BASIC ENGINE COMPONENTS

The engine provides the power to drive the wheels of the vehicle. All automobile engines, both gasoline and diesel, are classified as internal combustion engines because the combustion or burning that creates energy takes place inside the engine. Combustion is the burning of an air and fuel mixture. As a result of combustion, large amounts of pressure are generated in the engine. This pressure or energy is used to power the car. The engine must be built strong enough to hold the pressure and temperatures formed by combustion.

Figure 1

Diesel engines have been around a long time and are mostly found in big heavy-duty trucks. However, they are also used in some pickup trucks and will become more common in automobiles in the future (Figure 1). Although the construction of gasoline and diesel engines is similar, their operation is quite different.

A gasoline engine relies on a mixture of fuel and air that is ignited by a spark to produce power. A diesel engine also uses fuel and air, but it does not need a spark to cause ignition. A diesel engine is often called a compression ignition engine. This is because its incoming air is tightly compressed, which greatly raises its temperature. The fuel is then injected into the compressed air. The heat of the compressed air ignites the fuel and combustion takes place. The following sections cover the basic parts and the major systems of a gasoline engine.

Cylinder Block

The biggest part of the engine is the cylinder block, which is also called an engine block (Figure 2). The cylinder block is a large casting of metal (cast iron or aluminum) that is drilled with holes to allow for the passage of lubricants and coolant through the block and provide spaces for movement of mechanical parts. The block contains the cylinders, which are round passageways fitted with pistons. The block houses or holds the major mechanical parts of the engine.

Figure 2

Cylinder Head

The cylinder head fits on top of the cylinder block to close off and seal the top of the cylinders (Figure 3). The combustion chamber is an area into which the air-fuel mixture is compressed and burned. The cylinder head contains all or most of the combustion chamber. The cylinder head also contains ports, which are passageways through which the air-fuel mixture enters and burned gases exit the cylinder. A cylinder head can be made of cast iron or aluminum.

Figure 3

Piston

The burning of air and fuel takes place between the cylinder head and the top of the piston. The piston is a can-shaped part closely fitted inside the cylinder (Figure 4). In a four-stroke cycle engine, the piston moves through four different movements or strokes to complete one cycle. These four are the intake, compression, power, and exhaust strokes. On the intake stroke, the piston moves downward, and a charge of air-fuel mixture is introduced into the cylinder. As the piston travels upward, the air-fuel mixture is compressed in preparation for burning. Just before the piston reaches the top of the cylinder, ignition occurs and combustion starts. The pressure of expanding gases forces the piston downward on its power stroke. When it reciprocates, or moves upward again, the piston is on the exhaust stroke. During the exhaust stroke, the piston pushes the burned gases out of the cylinder.

Connecting Rods and Crankshaft

The reciprocating motion of the pistons must be converted to rotary motion before it can drive the wheels of a vehicle. This conversion is achieved by linking the piston to a crankshaft with a connecting rod. As the piston is pushed down on the power stroke, the connecting rod pushes on the crankshaft, causing it to rotate. The end of the crankshaft is connected to the transmission to continue the power flow through the drivetrain and to the wheels.

Figure 4

Valve Train
A valve train is a series of parts used to open and close the intake and exhaust ports. A valve is a movable part that opens and closes a passageway. A camshaft controls the movement of the valves (Figure 4), causing them to open and close at the proper time. Springs are used to help close the valves. 

Figure 5

Manifolds 

A manifold is metal ductwork assembly used to direct the flow of gases to or from the combustion chambers. Two separate manifolds are attached to the cylinder head (Figure 5). The intake manifold delivers a mixture of air and fuel to the intake ports. The exhaust manifold mounts over the exhaust ports and carries exhaust gases away from the cylinders.

FOUR STROKE ENGINE

In a passenger car or truck, the engine provides the rotating power to drive the wheels through the transmission and driving axles. All automotive engines, both gasoline and diesel, are classified as internal combustion because the combustion or burning takes place inside the engine. These systems require an air-fuel mixture that arrives in the combustion chamber at the correct time and an engine constructed to withstand the temperatures and pressures created by the burning of thousands of fuel droplets.

The combustion chamber is the space between the top of the piston and the cylinder head. It is an enclosed area in which the fuel and air mixture is burned. The piston fits into a hollow metal tube, called a cylinder. The piston moves up and down in the cylinder. This reciprocating motion must be converted to a rotary motion before it can drive the wheels of a vehicle.

This change of motion is accomplished by connecting the piston to a crankshaft with a connecting rod (Figure). The upper end of the connecting rod moves with the piston as it moves up and down in the cylinder. The lower end of the connecting rod is attached to the crankshaft and moves in a circle. The end of the crankshaft is connected to the flywheel, which transfers the engine’s power through the drivetrain to the wheels.

In order to have complete combustion in an engine, the right amount of fuel must be mixed with

the right amount of air. This mixture must be compressed in a sealed container, then shocked by the right amount of heat (spark) at the right time. When these conditions exist, all the fuel that enters a cylinder is burned and converted to power, which is used to move the vehicle. Automotive engines have more than one cylinder. Each cylinder should receive the same amount of air, fuel, and heat, if the engine is to run efficiently.

Although the combustion must occur in a sealed cylinder, the cylinder must also have some means of allowing heat, fuel, and air into it. There must also be a means to allow the burnt air-fuel mixture out so a fresh mixture can enter and the engine can continue to run. To accommodate these requirements, engines are fitted with valves.

There are at least two valves at the top of each cylinder. The air-fuel mixture enters the combustion chamber through an intake valve and leaves (after having been burned) through an exhaust valve (Figure). The valves are accurately machined plugs that fit into machined openings. A valve is said to be seated or closed when it rests in its opening. When the valve is pushed off its seat, it opens.

A rotating camshaft, driven and timed to the crankshaft, opens and closes the intake and exhaust valves. Cams are raised sections of a shaft that have high spots called lobes. Cam lobes are oval shaped. The placement of the lobe on the shaft determines when the valve will open. The height and shape of the lobe determines how far the valve will open and how long it will remain open in relation to piston movement.

As the camshaft rotates, the lobes rotate and push the valve open by pushing it away from its seat. Once the cam lobe rotates out of the way, the valve, forced by a spring, closes. The camshaft can be located either in the cylinder block or in the cylinder head. When the action of the valves and the spark plug is properly timed to the movement of the piston, the combustion cycle takes place in four strokes of the piston: the intake stroke, the compression stroke, the

power stroke, and the exhaust stroke. 

The camshaft is driven by the crankshaft through gears, or sprockets, and a cogged belt, or timing chain. The camshaft turns at half the crankshaft speed and rotates one complete turn during each complete four-stroke cycle.

Four-Stroke Cycle A stroke is the full travel of the piston either up or down in a cylinder’s bore. The reciprocal movement of the piston during the four strokes is converted to a rotary motion by the crankshaft. It takes two full revolutions of the crankshaft to complete the four-stroke cycle. One full revolution of the crankshaft is equal to 360 degrees of rotation; therefore, it takes 720 degrees to complete the four-stroke cycle. During one piston stroke, the crankshaft rotates 180 degrees.

Flywheel The piston moves by the pressure produced during combustion, but this moves the piston only about half a stroke or one-quarter of a revolution of the crankshaft. This explains why a flywheel is needed. The flywheel stores some of the power produced by the engine. This power is used to keep the pistons in motion during the rest of the four-stroke cycle. A heavy flywheel is only found on engines equipped with a manual transmission. Engines with automatic transmissions have a flexplate and a torque converter. The weight and motion of the fluid inside

the torque converter serve as a flywheel.

Intake Stroke The first stroke of the cycle is the intake stroke. As the piston moves away from top dead center (TDC), the intake valve opens (Figure 9 – 8A). The downward movement of the piston increases the volume of the cylinder above it, reducing the pressure in the cylinder. This reduced pressure, commonly referred to as engine vacuum, causes the atmospheric pressure to push a mixture of air and fuel through the open intake valve. (Some engines are equipped with a super- or turbocharger that pushes more air past the valve.) As the piston reaches the bottom of its stroke, the reduction in pressure stops, causing the intake of air-fuel mixture to slow down. It does not stop because of the weight and movement of the air-fuel mixture. It continues to enter the cylinder until the intake valve closes. The intake valve closes after the piston has reached bottom dead center (BDC). This delayed closing of the valve increases  the volumetric efficiency of the cylinder by packing as much air and fuel into it as possible.

Compression Stroke The compression stroke begins as the piston starts to move from BDC. The intake valve closes, trapping the air-fuel mixture in the cylinder (Figure). The upward movement of the piston compresses the air-fuel mixture, thus heating it up. At TDC, the piston and cylinder walls form a combustion chamber in which the fuel will be burned. The volume of the cylinder with the piston at BDC compared to the volume of the cylinder with the piston at TDC determines the compression ratio of the engine.

Power Stroke The power stroke begins as the compressed fuel mixture is ignited (Figure). With the valves still closed, an electrical spark across the electrodes of a spark plug ignites the air-fuel mixture. The burning fuel rapidly expands, creating a very high pressure against the top of the piston. This drives the piston down toward BDC. The downward movement of the piston is transmitted through the connecting rod to the crankshaft.

Exhaust Stroke The exhaust valve opens just before the piston reaches BDC on the power stroke (Figure). Pressure within the cylinder causes the exhaust gas to rush past the open valve and into the exhaust system. Movement of the piston from BDC pushes most of the remaining exhaust gas from the cylinder. As the piston nears TDC, the exhaust valve begins to close as the intake valve starts to open. The exhaust stroke completes the four-stroke cycle. The opening of the intake valve begins the cycle again. This cycle occurs in each cylinder and is repeated over and over, as long as the engine is running.

FASTENER

Fasteners are used to secure or hold different parts together or to mount a component. Many types and sizes of fasteners are used in automobiles. Each fastener is designed for a specific purpose and condition. The most commonly used is the threaded fastener. Threaded fasteners include bolts, nuts, screws, and similar items that allow for easy removal and installation of parts (Figure).

The threads can be cut or rolled into the fastener. Rolled threads are 30% stronger than cut threads. They also offer better fatigue resistance because there are no sharp notches to create stress points. There are four classifications for the threads of Imperial fasteners: Unified National Coarse (UNC), Unified National Fine (UNF), Unified National Extrafine (UNEF), and Unified National Pipe Thread (UNPT or NPT). Metric fasteners are also available in fine and coarse threads.

NPT is the standard thread design for joining pipes and fittings. There are two basic designs: tapered and straight cut threads. Straight cut pipe thread is used to join pipes but it does not provide a good seal at the joining point. Tapered pipe threads provide a good seal because the internal and external threads compress against each other as the joint is tightened. Most often a sealant is used on pipe threads to provide a better seal. Pipe threads are commonly used at the ends of hoses and lines that carry a liquid or gas (Figure)

Coarse (UNC) threads are used for general- purpose work, especially where rapid assembly and disassembly are required. Fine threads (UNF) are used where greater holding force is necessary. They are also used where greater vibration resistance is desired.

MEASUREMENT SYSTEM

Two systems of weights and measures exist side by side in the United States—the Imperial or U.S. customary system and the international or metric system. The basic unit of linear measurement in the Imperial system is the inch. The basic unit of linear measurement in the metric system is the meter. The meter is easily broken down into smaller units, such as the centimeter (1⁄100 meter) and millimeter (1⁄1,000 meter).

All units of measurement in the metric system are related to each other by a factor of 10. Every metric unit can be multiplied or divided by the factor of 10 to get larger units (multiples) or smaller units (submultiples). This makes the metric system much easier to use, with less chance of math errors than when using the Imperial system (Figure).

The United States passed the Metric Conversion Act in 1975 in an attempt to get American industry and the general public to use the metric system, as the rest of the world does. While the general public has been slow to drop the customary measuring system of inches, gallons, and pounds, many industries, led by the automotive industry, have now adopted the metric system for the most part.

Nearly all vehicles are now built to metric standards. Technicians must be able to measure and work with both systems of measurement. The following are some common equivalents in the two systems:

Linear Measurements

1 meter (m) 39.37 inches (in.)

1 centimeter (cm) 0.3937 inch

1 millimeter (mm) 0.03937 inch

1 inch 2.54 centimeters

1 inch 25.4 millimeters

1 mile 1.6093 kilometers

Square Measurements

1 square inch 6.452 square centimeters

1 square centimeter 0.155 square inch

Volume Measurements

1 cubic inch 16.387 cubic centimeters

1,000 cubic centimeters 1 liter (l)

1 liter (l) 61.02 cubic inches

1 gallon 3.7854 liters

Weight Measurements

1 ounce 28.3495 grams

1 pound 453.59 grams

1,000 grams 1 kilogram

1 kilogram 2.2046 pounds

Temperature Measurements

1°Fahrenheit (F) 9⁄5C 32°

1°Celsius (C) 5⁄9(F – 32°)

Pressure Measurements

1 pound per square inch (psi) 0.07031 kilogram(kg) per square centimeter

1 kilogram per square centimeter 14.22334 pounds per square inch

1 bar 14.504 pounds per square inch

1 pound per square inch 0.06895 bar

1 atmosphere 14.7 pounds per square inch

Torque Measurements

10 foot-pounds (ft.-lb) 13.558 Newton meters (N-m)

1 N-m 0.7375 ft.-lb

1 ft.-lb 0.138 m kg

1 cm kg 7.233 ft.-lb

10 cm kg 0.98 N-m