Study Unit
Small Engine Parts
and Operation
In this study unit, you’ll learn about the construction of small gasoline engines. The first section of your text is a review of basic engine operation. Next, you’ll learn about all the important engine components and their functions in engine operation. Most of these basic components are found in both four-stroke engines and two-stroke engines. Then, you’ll learn a bit about the operation of two-stroke engines and diesel engines.
Pr e view Pr e view
When you complete this study unit, you’ll be able to
• Explain how the basic operation of the two-stroke engine and the four-stroke engine differs
• Describe how fuel is used to create energy to make an engine operate
• Visually identify the external components of a small engine, including the muffler, air cleaner, carburetor, fuel tank, spark plug wire, cylinder head, and blower housing
• Visually identify the internal components of a small engine, including the piston, connecting rod, crankshaft, camshaft, and valves
• Describe the basic operation of carburetors, ignition sys- tems, and starting systems
• Explain how a transmission can be used to increase the output horsepower and torque of an engine
• Describe the basic operation of a diesel engine and explain how it differs from a gasoline engine
Remember to regularly check your student portal. Your instructor may post additional resources that you can access to enhance your learn- ing experience.
INTRODUCTION 1
The Four Stages of Engine Operation 2 Two-Stroke Engines and Four-Stroke Engines 4 Basic Four-Stroke Engine Operation 5
Basic Two-Stroke Engine Operation 9
Engine Power and Speed 13
BASIC ENGINE COMPONENTS 18
The Blower Housing 18
The Cylinder Block 21
The Cylinder 21
The Cylinder Head 23
The Crankcase 25
The Piston 26
The Connecting Rod 27
The Crankshaft 28
The Bearings 31
The Flywheel 34
The Camshaft and the Timing Gears 35
The Valves 38
The Muffler 43
ENGINE SYSTEMS 45
The Ignition System 45
The Starter System 50
The Fuel System 52
The Speed Control System 56
The Lubrication System 58
The Transmission System 59
TWO-STROKE ENGINES 63
Piston-Port Engines 64
Reed-Valve Engines 68
Rotary Valve Engines 69
DIESEL ENGINES 70
Types of Diesel Engines 71
Diesel Engine Components 72
POWER CHECK ANSWERS 77
Contents Contents
INTRODUCTION
The small engines used in lawn mowers, garden tractors, chain saws, and other such machines are called internal com- bustion engines. In an internal combustion engine, fuel is burned inside the engine to produce power. The internal com- bustion engine produces mechanical energy directly by
burning fuel.
In contrast, in an external combustion engine, fuel is burned outside the engine. A steam engine and boiler is an example of an external combustion engine. The boiler burns fuel to produce steam, and the steam is used to power the engine.
An external combustion engine, therefore, gets its power indi- rectly from a burning fuel.
In this course, you’ll only be learning about small internal combustion engines. A “small engine” is generally defined as an engine that produces less than 25 horsepower.
In this study unit, we’ll look at the parts of a small gasoline engine and learn how these parts contribute to overall engine operation. A small engine is a lot simpler in design and func- tion than the larger automobile engine. However, there are still a number of parts and systems that you must know about in order to understand how a small engine works.
The most important things to remember are the four stages of engine operation. Memorize these four stages well, and every- thing else we talk about will fall right into place. Therefore, because the four stages of operation are so important, we’ll start our discussion with a quick review of them. We’ll also talk about the parts of an engine and how they fit into the
Small Engine Parts and
Operation
four stages of operation. Note that this will just be a simple overview. Later in the study unit, we’ll talk about the parts of an engine and their operation in much more detail.
The Four Stages of Engine Operation
Let’s start by discussing some basic ideas we covered earlier in the course. These topics are so important that it won’t hurt to review them. First, you know that the main moving part in a small engine is the piston. The piston is a round, can-shaped metal device that fits into a round metal opening called the cylinder. The piston can move up and down inside the cylinder. The top of the cylinder is sealed by the cylinder head. The other end of the piston is connected to a rod and crankshaft assembly.
Remember that when a piston is at its lowest position in the cylinder, it’s said to be at bottom dead center (BDC). When the piston is at its highest position in the cylinder, it’s said to be at top dead center (TDC).
Inside the cylinder, the space above the piston is called the combustion chamber. In the combustion chamber, a mixture of air and fuel is burned to produce power. When the air-and- fuel mixture burns in the combustion chamber, it actually produces a small, contained explosion. This explosion is strong enough to force the piston downward in the cylinder.
When the piston is forced downward in the cylinder, the pis- ton’s downward motion is transferred to the rod and
crankshaft. The rod and crankshaft then convert the up-and- down motion of the piston into rotary motion (circular
motion). A simplified drawing of the cylinder, piston, and crankshaft is shown in Figure 1.
This conversion of up-and-down motion to rotary motion can be compared to the motion produced by a regular bicycle.
When you pedal a bike, you push down on the pedals with your feet. The downward motion of your feet on the pedals is converted into circular motion in the rear wheel of the bike.
The same principle applies to an engine.
The downward motion of the piston is converted to circular
motion that can be used to power a piece of equipment.
In order to work, all gasoline engines must do four basic actions. An engine must
1. Take in fuel
2. Squeeze or compress the fuel 3. Ignite and burn the fuel 4. Get rid of the burned gases
The engine actions we’ve just described are the four stages of engine operation. The proper names for these stages are intake, compression, power, and exhaust. When an engine is operating, it continually runs through these four stages, over and over again.
Stage 1: In the intake stage, air that has been mixed with fuel is drawn into the cylinder.
Stage 2: In the compression stage, the piston rises, compress- ing the air-and-fuel mixture trapped in the combustion
chamber.
FIGURE 1—A simplified drawing of the cylinder, piston, and crankshaft is shown here.
Stage 3: During the power stage, the air-and-fuel mixture is ignited, and the contained explosion of the fuel presses the piston back down in the cylinder. The downward motion of the piston is transferred to the rod and crankshaft.
Stage 4: During the exhaust stage, the exhaust gases are released from the cylinder. The four stages then begin all over again.
One engine cycle is a complete “run” through all four stages of operation. Note that the four stages of operation that we’ve described occur very quickly, and they repeat continuously for as long as the engine is running.
Now that you understand the basics of engine operation, let’s look at a comparison of the two-stroke engine and the four- stroke engine.
Two-Stroke Engines and Four-Stroke Engines
Two-stroke engines and four-stroke engines are both used in outdoor power equipment applications. However, the internal operation of the two types of engines are different. The basic difference between two-stroke engines and four-stroke
engines is the way in which they run through the four stages of operation. (When we’re talking about engines, remember that the stroke of an engine is the distance that the piston travels up and down in a cylinder.)
In order for any engine to operate, it must run through all four stages of operation. In a four-stroke engine, four strokes of the piston are required to complete the four stages of oper- ation, as follows:
Stroke 1: Intake stage
Stroke 2: Compression stage Stroke 3: Power stage
Stroke 4: Exhaust stage
In contrast, in a two-stroke engine, only two strokes of the piston are needed to complete the four stages of operation:
Stroke 1: Intake stage/Compression stage
So, in a two-stroke engine, the intake and compression stages are completed by one stroke of the piston. The power and exhaust stages are completed by the second stroke of the piston.
Basic Four-Stroke Engine Operation
Let’s take a closer look at the operation of the four-stroke engine. A simplified drawing of a four-stroke engine is shown in Figure 2. Note the position of the piston, the cylinder, and the crankshaft. The crankshaft is connected to the piston by a connecting rod. The spark plug is positioned at the top of the engine over the combustion chamber.
The four-stroke engine contains two mechanical valves: the intake valve and the exhaust valve. These valves lift up and down to open and close during engine operation. The intake
FIGURE 2—The parts of the four-stroke engine are clearly labeled for you in this illustration. During the intake stage, the piston lowers to suck the air-and- fuel mixture into the cylinder. The intake valve is open and the exhaust valve is closed.
valve opens to allow the air-and-fuel mixture to flow into the combustion chamber, and the exhaust valve opens to allow exhaust gases to flow out of the engine after the fuel is burned.
The intake and exhaust valves are mechanically lifted to make them open and close. The valves are lifted by valve lifters that rest on the lobes of the camshaft. As the camshaft turns, the lobes lift the valves in a timed sequence to match up properly with the up-and-down motion of the piston.
In order to burn properly in an engine, fuel must be mixed with air. The part that does this is the carburetor. Fuel moves from the fuel tank into the carburetor where it’s vaporized and mixed with air. The air is taken in through an air intake port. The air-and-fuel mixture is then transferred out of the carburetor and into the cylinder through the intake valve.
A four-stroke engine completes the four stages of engine operation in four piston strokes. During the intake stage, the intake valve opens and the piston moves down in the cylin- der. As the piston moves down, a vacuum is created that sucks the air-and-fuel mixture into the cylinder through the intake valve. The intake valve is held open at this time by one of the lobes on the camshaft. The exhaust valve is closed.
Figure 2 illustrates the intake stage.
When the piston reaches bottom dead center, both valves are closed. The air-and-fuel mixture is now trapped inside the sealed combustion chamber. At this point, the piston begins to rise, compressing the air-and-fuel mixture tightly. This is the compression stage, which is illustrated in Figure 3.
The piston rises until it reaches top dead center. At that moment, the engine’s ignition system is timed to make the spark plug “fire.” That is, the ignition system produces elec- tric current that causes a spark to jump across the two electrodes of the spark plug. Naturally, when a spark is applied to a compressed mixture of fuel and air, an explosion occurs and the fuel mixture is burned.
When gases explode, they expand rapidly. The force of this contained explosion forces the piston down in the cylinder.
Since the piston is connected to the crankshaft through the
connecting rod, the piston’s downward movement causes the crankshaft to turn around. This is the power stage, which is illustrated in Figure 4.
As the piston moves downward during the power stage, the exhaust valve opens. By the time the piston reaches bottom dead center, the exhaust valve is completely open. As the pis- ton rises back up, it pushes the burned gases out of the exhaust valve. The exhaust gases then pass out of the engine. This is the exhaust stage, which is illustrated in Figure 5.
Once the exhaust stage is completed, the four stages of oper- ation begin all over again. The movement of the camshaft closes the exhaust valve and opens the intake valve, and the piston moves down to begin a new intake stage.
FIGURE 3—In the compres- sion stage, both valves are closed. As the piston rises, it compresses the air-and- fuel mixture in the sealed combustion chamber.
FIGURE 4—When the piston reaches top dead center, 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.
FIGURE 5—When the piston moves down during the power stage, the exhaust valve opens. The piston then rises again and pushes the burned gases out of the exhaust valve.
Basic Two-Stroke Engine Operation
Although a two-stroke engine has many of the same compo- nents as a four-stroke engine, its operation is very different.
In a two-stroke engine, one power stroke occurs for each rotation of the crankshaft. (In contrast, you’ll remember that in a four-stroke motor, one power stroke occurs for every two rotations of the crankshaft.)
So, remember that the two-stroke engine must go through the same four stages of engine operation—intake, compres- sion, power, and exhaust—that a four-stroke engine does.
However, the two-stroke engine goes through all four stages in just two piston strokes. Each time the piston moves up, it completes the intake and compression stages. Each time the piston moves down, it completes the power and exhaust stages.
Two-stroke engines are much simpler in design than four- stroke engines. The simplest two-stroke engine has only three moving parts:[ensp ]the piston, the connecting rod, and the crankshaft. Two-stroke engines don’t use the same type of mechanical valves in the combustion chamber that a four- stroke engine does. Instead, the two-stroke engine has holes in the cylinder wall called ports. As the piston slides up and down in the cylinder, it covers and uncovers the ports, allow- ing the air-and-fuel mixture into the cylinder and forcing the exhaust gases out.
The intake/compression stage of a two-stroke engine is illus- trated in Figure 6. During the intake/compression stage, the piston begins to move up in the cylinder, creating a vacuum in the crankcase below the piston. The vacuum sucks in fuel and air through the carburetor to fill the crankcase. Note the small reed valve that’s positioned at the point where the air- and-fuel mixture enters the crankcase. This valve is a small piece of fiberglass or spring steel that bends back to allow fuel to enter the crankcase.
During this stage, note that the piston covers both the intake
port and the exhaust port. Some air-and-fuel mixture is
already in the combustion chamber. As the piston rises, it
compresses this air-and-fuel mixture that’s already in the
combustion chamber. (In the figure, observe the positions of the intake and exhaust ports—they’re not directly across from each other. This allows the piston to cover one of the ports at a time.)
The power/exhaust stage of the two-stroke engine is illus- trated in Figure 7. During this stage, the spark plug fires and the air-and-fuel mixture burns. The expanding gases pro- duced by the burning air-and-fuel mixture force the piston down in the cylinder. As the piston begins to move down in the cylinder, note that the two ports are still closed.
Because the two ports are still closed, the downward motion of the piston creates pressure that presses down on the air- and-fuel mixture in the crankcase. At this time, the reed valve in the crankcase is also closed because the air-and-fuel mixture is pressing down on it.
FIGURE 6—The intake/com- pression stage of a
two-stroke engine is shown here. The piston rises, cre- ating a vacuum in the crankcase. Air and fuel are sucked into the crankcase through the reed valve. At the same time, the air-and- fuel mixture that’s already in the combustion chamber is compressed. Both the intake port and the exhaust port are closed. The reed valve is open.
The end of the power/exhaust stage is shown in Figure 8. As the piston reaches bottom dead center, the two ports are opened. The exhaust gases are expelled out of the engine, and at the same time, the air-and-fuel mixture from the crankcase flows into the combustion chamber through the intake port. This occurs because the piston is pressing down on the air-and-fuel mixture in the crankcase and forcing it into the transfer tube. The transfer tube is simply a tube that connects the crankcase to the intake port. The air-and-fuel mixture being forced into the combustion chamber helps force the exhaust gases out. The intake/compression stage then begins again.
Note that the four stages of operation in a two-stroke engine overlap each other a bit. In the intake/compression stage of the two-stroke engine, air-and-fuel mixture is sucked into the crankcase at the same time that the piston is compressing the air-and-fuel mixture in the combustion chamber.
FIGURE 7—The beginning of the power/exhaust stage of the engine is shown here.
Both ports are still closed.
The reed valve is also closed during this stage.
The downward motion of the piston forms a vacuum that presses down on the air-and-fuel mixture in the crankcase.
During the power/exhaust stage, the air-and-fuel mixture flows into the combustion chamber at the same time that the exhaust gases are escaping. In contrast, in the four-stroke engine, these four stages are more clearly separated from each other.
The term “intake” also has a slightly different meaning in the two-stroke engine than in a four-stroke engine. In the two- stroke engine, air-and-fuel mixture is taken into the
crankcase during the intake stage. In contrast, in a four- stroke engine, the air-and-fuel mixture enters the cylinder during the intake stage.
Two-stroke engines are usually simple in construction and they operate efficiently. However, these engines do have some disadvantages. First, a two-stroke engine must rotate faster to produce the same power output as a four-stroke engine.
This faster rotation causes quicker wear of the engine’s mov-
FIGURE 8—The end of the power/compression stroke is shown here. The exhaust gases escape through the exhaust port, and at the same time, more air-and- fuel mixture enters the combustion chamber through the intake port.
ing parts. Second, in a two-stroke engine, the lubricating oil must be mixed with the air-and-fuel mixture. Thus, the oil burns along with the air-and-fuel mixture, causing pollution.
In contrast, the crankcase of a four-stroke motor is filled with oil that’s not burned, but simply splashed on the internal engine parts to lubricate them.
Engine Power and Speed
Let’s take a moment now to review the factors that determine an engine’s power and speed. The intake, compression, power, and exhaust stages of operation cause an engine to rotate at a certain speed. The speed of operation depends on the amount of air-and-fuel mixture that’s allowed to enter the cylinder.
The more air-and-fuel mixture that reaches the cylinder, the faster the engine’s crankshaft will rotate. The faster the crankshaft rotates, the more air that will be drawn into the engine. If the amount of air-and-fuel mixture that reaches the cylinder is reduced, the crankshaft will turn slower.
When an engine is at its lowest operating speed, the engine is said to be at idle speed. At idle speed, an engine produces very little power. However, once the speed of the engine increases, the power of the output shaft of the motor also increases. In outdoor power equipment, this power increase is used to turn a lawn mower blade, operate a chain saw, or perform other such work.
The speed at which an engine operates is measured in units called revolutions per minute (rpm). The rpm is a measure of how fast the crankshaft is turning, that is, how many com- plete turns the crankshaft can make in one minute. At idle speed, a typical motor revolves at approximately 700 rpm. As the flow of air and fuel increases at the intake port of the engine, the engine rotates at a much higher rpm until it reaches the maximum value set by its manufacturer.
Depending on the engine model, the maximum rpm value is
between approximately 3,000 rpm and 7,000 rpm. Note that
if an engine is modified to run faster than its rated rpm, the
engine may be destroyed by the large reciprocating and cen-
trifugal forces present within its components.
Horsepower
Most small engines are rated in units called horsepower (hp).
One horsepower is therefore equal to 550 foot-pounds of work per second. Today, almost all small engines are identified by their horsepower output. For example, many lawn mowers contain engines with ratings between 3 hp and 5 hp. Garden tractors are available with engines rated at 9 hp, 10 hp, 12 hp, or more. The higher the horsepower of the engine, the
stronger the engine is and the more work it can perform.
Remember that the maximum power output of an engine is the brake horsepower (bhp). You’ll usually see the specifica- tions for an engine given in units of bhp. In practical use, an engine that was always run at maximum would have a very short life span. For this reason, engine specifications are also given in units called rated horsepower. The rated horsepower of an engine is normally considered to be about 80 percent of the engine’s maximum horsepower.
For example, an engine with a maximum brake horsepower of 10 bhp would have a rated horsepower of 8 hp, since 8 is equal to 80 percent of 10. The rated horsepower of an engine is often printed on the outside of the engine and can also be found in the manufacturer’s manual.
Rated horsepower is useful when you need to determine what size engine is needed for a particular machine. If the machine requires 8 hp to operate, you would choose an engine that has a rated horsepower of 8 hp or a maximum horsepower of 10 bhp.
The horsepower rating of an engine is normally given along with the rpm value of the engine. For example, an engine may be rated for 10 hp at 4,000 rpm. Figure 9 displays a graph of horsepower versus rpm. Note that this graph dis- plays a curve rather than a straight line. Also, note that horsepower will decrease after a certain rpm value is reached.
Torque
Another quantity that’s often used to rate engines is torque.
Torque is the twisting force produced at the output shaft of
the motor. Engine torque values are normally measured in
units called foot-pounds. The torque value of an engine increases with an increase in rpm.
As you’ve probably figured out by now, the ideal engine would have high horsepower and lots of torque. Unfortunately, this combination of qualities doesn’t happen often in real life. In a typical engine, horsepower generally increases as the rpm increases. The maximum horsepower develops near the maxi- mum rpm limit of the engine. Torque, on the other hand, is produced somewhat differently. In a typical engine, the maxi- mum torque will normally be produced at a lower rpm, then decline as the rpm increases. This means that the maximum torque and the maximum rpm don’t usually occur at the same time. Thus, engine selection is a compromise, depend- ing on the particular application.
Displacement
Engine displacement is the volume of space that the piston moves as it moves from the bottom dead center to top dead center. The distance that the piston travels up and down in a cylinder is called the stroke of the engine. Displacement is measured in cubic inches.
The displacement of an engine will usually be stated in the service manual for an engine, or printed on the engine itself.
FIGURE 9—As you can see by this graph, horsepower increases with engine rpm.
You can calculate the displacement of an engine if you know the diameter of the cylinder and the length of the stroke of the engine. The displacement of an engine can be calculated by using the following formula:
π(d
2)(s)(n) displacement —————
4
In the formula, the symbol π stands for the constant pi, which is always equal to 3.14. The letter d stands for the diameter of the cylinder. The 2 after the letter d means that the diame- ter must be squared in the equation. (To square a number, simply multiply the number times itself). The letter s stands for the length of the stroke of the engine. The letter n stands for the number of cylinders in the engine.
An engine’s displacement value has an effect on the power that the engine develops. In most cases, the larger the dis- placement, the more power the engine will develop.
Compression Ratio
When a piston is at its lowest point in the cylinder (BDC), the volume of the cylinder is at its largest. When the piston is at its highest point in the cylinder (TDC), the volume of the cylin- der is at its smallest. The ratio of the largest cylinder volume to the smallest cylinder volume is called the compression ratio.
An engine’s compression ratio will determine how much the fuel mixture is compressed when the piston rises. The higher the compression ratio, the more the fuel mixture will be com- pressed. So, if an engine has a compression ratio of 5 to 1, it means that the volume of the cylinder at BDC is 5 times higher than the volume of the cylinder at TDC.
When the air and fuel mixture in the cylinder is compressed, the pressure of the mixture increases dramatically. This large increase in pressure will make the mixture burn stronger when it’s ignited. In general, the higher the compression ratio of the engine, the stronger the fuel mixture will burn and the faster the engine will run. Most small gasoline engines have a compression ratio of between 5 to 1 and 6 to 1.
Now, take a few moments to review what you’ve learned by
Power Check 1
At the end of each section of your Small Engine Parts and Operation text, you’ll be asked to check your understanding of what you’ve just read by completing a “Power Check.”
Writing the answers to these questions will help you review what you’ve learned so far.
Please complete Power Check 1 now.
1. True or False? In an external combustion engine, fuel is burned inside the engine.
2. In the _______ stage, the piston rises, compressing the air-and-fuel mixture trapped in the combustion chamber.
3. When a piston is at its lowest position in the cylinder, it’s said to be at _______.
4. During the _______ stage, gases are released from the cylinder.
5. A typical one-cylinder, four-stroke engine contains two valves called the _______ valve and the _______ valve.
6. In a four-stroke engine, the part that mixes fuel with air is the _______.
7. In the _______ stage, air mixed with fuel is drawn into the cylinder.
8. True or False? A two-stroke engine completes all four stages of engine operation in four piston strokes.
9. The units of rpm measures how fast the _______ is turning in an engine.
10. During the _______ stage, the mixture of air and fuel is ignited, and the contained explosion of the fuel presses the piston back down in the cylinder.
Check your answers with those in the back of this book.
BASIC ENGINE COMPONENTS
Now that we’ve reviewed the important basics of engine oper- ation, let’s begin our discussion about small engine parts.
We’ll cover all the basic parts of an engine and explain how they function. Note that almost all of these basic parts are found in both four-stroke engines and two-stroke engines.
However, the arrangement of the parts will vary slightly in different types of engines.
Figure 10 shows four views of the same four-stroke engine.
Figure 10A is an external view of the front of the engine, and Figure 10B is an external view of the rear of the engine. In Figure 10C, the blower housing has been removed from the engine, and several internal components are visible. Finally, in Figure 10D, the crankcase cover (the very bottom cover) of the engine has been removed, and other components are visi- ble. Many of the major components are labeled in these photos for you.
We’ve provided you with these illustrations so you can have reference points to look at during our discussion of engine parts. Naturally, not all engines look exactly the same as this one; however, this engine is typical of many of the four-stroke engines you’ll see. Refer back to these photos as needed to locate and observe the components as we talk about them.
The Blower Housing
Usually, one of the first components you see when you look at an engine is the blower housing. The blower housing is a metal cover that covers the top of an engine. The blower housing serves several purposes. Its most important function is engine cooling. The engine’s flywheel acts like a fan to pro- duce a flow of air, and the blower housing helps direct this flow of air over the engine to keep it cool. Another function is protection. The blower housing covers an engine’s internal components, preventing dirt and debris from getting it.
Finally, in many engines, the blower housing holds the
starter assembly (the pull rope or line that’s used to start
the engine).
FIGURE 10—These photos show four views of the same four-stroke engine. Figure 10A shows an external front view of the engine, and Figure 10B shows the rear of the engine. In Figure 10C , the blower housing has been removed from the top of the engine, and several components are visible.
Finally, in Figure 10D, the crankcase cover has been removed, allowing you to see several other com- ponents clearly.
FIGURE 10—Continued.
The Cylinder Block
The cylinder block is the most basic part of any engine. All of the other engine parts are fastened to the inside or outside of the cylinder block. In the past, some cylinder blocks were made of cast iron; however, now most are made from cast aluminum and similar light metals.
The metal is cast in a mold to produce the unique shape of the cylinder block. A cylinder from a typical small engine is shown in Figure 11.
The Cylinder
In very simple terms, the cylinder of an engine is a large hole that’s bored in the metal cylinder block. The cylinder serves two main purposes in a small engine:
The cylinder supports the cylinder head The cylinder holds the piston
FIGURE 11—A cylinder block from a typical small engine is shown here. Note the location of the cylinder and the crankcase.
The proper name for the hole in the cylinder block is the bore. The inside surfaces of the bore are called the cylinder walls. The bore is machined to an exact diameter and is made to be very smooth. The smooth inside surface allows the piston to slide easily up and down inside the cylinder.
The outside surface of the cylinder block is deeply finned.
These fins increase the surface area of the cylinder block and expose more of the surface area to the air. The metal cylinder block gets very hot when the engine is running, and this heat must be dissipated away from the cylinder block to prevent the engine from overheating and burning up. The more sur- face area of the cylinder block that’s exposed to the air, the more the heat will dissipate. Thus, the fins are an important part of the cooling system of an engine. Some cylinder blocks use an aluminum alloy for the finned portion of the cylinder, because aluminum is a metal that dissipates heat well.
Most small engines contain one cylinder and one piston.
These engines are called single-cylinder engines. However, an engine may have two, four, or even more pistons and cylin- ders. These multi-cylinder engines are usually classified by the positions of their cylinders.
There are three types of cylinder arrangements: the straight or in-line type, the V-type, and the opposed type. These three types of multi-cylinder arrangements are shown in Figure 12.
In the straight or in-line arrangement, all the cylinders are positioned in a row (Figure 12A). In the V-type arrangement, the cylinders are angled to create a V shape (Figure 12B). In the opposed arrangement, a pair of cylinders is positioned with one directly opposite the other (Figure 12C).
FIGURE 12—Multi-cylinder engines are usually classi- fied according to their cylinder positions. Three cylinder arrangements are shown here. Figure 12A shows the straight or in- line arrangement, Figure 12B shows the V arrange- ment, and Figure 12C shows the opposed arrangement.
Of the three multi-cylinder arrangements shown, the in-line arrangement is the most common. In all of the cylinder arrangements, the pistons and connecting rods are attached to one crankshaft. The crankshaft has offset counterweights and crank pins that stagger the firing order of the cylinders.
When one cylinder is on the power stroke, the second cylin- der will be on the intake stroke, and so on. This staggering helps to make the crankshaft rotate smoothly.
The Cylinder Head
The cylinder head is a metal cover that’s used to cover the cylinder (Figure 13). The cylinder head is bolted to the top of the cylinder block. The cylinder head is generally made of the same metal as the cylinder block. Like the cylinder block, the cylinder head is also finned to promote cooling.
FIGURE 13—This photo clearly shows the cylinder, the cylinder head, and the gasket that lies between them.
The underside of a cylinder head is hollowed out to form a combustion chamber. Inside this dome (normally at its cen- ter) is a threaded hole that accepts the spark plug. A gasket called the head gasket is placed between the cylinder head and the cylinder to seal the head to the cylinder and to form an airtight combustion chamber.
Gaskets are used at different places in engines to form seals between connected metal parts. The head gasket is thicker than many other types of gaskets and is made of heat-resist- ant materials, including asbestos fibers and thin sheets of aluminum.
As we discussed earlier, a four-stroke engine requires two valves to operate: the intake valve and the exhaust valve. Two openings for the valves must be drilled in the cylinder head.
Depending on the type of engine, the valves may be posi- tioned in different locations in the cylinder head. Each valve arrangement requires a different shape of cylinder head.
There are three cylinder head shapes: the L-type, the T-type, and the I-type. These three shapes are shown in Figure 14.
First, in an L-type cylinder head, the combustion chamber extends to one side of the head. The intake and exhaust valves are positioned vertically in the cylinder block, and the valves open upward. This type of cylinder head is used in the side-valve engine. This type of cylinder head is the one that’s most often used in small, four-stroke engines.
FIGURE 14—The three types of cylinder heads used in four-stroke engines are shown here. Figure 14A is the L-type, Figure 14B is the I-type, and Figure 14C is the T-type.
In the T-type cylinder head, the combustion chamber extends to both sides of the head. The intake and exhaust valves are posi- tioned on opposite sides of the cylinder head and open upward.
This type of arrangement is rarely used in small engines.
In the I-type cylinder head, the intake and exhaust valves are positioned vertically at the top of the cylinder head. Both valves move down into the combustion chamber to open. This type of cylinder head is used in overhead valve (OHV) engines, which are found most often in automobiles. However, some small engines do contain overhead valve arrangements. Note that if an engine contains an overhead valve arrangement, the abbreviation OHV will often be seen printed on the blower housing.
The Crankcase
The crankcase is located at the lower end of the cylinder block. The crankcase may be molded as a part of the cylinder block, or it may bolt on separately.
The main purpose of the crankcase is to enclose and support the crankshaft. This support is provided by holes bored into the crankcase. The two ends of the crankshaft may rest directly in these holes, or the holes may contain bearings that the ends of the crankshaft fit into.
Another important function of the purpose of the crankcase is to hold engine oil. The crankcase is filled with a certain quantity of oil that’s used to lubricate engine parts such as the connecting rod bearings, piston, and camshaft assembly.
Oil seals are installed in the crankcase to prevent engine oil from leaking out. Because the crankcase is used as a sort of oil reservoir, it’s sometimes called the sump.
The crankcase cover is the detachable bottom part of the crankcase. The crankcase cover has a hole in its center that holds and supports one end of the crankshaft. Because the crankshaft rotates constantly in this hole, the hole is often lined with a bearing.
In order for the crankshaft to turn properly and for the engine
to perform correctly, the crankcase cover must be firmly
attached and precisely aligned with the crankcase. To provide
this precise alignment, small steel pins called alignment dow- els are fitted between the crankcase and the crankcase cover to hold the two parts tightly together.
The Piston
The piston (Figure 15) is a can-shaped object made of a cast piece of steel or aluminum. The top of the piston is called the head or dome. Most piston heads have a slightly elevated surface toward their centers. Further down on the piston is the hole that holds the wrist pin (also called the piston pin).
The wrist pin holds the connecting rod to the piston. Inside the hole are two machined areas that hold the retaining clips for the wrist pin. The bottom surfaces of the piston are
termed piston skirts.
Pistons come in many different sizes. Small engines like the ones used in chain saws and weed trimmers have very small pistons. Garden tractors, industrial pumps, and other larger types of equipment have engines with larger pistons.
On the outside of a piston near its top are two or more
grooves that hold steel piston rings. The piston rings are used to form a seal between the piston and the cylinder. The mate- rial between the ring grooves is often called the ring lands.
The number of rings a piston has depends on the type of engine. Some two-stroke, high rpm engines have just one ring. This ring is called a compression ring. Other types of
FIGURE 15—The basic parts of a piston are shown here.
engines contain pistons that have two or more rings. In a three- ring piston, the top two rings are compression rings. These two rings are used to seal the cylinder and to compress the air and fuel mixture within the cylinder. The bottom ring is a special oil control ring. When the piston rises, the oil control ring floats in the groove in the piston. After the compression stage, the oil control ring makes contact with the cylinder walls and scrapes the oil back down into the crankcase.
On some engines, two pins or stakes are located inside the top two ring lands (where the compression rings are located).
These pins prevent the rings from rotating on the piston. The third groove or land on a piston is slightly wider. The lower land holds the oil-control ring.
When piston rings are being installed on a piston, the rings are fitted over the top of the piston and are placed into the ring lands. The oil-control ring is normally placed on the pis- ton first. Then, the compression rings are gently expanded and placed over the piston. If the piston is the type that has pins, each ring will be rotated on the piston until the ring
“split” lines up with the pins on the piston lands.
The Connecting Rod
A connecting rod is used to connect the piston to the crank- shaft. One end of the connecting rod is connected to the inside of the piston by the wrist pin. The wrist pin passes through both the piston and the top of the rod. The pin can be supported by a bearing in the end of the rod, or it may be supported directly by a hole in the rod.
Figure 16 displays two types of connecting rods used on small engines. A two-piece or split connecting rod is shown in Figure 16A, and a one-piece connecting rod is shown in
Figure 16B. In each illustration, the top of the connecting rod would be the piston side of the connecting rod.
In the split-type connecting rod, the bottom end of the con-
necting rod splits apart and comes off. The end part is called
the connecting rod cap. The end of the connecting rod is split
apart and clamped around the crankpin, and the cap is held
to the main part of the connecting rod by two bolts.
Look at the long section of the connecting rod that’s located between the wrist pin and the crankpin hole. Note that the edges of the connecting rod are thicker than the center. This thickness adds strength to the connecting rod to prevent it from bending or breaking.
When the piston and connecting rod are joined together, the piston fits over the top part of the connecting rod. The wrist pin fits through the hole in the piston and passes through the top end of the connecting rod. Two small, spring steel clips fit into the machined recesses in the hole in the piston.
These clips hold the wrist pin centered in the piston. This construction is illustrated in Figure 17.
The Crankshaft
The crankshaft is the main rotating part in the engine. The crankshaft converts the up-and-down motion of the piston into rotary motion. The crankshaft is also used to drive the flywheel, the camshaft, and other parts of an engine that are timed to function with the movement of the piston.
A typical crankshaft for a single-cylinder engine is shown in Figure 18. This type of crankshaft is made in one solid piece, and is therefore intended to be used with a two-piece con- necting rod. The two cranks are joined together by a crankpin
FIGURE 16—A two-piece connecting rod is shown in Figure 16A, and a one-piece connecting rod is shown in Figure 16B.
(also called the connecting rod journal). The two-piece con- necting rod is clamped around the crankpin. The entire structure that extends off from the crankshaft and that con- tains the crankpin is called the throw.
FIGURE 17—The construc- tion of the piston and rod assembly is shown here.
When the wrist pin is removed as shown here, the connecting rod will fall out of the bottom of the piston.
FIGURE 18—This type of crankshaft is used with a two-piece connecting rod.
Heavy counterweights are used on the crankshaft to balance the weight of the connecting rod. Note how the counterweight is positioned opposite the connecting rod and piston assem- bly. This creates a balanced system when the crankshaft rotates. Also, note how the crankpin is offset from the center- line of the crankshaft. The off-center positioning of the
crankpin makes the crankshaft turn as the piston moves up and down inside the cylinder.
Another type of crankshaft is used with a one-piece connect- ing rod (Figure 19). This type of crankshaft is sometimes called a built-up crankshaft. The built-up crankshaft comes apart to allow the connecting rod or connecting rod bearing to be removed. During assembly, the crankpin is inserted through the hole in one half of the crankshaft, through the roller bearing, and then through the hole in the other half of the crankshaft.
On some older small engines, you may find a one-piece con- necting rod attached to a single counterweight. A bolt holds the connecting rod to this single crankshaft counterweight.
FIGURE 19—The crankshaft shown here is used with a one-piece connecting rod.
The crankshaft comes apart to allow the connecting rod to be removed.
The bolt passes through a spacer, and on the spacer rides the bearing that supports the connecting rod. A thrust washer is used to keep the connecting rod from coming off the bearing.
The crankshaft in any small engine is always positioned at a 90º angle to the cylinder. This position is necessary to move the pistons up and down properly. However, the position of the crankshaft to the other parts of a piece of equipment may vary. There are three different operating positions for the crankshaft: horizontal, vertical, and multi-positional. Note that these terms refer to the normal position of the crank- shaft when the piece of equipment is operating.
In a horizontal crankshaft engine, the crankshaft is positioned horizontally (side-to-side) inside the engine. Some tractors use this type of crankshaft to operate a horizontal transmis- sion shaft. In a vertical crankshaft engine, the crankshaft is positioned vertically (straight up-and-down) inside the engine.
Many lawn mowers use a vertical crankshaft because it can be used to directly operate a blade. The multipositional engine is designed to operate in a variety of positions.
Multipositional engines are found in equipment such as chain saws, where the engine may be held and operated at an angle or even upside-down.
The crankcase is located below the cylinder on horizontal engines and to the side of the cylinder head on vertical engines. These two arrangements are shown in Figure 20.
The Bearings
Bearings are ring-shaped metal components that are designed to support engine parts that turn or slide against each other.
The bearings absorb the heat and friction created by moving engine parts and prevent wear and tear on these parts.
Bearings are highly resistant to corrosion and scoring.
There are several different types of bearings that are used for
different purposes. Ball bearings, roller bearings, and needle
bearings are among the most common types. Many bearings
are composed of two metal rings called races. The inner race
fits inside the outer race. The metal balls or rollers are then
positioned between the inner race and the outer race. (Note that some needle bearings don’t have an inner race—the nee- dles are simply fitted into the outer race.) Figure 21 shows three different types of bearings.
FIGURE 20—The two different positions for a crankshaft are shown here. Figure 20A shows a vertical crankshaft engine, and Figure 20B shows a horizontal crankshaft engine. (Courtesy of Kawasaki Motors Corp., U.S.A.)
FIGURE 21—Clockwise starting from the left, this photo shows a ball bearing, a needle bearing, and a plain bushing.
The balls used in ball bearings are spheres of very hard metal. The rollers and needles used in roller and needle bear- ings are small, hard metal cylinders. Needles are generally longer and thinner than rollers. Ball bearings and roller bear- ings may be used to support crankshafts, camshafts, and transmissions. Needle bearings are generally used to support transmissions and connecting rods.
A crankshaft may be supported at each end by ball bearings or roller bearings located in the crankcase. In the engine shown earlier in Figure 10, one crankshaft bearing is located in the crankcase and the other bearing is positioned in the crankcase cover. In some engine models, however, the ends of the crankshaft simply rest in holes in the crankcase. In such an engine, the crankcase will usually be made of alu- minum, which is a soft metal, while the crankshaft will be made of a harder metal. Since the metal of the crankcase is soft, the crankshaft will rotate in the holes in the crankcase easily without requiring the use of a separate bearing.
Plain bearings are thin metal rings that are used for the same purposes as other bearings. However, plain bearings are made very differently than ball, roller, and needle bearings. A plain bearing is made with a layer of hard metal on the out- side surface (usually steel) and a layer of softer metal on the inner surface (alloys of aluminum, lead, or zinc). The softer metal on the inside surface allows the shaft to turn easily inside the bearing. Plain bearings may have either a one- piece or two-piece construction. The two-piece type of bearing is sometimes called a split-sleeve bearing. Split-sleeve bear- ings are often used with two-piece connecting rods.
Another type of bearing commonly used in small engine applications is the bushing. A bushing is a one-piece bearing that’s designed to handle lower-speed rotary loads. Bushings are often made of bronze and may or may not require oil lubrication. One commonly used bushing is the valve guide, a component we’ll discuss in the section about valves.
Bearings must be properly lubricated with oil in order to func-
tion properly. In small engines, engine oil from a pool in the
crankcase may be splashed or pumped onto the bearings. The
inside surface of a plain bearing must also be lubricated with
oil to form a cushioning film. The rotating shaft then “floats”
in this film of oil. Some plain bearings contain small oil holes that allow oil to reach the inside surface of the bearing.
Bearings are manufactured to precise standards and must be installed carefully in order to work properly. Also, a bearing must always be replaced with a bearing that has the same specific ratings.
The Flywheel
A flywheel is a heavy, cast metal disk that’s attached to one end of the crankshaft (Figure 22). A tapered hole in the cen- ter of the flywheel fits onto the tapered end of the crankshaft.
The flywheel is located directly under the blower housing.
The flywheel performs several different functions in an engine.
First, the flywheel acts as a balancing weight for the crank- shaft. As the crankshaft turns, the flywheel spins. The spinning weight of the flywheel balances out the jerking or pulsing motion of the crankshaft that’s produced by each
FIGURE 22—The flywheel performs several functions in an engine.
turning smoothly between power strokes. In addition, the weight of the spinning flywheel helps to keep the crankshaft in motion, which improves engine performance and efficiency.
Another function of the flywheel is to provide some cooling for the engine. Like the outside surface of the cylinder block, the metal surface of the flywheel is deeply finned. When the fly- wheel spins, these fins act like the paddles of a fan, blowing cool air past the cylinder head and cylinder block.
The flywheel is also a part of an engine’s starter system. In pull-start engines, the starter rope may be wound around a pulley that’s attached to the flywheel. In some engines, magnets are cast into the rim of the flywheel, so the flywheel becomes part of the magneto system in the starter. (We’ll discuss the magneto in more detail a little later in the study unit.)
The Camshaft and the Timing Gears
The camshaft is a turning shaft in a four-stroke engine that’s used to push open the intake and exhaust valves. Structures called cam lobes are positioned along the length of the
camshaft. The lobes are somewhat oval-shaped; that is, one side of the lobe projects out from the center. The lobes are positioned off-center on the camshaft. When the camshaft turns, the turning lobes lift the intake and exhaust valves in a timed sequence. A camshaft will contain one lobe for each valve in the engine.
Refer back to Figures 2 through 5 to observe how the turning camshaft opens and closes the valves. These four figures show all four positions of the lobes. Note that in these views, the crankshaft is turning in a clockwise rotation, so if the camshaft is driven by a gear on the crankshaft, the camshaft will rotate counterclockwise.
Figure 2 shows the intake stroke of a four-stroke engine.
Note the position of the two lobes on the camshaft. The lobe
under the intake valve has turned so that it can lift the
intake valve up, opening the valve. In Figure 3, the compres-
sion stroke of the engine is shown. Note how the lobes have
now turned to close both valves. In Figure 4, the power
stroke is shown. The lobes have now turned again, but they
aren’t lifting the valves yet. Both valves are still closed.
Finally, in Figure 5, the lobe under the exhaust valve turns so that it lifts the exhaust valve.
The turning motion of the camshaft is linked to the motion of the crankshaft by gears. A small gear at the end of the crank- shaft connects with a larger gear at the end of the camshaft.
These gears are called the timing gears. The timing gears con- trol when the intake and exhaust valves open and close.
A gear drive system for turning a camshaft is shown in Figure 23A. Observe the timing marks on the gears. The timing marks are small dents or dots that are punched into the metal sur- face of the gears. These marks must be aligned when the gears are installed to ensure proper operation of the gears.
To understand how these gears work, think back to our dis- cussion of the four stages of engine operation. Remember that in a four-stroke engine, four strokes of the piston are required to complete one engine cycle. The four piston strokes can be summarized as follows:
DOWN—intake UP—compression DOWN—power UP—exhaust
FIGURE 23—Figure 23A shows a gear drive system for turning a camshaft. Figure 23B shows how a chain is used on two sprockets to rotate a camshaft.
So, in one four-stroke engine cycle, the piston moves down twice and up twice. Each time the piston moves up and down once, the crankshaft makes one complete rotation. When the piston moves up and down twice, the crankshaft makes two complete rotations. This means that for every four-stroke engine cycle, the crankshaft makes two complete rotations.
During each cycle of the four-stroke engine, the intake valve must open once and the exhaust valve must open once.
Remember that the valves are lifted and opened by the lobes on the camshaft. To open and close the valves prop- erly, the camshaft must make one complete rotation in each engine cycle.
So, in order for a four-stroke engine to operate properly, the crankshaft must make two complete rotations and the camshaft must make one complete rotation in each engine cycle. The crankshaft must therefore turn twice as fast as the camshaft in order to provide correct timing of the opening and closing of the valves. The job of the timing gears is to link the camshaft and crankshaft so that they rotate in the proper way.
Look at Figure 23A again. Note that the camshaft gear is twice the size of the crankshaft gear (that is, the camshaft gear has two times as many teeth as the crankshaft gear).
This arrangement guarantees that the camshaft will make one complete revolution for every two revolutions of the crankshaft.
Some engines connect the camshaft and the crankshaft by using a chain and sprockets rather than gears. In such an arrangement, the chain is called the timing chain. Figure 23B shows how a chain is used on two sprockets to connect the crankshaft and camshaft. Note that the camshaft sprocket is twice as large as the crankshaft sprocket. This ensures that the camshaft makes one rotation for every two rotations of the crankshaft. Also, note the position of the timing marks on the timing chain and the camshaft sprocket.
Now, let’s look at two special types of camshaft arrange- ments. First, remember that we discussed the overhead valve engine a little earlier in the study unit. In an overhead valve engine, the camshaft is normally mounted within the
crankcase, and tubes called push rods are driven by the
lobes. The push rods press upward on rocker arms that in turn push downward on the valves. This arrangement is shown in Figure 24. Four-stroke, overhead valve engines are often used in riding mowers, garden tractors, and other large pieces of equipment. In an overhead camshaft (OHC) engine, the camshaft, rocker arms, valve springs, and valves are all located in the cylinder head. The camshaft is driven off the crankshaft by using a timing chain. The lobes press directly up onto rocker arms, and the rocker arms push down on the valves to open them. This arrangement is shown in Figure 25.
The overhead camshaft engine is performance-based and is often used in go-carts.
The Valves
In a typical four-stroke engine, valves open to allow the air- and-fuel mixture to enter the cylinder and close to allow exhaust gases to escape. The valves used to perform this function are called poppet valves. A diagram of the parts of a poppet valve is shown in Figure 26. Valves are machined from a high grade of steel.
FIGURE 24—In an overhead valve engine, the lobes drive push rods that actu- ate rocker arms. The rocker arms push down on the valves to open them.
Remember that in a two-stroke engine, ports perform the intake and exhaust functions. Two-stroke engines have no valves in the combustion chamber area.
FIGURE 25—In an overhead camshaft engine, the camshaft components and the valves are mounted in the cylinder head.
FIGURE 26—The parts of a poppet valve are shown here.
The top portion of a valve is called the head. The small flat section just below the top surface of the head is called the margin. The valve face is just below the margin. The face is machined at an angle to the margin. The face is the combus- tion chamber sealing surface of the valve. A pocket on the cylinder is machined to exactly the same angle as the face of the valve. The valve seat is the opening in the cylinder head block that accepts the valve. When the valve is fitted into the valve seat, the valve face will fit so tightly against the cylinder head that no compression pressure will be lost.
Note that in a four-stroke engine, the head of the intake valve is always larger than the head of the exhaust valve.
There are two reasons for this. First, the intake valve needs to be large enough to allow enough air-and-fuel mixture into the cylinder for combustion. The exhaust gases are expelled from the combustion chamber under high pressure, so the head of the exhaust valve can be smaller than the head of the intake valve.
Second, since exhaust gases are very hot, the head of the exhaust valve is kept smaller so that it can transfer heat to the cylinder head properly.
The long part of the valve is the stem. The valve stem moves up and down in the body of the cylinder. A valve guide is used to prevent the steel valve stem from rubbing against the steel of the cylinder, as shown in Figure 27. This valve guide
FIGURE 27—This illustra- tion shows the top section of a valve assembly as it fits into the cylinder block.
is made of a softer metal than either the valve stem or the cylinder. Lubricating oil helps suspend the valve in the valve guide, preventing wear on the softer metal.
The intake and exhaust valves are driven by the camshaft, as we discussed earlier. Figure 28 shows an exploded diagram of a valve assembly. Note that the lobe doesn’t directly make contact with the valve stem. Instead, the valve lifter or tappet rests on the lobe. As the lobe rotates, the valve lifter rises and presses up on the valve to open it.
The valve spring serves two purposes in the valve assembly.
First, the tension of the valve spring continuously pushes down on the valve, keeping it closed until the valve lifter pushes up on it. When the lobe turns and releases the pres- sure on the valve lifter, the tension of the spring returns the valve to its original position. The valve spring is held in place on one end by the crankcase. The other end of the spring is held on the valve stem by a retainer and a retainer lock.
Second, the pressure of the valve spring allows the valve to closely follow the shape of the camshaft lobe as it rotates. It’s very important that the spring pressure be tight enough so
FIGURE 28—This exploded diagram shows how the parts of the valve assembly fit together.
that the valve follows the movement of the lobe in a smooth and controlled way. If the spring pressure isn’t tight enough, the valve will bounce up off the lobe or “float” when the engine runs fast. This floating action decreases engine per- formance because the valve doesn’t close completely at the correct times.
Figure 29 shows the operation of the valve and its parts as the lobe rotates. In Figure 29A, the lobe isn’t pushing the valve lifter up, and the valve spring is holding the valve closed. In Figure 29B, the lobe is pushing up on the valve lifter, compressing the spring and opening the valve.
Note that the period of time that an intake valve stays open is called the duration. The duration is measured in degrees of crankshaft rotation, not seconds. The vertical travel distance of a valve is called the lift.
The valve system in a four-stroke engine is timed in such a manner as to perfectly control the flow of air into and out of the engine. In our previous discussion of the four stages of
FIGURE 29—This illustra- tion shows the action of the valve assembly as the lobe turns. In Figure 29A, the lobe isn’t pressing up on the valve lifter, and the valve is held closed by the valve spring. In Figure 29B, the lobe is pressing up on the valve lifter. This action compresses the valve spring and opens the valve.
engine operation, it may have seemed that the valves are timed at exact intervals and no two valves will be open at the same time. However, this isn’t exactly true. At the beginning of the intake stage, the intake valve is opening and the exhaust valve is closing. Both of these valves are timed so that they’re both slightly open at the same instant. This allows the inrush of the fresh air-and-fuel mixture to help push out any exhaust gases that might be left in the com- bustion chamber. The time period during which both valves are slightly open is called valve overlap.
The Muffler
The muffler on an engine is a component of the exhaust sys- tem. As an engine burns fuel, it produces large amounts of very hot gases. These gases must be released from the com- bustion chamber before more fuel can be burned. The efficient removal of these gases is an important factor in an engine’s performance. On most small engines, the exhaust system consists of an exhaust port and a small muffler.
The muffler is a filter-like device with a metal outside shell.
Exhaust gases are expelled through the muffler, which reduces the noise of the process. Some mufflers are bolted onto an engine, while others are designed to screw directly onto the engine.
Now, take a few moments to review what you’ve learned by
completing Power Check 2.
Power Check 2
1. The proper name for the hole in the cylinder block is the _______.
2. The inside surfaces of the cylinder are called the _______.
3. A piston is held to a connecting rod by a _______.
4. A _______ is placed between the cylinder head and the cylinder block to form a seal against the loss of compression.
5. The surfaces of the cylinder block and cylinder head are _______ to help cool the engine.
6. The _______ is a turning shaft that’s used to push open the intake and exhaust valves.
7. The material between the grooves of a piston that hold the rings are called the _______.
8. In a four stroke engine, if the valves are moved by push rods and rocker arms, the engine is the _______ type.
9. In an L-type cylinder arrangement, the valve assembly is placed in the _______.
10. The underside of a cylinder head is hollowed out to form a _______.
11. The part of a valve assembly that rests on the lobe is called the _______.
12. Many lawnmowers use a _______ shaft engine because it can be used to directly operate a blade.
Check your answers with those in the back of this book.
