ASE 8 - Engine Performance. Module 1 Engine Mechanical

Transcription

1 Module 1 Engine

2 Acknowledgements General Motors, the IAGMASEP Association Board of Directors, and Raytheon Professional Services, GM's training partner for GM's Service Technical College wish to thank all of the people who contributed to the GM ASEP/BSEP curriculum development project This project would not have been possible without the tireless efforts of many people. We acknowledge: The IAGMASEP Association members for agreeing to tackle this large project to create the curriculum for the GM ASEP/BSEP schools. The IAGMASEP Curriculum team for leading the members to a single vision and implementation. Direct contributors within Raytheon Professional Services for their support of translating a good idea into reality. Specifically, we thank: Chris Mason and Vince Williams, for their leadership, guidance, and support. Media and Graphics department under Mary McClain and in particular, Cheryl Squicciarini, Diana Pajewski, Lesley McCowey, Jeremy Pawelek, & Nancy DeSantis. For their help on the Engine curriculum volume, Subject Matter Experts, John Beggs and Stephen Scrivner, for their wealth of knowledge. Finally, we wish to recognize the individual instructors and staffs of the GM ASEP/BSEP Colleges for their contribution for reformatting existing General Motors training material, adding critical technical content and the sharing of their expertise in the GM product. Separate committees worked on each of the eight curriculum areas. For the work on this volume, we thank the members of the Engine committee: Jamie Decato, New Hampshire Community Technical College Lorenza Dickerson, J. Sargeant Reynolds Community College Marvin Johnson, Brookhaven College Jeff Rehkopf, Florida Community College at Jacksonville David Rodriguez, College of Southern Idaho Paul Tucker, Brookdale Community College Kelly Smith, University of Alaska Ray Winiecki, Oklahoma State University - Okmulgee

3 Contents Module 1 Engine Acknowledgements... 2 Module Objectives:... 4 Introduction... 5 Engine Basics... 5 The Combustion Process... 6 Volumetric Efficiency Air Intake Systems Forced Induction Combustion Chamber Design Exhaust Systems Valvetrain and Valve Timing Camshaft Timing and Balance Shafts Valve Deposits Manifold Vacuum Diagnosis Analysis Cylinder Leakage Testing Running Compression Testing Module 1 Test Exercise 1-1 Vacuum Testing Exercise 1-2 Compression and Cylinder Leakage Testing Cylinder Leakage Running Compression Testing Exercise 1-3 Exhaust Back Pressure... 38

4 Module Objectives: Upon completion of this module, the successful learner will be able to: Explain each of the four cycles of the combustion process. Describe the effects of barometric pressure and combustion chamber design on volumetric efficiency. Identify sources of abnormal combustion NATEF Area VIII tasks: A7: Perform engine absolute manifold pressure tests; determine necessary action. A9: Perform cylinder compression tests; determine necessary action. A10: Perform cylinder leakage test; determine necessary action. A12: Perform exhaust system backpressure test; determine necessary action. 1-4

5 Introduction In today' s automobile we rely heavily on computers and electronics to control the performance of our engines. For this reason, technicians often blame the computer or one of its input/output devices when an engine performance issue arises. This leads to the unnecessary replacement of components, a dis-satisfied customer, and a frustrated technician. It is important to understand that not even the best computer can compensate for a worn camshaft, piston rings, intake manifold gasket leak, burnt valve, or any other base engine mechanical problem. Before we look at the intricate electronics, let's first examine the basics. Engine Basics The engine generates power to move the vehicle by burning fuel. The engine converts the chemical energy of the fuel into mechanical energy (motion). Gasoline is mixed with air and burned in hollow chambers of the engine block, called cylinders. These cylinders are closed at the top by a cylinder head. The cylinder head contains valves that open and close to allow airflow in and out of the cylinders. The opening and closing of the valves must exactly correspond to the up-and-down movement of the piston in the cylinder for optimum performance. The camshaft and other valve train components handle this very important job. A piston inside each cylinder is put into motion by expanding gasses when the air /fuel charge is ignited. This process is called combustion. A connecting rod provides a link between the piston and crankshaft. The movement of the piston/rod assembly exerts force on the crankshaft and causes it to rotate. The vehicle is propelled by the collective movements of the pistons turning the crankshaft and transmitting this torque through the rest of the powertrain. Figure

6 The Combustion Process Most automotive engines are four stroke internal combustion engines. The combustion process consists of four cycles: 1) intake, 2) compression, 3) power, and 4) exhaust. Figure 1-2, Cycles of the Combustion Process The process begins with the Intake stroke, when the air/fuel mixture is drawn in to the cylinder. The piston moving down in the cylinder from top dead center (TDC) toward bottom dead center (BDC) creates a partial vacuum (negative pressure) in the cylinder. Atmospheric pressure pushes the air/fuel charge into the cylinder past the open intake valve. The exhaust valve remains closed during this cycle. The intake valve is closed near the end of the cycle. The second movement of the piston during the four-stroke cycle is the compression stroke. During the compression stroke, both the intake and exhaust valves are closed. The piston moves from the Bottom Dead Center position toward the Top Dead Center position. As the piston moves up, the air fuel mixture is compressed due to the decreasing volume of the cylinder. 1-6

7 Compressing the air/fuel mixture raises its pressure, simultaneously raising its temperature. This allows the fuel to reach a point close to selfignition as the piston moves upwards to top dead center. Compression ratios and fuel volatility are closely matched to produce optimum power. Figure 1-3, Effects of Pressure on Temperature Just before the piston reaches TDC, the spark generated by the ignition system at the spark plug ignites the compressed air/fuel charge. This allows the air/fuel mixture to begin burning. The burning of the air/fuel mixture is called combustion. Combustion is started before the piston reaches Top Dead Center to allow complete combustion of the air/fuel mixture during the power stroke. Combustion creates very high temperatures in the cylinder. The expanding gases produce high pressure, which forces the piston down. The movement of the piston provides useable power from the combustion of the air/fuel mixture. This cycle is called the power stroke. Each power stroke slightly increases the speed of the crankshaft. Since most engines have more than one cylinder, there will be a slight increase in crankshaft speed during each cylinder's power stroke. If a cylinder does not provide power during its power stroke, there will be a noticeable slowing of the crankshaft. The fourth and final cycle is called the exhaust stroke. Since the cylinder is filled with the exhaust gases created by combustion, these gases must be removed from the cylinder before the intake stroke can start again. As the piston nears BDC the exhaust valve is opened to begin the exhaust process. The exhaust will begin to exit the cylinder even though the piston is still moving down because the pressure is less in the exhaust manifold than it is in the cylinder. The upward movement of the piston continues to push the spent air/fuel charge out through the exhaust port past the open exhaust valve. When the piston reaches near the Top Dead Center position, the exhaust valve closes. The engine has completed one full power cycle, and the crankshaft has rotated twice. The four-stroke cycle can now begin again with the intake stroke. 1-7

8 Normal Combustion Normal combustion occurs as the flame front burns across the combustion chamber from the point of ignition. Normal propagation, or speed of the flame front, is between 45 and 90 miles per hour. This produces heat, which causes the burned gasses to expand. As this expansion occurs, the unburned end gases are compressed even further to higher temperatures and pressures. This process creates a great force on the top of the piston, forcing it downward. However, not all of the heat is transferred into energy. As the hot end gases come in contact with the cylinder head, heat is conducted through the cylinder head to the coolant passages where it is carried away to the radiator by the coolant. The piston also conducts heat to the piston rings, which are in contact with the cylinder walls. This heat is conducted through the cylinder to the coolant passages in the block where the coolant circulating through the engine carries it away. The engine oil under the piston also absorbs some of the heat generated by combustion. Another way of controlling heat is turbulence. Since stagnant air can only dissipate heat at its outer edges, it is important to keep it circulating. If the air/fuel mixture is allowed to become stagnant, the center may become hot enough to self-ignite, creating abnormal combustion. Abnormal Combustion There are typically two types of abnormal combustion; pre-ignition and post-ignition or spark knock. Pre-ignition occurs when the air/fuel charge self ignites during the compression stroke before the spark plug fires. Pre-ignition starts from a hot surface in the combustion chamber and causes extremely advanced combustion. This creates abnormally high cylinder pressures and temperatures that cannot be sustained, and will quickly lead to severe engine damage. Engine damage resulting from pre-ignition may include melted spark plug electrodes, melted/scuffed pistons, piston ring damage, and distorted valve heads. Causes of pre-ignition include "hot" spark plugs (incorrect heat range), excessive accumulation of carbon deposits in the combustion chamber, overheated exhaust valves, and cooling system malfunctions. Pre-ignition is at its worst at during high speed and load conditions. Spark knock is caused by the flame front initiated by the spark plug collides with an undesired flame front. The undesired flame front occurs when part of the unburned air/fuel mixture is compressed to a pressure and temperature that exceeds the "self ignition" point causing it to spontaneously ignite. 1-8

9 Remember that when the air/fuel mixture is ignited, the pressure produced tends to compress the gasses that are still unburned. If the heat is not removed fast enough the mixture can self ignite. As the flame fronts collide an audible knock can be heard. Spark knock generally occurs during conditions of heavy load at low to medium engine speeds. Common causes of spark knock include low octane fuel, excessive carbon deposits, EGR malfunctions, high combustion chamber temperature, over advanced ignition timing, and high compression ratios. Spark knock causes a rapid rise in temperature and pressure that can damage spark plug electrodes, pistons, rings, valves, and valve seats. Figure 1-4, Pre-Ignition Figure 1-5, Post-Ignition Note: Two terms that you should be familiar with are quenching and turbulence. Quenching occurs as the heat generated by the compression of the air/ fuel mixture between the cylinder head and the piston is transferred to the cylinder head and cooling jackets. This heat transfer helps to control combustion chamber temperature and prevent abnormal combustion. Turbulence refers to the swirling or tumbling of the combustion mixture that improves air/fuel mixing and helps to control combustion chamber temperature. 1-9

10 Volumetric Efficiency When the piston is at Bottom Dead Center the space above the piston is the volume of that cylinder. This volume is the maximum amount of air/fuel mixture that can be placed within the cylinder. The volume of a cylinder is divided between the piston displacement and the combustion chamber. Piston displacement contains the volume from bottom dead center to top dead center. The combustion chamber is the volume above the piston at top dead center. The volume of a cylinder is measured in cubic centimeters or liters. The difference between cylinder displacement at BDC and TDC is called compression ratio. Note: Engine displacement is the sum of each cylinder's piston displacement. Engine displacement does not account for the volume of the cylinder head. A common way to describe an engine is that it is just a big air pump. The ability of an engine to draw in air under various running conditions is referred to as its volumetric efficiency rating. An engine that can completely fill the cylinders with a fresh air/fuel charge during the intake stroke is said to have high volumetric efficiency. Since only about 25% of the energy produced during combustion is used to propel the vehicle, a small change its ability to "breathe" will have a very large impact on engine performance. The actual volume of air/fuel mixture that can enter a cylinder is usually less than the maximum volume of the cylinder. Volumetric efficiency is expressed in a percentage. If a cylinder has a volumetric efficiency of 80 percent, the actual volume is 20 percent less than the maximum volume. The actual volume is determined by a combination of outside conditions, engine design and engine speed. Figure 1-6, Volumetric Efficiency 1-10

11 Conditions Outside of the Engine Since oxygen is needed for burning fuel, the amount of oxygen determines how much fuel can be burned during combustion, regardless of the quantity of fuel. Air temperature, atmospheric pressure and atmospheric conditions control how much oxygen is in a cubic centimeter of air. This is called air density. High temperatures result in less oxygen in a cubic centimeter of air. Low temperatures result in more oxygen in a cubic centimeter of air. Since a cylinder can only hold a specific volume of air, air temperature will limit the amount of oxygen in that volume and the amount of combustion that can occur. The amount of combustion determines the power output. The affect of atmospheric pressure on air density is slightly different than air temperature. When atmospheric pressure is high there is more oxygen per cubic centimeter of air. When atmospheric pressure is low there is less oxygen per cubic centimeter of air. Atmospheric pressure changes based on altitude. At sea level the average atmospheric pressure is 14.7 pounds per square inch and decreases as the altitude increases. In an engine's cylinder, high atmospheric pressure results in an air/fuel mixture with more oxygen. This produces more efficient combustion and higher power output. When the atmospheric pressure is low, the power output is lower. An engine that "breathes" at atmospheric pressure is referred to as a naturally aspirated engine. Note: Engine horsepower of a naturally aspirated engine is reduced by about 3% per one thousand feet above sea level. For example, an engine that produces 150 bhp at sea level will only produce 85 bhp at the top of Pike's Peak Colorado (14,110 ft.). Humidity is a part of atmospheric condition that can also affect the amount of oxygen in a cubic centimeter of air. When humidity is high, there is an increase of water vapor in the air. The water vapor displaces oxygen. With less oxygen per cubic centimeter, high humidity will reduce combustion efficiency and reduce power output of the engine. We are not the only ones that have a hard time breathing on hot humid days. Engine Speed and Its Affect on Volumetric Efficiency Since flow is based on time, engine speed will determine the amount of time for both air/fuel mixture and exhaust gas flow. As the engine speed increases less time is allowed for flow through the engine. Volumetric efficiency typically decreases at high engine speeds since there is less time for air to fill the cylinders. 1-11

12 Engine Design Factors for Volumetric Efficiency Although air temperature and atmospheric pressure determines the amount of available oxygen per cubic centimeter, the engine design and engine speed determines the volume of air/fuel mixture that can be delivered to the cylinder. The design of the intake system, combustion chamber and the exhaust system all determine the flow of the air/fuel mixture. Air Intake Systems Before outside air can enter the engine cylinder it must flow through the components of the intake system. The intake system limits the air/fuel mixture available to the combustion chamber. The intake system can only allow a limited amount of air/fuel mixture per second to the combustion chamber. Induction systems are matched, or tuned, to a particular application. Air filter surface area, throttle body bore diameter, and even the tubing connecting the air filter housing to the throttle body control the air charge to the engine. Once the air enters the throttle body it is the intake manifold's job to bring it the rest of the way to the combustion chamber when the intake valve opens. Figure 1-7, TBI Airflow On engines equipped with TBI, the intake manifold must carry both air and atomized fuel to the combustion chamber. These manifolds must be designed with compromises to meet the needs of both air and fuel delivery. "Wet" manifolds, as they are known, must maintain enough velocity throughout the desired operating range to hold fuel in suspension while maintaining sufficient air capacity to obtain peak horsepower. If air velocity is not great enough the fuel will fall out of the air and condense of the manifold walls, causing a lean condition. Intake manifolds for Port Fuel Injection, PFI, systems do not have to carry fuel and can be tuned for either maximum torque or horsepower. Long intake runners, for increased low-end and mid-range torque, are possible without the concern of fuel condensing on the manifold walls. 1-12

13 Figure 1-8, AFI Airflow Recent changes in manifold design have helped to improve volumetric efficiency. Many of today's manifolds are larger and have bigger runners to handle an increased volume of air. The use of thermoplastic manifolds instead of cast aluminum is also a growing trend. Manifolds made from thermoplastics are lighter, dissipate heat better, have smoother internal surfaces, and are much cheaper to manufacture than their aluminum counterparts. An Over-Pressurization Relief valve may be used to reduce the chance of damage if combustion pressures enter the intake manifold. The more turbulent, or erratic the air is as it moves through the manifold, the greater velocity it will have moving into the combustion chamber when the intake valve opens. This helps to atomize the fuel and fill the cylinder more completely on the intake stroke. Combined with the proper combustion chamber design, turbulence also helps to reduce combustion chamber temperatures as the air/fuel mixture burns, reducing NOx emissions. The air intake system determines the engines torque curve. A longer intake manifold results in higher torque at lower RPM, and a shorter intake manifold results in higher torque at higher RPM. Some intake designs can change the way air flows through them at different RPM's to optimize torque output. One such manifold design can be found on the Cadillac Catera with the 3.0L (L81) and CTS with the 2.6L (LY9) and the 3.2 (LA3) engines. The intake manifolds on these engines are fitted with 2 valves that can be set for 4 different intake manifold lengths. The 4 different manifold lengths obtainable result in different torque curves with maximum torque at different engine RPM. The intake plenum switchover valve is located in the intake manifold between the cylinder heads. The intake resonance switchover valve is located between the 2 resonating pipes connected to the intake manifold. The control module controls the 2 valves by means of solenoid valves and vacuum operated diaphragm units. 1-13

14 Figure 1-9, Resonator Valve and Related Parts 3.0L (L81) (1) Air Intake Duct (5) Bracket Mounting Bolts (2) Retaining Clip (6) Bracket (3) Intake Resonator (7) Resonator Valve Actuator (4) Vaccum Hose Figure 1-10, Plenum Switch-Over Valve, Actuator and Solenoid 1-14

15 At idle and engine speeds up to 3200 RPM, the resonance valve is positioned for air flow to continue through the separate passages of the resonance valve assembly. The air flows from the resonance valve assembly through the throttle body ducts to the throttle body. At idle, air flows through the idle air passage or past the throttle plates into the intake manifold. The plenum switch-over valve is positioned so that idle airflow is distributed equally between all of the cylinders. As the engine speed increases to 3200 rpm, the plenum switch-over valve moves to separate the two halves of the intake manifold. The long airflow path provides higher engine torque at low rpm. Figure 1-11, Muti-Ram System Operation When the engine speed is between 3200 and 4100 RPM, the resonance switch-over valve is positioned to connect the two passages of the resonance valve assembly. The active length of the air intake is shortened. This provides the airflow necessary for maximum torque in this rpm range. At 4100 RPM the plenum switch-over valve is positioned to connect the two sides of the intake manifold. This provides the shortest active air intake length and provides the correct amount of airflow for maximum torque at this engine rpm range. Forced Induction To get more power from an engine, more air and more fuel must move into the combustion chamber during the intake stroke. Engines with forced air induction have intake manifold pressure values that vary from below atmospheric pressure, or a vacuum, to higher than atmospheric pressure, or boost pressure. To overcome the limitations of natural aspiration turbochargers and superchargers act as a pump to increase air intake pressure and the amount of air that goes into the cylinders. 1-15

16 Turbochargers The turbocharger is mounted between the air cleaner assembly and throttle plates. It operates as a centrifugal pump to increase the pressure in the air intake and move more air into the cylinders. Exhaust gases are directed to the turbocharger. The flow of the exhaust gases turns the turbine on the exhaust side of the turbocharger. The compressor turns with the turbine. Air enters the compressor from the air cleaner assembly. The rotating force of the compressor boosts the pressure of the intake air. At a higher pressure, the air is denser and contains more oxygen molecules. With an increase in air density, more fuel can be added to the mixture. Forcing more air and fuel into the cylinder increases the volumetric efficiency. During combustion, more power is produced to push the piston on the power stroke. Figure 1-12, Turbocharger Operation The rotational force of the compressor varies with exhaust flow. When the engine speed is low or under light load, the exhaust flow turns the turbocharger at a slower speed. The boost pressure is low at this time. As the throttle is opened, the increased engine load and speed creates more exhaust flow. The increased exhaust flow causes an increase in boost pressure. The turbocharger has the ability to increase boost pressure high enough to cause detonation and engine damage. A wastegate allows some exhaust gases to bypass the turbine. The wastegate opens to limit the boost pressure to a predetermined amount. 1-16

17 Note: A turbocharger does not provide boost when the engine is at idle or during light load. When the engine load increases, the turbocharger speed must increase before boost pressure is available. The time that elapses between when the load increases and the boost increases is referred to as "turbo-lag". Because the wheel/shaft may spin at speeds in excess of 100,000 rpm, the bearings are cooled and lubricated with engine oil. Constant lubrication of the turbocharger is important to avoid failure. To help cool the turbocharger bearings and prevent damage, you should allow the engine to idle before shutting it off. Superchargers A supercharger is a positive displacement pump. Some 3800 engines use a supercharger that is mounted to the intake manifold and driven with a belt from the engine crankshaft. With the engine running, the rotors rotate in opposite directions. Intake air is compressed as it moves through the supercharger housing. Compressing the air boosts its pressure. At a higher pressure, air is denser and contains more oxygen molecules. More fuel can be added to the air. During combustion more power is produced to push the piston on the power stroke. Figure 1-13, Supercharger Operation 1-17

18 Because the supercharger is connected directly to the engine crankshaft, boost pressure is available at low engine rpm as well as through the whole range of engine speeds. As a positive displacement pump the supercharger does have a maximum boost pressure that it can produce. To limit boost pressure, a bypass valve operates to recirculate boost pressure to the supercharger inlet. Limiting boost reduces the amount of drag put on the engine by the supercharger, as well as maximizes fuel economy. The supercharger used on the 3800 engine is a Roots type. Two rotors with three lobes each have a helical twist to quiet the operation and smooth air pulses. The rotors run at a minimal clearance to each other and the supercharger housing. The rotors are timed to each other with a pair of gears. The gears are turned with the input shaft, which is driven by the engine drive belt. The rotors spin at approximately two times the engine's rpm. Since the supercharger is driven off the crankshaft, boost is always available. There isn't any "lag" time to get the compressor up to speed as there is on a turbocharger. However, the overall gain in volumetric efficiency is slightly overshadowed by the power it robs from the engine to turn the gears of the supercharger. Combustion Chamber Design The combustion chamber limits the flow of the air/fuel mixture into the cylinder. The size of the intake valve or valves and the design of the combustion chamber will only allow a limited amount of flow into the cylinder per second. There are many different types of combustion chambers. Each design has specific advantages and disadvantages. However, the ultimate goal of any combustion chamber design is to maximize volumetric efficiency and create conditions within the cylinder that allow complete combustion. The hemispherical design combustion chamber uses one intake valve and one exhaust valve. The combustion chamber has a half ball shape with the spark plug located between the valves. This design provides very good airflow, but offers very little turbulence of the air/fuel charge entering the cylinder. Under typical driving conditions combustion often suffers because of the large volume of the combustion chamber. This design requires more time for the flame to completely burn the fuel and generally results in higher emissions. 1-18

19 Figure 1-14, Combustion Chamber Design GM uses a modified version of the hemispherical design in its four-valve per cylinder applications, known as the pentroof design. This design differs from the hemispherical design in that the two intake and two exhaust valves are seated on a plane with the spark plug in the center. This creates a flat "pentroof" shape combustion chamber instead of spherical shape. This four-valve design combines improved airflow and turbulence to provide greater volumetric efficiency at high RPM. As with the pentroof design the wedge design is named after the shape in creates. This design works well for a two-valve system. The wedge shape design offers slightly lower volumetric efficiency at high RPM. However, it provides increased air/fuel turbulence for better mixing and more complete burning. This results in better performance at lower RPM. Note: The GM "Small Block" engine family is well known for this type of cylinder head. Exhaust Systems The exhaust system has a major affect on the air flowing through the engine. The exhaust must be the correct size to allow the engine to expel the exhaust gases at the proper rate. It must allow for good flow with minimum backpressure. Backpressure is created when exhaust gases back up in the exhaust system because they can't be expelled as fast as they are produced. If exhaust gases are not expelled effectively, the air/ fuel charge to the cylinder will be diluted and the velocity of the in coming intake charge will be drastically reduced. The resulting performance will be less than satisfactory. If the diameter of the exhaust pipes is too big, the engine may run cooler, resulting in increased emissions levels. 1-19

20 Although the operation is simple, the design of the exhaust system is often more complex. High-pressure pulses produced by the exhaust valves opening and closing must be compensated for to reduce exhaust noise. This is accomplished through manifold design and the muffler. Exhaust manifolds must fit in and around cramped engine compartments, leave adequate space for serviceability, and contain no sharp bends that could restrict flow. Figure 1-15, Exhaust System A major component in the exhaust system is the catalytic converter. Its primary function is to chemically convert harmful exhaust gases such as hydrocarbons, carbon monoxide, and NOx into more environmentally friendly water, carbon dioxide, and nitrogen. Deterioration of the materials inside the converter, possibly due to a sustained misfire condition or rich mixture, can cause the exhaust to become restricted. If an Exhaust restriction occurs, the customer concern will most likely be lack of power. Follow service manual procedures to check for a restriction. Although specifications vary between vehicle platforms, typically there should be less than 1.25 psi of backpressure at 2,500 RPM with the engine at normal operating temperature with no load on the engine. 1-20

21 Valvetrain and Valve Timing The final area in engine design that affects volumetric efficiency is the valvetrain. Valvetrains come in various configurations including push rod, single and dual over-head overhead camshaft designs. No matter which design is used, proper camshaft timing, valve lift, and the duration of lift are necessary for good engine performance and emissions control. The camshaft is responsible for controlling the rate of valve opening and closing. It also determines the length of time that the valve is open. These functions are identified through the terms lift, duration, and overlap. LIFT is the distance that the valve is moved off of its seat when fully open. Valve lift is generally measured at the camshaft lobe, but can also be measured at the valve. DURATION is the length of time that the valve remains open. Since the actual time in seconds varies with engine speed, duration is measured in degrees of crankshaft rotation. Ideally, the duration event begins before the piston changes direction during a stroke. OVERLAP is the amount of time (in crankshaft degrees) that both the intake and exhaust valves are open at the same time. As we learned earlier, the intake stroke begins before the piston reaches TDC of the exhaust stroke. Since the exhaust is hot and under pressure, the inertia of the gasses actually causes their flow to continue after the piston passes TDC. Just before TDC (about 12 ) the intake valve opens. The air/fuel mixture begins to fill the cylinder even though the piston is still moving upward. This is because the inertia created by the exiting exhaust gases create a siphoning or venture effect increasing volumetric efficiency. This is called scavenging. Figure 1-16, Valve Timing Figure 1-17, Camshaft Lobe A worn camshaft lobe will cause the valve to open later, close sooner, and reduce valve lift. Engine performance will suffer due to the loss in volumetric efficiency. 1-21

22 Camshaft Timing and Balance Shafts In order for the engine to perform correctly, the camshaft must be synchronized to the movements of the crankshaft. This will ensure that the valves open at the correct time relative to piston position. The camshaft(s) is/are driven by a chain or belt at one-half crankshaft speed. Aligning the timing marks on the camshaft and crankshaft sprockets to their correct position will set static timing. Static timing provides a baseline relationship between the crankshaft and camshaft(s). A worn timing chain or sprockets will cause the camshaft to lag behind crankshaft rotation changing when the valves open and close in reference to piston position. This will reduce the volumetric efficiency and power output of the engine. Figure 1-18, Static Timing Marks 4.2L (LL8) 1-22

23 Some engines have a natural dynamic engine imbalance due to the firing pulses and the forces exerted on the crankshaft. This can cause customer concerns of "running rough" or "missing" when no problem actually exists. Engines that have a strong natural imbalance may contain one or more balance shafts to minimize vibration levels. These balance shafts must be properly timed to the crankshaft to counteract the deflections produced during rotation. One such engine that utilizes this feature is the 2.2L (L61) Ecotec engine used in J and N body passenger cars. It is important for the technician to be able to differentiate between normal engine imbalance and abnormal imbalance such as a misfire to prevent misdiagnosis. Figure 1-19, Balance Shafts w/chain (2.2 L61) Valve Deposits Carbon deposits on the valves can cause drivability concerns such as cold stumbles, hesitation, and stalling. This carbon absorbs the fuel intended for combustion in the cylinder causing a lean condition. As the deposit grows, fuel will become further restricted causing increased combustion chamber temperatures and higher NOx emissions. Excessive valve deposits can be caused by oil, fuel, excessive heat, and PCV system malfunctions. 1-23

24 Manifold Vacuum The intake stroke of the piston creates a vacuum in the manifold. Vacuum is any pressure lower than atmospheric pressure. The greater the difference between the low pressure in the cylinder and high pressure in the atmosphere, the better the distribution of air and fuel to the cylinders. An engine's ability to form and hold a vacuum is directly related to its ability to form and hold compression. When an engine loses the ability to create vacuum, performance suffers. The amount of vacuum formed in the manifold depends on several things. First, the cylinders must be sealed. If a cylinder has low compression or high leakage, it will not produce sufficient vacuum to draw in the air/fuel mixture. If the manifold is not sealed, vacuum will be lower than normal. Vacuum hoses, vacuum operated systems, and accessories that operate on vacuum may also leak, causing lower manifold vacuum. In the manifold, when the throttle plate is closed at idle, the vacuum is greater. When the throttle plate is open and atmospheric pressure enters the manifold, vacuum is lower. Monitoring vacuum is a quick and easy way to test an engine. It is a good indicator of the engine's ability to run efficiently. Typical engine vacuum is a steady reading between 15 and 22 inches Hg with the engine at normal operating temperatures, idle, and in drive. Vacuum changes with load, so operating accessories when monitoring vacuum will change the readings. Vacuum readings will vary between engines. One reason may be differences in the compression ratio. If the engine has higher compression, it will have 1 to 2 inches Hg higher vacuum. Altitude also affects vacuum. For every 1,000 feet above sea level, vacuum will be lower by 1 inch Hg. Some engines use a high lift cam or have considerable valve overlap. This will produce a slightly lower, erratic needle reading on the gauge. Some things that can be diagnosed using vacuum readings are engine components (i.e., valves, valve guides and springs, piston rings), manifold leaks, timing, and a restricted exhaust system. In addition, a lower manifold vacuum can also Figure 1-20, Manifold Vacuum 1-24

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26 Diagnosis Cranking "static" Compression Testing As we have learned, compression is a critical component of combustion. If a combustion chamber is not properly sealed there will be a loss of compression resulting in a loss of performance. When an engine mechanical issue is suspected or a vacuum test indicates a problem, a cranking compression test should be performed. 1. Charge the battery if the battery is not fully charged. 2. Disable the ignition system. 3. Disable the fuel injection system. 4. Remove all the spark plugs. 5. Block the throttle plate wide open. 6. Start with the compression gauge at zero and crank the engine through four compression strokes (four puffs). 7. Make the compression check for each cylinder. Record the reading. 8. If a cylinder has low compression, inject approximately 15 ml (one tablespoon) of engine oil into the combustion chamber through the spark plug hole. Recheck the compression and record the reading. This is called a "wet" compression test. Analysis The minimum compression in any one cylinder should not be less than 70 percent of the highest cylinder. No cylinder should read less than 690 kpa (100 psi). For example, if the highest pressure in any one cylinder is 1035 kpa (150 psi), the lowest allowable pressure for any other cylinder would be 725 kpa (105 psi). (1035 x 70% = 725) (150 x 70% = 105). Normal - Compression builds up quickly and evenly to the specified compression for each cylinder. Piston Rings Leaking - Compression is low on the first stroke. Compression then builds up with the following strokes but does not reach normal. Compression improves considerably when you add oil. Valves Leaking - Compression is low on the first stroke. Compression usually does not build up on the following strokes. Compression does not improve much when you add oil. If two adjacent cylinders have lower than normal, suspect leaking head gasket 1-26

27 Cylinder Leakage Testing When a cranking compression test indicates that a compression leak is present, it is necessary to determine its source before engine disassembly. The more you know about the area of the fault, the easier and less time consuming it will be to correct it. By introducing compressed air into the cylinder and monitoring the amount of pressure loss using the - J A Cylinder Leak down Tester (or equivalent) and listening for escaping air the exact location of the compression loss can be determined. Figure 1-21, Cylinder Leakage Tester 1. Disconnect the negative battery cable. 2. Remove the spark plugs. Refer to Spark Plug Replacement in Engine Controls. 3. Install the J A. 4. Measure each cylinder on the compression stroke, with both valves closed. Note: Hold the crankshaft balancer bolt in order to prevent piston movement. 5. Apply air pressure, using the J A. Refer to the manufacturer's instructions. 6. Record the cylinder leakage readings for each cylinder. Note:Normal cylinder leakage is from 12 to 18 percent. Make a note of any cylinder with more leakage than the other cylinders. Any cylinder with 30 percent leakage or more requires service. 7. Inspect the 4 primary areas in order to properly diagnose a leaking cylinder. 8. If air is heard from the intake or exhaust system, perform the following procedure: Remove the valve rocker arm cover of the suspect cylinder head. Ensure that both valves are closed. Inspect the cylinder head for a broken valve spring. Remove and inspect the suspect cylinder head. Refer to Cylinder Head Cleaning and Inspection. 1-27

28 9. If air is heard from the crankcase system at the crankcase (oil filler tube), perform the following procedure: Remove the piston from the suspect cylinder. Inspect the piston and connecting rod assembly. Refer to Piston, Connecting Rod, and Bearings Cleaning and Inspection. Inspect the engine block. Refer to Engine Block Cleaning and Inspection. 10. If bubbles are found in the radiator, perform the following procedure: Remove and inspect both cylinder heads. Refer to Cylinder Head Cleaning and Inspection. Inspect the engine block. Refer to Engine Block Cleaning and Inspection. 11. Remove the J A. 12. Install the spark plugs. Refer to Spark Plug Replacement in Engine Controls. 13. Connect the negative battery cable. Refer to Battery Negative Cable Disconnect/Connect Procedure in Engine Electrical. Caution: Before servicing any electrical component, the ignition key must be in the OFF or LOCK position and all electrical loads must be OFF, unless instructed otherwise in these procedures. A tool or equipment could easily come in contact with a live exposed electrical terminal; also disconnect the negative battery cable. Failure to follow these precautions may cause personal injury and/or damage to the vehicle or its components. Running Compression Testing The typical static cranking compression test does a good job of checking overall cylinder seal, and will identify gross compression pressure leaks. However, we have learned that volumetric efficiency is highest at slow engine speeds because the piston moves slower and allows more time for the air/fuel charge to enter and the exhaust to leave the cylinder. In order to determine the overall breathing ability we must measure the actual amount of air moving through the engine. We do this by checking compression produced with the engine running. Start with a normal ( static ) compression test. To eliminate rings, valves, holes in pistons, that sort of thing. A normal cylinder balance test is also helpful (so you know which, if any, cylinder is presenting a problem). Engine should be warm. Put all spark plugs but one back in. Ground that plug wire to prevent module damage.disconnect that injector on a port fuel system. 1-28

29 Put your compression tester into the empty hole. The test can be done without a Schrader Valve, but most people recommended leaving the valve in the gauge and burping the gauge every 5-6 puffs. Start the engine and take a reading. Write it down. Now goose the throttle for a snap acceleration reading. Reading should rise. Write it down. Note: Don t use the gas pedal for this snap acceleration. The idea is to manually open then close the throttle as fast as possible without speeding up the engine. This forces the engine to take a gulp of air. Now, write down your readings for at least the bad cylinder (if there is a single bad cylinder) and maybe 2-3 good ones. Make a chart like this: CYL STATIC COMPR IDLE-RUNNING COMPR SNAP Cyl Cyl Cyl Cyl Analysis: Running compression at idle should be psi (about half cranking compression). Snap throttle compression should be about 80% of cranking compression. Sample 1 - Restricted Exhaust CYL STATIC COMPR IDLE - RUNNING COMPR SNAP Cyl If snap measurements are higher than 80% of cranking measurements, look for restricted exhaust on that cylinder - such as worn exhaust cam lobe, or collapsed lifter. Or, if these are all high, look for a clogged catalytic converter or collapsed exhaust pipe. 1-29

30 Sample 2 Restricted Intake CYL STATIC COMPR IDLE-RUNNING COMP SNAP Cyl If snap measurements are less than 80% of cranking measurements, look for restrictions in the intake side of that cylinder such as a worn intake cam lobe, collapsed lifter, bent pushrod, or excessive carbon build up on the valves. If all measurements are low, look for a dirty air filter or collapsed air inlet hoses. 1-30

31 Module 1 Test 1. A manifold vacuum test should be used to check the condition of the: a. ignition system b. fuel system c. base engine d. cooling system 2. In addition to valve and head gasket concerns, irregular, low, or fluctuation manifold vacuum test readings may be the result of: a. plugged PCV valve b. late ignition timing c. excessive compression d. exhaust leak 3. Which of the following will cause excess exhaust back pressure? a. Exhaust valve b. Defective HO2S sensor c. Plugged catalytic converter d. Worn valve guides 4. Excessive exhaust back pressure is most noticeable during speed, load conditions. a. low, low b. low, high c. high, high d. high, low 5. Why is spark knock a potential result of too little EGR? a. Excessive exhaust manifold/catalyst temperature b. Excessive combustion chamber temperature c. Air/fuel ratio too rich d. Reduced emission levels 1-31

32 6. What occurs when the air-fuel mixture self ignites during the compression stroke before the spark plug fires? a. Spark knock b. Misfire c. Pre-ignition d. Valve slap 7. Short intake runners are desirable for peak performance at RPM. a. Low b. High c d. mid-range 8. Which of the following is NOT a benefit of recent changes in intake manifold design?. a. Lighter b. Stronger c. Cheaper d. Increased air capacity 9. Technician A says atmospheric pressure decreases as altitude is increased. Technician B says that atmospheric pressure at sea level is about 14.7 lbs. per square inch. Who is Correct? a. Technician A b. Technician B c. Both d. Neither 10. Humid air has more oxygen molecules per cubic centimeter than dry air. a. True b. False 1-32

33 Exercise 1-1 Vacuum Testing Objectives: Following completion of this worksheet, you will be able to: Perform a manifold vacuum test Use your knowledge and critical thinking skills to identify possible engine performance problems associated with engine mechanical condition based on test results Locate an unrestricted intake vacuum port and connect a vacuum gauge with three feet of hose to it. Try to find a source as close to the manifold as possible. Start the engine and let it reach normal operating temp. 1. With the engine idling in drive (neutral for M/T), with all accessories off, and the parking brake set for safety, observe the gauge. What is the reading on the vacuum gauge? 2. Does the reading fall within an acceptable range and remain steady? 3. If the vacuum gauge needle fluctuates or drops 1 to 2 inches Hg lower than normal at regular intervals what could be the cause? 4. What would cause a steady reading that was lower than normal (12-15" Hg)? Place the transmission in park and quickly open then close snap the throttle. 5. What does the vacuum gauge read at wide open throttle? 6. What caused this change in vacuum? 7. Observe the vacuum reading just after snapping the throttle closed. What is the reading on the gauge? What is this an indication of? 8. If the vacuum did not reach inches Hg when the throttle is snapped shut, what would be the next logical diagnostic test to perform? 1-33

34 Gradually increase engine speed to 2000 RPM, hold it there for one full minute. 9. What is the vacuum reading at 2000 RPM? 10.Earlier we saw the vacuum drop off completely when the throttle was fully opened. Why did that not occur as the throttle was opened to 2000 RPM? 11. What would be indicated if the vacuum gauge read less than 5 inches Hg at 2000 RPM? 12.If the reading was as indicated above, what test would you perform next? Stop the engine and disable the fuel and ignition systems. Restrict the airflow into the intake by using the IAC valve tester to bottom out the IAC valve in its bore or by covering the throttle body with your hand. Observe the reading on the vacuum gauge while cranking the engine. This is called a cranking vacuum test. This test can provide a good indication of overall cylinder seal. The gauge reading should be above 2.5 inches Hg. 13.What is the vacuum reading on the gauge during cranking? 14.What does each fluctuation of the needle on the gauge indicate? 15.Is the gauge reading above 2.5 inches Hg? 1-34

35 Exercise 1-2 Compression and Cylinder Leakage Testing Objectives: Following completion of this worksheet, you will be able to: Perform a cranking compression test (wet and dry) Perform a cylinder leakage test Perform a running compression test Use acquired knowledge and critical thinking skills to identify performance problems based on the results of the compression and leakage tests Use Service Information to locate the procedure for compression testing on the chosen ASEP vehicle. 1. List four steps necessary when preparing for a cranking compression test. 2. Install the compression gauge into #1 spark plug hole and crank the engine through four compression strokes puffs. Observe the reading on the first puff, then after four. Record your results below. Repeat for the remaining cylinders. Cylinder 1 st Puff After 4 Cylinder 1 st Puff After 4 #1 #5 #2 #6 #3 #7 #4 #8 3. What is the minimum allowable compression according to the service information? 4. The first puff should be within % of the final reading. 5. All cylinders should be within what percentage of the highest cylinder? 6. Were all of the cylinders within the specified range of each other? 1-35

36 6. What would be the next step in your diagnosis if any cylinders were lower than specified? 7. If compression increases after performing the test in the above question, what is the most likely cause of the low compression reading? 8. If cylinder compression remains abnormally low, what test should be performed to isolate the cause of the poor cylinder seal? Cylinder Leakage Rotate the crankshaft so the #1 piston is at TDC compression stroke. Calibrate and install the J A, or equivalent, apply air to the cylinder. To ensure accuracy the engine should be warm. 1. Record your leakage readings below: Cylinder Reading Cylinder Reading #1 #5 #2 #6 #3 #7 #4 #8 2. No cylinder should have more than % leakage. 3. List three possible causes for high leakage and air bubbles present in the radiator or surge tank. 4. List the most likely cause of air escaping at the throttle body. 5. While performing a cylinder leakage test you measure 18% leakage and hear escaping at the oil fill. What is indicated? 6. What is the correct spark plug torque for this engine? 1-36