Automotive Engine
Parts and Operation
By far, the most important component in any car is the engine. If the engine isn’t operating prop-erly, the car isn’t going anywhere. A car’s engine must be reliable and economical to use, as well as powerful enough to perform under all sorts of driving conditions. In addition, an automotive en-gine must be able to keep running in all temperatures, through the heat of summer and the bitter cold of winter.
Because the engine performs more and works harder than any other automotive component, it’s serviced more often. As an automotive technician, engine repair and troubleshooting will be one of your chief concerns. Because of this, you must thoroughly understand how automotive engines operate and how they’re repaired.
In this study unit, you’ll begin your examination of the engine. You’ll start with a discussion of the basic parts of the gasoline engine. Then, you’ll learn how these engine components work together to make an engine run. A complete understanding of engine operation will be your first step to-ward engine repair. When diagnosing engine problems, knowing exactly how an engine operates will be your most important tool.
When you complete this study unit, you’ll be able to · Explain the basic operation of the four-stroke engine
· Visually identify the components of the lower-end assembly, including the engine block, crankshaft, connecting rods, piston assembly, engine bearings, and vibration dampeners · Visually identify the components of the upper-end assembly, including the cylinder head,
valve assembly, camshaft, and the intake and exhaust manifolds
· Name and describe the functions of the primary engine systems, including the fuel, ignition, cooling, and lubrication systems
THE FOUR STAGES OF ENGINE OPERATION· · · · 1 Introduction
Review of the Basic Parts of an Engine Review of the Four Stages of Operation Engine Parts
THE LOWER-END ASSEMBLY · · · · 11 The Engine Block and the Cylinders
The Piston The Piston Rings
The Movement of the Connecting Rod and Crankshaft The Connecting Rod
The Crankshaft The Engine Bearings Engine Vibration The Oil Pan Crankshaft Seals
THE UPPER-END ASSEMBLY · · · · 50 The Cylinder Head
The Intake and Exhaust Valves The Camshaft
The Valve Train Assembly
The Valve Train in an Overhead Valve Engine The Valve Train in an Overhead Camshaft Engine The Manifolds
The Engine Covers
SUMMARY · · · · 94 POWER CHECK ANSWERS · · · · 97 EXAMINATION · · · · 99
Contents
THE FOUR STAGES OF ENGINE OPERATION
Introduction
As you learned in an earlier study unit, the engine is the automotive system that produces the power for operation. This power is produced when a mixture of air and fuel is burned inside the engine. Since the fuel is burned inside the engine to produce power, an automotive en-gine is called an internal combustion enen-gine. The term combustion refers to the burning of the air-and-fuel mixture.
The engines used in automobiles, light trucks, and sport-utility vehi-cles are all gasoline-powered, internal combustion engines. In contrast, the engines used in large trucks are usually powered by diesel fuel. These engines are called diesel engines, and they’re different in con-struction and operation from gasoline engines. Note that we’ll discuss only gasoline-powered engines in this program.
In this study unit, we’ll look at the parts of an automotive engine and learn how these parts contribute to overall engine operation. However, before we begin to look at the parts of an engine, let’s quickly review the four stages of engine operation. These stages of operation are called the four-stroke engine cycle. When you’re studying engine con-struction and repair, it’s very important to remember the four stages of engine operation. Memorize these four stages well, and everything else we talk about will fall right into place.
Review of the Basic Parts of an Engine
Let’s start by discussing some basic ideas we covered earlier in your program. These topics are so important that it won't hurt to review them. First, you know that an automotive engine produces power by burning a mixture of air and fuel inside a closed cylinder. The basic parts that make up an engine cylinder assembly are shown in Figure 1. Note the round cylinder with the piston positioned inside it. The cylin-der is a hollow metal tube that’s drilled into the metal engine block. The piston is a can-shaped metal component that can move up and down inside the cylinder. The piston is the main moving part in an engine.
The top of the cylinder is sealed by a metal cover that’s called the
cylin-der head. The cylincylin-der head is bolted onto the top of the cylincylin-der.
When the piston is positioned at the very top of the cylinder, an open area is still left above the piston. This area is in the cylinder head and is called the combustion chamber. In the combustion chamber, a mixture of air and gasoline is burned to produce power.
Note that the spark plug is screwed into a threaded hole in the cylinder head. The end of the spark plug protrudes through the cylinder head and into the combustion chamber. The spark plug is used to make sparks that will ignite the air-and-fuel mixture in the cylinder and cause it to burn.
The piston is connected to the crankshaft by the connecting rod. When the piston is forced downward in the cylinder, the piston's downward motion is transferred through the connecting rod to the crankshaft. The rod and crankshaft then convert the up-and-down motion of the piston into rotary motion.
Remember that some special terms are used to describe the exact posi-tion of the piston in the cylinder. When a piston is at its lowest posiposi-tion in the cylinder, it's said to be at bottom dead center (BDC). When the pis-ton is at its highest position in the cylinder, it's said to be at top dead
center (TDC). The total distance that the piston moves from the top of
the cylinder to the bottom of the cylinder is called the stroke.
Note that the cylinder head contains valves that open and close during engine operation. A four-stroke engine will contain two types of valves: intake valves and exhaust valves. These valves are moved up and down to make them open and close. The intake valve opens to allow the air-and-fuel mixture to enter the cylinder. The exhaust valve opens to allow the exhaust gases produced by the burning mixture to exit the cylinder. When both valves are closed, the cylinder is sealed tightly. In order to burn properly in an engine, fuel must be mixed with air. Fuel from the fuel tank is vaporized and mixed with air in the engine. The air-and-fuel mixture is then delivered to the cylinder through the intake valve.
Review of the Four Stages of Operation
Now that we’ve reviewed some basic components of a typical engine, let’s take a look how these components make the engine work. In or-der to produce power, an engine must continually go through the same four stages of operation. The stages are
1. Intake 2. Compression
3. Power 4. Exhaust
In the intake stage, air that has been mixed with fuel is drawn into the cylinder. In the compression stage, the piston rises and compresses the air-and-fuel mixture trapped in the combustion chamber. During the
power stage, the air-and-fuel mixture is ignited, and the contained
ex-plosion of the fuel forces the piston back down in the cylinder. The downward motion of the piston is transferred to the rod and crank-shaft. During the exhaust stage, the exhaust gases are released from the cylinder. The four stages then begin all over again.
In order for any engine to operate, it must run through all four stages of operation. In a four-stroke engine, the piston requires four strokes (that is, two complete up movements and two complete down
movements) to complete one full engine cycle. In a four-stroke engine, the four piston strokes complete the four stages of operation as follows:
Piston stroke 1: Intake stage
Piston stroke 2: Compression stage
EXHAUST VALVE INTAKE VALVE SPARK PLUG CYLINDER PISTON CONNECTING ROD CRANKSHAFT CYLINDER HEAD COMBUSTION CHAMBER
FIGURE 1—A simplified drawing of part of a four-stroke engine is shown here. Note the location of the cylinder, piston, connecting rod, crankshaft, combustion chamber, cylinder head, spark plug, intake valve, and exhaust valve.
Piston stroke 3: Power stage
Piston stroke 4: Exhaust stage
Now, let’s look at the four stages of operation more closely. During the intake stage, the intake valve opens and the piston moves down in the cylinder. As the piston moves down in the cylinder, a vacuum is cre-ated. This vacuum draws the air-and-fuel mixture into the cylinder through the open intake valve. (This is something like the vacuum cre-ated when you use a straw to draw liquid up out of a glass.) The intake stage continues until the piston reaches BDC. During the intake stage, the engine’s exhaust valve remains closed. The intake stage is illus-trated in Figure 2.
EXHAUST VALVE (CLOSED) INTAKE VALVE (OPEN)
SPARK PLUG CONNECTING ROD COMBUSTION CHAMBER BLOCK PISTON (MOVES DOWN) CRANKSHAFT AIR-AND-FUEL MIXTURE
FIGURE 2—During the intake stage, the piston lowers and causes the air-and-fuel mixture to be drawn into the cylinder. The intake valve is open at this time, and the exhaust valve is closed.
When the piston reaches BDC, the intake stage is completed and the compression stage of the cycle begins. During this stage, both the in-take and the exhaust valves are closed. The air-and-fuel mixture is now trapped inside the sealed combustion chamber. At this point, the pis-ton begins to rise, compressing the air-and-fuel mixture tightly. Com-pressing the air-and-fuel mixture will make it ignite easier and burn more efficiently, thus producing more power. The compression stage is illustrated in Figure 3.
The compression stage continues as the piston rises to the top of the cylinder. As the piston approaches TDC, the engine's ignition system causes the spark plug to “fire.” That is, the ignition system causes the spark plug to make a spark. Naturally, when a spark is applied to a compressed mixture of fuel and air, an explosion immediately occurs. Remember that the process of burning the air-and-fuel mixture in the cylinder is called combustion.
When the air-and-fuel mixture is ignited, the burning gases expand rapidly with great force. Therefore, the contained explosion inside the cylinder forces the piston down hard and fast. Since the piston is con-nected to the crankshaft through the connecting rod, the piston's downward movement causes the crankshaft to turn around, just like
EXHAUST VALVE (CLOSED)
INTAKE VALVE (CLOSED)
SPARK PLUG CONNECTING ROD CRANKSHAFT CYLINDER HEAD COMBUSTION CHAMBER BLOCK PISTON (MOVES UP) AIR-AND-FUEL MIXTURE
(IN COMBUSTION CHAMBER)
FIGURE 3—During the compression stage, both the intake and exhaust valve are closed. As the piston rises, it compresses the air-and-fuel mixture in the sealed combustion chamber.
pushing down on the pedals of a bicycle. This stage of burning the mixture in the cylinder and forcing the piston downward is called the power stage, which is illustrated in Figure 4.
The power stage continues until the piston reaches BDC. At this point, the piston begins to move back upward in the cylinder, and the ex-haust valve opens. However, the intake valve remains closed at this time. As the piston moves upward in the cylinder, it forces the remain-ing burned gases in the cylinder out through the open exhaust valve. This is the exhaust stage, which is illustrated in Figure 5.
The exhaust stage continues until the piston reaches TDC. Once the ex-haust stage is completed, the four stages of engine operation are fin-ished, and the cycle begins all over again. The intake valve opens, the piston moves downward to draw in more fuel, and a new intake stage begins.
EXHAUST VALVE (CLOSED) INTAKE VALVE (CLOSED)
SPARK PLUG FIRES CONNECTING ROD COMBUSTION CHAMBER BLOCK PISTON (MOVES DOWN) CRANKSHAFT
FIGURE 4—When the piston reaches TDC, the spark plug fires and ignites the air-and-fuel mixture. The force created by the exploding fuel gases pushes the piston down in the cylinder. This is the power stage.
One engine cycle is a complete run through all four stages of opera-tion—intake, compression, power, and exhaust. After the completion of the fourth piston stroke, the engine has gone through all four stages of operation, and the cycle will begin again. Note that the four stages of operation occur very quickly, and they repeat continually for as long as the engine is running. All automotive engines operate in these same four basic stages, and all the stages must occur in order for the engine to run properly.
Engine Parts
Virtually all modern automobiles contain the four-stroke engines you just learned about. A typical automotive engine is shown in Figure 6. You'll see that an automotive engine is actually quite complex. Figure 7 shows a disassembled view of a typical engine. In the disassembled
EXHAUST VALVE (OPEN)
SPARK PLUG CONNECTING ROD COMBUSTION CHAMBER BLOCK PISTON (MOVES UP)
INTAKE VALVE (CLOSED)
EXHAUST MANIFOLD
CRANKSHAFT
FIGURE 5—During the exhaust stage, the piston rises and the exhaust valve opens. The rising piston pushes the remaining burned gases in the cylinder out through the open exhaust valve.
view, you can see all the component parts that make up the engine. In this study unit, we’ll look at each of these components and how they relate to the overall operation of the engine.
Since an engine contains so many different parts, it's usually easier to break the engine into two separate sections for discussion. These two sections are the lower end and the upper end. An engine’s lower end contains the cylinder block, the cylinders, the piston, the connecting rods, and the crankshaft assemblies. This area is called the lower end because these components are usually located near the bottom of the engine assembly. The basic function of the lower-end components is to convert the up-and-down motion of the pistons into rotary motion that can be used to power a vehicle. An engine’s upper end contains the
cylinder head, the valves, the valve train components, the manifolds, and the engine covers. This area is called the upper end because these components are located at the top of the engine assembly. The basic function of the upper-end components is to control the flow of air-and-fuel mixture into the engine, and the flow of exhaust gases out of the engine after combustion.
Now, take a few moments to review what you’ve learned by complet-ing Power Check 1.
FIGURE 6—An external view of a typical automotive engine is shown here.
Power Check 1
At the end of each section of Automotive Engine Parts and Operation, you’ll be asked to pause and check your understanding of what you’ve just read by completing a “Power Check” exercise. Writing the answers to these questions will help you to review what you’ve studied so far. Please complete Power Check 1 now.
1–12: Fill in the blanks in each of the following statements.
1. One engine _______ is a complete run through all four stages of operation—intake, compression, power, and exhaust.
2. When the piston is at its highest position in the cylinder, it's said to be at _______. 3. During the _______ stage, the air-and-fuel mixture is ignited, and the contained
explo-sion of the fuel presses the piston back down in the cylinder.
4. The burning of the air-and-fuel mixture in the cylinder is called _______.
5. The _______ is the total distance that the piston moves from the top of the cylinder to the bottom of the cylinder.
6. The small open space between the top of the piston and the cylinder head is called the _______.
7. The basic function of the _______ -end components is to control the flow of air-and-fuel mixture into the engine, and the flow of exhaust gases out of the engine after com-bustion.
8. In an engine, the _______ ignites the air-and-fuel mixture in the cylinder and causes it to burn.
9. Air that has been mixed with fuel is drawn into the cylinder during the _______ stage. 10. In a four-stroke engine, the piston requires four strokes to complete one full _______. 11. During the _______ stage, burned gases are released from the cylinder.
12. An engine’s _______ contains the cylinder block, the cylinders, the piston, the connect-ing rods, and the crankshaft assemblies.
THE LOWER-END ASSEMBLY
An engine’s lower end contains several of the engine’s most important parts. You’ve already learned the operation of some of these parts. However, in this section you’ll learn about the following lower-end components in more detail.
· Engine block · Crankshaft · Connecting rods · Piston assembly · Engine bearings · Vibration dampeners
As you read through this section of your study unit, try to understand what each individual part does, and also how each part works with the other components to make the engine run. It’s important to remember that the performance of one component often relies on the perform-ance of another. Therefore, the way that the parts work together is just as important as the individual function of each part.
The Engine Block and the Cylinders
If any one part could be considered to be the “main component” of an engine, it would be the engine block. Almost all other engine compo-nents are attached in some way to the engine block. Some parts fit in-side the block, while others are bolted to the outin-side of the block. The most important function of the engine block is to hold the pistons inside the cylinders. The cylinders are deep, can-shaped holes that are drilled into the engine block. The up-and-down movement of the pis-tons inside the cylinders produces the power needed to operate the ve-hicle. Therefore, the more cylinders an engine contains, the more power it will produce.
A typical engine block is shown in Figure 8. Note that this block con-tains four separate cylinders. Each cylinder will hold a piston that will be used to produce power. An engine may contain any number of cyl-inders, although modern automobiles usually contain either four, six, or eight cylinders. Most engines will contain an even number of cylin-ders in order to maintain a proper balance of weight and forces. Some
more unusual engines may contain up to twelve cylinders, but these engines are usually found only in high-performance vehicles. Since an engine block supports all the other engine components, it must be solidly built. For this reason, most of today’s engine blocks are made of cast iron. Cast-iron blocks are very strong and can be mass-produced easily. However, one disadvantage of cast-iron engine blocks is their extreme weight. Because cast iron is so heavy, some en-gine blocks are made of aluminum instead of cast iron. However, alu-minum engine blocks are quite expensive, so they’re usually found only in expensive or high-performance automobiles.
Because aluminum is a very soft metal, each cylinder in an aluminum engine block will usually be lined with a thin layer of steel that's pressed into the block. This steel lining is called a cylinder sleeve. The function of the cylinder sleeve is to prevent the soft aluminum cylinder from wearing excessively as the piston rubs against it. In cast-iron blocks, the cylinders may be machined directly into the block. Now, let’s take a moment to examine the engine’s cylinders. In very simple terms, an engine cylinder is a large hole that’s bored into the cylinder block. The proper name for this hole in the engine block is the
cylinder bore, but it’s usually referred to as the cylinder. The inside
sur-faces of the bore are called the cylinder walls. Each bore is machined to a precise diameter (width) so that a piston will fit inside it properly. The bores are also very smooth inside, so the piston can slide up and down easily inside the cylinder.
As you learned earlier, many vital engine parts are mounted on the en-gine block. Therefore, an enen-gine block must be carefully cut, drilled, and machined so that it can hold all the engine components in the proper alignment. The engine block must also be able to maintain its
proper alignment when it’s exposed to strong physical forces. For ex-ample, the small explosions produced by the fuel burning inside the cylinders apply very high stresses to the engine block. In order for the engine to operate properly, the block must be able to maintain its alignment even when it’s exposed to these stresses.
The burning of the fuel inside the engine creates a lot of heat as well as a lot of force. Therefore, engine blocks must also be designed to with-stand a wide range of temperatures. Take a moment to think about all the different types of weather an automobile is driven in. Automobiles are driven on cold days when the temperature is below freezing, and on hot days when the temperature inside the engine can reach 200 de-grees Fahrenheit or more. The engine block must be strong enough to tolerate these temperature changes and the stresses they produce. The engine block also contains the engine’s cooling system and lubrica-tion system. The cooling system regulates the engine’s temperature, and the lubrication system circulates lubricating oil to all of the engine’s moving parts.
At this point, you may be wondering if all engine blocks are the same. Well, all engine blocks perform the same functions and are designed in a similar way. However, the exact shape, size, and features of an en-gine block vary depending on the enen-gine model and manufacturer. Automotive engine blocks are usually classified in the following two ways:
1. By the number of cylinders the block has
2. By the way the cylinders are arranged in the block
Most of us have heard an engine described by the number of cylinders it has. When you purchase a car, one of the features that’s often
pointed out to you is the number of cylinders in the engine. For exam-ple, a car may be described as having a six-cylinder engine or an eight-cylinder engine. The number of cylinders simply refers to the to-tal number of cylinder bores in the engine block.
The number of cylinders isn't the only factor that may differ between engine blocks—the size of the cylinders may also vary. Therefore, it’s possible for two different engines to contain the same number of cylin-ders, but have different sizes. Remember that the size of an engine is called the engine’s displacement. When you’re comparing two different engines, the engine with the larger displacement will generally be more powerful.
Engine blocks are often classified according to the arrangement of their cylinders. The three most common cylinder arrangements used today are the in-line arrangement, the V-type arrangement, and the horizontally
opposed arrangement. Let’s take a look at each of these types of engine
blocks.
An in-line engine (also called a straight engine) is an engine whose cyl-inders are arranged in a straight line. An in-line engine block is shown in Figure 9A. Note that all the cylinders in the engine block shown are arranged ina straight line. In-line engines may contain any number of cylinders; however, most automotive in-line engines contain either four or six cylinders.
In contrast to the in-line engine just described, the cylinders in a V-type engine are arranged in two separate rows, with an equal number of cylinders in each row. The two cylinder rows are placed together at an angle that resembles a letter V. A typical V-type engine block is shown in Figure 9B. Like in-line engines, V-type engines can have any
number of cylinders. However, most V-type engines contain six or eight cylinders. These engines are often called V-6 or V-8 engines. The final type of cylinder arrangement is the horizontally opposed engine or opposed engine. The cylinders in an opposed engine are ar-ranged so that they’re directly across from one another. In most op-posed engines, the cylinders are placed as shown in Figure 9C. These engines aren’t as common as in-line and V-type engines; however,
(A) CYLINDERS
CYLINDERS
(B)
(C)
FIGURE 9—An example of an in-line engine block is shown in Figure 9A. Figure 9B shows the arrangement of cylinders in a V-type engine. Figure 9C shows the arrangement of cylinders in an opposed engine.
opposed engines are used widely by manufacturers such as Volks-wagen and Subaru.
Now that you have a basic understanding of the engine block, let’s move on to other lower-end components.
The Piston
The piston is one of the hardest-working parts in an engine. Remember that the piston forms the bottom part of the combustion chamber where the fuel is burned. This means that a piston must be able to withstand the extreme heat produced by the burning air-and-fuel mixture as well as the physical forces produced during the power stage.
Each cylinder in an engine contains its own piston. The parts of a typi-cal automotive piston are shown in Figure 10A. As you can see, a pis-ton is shaped rather like a soup can. The pispis-tons used in modern automotive engines are usually made of aluminum.
Now, observe all the parts of the piston carefully. Let’s begin with the very top of the piston, which is called the piston head. Depending on the type of engine, a piston head may be perfectly flat, like the one shown in Figure 10B, or it may be slightly raised, like the one shown in Figure 10C. A piston with a flat head is called a flat-topped piston, while a piston with a raised head is called a domed piston. You may also see small indentations in the piston head. These indentations allow extra
FIGURE 10—The parts of a typical automotive piston are shown in Figure 10A. Some common piston head shapes are shown in Figure 10B and 10C.
piston-to-valve clearance for the engine valves so they won’t strike the piston head when the engine is running.
Note that the piston isn’t a completely solid metal “can.” The top half of the piston is solid metal, but the lower portion of the piston is hol-low inside. The sides of the piston that extend downward from the solid portion are called the piston skirts. The skirts prevent the piston from rocking in the cylinder as it moves up and down. If a piston didn't have a skirt, it would wobble from side to side inside the cylin-der instead of moving straight up and down. Also, note that the skirt doesn’t go all the way around the bottom of the piston. To save weight and allow clearance for the crankshaft, most pistons have skirts only on the sides that receive the most forces. The exact design of the skirt-ing varies from manufacturer to manufacturer, but most engine pis-tons have a similar shape.
All automotive pistons are fitted with steel rings called piston rings. The piston rings are placed around the outside of the piston, and fit into slots or grooves that are machined into the outside of the piston. These machined slots are called the piston ring grooves, and the small areas between the grooves are called the ring lands. The typical piston has three grooves, and therefore holds three rings. When the piston— with its rings installed—is placed in a cylinder, the piston rings seal the area between the cylinder wall and the side of the piston.
Also note that a piston has a hole machined through it. This hole, called the piston pin hole or the wrist pin hole, is used to connect the pis-ton to the connecting rod. A round, thick steel pin called a pispis-ton pin or a wrist pin is placed through the hole to connect the piston to the con-necting rod. When the piston and the concon-necting rod are joined to-gether, the piston fits over the top of the connecting rod. The piston pin fits through the hole in the piston, and then passes through the hole in the top end of the connecting rod. Several methods are used to keep the wrist pin from falling out while the engine is running. These will be discussed later in this unit.
Most pistons aren’t perfectly round, although they may appear to be round at first glance. Actually, most pistons are slightly oval in shape (Figure 11A). You can see in Figure 11A that an oval-shaped piston has different values for dimensions D1and D2.The third piston shown in
Figure 11A is a circle with the diameter, D1equal to D2. Pistons are
oval for two basic reasons. First, a piston comes into direct contact with the burning fuel in a cylinder, and the high combustion temperatures cause the piston to expand as an engine runs. Second, a great deal of stress is placed on a piston as it moves up and down in a cylinder. The heat and forces placed on a piston distort it as it moves up and down in the cylinder. The oval shape takes this distortion into account, so when an oval piston is distorted by heat and stress, it actually becomes almost perfectly round. So, to make the piston almost perfectly round when it gets hot, the piston is deliberately made out-of-round.
These oval-shaped pistons are often called cam ground. The smaller di-ameter of the oval is along the same line as the piston pin hole, while the larger diameter of the oval is directly opposite the piston pin hole. Most of the stress that’s placed on a piston during the power stroke is applied to the side of the piston that’s opposite from the piston pin. This area opposite the piston pin is called the thrust face of the piston. Because the thrust face receives most of the forces during the power stroke, this side of the piston is made wider.
Pistons aren’t only made slightly out-of-round, they’re also usually tapered slightly from top to bottom. That is, the diameter of the piston head is slightly smaller that the diameter of the piston skirt
(Figure 11B). Again, the piston is made with a taper because of heat ex-pansion. As the aluminum piston gets hot, it expands in the cylinder. However, the piston head is directly in contact with the burning air-and-fuel mixture, while the skirt is farther away from the heat source. As a result, the piston head will expand more than the piston skirt. To compensate for these differences in expansion, a piston will be made slightly larger at the bottom than at the top.
Now, let’s look at how the piston fits into the cylinder block. As stated earlier, each engine cylinder has its own separate piston. The piston fits
FIGURE 11—Most pistons are slightly oval in shape, as shown in Figure 11A. When an oval-shaped piston heats up during operation, it becomes almost perfectly round in shape. Pistons are also made with a slight taper. That is, the diameter of the piston head is slightly smaller than the diameter of the piston skirt, as shown in Figure 11B. Figure 11C shows the thrust face of the piston, located opposite the piston pin. The thrust face is pressed into the cylinder wall somewhat during the power stage.
into the cylinder as shown in Figure 12. Remember that the piston’s most important function is to transfer the force produced by the burn-ing air-and-fuel mixture into usable power. Durburn-ing the power stage, the piston is forced downward in the cylinder very quickly. As the pis-ton moves down, it transfers its force to the connecting rods, and then to the crankshaft. The piston also performs several other important func-tions during engine operation. For example, the piston helps to create a vacuum in the cylinder during the intake stage. It also compresses the air-and-fuel mixture during the compression stage, and forces the burned exhaust gases out of the cylinder during the exhaust stage.
The Piston Rings
In order for a piston to properly perform all of its functions, there must be a good seal between the side of the piston and the cylinder wall. If gases in the combustion chamber are allowed to leak past the piston, the engine will produce very little compression and very little power. Therefore, the piston rings are used to help form a seal between the piston and the cylinder walls. The piston rings are placed in grooves that are cut into the outside of the piston. As the piston moves up and down in the cylinder, the piston ring edges will slide along the cylin-der walls, which helps the piston form a tight seal against the walls. Note that a piston ring isn’t a completely solid ring; instead, it’s usu-ally split at one point. This split or gap in the ring is called the ring end
gap (Figure 13). The ring end gap allows the ring to expand and
con-tract as the temperature changes.
FIGURE 12—The piston fits into a cylinder as shown here.
Piston rings are made to be slightly larger in diameter than the cylinder. Since they’re larger in diameter, the piston ring ends must be pushed to-gether in order for the ring to fit into the cylinder. Once a ring is inside a cylinder, it will spring back out to return to its original size. Thus, the ring presses tightly outward against the cylinder wall. This spring pres-sure allows the piston rings to form a seal and prevent leakage between the rings and the cylinder wall (Figure 14).
FIGURE 13—A typical piston ring is shown here. This ring is made from layers of cast iron and an alloy called molybdenum. Note the position of the ring end gap. CYLINDER WALL OIL CONTROL RING COMPRESSION RINGS
FIGURE 14—Shown here is an automotive piston with its three rings installed on it. Note how the rings contact the cylinder walls to form a tight seal.
Since the alloy layer of the piston rings slide against a cast-iron cylin-der wall, you may woncylin-der about the problem of friction and wear. Well, over time, piston rings will wear out to a point where they can no longer seal the cylinder properly. Therefore, to help reduce the amount of piston ring wear, lubricating oil is splashed onto the cylin-der walls when the engine is running. This oil keeps the cylincylin-der walls well lubricated so the piston rings will slide along them easily. The oil reduces the amount of friction and wear on the rings, so piston rings usually last a very long time before they become too worn to seal properly.
The amount of time that piston rings last depends on the quality of the maintenance that's performed on the engine. Dirt is the greatest enemy of piston rings. Any dirt that’s allowed to get into the cylinder will wear out the rings faster. Therefore, it's very important to properly maintain an engine by changing the oil, oil filter, and air filter at regu-lar intervals. A well-maintained engine can usually run over 100,000 miles without much wear. However, an engine that isn't properly maintained will wear out much more quickly. Much of the wear that occurs will be in the piston rings.
Most automotive pistons contain three piston rings. The design of each ring varies slightly, and each has a specific purpose. The top two rings are called compression rings. The compression rings are used to prevent any gases from leaking out of the combustion chamber and past the rings. Since these two rings perform the same function, they’re usually similar in shape; in fact, in many engines, the two compression rings are identical. The third piston ring is an oil control ring. The oil control ring has a different function than the compression rings. The oil con-trol ring’s function is to prevent the oil on the cylinder walls from leak-ing past the rleak-ings and into the combustion chamber. To perform this function, the oil control ring scrapes excess oil down off the cylinder wall during each downward piston stroke.
Now, let’s examine the different types of piston rings in more detail.
Compression Rings
As we just mentioned, compression rings seal in the pressure created in the combustion chamber during the compression and power stages. During these stages, the pressure in the combustion chamber is quite high. If the pressure in the combustion chamber was allowed to leak out, the engine wouldn’t develop much power, and might actually fail to run. For this reason, the compression rings perform one of the most important functions inside the engine. Naturally, the piston rings can’t seal the cylinder completely; however, the amount of leakage must be kept to a minimum, or performance problems will occur. Leakage in only one cylinder is enough to cause a noticeable power loss in the engine.
Any compression gases that do find their way past the rings and into the engine block’s lower area are called blow-by gases. You may hear someone say that an engine “has by” or “has too much blow-by.” This means that the engine’s piston rings are worn or damaged in some way, allowing combustion gases to leak past the rings and into the engine block’s lower area. (The engine block’s lower area is called the crankcase, since this part of the engine block holds the crankshaft.) Compression rings are made in many different styles, depending on the engine make and model; however, their overall shapes are similar. Figure 15 shows some typical compression rings. Note that some rings have square edges, and some have beveled edges. Compression rings can also be made of different metals. Some rings are made of cast iron, but most are made of iron or steel alloys. Since the exact shape of the rings varies slightly depending on the engine make and model, when you're replacing rings, it's important to use the exact rings that were intended for the vehicle. Also, you must always make sure that com-pression rings are reinstalled in their correct positions.
Let’s take a closer look at exactly how compression rings seal the cylin-der. First, remember that spring tension in the rings keeps them
pressed out against the cylinder walls to form a seal. Compression rings must be flexible so that they can flex slightly without breaking. As the piston moves up and down in the cylinder, this flexibility tends to force one edge of the compression ring against the cylinder wall, thus forming a tighter seal.
VERTICAL GAP
DIAGONAL GAP
STEP GAP
FIGURE 15—Some typi-cal compression rings are shown here.
The compressed gases in the combustion chamber also help the rings form a seal. In most cases, when a ring is installed on a piston, a small space is left between the groove and the piston ring. When the engine is in its compression and power stages, the pressurized gases in the com-bustion chamber force their way into the small space behind the piston ring (Figure 16). This pressure behind the ring pushes the ring harder into the cylinder wall. This action helps the rings seal the cylinder better. In the other stages of the engine cycle, no pressure is created behind the ring, so the ring can easily glide along the lubricated cylinder wall.
Oil Control Rings
As you’ve learned, wear on the compression rings is reduced by the coating of oil on the cylinder walls. Well, the oil coating also assists in sealing the cylinder. Since oil is a thick liquid, it tends to stick to both the cylinder wall and the piston rings, which helps to seal the cylinder better.
However, what happens if there’s too much oil on the cylinder walls? The rings will slip and won’t be able to seal properly. Also, oil may leak past the rings and into the combustion chamber. Therefore, a spe-cial piston ring called an oil control ring is used to control this prob-lem. The oil control ring scrapes most of the oil off the cylinder wall
CYLINDER WALL COMBUSTION CHAMBER PISTON (A) RING DURING INTAKE STROKE (B) RING DURING COMPRESSION AND EXHAUST STROKE (C) RING DURING POWER STROKE PISTON RING
FIGURE 16—When the piston moves down during the intake stage, the piston rings scrape excess oil off the cylinder walls. During the compression and exhaust stages, the rings glide along the walls. During the power stage, the pressurized gases in the combustion chamber get into the small spaces behind the piston rings and help them form a tighter seal against the cylinder wall.
with each downward piston stroke. Naturally, the rings don’t scrape off all the oil—a film of oil is left on the wall to lubricate the compres-sion rings.
Oil control rings are designed differently from compression rings. A typical oil control ring consists of three separate pieces (Figure 17): two very thin metal rings called scrapers, and a spacer ring called an
ex-pander placed in the groove between them. The scraper rings are much
thinner than the compression rings, and a lot more flexible. As the pis-ton moves up and down in the cylinder, the scraper rings flex, which forces their edges into the cylinder wall. The edges of the scraper rings slide along the cylinder wall, scraping the majority of the oil off the cylinder wall, and leaving only a thin coating of oil behind. The ex-pander ring doesn’t contact the cylinder wall—it simply keeps the two scraper rings in their proper positions.
You may wonder what happens to the oil that’s scraped off the cylin-der wall. In most cases, the oil simply drops down into the crankcase. However, some oil will be caught between the two scraper rings. This oil then escapes through drilled holes near the oil control ring grooves. These holes allow any oil that’s caught between the two scraper rings to pass through the piston, and then drop down into the crankcase. As you can see, oil control rings are important because they keep the cylinder walls lubricated. In addition, they also help to keep the engine running properly. During the intake stage, the piston moves down-ward to create a vacuum in the combustion chamber. If the oil control rings didn't keep excess oil off the cylinder walls at that time, the ex-cess oil would be drawn up into the combustion chamber. In the cham-ber, the oil would be burned along with the air-and-fuel mixture. Burning oil will produce a blue-colored smoke from the exhaust pipe, and also cause the engine to consume an excessive amount of oil. In fact, if a large amount of oil enters the combustion chamber, it can coat the end of the spark plug and prevent it from igniting the air-and-fuel mixture properly. This would reduce the amount of power the engine produces.
SCRAPER EXPANDER
SCRAPER
FIGURE 17—A typical oil control ring consists of three separate pieces as shown.
Now that you understand the function of the piston and rings, let’s re-view how the piston works with the other engine components to pro-duce usable power in a vehicle.
The Movement of the Connecting Rod and Crankshaft
Remember that a piston is forced down in its cylinder by expanding gases in the combustion chamber. This downward movement of the piston is then converted to rotary motion by the crankshaft. In order for this to occur, the piston must somehow be connected to the crank-shaft. Figure 18 shows a typical piston-and-crankshaft system. Note that the pistons are connected to the crankshaft by the connecting rods. As the pistons move up and down in the cylinder, the connecting rods push up and down on the crankshaft to turn it.
The action of the piston and crankshaft is similar to the pedaling of a bicycle. When you’re on a bicycle, your upper body can be compared to a piston, your legs can be compared to the connecting rods, and the bicycle pedals can be compared to the crankshaft. As you pedal the bi-cycle, your legs move up and down to push on the pedals. As you push on the pedals, the pedal assembly rotates and moves the bicycle forward (Figure 19). The same action occurs in an automotive engine. As the pistons are forced downward, the connecting rods push on the crankshaft and cause the crankshaft to rotate, thus moving the vehicle. As the crankshaft rotates, the lower ends of the connecting rods move around with the crankshaft, and eventually return to their original po-sition. You may wonder how the pistons are moved back up in their cylinders. Well, in general, the crankshaft rotation itself is sufficient to force the pistons back up in their cylinders. During the power stage, the pistons move downward with so much force that the crankshaft is
rotated. This force is so great that the crankshaft will continue to rotate even after the pistons stop moving downward. Therefore, even if an engine has only one cylinder, enough rotational force is placed on the crankshaft to keep it turning so it pushes the piston back upward in the cylinder. You can observe a similar situation with a bicycle. If you pedal a bike fast and then take your feet off the pedals, the pedals will keep rotating due to the force that was created.
Naturally, automotive engines have more than one cylinder. The more cylinders the engine has, the more power it will produce. This can again be compared to the pedaling of a bicycle. When you pedal a bi-cycle with both legs, more power will be produced than if you pedal with just one leg. It’s also important to remember that power is applied unevenly in an engine. That is, the cylinders go through their power stages at different times. This is the same in a bicycle—first, you push one pedal down, then the other. This alternating between pedals is the same thing that occurs with the power stages in an engine. In an en-gine with several cylinders, the power stage of each cylinder occurs at a different time. This method provides smooth engine operation and helps to keep engine vibration to a minimum.
In a multicylinder engine, the process of moving the pistons back up in the cylinders is easier than in a single-cylinder engine, because all the pistons are connected to the same crankshaft. Therefore, since the power stages in all the cylinders occur at different times, an almost constant force is placed on the crankshaft. This constant force keeps the crankshaft rotating smoothly, and thus forces the pistons back up in the cylinders.
The Connecting Rod
As you’ve learned, the connecting rod connects the piston to the crank-shaft. The connecting rod must be strong and capable of withstanding a lot of force, because all the combustion forces placed on the piston will be transferred to the connecting rod. Almost all automotive con-necting rods today are made of cast iron, although some high-performance engines may contain steel, titanium, or even aluminum connecting rods. In addition, since the lower end of the connecting rod must rotate with the crankshaft, the connecting rod must be able to pivot at each end.
A view of all the parts of a connecting rod is shown in Figure 20. Note that most connecting rods are made in two pieces. The top end of the rod has a small hole machined through it. A round steel pin is placed through this hole to connect the rod to the piston. The opposite end of the rod connects to the crankshaft. That end of the rod is usually split into two halves as shown in the figure. The top half of the rod end fits over the crankshaft journal. The lower half of the rod end, called the
bearing cap, is then placed under the crankshaft journal and is fastened
with the two retaining bolts (Figure 21). These retaining bolts are called rod bolts. Once connected, the connecting rod can rotate freely on the crankshaft journal.
Remember that as the connecting rod rotates on the crankshaft journal, wear will rapidly occur unless something is placed between the parts to reduce the friction. Unlike piston rings, which have relatively small forces applied to them, connecting rods receive very high stresses. Therefore, a coating of oil isn’t enough to protect a connecting rod.
For this reason, bearings are used to reduce friction. A bearing is a com-ponent that helps reduce the friction between two metal parts. Many types of bearings are used in an engine. A bearing can be either a one-piece or two-one-piece type. The one-one-piece type of bearing is called a
bush-ing. In the connecting rod, a two-piece bearing called a shell-type bearing
or a bearing insert is used to reduce friction. In Figure 20, note the two thin, half-moon-shaped bearing inserts in the lower end of the connect-ing rod. The bearconnect-ing inserts are made of a softer metal than the con-necting rod, so they form a lining that helps to reduce wear on the connecting rod itself. The bearing inserts, along with proper oil lubri-cation, reduce the amount of friction between the connecting rod and the crankshaft journal while still allowing the rod to rotate freely. Now that you’ve seen how the connecting rod attaches to the crank-shaft, let’s take a closer look at how the other end of the rod attaches to the piston. Remember that the connecting rod is attached to the piston with a piston pin. Once in place, the connecting rod can pivot back and forth freely under the piston, similarly to the way your hand can pivot back and forth on your wrist. To prevent friction and wear, some type of bearing must be used on this end of the connecting rod. The type of bearing that’s used depends on the design of the rod.
In general, the connecting rod can be attached to the piston in one of three ways. Each of these ways uses a wrist pin—the difference is in how the two parts are connected. The first and most common method of connecting the piston to the connecting rod is shown in Figure 22. In this method, the piston is placed over the end of the connecting rod; then, the wrist pin is inserted through the hole in the piston and into the hole in the end of the rod. The diameter of the piston pin hole is slightly larger than the diameter of the piston pin, so the pin can rotate freely in the hole. Since the wrist pin is tightly press-fitted into the end of the connecting rod, the pin will move and pivot with the connecting
FIGURE 21—The piston and connecting rod are attached to a crankshaft journal as shown. The rod fits over the top of the rod jour-nal, and the cap fits up under the journal. The rod bolts are then used to fasten the cap to the rod.
rod. This tight fit prevents the wrist pin from falling out when the en-gine is running. In this method, the soft aluminum of the piston itself is used as the bearing—no additional bearing insert is necessary.
Now, let’s look at another method of connecting the piston and the connecting rod. In this second method, a thin bushing of soft material is inserted into the hole on the end of the connecting rod. The hole in the bushing is slightly larger in diameter than the wrist pin, which al-lows the wrist pin to rotate easily in the bushing. The diameter of the piston hole is slightly smaller than the wrist pin. To connect the two parts together, the piston is placed over the end of the connecting rod, and the wrist pin is then press-fitted into the piston hole and through the bushing in the end of the connecting rod. Since the wrist pin is tightly pressed into the piston hole, the pin moves and rotates with the piston rather than with the connecting rod, and the connecting rod ro-tates easily on the pin. Since the wrist pin is press-fitted into the piston, it’s held tightly in place and won’t fall out during engine operation. The third method of connecting the connecting rod and the piston is a combination of the two methods previously described. In this method, the piston is placed over the end of the connecting rod, and the piston pin is inserted through the holes in the piston and the rod end. Because the diameter of the pin is smaller than both the piston hole and the bushing, the pin slides easily through the holes. To hold the pin in place, small wire retaining clips are placed at each end of the wrist pin. The retaining clips fit into small grooves that are machined into the two ends of the piston hole. Once in place, the clips prevent the pin from falling out. In this method, the wrist pin can rotate freely in both the piston and the end of the connecting rod.
As you can see, several different methods can be used to attach a con-necting rod to a piston. However, no matter what method is used, the overall function is the same—to attach the piston securely to the con-necting rod while still allowing the rod to pivot back and forth.
FIGURE 22—In the most common method of connecting the piston to the connecting rod, the piston is placed over the end of the connecting rod. The wrist pin is then inserted thorough the hole in the piston and pressed into the hole in the end of the rod.
The Crankshaft
The crankshaft is the main rotating part in the engine. The crankshaft’s function is to convert the up-and-down motion of the piston into ro-tary motion that can be used to power the vehicle. A crankshaft is mounted in an engine’s crankcase (the lower area of the engine block). Bearing inserts are used to allow it to rotate freely. The connecting rods are attached to the crankshaft, and when the force from the pistons is transmitted to the crankshaft, it makes the crankshaft rotate.
Now, let’s take a look at the parts of a typical crankshaft. The exact crankshaft design depends on the engine’s make and model; however, all crankshafts have the same basic parts. A typical automotive crank-shaft is shown in Figure 23. The most important parts of the crankcrank-shaft are the crankshaft journals. There are two types of journals on a crank-shaft—the rod journals and the main journals. The connecting rods are attached to the rod journals. The main journals are used to mount the crankshaft to the engine block. Each journal has a wide, smooth sur-face that’s machined to the exact size to hold a bearing. The journals are all highly polished and are machined to be perfectly round so that they’ll rotate easily inside the bearings.
Since the crankshaft must be able to rotate within the engine block, the main journals are placed directly in line with the crankshaft’s center-line. In contrast, the rod journals are offset slightly from the crank-shaft’s centerline. This offset placement provides the means to convert the rod’s up-and-down motion into rotary motion. Just as the pedals of a bicycle are placed away from the centerline, the connecting rods are mounted so that they’re offset slightly from the crankshaft’s center. The distance the rod journals are offset from the centerline is called the
crankshaft throw.
The rod journals are not only spaced away from the crankshaft’s cen-terline—they’re also spaced apart from one another. This staggering of the rod journals is the feature that allows each cylinder to reach its
power stage at a different time. In almost all engines, the rod journals are spaced evenly around the crankshaft.
Figure 24 contains a simplified drawing of the parts of a typical crank-shaft in a four-cylinder engine. Notice that the rod journals on this crankshaft are placed directly opposite one another. When one piston is at TDC, another is at BDC. Remember that the power stage in each cylinder occurs once for every two rotations of the crankshaft. Since the crankshaft’s rod journals are spaced apart from one another, each cylinder will be at a different point in the engine cycle at any given time. For example, Cylinder 1 could be performing its power stage, while Cylinder 4 is on its intake stage. The crankshaft design, espe-cially the rod journal spacing, makes this possible.
To get a better understanding of just how this works, let’s look at the crankshaft of a typical four-cylinder, in-line engine as it performs two complete rotations. In Figure 25, the crankshaft is shown in its “start-ing” position. At this time, because of the crankshaft design, you can see that two pistons are at TDC and two pistons are at BDC. Let’s as-sume that Cylinder 1 is now in its power stage, and Piston 1 is being forced downward.
Now, let’s look at what’s occurring in the other cylinders at this same time. Piston 2 is now moving up, and Cylinder 2 is currently in its ex-haust stage. Piston 3 is also moving up, but Cylinder 3 is now in its compression stage. Finally, Piston 4 is moving down, and Cylinder 4 is in its intake stage.
Next, let’s rotate the crankshaft one-half turn. The crankshaft in this new position is shown in Figure 26. At this time, you can see that Cylinder 1 has completed its power stage. Piston 1 is moving up, and Cylinder 1 is entering its exhaust stage. Cylinder 2 has completed its exhaust stage, and Piston 2 is moving down in the cylinder to perform the intake stage. Cylinder 3 has completed its compression stage, the spark plug has ignited the fuel mixture, and Piston 3 is moving down to start its power stage. Finally, Cylinder 4 has completed its intake stage, and Piston 4 is moving up to perform the compression stage.
FRONT END
THROW
ROD BEARING JOURNAL MAIN BEARING JOURNAL
ROD BEARING JOURNAL
MAIN BEARING JOURNAL FLYWHEEL END
THROW
MAIN BEARING JOURNAL
Now, let’s rotate the engine another one-half turn. This new crankshaft position is illustrated in Figure 27. At this point, we’ve rotated the en-gine a total of one complete turn from where we started. Notice that the cylinders appear to be in the same position as when we started. That is true; however, the stages that each cylinder is currently under-going are quite different. For example, Piston 1 is beginning to move down again; however, this time it’s performing the intake stage in Cylinder 1. Piston 2 is moving up, performing the compression stage in Cylinder 2. Cylinder 3 has completed its power stage, and Piston 3 is moving back up to perform its exhaust stage. Finally, Cylinder 4 has completed its compression stage, the spark plug has ignited the fuel mixture, and Piston 4 is moving down to perform its power stage. Next, let’s rotate the engine another one-half turn. This will make a to-tal of 112revolutions from where we started in Figure 25. This new
crankshaft position is shown in Figure 28. In this position, Cylinder 1 has completed its intake stage, and Piston 1 is moving upward to per-form its compression stage. Cylinder 2 has completed its compression stage, the spark plug has ignited the mixture, and Piston 2 is now mov-ing down to perform its power stage. Cylinder 3 has competed its ex-haust stage, and Piston 3 is moving down to perform its intake stage.
PISTON 1 CYLINDER 1
POWER
PISTON 2 CYLINDER 2
EXHAUST COMPRESSION INTAKE
PISTON 3 CYLINDER 3
PISTON 4 CYLINDER 4
FIGURE 25—At the beginning of the crankshaft rotation, two pistons are at TDC and two pistons are at BDC. Piston 1 is moving down, and Cylinder 1 is in its power stage. Piston 2 is moving up, and Cylinder 2 is in its exhaust stage. Piston 3 is moving up, and Cylinder 3 is in its compression stage. Piston 4 is moving down, and Cylinder 4 is in its intake stage.
Finally, Cylinder 4 has completed its power stage, and Piston 4 is mov-ing up to perform the exhaust stage.
Now, let’s rotate the crankshaft an additional one-half turn as shown in Figure 29. This makes a total of two complete crankshaft revolu-tions, and as you can see, all the cylinders are back to the stages they started from in Figure 25. Cylinder 1 is in its power stage, Cylinder 2 is in its exhaust stage, Cylinder 3 is in its compression stage, and Cylin-der 4 is in its intake stage.
Notice that in the two complete crankshaft revolutions we observed, each separate cylinder completed all four engine cycles in the proper order (intake, compression, power, exhaust). However, each cylinder completed its stages at a different time from the other three cylinders. That is, each cylinder completed its power stage at a different time. En-gine cylinders are designed to work in this way in order to keep the physical forces in the engine balanced. If all the cylinders entered their power stages at the same time, the force of all four power stages com-bined would throw the engine off balance and probably damage it.
POWER INTAKE PISTON 1 CYLINDER 1 PISTON 2 CYLINDER 2 COMPRESSION PISTON 3 CYLINDER 3 PISTON 4 CYLINDER 4 EXHAUST
FIGURE 26—In this illustration, the crankshaft has rotated one-half turn from its previous position in Figure 25. The power stage in Cylinder 1 has now been completed. Piston 1 is now coming up, and Cylinder 1 is enter-ing its exhaust stage. The exhaust stage has been completed in Cylinder 2, and Piston 2 is moventer-ing down to perform its intake stage. The compression stage has been completed in Cylinder 3, and Piston 3 is moving down to perform its power stage. The intake stage has been completed in Cylinder 4, and Piston 4 is now moving up to perform its compression stage.
The order in which an engine’s cylinders complete their power stages is called the firing order for the engine. In our example, Cylinder 1 com-pleted its power stage first, followed by Cylinder 3, Cylinder 4, and finally, Cylinder 2. This means that the firing order for this particular engine is 1-3-4-2. You’ll learn about the firing order of engines more in a later study unit. The important point to remember for now is that the crankshaft's journals are spaced on the crankshaft in order to allow each cylinder to complete all four stages of the engine cycle within two crankshaft revolutions. This applies to any type of automotive engine. A V-type engine operates in the same manner. Each cylinder has its power stage at a different time, and all cylinders complete the four stages within two crankshaft revolutions.
The appearance of an engine’s crankshaft depends on the number of cylinders the engine has. For example, a six-cylinder, in-line engine has six crankshaft rod journals spaced equally on the crankshaft (Figure 30). Another example is the V-8 crankshaft shown in Figure 31. Note that this crankshaft looks different from the in-line crankshaft. In a V-8 en-gine, when the rod journals are spread evenly around the crankshaft, some of the rod journals end up in the same positions. Therefore,
POWER COMPRESSION EXHAUST INTAKE PISTON 1 CYLINDER 1 PISTON 2 CYLINDER 2 PISTON 3 CYLINDER 3 PISTON 4 CYLINDER 4
FIGURE 27—In this illustration, the crankshaft has rotated another one-half turn. The exhaust stage in Cylin-der 1 has now been completed, and Piston 1 is now moving down to perform its intake stage. The intake stage has been completed in Cylinder 2, and Piston 2 is moving up to perform its compression stage. The power stage has been completed in Cylinder 3, and Piston 3 is moving up to perform its exhaust stage. The compression stage has been completed in Cylinder 4, and Piston 4 is now moving down to perform its power stage.
instead of having eight separate rod journals, the crankshaft simply uses four extra-wide rod journals. These journals are wide enough to allow two connecting rods to be attached to each journal. Therefore, even though the crankshaft has only four rod journals, eight connect-ing rods (one for each cylinder) can be attached to the crankshaft. Because the crankshaft must be able to withstand the tremendous forces applied during the engine’s power stages, it must be very strong. A typical automotive crankshaft is usually made of cast iron or forged steel, and is usually fairly heavy. In addition, because a crank-shaft rotates very fast when it’s operating (sometimes thousands of times per minute), the crankshaft must be balanced so that it won’t vi-brate. Most crankshafts contain built-in weights called counterweights to help maintain their balance during engine operation. These weights compensate for the weight of the pistons and connecting rods that are placed on the crankshaft, keeping it perfectly balanced.
The crankshaft also contains a stub on its front end that’s used for mounting other engine components, and a flange on its rear end that's used to hold the attachment components that connect the engine to the transmission. EXHAUST POWER INTAKE COMPRESSION PISTON 1 CYLINDER 1 PISTON 2 CYLINDER 2 PISTON 3 CYLINDER 3 PISTON 4 CYLINDER 4
FIGURE 28—In this illustration, the crankshaft has rotated another one-half turn. The intake stage in Cylinder 1 has now been completed, and Piston 1 is moving up to perform its compression stage. The compression stage has been completed in Cylinder 2, and Piston 2 is moving down to perform its power stage. The ex-haust stage has been completed in Cylinder 3, and Piston 3 is moving down to perform its intake stage. The power stage has been completed in Cylinder 4, and Piston 4 is now moving up to perform its exhaust stage.
POWER EXHAUST COMPRESSION INTAKE PISTON 1 CYLINDER 1 PISTON 2 CYLINDER 2 PISTON 3 CYLINDER 3 PISTON 4 CYLINDER 4
FIGURE 29—In this illustration, the crankshaft has rotated another one-half turn. At this point, we’ve rotated the crankshaft two complete turns, and are back to the position we started at in Figure 25. Cylinder 1 is in its power stage, Cylinder 2 is in its exhaust stage, Cylinder 3 is in its compression stage, and Cylinder 4 is in its intake stage.
FIGURE 30—A six-cylinder, in-line engine crankshaft will contain six rod jour-nals spaced equally along the crankshaft.
FIGURE 31—This V-8 crank-shaft has only four rod journals. However, each rod journal is designed double-wide to hold two connecting rods. Thus, the crankshaft will be turned by a total of eight rods.
As you learned earlier, the crankshaft is mounted in the crankcase, and soft bearing inserts are used to help reduce friction. To further reduce friction, a film of oil is applied between each bearing and the crank-shaft. Because the space between the bearing and the crankshaft journal (called the bearing clearance) is so small, it isn't possible to splash oil onto the surface. Instead, a small pump is used to pump pressurized oil to the bearing inserts. This oil enters the bearing clear-ance area through small passages that are drilled inside the crankshaft. Figure 32 shows the oil passages of a typical crankshaft. Notice how the passages deliver oil to both the main bearing journals and the con-necting rod journals.
The bearing clearance is small enough to prevent the oil from flowing out easily, therefore maintaining lubrication. In fact, the bearing clear-ance is so small that the oil pressure is enough to keep the space filled with oil. Thus, when the engine operates, the crankshaft journal and the bearing insert rarely touch each other. Instead they ride on a thin film of oil. This allows the components to operate over many miles with very little wear to the bearing surface.
Now, let’s take a closer look at how the crankshaft is mounted inside the engine block. Remember that the crankshaft is mounted in the crankcase in the same way as the connecting rods. Special crankshaft bearing supports are machined into the bottom of the engine block (Figure 33). Two-piece bearing supports are used to reduce the amount of friction. The upper half of the bearing insert is placed onto the en-gine block’s bearing support. Then, the crankshaft is placed into the engine block on the bearing supports and inserts. The crankshaft is then secured with bearing caps, called main bearing caps. The main bearing caps contain the other halves of the bearing inserts. The main bearing caps are held onto the engine block using bolts threaded into the block. An exploded view of a typical crankshaft installation is shown in Figure 34. Once the crankshaft is installed in the block, it can be rotated easily due to the bearing inserts that are used in the supports.
Usually, two bolts are used to hold each bearing cap to the engine block and to hold the crankshaft in the block. However, in some situations, four bolts are used to retain each bearing cap. This type of bearing cap is usually found in high-performance engines that need the strength provided by the extra bolts. When four bolts are used to retain each main bearing cap, the engine is said to have a four-bolt main engine block. The number of main bearings and supports an engine has depends on the design and size of the engine. Most four-cylinder engines have four main bearing supports, while larger six-cylinder and eight-cylinder engines may have five, six, or seven supports. An example of this is shown earlier in Figure 30. The crankshaft shown in Figure 30 would be found in an in-line, six-cylinder engine with a total of five main bearing supports for the crankshaft. Since this is a six-cylinder, in-line
FIGURE 33—Special crankshaft bearing supports are machined into the bottom of the engine block as shown in Figure 33A. Figure 33B shows a closer view of the bearing caps and bolts that hold the crankshaft in the block.
engine, its crankshaft is quite a bit longer than the crankshaft found in a four-cylinder or V-type engine. Therefore, more crankshaft supports are required.
The Engine Bearings
As you learned earlier, the bearings used with a typical connecting rod are two-piece inserts. Once installed, the two-bearing inserts com-pletely surround the crankshaft journal, providing a good support surface all the way around the journal. The same type of two-piece bearings are used to support the crankshaft in the engine block. These crankshaft support bearings are called main bearings. The upper half of the main bearing insert fits into the engine block, and the lower half fits into the bearing cap.
The main bearing inserts are made of a softer metal than the crank-shaft. The bearing insert is actually made up of layers of different ma-terial. However, most engine bearings will contain a layer of copper material covered with a layer of soft material called babbitt.
Bearing inserts offer several important benefits. Even under ideal con-ditions, the engine bearings experience some wear. Although a supply of oil is pumped into the bearing clearance, the metal of the bearing sometimes comes into direct contact with the metal of the crankshaft journal, especially during starting. Over time, the bearings will wear so much that they won’t be able to properly support the crankshaft and reduce the friction. If not corrected, this excessive bearing wear will eventually cause serious damage to the engine. The main benefit to us-ing a bearus-ing insert is that if wear does occur, only the bearus-ing insert
FIGURE 34—This illustra-tion shows an ex-ploded view of a crankshaft installation. Note the location of the main bearing caps and the bearing inserts.
