<ul><li>Prepare for ASE Engine Performance (A8) certification test content area “C” (Air Induction and Exhaust Systems Diagnosis and Repair). </li></ul><ul><li>Discuss the purpose and function of intake manifolds. </li></ul><ul><li>Explain the differences between throttle fuel-injection manifolds and port fuel-injection manifolds. </li></ul>OBJECTIVES: After studying Chapter 22, the reader should be able to: Continued
<ul><li>Describe the operation of the exhaust gas recirculation system in the intake manifold. </li></ul><ul><li>List the materials used in exhaust manifolds and exhaust systems. </li></ul>OBJECTIVES: After studying Chapter 22, the reader should be able to:
AIR INTAKE FILTRATION <ul><li>Gasoline must mix with air to form a combustible mixture. Air movement into an engine occurs due to vacuum being created in the engine. </li></ul>Figure 22–1 Downward movement of the piston lowers the air pressure inside the combustion chamber. The pressure differential between the atmosphere and the inside of the engine forces air into the engine. Like gas, air contains dirt and other materials which cannot be allowed to reach the engine. Just as fuel filters are used to clean impurities from gas, an air cleaner and filter are used to remove contaminants from the air. Continued
<ul><li>The three main jobs of the air cleaner and filter are to: </li></ul><ul><li>Clean the air before it is mixed with fuel </li></ul><ul><li>Silence intake noise </li></ul><ul><li>Act as a flame arrester in case of a backfire </li></ul>The automotive engine uses about 9,000 gallons (34,069 liters) of air for every gallon of gasoline burned at a ratio of 14.7 to 1. Without proper filtering, dust and dirt in the air seriously damage engine parts and shorten engine life. While abrasive particles can cause wear any place inside the engine where two surfaces move against each other, they first attack piston rings and cylinder walls. Contained in blowby gases, they pass by the piston rings and into the crankcase. Continued
<ul><li>From the crankcase, the particles circulate throughout the engine in the oil. Large amounts of abrasive particles in the oil can damage other moving engine parts. </li></ul>Figure 22–2 Dust and dirt in the air are trapped in the air filter so they do not enter the engine. Continued Filter Replacement Manufacturers recommend cleaning or replacing air filter elements at periodic intervals, usually in terms of distance driven or months of service. It is best to replace a filter element before it becomes too dirty to be effective. A dirty air filter passes contaminants that cause engine wear.
<ul><li>Air Filter Elements The paper air filter element is the most common type of filter, made of chemically treated paper stock that contains tiny passages in the fibers. </li></ul>NOTE: Do not attempt to clean a paper element filter by rapping it on a sharp object to dislodge the dirt, or blowing compressed air through the filter. This tends to clog the paper pores and further reduce the airflow capability of the filter. NOTE: A person can only see objects that are 40 microns or larger in size. A human hair is about 50 microns in diameter. These passages form an indirect path for the airflow to follow. The airflow passes through several fiber surfaces, each of which traps microscopic particles of dust, dirt, and carbon. Most filters are capable of trapping dirt and other particles larger than 10 to 25 microns in size. One micron is equal to 0.000039 in.
<ul><li>Remotely Mounted Air Filters and Ducts Air cleaner and duct design depend on a number of factors such as the size, shape, and location of other engine compartment components, as well as the vehicle body structure. </li></ul>Figure 22–3 Most air filter housings are located on the side of the engine compartment and use flexible rubber hose to direct airflow to the throttle body of the engine. Port fuel-injection systems generally use a horizontally mounted throttle body. Some systems also have a mass airflow (MAF) sensor between throttle body and air cleaner. Remote air cleaners are connected by composite ducting. The ducting may be rigid or flexible.
Always inspect the air filter and the air intake system carefully during routine service. Debris or objects deposited by animals can cause a restriction to the airflow and can reduce engine performance. Always Check the Air Filter Figure 22–4 (a) Note the discovery as the air filter housing was opened during service on a Pontiac Bonneville. The nuts were obviously deposited by squirrels (or some other animal). Figure 22–4 (b) Not only was the housing filled with nuts, but also this air filter was extremely dirty, indicating that this vehicle had not been serviced for a long time.
ENGINE AIR TEMPERATURE REQUIREMENTS <ul><li>Some form of thermostatic control has been used on vehicles equipped with a throttle-body fuel injection to control intake air temperature for improved driveability. </li></ul><ul><li>In a throttle-body fuel injection system, fuel and air are combined above the throttle plate and must travel through the intake manifold before reaching the cylinders. Air temperature control is needed under these conditions to keep the gas & air mixture combined. </li></ul>Heat from the exhaust manifold is retained and sent to the air cleaner inlet to provide heated air to the throttle body. While the air control valve generally is located in the air cleaner snorkel, it may be in the air intake housing or ducting of remote air cleaners. Most fuel-injection systems don’t use temperature control.
What is the purpose of the odd-shaped tube attached to the inlet duct between the air filter and the throttle body, as seen here? What Does This Tube Do? - Part 1 Figure 22–5 A resonance tube, called a Helmholtz resonator, is used on the intake duct between the air filter and the throttle body to reduce air intake noise during engine acceleration.
The tube shape is designed to dampen out certain resonant frequencies that can occur at certain engine speeds. The length and shape of this tube are designed to absorb shock waves that are created in the air intake system and to provide a reservoir for the air that will then be released into the airstream during cycles of lower pressure. This resonance tube is often called a Helmholtz resonator, named for the discoverer of the relationship between shape and value of frequency Herman L. F. von Helmholtz (1821–1894) of the University of Hönizsberg in East Prussia. The overall effect of these resonance tubes is to reduce the noise of the air entering the engine. What Does This Tube Do? - Part 2
<ul><li>The intake manifold is also called the inlet manifold . Smooth operation can only occur when each combustion chamber produces the same pressure as every other chamber in the engine. </li></ul>THROTTLE BODY INJECTION INTAKE MANIFOLDS For this to be achieved, each cylinder must receive a charge exactly like the charge going into the other cylinders in quality and quantity. The charges must have the same physical properties and the same air–fuel mixture. A throttle-body fuel injector forces finely divided droplets of liquid fuel into the incoming air to form a combustible air–fuel mixture. Continued
<ul><li>For this to be achieved, each cylinder must receive a charge exactly like the charge going into the other cylinders in quality and quantity. </li></ul>Figure 22–6 A throttle-body injection (TBI) unit used on a GM V-6 engine. The charges must have the same physical properties and the same air–fuel mixture. A throttle-body fuel injector forces finely divided droplets of liquid fuel into the incoming air to form a combustible air–fuel mixture. These droplets start to evaporate as soon as they leave the throttle-body injector nozzles. The droplets stay in the charge as long as the charge flows at high velocities . These velocities may reach 300 feet per second.
<ul><li>Separation of the droplets from the charge as it passes through the manifold occurs when the velocity drops below 50 feet per second. Intake charge velocities at idle speeds are often below this value. When separation occurs—at low engine speeds—extra fuel must be supplied to the charge in order to have a combustible mixture reach the combustion chamber. </li></ul>Manifold sizes represent a compromise. They must have a cross-section large enough to allow charge flow for maximum power. The cross-section must be small enough that the flow velocities of the charge will be high enough to keep the fuel droplets in suspension. Continued This is required so that equal mixtures reach each cylinder. Manifold cross-sectional size is one reason why engines designed especially for racing will not run at low engine speeds. Racing manifolds must be large enough to reach maximum horsepower.
<ul><li>This size, however, allows the charge to move slowly, and the fuel will separate from the charge at low engine speeds. Fuel separation leads to poor accelerator response. Standard passenger vehicle engines are primarily designed for economy during light-load, partial-throttle operation. </li></ul>Figure 22–7 Heavy fuel droplets separate as they flow around an abrupt bend in an intake manifold. Their manifolds, therefore, have a much smaller cross-sectional area than do those of racing engines. This small size will help keep flow velocities of the charge high throughout the normal operating speed range of the engine. Continued
Because many V-type engines equipped with a throttle-body injection and/or EGR valve use a crossover exhaust passage, a leak around this passage will create an exhaust leak and noise. Always check for evidence of an exhaust leak around the intake manifold whenever diagnosing an exhaust sound. Check the Intake If an Exhaust Noise
PORT FUEL-INJECTION INTAKE MANIFOLD <ul><li>The size and shape of port fuel-injected engine intake manifolds can be optimized because the only thing in the manifold is air. The fuel injection is located in the intake manifold about 3 to 4 inches (70 to 100 mm) from the intake valve. </li></ul>The runner length and shape are designed for tuning only. There is no need to keep an air–fuel mixture homogenized through its trip from TBI unit to intake valve. Long runners build low-rpm torque while shorter runners provide maximum high-rpm power. The runner length and shape are designed for tuning only. There is no need to keep an air–fuel mixture homogenized through its trip from the TBI unit to the intake valve. Typically, long runners build low-rpm torque while shorter runners provide maximum high-rpm power. Continued
Figure 22–8 The graph shows the effect of sonic tuning of the intake manifold runners. The longer runners increase the torque peak and move it to a lower rpm. The 600-mm-long intake runner is about 24 inches long. Continued
Figure 22–9 Airflow through the large diameter upper intake manifold is distributed to smaller diameter individual runners in the lower manifold in this two-piece manifold design.
VARIABLE INTAKES <ul><li>Many intake manifolds are designed to provide both short runners, best for higher engine speed power, and longer runners, best for lower engine speed torque. </li></ul>Figure 22–10 The air flowing into the engine can be directed through long or short runners for best performance and fuel economy. The valve(s) that control the flow of air through the passages of the intake manifold are computer controlled.
PLASTIC INTAKE MANIFOLDS <ul><li>Most thermoplastic intake manifolds are molded from fiberglass reinforced nylon. They can be cast or injection molded. Plastic manifolds are lighter than aluminum and can better insulate engine heat from fuel injectors. </li></ul>Figure 22–11 Many plastic intake manifolds are constructed using many parts glued together to form complex passages for airflow into the engine. Plastic manifolds have smoother interior surfaces than other types, resulting in greater airflow.
EXHAUST GAS RECIRCULATION PASSAGES <ul><li>To reduce the emission of oxides of nitrogen (NOx), engines have been equipped with exhaust gas recirculation ( EGR ) valves. From 1973 until recently, they were used on almost all vehicles. </li></ul>Some engines use intake and exhaust valve overlap as a means of trapping some exhaust in the cylinder as an alternative to using an EGR valve. The EGR valve opens at speeds above idle on a warm engine. When open, the valve allows a small portion of the exhaust gas (5% to 10%) to enter the intake manifold. Here, the exhaust gas mixes with and takes the place of some of the intake charge. Recirculated exhaust gas is inert and does not enter the combustion process. The result is a lower peak combustion temperature and lowered production of oxides of nitrogen. Continued
Figure 22–12 The exhaust gas recirculation system is more efficient at controlling Nox emissions if the exhaust gases are cooled. A long metal tube between the exhaust manifold and the intake manifold allows the exhaust gases to cool before entering the engine. <ul><li>The EGR system has means of interconnecting exhaust and intake manifolds. The passage is controlled by the EGR valve. On V-type engines, the intake manifold crossover is used as a source of exhaust gas for the EGR system. A cast passage connects the exhaust crossover to the EGR valve. </li></ul>On inline-type engines, an external tube is used to carry exhaust gas to the EGR valve.
UPPER AND LOWER INTAKE MANIFOLDS <ul><li>Many intake manifolds are constructed in two parts: </li></ul><ul><li>A lower section, usually called the plenum , attaches to the cylinder heads and includes passages from the intake ports. </li></ul><ul><li>An upper manifold connects to the lower unit and includes the long passages needed to help provide the ram effect that helps the engine deliver maximum torque at low engine speeds. The throttle body attaches to the upper intake. </li></ul>Use of a two-part intake manifold allows for easier manufacturing as well as assembly, but can create additional locations for leaks. If the lower intake manifold gasket leaks, not only could a vacuum leak occur affecting the operation of the engine, but a coolant leak or an oil leak can also occur. A leak at the gasket(s) of the upper intake manifold usually results in a vacuum (air) leak only.
EXHAUST MANIFOLD DESIGN <ul><li>The exhaust manifold is designed to collect high-temperature spent gases from the head exhaust ports. </li></ul>Figure 22–13 Exhaust gases are pushed out of the cylinder by the piston on the exhaust stroke. The hot gases are sent to an exhaust pipe, then a catalytic converter, the muffler, to a resonator, and to the tailpipe, where they are vented to the atmosphere. This must be done with the least-possible amount of restriction or back pressure while keeping the exhaust noise at a minimum.
<ul><li>Exhaust gas temperature will vary according to power produced by the engine. The manifold must operate at both engine idle and continuous full power. Under full-power conditions, the exhaust manifold will become red-hot, causing a great deal of expansion. At idle, the exhaust manifold is just warm, causing little expansion. </li></ul>After casting, the manifold may be annealed. Annealing is a heat-treating process that takes out the brittle hardening of the casting to reduce the chance of cracking from the temperature changes. During vehicle operation, manifold temperatures usually reach the high-temperature extremes. Most exhaust manifolds are made from cast iron to withstand extreme and rapid temperature changes. The manifold is bolted to the head in a way that will allow expansion and contraction. NOTE: T emperature of an exhaust manifold can exceed 1500°F (815°C). Continued
<ul><li>The exhaust manifold is designed to allow the free flow of exhaust gas. Some manifolds use internal cast-rib deflectors or dividers to guide the exhaust gases toward the outlet as smoothly as possible </li></ul>Figure 22–14 This exhaust manifold has a heat shield to help retain the heat and reduce exhaust emissions. Some are designed to go above the spark plug, others below. The spark plug and ignition wires are usually shielded. Many exhaust manifolds have heat shields as seen here. Exhaust systems are especially designed for the engine-chassis combination.
<ul><li>The exhaust system length, pipe size, and silencer are designed, where possible, to make use of the tuning effect of the gas column resonating within the exhaust system. Tuning occurs when the exhaust pulses from the cylinders are emptied into the manifold between the pulses of other cylinders. </li></ul>Figure 22–15 Many exhaust manifolds are constructed of pressed steel and are free flowing to improve engine performance.
Figure 22–16 A crack in an exhaust manifold is often not this visible. A crack in the exhaust manifold upstream of the oxygen sensor can fool the sensor and affect engine operation.
A crack in an exhaust manifold will not only allow exhaust gases to escape and cause noise but the crack can also allow air to enter the exhaust manifold. Exhaust flows from the cylinders as individual puffs or pressure pulses. Behind each of these pressure pulses, a low pressure (below atmospheric pressure) is created. Outside air at atmospheric pressure is then drawn into the exhaust manifold through the crack. This outside air contains 21% oxygen and is measured by the oxygen sensor (O 2 S). The air passing the O 2 S signals the engine computer that the engine is operating too lean (excess oxygen) and the computer, not knowing that the lean indicator is false, adds additional fuel to the engine. The result is that the engine will be operating richer (more fuel than normal) and spark plugs could become fouled, causing poor operation. How Can a Cracked Exhaust Manifold Affect Engine Performance?
EXHAUST MANIFOLD GASKETS <ul><li>Exhaust heat will expand the manifold more than it will expand the head. It causes the exhaust manifold to slide on the sealing surface of the head. The heat also causes thermal stress. </li></ul>Continued When the manifold is removed from the engine for service, the stress is relieved and this may cause the manifold to warp slightly. Exhaust manifold gaskets are included in gasket sets to seal slightly warped exhaust manifolds. These gaskets should be used, even if the engine did not originally use exhaust manifold gaskets. When a perforated core manifold gasket has facing on one side only, put the facing side against the head and put the manifold against the perforated metal core. The manifold can slide on the metal of the gasket as it slid on the sealing surface of the head.
<ul><li>Gaskets are used on new engines with tubing- or header-type exhaust manifolds, and often include heat shields to keep exhaust heat from the spark plugs and cables. </li></ul>Figure 22–17 Typical exhaust manifold gaskets. Note how they are laminated to allow the exhaust manifold to expand and contract due to heating and cooling. They may have several layers of steel for high-temperature sealing, spot-welded together. Some are embossed for special sealing. Many new engines do not use gaskets with cast exhaust manifolds. The flat surface of the new cast-iron exhaust manifold fits tightly against the flat surface of the new head.
When cast-iron exhaust manifolds are removed, the stresses built up in the manifolds often cause the manifolds to twist or bend. This distortion even occurs when the exhaust manifolds have been allowed to cool before removal. Attempting to reinstall distorted exhaust manifolds is often a time-consuming and frustrating exercise. However, special spreading jacks can be used to force the manifold back into position so that the fasteners can be lined up with the cylinder head. The Correct Tools Save Time Figure 22–18 An exhaust manifold spreader tool is a tool that is absolutely necessary to use when reinstalling exhaust manifolds. When they are removed from the engine, they tend to warp slightly even though the engine is allowed to cool before being removed. The spreader tool allows the tech to line up the bolt holes without doing any harm to the manifold.
MUFFLERS <ul><li>When the exhaust valve opens, it rapidly releases high-pressure gas sending a strong air pressure wave through the atmosphere, which produces a sound we call an explosion. It is the same sound produced when the high-pressure gases from burned gunpowder are released from a gun. </li></ul>Continued In an engine, the pulses are released one after another. The explosions come so fast that they blend together in a steady roar. Sound is air vibration. When vibrations are large, the sound is loud. The muffler catches the large bursts of high-pressure exhaust gas from the cylinder, smoothing out the pressure pulses and allowing them to be released at an even and constant rate. It does this through the use of perforated tubes within the muffler chamber. The gases are released to the tailpipe, silencing engine exhaust noise.
<ul><li>Sometimes resonators are used in the exhaust system and the catalytic converter also acts as a muffler. They provide additional expansion space at critical points to smooth out exhaust gas flow. </li></ul>Figure 22–19 Exhaust gases expand and cool as they travel through the passages in the muffler. Most mufflers have a larger inlet diameter than outlet diameter. As the exhaust enters the muffler, it expands and cools. The cooler exhaust is more dense and occupies less volume. The diameter of the outlet of the muffler and the diameter of the tailpipe can be reduced with no decrease in efficiency. The muffler and tailpipe are supported with brackets called hangers , made of rubberized fabric with metal ends that hold the muffler and tailpipe in position so that they do not touch any metal part.
Many mufflers are equipped with a small hole in the lower rear part to drain accumulated water. About 1 gallon of water is produced in the form of steam for each gallon of gasoline burned. The water vapor often condenses on the cooler surfaces of the exhaust system unless the vehicle has been driven long enough to fully warm the muffler above the boiling point of water (212°F [100°C]). Why is There a Hole in My Muffler? Figure 22–20 A hole in the muffler allows condensed water to escape.
<ul><li>One of the most popular high-performance modifications is to replace the factory exhaust system with a low-restriction design and to replace the original air filter and air filter housing with a low-restriction unit. </li></ul>Figure 22–21 A high-performance aftermarket air filter often can increase airflow into the engine for more power. More Airflow = More Power The installation of an aftermarket air filter not only increases power, but also increases air induction noise, which many drivers prefer. The filter housing, however, may not be able to effectively prevent water from being drawn into the engine if the vehicle is traveling through deep water. Almost every modification that increases performance has a negative effect on some other part of the vehicle, or else the manufacturer would include the change at the factory.
SUMMARY <ul><li>All air entering an engine must be filtered. </li></ul><ul><li>Engines that use throttle-body injection units are equipped with intake manifolds that keep the airflow speed through the manifold at 50 to 300 feet per second. </li></ul><ul><li>Most intake manifolds have an EGR valve that regulates the amount of recirculated exhaust that enters the engine to reduce NOx emissions. </li></ul><ul><li>Exhaust manifolds can be made from cast iron or stainless steel. </li></ul>Continued
SUMMARY <ul><li>The exhaust system also contains a catalytic converter, exhaust pipes, and muffler. The entire exhaust system is supported by rubber hangers that isolate the noise and vibration of the exhaust from the rest of the vehicle. </li></ul>( cont. )