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| Surging of turbocharger occurs when the air pressure after the compressor is higher than the pressure compressor can internally maintain. This means, when the pressure of the air delivered by the compressor is higher than the pressure inside the compressor a reverse flow of air is created towards the impeller and inlet of the compressor, which reduces the speed of the turbine shaft and creates noise and vibration. Surging can better be understood by drawing a graph of pressure ratio against mass airflow of the system. From the graph it can be seen that surging is an unavoidable phenomena. The efficiency of compressor is highest near the surge line. This means that if high turbocharger efficiency is desired, a compromise between high efficiency or surging needs to be made. Surging leads to a sharp fall in the flow and acceleration of air mainly because of the reversal pressure. This imbalance in the demand and supply also leads to heavy damage of the turbocharger It can also happen due to sudden change in the engine load or speed.
Main Reasons for SurgingThe main reason , as discussed above, is the deviation of the pressure and operating condition of the turbocharger from the set condition. Apart from that surging can also occur due to any or all of the following reasons.
Sometimes a dirty hull that makes the ship run at full torque has also been shown as the reason for surging. Malfunction of engine's fuel system may also lead to surging.
Surging ProcessThere are mainly three things on which the functioning of a turbocharger depends. They are :
When the air enters the compressor it follows the direction of diffuser vanes. The radial velocity attained by the rotational motion of the impeller is converted into pressure by the diffuser. This increases pressure at the compressors outlet. When surging occurs, due to the reverse air flow the velocity angles are disturbed which causes breakdown of the boundary layers. Turbulence is created near the boundary which reduces the air flow area, causing resistance. When the turbulence increases beyond a certain limit, the diffusion of air drastically reduces leading to reduced pressure. Thus the pressure downstream of the diffuser goes higher than the diffuser pressure, leading to increase in reversal of air flow. ReferencesIntroduction to Marine Engineering 2nd Edition by D.A Taylor Image credits http://superhachi.com/theory/compmap/higheff.gif http://www.turbo.com.sg/genuine/iog_eng_1.jpg |
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| When Caterpillar needed a turbocharger developed for its D9 tractor in 1950, it didn’t call an automotive supplier. Instead it called an aerospace company — Garrett Corporation, whose successful development of turbochargers led to the formation of AiResearch Industrial Division, which later became part of Honeywell. Known at the time for producing gas turbine engines and cabin pressurization systems, Garrett had well-developed expertise in turbo-machinery and aerodynamics: areas that would prove essential in developing a mainstream turbocharger application. Following the first phase of the Caterpillar project, Garrett turbochargers saw ever wider use on earth-moving equipment, in tractors, stationary powerplants, railroad locomatives and ships. The Garrett T11 automotive turbocharger came into being in 1960 and promtly became popular with diesel truck operators. By 1962, Garrett was powering the world’s first turbocharger production car, the Oldsmobile Jetfire Rocket. This was followed by several other firsts, including the first turbocharged car to win the Indianapolis 500 (1968), the first turbo for a non sports car application (1977-Saab 99), the first mass production turbo for diesel engines (1978-Mercedes 300TD), and the first turbo to win the 24 Hours of Le Mans (1978-Renault). The real game changer came in 1990s with the development of VNT™ (Variable Nozzle Turbine) technology. First introduced on Chrysler Dodge Daytona Shelby Turbo Z in 1990 and a year later on the Fiat Croma in Europe, VNT™ allowed a turbocharger to adjust to an engine’s air boosting requirements as needed, providing superior performance and fuel economy. While the commitment to developing and manufacturing leading-edge turbochargers has remained constant, the years have seen Garrett evolve into The Signal Companies in the 1970s, and then AlliedSignal in the 1980s. In 1999 it then became Honeywell when that company merged with AlliedSignal. Since the beginning of the new century, the innovation pace at Honeywell Turbo Technology has quickened. In 2002, the variable geometry technology was successfully adapted for commercial vehicles (AVNT™) on the Ford F250 and F350. The third-generation VNT™ for passenger cars was introduced in 2004 for the BMW 1 Series. In 2006, Honeywell launched the world’s first diesel Parallel Sequential Dual-Stage turbo technology on Peugeot 407 and 607 and the Citroen C5 and C6. In 2008, Honeywell turbos are equipped on Ford’s new EcoBoost engine, ushering a new turbocharging era in the US. Regardless of the name, each Honeywell turbocharger continues to carry the pioneering spirit of its early engineers that has made turbocharging more relevant than ever. |
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| A turbocharger is basically a combination of a compressor and a turbine, both mounted on a common shaft. Turbocharger uses the exhaust gases of the engine itself, to rotate the turbine which in turn moves the compressor. Mainly two type of compressors are used in a turbocharger.
Centrifugal compressors are generally used in applications where the size of turbocharger is to be kept small, for e.g., turbocharger in automotive system. Axial flow compressors are used in applications of larger radial units where internal modifications might be needed. They are most efficient with engines using heavy oils.
Main PartsThere are three main parts of a turbocharger :
The wheels of the turbine and compressor are contained in their own conical housing. The amount of air that is to be submitted depends on the sizes of these wheels. The shaft is contained in the central hub with the help of bearings and connects the turbine and impeller wheel on the opposite sides. Due to high speed of rotation, extreme heat is generated in the hub. Water cooling or any other form of cooling system is provided to prevent temperatures from rising. Sufficient sealing arrangements are made between the compressor and turbine side to prevent mixing of gases. A filter is provided on the turbine side to ensure that the air going to the compressor side is free of any impurities.
Turbine Side The turbine side is usually made of cast iron material. The inlet side of the turbine have nozzle blade ring which is used for two purposes -
The outlet side of the turbine casing consists of blower and air passages to supply air to labyrinths seals. Compressor Side The compressor side is usually made of aluminum alloys and it also consists of two parts. The inlet part or casing deals with drawing air from the surrounding areas i.e engine room or deck spaces. If air is drawn from the deck spaces, special ducting is made for the same. The advantage of drawing air from the deck spaces is low air temperature and humidity. While the advantage of drawing air from the engine space is that the air is pressurized and there is no need for long and complex ducting arrangements. The main parts on the compressor side are inducer, impeller, diffuser and inlet and outlet casing.
WorkingThe turbine uses energy from the exhaust gases to convert heat energy into rotational motion. This rotational motion of turbine drives the compressor, which draws in ambient air from the surrounding and pumps compressed air with high density and pressure into the intake manifold. The exhaust gas enters the turbine inlet side of the turbocharger through a pressurized chamber and a series of filters. The nozzle blade rings concentrates the exhaust gas on to the turbine wheel. The movement of the turbine wheel rotates the shaft which in turn rotates the impellor of the compressor. A part of this air goes to the labyrinths seal from the outlet side of the turbine. As the impeller rotates, air is sucked in through the center of the impeller and due to the heavy rotational movement, experiences circumferential velocity which pushes it outwards. A radial velocity is gained which pushes the air further outwards on to the inducer. An additional resultant velocity is gained due to the accurately designed inducer inlet angle which gives maximum compressor efficiency. Excessive pressure leads to spoiling or fouling of the impeller and inducer surfaces. This results in change in angle of incidence and thus drop in efficiency. All heavy fuel engines are subjected to heavy load variations which results in fluctuation of exhaust. gas pressure. A prolonged fluctuation in pressure leads to detrimental effects on the internal parts of the compressor. For this reason, constant pressure chambers are provided in most of the engines. The exhaust gas , instead of directly entering from the engine, first goes to the pressure chamber and from there it is circulated to the turbine at constant pressure. This reduces the excessive stress that gets created on the shaft bearing and sealing. We will learn about turbocharger surging in our next article.
ReferencesIntroduction To Marine Engineering - 2nd Edition by D.A Taylor Image Credits http://www.lotusespritturbo.com/Garrett_AiResearch_T3_Turbocharger.jpg http://www.paxmanhistory.org.uk/images/turbochr.gif http://upload.wikimedia.org/wikipedia/commons/7/76/Turbocharger.jpg |
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