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Turbocharger History  The first exhaust-driven supercharger was developed by Dr. Alfred J. Buchi of Switzerland between 1909 and 1912, long before Garrett products entered the turbocharger picture. Dr. Buchi was Chief Engineer of Sulzer Brothers Research Department and in 1915 proposed the first prototype of a turbocharged diesel engine, but his ideas gained little or no acceptance at that time. General Electric began developing turbochargers during the late 1910's. In 1920, a LePere bi-plane that was equipped with a Liberty engine and a General Electric turbocharger set a new altitude record of 33,113 feet (10092m). Turbochargers were used sparingly on aircraft in World War I, but their development occurred on a widening scale in the 1930's and 1940's - first in Europe and then in the United States. In the United States, General Electric developed turbochargers for military aircraft, and in World War II, thousands were used on fighter aircraft and bombers, such as the B-17. The Garrett Corporation, formed in 1936 by J. C. "Cliff" Garrett, supplied the charge air cooler (aftercooler) for the B-17, located between the General Electric turbocharger and the Pratt and Whitney engine. In the late 1940's and early 1950's, Garrett was heavily committed to the design of small gas turbine engines from 20 - 90 horse power (15 - 67 kw). The engineers had developed a good background in the metallurgy of housings, high speed seals, radial inflow turbines, and centrifugal compressors. On September 27, 1954, Cliff Garrett made the decision to separate the turbocharger group from the Gas Turbine department due to commercial diesel turbocharger opportunities. That was the beginning of the new AiResearch Industrial Division - for turbocharger design and manufacturing. AiResearch Industrial Division would later be named Garrett Automotive. The Chevrolet Corvair Monza and the Oldsmobile Jetfire were the first turbo-powered passenger cars, and made their debut on the US market in 1962/63. Despite maximum technical outlay, however, their poor reliability caused them to disappear quickly from the market. The internal combustion engine is an air consuming machine. This is because the fuel that is burned requires air with which it can mix to complete the combustion cycle. Once the air/fuel ratio reaches a certain point, the addition of more fuel will not produce more power, but only black smoke or unburned fuel into the atmosphere. The more dense the smoke, the more the engine is being over fueled. Therefore, increasing the fuel delivery beyond the air/fuel ratio limit results in excessive fuel consumption, pollution, high exhaust temperature (diesel) or low exhaust temperature (gasoline), and shortened engine life. After the first oil crisis in 1973, turbocharging became more acceptable in commercial diesel applications. Until then, the high investment costs of turbocharging were offset only by fuel cost savings, which were minimal. Increasingly stringent emission regulations in the late 80's resulted in an increase in the number of turbocharged truck engines, so that today, virtually every truck engine is turbocharged. In the 70's, with the turbocharger's entry into motor sports, especially into Formula I racing, the turbocharged passenger car engine became very popular. The word "turbo" became quite fashionable. At that time, almost every automobile manufacturer offered at least one top model equipped with a turbocharged petrol engine. However, this phenomenon disappeared after a few years because although the turbocharged petrol engine was more powerful, it was not economical. Furthermore, the "turbo-lag", the delayed response of the turbochargers, was at that time still relatively large and not accepted by most customers. The real breakthrough in passenger car turbocharging was achieved in 1978 with the introduction of the first turbocharged diesel engine passenger car in the Mercedes-Benz 300 SD, followed by the VW Golf Turbodiesel in 1981. By means of the turbocharger, the diesel engine passenger car's efficiency could be increased, with almost petrol engine "driveability", and the emissions significantly reduced. Today, the turbocharging of petrol engines is no longer primarily seen from the performance perspective, but is rather viewed as a means of reducing fuel consumption and, consequently, environmental pollution on account of lower carbon dioxide (CO2) emissions. Currently, the primary reason for turbocharging is the use of the exhaust gas energy to reduce fuel consumption and emissions. Turbocharger Theory A turbo charger is basically an exhaust gas driven air compressor and can be best understood if it is divided into its two basic parts, the exhaust gas driven turbine and its housing, and the air compressor and its housing. I did say divided didn't I. Well I should have said like a set of Siamese twins because each of them perform different functions but, because they are joined together at the hip via a common shaft, the function of one impacts the function of the other. How? Take a perfectly set up compressor section and mate it with an incorrect turbine section, or visa versa, and you end up with with our Siamese twins trying to go in different directions. The result is that our Siamese twins end up wasting all of their energy fighting each other and go nowhere. When considering a turbo charger most folks tend to look at the maximum CFM rating of the compressor and ignore everything else under the assumption that the compressor and the exhaust turbine are perfectly matched out of the box. I will grant you that in stock factory applications that is probably close to the truth but, in all out performance applications, nothing could be further from the truth because of the extremes of operation in a performance application. The goal in a performance application is to get the exhaust turbine up to speed as quickly as possible however, it must be mated to a compressor wheel that will generate as much pressure as it can as soon as possible. This is a contradiction because the exhaust turbine generates the drive power and the compressor consumes that power. The larger the compressor and the higher the pressure (boost) we want, the quicker the power from the exhaust turbine is used up. Put in a larger exhaust turbine and it will take the engine longer to develop enough hot expanding exhaust gas to spin it, slowing down the compressor and causing turbo lag. At this point I am going to repeat something stated earlier, do not think of a turbo charger as a bolt on piece of equipment, think of it as a system. The turbine is powered by hot expanding exhaust gas, a lot of hot expanding exhaust gas, the more and the hotter the expanding exhaust gas the better. I am sure many of you have seen pictures of turbo charged engines with cherry red hot exhaust systems and turbo housings. The captions under most of these types of pictures proclaim outstanding horse power numbers. What most of the articles related to these pictures do not tell you is that the engine was under an extreme load. A load so heavy that the engine was almost at its stall point for a prolonged period of time. A condition that most turbo charged engines will never see. The real point I am trying to make is that the exhaust turbine will not generate enough power to turn the air compressor fast enough for it to work properly unless the engine is feeding the exhaust turbine a lot of hot expanding exhaust gas, a condition that can only be created when the engine is under a load. There is where the selection of transmission gear ratios and the ring and pinion ratio play a critical part. The fact that the engine must be under a load is the reason why, no matter how high you rev a turbo charged engine with no load on it, you will not see the boost gauge move. This is also where the term 'turbo lag' came from. Turbo lag is basically the amount of time it takes from the time you place a load on the engine (stomp the gas peddle to the floor and dump the clutch or, get full converter lock up with your automatic trans) until the time the engine develops enough hot expanding exhaust gas to spin the turbine fast enough for the compressor to do its job. Effectively, a turbo charged engine is a normally aspirated engine until the turbine and compressor spin up. To minimize turbo lag, it is imperative that the turbine and the compressor are properly matched to the engine as well as the engine being properly matched to the transmission gears, the ring and pinion gears, and the tires. Principles of Turbocharging To better understand the technique of turbocharging, it is useful to be familiar with the internal combustion engine's principles of operation. Today, most passenger car and commercial diesel engines are four-stroke piston engines controlled by intake and exhaust valves. One operating cycle consists of four strokes during two complete revolutions of the crankshaft. Schematic of a four stroke piston engine Suction (charge exchange stroke) When the piston moves down, air (diesel engine or direct injection petrol engine) or a fuel/air mixture (petrol engine) is drawn through the intake valve. Compression (power stroke) The cylinder volume is compressed. Expansion (power stroke) In the petrol engine, the fuel/air mixture is ignited by a spark plug, whereas in the diesel engine fuel is injected under high pressure and the mixture ignites spontaneously. Exhaust (charge exchange stroke) The exhaust gas is expelled when the piston moves up. These simple operating principles provide various possibilities of increasing the engine's power output: Swept volume enlargement Enlargement of the swept volume allows for an increase in power output, as more air is available in a larger combustion chamber and thus more fuel can be burnt. This enlargement can be achieved by increasing either the number of cylinders or the volume of each individual cylinder. In general, this results in larger and heavier engines. As far as fuel consumption and emissions are concerned, no significant advantages can be expected. Increase in engine rpm Another possibility for increasing the engine's power output is to increase its speed. This is done by increasing the number of firing strokes per time unit. Because of mechanical stability limits, however, this kind of output improvement is limited. Furthermore, the increasing speed makes the frictional and pumping losses increase exponentially and the engine efficiency drops. Turbocharging In the above-described procedures, the engine operates as a naturally aspirated engine. The combustion air is drawn directly into the cylinder during the intake stroke. In turbocharged engines, the combustion air is already pre-compressed before being supplied to the engine. The engine aspirates the same volume of air, but due to the higher pressure, more air mass is supplied into the combustion chamber. Consequently, more fuel can be burnt, so that the engine's power output increases related to the same speed and swept volume. Basically, one must distinguish between mechanically supercharged and exhaust gas turbocharged engines. Mechanical supercharging With mechanical supercharging, the combustion air is compressed by a compressor driven directly by the engine. However, the power output increase is partly lost due to the parasitic losses from driving the compressor. The power to drive a mechanical turbocharger is up to 15 % of the engine output. Therefore, fuel consumption is higher when compared with a naturally aspirated engine with the same power output. Schematic of a mechanically supercharged four-cylinder engine Exhaust gas turbocharging In exhaust gas turbocharging, some of the exhaust gas energy, which would normally be wasted, is used to drive a turbine. Mounted on the same shaft as the turbine is a compressor which draws in the combustion air, compresses it, and then supplies it to the engine. There is no mechanical coupling to the engine. Schematic of an exhaust gas turbocharged four-cylinder Common Terms Adiabatic Efficiency A 100% adiabatic efficiency means that there is no gain or loss of heat during compression. Most turbochargers will have a 65-75% adiabatic efficiency. Some narrow range turbo's can get higher, these types of turbo's work well in engines that operate over a narrow rpm range. In general the wide range turbo's don't have as good peak efficiency, but have better average efficiency and work better on engine that operate over a wide rpm range. Pressure Ratio This is the inlet pressure compared to the outlet pressure of the turbocharger's compressor. For single stage turbo's, the inlet pressure will usually be atmospheric (14.7 psi) and the outlet will be atmospheric + boost pressure. For staged turbo's the inlet pressure will be the outlet pressure of the turbo before it + atmospheric, and the outlet will be inlet pressure + additional boost from that turbo. Density Ratio Turbochargers compress the air to make it more dense, this is what allows more oxygen in the engine and give the potential to make more power. The density of the inlet air compared to the density of the outlet air is the density ratio. Turbine The side of the turbocharger that converts the energy of the exhaust into mechanical energy to turn the compressor. Compressor The side of the turbocharger that compresses the incoming air charge and directs it to your engine. Cartridge This is the center section of your turbocharger. It houses the bearings for your turbocharger, they have oil passages to lubricate the bearings and some have water jackets for water cooling. Intercooler When intake air is compressed by a turbocharger it is also heated, even more so than when supercharging due to the turbo being heated by the exhaust. Hot intake air is not good for power and will increase the chance of detonation. An intercooler reduces the intake temperature by pushing the air through a heat exchanger (much like a small radiator) that absorbs some of the heat out of the charge. With less heat, you'll need less boost pressure to get the desired power and decrease the chance of detonation. Anything that reduces the intake temperature is a big plus in a supercharged engine. Turbo Lag A turbocharger uses a centrifugal compressor, which needs rpm to make boost, and it is driven off the exhaust pressure, so it cannot make instant boost. It is especially hard to make boost at low rpm. The turbo takes time to accelerate before full boost comes in, it is this delay that is known as turbo lag. To limit lag, it is important to make the rotating parts of the turbocharger as light as possible. Larger turbo's for high boost applications will also have more lag that smaller turbo's, due to the increase in centrifugal mass. Impeller design, and the whole engine combo also have a large effect on the amount of lag. Turbo lag is often confused with the term boost threshold, but they are not the same thing, lag is nothing more the the delay from when the throttle is opened to the time noticeable boost is achieved. Turbo Boost Usually measured in pounds per square inch, it is the pressure the turbocharger makes in the intake manifold. One of the ways to increase airflow through a passage is to increase the pressure differential across the passage. By boosting the intake manifold pressure, airflow into the engine will increase, making more power potential. Boost is also measured in Bar. One Bar equals 14.5 psi. Boost Threshold Unlike turbo lag, which is the delay of boost, boost threshold is the lowest possible rpm at which there can be noticeable boost. A low boost threshold is important when accelerating from very low rpm, but at higher rpm, lag is the delay that you feel when you go from light to hard throttle settings. Wastegate The wastegate is a valve that allows the exhaust gasses to bypass the turbine. The waste gate relies on boost pressure to open it. Spliced into the wastegate pressure feed there must be some form of pressure bleed. By bleeding pressure to the wastegate, it is possible to control the amount of boost by reducing the pressure at the wastegate. Turbo Cool Down A turbocharger is cooled by engine oil, and in many cases, engine coolant as well. Turbo's get very hot when making boost, when you shut the engine down the oil and coolant stop flowing. If you shut the engine down when the turbo is hot, the oil can burn and build up in the unit (known as "coking") and eventually cause it to leak oil (this is the most common turbocharger problem). It is a good idea to let the engine idle for at least 2 minutes after any time you ran under boost. This will cool the turbo down and help prevent coking. Advantages of Exhaust Gas Turbocharging Compared with a naturally aspirated engine of identical power output, the fuel consumption of a turbo engine is lower, as some of the normally wasted exhaust energy contributes to the engine's efficiency. Due to the lower volumetric displacement of the turbo engine, frictional and thermal losses are less. The power-to-weight ratio, i.e. kilowatt (power output)/kilograms (engine weight), of the exhaust gas turbocharged engine is much better than that of the naturally aspirated engine. The turbo engine's installation space requirement is smaller than that of a naturally aspirated engine with the same power output. A turbocharged engine's torque characteristic can be improved. Due to the so-called "maxi dyne characteristic" (a very high torque increase at low engine speeds), close to full power output is maintained well below rated engine speed. Therefore, climbing a hill requires fewer gear changes and speed loss is lower. The high-altitude performance of a turbocharged engine is significantly better. Because of the lower air pressure at high altitudes, the power loss of a naturally aspirated engine is considerable. In contrast, the performance of the turbine improves at altitude as a result of the greater pressure difference between the virtually constant pressure upstream of the turbine and the lower ambient pressure at outlet. The lower air density at the compressor inlet is largely equalized. Hence, the engine has barely any power loss. Because of reduced overall size, the sound-radiating outer surface of a turbo engine is smaller, it is therefore less noisy than a naturally aspirated engine with identical output. The turbocharger itself acts as an additional silencer. Development, Matching and Testing Development As turbochargers have to meet different requirements with regard to map height, map width, efficiency characteristics, moment of inertia of the rotor and conditions of use, new compressor and turbine types are continually being developed for various engine applications. Furthermore, different regional legal emission regulations lead to different technical solutions. The compressor and turbine wheels have the greatest influence on the turbocharger's operational characteristics. These wheels are designed by means of computer programs which allow a three-dimensional calculation of the air and exhaust gas flows. The wheel strength is simultaneously optimized by means of the finite-element method (FEM), and durability calculated on the basis of realistic driving cycles. CAD-assembled model of a turbocharger Despite today's advanced computer technology and detailed calculation programs, it is testing which finally decides on the quality of the new aerodynamic components. The fine adjustment and checking of results is therefore carried out on turbocharger test stands. Matching The vital components of a turbocharger are the turbine and the compressor. Both are turbo-machines which, with the help of modeling laws, can be manufactured in various sizes with similar characteristics. Thus, by enlarging and reducing, the turbocharger range is established, allowing the optimal turbocharger frame size to be made available for various engine sizes. However, the transferability to other frame sizes is restricted, as not all characteristics can be scaled dimensionally. Furthermore, requirements vary in accordance with each engine size, so that it is not always possible to use the same wheel or housing geometries. The model similarity and modular design principle, however, permit the development of turbochargers which are individually tailored to every engine. This starts with the selection of the appropriate compressor on the basis of the required boost pressure characteristic curve. Ideally, the full-load curve should be such that the compressor efficiency is at its maximum in the main operating range of the engine. The distance to the surge line should be sufficiently large. The thermodynamic matching of the turbocharger is implemented by means of mass flow and energy balances. The air delivered by the compressor and the fuel fed to the engine constitute the turbine mass flow rate. In steady-state operation, the turbine and compressor power outputs are identical (free wheel condition). The matching calculation is iterative, based on compressor and turbine maps, as well as the most important engine data. The matching calculation can be very precise when using computer programs for the calculated engine and turbocharger simulation. Such programs include mass, energy and material balances for all cylinders and the connected pipe work. The turbocharger enters into the calculation in the form of maps. Furthermore, such programs include a number of empirical equations to describe interrelationships which are difficult to express in an analytical way. Testing The turbocharger has to operate as reliably and for as long as the engine. Before a turbocharger is released for series production, it has to undergo a number of tests. This test program includes tests of individual turbocharger components, tests on the turbocharger test stand and a test on the engine. Some tests from this complex testing program are described below in detail. Containment test If a compressor or turbine wheel bursts, the remaining parts of the wheel must not penetrate the compressor or turbine housing. To achieve this, the shaft and turbine wheel assembly is accelerated to such a high speed that the respective wheel bursts. After bursting, the housing's containment safety is assessed. The burst speed is typically 50 % above the maximum permissible speed. Low-Cycle Fatigue Test (LCF test) The LCF test is a load test of the compressor or turbine wheel resulting in the component's destruction. It is used to determine the wheel material load limits. The compressor or turbine wheel is installed on an overspeed test stand. The wheel is accelerated by means of an electric motor until the specified tip speed is reached and then slowed down. On the basis of the results and the component's S/N curve, the expected lifetime can be calculated for every load cycle. Rotor dynamic measurement The rotational movement of the rotor is affected by the pulsating gas forces on the turbine. Through its own residual imbalance and through the mechanical vibrations of the engine, it is stimulated to vibrate. Large amplitudes may therefore occur within the bearing clearance and lead to instabilities, especially when the lubricating oil pressures are too low and the oil temperatures too high. At worst, this will result in metallic contact and abnormal mechanical wear. The motion of the rotor is measured and recorded by contactless transducers located in the suction area of the compressor by means of the eddy current method. In all conditions and at all operating points, the rotor amplitudes should not exceed 80 % of maximum possible values. The motion of the rotor must not show any instability. Start-stop test The temperature drop in the turbocharger between the gases at the hot turbine side and at the cold compressor inlet can amount to as much as 1000 °C in a distance of only a few centimeters. During the engine's operation, the lubricating oil passing through the bearing cools the center housing so that no critical component temperatures occur. After the engine has been shut down, especially from high loads, heat can accumulate in the center housing, resulting in coking of the lubricating oil. It is therefore of vital importance to determine the maximum component temperatures at the critical points, to avoid the formation of lacquer and carbonized oil in the turbine-side bearing area and on the piston ring. After the engine has been shut down at the full-load operating point, the turbocharger's heat build-up is measured. After a specified number of cycles, the turbocharger components are inspected. Only when the maximum permissible component temperatures are not exceeded and the carbonized oil quantities around the bearing are found to be low, is this test considered passed. Cyclic endurance test During engine operation, the waste gate is exposed to high thermal and mechanical loads. During the waste gate test, these loads are simulated on the test stand. The checking of all components and the determination of the rates of wear are included in the cycle test. In this test, the turbocharger is run on the engine for several hundred hours at varying load points. The rates of wear are determined by detailed measurements of the individual components, before and after the test. Recommendations for Servicing and Care What is good for a turbocharger? The turbocharger is designed such that it will usually last as long as the engine. It does not require any special maintenance; and inspection is limited to a few periodic checks. To ensure that the turbocharger's lifetime corresponds to that of the engine, the following engine manufacturer's service instructions must be strictly observed: - Oil change intervals - Oil filter system maintenance - Oil pressure control - Air filter system maintenance What is bad for a turbocharger? 90 % of all turbocharger failures are due to the following causes: - Penetration of foreign bodies into the turbine or the compressor - Dirt in the oil - Inadequate oil supply (oil pressure/filter system) - High exhaust gas temperatures (ignition system/injection system) - These failures can be avoided by regular maintenance. When maintaining the air filter system, for example, care should be taken that no tramp material gets into the turbocharger. Failure diagnosis If the engine does not operate properly, one should not assume that the turbocharger is the cause of failure. It often happens that fully functioning turbochargers are replaced even though the failure does not lie here, but with the engine. Only after all these points have been checked should one check the turbocharger for faults. Since the turbocharger components are manufactured on high-precision machines to close tolerances and the wheels rotate up to 300,000 rpm, turbochargers should be inspected by qualified specialists only. Turbocharger Turbine The turbocharger turbine, which consists of a turbine wheel and a turbine housing, converts the engine exhaust gas into mechanical energy to drive the compressor. The gas, which is restricted by the turbine's flow cross-sectional area, results in a pressure and temperature drop between the inlet and outlet. This pressure drop is converted by the turbine into kinetic energy to drive the turbine wheel. There are two main turbine types: axial and radial flow. In the axial-flow type, flow through the wheel is only in the axial direction. In radial-flow turbines, gas inflow is centripetal, i.e. in a radial direction from the outside in, and gas outflow in an axial direction. Up to a wheel diameter of about 160 mm, only radial-flow turbines are used. This corresponds to an engine power of approximately 1000 kW per turbocharger. From 300 mm onwards, only axial-flow turbines are used. Between these two values, both variants are possible. As the radial-flow turbine is the most popular type for automotive applications, the following description is limited to the design and function of this turbine type. In the volute of such radial or centripetal turbines, exhaust gas pressure is converted into kinetic energy and the exhaust gas at the wheel circumference is directed at constant velocity to the turbine wheel. Energy transfer from kinetic energy into shaft power takes place in the turbine wheel, which is designed so that nearly all the kinetic energy is converted by the time the gas reaches the wheel outlet. Operating characteristics The turbine performance increases as the pressure drop between the inlet and outlet increases, i.e. when more exhaust gas is dammed upstream of the turbine as a result of a higher engine speed, or in the case of an exhaust gas temperature rise due to higher exhaust gas energy. Turbocharger turbine map The turbine's characteristic behaviors is determined by the specific flow cross-section, the throat cross-section, in the transition area of the inlet channel to the volute. By reducing this throat cross-section, more exhaust gas is dammed upstream of the turbine and the turbine performance increases as a result of the higher pressure ratio. A smaller flow cross-section therefore results in higher boost pressures. The turbine's flow cross-sectional area can be easily varied by changing the turbine housing. Besides the turbine housing flow cross-sectional area, the exit area at the wheel inlet also influences the turbine's mass flow capacity. The machining of a turbine wheel cast contour allows the cross-sectional area and, therefore, the boost pressure, to be adjusted. A contour enlargement results in a larger flow cross-sectional area of the turbine. Turbines with variable turbine geometry change the flow cross-section between volute channel and wheel inlet. The exit area to the turbine wheel is changed by variable guide vanes or a variable sliding ring covering a part of the cross-section. In practice, the operating characteristics of exhaust gas turbocharger turbines are described by maps showing the flow parameters plotted against the turbine pressure ratio. The turbine map shows the mass flow curves and the turbine efficiency for various speeds. To simplify the map, the mass flow curves, as well as the efficiency, can be shown by a mean curve For a high overall turbocharger efficiency, the co-ordination of compressor and turbine wheel diameters is of vital importance. The position of the operating point on the compressor map determines the turbocharger speed. The turbine wheel diameter has to be such that the turbine efficiency is maximized in this operating range. Twin-entry turbines Turbocharger with twin-entry turbine The turbine is rarely subjected to constant exhaust pressure. In pulse turbocharged commercial diesel engines, twin-entry turbines allow exhaust gas pulsations to be optimized, because a higher turbine pressure ratio is reached in a shorter time. Thus, through the increasing pressure ratio, the efficiency rises, improving the all-important time interval when a high, more efficient mass flow is passing through the turbine. As a result of this improved exhaust gas energy utilization, the engine's boost pressure characteristics and, hence, torque behavior is improved, particularly at low engine speeds. To prevent the various cylinders from interfering with each other during the charge exchange cycles, three cylinders are connected into one exhaust gas manifold. Twin-entry turbines then allow the exhaust gas flow to be fed separately through the turbine. Water-cooled turbine housings Turbocharger with water-cooled turbine housing for marine applications Safety aspects also have to be taken into account in turbocharger design. In ship engine rooms, for instance, hot surfaces have to be avoided because of fire risks. Therefore, water-cooled turbocharger turbine housings or housings coated with insulating material are used for marine applications. Turbocharger Bearing System Turbocharger bearing system (cut-away model) The turbocharger shaft and turbine wheel assembly rotates at speeds up to 300,000 rpm. Turbocharger life should correspond to that of the engine, which could be 1,000,000 km for a commercial vehicle. Only sleeve bearings specially designed for turbochargers can meet these high requirements at a reasonable cost. Radial bearing system With a sleeve bearing, the shaft turns without friction on an oil film in the sleeve bearing bushing. For the turbocharger, the oil supply comes from the engine oil circuit. The bearing system is designed such that brass floating bushings, rotating at about half shaft speed, are situated between the stationary center housing and the rotating shaft. This allows these high speed bearings to be adapted such that there is no metal contact between shaft and bearings at any of the operating points. Besides the lubricating function, the oil film in the bearing clearances also has a damping function, which contributes to the stability of the shaft and turbine wheel assembly. The hydrodynamic load-carrying capacity and the bearing damping characteristics are optimized by the clearances. The lubricating oil thickness for the inner clearances is therefore selected with respect to the bearing strength, whereas the outer clearances are designed with regard to the bearing damping. The bearing clearances are only a few hundredths of a millimeter. The one-piece bearing system is a special form of a sleeve bearing system. The shaft turns within a stationary bushing, which is oil scavenged from the outside. The outer bearing clearance can be designed specifically for the bearing damping, as no rotation takes place. Axial-thrust bearing system Neither the fully floating bushing bearings nor the single-piece fixed floating bushing bearing system support forces in axial direction. As the gas forces acting on the compressor and turbine wheels in axial direction are of differing strengths, the shaft and turbine wheel assembly is displaced in an axial direction. The axial bearing, a sliding surface bearing with tapered lands, absorbs these forces. Two small discs fixed on the shaft serve as contact surfaces. The axial bearing is fixed in the center housing. An oil-deflecting plate prevents the oil from entering the shaft sealing area. Oil drain The lubricating oil flows into the turbocharger at a pressure of approximately 4 bar. As the oil drains off at low pressure, the oil drain pipe diameter must be much larger than the oil inlet pipe. The oil flow through the bearing should, whenever possible, be vertical from top to bottom. The oil drain pipe should be returned into the crankcase above the engine oil level. Any obstruction in the oil drain pipe will result in back pressure in the bearing system. The oil then passes through the sealing rings into the compressor and the turbine. Sealing The center housing must be sealed against the hot turbine exhaust gas and against oil loss from the center housing. A piston ring is installed in a groove on the rotor shaft on both the turbine and compressor side. These rings do not rotate, but are firmly clamped in the center housing. This contactless type of sealing, a form of labyrinth seal, makes oil leakage more difficult due to multiple flow reversals, and ensures that only small quantities of exhaust gas escape into the crankcase. Water-cooling Turbocharger for passenger car gasoline applications with water-cooled bearing housing Petrol engines, where the exhaust gas temperatures are 200 to 300 °C higher than in diesel engines, are generally equipped with water-cooled center housings. During operation of the engine, the center housing is integrated into the cooling circuit of the engine. After the engine's shutdown, the residual heat is carried away by means of a small cooling circuit, which is driven by a thermostatically controlled electric water pump. |
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