Ch07

OBJECTIVES After studying Chapter 7, 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. Describe how the boost levels are controlled. Discuss maintenance procedures for turbochargers and superchargers.

AIRFLOW REQUIREMENTS Naturally aspirated engines with throttle bodies rely on atmospheric pressure to push an air-fuel mixture into the combustion chamber vacuum (low pressure) created by the down stroke of a piston. The mixture is then compressed before ignition to increase the force of the burning, expanding gases. The greater the mixture compression, the greater the power resulting from combustion.

Engineers calculate engine airflow requirements using these three factors: Engine displacement Engine revolutions per minute (RPM) Volumetric efficiency (Continued)

Volumetric Efficiency Volumetric efficiency is a comparison of the actual volume of air-fuel mixture drawn into an engine to the theoretical maximum volume that could be drawn in. Volumetric efficiency is expressed as a percentage, and changes with engine speed. (Continued)

The average street engine the volumetric efficiency is about 75% at maximum speed, or 80% at the torque peak. turbocharged and supercharged engines easily achieve more than 100% volumetric efficiency. (Continued)

Engine Compression Higher compression increases the thermal efficiency of the engine because it raises compression temperatures, resulting in hotter, more complete combustion. However, a higher compression can cause an increase in NOX emissions and would require the use of high-octane gasoline with effective antiknock additives. (Continued)

Figure 7-1 The greater the amount of air into the cylinders, the more fuel that can be added, thereby increasing the torque output of the engine.

SUPERCHARGING PRINCIPLES The amount of force an air-fuel charge produces when it is ignited is largely a function of the charge density. Density is the mass of a substance in a given amount of space. An engine that uses atmospheric pressure for intake is called a naturally (normally) aspirated engine. (Continued)

When air is pumped into the cylinder, the combustion chamber receives an increase of air pressure known as boost and is 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. (Continued)

This increase in temperature works in the opposite direction, decreasing air density. There are several other advantages of supercharging an engine including: It increases the air-fuel charge density to provide high-compression pressure when power is required, but allows the engine to run on lower pressures when additional power is not required. (Continued)

The pumped air pushes the remaining exhaust from the combustion chamber during intake and exhaust valve overlap. The forced airflow and removal of hot exhaust gases lowers the temperature of the cylinder head, pistons, and valves, and helps extend the life of the engine. (Continued)

A supercharger pressurizes air to greater than atmospheric pressure. The pressurization above atmospheric pressure, or boost, can be measured in the same way as atmospheric pressure. Atmospheric pressure drops as altitude increases, but boost pressure remains the same. (Continued)

If a supercharger develops 12 psi (83 kPa) boost at sea level, it will develop the same amount at a 5000-foot altitude because boost pressure is measured inside the intake manifold. (Continued)

Figure 7-2 The more air and fuel that can be packed in a cylinder, the greater the density of the air-fuel charge.

Figure 7-3 Atmospheric pressure decreases with increases in altitude.

SUPERCHARGERS There are two general types of superchargers: Roots-type. The roots-type supercharger is called a positive-displacement design because all of the air that enters is forced through the unit. (Continued)

Centrifugal supercharger. A centrifugal supercharger is not a positive displacement pump and all of the air that enters is not forced through the unit. Air enters a centrifugal supercharger housing in the center and exits at the outer edges of the compressor wheels at a much higher speed due to centrifugal force. (Continued)

Centrifugal supercharger. The speed of the blades has to be higher than engine speed, so a smaller pulley is used on the supercharger and the crankshaft overdrives the impeller through an internal gear box achieving about seven times the speed of the engine. Examples of centrifugal superchargers include Vortech and Paxton. (Continued)

Supercharger Boost Control Many factory installed superchargers are equipped with a bypass valve that allows intake air to flow directly into the intake manifold, bypassing the supercharger. (Continued)

The airflow is directed around the supercharger whenever any of the following conditions occur: The boost pressure, as measured by the MAP sensor, indicates that the intake manifold pressure is reaching the predetermined boost level During deceleration Whenever reverse gear is selected (Continued)

Figure 7-4 A roots-type supercharger uses two lobes to force the air around the outside of the housing and forces it into the intake manifold.

Figure 7-5 The bypass actuator opens the bypass valve to control boost pressure.

TURBOCHARGERS The major disadvantage of a supercharger is its reliance on engine power to drive the unit. In some installations, as much as 20% of the engine’s power is used by a mechanical supercharger. (Continued)

The combustion heat energy lost in the engine exhaust (as much as 40% to 50%) can then be harnessed to do useful work. This is the concept of a turbocharger. In a naturally aspirated engine, about half of the heat energy contained in the fuel goes out the exhaust system. (Continued)

Another 25% is lost through radiator cooling. Only about 25% is actually converted to mechanical power. A turbocharger turbine looks much like a typical centrifugal pump used for supercharging. (Continued)

As exhaust gas enters the turbocharger, it rotates the turbine blades. The turbine wheel and compressor wheel are on the same shaft so that they turn at the same speed. (Continued)

Rotation of the compressor wheel draws air in through a central inlet and centrifugal force pumps it through an outlet at the edge of the housing. A pair of bearings in the center housing supports the turbine and compressor wheel shaft, and is lubricated by engine oil. (Continued)

As the engine runs faster or load increases, both exhaust heat and flow increases, causing the turbine and compressor wheels to rotate faster. When an engine is running at full power, the typical turbocharger rotates at speeds between 100,000 and 150,000 RPM. (Continued)

If the engine is decelerated to idle and then shut off immediately, engine lubrication stops flowing to the center housing bearings while the turbocharger is still spinning at thousands of RPM. The oil in the center housing is then subjected to extreme heat and can gradually “coke” or oxidize. (Continued)

The coked oil can clog passages and will reduce the life of the turbocharger. The high rotating speeds and extremely close clearances of the turbine and compressor wheels in their housings require equally critical bearing clearances. (Continued)

To prevent problems, four conditions should be considered: The turbocharger bearings must be constantly lubricated with clean engine oil—turbocharged engines should have regular oil changes at half the time or mileage intervals specified for non-turbocharged engines. Dirt particles and other contamination must be kept out of the intake and exhaust housings. (Continued)

Whenever a basic engine bearing (crankshaft or camshaft) has been damaged, the turbocharger must be flushed with clean engine oil after the bearing has been replaced. If the turbocharger is damaged, the engine oil must be drained and flushed and the oil filter replaced as part of the repair procedure. (Continued)

Turbocharger Size and Response Time A time lag occurs between an increase in engine speed and the increase in the speed of the turbocharger. This delay between acceleration and turbo boost is called turbo lag. Unlike a supercharger, the turbocharger cannot supply an adequate amount of boost at low speed. (Continued)

Turbocharger response time is directly related to the size of the turbine and compressor wheels. Small wheels accelerate rapidly; large wheels accelerate slowly. While small wheels would seem to have an advantage over larger ones, they may not have enough airflow capacity for an engine. (Continued)

Figure 7-6 A turbocharger uses some of the heat energy that would normally be wasted.

Figure 7-7 A turbine wheel is turned by the expanding exhaust gases.

Figure 7-8 A cutaway of a typical turbocharger. The exhaust from the engine turns the turbine on the left side over 100,000 revolutions per minute. The turbine is connected by a shaft to a compressor located on the right side of the turbocharger. The compressor blades draw air from the air filter housing and force it into the intake manifold to give the engine extra power.

BOOST CONTROL Both supercharged and turbocharged systems are designed to provide a pressure greater than atmospheric pressure in the intake manifold. This increased pressure forces additional amounts of air into the combustion chamber over what would normally be forced in by atmospheric pressure. (Continued)

This increased charge increases engine power. The amount of "boost" (or pressure in the intake manifold) is measured in pounds per square inch (psi), in inches of mercury (in. Hg), in bar's, or in atmospheres. 1 atmosphere = 14.7 psi 1 atmosphere = 30 in. Hg 1 atmosphere = 1.0 bar 1 bar = 14.7 psi (Continued)

INTERCOOLER The higher the level of boost (pressure), the greater the horsepower potential. However, other factors must be considered when increasing boost pressure: As boost pressure increases, the temperature of the air also increases. (Continued)

As the temperature of the air increases, combustion temperatures also increase, which increases the possibility of detonation. Power can be increased by cooling the compressed air after it leaves the turbocharger. The power can be increased about 1% per 10  F by which the air is cooled. A typical cooling device is called an intercooler and is similar to a radiator, wherein outside air can pass through, cooling the pressurized heated air. (Continued)

As boost pressure increases, combustion temperature and pressures increase, which, if not limited, can do severe engine damage. The maximum exhaust gas temperature must be 1550  F (840  C ). Higher temperatures decrease the durability (Continued)

Figure 7-9 An intercooler on a vehicle equipped with an aftermarket turbocharger shown with the bumper and grill removed.

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 turbocharger systems use a wastegate. A wastegate is a valve similar to a door that can open and close. (Continued)

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. (Continued)

The wastegate is the pressure control valve of a turbocharger system. The wastegate is usually controlled by the onboard computer through a boost control solenoid. (Continued)

Figure 7-10 The boost pressure on most turbocharged engines is controlled by the engine computer by pulsing the boost control solenoid on and off based on signals received by the computer from the manifold absolute pressure (MAP) sensor.

Relief Valves A wastegate controls the exhaust side of the turbocharger. A relief valve controls the intake side. 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. (Continued)

There are two basic types of relief valves including: 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. 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. (Continued)

A V-type engine has two exhaust manifolds and so two small turbochargers can be used to help force greater quantities of air into an engine (Continued)

Figure 7-11 A blow-off valve vents pressure to the atmosphere when the throttle is closed to help keep the turbine blade from slowing when the pressurized air backs up after striking the closed throttle plate.

Figure 7-12 A dual turbocharger system installed on a small block Chevrolet V-8 engine.

TURBOCHARGER FAILURES When turbochargers fail to function correctly, a drop in power is noticed. To restore proper operation, the turbocharger must be rebuilt, repaired, or replaced. (Continued)

Because there are no seals to keep oil in, excessive oil consumption is usually caused by: 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. (Continued)

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. (Continued)

Obviously, oil being forced into both the intake and exhaust would create lots of smoke. (Continued)

Ch07

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