车用增压器简析 HOW A TURBO WORKS

kind of output improvement is limited. Furthermore, the increasing speed makes the frictional and pumping losses increase exponentially and the engine efficiency drops.TurbochargingIn 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 superchargingWith 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 engineExhaust gas turbochargingIn 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-cylinderThe Turbine sectionThe turbine stage comprises of two components; the turbine 'wheel' and the collector, commonly referred to as a 'housing'. The turbine wheel can be of radial mixed or axial design. Generally, in turbochargers used on high speed engines the turbines are of radial design. On larger engines such as ship propulsion axial turbines are used.The exhaust gas is guided into the turbine wheel by the housing. The energy in the exhaust gas turns the turbine. Significant amounts of power can be generated in the region of 50kW on a typical 12 litre diesel engine.Once the gas has passed through the blades of the wheel it leaves the turbine housing via the exhaust outlet area.The speed of the engine determines how fast the turbine wheel spins. If the engine is in idle mode the wheel will be spinning but at a minimal speed. As you put your foot on the accelerator the wheel starts spinning faster. As more gas passes through the turbine housing, the faster the turbine wheel rotates.Compressor sectionCompressors are the opposite of turbines. Again the compressor stage comprises of two sections, the impeller or 'wheel' and the 'housing'. The compressor wheel is connected to the turbine by a forged steel shaft. As the compressor wheel spins air enters through an area known as the inducer and is compressed through the blades leaving the exducer at a high velocity. The housing is designed to convert the high velocity, low pressure air stream into a high pressure, low velocity air stream through a process called diffusion.Air enters the compressor at a temperature equivalent to atmosphere, however it leaves the compressor cover at a temperature up to 200 degrees celsius.Because the density of the air decreases as it is heated up, even more air can be forced into the engine if the air is cooled after the compressor. This is called intercooling or aftercooling and is achieved either by cooling the charge air with water or air.The oil supplyThe turbocharger bearing system is lubricated by oil from the engine. The oil is fed under pressure into the bearing housing, through to the journal bearings and thrust system. The oil also acts as a coolant taking away heat generated by the turbine.The Journal Bearings are a free-floating rotational type. To perform correctly, the journal bearings should float between a film of oil (i.e. between bearing & shaft, and bearing & bearing housing.) The bearing clearances are very small, less than the width as a human hair.Dirty oil, or blockages in the oil supply holes, can cause serious damage to the turbocharger.A rubber former is made to replicate the impeller around which a casting mould is created. The rubber former can then be extracted from the mould into which the metal is poured. Accurate blade sections and profiles are important in achieving compressor performance. Back face profile machining optimises impeller stress conditions. Boring to tight tolerance and burnishing assist balancing and fatigue resistance. The impeller is located on the shaft assembly using a threaded nut.Bearing HousingA grey cast iron bearing housing provides locations for a fully floating bearing system for the shaft, turbine and compressor which can rotate at speeds up to 170,000 rev/min. Shell moulding is used to provide positional accuracy of critical features of the housing such as the shaft bearing and seal locations. CNC machinery mills, turns, drills and taps housing faces and connections. The bore is finish honed to meet stringent roundness, straightness and surface finish specifications.Bearing SystemsThe bearing system has to withstand high temperatures, hot shut down, soot loading in the oil, contaminants, oil additives, dry starts.Journal bearings are manufactured from specially developed bronze or brass bearing alloys. The manufacturing process is designed to create geometric tolerances and surface finishes to suit very high speed operation.Hardened steel thrust collars and oil slingers are manufactured to strict tolerances using lapping. End thrust is absorbed in a bronze hydrodynamic thrust bearing located at the compressor end of the shaft assembly. Careful sizing provides adequate load bearing capacity without excessive losses.Experimental turbocharging plant opened in Sulzer Bros Ltd, Winterthur, Switzerland in 19111920's1925 marked the first successful application on 2 German ships fitted with 2,000 hp turbocharged diesel engines. This success led to Buchi licensing many manufacturers in Europe, USA, Japan1930'sTurbochargers with axial turbines used in marine, railcar and large stationary applications.1940'sThe advent of the aircraft gas turbine led to major advances in materials technology and design. This had the following implications for turbocharging : ? development of improved heat resisting materials? development of precision casting techniques for high temp materials? this allowed the development of radial turbines and led to the use of radial flow turbos on small automotive diesel engines1950'sMajor engine producers such as Cummins, Volvo and Scania start experimenting with turbocharged engines for trucks using turbochargers supplied by Elliot and Ebersp鋍her. These early designs were unsuccessful due to the large size of the turbocharger. German engineer, Kurt Beirer produces an innovative compact design that is taken up by Schwitzer Corporation, Indianapolis. In 1954 Cummins offer a range of turbocharged engines, the VT12, six cylinder NT, NRT's and JT's.Also in 1954, Volvo offered their first turbocharged truck diesel, the TD96AS, rated at 185 bhp compared with the 150 bhp naturally aspirated D96AS.Pole position at Indianapolis in 1952 won by a car powered by a turbocharged Cummins engine.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.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.Oil change intervalsOil filter system maintenanceOil pressure controlAir filter system maintenanceTROUBLESHOOTINGBLACK SMOKEBLUE SMOKEBOOST PRESSURE TOO HIGHCOMPRESSOR/TURBINE WHEEL DEFECTIVEHIGH OIL CONSUMPTIONINSUFFICIENT POWER/BOOST PRESSURE TOO LOWOIL LEAKAGE AT COMPRESSOROIL LEAKAGE AT TURBINETURBOCHARGER GENERATES ACOUSTIC NOISEBlack smoke possible cause:Dirty air filter systemSuction and pressure line distorted or leakingExcessive flow resistance in exhaust system/ leakage upstream of turbineFuel system/injection feed system defective or incorrectly adjusted Valve guide, piston rings, engine or cylinder liners worn/increasedblow byDirty compressor or charge air coolerBoost pressure control swing valve/poppet valve does not close Turbocharger bearing damageForeign body damage on compressor or turbineEngine air collector cracked/missing or loose gasketsTurbine housing/flap damagedInsufficient oil supply of turbochargerBlue smoke possible cause:Dirty air filter systemExcessive flow resistance in exhaust system/ leakage upstream of turbineOil feed and drain lines clogged, leaking or distorted Crankcase ventilation clogged and distortedCoke and sludge in turbocharger center housingValve guide, piston rings, engine or cylinder liners worn/increased blow byDirty compressor or charge air coolerPiston ring sealing defectiveTurbocharger bearing damageBoost pressure too high possible cause:Fuel system/injection feed system defective or incorrectly adjusted Boost pressure control swing valve/poppet valve does not open Pipe assy. to swing valve/poppet valve defectiveCompressor/turbine wheel defective possible cause: Turbocharger bearing damageForeign body damage on compressor or turbineTurbine housing/flap damagedInsufficient oil supply of turbochargerHigh oil consumption possible cause:Dirty air filter systemExcessive flow resistance in exhaust system/ leakage upstream of turbineOil feed and drain lines clogged, leaking or distortedCrankcase ventilation clogged and distortedCoke and sludge in turbocharger center housingValve guide, piston rings, engine or cylinder liners worn/increased blow byDirty compressor or charge air coolerPiston ring sealing defectiveTurbocharger bearing damageInsufficient power/boost pressure too low possible cause:Dirty air filter systemSuction and pressure line distorted or leakingExcessive flow resistance in exhaust system/ leakage upstream of turbineFuel system/injection feed system defective or incorrectly adjusted Valve guide, piston rings, engine or cylinder liners worn/increased blow byDirty compressor or charge air coolerBoost pressure control swing valve/poppet valve does not close Pipe assy. to swing valve/poppet valve defectiveTurbocharger bearing damageForeign body damage on compressor or turbineEngine air collector cracked/missing or loose gasketsTurbine housing/flap damagedInsufficient oil supply of turbochargerOil leakage at compressor possible cause:Dirty air filter systemExcessive flow resistance in exhaust system/ leakage upstream of turbineOil feed and drain lines clogged, leaking or distortedCrankcase ventilation clogged and distortedCoke and sludge in turbocharger center housingValve guide, piston rings, engine or cylinder liners worn/increased blow byDirty compressor or charge air coolerPiston ring sealing defectiveTurbocharger bearing damageOil leakage at turbine possible cause:Oil feed and drain lines clogged, leaking or distortedCrankcase ventilation clogged and distortedCoke and sludge in turbocharger center housingValve guide, piston rings, engine or cylinder liners worn/increased blow byPiston ring sealing defectiveTurbocharger bearing damageTurbocharger generates acoustic noise possible cause:Suction and pressure line distorted or leakingExcessive flow resistance in exhaust system/ leakage upstream of turbineDirty compressor or charge air coolerTurbocharger bearing damageForeign body damage on compressor or turbineExhaust gas leakage between turbine outlet and exhaust pipe Engine air collector cracked/missing or loose gasketsTurbine housing/flap damagedInsufficient oil supply of turbocharger。