chap23 pps Automotive technology at University of Cambridge

start Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 OBJECTIVES: After studying Chapter 23, the reader should be able to: • Prepare for ASE Engine Performance (A8) certification test content area “C” (Fuel, Air Induction, and Exhaust Systems Diagnosis and Repair) • Explain the difference between a turbocharger and a supercharger Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 OBJECTIVES: After studying Chapter 23, the reader should be able to: • Describe how the boost levels are controlled • Discuss maintenance procedures for turbochargers and superchargers Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 KEY TERMS: bar • boost • bypass valve compressor bypass valve (CBV) • dump valve intercooler • naturally (normally) aspirated • positive displacement relief valve • roots-type • supercharger turbo lag • turbocharger vent valve • volumetric efficiency wastegate Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 AIRFLOW REQUIREMENTS Naturally aspirated engines with throttle bodies rely on atmospheric pressure to push an air–fuel mixture into the combustion chamber vacuum created by the downstroke of a piston The mixture is compressed before ignition to increase force of the burning, expanding gases The greater the compression, the greater the power resulting from combustion All gasoline automobile engines share certain air–fuel requirements A four-stroke engine can take in only so much air, and the fuel it consumes depends on how much air it takes in Engineers calculate engine airflow requirements using three factors: Engine displacement Engine revolutions per minute (rpm) Volumetric efficiency Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman Continued ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Volumetric Efficiency A comparison of volume of air–fuel mixture drawn into an engine to the theoretical maximum volume that could be drawn in is called volumetric efficiency It changes with engine speed and is expressed as a percentage An engine might have 75% volumetric efficiency at 1000 rpm The same engine might rate 85% at 2000 rpm and 60% at 3000 rpm If the engine takes airflow volume slowly, a cylinder might fill to capacity as it takes a definite time for airflow to pass through the curves of the intake manifold and valve port Volumetric efficiency decreases as engine speed increases At high speed, it may drop to as low as 50% The average street engine never reaches 100% efficiency and is about 75% at maximum speed, or 80% at the torque peak Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 23–1 A supercharger on a Ford V-8 Many vehicles are equipped with a supercharger or a turbocharger to increase power Figure 23–2 A turbocharger on a Toyota engine Turbocharged and supercharged engines easily achieve more than 100% volumetric efficiency Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Engine Compression Higher compression increases thermal efficiency of the engine because it raises compression temperatures, resulting in hotter, more complete combustion Higher compression can cause an increase in NOx emissions and would require the use of high-octane gasoline with effective antiknock additives Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 SUPERCHARGING PRINCIPLES The force an air–fuel charge produces when ignited is largely a function of the charge density Density is the mass of a substance in a given amount of space The greater the density of an air–fuel charge forced into a cylinder, the greater the force and engine power it produces when ignited Figure 23–3 The more air and fuel that can be packed in a cylinder, the greater the density of the air–fuel charge An engine using atmospheric pressure for intake is called naturally (normally) aspirated A better way to increase air density in the cylinder is to use a pump Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 When air is pumped into the cylinder, the combustion chamber receives an increase of air pressure known as boost, measured in pounds per square inch (psi), atmospheres (ATM), or bar While boost pressure increases air density, friction heats air in motion and causes an increase in temperature This increase in temperature works in the opposite direction, decreasing air density Because of these and other variables, an increase in pressure does not always result in greater air density Another way to achieve an increase in mixture compression is called supercharging and uses a pump to pack a denser air–fuel charge into the cylinders Since the density is greater, so is weight— and power is directly related to the weight of an air–fuel charge consumed within a given time period Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 10 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Turbocharger Size and Response Time A time lag between an increase in engine speed and the increase in the speed of the turbocharger is called turbo lag Moving exhaust gas has inertia Inertia is present in turbine and compressor wheels, as well as intake airflow Unlike a supercharger, the turbocharger can’t supply adequate boost amount at low speed Turbocharger response time is directly related to the size of the turbine and compressor wheels Small wheels accelerate rapidly; large wheels accelerate slowly To minimize turbo lag, the intake and exhaust breathing capacities of an engine must be matched the exhaust and intake airflow capabilities of the turbocharger Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 27 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 BOOST CONTROL Supercharged and turbocharged systems provide pressure greater than atmospheric pressure in the intake manifold The increased pressure forces additional amounts air into the combustion chamber The “boost” amount (pressure in the intake manifold) is measured in pounds per square inch (psi), in inches of mercury (in Hg), in bars, or in atmospheres The following values vary due to altitude and weather conditions: atmosphere = 14.7 psi atmosphere = 29.50 in Hg atmosphere = 1.0 bar bar = 14.7 psi Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 28 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 The higher the level of boost (pressure), the greater the horsepower potential Other factors to consider when increasing boost pressure: As boost pressure increases, temperature of the air also increases As the temperature of the air increases, combustion temperatures also increase, which increases the possibility of detonation Power can be increased by cooling compressed air after it leaves the turbocharger As boost pressure increases, combustion temperature and pressures increase, which can severe engine damage The maximum exhaust gas temperature must be 1550°F (840°C) Higher temperatures decrease the durability of the turbocharger and the engine Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 29 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Power can be increased about 1% per 10°F by which air is cooled A typical cooling device, called an intercooler, similar to a radiator, allows outside air to pass through, cooling pressurized heated air Some intercoolers use engine coolant to cool the hot compressed air that flows from the turbocharger to the intake Figure 23–10 The unit on top of this Subaru that looks like a radiator is the intercooler, which cools the air after it has been compressed by the turbocharger Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 30 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Boost is the Result of Restriction The boost pressure of a turbocharger (or supercharger) is commonly measured in pounds per square inch If a cylinder head is restricted because of small valves and ports, the turbocharger will quickly provide boost Boost results when the air being forced into the cylinder heads cannot flow into the cylinders fast enough and “piles up” in the intake manifold, increasing boost pressure If an engine had large valves and ports, the turbocharger could provide a much greater amount of air into the engine at the same boost pressure as an identical engine with smaller valves and ports Therefore, by increasing the size of the valves, a turbocharged or supercharged engine will be capable of producing much greater power Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 31 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Wastegate A turbocharger uses exhaust gases to increase boost, which causes the engine to make more exhaust gases, which in turn increases the boost from the turbocharger To prevent overboost and severe engine damage, most systems use a wastegate, a valve similar to a door that can open and close The wastegate is a bypass valve at the exhaust inlet to the turbine It allows all of the exhaust into the turbine, or it can route part of the exhaust past the turbine to the exhaust system If the valve is closed, all of the exhaust travels to the turbocharger When a predetermined boost pressure develops in the intake, the wastegate valve is opened As the valve opens, most of the exhaust flows directly out the exhaust system, bypassing the turbocharger Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 32 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 23–11 A wastegate is used on the firstgeneration Duramax diesel to control maximum boost pressure When boost pressure drops, the wastegate valve closes to direct exhaust over the turbocharger vanes and allow boost pressure to rise Wastegate operation is a continuous process to control boost pressure It is the pressure control valve of a turbocharger system The wastegate is usually controlled by the onboard computer through a boost control solenoid Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 33 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Relief Valves Controls the intake side of the turbocharger A relief valve vents pressurized air from the connecting pipe between the outlet of the turbocharger and the throttle whenever the throttle is closed during boost, such as during shifts If the pressure is not released, the turbocharger turbine wheel will slow down, creating a lag when the throttle is opened again after a shift has been completed There are two basic types of relief valves: Compressor bypass valve or CBV This type of relief valve routes the pressurized air to the inlet side of the turbocharger for reuse and is quiet during operation Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 34 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Blow-off valve or BOV This is also called a dump valve or vent valve and features an adjustable spring design that keeps the valve closed until a sudden release of the throttle The pressure increase opens the valve, and vents pressurized air directly to the atmosphere This operation of this type of relief valve is noisy, and creates a whooshing sound when the valve opens Figure 23–12 A blow-off valve is used in some turbocharged systems to relieve boost pressure during deceleration Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 35 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 If One is Good, Two are Better A turbocharger uses exhaust from the engine to spin a turbine, connected to an impeller inside a turbocharger This impeller then forces air intopressure the under higher than is achieved without a turbocharger The more air that can be forced into an engine, the greater the power potential A V-type engine has two exhaust manifolds, and so two small turbochargers can be used to force greater quantities of air into an engine Figure 23–13 A dual turbocharger system installed on a small block Chevrolet V-8 engine Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 36 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 TURBOCHARGER FAILURES When turbochargers fail, a drop in power is noticed To restore operation, the turbocharger must be rebuilt, repaired, or replaced It is not possible to simply remove the turbocharger, seal openings, and still maintain decent driveability Bearing failure is a common cause of turbocharger failure, and replacement bearings are usually only available to rebuilders Another common problem is excessive, continuous oil consumption resulting in blue exhaust smoke Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 37 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Turbochargers use small rings similar to piston rings on the shaft to prevent exhaust (combustion gases) from entering the central bearing With no seals, excessive oil consumption is usually due to: A plugged positive crankcase ventilation (PCV) system resulting in excessive crankcase pressures forcing oil into the air inlet This failure is not related to the turbocharger, but the turbocharger is often blamed A clogged air filter, which causes a low-pressure area in the inlet, which can draw oil past the turbo shaft rings and into the intake manifold A clogged oil return (drain) line from the turbocharger to the oil pan (sump), which can cause the engine oil pressure to force oil past the turbocharger’s shaft rings and into the intake and exhaust manifolds Obviously, oil being forced into both the intake and exhaust would create lots of smoke Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 38 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 SUMMARY Volumetric efficiency is a comparison of the actual volume of air fuel mixture drawn into the engine to the theoretical maximum volume that can be drawn into the cylinder A supercharger operates from the engine by a drive belt and, while it does consume some engine power, it forces a greater amount of air into the cylinders for even more power A turbocharger uses the normally wasted heat energy of the exhaust to turn an impeller at high speed The impeller is linked to a turbine wheel on the same shaft and is used to force air into the engine Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 39 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 SUMMARY (cont.) There are two types of superchargers: roots-type and centrifugal A bypass valve is used to control the boost pressure on most factory-installed superchargers An intercooler is used on many turbocharged and some supercharged engines to reduce the temperature of air entering the engine for increased power A wastegate is used on most turbocharger systems to limit and control boost pressures, as well as a relief valve, to keep turbine wheel speed from slowing down during engine deceleration 40 Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 end Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 41 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458