Precision castings as Automotive Turbocharger component

Motoki Ebisu

Mitsubishi Heavy Industries Ltd.

Japan Foundry Society, Inc.

13th WORLD CONFERENCE ON INVESTMENT CASTING

Paper: U1

Copyright reserved: Neither the Japan Foundry Society, Inc. nor its officers accept legal responsibility for information, advice given or opinions expressed.

Precision castings as Automotive Turbocharger component

Motoki Ebisu，Hiroshi Suzuki，Atsushi Hagita Mitsubishi Heavy Industries, Ltd.

Abstract The market requirements to reduce carbon dioxide (CO2) from automobiles have been

increasing, and at the same time, the reduction of fuel consumption in eco-cars including hybrid vehicle (HV), plug-in hybrid vehicle (PHV) and clean diesel cars becomes a severe competition. The regulations of CO2, 130 g/km from 2012 and in 2020 95 g/km of average emission of an automobile company are said to the target in EU countries. And a company which can not satisfy the regulation limit will be liable to a fine. The improvement of fuel consumption is the absolute must for the automobile company, and the fuel consumption value is the most important factor to decide the sales of automobiles with the enhancing environmental consciousness in the world. The turbocharger was originally an indispensable tool to reduce the fuel consumption

and improve the exhaust gas of diesel engines. Recently it has been attracting attention to be a very important component for a better fuel consumption of gasoline engines. As the turbocharger has become to show a great improvement of fuel consumption by downsizing of the engine, while keeping the torque and output power. The installation rate of turbochargers on gasoline engines has been increasing year by year, and the requirements of technology, cost and quality have become demanding accordingly. The precision casting parts such as turbine wheel is an important component to meet these requirements and they have become indispensable parts in the turbocharger. In this report the design, function and quality of turbine wheel, which is an important component to support the performance and durability of turbocharger, and future tasks are described. 2. What is the turbocharger? 2.1. Construction and working principle The turbocharger recovers energy in exhaust gas from reciprocal engines such as diesel

and gasoline engines. The turbine in turbocharger, which is rotated by exhaust gas, drives the coaxial compressor to supply compressed air to the engine. And increase of power and torque, clean exhaust gas and reduction of fuel consumptions are attained. Fig. 1 shows the turbocharger working principle and Fig.2 shows the consisting parts with waste gate.

Generally speaking the exhaust gas temperature of a passenger car is approximately 850°C at the maximum in the case of diesel engine. And in the case of gasoline engine it sometimes reaches 1050°C. For this reason the consisting parts on turbine side such as the turbine housing, turbine wheel and waste gate valve are usually casting parts made of heat-resistant cast iron, cast steel and nickel alloy. Among them, the turbine wheel is generally made by precision casting to meet the high form accuracy, strength and quality as one of the important parts which dominate the aerodynamic performance of turbocharger. Recently, the turbocharger with the flow rate characteristics optimization by variable

nozzle, which is called a variable geometry (VG) turbocharger, has become widely used for diesel engines. The VG turbocharger allows an efficient supercharging from a low-speed to a high-speed of engine, and provides effects of not only the improvement of torque and power but also the significant reduction of particulate matter (PM) and nitrogen oxide (NOx) by increasing the exhaust gas recirculation (EGR) rate. Fig.3 shows the VG turbocharger working principle, and Fig.4 shows a VG turbocharger

construction using a nozzle link mechanism. The nozzle link mechanism, which is a typical variable mechanism of VG turbocharger, must be operated without lubrication in a high-temperature exhaust gas environment. For this reason, high accuracy is required in each part, and the nozzle vane, which effects the aerodynamic performance, is usually made by precision casting.

Fig.4 VG turbocharger structure

2.2. Market trend The turbocharger is indispensable for diesel engines to comply with the exhaust gas

regulations. And almost of all passenger car diesel engines are equipped with turbochargers. Contrarily, gasoline engines do not need turbochargers from a viewpoint of exhaust gas emission. And the installation rate is approximately 30% even in Europe where the rate is relatively high. Although the diesel engine is superior to gasoline engine in fuel consumption, it must be

equipped with expensive devices such as Low Pressure Loop EGR (LPL-EGR), Diesel Particulate Filter (DPF), Urea Selective Catalytic Reduction (Urea SCR) and NOx storage and reduction catalysis to comply with the exhaust gas regulations. As a result the price of the diesel engine is much higher than that of the gasoline engine. On the other hand, the gasoline engine is inferior to diesel engine in fuel consumption, but it can be said to be superior in cost because it can purify the exhaust gas using a three-way catalyst. Recently, the fuel consumption reduction of a vehicle by replacing a large naturally

aspirated engine with a turbocharged small engine, so called the downsizing with turbocharger, has become popular mainly in Europe. And the installation rate of

Drive Ring

Lever Plate

Lever

Mount

Actuator

Bearing Housing

Ring

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Turbine Housing

turbochargers on gasoline engines is in an upward trend. Fig.5 shows the automobile engine trend in each region. The turbocharged gasoline engines will increase in number especially in Europe, and the number will be doubled or more in 2016 compared to the year 2011. The turbocharger installation rate including diesel engines is supposed to be rise to approximately 76%.

Fig.5 Worldwide turbocharger demand forecast 3. Technological requirements to turbine wheel 3.1.1 Operational environment The exhaust gas temperature of a passenger car is approximately 850°C at the

maximum in the case of diesel engine. And in the case of gasoline engine it sometimes reaches up to 1050°C. The rotational speed of some high-speed turbine wheels is 250,000 rpm or higher in such a high-temperature condition. The required boost pressure is in an uprising trend in accordance with the rising torque and output power. The turbocharger rotational speed is rising accordingly, and the centrifugal force generated in the turbine wheel is also increasing. In the case of gasoline engine, the exhaust gas temperature tends to rise for the fuel consumption reduction in high-speed and high-load operation. From these points, the operational environment of turbine wheel has been becoming increasingly severe. 3.2. Point of strength design The items below must be considered for the strength design of turbine wheel. This

chapter describes the theory of strength design. (1) Fatigue strength (2) Creep strength (3) Resonance

3.2.1 Fatigue strength The turbocharger rotational speed changes according to the engine speed and load. The

rotational speed of turbocharger in a vehicle changes time to time from starting and

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(Copyright IHS, Inc., 2012 Database on Jul., 2010)

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various running conditions in urban area, high-way, hill-climbing and others. The centrifugal force generated in the turbine wheel changes when the rotational speed changes. Fig.6 shows an example of turbocharger rotational speed change in urban area running. The turbocharger rotational speed and centrifugal force deviations are calculated from the number of load fluctuation cycles. And the fatigue life of turbine wheel can be attained by the material fatigue strength diagram. The material fatigue strength diagram of 713C alloy as a representative example of turbine wheel material is shown in Fig.7. (Aerospace Structural Metals Handbook) In the strength design, the average level of centrifugal stress which satisfies the required value of fatigue life is defined as the strength criteria. The shape parameters such as the blade thickness are decided within the range not to exceed the criteria stress. Fig.8 shows a calculation result example of stress distribution.

Fig.6 a calculation result example of stress distribution

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Fig.8 Turbocharger speed fluctuation in urban district 3.2.2 Creep strength The turbine wheel changes its rotational speed with the running conditions, and the

fluctuating stress and steady state stress are applied to the wheel. The fluctuating stress affects the fatigue life, and the steady state stress affects the creep life. The consideration of creep life is important in the case of turbocharger for a gasoline engine with high exhaust temperature. The creep strength of material is studied by using a summarized diagram using the Larson-Miller Parameter. Fig.9 shows the creep curb of 713C alloy. With this curb the life to the creep rupture can be calculated under a certain temperature and stress. 3.2.3. Resonance The turbine wheel is at the down stream of the turbine scroll, and the turbine blade is

subjected to a pressure fluctuation as the pressure is not uniformly distributed at the outlet port of the turbine scroll. The pressure fluctuation is especially significant at the tongue portion of scroll, and the scroll must be designed to provide a pressure distribution

Low

High

as smooth as possible in the circumferential direction at the outlet port. Fig. 10 shows the outline of the turbine scroll, and the pressure distribution measured at the scroll outlet port. When turbine blades pass through the pressure distribution, the turbine is subjected to multiple number of repeating forces (excitation force) at every rotation. When the force cycle coincides with the blade natural frequency, the blade resonates and breaks in a short period. When the blade resonates with N-time excitation force in one rotation, it is called an Nth harmonics resonance. Fig.11 shows the measured results of resonance stress on the Campbell diagram.

Fig.10 Turbine Scroll Static Pressure Disribution

3.3. Exhaust gas temperature rise and material Although the air fuel ratio of a gasoline engine is generally the stoichiometric ration (λ =

1), the excessive fuel supply to lower the combustion temperature has been a common practice to protect engine parts in a raised combustion temperature at full-load and high-speed operational ranges. In recent years, however, the excessive fuel supply is not

allowed to attain a better fuel consumption ratio, and the exhaust temperature tends to be raised. For this reason, a material with higher creep strength is required. Generally speaking, the 713C alloy is usually used as a turbine wheel material, the MarM alloy having better high-temperature strength has become frequently used to cope with the raised exhaust temperature. Fig.12 shows the comparison of 713C and MarM alloy creep strength. Although the MarM alloy has higher creep strength than 713C alloy, its lower castability

(fluidity) tends to cause the micro shrinkage. Fig.13 shows the micro shrinkage of MarM alloy. The Hot Isostatic Pressing (HIP) treatment is sometimes applied to the MarM alloy to remove casting defects.

Fig.12 Comparison of the creep strain

Cut Section

Fig.13 Micro shrinkages of MarM turbine wheel

Creep strain = (creep elongation) / (initial exit blade height)

3.4. Reduction of inertia moment and improvement of transient response The transient response improvement of turbocharger is indispensable to promote the

engine downsizing without a degradation of drivability. The improvement of transient response is to shorten the time of turbocharger rotational speed rise. The reduction of inertia moment of rotational parts such as the turbine wheel, shaft, sleeve and compressor wheel as well as the reduction of each parts loss such as the frictional resistance are effective to reduce the inertia moment. The turbine wheel is the heaviest part in the rotational parts, and the inertia moment reduction of turbine wheel is very effective to improve the turbocharger transient response. The shape optimization of turbine wheel such as the size reduction and the lowering of material density are two approaches to reduce the inertia moment. The use of ceramics and titanium-aluminum (TiAl) alloy are generally the means to reduce the turbine wheel material density. The ceramics has a high-temperature strength, but it breaks when a foreign subject flies in as it is a brittle material. Then the blade must be thick for the strength and also for the manufacturing reason, and it is disadvantageous from the viewpoint of performance. Recently TiAl is often used for this reason. The specific gravity of 713C alloy is 7.9, and the TiAl alloy is 4.2, which reduces the inertia moment by approximately 47%. The transient response of turbocharger is improved by reducing the turbine inertia moment, which results in a shorter response time of torque rise. Fig.14 shows the comparison of transient performances of engine torque rise.

4. REGUIRE QUALITY OF TURBINE WHEEL 4.1 MATERIAL STRENGTH DEVITION The turbine wheel is a product of precision investment casting by the lost-wax method.

The consideration of material strength deviation must be sufficient to cope with the very severe operational environment. Fig.15 shows the material strength deviation of each casting lot. To minimize the material strength deviation and to ensure the minimum strength, the process control of temperature, humidity and others are important from the base metal control to wax, tree shape, size, mould, dewaxing/burning process, casting and knock-out. So, the sufficient assessment is necessary at every process change.

4.2. Defects Although the product without any defect is ideal, the defect exists even in the precision

molding. Minute defects can be ignored, but a relatively large defect or aggregated defects are not allowed. The consideration is made to avoid an excessive quality by specifying standards according to the positions of turbine wheel. And casting appearance limit samples for relatively easy judgments are made for the control. 4.3. Structure The microscopic structure changes according to the casting condition, and the

optimization of structure is required for the turbocharger characteristic. The fatigue strength and creep strength are conflicting characteristics, and a tuning is sometimes required for some applications. The grain size miniaturization is effective for an application that requires a long fatigue life. Contrarily, the grain size adjustment is sometimes required to maintain the creep strength for gasoline engines in which the exhaust gas temperature is rising.

4.4. Balance The balance of machining stock of turbine wheel used in a turbocharger is required for a

high-speed rotation. The turbocharger unbalancing noise for a high-power vehicle was allowed as a proof of high-performance in 1980s. However the noise indicating the presence of turbocharger is no more allowed these days. The reduction of imbalance in the machining stock contributes to a better yield and noise reduction. The counter measures for deviation of each blade, hub bulge, mould mismatch and others are required according to the necessity. The machining stock imbalance can be measured these days, and the further quality improvement is expected.

5. Failure examples 5.1. Cause of breakage Refer to the FTA diagram in Fig.16 for the cause of turbocharger breakage.

Fig.16 Turbine Scroll Static Pressure Distribution

5.2. Fragment and its cause Failure examples are introduced as below: The most frequent trouble is the sucking of

foreign substances, then defective casting, over speeding, resonance and interference with casing.

Fig.17 to Fig.25 show the breakage examples.

Fig.17 Cast problem ① Fig.18 Cast problem ② Fig.19 Cast problem ③

T/W breakage foreign objectdamage

excess centrifugal stress over revolution high centrifugal stress design

creep

resonance

interfere with T/H

Cast problem

stress concentration

1st natural frequency 2nd natural frequency high pressure fluctuation

material problem casting defect macro structure problem

Fig.24 Resonance Fig.23 Resonance Fig.25 Interfere with

Fig.21 Excess centrifugal Fig.22 Fig.20 Foreign object damage

6. Introduction of precision casting parts for turbocharger The precision casting parts other than the turbine wheel are used in the turbocharger.

For example, a nozzle vane for a variable geometry turbocharger, insert turbine, flow control valve, waste gate valve and connecting lever.

7. Conclusion The precision casting, which can mass-produce a high-quality product of sophisticated

form at a moderate price, is an essential technology for the turbocharger which is subjected to demanding quality, accuracy and cost. As well as for diesel engines, the turbocharger is becoming an indispensable device for gasoline engines in the trend of less fuel consumption of passenger cars. The quality of investment casting parts are considered to be relatively in a high level compared to other casting methods. However, many problems are still remaining. The turbocharger is in an increasing demand. Accordingly the requirements of supply and quality stabilization of precision casting parts such as the turbine wheel are thought to increase. We would like to proceed with the development and supply of products, which can fulfill the demand of performance, quality and cost from automobile companies in cooperative engineering works with casting part manufactures.

Reference

(1) S.Ibaraki, D.Watanabe, T.Yokoyama, T.Arai, M.Ebisu, M.Tojo : 13th Supercharging Conference 2008, Dresden, 7 (2008)

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