Efficiency Of Internal Combustion Engines Research Articles

Electric Turbocharger Concept for Highly Efficient Internal Combustion Engines

Electric Turbocharger Concept for Highly Efficient Internal Combustion Engines

Emission characteristics and engine performance of gasoline DICI engine in the transition from HCCI to PPC

The injection strategy is commonly manipulated to control the stratification level to simultaneously achieve a low pollutant emission and a high efficiency in internal combustion engines. This paper reports on a joint experimental and numerical study of the emission characteristics and engine performance in a heavy-duty direct-injection compression ignition (DICI) engine operating in the HCCI and PPC regimes, with a primary reference fuel made up of iso-octane (81% by volume) and n-heptane (19%) as a model gasoline fuel. The injection timing was varied to achieve different levels of stratification of the charge in the cylinder while the intake air temperature was adjusted accordingly to keep the same combustion phasing at 3° crank angle (CA) after top dead centre (ATDC). The main results of the present study are: (1) In gasoline DICI engines, the fuel is injected into the cylinder at an earlier SOI. The combustion process can be divided into different regimes, HCCI, PPC, and transition from HCCI to PPC, depending on SOI. Two distinctive classes of in-cylinder combustion temperature distributions could be found from the simulation results for the studied engine: one was for the SOI range from -100 to -48 °CA ATDC, which was the HCCI regime and/or the transition from HCCI to PPC regime, where the mean effective in-cylinder temperature was lower than 1700 K. The second class was for the SOI range from -44 to -20 °CA ATDC, where the combustion temperature was higher than 1850 K. This corresponded to the PPC regime. (2) The NOx emission was not only affected by the mean temperature but also the distribution of temperature in the cylinder. (3) The main source of unburned hydrocarbon (UHC) emission in the HCCI and the transition regimes was the fuel trapped in the crevice region where the oxidation process could not function properly. (4) Carbon monoxide (CO) emissions showed a non-monotonic variation with the injection timing, with one peak forming in the bowl region and one forming in the squish region during the transition from HCCI to PPC. The main source of CO emission was in the low temperature and fuel-lean region in the cylinder. (5) In the present engine operation, the pressure rise rate (PRR) in the PPC regime was higher than that in the HCCI regime, which is contrary to most results reported in the literature. This is a combined effect of low equivalence ratio in the piston bowl and in the squish region, and the stratification of the ignition delay time in the mixture. (6) Highest thermodynamic efficiency was achieved in the PPC regime with SOI from -44 to -31 °CA ATDC. In the transition from HCCI to PPC regime, the thermodynamic efficiency reached its lowest due to the poor combustion efficiency.

Validation of TRF-E as gasoline surrogate through an experimental laminar burning speed investigation

Nowadays, one of the main challenge in the automotive sector concerns the increase of Internal Combustion Engine efficiency. Present and future stringent standards call for reduction of fuel consumption and cut of pollutant emissions. In this respect, one technical solution is the downsizing of the engines that consists in increasing the intake pressure and temperature, through a turbocharger, while reducing the displacement. To avoid abnormal combustion occurrences, the use of high dilution rates is required. It is evident that due to these drastic constraints, high pressure and high dilution, the propagation and the stability of the premixed turbulent flame are strongly affected. Beyond this case, the laminar flamelet regime concept remains the base for the models used in industrial applications. Therefore, the knowledge of laminar burning speed is still crucial and an open research topic. As this parameter is dependent of fuel, its investigation is fundamental in order to evaluate potential suitable surrogates or/and alternative fuels. The present study aims to validate the TRF-E (a Toluene Reference Fuel blended with 5%vol ethanol) as suitable surrogate of a commercial gasoline and to gather a wide laminar burning velocities database for the TRF-E.For that, the laminar flame propagation inside a spherical stainless steel vessel was observed via a Double-View Schlieren technique. The temperature was set at 373, 423 and 473 K, while pressure and dilution were, respectively, 0.1, 0.2, 0.3 and 0.5 MPa and 0, 10 and 20%, for equivalence ratio range from 0.8 to 1.3. Thanks to this large laminar burning velocity database, it was possible to determine coefficients from the mathematical correlation proposed by Metghalchi and Keck (1982) with high precision by solving a global optimization problem. Reference conditions were fixed at 473 K, 0.1 MPa and 0% of dilution. In this case, the laminar burning velocity difference between TRF-E and gasoline is less than 4%. But the decrease of temperature and the increase of pressure or dilution affect the laminar burning velocity of these two fuels in a different way. Anyway, this gap is limited to 10–15% in the worst cases, confirming that TRF-E may be considered as a suitable surrogate of the investigated gasoline.

The role of electric vehicles in near-term mitigation pathways and achieving the UK’s carbon budget

The decarbonisation of the road transport sector is increasingly seen as a necessary component to meet global and national targets as specified in the Paris Agreement. It may be achieved best by shifting from Internal Combustion Engine (ICE) cars to Electric Vehicles (EVs). However, the transition to a low carbon mode of transport will not be instantaneous and any policy or technological change implemented now will take years to have the desired effect. Within this paper we show how on-road emission factors of EVs and models of embedded CO2 in the vehicle production may be combined with statistics for vehicle uptake/replacement to forecast future transport emissions. We demonstrate that EVs, when compared to an efficient ICE, provide few benefits in terms of CO2 mitigation until 2030. However, between 2030 and 2050, predicted CO2 savings under the different EV uptake and decarbonisation scenarios begin to diverge with larger CO2 savings seen for the accelerated EV uptake. This work shows that simply focusing on on-road emissions is insufficient to model the future CO2 impact of transport. Instead a more complete production calculation must be combined with an EV uptake model. Using this extended model, our scenarios show how the lack of difference between a Business as Usual and accelerated EV uptake scenario can be explained by the time-lag in cause and effect between policy changes and the desired change in the vehicle fleet. Our work reveals that current UK policy is unlikely to achieve the desired reduction in transport-based CO2 by 2030. If embedded CO2 is included as part of the transport emissions sector, then all possible UK EV scenarios will miss the reduction target for 2050 unless this is combined with intense decarbonisation (80% of 1990 levels) of the UK electricity grid. This result highlights that whilst EVs offer an important contribution to decarbonisation in the transport sector it will be necessary to look at other transport mitigation strategies, such as modal shift to public transit, car sharing and demand management, to achieve both near-term and long-term mitigation targets.

The potential of statistical RANS to predict knock tendency: Comparison with LES and experiments on a spark-ignition engine

Pollutant regulations and fuel consumption concerns are the driving guidelines for increased thermal efficiency and specific power in current internal combustion engines. The achievement of such challenging tasks in Spark Ignition units is often limited by the onset of knock, which hinders the possibility to operate the engine with the optimal combustion phasing. The sporadic occurrence of individual knocking events is related to cycle-to-cycle variability of turbulent combustion. This is avoidable by only accepting a safety margin from its earliest onset.On one side, the stochastic nature of knock and turbulence-related combustion variability would indicate Large-Eddy Simulation (LES) as the most appropriate technique for CFD analyses. Nevertheless, Large-Eddy Simulation remains a very time- and CPU-demanding approach, hardly integrated in the industrial timeframe for the design exploration and development of new units. Therefore, Reynolds Averaged Navier Stokes (RANS) models representing the average flow are chosen to limit CPU and development times, though they suffer from the intrinsic inability to account for cycle-dependent phenomena (e.g. knock). This is particularly critical at knock-borderline conditions, where far-from-average knocking events may occur. A previously developed statistical RANS-PDF knock model partly overcomes this limitation using equations for mixture fraction and enthalpy variance, ultimately reconstructing log-normal distributions of knock intensity. This allows RANS simulations to be directly compared to the usual statistical knock analysis at the test-bench.In this paper all the mentioned modelling techniques (LES, RANS and RANS-PDF) are applied to simulate combustion and knock in a currently made turbocharged GDI engine under knock-safe, knock-limited and light-knocking conditions. The study relevance lays in the critical comparison of the results. The full potential of the statistical RANS-PDF model for engine development is highlighted on a coherent basis. The possibility to preserve the RANS formalism while enriching the results with knock statistical description is a relevant advancement in the virtual design of high-efficiency engines.

Performance estimation of an engine with magnetorheological variable valve train

Researchers are continuously developing various energy-saving technologies to enhance the internal combustion engine efficiency. Among them, continuous variable valve timing and variable valve lift system is one potential technology. To achieve this ability, a new type of electric valve, known as magnetorheological valve train, was introduced, which is based on the magnetorheological fluid’s active behavior. In this study, gasoline engine models are established for dynamic simulations of an engine with the magnetorheological valve train. Volumetric efficiency and performance of the engine with different magnetorheological valve train valve lifts and valve timings at engine speeds between 2000 and 6000 r/min are also analyzed. Simulation results show that, at low engine speeds, the lift of the valve has little effect on the engine performance. However, under high and medium engine speeds, the lift of the valve significantly affects the performance; the smaller the valve lift, the smaller the volumetric efficiency. Likewise, an adjustment of valve opening angle timing to reduce the valve overlap angle effectively increased the engine volumetric efficiency at low engine speeds.

CFD Simulation of Oil Jets for Piston Cooling Applications Comparing the Level Set and the Volume of Fluid Method

<div class="section abstract"><div class="htmlview paragraph">A new CFD simulation model and methodology for oil jet piston cooling has been developed using the modern level set approach. In contrast to the widely used volume of fluid (VOF) method, the level set approach explicitly tracks the interface surface between oil and air, using an additional field equation. The method has been extensively tested on two- and three-dimensional examples using results from literature for comparison. Furthermore, several applications of oil jet piston cooling on Ford engines have been investigated and demonstrated. For example, three-dimensional simulations of piston cooling nozzle jets on a diesel engine have been calculated and compared to test-rig measurements. Laminar jets, as well as jets with droplets and fully atomized jets, have been simulated using realistic material properties, surface tension, and gravity. Simulations of cooling jets on the undercrown of a gasoline piston and on a moving piston with a cooling gallery have been investigated and compared to test-rig measurements. Results of a VOF model with the CFD software STAR-CCM+ used in the Ford CAE workflow have been compared to the new level set method. Despite using different computational approaches (level set versus volume of fluid method), the results are similar for laminar jets. Differences occur for semi-turbulent and atomized oil jets with many droplets, which need highly resolved meshes. All in all, the modern CFD tools are a powerful way for investigating active cooling strategies for pistons in order to improve the efficiency of internal combustion engines and to reduce emissions.</div></div>

Automotive Waste Heat Recovery after Engine Shutoff in Parking Lots

<div class="section abstract"><span class="label">1</span><div class="htmlview paragraph">The efficiency of internal combustion engines remains a research challenge given the mechanical friction and thermodynamic losses. Although incremental engine design changes continue to emerge, the harvesting of waste heat represents an immediate opportunity to address improved energy utilization. An external mobile thermal recovery system for gasoline and diesel engines is proposed for use in parking lots based on phase change material cartridges. Heat is extracted via a retrofitted conduction plate beneath the engine block after engine shutoff. An autonomous robot attaches the cartridge to the plate and transfers the heat from the block to the Phase Change Material (PCM) and returns later to retrieve the packet. These reusable cartridges are then driven to a Heat Extraction and Recycling Tower (HEART) facility where a heat exchanger harvests the thermal energy stored in the cartridges. A series of mathematical models are created to estimate the recoverable heat from a standard parking lot configuration. A representative case study that considered 500 cars with periodic traffic flow over a period of 16 hours can heat approximately 25 kiloliters of potable water from 15°C to 50°C. Future development will involve the creation of strategies to extract heat during engine idling at traffic stops and drive through lanes.</div></div>

A Study of Exhaust Waste Heat Recovery in Internal Combustion Engines

This research presents an investigation of an energy recovery solution from exhaust gases in internal combustion based on the heat exchange to distil fresh water from the sea water. Consequently, an optimization of the flow field design of the heat exchanger was performed using the commercial computational fluid dynamics (CFD) software. The result of this research showed that the energy recovery performance of the exchanger strictly depends on the engine load, which can reach a value of approximately 33% at the full load condition of the research engine. These results might forecast a future application of the optimized exchanger’s flow-field design in automotive waste heat recovery in order to improve the internal combustion engine efficiency.

Comparison of different layouts for the integration of an organic Rankine cycle unit in electrified powertrains of heavy duty Diesel trucks

In the last years an increasing interest has grown about the organic Rankine cycle (ORC) for waste heat recovery from the engine exhausts, as a feasible way to improve the efficiency of internal combustion engines (ICE). However, the highly variable working conditions in which an engine usually works make the operation of the ORC unit very challenging. In the management of ORC units, optimization and control are considered as an essential step in order to attain good performance during the whole operation of the ICE on a driving cycle. In this paper a GT-Power model of a 12.6 l turbocharged Diesel engine is used as a typical heavy duty Diesel engine to retrieve the temperature and the mass flow rate of the exhaust gases during a heavy heavy-duty driving cycle. An optimization process is performed to maximize the power output of the ORC as a function of the revolution speeds of the pump and of the turbine. For powertrain electrification purposes, two different ORC layouts are proposed in this paper, both seeking to store the recovered energy into a battery pack. The first layout is optimized based on mean values of exhausts temperature and mass flow rate, while the second layout is designed to control independently the revolutions of the pump and of the turbine of the ORC unit toward optimal values for maximum net power output. Both the layouts allow to recover an interesting amount of energy, in spite of a straightforward control strategy. Nonetheless, results show that optimizing the net power of the ORC cycle by decoupling the turbine and the pump may not ensure the maximization of the recovered energy. In fact, the storable energy from the ORC unit is strongly affected by the conversion efficiencies related to the specific layout.

Effect of renewable fuel and injection strategies on combustion characteristics and gaseous emissions in diesel engines

ABSTRACT‏Renewable fuels and injection strategies can both provide cleaner combustion and more efficient internal combustion (IC) ‎engines. The effect of fuel injection strategies (injection timings and pressures) and renewable fuel (B20)‎ on engine performance, and gaseous emissions have been studied ‎in this experimental work. The results showed that the combustion performance improved from the combustion of B20 than to the diesel fuel under advanced injection timings. Furthermore, the high fuel injection pressure (100 MPa) was enhances the combustion performance more comparedthan low injection pressure (65 MPa) for both fuels studied. It can be concluded that B20 improved the combustion performance rather than diesel fuel for different injection pressures. Therefore, the experimental results showed that the combination of advanced injection timing and B20 were beneficial to improve the combustion performance, which in turn reduced the level of harmful gaseous emissions such as total hydrocarbons (THC) and carbon monoxide (CO) in the exhaust duct. In addition, it can be concluded that the high injection pressure reduced the level of the engine-out emissions. It was found that gaseous emissions such as CO and THC were reduced from the combustion of B20 fuel. In contrast, the nitrogen oxide (NOx) was found to be slightly higher than diesel fuel for different injection timings and pressures.

Effects of electrolytic jet plasma oxidation (EJPO) coatings on thermal behavior of engine cylinders

Aluminum engine blocks with ceramic-coated cylinder bores can effectively reduce the self-weight and largely improve the wear resistance. However, the presence of coatings to some extent also affects the heat transfer of internal combustion (IC) engine between the cylinder and the coolant due to their low thermal conductivities. In this paper, electrolytic jet plasma oxidation (EJPO) coatings were adopted to deposit on the engine cylinder bores. A three-dimensional transient finite element analysis (FEA) thermal model was developed to investigate the thermal behavior of EJPO-coated cylinders. It was found that both coating thickness and thermal conductivity could significantly influence the bore surface temperature and temperature fluctuation behaviors at the different crank angles. A possible selection of the parameter combination of coating thickness and thermal conductivity would provide a newly promising opportunity in the design of a high-quality coating for the cylinder bores to improve IC engine efficiency.

Novel Mechanical Interface Design for Automotive Starting Systems

Stop–start vehicles (SSVs) represent a potential alternative for improving internal combustion engine (ICE) efficiency. SSVs provide ICEs with the functionality of turning the engine off during traffic halts and restarting it without intervention by the driver. This strategy reduces fuel consumption, especially in dense urban traffic areas, and contributes to emissions reduction to meet green emissions targets. The most widely adopted SSV system has a mechanical interface to connect the electric starter motor to the ICE, which requires increased robustness compared with standard starting motors. This requirement allows the motor to withstand a higher number of engine start cycles compared with a standard starting motor. Nevertheless, it is a critical problem for wider adoption of SSVs. As SSV systems usually are based on the conventional starting system, its durability and noise remains a critical issue to be addressed by automakers. The typical pinion–ring gear interface uses intermittent gear meshing to form a transient coupling interface. The research reported here presents the development of an innovative mechanical interface for starting systems, called the permanent coupling (PC)-type interface, which reduces noise and increases durability compared with the existing design. The results obtained by a functional prototype of the PC-type mechanical interface confirm the feasibility of the proposed concept. The methodology is based on a product development process integrated with lumped-parameter modeling and virtual simulation aimed at reducing failures during prototype testing. The new mechanical interface was proven to be a good candidate for increasing the use of SSVs in the automotive market.

Morphology analysis on the effect of injection pressure on jet trajectory deviation of multi-jet sprays

Fuel spray behaviors play an important role in mixture formation, efficiency and emission of internal combustion engines. In this study, the effect of injection pressure on spray jet trajectory was investigated by using an asymmetric five-hole GDI injector. With schlieren imaging method, the ethanol spray development was captured under superheat degrees from 0.46 to 9.48, and the ambient pressure was fixed at 101.3 kPa. For the non-flash-boiling cases, the jet trajectory gradually deviated inward in the far field with the increase of injection pressure. For the flash boiling cases, the jet trajectory near the nozzle exit deviated outward under the relatively low injection pressures and deviated inwards with the increase of injection pressure. The outward deviation under relatively low injection pressure was attributed to the non-uniform vapor formation inside the nozzle orifice. The disappearance of near-field deviation under flash boiling conditions under high injection pressures could be attributed to the reduced time for nucleation and bubble growth inside the nozzle. Additionally, under both non-flash-boiling and flash-boiling conditions, the far-field deviation under high injection pressures could be attributed to the increased jet speed and the stronger jet-to-jet interaction. Compared with non-flash-boiling cases, the far field deviation under flash boiling conditions were more intense because of the enhanced atomization and it is believed that the smaller droplets were more inclined to move inwards. The remarkable effect of injection pressure on jet interaction of flash boiling sprays shows the potential of flexible jet targeting control at different engine loads.

Flash boiling steam engine

Increasing the efficiency of internal combustion engines and reducing their pollution is the key issue nowadays. As a result, researchers have focused their attention on energy recovery either thermal or kinetic. In terms of thermal energy recovery, this can be done with promising results, especially as most of the potential energy of the fuel is lost as heat. Energy recovery can be done either using the Clausius-Rankine cycle or the Organic Rankine Cycle, the use of thermoelectric generators that would convert thermal energy directly into electricity, Sterling engines or even steam engines. In this paper, we propose a steam engine in which water vaporization takes place directly in the cylinder. The engine, unlike the classic locomotive engines is working in a closed cycle. The condensate is preheated and re-injected into the engine. Such an engine has the advantage of a known, cheap manufacturing technology and simple maintenance. A steam engine efficiency of only 10% of the energy dissipated on the exhaust would provide a global increase in engine efficiency by about 3%, taking into ac-count that about 30% of the potential energy of the fuel is dissipated on the exhaust.

Analysis of spray combustion characteristics of diesel, biodiesel and their n-pentanol blends based on a one-dimensional semi-phenomenological model

Spray mixing and combustion processes are regarded as the major factors that determine the energy conversion efficiency and emission level of internal combustion engines. However, the related researches mainly rely on 3D flow simulation or optical engine experiments, which are highly expensive and time-consuming. In order to provide a more convenient solution, this paper developed a one-dimensional semi-phenomenological model to achieve accurate and efficient predictions on spray combustion characteristics. The model was constructed by a dimensionless spray theoretical model and limited experimental data of macroscopic spray characteristics. Inside the proposed model, a physical module was used to physical module spray air-fuel mixing conditions like fuel distribution and evaporation. Besides, an improved Arrhenius-typed correlation concerning injection parameters and environment variables was introduced to describe the chemical ignition characteristic of spray combustion. The optical experiment results showed that this method is simple and effective, and the forecasting is satisfactory.Based on the new model, the influences of blending n-pentanol on the spray combustion characteristics of diesel and biodiesel were investigated. On the physical aspect, the critical fuel concentration at liquid length position decreases with n-pentanol is blended into both fuels in the most cases, but witnessed a significant leap in 1200 K condition, especially for diesel blends; the physical ignition delay times of both diesel and biodiesel blends decreases first and go up later. On the chemical aspect, the join of n-pentanol caused the gap between the physical delay time and the chemical delay time much narrower, but the ignition patterns were different in diesel blends and biodiesel blends. Moreover, this paper proposed a series of correlations focused on the liquid length and ignition delay of n-pentanol blends sprays. These correlations covered wide conditions and showed better prediction accuracy than traditional ones.

Különböző súrlódáscsökkentő kenőolajadalékok tribológiai hatásainak vizsgálata

In modern vehicle and engine development one of the most important improvement tendencies is to increase the fuel efficiency of internal combustion engines. One of these possibilities is decreasing the frictional losses of engines, which can be achieved by decreasing the viscosity of lubricants used in engines (ultra-low-viscosity) and the usage of various additives in the lubricants. To choose the right type of additives and their concentrations, tribological investigation of these lubricants is necessary. In this article the investigation of the tribological behaviour of the chosen friction-modifier (FM) additives on several surface materials is published. The additives were investigated with several tribometers and with the use of self-developed test methods in the Department of Internal Combustion Engines at the Szechenyi Istvan University in Győr. The lubricant probes used for the tribological investigations are made in the lubricant laboratory of MOL-Lub Ltd.

Environmental impact analysis of food waste anaerobic digestion and products utilization process

Anaerobic digestion technology is a widely used technology for food waste treatment. It can produce clean fuel gas and realize harmless treatment of waste. Different ways of utilizing the digestion products vary depending on the market demand and production technology. In this paper, life cycle assessment (LCA) was conducted to analyse the environmental impacts of different processes in food waste anaerobic digestion treatment and compare different utilization strategies of digestion products especially the digestate. The results of this study indicated that the incineration of digestate had advantages in most environmental impacts except global warming potential (GWP), compared with composting as fertilizers. Besides that, incineration had the lower energy efficiency due to drying heat demand. The high energy conversion efficiency of internal combustion engine contributed much to reducing environmental impacts. It’s worth mentioning that the sufficient combustion performance of micro-turbine resulted in smaller global warming potential (GWP) and eutrophication potential (EP).

Modeling investigation of thermal insulation approaches for low heat rejection Diesel engines using a conjugate heat transfer model

Heat losses through combustion chamber walls are a well-known limiting factor for the overall efficiency of internal combustion engines. Thermal insulation of the walls has the potential to decrease substantially these heat losses. However, evaluating numerically the effect of coating and of its location in the combustion chamber and then design an optimized combustion system require the use of high-fidelity engine models. The objective of this article is to present the whole workflow implying the use of three-dimensional computational fluid dynamics techniques with conjugate heat transfer (CHT) models to investigate the potential benefits of a coating on a passenger car Diesel engine. First, the baseline combustion system is modeled, using CHT models to solve in a coupled simulation the heat transfers between the fluid in the intake and exhaust lines and in the combustion chamber, on one hand, and the solid piston, head and valves, on the other hand. Based on this setup, a second simulation is performed, modeling a thermo-swing insulation on all combustion chamber walls by a contact resistance, neglecting its thermal inertia to keep a manageable computational cost. Results show a decrease of 3.3% in fuel consumption with an increase in volumetric efficiency. However, decoupled one-dimensional/three-dimensional simulations highlight the inaccuracy of these results and the necessity to model the coating thermal inertia, as they show an overestimation of the heat insulation rate and, consequently, of the gain in fuel consumption (−2.1% instead of −1.6%), for a coating on the piston with no thermal inertia.

High efficient internal combustion engine using partially premixed combustion with multiple injections

Improving the efficiency of the powertrain system is of great importance to reduce the greenhouse gas CO2. Advanced combustion engine with Partially Premixed Combustion (PPC) is one of the best solutions. It is proved to have a high engine efficiency and low emission level. Using multiple injections is a good way to achieve PPC. The efficiencies using multiple injections were evaluated on a metal engine with modern architecture and the reasoning behind that was explored on an optical engine. The metal engine results shown that the point with optimized multiple injections is of higher efficiency than a single injection. Optical results demonstrated that the direct interaction of the first and later injection, as well as the interactions of the fuel and the in-cylinder bulk flow fields and surfaces, could affect mixing and fuel movement and, hence the efficiency. One of the reasons why the optimized multiple injections have a higher efficiency is that the center of the fuel is moved close to the center of the cylinder. Thus, the heat transfer between the heat produced from the fuel-gas mixture and the cylinder liner can be reduced by the isolation. This explains how the injections influence the fuel distribution and the heat transfer and, hence, the engine efficiency.