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A. What is the impact of the thermal behavior of materials on the thermodynamic efficiency and emissions of internal combustion engines, and how can advancements in material science be utilized to improve the performance and sustainability of these engines? In the current drive for cleaner energy use, the application of…
Mohamed Afsal Khan Mohamed Yousuf
updated on 10 Aug 2023
A. What is the impact of the thermal behavior of materials on the thermodynamic efficiency and emissions of internal combustion engines, and how can advancements in material science be utilized to improve the performance and sustainability of these engines?
In the current drive for cleaner energy use, the application of lightweight materials in internal combustion engines becomes imperative as it makes for greater fuel efficiency which results in pollution reduction.
Advanced materials are essential for boosting the fuel economy of modern automobiles while maintaining safety and performance. Because it takes less energy to accelerate a lighter object than a heavier one, lightweight materials offer great potential for increasing vehicle efficiency. A 10% reduction in vehicle weight can result in a 6%-8% fuel economy improvement. Replacing cast iron and traditional steel components with lightweight materials such as high-strength steel, magnesium (Mg) alloys, aluminum (Al) alloys, carbon fiber, and polymer composites can directly reduce the weight of a vehicle's body and chassis by up to 50 percent and therefore reduce a vehicle's fuel consumption. Using lightweight components and high-efficiency engines enabled by advanced materials in one quarter of the U.S. fleet could save more than 5 billion gallons of fuel annually by 2030.
By using lightweight structural materials, cars can carry additional advanced emission control systems, safety devices, and integrated electronic systems without increasing the overall weight of the vehicle. While any vehicle can use lightweight materials, they are especially important for hybrid electric, plug-in hybrid electric, and electric vehicles. Using lightweight materials in these vehicles can offset the weight of power systems such as batteries and electric motors, improving the efficiency and increasing their all-electric range. Alternatively, the use of lightweight materials could result in needing a smaller and lower cost battery while keeping the all-electric range of plug-in vehicles constant.
In the short term, replacing heavy steel components with materials such as high-strength steel, aluminum, or glass fiber-reinforced polymer composites can decrease component weight by 10-60 percent. Scientists already understand the properties of these materials and the associated manufacturing processes. Researchers are working to lower their cost and improve the processes for joining, modeling, and recycling these materials.
In the longer term, advanced materials such as magnesium and carbon fiber reinforced composites could reduce the weight of some components by 50-75 percent. The Office is working to increase our knowledge of these materials' chemical and physical properties and reduce their cost.
Optimising the material of piston for better fuel economy & performance:
The design and materials used for the pistons exert a significant influence on the value of the clearance of the piston-cylinder assembly. Clearances have an impact on the lubricating oil consumption, blow-up into the crankcase, exhaust gases toxicity and noise of engine operation.
Hysteresis of pistons dimensions due to changes in temperature distribution and stress has a major impact on the value of the clearances. In the most internal combustion engines, the pistons are made f alloys based on aluminum, whereas the cylinder liners are made of cast iron, which has different expansion coefficient than the aluminum alloys expansion. Therefore, the attempts of the steel pistons application are investigated.
However, these pistons have a much larger mass, which increases the load of the crank mechanism and reduces the nominal speed of the engine. Moreover, the implementation of steel pistons requires new and expensive production methods. Therefore, the use of composite pistons of the aluminum is a better solution. It allows to increase the strength of the material at elevated temperature and to reduce the changes in the piston dimensions. Furthermore, the use of a new alloy allows the use of smaller assembling clearances between the piston and the cylinder.
The composite alloy is obtained by adding to the base material (aluminum) of the refractory elements such as chromium, molybdenum, nickel, tungsten. These elements form the intercrystalline compounds, which are located on the grain boundaries. Moreover, the fibres, such as silicon carbide are added, which strengthens the structure of the alloy. As a result, strength of the alloy and dimensional stability is increased, and the deformations of the pistons are reduced.
OPERATING CONDITIONS OF THE COMBUSTION ENGINE PISTONS
Pistons in internal combustion engines operate under permanent changes in their temperature and structural stresses distribution, due to changes in engine external loads and engine speed. The materials of the pistons must therefore have appropriate properties not only at the room temperature but also at the operating temperature of the pistons. The maximum temperature of the pistons, which are present on the piston crown, can reach 320-350oC. In the lower part of the piston, at the piston skirt, the temperature reaches a value of 100-140oC. Very important is the temperature in the region of the piston ring grooves, which is determined and limited by the properties of the lubricating oil, it should be a maximum of 230-240oC. If the temperature in this area exceeds this value, the piston must be internally cooled. Large temperature differences in the piston cause the occurrence of high thermal stresses, which superimpose to the mechanical stresses. Large differences in the piston stresses and temperature gradients cause deformation of the pistons, which should disappear after the withdrawal of stresses and temperature. The presence of pistons permanent dimensional changes, because of pistons permanent heating and cooling, known as hysteresis, is a serious problem that needs to be taken into account in the pistons design process. The value of these distortions, determine in fact the dimension of the clearance in a piston-cylinder liner assembly, which affect on piston seizure, lubricating oil consumption, exhaust gases blowby into the crankcase and harmful emissions, mainly unburned hydrocarbons. The value of the pistons hysteresis can be limited, primarily by proper selection of the pistons chemical composition and by selection of the appropriate piston manufacturing processes, including the piston heat treatment processes. The long experience in the pistons manufacturing and testing led to the selection of different aluminum alloys, which is used in the pistons mass production. Specific designs and the usage of the engines, however, require the introduction of the additional requirements that result, that it is necessary to correct the chemical composition of the piston alloys. The most commonly used piston aluminum alloys are Al-Si alloys, containing about 12% Si. They are near eutectic alloys, further more comprising a number of alloying additives.
THE SCOPE OF THE TESTING
The work aim was to replace pistons of the military applications engines, which were
produced by forging a wrought aluminum alloy PA12. The pistons produced by casting a near
eutectic Al-Si alloy should be characterized by similar strength properties and improved
functional properties, which are primarily enabled by reducing the clearances between the
piston and the cylinder.
The research program contained large range of metallographic research, the strength testing of materials in terms of the influence of various alloying elements on the strength of the final material, mathematical modelling of temperature distributions and stresses in the pistons and engine brake testing of the new pistons. As a basic alloy to manufacture the test samples, the near-eutectic Al-Si alloy, labelled AK12 was taken. Table 1 shows the PA12 alloy chemical composition, a global average of the near-eutectic alloys composition, and the alloy composite, which is the final result of the work carried out, and which was selected to make the new pistons. It should be noted that the results of materials tests have been used in the processes of mathematical modelling. Modelling make possible to determine the outer contour of the piston, whereby space between the piston and the cylinder has been reduced, which had a beneficial effect on the limitation exhaust emissions limitation, fuel economy, also reducing the noise of the engine. In this article, the main attention has been devoted to metallographic research, the strength research and engine research.
METALLOGRAPHIC TESTING
In a study of casting pistons processes and preparing samples for the strength tests, the derivative thermal method of analysis was used (Derivative Thermo Analysis − DTA), which allows one to track the process of the alloy crystallization and the setting temperatures of phase transitions. This also facilitates suitable design of the casting moulds, to obtain a desired speed of creation for specific structural components. On the other hand, from the alloy-cooling graph, the changes in the microstructure of the alloy can be predicted. Fig. 1 shows the alloy cooling curve and the derivation curve of the silumin composite alloy AlSi12Cu4Ni4MgCrMoVW (Mg 0.5%, Cr, Mo, W and V, 0.05%) modified with Sb, Ti, B and comparison of microstructure of novel alloy (left) and standard one. Table 2 shows the characteristic temperatures, corresponding to the markings on the cooling curve. The DTA
curve, in conjunction with graphs of phase equilibrium and metallographic tests and X-rays tests, allows the interpretation of type and sequence of crystallization of the individual phases in the alloy.
STRENGTH TESTS
High demands on the tensile strength and hardness of the piston material resulted in this that was necessary to increase the strength of the basic, near-eutectic Al-Si alloy, by adding suitable alloying additives. It was also necessary to obtain an alloy, characterized by high dimensional stability in an environment of continuously changing thermal and mechanical loads. Such additives as chromium, molybdenum, tungsten, nickel, and copper were specifically evaluated for their influence. There were tested: tensile strength Rm, yield strength R0.2, relative elongation A5, the Brinell hardness HB. The tests were carried out using a strength-testing machine with a hydraulic drive. Static tensile test was performed at a constant loading rate of 5 mm / min. In studying the effect of temperature on the material strength, the thermal chamber was used.
The testing machine view is shown in Fig. 2, and the view of the sample (on which the extensometer is installed) in the machine head is shown in Fig. 3. Test samples were cast in a chill mould and then heat-treated and machined. Investigations of the effect of chromium content in near-eutectic silumin on its strength were carried out in the range from 0% to 1.75% of chromium.
It has been found, that in the case of increasing the chromium content ranging from 0% to 0.6%, the strength increased from 200 MPa to 490 MPa. After crossing the Cr content of 0.6%, the strength rapidly decreased until 110 MPa with Cr content of 1.75%. At the same time, with the alloy chromium content increase, elongation of 0.95% at zero chromium level was reduced to the relative elongation equal zero at the chromium level of 1.2%. With further increase of the chromium content, the silumin sample acted like a fragile body. Fig. 4 shows the changes of the strength as the function of temperature and Fig. 5 the changes of elongation as the function of temperature, in relation to the silumin alloy, containing 0.6% Cr. As can be seen, during heating of the sample to a temperature of 350°C, the strength of the material systematically decreased and the elongation increased. To reach a temperature of 150°C the rate of change was small, but there has been a sharp rise above a temperature of 150°C.
When adding molybdenum to eutectic silumin, the strength increased with increasing molybdenum content up to 0.5% Mo, from 200 MPa to 480 MPa. After exceeding the 0.5% molybdenum content, the strength decreased to 200 MPa at a content of 1.4% Mo. In contrast, the relative elongation value at the increased molybdenum content, decreased from the value 0.95 with 0.0% Mo content to the relative elongation value of zero at the content of 1.04% Mo. The hardness of the alloy increased with a molybdenum content increase, from hardness of 98 HB with a molybdenum content of zero to 139 HB with a content of 1.8% Mo. In studying the effect of temperature on the strength, in the range from 20°C to 350°C, two silumin alloys, one containing 0.2% Mo and the other one containing 1.1% Mo, the silumin alloy containing 0.2% Mo, reduced its strength to the value of about one-sixth, and silumin alloy containing 1.1% Mo, the strength decreased by the value of about 40%. The effect of addition of tungsten to near-eutectic silumin was similar as in the case of molybdenum. Strength of the alloy increased with a tungsten content, ranging from the 210 MPa with zero tungsten content to 460 MPa with a tungsten content of 0.65%. Then the strength decreased, reaching a value of 215 MPa with a tungsten content of 1.4%. Elongation reached zero with tungsten content of 1.4% and grew linearly to the value of 0.96 when the content of tungsten in alloy was reduced to the value of 0.05%. Silumin hardness increased with the increase in tungsten content, from 98 HB with a content of 0.05% W to 139 HB with a content of 1.4% W. Tests were also carried on impact of other alloying elements to the near-eutectic silumin on the strength of the alloy, wherein on these samples the metallographic examination and thermal expansion and dimensional stability testing were also conducted. Based on the analysis of the test results, it was found that the most promising appears to be silumin alloy, containing 4% Ni and 4% Cu and small additives of other refractory elements, such as Cr, Co, Mo, W, and V. For testing, samples were prepared containing 2% Ni, also 4% Ni and 4% Cu and varying amounts of Co, Cr, Mo, W. Fig. 6 shows the tests results of strength as a function of temperature for these two silumin alloys.
The drawing shows, that increasing alloy nickel content from 2% to 4�used a 65% increase in strength at room temperature, while at temperature of 350°C, the strength of alloy containing 4% Ni was higher by 91%. Fig. 7 shows the dependence of the tensile strength of silumins, containing 4% Ni, 4.5% Cu and the different content of alloying elements, such as Co, Cr, Mo, W: 0.5%, 0.25% and 0.1%. Fig. 8 shows the dependence of elongation versus temperature for these alloys. In turn, Fig. 9 shows the dependence of the hardness on temperature with regard to alloys, such as before.
The results show, that with the increase in temperature, almost linear decrease in tensile strength and hardness appears. Interestingly, samples containing more alloying elements have higher strength and hardness than those containing less alloying additives but in contrast, as the temperature increases, the strength of the samples containing more alloying elements decreases faster than samples containing smaller amounts of alloying elements.
Nevertheless, at any temperature up to 350°C, the strength of the samples will be greater the more alloying elements comprised silumin. As for elongation, its value increased with increasing temperature. The increase was small up to 250°C and rapidly increased after the temperature exceeds 250°C, reaching, in the case containing 0.5% Co, Cr, Mo, W − 2.8 times, in the case containing 0.25% Co, Cr, Mo, W −3,3 times, and in the case containing 0.1 Co, Cr, Mo, W − 3.5 times. Fig. 10 presents the results of a hardness of near-eutectic silumin, containing 4% Ni, 4% Cu and 0.1% Cr, Mo, W, V modified with strontium, titanium and boron: in the rough-cast state, after various heat treatment methods, including a multi-stage methods. Differences in hardness for each sample approach the 40%. The results of strength and the hardness tests carried out of the cast samples at ambient temperature and at increased temperatures up to 350°C, show that the higher strength and hardness values were obtained for silumin, comprising high amounts of alloying elements. The strength of such silumin alloys was comparable to the strength of low alloy steels. This however required the use of large amounts of alloying elements. In selected to manufacture piston alloy, the content of alloying elements reached 22% (including the Si element content). Based on the effect of alloying element content on the alloy strength at room and elevated temperatures and the results of metallographic and the dimensional stability research was decided to choose the chemical composition and manufacturing technology of the research pistons.
RESEARCH OF STRENGTH AND HARDNESS OF PISTON TEST SPECIMENS
The chemical composition of the material, from which the new pistons were manufactured, is shown in Table 1. After determining the chemical composition of the composite silumin and the pistons manufacturing process, the experimental batch of the pistons was manufactured, from which the pistons for the strength tests were selected. Due to the specific dimensions of the pistons, the specimens could not be manufactured to the standard dimensions. Specimens were cut from the piston crown and piston skirt, from pistons made of the new composite silumin and from the old pistons, manufactured of PA12 alloy. The study allowed us to compare the strength properties of the new piston material and the old pistons alloy PA12. The tests determined tensile strength Rm, yield strength R0.2, relative elongation A5 and Brinell hardness HB at ambient temperature and at 250°C. Fig. 11 shows a sketch of a piston with marked sites from which specimens were cut out for the strength tests. Fig. 12 shows the specimen drawing, on which the tests were performed. Five specimens of each piston crown and 3 specimens of each piston skirt were conducted.
As shown by the test results, the pistons of the new material have a higher strength than pistons of PA12 alloy. These differences at room temperature are about 22% and at a temperature of 250°C about 45%. The decrease in the strength with heating specimens from 20°C to 250°C has reached about 24% with respect to the composite material and about 48% for PA12 alloy. It should be emphasized that the spread of the results of the examinations in respect of tensile strength was small and at room temperature was about 9% for samples from new material and about 3% for samples from PA12 material. As regards elongation, its values at room temperature were the same for both materials (3.7%). At a temperature of 250°C, elongation of samples made from PA12 material was about two times greater than the samples made with the new material. Young's modulus of the new material was higher by about 5% since the PA12 Young’s modulus. The hardness of the new material was approximately 10% higher than that of the PA12 material.
TESTS OF THE COMBUSTION ENGINE
The final verification of the quality of materials and newly developed pistons were tests of the
engines, equipped with new pistons. The investigations were carried out on the engine
dynamometer bench, installed in the engine test cell. The pistons have the corrected outline
external surface, obtained by mathematical modelling. The results of material and strength
research also have been applied.
On Fig. 15 the shape of the outer contour of the newly developed pistons to the shape of the outer contour of previously used pistons are compared. The performed correction allowed reduction of the clearance between the piston and the cylinder and the space between the piston and the cylinder in the area of the fire threshold by about 10-20%, which should have a positive impact on exhaust emissions of harmful gases.
The view of the novel pistons before engine tests is shown on Fig. 16. The pistons were carefully measured, whereupon one fitted them in the engine. After testing, the pistons were removed and measured to determine the pistons wear and cylinder wall interference signatures. After reaching the engine and conducting factory acceptance tests, the load characteristics were defined (at constant speeds) and external characteristics (full load for different engine speeds). During the tests were recorded: engine speed, engine load (torque), fuel consumption, oil consumption and characteristic pressure and temperature values at specific measurement points. Fig. 17 presents the engine on the test bench. The test stand was equipped with a set of modern measuring and recording instrumentation.
The course of engine power as a function of engine speed at full load is shown in Fig. 18, Fig. 19 presents the course of engine torque and fuel consumption graph is in Fig. 20. The tests results show, what was achieved because of the new pistons: the increase in power and torque and a reduction in specific fuel consumption, a significant reduction emission of unburned hydrocarbons (HC), as shown in Fig. 21, as well as smoke opacity and exhaust blowby into the crankcase. Oil consumption was reduced by about 17% in comparison to pistons made of the PA12 alloy.
B. How can the design and operation of internal combustion engines be optimized to improve their performance, efficiency, and environmental impact, and what technological innovations can be employed to achieve these goals?
The turbocharged diesel engine is one of the most efficient engines among internal or external combustion engines. Engine performance improvements through turbochargers are still active research topics. Engine altitude adaptability and engine loading schedule can further improve the engine performance. One of the biggest advantages of the diesel engine is that it can burn a large variety of fuels. In a turbocharged diesel engine, the air passes through a turbocharger compressor to increase the air pressure and then flows into the engine cylinders during the downward stroke of the pistons. Air and fuel mix in the cylinders and then burn to produce the thermal energy. The more mass flow of air is sucked into the cylinders, the more powerful the engine is. The exhaust gas discharged from diesel engines contains energy and pollution. Climate change resulting from CO2 emissions to the sky from burning fuels is impacting the world's inhabitants. Carbon dioxide (CO2) and Nitrous oxide (NOx) emissions from diesel engines are the biggest sources of pollution. Improving engine efficiency and reducing emissions are always the goal of diesel engine manufacturers. Research in diesel engine performance is still an active field.
urbochargers can make significant contributions to overall engine performance. Turbocharger for diesel engines was used for the purpose to increase both power density and efficiency. A turbocharger turbine converts thermal energy from exhausted gas to mechanical energy and drives the compressor. The compressor increases the air pressure before the air enters into the engine intake manifold and increases the air mass flow entering the cylinders. Turbochargers can enhance the output of an internal combustion engine without increasing the cylinder capacity and make it possible to downsize the engine. Turbocharger provides a feasible engineering solution to engine economics and emission reductions. For achieving emission and fuel consumption reductions, all potential areas of improvement must be identified for both core engines and turbochargers. Developments of the new diesel engines are undergoing rapid transformation due to emission regulations and fuel efficiency. Many studies have been done to improve engine performance and emissions. Gupta and Mishra modified the design of the intake manifold to qualify the engine noise and performance requirements. Kesgin studied the design of the intake and exhaust of diesel engines for possible performance improvements. However, engine system modification requires modifications to the engine architecture. In a turbocharged engine, high gas pressure can result in an excessively hot intake charge that significantly reduces the performance gains from turbochargers. Therefore, turbocharged engines always need a cooling system to drop the air temperature of the turbocharger compressor outlet. The cooling effects of the engine inlet cooler impact the performance of the engine system. Bevilacqua et al. used the GT-POWER simulation tool to study the engine performance improvement by reducing the engine inlet temperature. But changing coolers is impossible for an existing engine.
A significant contribution has been made to overall engine performance improvements through turbochargers. Papagiannakis used natural gas as a supplement for the diesel fuel in a compression–ignition environment to improve the natural gas/diesel engine with an optimum combination. Senthil et al. studied the performance and emission improvement through boost pressure and injection of high-pressure gas in the combustion process for various engine loads and speeds. Emara et al. performed the research on the power increments of a six-cylinder turbocharged four-stroke direct-injection heavy-duty diesel engine by replacing it with a better-matched market-available turbocharger. Muqeem et al. provided a review of the current techniques used in the turbocharger to improve diesel engine efficiency and reduce emissions. The most of research for turbocharger efficiency improvements were focused on turbine and turbine vane designs; compressor turbine empirical design; wheel, meridional, clearance, and splitter optimization for performance improvements. There is very limited literature discussing a systematic way to obtain the design targets for a better-matched turbocharger wheel diameter ratio to improve the engine performance. Traditionally, engine simulations used by engine manufacturers were used to choose an existing turbocharger to meet the engine needs. In this study, the simulations were used to obtain the turbocharger turbine design targets. This study first proposed the velocity ratio to systematically consider the overall turbocharger performance and proposed an optimal turbine design target for the new turbine design.
Recently, the engine energy savings through turbochargers has been studied extensively. Darlington et al. used a one-dimensional (1D) model that studied the injection of compressed air into a turbocharger turbine to increase the shaft torque and performance. Katrasnik et al. discussed using an electric motor mounted on the turbocharger shaft or a separated electric supercharger to improve the engine performance. Vitek et al. performed an investigation of turbocharger selection procedures using corrected 1D simulation tools to select the turbocharger to better fit the engine system. Xu and Amano performed engine energy-saving studies by optimizing a turbocharger compressor. Lecuona et al. proposed an improved thermodynamic open dual cycle to simulate the working of internal combustion engines. offers a significant advantage for more accurate engine simulations. The mixed flow turbines provide the advantages for turbocharger engine low-speed performance and good unsteady characteristics. In this study, the turbomachinery design system developed in the past was used for turbine performance optimization. The mixed flow turbine was selected and optimized for the current turbocharger.
Most engine manufacturers start with the selection of the appropriate compressor on the basis of the required boost pressure characteristic curve. The full-load curve should be such that the compressor efficiency is at its reasonable levels in the main operating range of the engine. The surge margin should be sufficiently large to allow the engine to operate at high altitude. After the compressor was selected, a proper turbine flow capacity was selected. The compressor and turbine were chosen independently. This method is difficult to optimize the engine turbochargers matching. There was limited research available through engine match to propose an optimal turbocharger target based on turbocharger system efficiency. The current study is focused on an existing 9-L Tier IV diesel engine performance improvement through a better matching turbocharger and redesign turbocharger. The details of the engine parameters are listed in Table 1. For turbocharger development, low cost, low weight, low inertia, and high performance, meeting engine packaging requirements, and being mechanically robust are keys for mass production. In this study, the turbocharger package needs to keep the same. For reducing the development time, a newly designed compressor was used. A new turbine wheel was redesigned in this study. Design optimization with a fast turnaround method was developed to use in the turbine design. An optimization with several penalty functions is used to optimize the turbine design.
Table 1. Engine parameters
Type | Parameters |
---|---|
Engine | Six-cylinder, four-stroke |
Fuel type | Diesel |
Number of valves | Four |
Bore diameter × Stroke length (mm) | 126 × 130 |
Displacement | 9.73 L |
Calibration power | 130 kw |
Maximum Torque | 950 Nm |
A systematic way to optimize the engine and turbocharger matching was proposed to define the requirements for turbocharger design. A diameter ratio of compressor and turbine wheels were incorporated in the performance simulation to better optimize the turbocharger performance. In the performance optimization, the constraints were added for the availability of current technologies and manufacturing capacity, emissions, and cost. The multidisciplinary optimization method was used to design a new turbine wheel. The new turbine was designed using a mixed flow type to improve the compressor and turbine matching. This study showed that not only turbocharger compressor and turbine efficiency impact engine performance, but also wheel diameter matching between compressor and turbine is important. The new turbocharger was built and gas stand performance tests were performed to compare with the analysis. The final engine test indicated that the current study provided an opportunity to further improve the engine performance without scarifying cost and emission.
Reducing fuel consumption and emission at the same time is difficult because one may affect the other. High-efficiency combustion normally requires air-fuel ratios that are difficult to handle by downstream emission-reducing devices. The temperature for emission chemistry to work can postulate a high exhaust gas temperature, which requires more fuel that is not efficient for engines. The development of simulation tools has provided a way of conducting research to improve engine efficiency and reduce emissions at the same time. The engine simulation is implemented by means of mass flow and energy balances; the air is compressed by the compressor and the fuel is fed to the engine. The turbine power outputs and the power consumed from the compressor and bearing are identical. The engine simulation is an iterative process, based on the compressor and turbine maps to obtain the engine performance and emission data.
GT-POWER is one of the important engine simulation tools to help engine manufacturers to do engine turbocharger matching. The simulation is fast and allows for finding suitable turbochargers without a need to calculate all possible combinations of turbine and compressor performances. However, matching the results with existing engine tests is the key to simulation.
GT-Power is a well-known engine simulation tool used by engine makers and suppliers. It can be enhanced by integration with CFD and MS/EXCEL. GT-POWER has been used for a wide range of applications relating to engine design and development. Turbocharger matching is one of the important applications. The procedure of matching the turbocharger to an engine can be time-consuming and it will also be very expensive. If an analytical method is available for calculating performance with different turbocharger matches then actual engine testing may begin with a turbocharger close to the optimum match. A typical procedure for a four-stroke engine has been described and successfully predicated the engine performance. The calibration procedure is critical for the GT-Power simulation. In this study, the simulation was calibrated with original engine test data. GT-Power simulations of EGR systems to assess their strengths and weaknesses. Park et al. performed simulations of a light-duty diesel engine. The study was performed using GT-Power to visualize how different parameters affected parameters such as turbine efficiency, and break torque. In the current study, the GT-Power simulation was calibrated with existing engine test data. The intake environment conditions, intercooler, intake pipe, injector, cylinder, crankcase, exhaust gas pipeline, and exhaust environment were modeled with corresponding throttling modules as shown in Figure 1. The combustion model uses the in-cylinder direct injection diesel Weber combustion model. The mechanical loss is performed using the Chen-Flynn empirical model.
Figure 1
GT-Power model
For correctly matching simulation results with engine tests, it is critical to set up the geometry details, and turbocharger maps accurately. The three-dimensional models of the engine pipes were imported into GT-Power software to better consider the piping losses. The compressor and turbine maps of the original turbocharger were obtained from gas stand tests where the new turbocharger will test. Engine operating points such as low torque, maximum torque, and maximum power determine the operating points on the compressor map. The operating points of the compressor can be obtained through calculations of pressure ratio and mass flow rate. Then engine performance and emission can be calculated. The independent tuning multipliers for combustion and turbocharger performance were adjusted to match the emission and fuel consumption from the engine test. The important parameters such as fuel consumption, airflow, power, torque, turbocharger pressure ratio, exhaust gas temperature, and pressure loss across the intercooler, and intercooler outlet air temperature were matched by test data within 1%. The other parameters were within 3% compared with tests. The existing engine simulation results for fuel consumption and engine airflow were compared with engine tests as shown in Figures 2 and 3. It can be seen that the engine fuel consumption and airflow from simulations are matched with engine measurements within 1%. The turbocharger optimization can be performed.
Figure 2
Nondimensional fuel consumption
Figure 3
Nondimensional engine airflow
The engine improvement process developed in this study is shown Then turbocharger factors design of experiments (DOE) were set up as shown in Figure 4. The first step is to match the simulation with the engine test key data as mentioned earlier. The simulation tool then calculates the engine performance and emission with different turbocharger factors. After the engine performance was calculated for all factors, the DOE helped to find good matched turbocharger options. The turbocharger design parameters were defined. The new turbocharger could be designed and manufactured. The aerodynamic performances of the new turbocharger were tested in a gas stand. The new turbocharger and the original turbocharger were installed on the same engine for performance comparisons and tests.
Figure 4
Engine performance optimization process
The engine simulation provided the mass and energy balances for cylinders, piping, intercooler, and turbocharger. Turbochargers involve in simulation in the form of maps. The compressor and turbine maps are independently input into the simulation. In this study, the wheel size effects were considered during the simulation to obtain an optimal turbocharger design condition. The optimization factors were considered by adjusting the compressor and turbine efficiencies. The wheel diameter ratio was achieved to shift peak turbine efficiency with velocity ratio for each speed line.
Most turbocharger selection typically starts with the target power or torque curve of the engine along with overall engine parameters such as air fuel ratio (AFR), displacement volume, fuel heating value, and so on. For an existing turbocharged engine upgrade, it is good to make sure the emission is not to be impacted. The traditional selection method of the turbochargers is based on the original compressor and turbine maps. There is no detailed information regarding compressor and turbine wheel sizes and designs. It is hard to know whether the diameter of the compressor and turbine wheel are optimal for overall turbocharger efficiency. Traditionally, the wheel selections are based on the database of the designs of the turbocharger OEM. However, the diameters' relationship between compressor and turbine impacts the turbine matching point efficiency. Theoretically, turbocharger wheels should be designed to match their desired characteristics to minimize performance deterioration. For compressor and turbine match, the compressor and turbine power should equal, that is,
where ṁc and ṁt are the compressor and turbine inlet mass flow and Tc and Tt are the compressor and turbine inlet temperature, respectively. The left-hand side of the equation represents the energy needs for the compressor and the right-hand side represents the energy turbine can provide. It can be seen from Equation (1) that the turbocharger performance is strongly impacted by compressor and turbine efficiency. The compressor and turbine are linked with one shaft. Once one component is fixed by choosing an optimal parameter and other components may not be optimal. For a specified power level, the turbine may run faster or slower than its optimal speed at a cost of low turbine efficiency. Therefore, the compressor and turbine should match their rotating speed, that is, wheel diameter ratio should match well to get the best overall turbocharger efficiency. The blade speed ratio U/C0 is the inverse turbine isentropic loading coefficient. The turbine efficiency impacts due to the wheel diameter ratio of the turbine and the compressor can use velocity to identify. The U/C0 is defined as:
The numerical simulation was performed for the original turbocharger. The turbine performance plotted with a velocity ratio is shown in Figure 5. Each line of the plot represented the same ER with a different rotating speed. It can be seen that ER impacts the peak efficiency U/C0 value. As the expansion ratio increases, the U/C0 value at the peak efficiency point increases. This character is good for high engine speed but not good for low engine speed. When a turbocharger is installed on an engine, the engine exhaust valve opens, the turbine pressure ratio is relatively high compared to the turbine speed and U/C0 is low, especially at low engine speed where turbine rotational speed is low. When the cylinder empties, the pressure ratio falls; U/C0 increases especially at high engine speed. For convenience in the discussion, most of the time we can use one typical line to represent the turbine efficiency and U/C0 relationship as shown in Figure 6. for the original turbine. The U/C0 working range on the current engine is shown in Figure 6. The low U/C0 causes the turbine to operate away from the maximum efficiency point. For a conventional radial-inflow turbine, the U/C0 theoretical value for peak efficiency point is around 0.707, but turbocharger turbine on engine typically occurs between values of 0.3–0.9. A turbocharger turbine is desirable to have high efficiency at a high-pressure ratio and low speed. The turbine optimization goal is to have high peak efficiency at low U/C0, while simultaneously minimizing inertia and meeting material limits for stress and vibration. In this study, a new turbine is developed to have a high efficiency by introducing a forward-swept blade and mixed flow type. The turbocharger turbine U/C0 working range on engine changes.
Figure 5
Turbine efficiency as a function of velocity ratio
Figure 6
Turbine efficiency as a function of velocity ratio
In this study, a systematic simulation to optimize turbocharger overall efficiency was considered during the simulations. The performance impacts of diameter ratio between compressor and turbine wheels were considered through U/C0 in the simulation as shown in Figure 6. The original turbocharger compressor and turbine wheel diameters ratio is 1.14. A newly developed compressor wheel with a 5% smaller diameter was used for this application. The new compressor has better efficiency at an engine speed lower than 12,000 rpm as shown in Figure 7. The wheel diameter ratio was reduced by 5% to reduce peak efficiency point U/C0 and improve turbine matching efficiency by about 4.7% as shown in Figure 6.
Figure 7
Compressor efficiency versus engine rotational speed
In most diesel engines, there is an emission treatment device, such as catalytic reduction (SCR), downstream of the turbine. In this study, an existing 9-L off-road Tier IV engine with an SCR system to control the emissions was selected for performance improvement studies. SCR is a technology to control NOx emissions from diesel engines. The reductant source is usually automotive-grade urea, known as DEF. The DEF sets off a chemical reaction that converts nitrogen oxides into nitrogen, water, and small amounts of CO2. SCR technology is designed to permit NOx reduction reactions to take place in an atmosphere. SCR system in the exhausted pipe would increase the engine backpressure. The engine has to compress the exhaust gasses to a higher pressure, which involves additional mechanical work and less energy extracted by the exhaust turbine. This can lead to an increase in fuel consumption, CO emissions, and exhaust temperature. The engine back pressure needs to be within a certain range to keep overall engine emission to meet the standards. In this project, one constraint for performance optimization is to make sure the emission meets the emission standard.
Another constraint for engine performance optimization is overall engine cost. The simplest way to improve the engine performance is to optimize the turbocharger and engine match. In this study, the performance optimization goal is to use a newly developed compressor and redesign the turbine wheel to improve the engine performance without scarifying the cost and emissions. A simple factor DOE was used in the simulation and turbine wheel design. DOE establishes a cause-and-effect relationship between a number of independent variables and a dependent variable of interest. In the current study, six factors are selected and they are compressor diameter, turbine diameter, compressor efficiency, turbine efficiency, pressure ratio, and engine airflow. Three-factor levels are used for all factors. Based on the cause-and-effective relationship, a set of good turbocharger targets for the engine performance is selected in the simulation. Due to the original engine performance with the original turbocharger being calibrated with engine test and also the turbocharger maps of the original one and new one were from the same gas stand, the GT-POWER simulations provided good results for the new turbocharged engine.
In this study, a new mixed flow turbine wheel is designed to meet the upgraded engine performance targets. The mixed flow turbine can reduce the peak efficiency U/C0 value to improve the overall turbocharger efficiency. In this design, a turbomachinery multidisciplinary DOE optimization design process developed recently is used for the mixed flow turbine design. A Navier–Stokes solver with an SST turbulence model is used for the aerodynamic optimization. A finite element analysis is employed for the mechanical integrity assessment.
The geometry generation and parameterization have been used in the codes developed in the past. The turbine wheel hub, shroud, and other three meridional design sections are represented by using the Bezier curves. The Bézier curves are smooth parametric curves used in turbomachinery design as the curves are completely contained in the convex hull of its control points, the points can be displayed on the computer screen and used to manipulate the curve intuitively.
At each turbine wheel design section, the design optimization parameters are geometry control points, blade angles, and outflow angle. The objective functions which are made up of several penalty terms are optimized to be minimized:
In Equation (3), constants 2, 25, and 50 are the weighting constants of the optimization. The maximum exit flow angle is set as
to control the exit swirl within acceptable levels. It can be seen that the main goal is high turbine efficiency, by preserving as much as possible the mass flow rate and keeping, if possible, the outlet flow angle spanwise distribution in the desired range. The turbine design optimization point was set near the engine maximum torque point. The turbine wheel rotational speed is 95 krpm. The turbine inlet's total pressure and temperature were 2.2 kpa and 680 K, respectively. The turbine expansion ratio is 2.1 and the wheel tip cleanse was set as 0.5 mm and kept constant from wheel inlet to exit.
The twin housing was used to get better pulse effects for the turbine. During the turbine design, we kept the original turbine twin housing. Before turbine optimization, the grid independent analyses were performed. The calculations for nondimensional efficiency, which is defined as the calculated efficiency over the final calculated efficiency, for different grid sizes were tabled in Table 2. It is shown that the total grid nodes larger than 4.4, the calculated nondimensional efficiency is not changed significantly. The CFD total grid sizes were 5.0 million for the turbine stage. The turbine stage grid was shown in Figure 8. After aerodynamic optimization, the conjugate heat transfer structure approach was used to ensure the structure integrity.
Table 2. Grid independent test
Grid nodes (million) | 0.978 | 0.97 | 1.04 | 0.998 | 1 | 0.999 | 1 |
Nondimensional efficiency | 1.4 | 2.5 | 3.4 | 4 | 4.4 | 5 | 6 |
Figure 8
Turbine grids
After the turbine optimization, the final turbine flow characters and performance were analyzed in detail by using Ansys CFX. The k–ε turbulence model was used in the calculation. The mixing model was used in the domain interface between the rotating and stationary domains. The efficiencies at the design speedline were compared with the original design as shown in Figure 9. The results showed that the optimized turbine had 5�tter efficiency than the original design for the expansion ratio larger than 1.6 which was better than the design target.
Figure 9
Efficiency comparison between the original and final design
The entropy distributions for the meridional plan and downstream of the turbine wheel were shown in Figures 10 and 11. It can be seen from Figure 10 that the loss developed near the tip clearance area. The high loss area near tip clearance increased from inlet to wheel exit due to tip clearance flow development. The high loss core between the blades is near the middle between the blades as shown in Figure 11. However, the clearance is not avoidable, the tip clearance loss always exists. To further improve the turbine efficiency, we can reduce the tip clearance and add tip clearance control. The nonlinear clearance and improving the bearing symmetricity can reduce the clearance losses After aerodynamic optimization, the conjugate heat transfer wheel structure analysis was performed. The stress analysis and qualification tasks were performed. The final wheel life was estimated as shown in Figure 12. The turbine wheel life met the design target which required 1.0 × 106 cycles. However, the turbine life is shorter than the compressor wheel.
Figure 10
Entropy contour at meridional
Figure 11
Entropy at turbine wheel exit
Figure 12
Wheel life estimation
After the turbocharger was designed, the five prototypes were built. We chose one unit of turbochargers to perform gas-stand, engine, and reliability tests to verify the new turbocharger's performance. One unit was used to perform The other three units were backup units. The gas-stand test showed that the newly developed turbine had better performance than the original turbine. The turbocharger was tested on a gas stand which approved the performance and then installed on the engine. To ensure the test repeatability, the turbochargers on engine tests were performed in a sequence with installing the new turbocharger, the original turbocharger, and final repeat the new turbocharger. The test showed that the repeatability of the test was acceptable. The new turbocharger engine test results reported here were the averages of the two tests.
The new turbocharger airflow capacity was tested on the same engine as the original one and the test results were shown in Figure 13. On-engine tests demonstrated that the new turbocharger has a similar flow capacity as the original turbocharger. The low engine speed turbocharger efficiency is more critical than the engine speed over wastegate open engine rotational speed. The wastegate opens for this engine at engine speed over 1250 rpm. The turbocharger provides more than enough boost for the engine after the wastegate opens. The turbine efficiency in most of the engine speeds exceeded the design target and showed over 10�tter performance compared with the original turbocharger as shown in Figure 14. The turbine efficiency improvement came from both peak efficiency gain and U/C0 value drop at peak efficiency. The U/C0 drops at the peak efficiency point due to forward lean and the diameter ratio reduction of compressor and turbine as shown in Figure 6. The low to medium engine speeds and turbine efficiency gained significantly from the engine test as shown in Figure 14. This may be because the mixed turbine can handle engine pulse more effectively. This is good for low engine speed performance as well as emission reductions.
Figure 13
Engine airflow changes with engine speed
Figure 14
Turbine efficiency versus engine rotational speed
Figure 15 was the on-engine tests for compressor pressure ratios along the engine lugline at different engine speeds. It can be seen that the new compressor has a higher pressure ratio compared with the original compressor for engine match. The slightly higher-pressure ratios benefit the SCR functions. The tests and simulation also showed that NOx emission did not have significant change. Figure 16 is the engine torques vary with engine rotational speed for original and new turbocharger on-engine tests. It can be seen that the new turbocharger design meets the design intention and provides about 0.4% more torque at middle engine speed. Engine BSFC test results in Figure 17 show that the engine BSFC was reduced by using the new turbocharger. In the middle engine speed, the maximum BSFC reduction was about 1.5%. The engine tests showed that the engine backpressures increased because the compressor ratios on the engine increased as shown in Figure 18. The slightly higher back pressure provided margins to make sure the SCR system works properly for a long time. The engine tests for the new turbocharger were fairly agreed with the analysis target. It was also shown that the simulation results fairly agreed with the final engine tests. The turbine inlet and outlet temperature on-engine tests as shown in Figures 19 and 20 have slightly increased. It may be because of the pressure ratios of the compress increase. The high pressure caused the compressor exit temperature to increase. With the same cooler, the cooler exit temperature slightly increased. The emission test results demonstrated the emission is acceptable. The test results for exhaust smoke, unburned hydrocarbons (HC), and NOx are shown in Figure 21. HC and NOx are slightly increased, this may be because the engine back pressure slightly increases, and also the exhaust temperature for engine Rpm is lower than 1500 rpm.
Figure 15
Compressor pressure ratio versus engine rotational speed
Figure 16
Engine torque versus engine rotational speed
Figure 17
Engine BSFC versus engine rotational speed
Figure 18
Exhaust back pressure versus engine rotational speed
Figure 19
Turbine inlet temperature versus engine rotational speed
Figure 20
Turbine exit temperature versus engine rotational speed
Figure 21
Emission comparisons
In this study, an existing 9-L Tier IV existing turbocharged diesel engine performance improvement was studied through GT-power systematic simulation. The simulation first used newly test original gas stand maps that matched the engine-tested performance and emission of an existing engine. and then optimized the turbocharger and engine system to propose the turbocharger design targets. This study demonstrated that the current simulation method with matching the simulation the original engine test can be a good method to make sure that the simulation reasonably predicates the new engine system performance.
The current study streamlines engine matching with the existing engine performance calibration, turbocharger overall efficiency, and turbocharger design together. This method makes the turbocharger design more optimal for the engine system. The new turbocharger was designed and installed on the engine for performance vilifications. The studies demonstrated that the systematic engine and turbocharger simulations could optimize the turbocharger to better match the engine system.
Instead of the traditional way to use GT-power to choose a turbocharger from existing products, the systematic engine and turbocharger optimization were used to provide the best matching turbocharger for the engine. Furthermore, the traditional turbocharger simulation using the compressor and turbine maps was not easy to optimize the engine turbocharger system. In this study, the velocity ratio concept was introduced in the simulation to consider the performance impacts of the compressor and turbine wheel diameter ratio. The comparison tests for original and new turbochargers demonstrated that the analysis was fairly consistent with engine tests.
A multidisciplinary optimization method developed in the past was used for the new mixed-flow turbine design. The in-house code combined with a multidisciplinary optimization process can optimize the turbine efficiency at the same time to meet the structure requirements. The optimization with forward lean not only improved the turbine peak efficiency but also improved the turbocharger matching efficiency. The new turbine wheel with a mixed-flow turbine on engine test showed significant performance improvements. It is shown that the mixed-flow turbine wheel has more advantages than the original radial turbine. With the new turbocharger, the engine obtained about as much as a 1.5% reduction of the BSFC at middle engine speed. At the same time, the new engine system provided about 0.4% more torque at middle engine speed.
This study demonstrated that the turbocharger performance not only depended on the compressor and turbine efficiency but was also impacted by the diameter ratio of the compressor and turbine wheel. The diameter ratio impacts the turbine and compressor match point efficiency which is critical to turbocharger overall efficiency. The study demonstrated that future turbocharger development should consider the diameter ratio of compressor and turbine wheels. This concept can be extended to turbomachine designs.
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