of anticipated operating occurrences and design basis accidents for pebble-bed reactor systems requires knowledge of neutron point kinetics equations (PKE) parameters. Typically, the generation of PKE parameters is performed in a global manner using standalone neutronics calculations, without the inclusion of thermal fluid distributions. We utilize Griffin and Pronghorn for generating global and local PKE parameters which includes the use of thermal fluid distributions to account for localized effects. This work establishes a methodology for calculating PKE parameters for a pebble bed reactor with a coupled neutronics/thermal fluids analysis. PKE parameters generated on a global and local basis are compared against a diffusion solve for a typical load-following transient. Locally-generated neutron kinetic parameters are able to reduce the maximum error in the transient power level from 5% to below 1.5%; along with this, bulk temperature errors were reduced from 12 K to 4 K.

Analysis of Temperature Effect on Saturated Hydraulic Conductivity of the Chinese Loess

Saturated hydraulic conductivity (Ks) is a significant basic hydraulic parameter in the field of soil science. However, it is typically used as a constant, and that will lead to some inaccurate results in numerical simulations of water movement. In this study, a laboratory experiment was conducted to investigate the effect of temperature on Ks of Chinese loess. The results indicated that Ks had a special relationship with temperature for this soil: Ks increased in the range of temperatures (t) from 7 to 44 °C. A formula in Forest Standard of China LY/T 1218-1999 can well describe the distribution characteristics of the experimental data, but it may not apply if bulk density changes. The study improved the formula by analyzing another set of published experimental data. In the improved formula, the influence of bulk density and temperature on Ks is fully considered. The results show that the improved formula can be applied to the simulation of saturated water flow by accurately describing Ks variation with temperature and bulk density.

Investigation of the Effect of Altitude on In-Cylinder Heat Transfer in Heavy-Duty Diesel Engines Based on an Empirical Model

Abstract Diesel engines are the predominant power source for the trucking industry. Heavy-duty trucks carry more than 70% of all goods transported across the United States. Atmospheric conditions vary with altitude but are critical to diesel engine performance, efficiency, and emissions. Existing studies have reported reduced thermal efficiency and increased emissions when truck engines were operated at high altitudes. Since heat loss is a key parameter related to engine efficiency, the goal of this paper was to investigate the altitude impacts on the in-cylinder heat transfer characteristics. Experiments on a single-cylinder four-stroke heavy-duty diesel engine were conducted at a constant speed and load, but with different intake pressures to simulate the varying atmospheric conditions at different altitudes. The quantity of diesel fuel injected into the cylinder per cycle was increased to maintain the same power output at decreased atmospheric pressure and to compensate for the combustion deterioration that occurred inside the cylinder. Experimental results showed that the bulk temperature was higher at high altitudes because of less mass of the mixture trapped in the cylinder. Such a large temperature difference between the hot products and the cold walls increased the in-cylinder heat transfer to the coolant, especially during combustion. Specifically, the 2000 m altitude rise led to an increase in heat loss to the atmosphere of ∼2% per cycle under full load conditions. As a result, the application of thermal coatings to improve fuel economy is even more necessary in high altitude states, such as Colorado and Wyoming.