Halderman ch025 lecture

© 2011 Pearson Education, Inc.All Rights Reserved

Automotive Technology, Fourth EditionJames Halderman

TURBOCHARGING AND SUPERCHARGING

25

25 TURBOCHARGING AND SUPERCHARGING



ObjectivesObjectives• The student 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.




ObjectivesObjectives• The student should be able to:

– Describe how the boost levels are controlled.

– Discuss maintenance procedures for turbochargers and superchargers.




INTRODUCTIONINTRODUCTION




IntroductionIntroduction• Airflow Requirements

– Naturally aspirated engines use atmospheric pressure to push air-fuel mixture into combustion chamber

– Mixture is compressed to increase force of burning, expanding gases





– The greater the compression, the greater the engine power

– Four-stroke engines can only take in so much air





– Engine airflow requirements determined by• Engine displacement• Engine revolutions per minute (RPM)• Volumetric efficiency




IntroductionIntroduction• Volumetric Efficiency

– Measurement compares actual volume of air-gas mixture with theoretical maximum volume





– Volumetric efficiency decreases as engine speed increases

– Average engine never reaches 100% volumetric efficiency

• New engine is about 85% efficient





– Race engine is about 95% efficient– Turbochargers or supercharges allow

engines to exceed 100%




Figure 25-1 A supercharger on a Ford V-8.




Figure 25-2 A turbocharger on a Toyota engine.




FORCED INDUCTIONFORCED INDUCTIONPRINCIPLESPRINCIPLES




Forced Induction PrinciplesForced Induction Principles• Purpose and Function

– Force an air-fuel charge produces when ignited is a function of charge density

– Charge density is amount of air-fuel charge introduced into cylinders




Figure 25-3 The more air and fuel that can be packed in a cylinder, the greater the density of the air-fuel charge.





– The greater the density of an air-fuel charge the greater the force produced





– Engine using atmospheric pressure for intake charge is naturally aspirated





– A better way to increase air density is with air pump such as turbocharger or supercharger





– Pumping air into cylinder provides combustion chamber with increased air pressure known as boost





– Boost can be measured several ways• Pounds per square inch (PSI)





– Boost can be measured several ways• Atmospheres (ATM) (1 atmosphere is 14.7

PSI)





– Boost can be measured several ways• Bars (1 bar is 14.7 PSI)





– Boost increases air density thereby increasing friction that heats air

– Increased temperature decreases air density





– Increased pressure doesn’t always produce greater air density




Forced Induction PrinciplesForced Induction Principles• Forced Induction Principles

– Forced induction systems use air pump to pack denser air-fuel charge into cylinders

• The weight of the air-fuel charge is higher





– Forced induction systems use air pump to pack denser air-fuel charge into cylinders

• Power is increased because it is related to weight of air-fuel charge consumed within given time period





– Pumping air into intake system under pressure allows more air to enter intake port before valve closes





– Increased airflow allows more fuel to be added with same air-fuel ratio

– Denser air-fuel charge allows greater potential energy from combustion





– Advantages of Supercharging• Increases air-fuel charge density for high-

compression pressure when power is needed





– Advantages of Supercharging• Engine runs on lower pressures when

additional power is not needed





– Advantages of Supercharging• Pumped air pushes remaining exhaust from

combustion chamber





– Advantages of Supercharging• Forced airflow and removal of exhaust gases

lower temperature of cylinder head pistons and extends life of engine





– Supercharger or turbocharger pressurization can be measured like atmospheric pressure





– Atmospheric pressure drops with increased altitude

– Boost pressure remains the same regardless of altitude




Figure 25-4 Atmospheric pressure decreases with increases in altitude.




Forced Induction PrinciplesForced Induction Principles• Boost and Compression Ratios

– Boost increases amount of air drawn into cylinder during intake stroke

– The higher the boost pressure, the greater the compression ratio





– Higher compression ratio requires superchargers and turbochargers to use the following:

• Forged pistons






• Stronger than normal connecting rods






• Piston oil squirters to control temperatures






• Lower compression ratio compared to naturally aspirated engines




Chart 25-1 The effective compression ratio compared to the boost pressure.




SUPERCHARGERSSUPERCHARGERS




SuperchargersSuperchargers• Introduction

– Supercharger is engine-driven air pump– Boosts engine torque and power





– Provides instantaneous power increase– Requires horsepower to operate





– Is not as efficient as a turbocharger– Supercharger pumps air in direct relation to

engine speed




SuperchargersSuperchargers• Types of Superchargers

– Roots Type• Named for Philander and Francis Roots who

patented supercharger in 1860





– Roots Type• Patented as water pump for mines





– Roots Type• Changed to pump air





– Roots Type• Used on two-stroke-cycle Detroit diesel

engines and other supercharged engines





– Roots Type• Called positive displacement design: all air

that enters Roots supercharger is forced through the unit




Figure 25-5 A roots-type supercharger uses two lobes to force the air around the outside of the housing and into the intake manifold.





– Centrifugal Supercharger• Similar to turbocharger but mechanically

driven by engine instead of being powered by hot exhaust gases





– Centrifugal Supercharger• Not a positive displacement pump





– Centrifugal Supercharger• Air enters centrifugal supercharger housing

in the center and exits at the edges at a higher rate of speed





– Centrifugal Supercharger• Speed of blades is higher than engine speed




SuperchargersSuperchargers• Supercharger Boost Control

– Many factory installed superchargers have a bypass valve that allows air to bypass the supercharger




Figure 25-6 The bypass actuator opens the bypass valve to control boost pressure.




Superchargers Superchargers • Supercharger Boost Control

– Airflow is directed around supercharger under these conditions

• Boost pressure indicates intake manifold pressure is reaching boost levels






• Excessive pressure builds up during deceleration






• Reverse gear is selected




SuperchargersSuperchargers• Supercharger Service

– Usually lubricated with synthetic engine oil– Oil level should be checked and oil changed

as specified by manufacturer





– Drive belt should be inspected and replaced as necessary

– Air filter should be replaced regularly





– Service information should be used to service separate cooling system on many superchargers




Figure 25-7 A Ford supercharger cutaway display showing the roots-type blower and air charge cooler (intercooler). The air charge cooler is used to reduce the temperature of the compressed air before it enters the engine to increase the air charge density.




TURBOCHARGERSTURBOCHARGERS




TurbochargersTurbochargers• Introduction

– Major disadvantage of supercharger: it takes engine power to drive it

– Mechanical supercharger can take up to 20% of engine power





– Turbocharger uses heat from exhaust to power turbine wheel

– About half of heat energy in fuel goes out exhaust system





– Another 25% is lost through radiator cooling

– Only 25% is converted to mechanical power





– A mechanically driven pump uses some of the mechanical power

– Turbocharger gets energy from exhaust gases




Figure 25-8 A turbocharger uses some of the heat energy that would normally be wasted.




TurbochargersTurbochargers• Operation

– Turbocharger turbine looks like centrifugal pump for supercharging

– Hot exhaust gases flow from combustion chamber to turbine wheel





– Gases expand as they leave engine– The expansion of hot gases turn turbine

wheel’s blades





– Turbocharger has two chambers connected by center housing

– The two chambers contain turbine wheel and impeller (compressor) wheel connected by a shaft




Figure 25-9 A turbine wheel is turned by the expanding exhaust gases.





– Turbocharger must be positioned as close as possible to exhaust manifold

– Hot air passes into turbocharger with minimal heat loss





– The exhaust gas rotates the turbine blades– Turbine wheel and compressor wheel on

same shaft and turn at same speed





– Compressor wheel draws air in through central inlet

– Centrifugal force pumps it through outlet at edge of housing





– Pair of bearings in center housing supports turbine and compressor wheel shaft and is lubricated by engine oil




Figure 25-10 The exhaust drives the turbine wheel on the left which is connected to the impeller wheel on the right through a shaft. The bushings that support the shaft are lubricated with engine oil under pressure.





– Turbine and compressor wheels operate with extremely close clearances to minimize leakage





– Leakage around turbine blades causes dissipation of heat energy

– Leakage around compressor blades reduces full boost pressure




TurbochargersTurbochargers• Turbocharger Operation

– When engine is started, exhaust heat and pressure are low

– Turbocharger runs at low speed (about 1000 RPM)





– Engine works like naturally aspirated engine

– As engine speed and load increase, exhaust heat and flow increase





– Turbine and compressor wheels accelerate as heat energy increases

– At full engine power, turbocharger rotates between 100,000 and 150,000 RPM




Figure 25-11 Engine oil is fed to the center of the turbocharger to lubricate the bushings and returns to the oil pan through a return line.





– Engine deceleration from full power to idle takes a second or two because of friction, pumping resistance, and drivetrain load





– Turbocharger has no load– Turbocharger takes a minute or more to

slow to idle





– If engine is turned off, lubrication flow to turbocharger stops

– Oil in center housing reaches extreme heat and can coke or oxidize





– Coked oil can clog passages and reduce turbocharger life

– High rotating speed and close clearance of turbine and compressor require critical bearing clearances





– Bearings must keep radial clearances of 0.003 to 0.006 in. (0.08 to 0.15mm)

– Axial clearance must be maintained at 0.001 to 0.003 in. (0.025 to 0.08 mm)





– To avoid problems with turbocharger • Turbochargers need constant lubrication

with clean oil; frequent oil changes are called for





– To avoid problems with turbocharger • Dirt and other contaminants must be kept

out of intake and exhaust housing





– To avoid problems with turbocharger • When basic engine bearing has been

damaged, turbocharger must be flushed with engine oil





– To avoid problems with turbocharger • If turbocharger is damaged, engine oil must

be drained and flushed and oil filter replaced





– Late-model turbochargers have liquid-cooled center bearings

– Engine coolant is circulated through center housing




TurbochargersTurbochargers• Turbocharger Size and Response Time

– Time lag exists between increase in engine speed and increase in turbocharger speed

– The delay is called turbo lag





– Unlike supercharger, turbocharger cannot supply adequate boost at low speed

– Response time related to size of turbine and compressor wheels





– Small wheels accelerate rapidly; large wheels slowly

– Small wheels may not have enough airflow capacity for engine





– Intake and exhaust breathing of engine must be matched to capabilities of turbocharger




BOOST CONTROLBOOST CONTROL




Boost ControlBoost Control• Purpose and Function

– Supercharged and turbocharged systems provide pressure greater than atmospheric pressure

– Increased pressure forces additional air into combustion chamber





– Increased charge increases engine power– Amount of boost is measured in pound per

square inch (PSI), in inches of mercury (in. Hg), in bars, or in atmospheres





– 1 atmosphere = 14.7 PSI– 1 atmosphere = 29.50 in. Hg





– 1 atmosphere = 1 bar– 1 bar =14.7 PSI




Boost ControlBoost Control• Boost Control Factors

– Factors to consider when increasing boost pressure

• As boost pressure increases, air temperature increases

• As air temperature increases, combustion temperature increases






• Power can be increased by cooling compressed air after leaving turbocharger






• Typical cooling device is intercooler






• Intercooler is similar to radiator: outside air passes through cooling pressurized heated air






• Intercooler located between turbocharger and intake manifold






• Some intercoolers use engine coolant




Figure 25-12 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.






• Increased combustion temperature and pressure must be limited to avoid engine damage






• Maximum exhaust gas temperature is • 1,440°F (840°C)






• Higher temperatures can damage turbocharger and engine




Boost ControlBoost Control• Wastegate

– Turbochargers use exhaust gas to increase boost

– The increased boost increases exhaust gas which again increases boost





– To prevent overboost and engine damage most turbocharger systems have a wastegate

– A wastegate is a bypass valve at exhaust inlet to turbine





– A wastegate can route part of exhaust past the turbine to exhaust system

– With less exhaust, turbocharger slows





– Wastegate is continuous process to control boost pressure

– Wastegate is a pressure control valve usually controlled by computer




Figure 25-13 A wastegate is used on many turbocharged engines to control maximum boost pressure. The wastegate is controlled by a computer-controlled valve.




Boost ControlBoost Control• Relief Valves

– A relief valve controls intake side of turbocharger

– Relief valve vents pressurized air from connecting pipe between turbocharger outlet and throttle





– Relief valve functions when throttle is closed during boost

– If pressure is not released, turbocharger will lag when throttle is opened again





– Two basic types of relief valves• Compressor by pass valve (CBV) is quieter

and routes pressurized air to inlet for reuse





• Blow-off valve (BOV) --also called dump valve or vent valve--uses adjustable spring to close valve until release of throttle




Figure 25-14 A blow-off valve is used in some turbocharged systems to relieve boost pressure during deceleration.




Figure 25-15 A dual turbocharger system installed on a small block Chevrolet V-8 engine.




TURBOCHARGER TURBOCHARGER FAILURESFAILURES




Turbocharger FailuresTurbocharger Failures• Symptoms of Failure

– Turbocharger failure results in drop in power

– To restore operation, turbocharger must be rebuilt, repaired, or replaced





– Turbochargers cannot be removed from vehicle

– Bearing failure common cause of turbocharger failure





– Another problem is excessive and continuous oil consumption

– Turbochargers use small rings to prevent exhaust from entering central bearing





– Excessive oil consumption is usually caused by one of the following

• Plugged positive crankcase ventilation (PCV) system






• Clogged air filter, causing low-pressure area in inlet and drawing oil past turbo shaft rings






• Clogged oil return (drain) line from turbocharger to oil pan




Turbocharger FailuresTurbocharger Failures• Preventing Turbocharger Failures

– Regular oil changes (synthetic oil is best)– Regular air filter replacement intervals– Recommended inspections and services




NITROUS OXIDENITROUS OXIDE




Nitrous OxideNitrous Oxide• Introduction

– Nitrous oxide is for racing or high performance only

– Relatively inexpensive way to get additional power




Nitrous OxideNitrous Oxide• Introduction

– Serious engine damage can occur if used incorrectly or excessively




Nitrous OxideNitrous Oxide• Principles

– Nitrous oxide (N2O) is colorless, nonflammable gas

– Discovered by British chemist Joseph Priestly




Nitrous OxideNitrous Oxide• Principles

– Causes light-headedness if breathed (known as laughing gas)

– Once used in dentistry to reduce pain – Nitrous oxide is a manufactured gas




Nitrous OxideNitrous Oxide• Engine Power Adder

– Power adder is device or system added to engine, such as supercharger, turbocharger, or nitrous oxide to increase power





– Nitrous oxide injected into engine along with extra gasoline

– N2O adds extra oxygen for extra fuel





– NOTE: Nitrous oxide was used as power adder in World War II on some fighter aircraft. Having several hundred more horsepower for a short time saved many lives.




Nitrous OxideNitrous Oxide• Pressure and Temperature

– Requires about 11 lb of pressure per degree Fahrenheit to condense nitrous oxide gas into liquid nitrous oxide





– To change N2O from liquid to gas, all that is needed is to lower its pressure below the pressure it takes to cause it to become a liquid.




Chart 25-2 Temperature/pressure relation for nitrous oxide: The higher the temperature, the higher the pressure.





– Temperature affects pressure of N2O– N2O is stored in pressurized container





– Installed at angle so pickup tube is in the liquid

– Front or discharge end of bottle should be toward front of vehicle




Figure 25-16 Nitrous bottles have to be mounted at an angle to ensure that the pickup tube is in the liquid N2O.




Nitrous OxideNitrous Oxide• Wet and Dry System

– Two types of N2O systems• Wet system involves additional fuel being

injected





– Two types of N2O systems• Identified by two nozzles; red supplying

gasoline and blue supplying N2O





– Two types of N2O systems• Dry system does not include gasoline





– Two types of N2O systems• PCM can be commanded to provide more

fuel when N2O is sprayed




Nitrous OxideNitrous Oxide• Engine Changes Needed for N2O

– If N2O is used to increase horsepower more than 50 hp, engine must be designed and built for greater heat and pressure





– The following items should be considered for adding turbocharger, supercharger, or nitrous oxide

• Forged pistons to withstand increased pressure and temperature






• Cylinder-to-wall clearance increase to compensate for greater heat






• Forged crankshaft and connecting rods





– Check instructions from N2O supplier for details and other changes





– CAUTION: The use of nitrous oxide injection system can cause catastrophic engine damage. Always follow instructions that come with the kit and be sure all internal engine parts meet standard specified to help avoid severe engine damage.




Nitrous OxideNitrous Oxide• System Installation and Calibration

– Nitrous oxide systems usually purchased as kit

– Kit includes one or more sizes of nozzles calibrated to control flow of N2O





– Sizes of nozzles are calibrated in horsepower gained by their use

• 50 hp• 100 hp• 150 hp





– Installation of N2O kit includes on-off switch and switch on or near throttle

– Switch is activated only when throttle is fully opened (WOT)

Halderman ch025 lecture

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