Chapter 18 Prime Movers

Question 1

Define prime mover

Answer 1

Prime movers are any devices designed to drive other machines.

Question 2

From where do internal-combustion engines derive their energy?

Answer 2

Internal-combustion engines derive their energy from fuel which is burned within the engine itself.

Question 3

What sets Diesel engines apart from other internal combustion engines?

Answer 3

• Diesel engines use air alone to fill the cylinders during the intake stroke.

Therefore, they have no carburetor system but, instead, use a fuel-injection system.

• The fuel is ignited solely by compression. Because ofthe increased temperature and compression these engines must have heavier
construction to withstand the stress.
• All diesels are fuel injected. That is, they have a metered flow of fuel into the combustion chamber.

Question 4

What are the two basic types of Diesel engines?

Answer 4

Diesel engines may be divided into two basic types: four-stroke and two-stroke.

Question 5

Describe naturally-aspirated and supercharged engines

Answer 5

Four-stroke engines are either naturally-aspirated or supercharged:
• In naturally aspirated engines, fresh air is drawn into the cylinder by a vacuum. The vacuum is created when the piston moves down the cylinder away from the head and combustion area.
• In supercharged engines air is forced into the cylinder at higher than atmospheric pressure. This is done by means of a pump or blower similar to those used on two-stroke engines

Question 6

What are the compression ratios for Diesel engines?

Answer 6

16: I to 23:1
Diesel engines run at significantly higher compression ratios than
most other engines. The range of compression ratios for diesel engines is from 16: I to 23: I. Compare this with the range for gasoline engines, which is from 6:1 to 13:1

Question 7

Describe compression-combustion

Answer 7

When air is compressed rapidly to a significantly smaller volume, the temperature of the air increases dramatically. When fuel is sprayed into the cylinder(s), the temperature and compression of the air cause the fuel-air mixture to ignite and burn. The rapidly expanding mixture pushes the piston down the cylinder.

Question 8

What are the methods of fuel injection?

Answer 8

Fuel is supplied to the cylinders of a diesel engine by fuel injection There are two types of fuel-injection system:
• In air injection, an external source of air forces a measured amount of fuel into the cylinders.
• In mechanical injection, an injector pump forces fuel into the cylinders by applying a hydraulic force to the fuel.
The mechanical injection system is the most used.

Question 9

How are gasoline engines similar Diesel engines

Answer 9

As with diesel engines, gasoline engines can be divided into two basic classifications of four-stroke and two-stroke. Similarly, gasoline engines can be naturally aspirated or supercharged, and may be fuel injected. Other than appearance, this is where the similarities end.

Question 10

Describe how combustion is achieved with a gasoline engine

Answer 10

Gasoline engines require that fuel and air be mixed together before ignition can take place. Once the piston reaches the top of its stroke on the combustion cycle, a spark plug fires. This causes the air-fuel mixture to burn. As a result, compression is not used for ignition

Question 11

What are the compression ratios with gasoline engines and how does that alter their construction?

Answer 11

Generally compression ratios for gasoline engines are much lower-usually in a range of 6:1 to 9:1, although they may be up to 13:1. Compare this with the range for diesel engines, which is from 16: 1 to 23: 1. As a result then gasoline engines can be made of lighter construction.

Question 12

How is fuel supplied to gasoline engines?

Answer 12

Some gasoline engines use a carburetor to supply fuel to the cylinder. In the carburetor, the fuel is atomized. It is then drawn into the cylinder where the spark plug fires, causing the fuel to burn. Gasoline engines now often use fuel-injection systems to supply fuel to the cylinders. The gasoline still requires a spark plug for ignition

Question 13

Compare high-compression gas-burning engines to both diesel and gasoline engines

Answer 13

These engines are very similar to gasoline and diesel engines:
• They can be divided into four-stroke and two-stroke.
• They may be naturally aspirated or supercharged.
• They may differ in outward appearance but their basic principles remain constant.
Unlike diesel and gasoline engines (which serve multiple purposes) high-compression gas engines serve a specific purpose within a given industry.

Question 14

Describe the operating principles of four-stroke cycle Diesel engines

Answer 14

In a four-stroke cycle, four separate and distinct processes take place in two complete revolutions of the crankshaft.
INTAKE:
1. Once the piston has travelled up the cylinder to top dead centre (TDC),
the intake valve opens and air intake begins The intake valve can open before the TOe. This depends on the manufacturer’s timing specifications.
2. As the piston travels down the cylinder, air is drawn into the cylinder.
3. When the piston travel reaches bottom dead centre (BDC), the intake valve closes and the next stroke begins.
COMPRESSION:
4. At this point the piston begins its second travel up the cylinder. Both the intake and exhaust valves are closed.
5. As the piston travels toward TDC the air is being compressed many times.
6. Once the piston reaches TDC the crankshaft has completed one revolution and the next step in the process begins.
Remember that, due to the extreme compression ratio in the diesel engine, the compressed air is hot. The fuel ignites when it contacts the hot air. The hot expanding gases react against the piston, forcing the piston down toward BDC. Once the piston reaches BDC the exhaust valve in the head begins to open and the last (exhaust) stroke begins.
Exhaust
On this stroke the exhaust valve is open. As the piston travels up the cylinder, the exhaust gases are forced out through the exhaust valve

Question 15

Describe the operating principles of two-stroke cycle diesel engines with blowers

Answer 15

Two-stroke diesel engine with blower
COMPRESSION
With the piston at BDC, ports in the walls of the cylinder are exposed. Notice that there are two valves iu the cylinder head-both are exhaust valves. Note that the exhaust valves are open. Air is pumped into the cylinder by a pump Or blower. Air is pushed into the cylinder, replacing the
spent fuel which in turn forces the spent fuel out through the exhaust valves. As the piston moves up the cylinder the first stroke begins. Because exhaust and intake take place at the same time with the engine at BDC, no stroke takes place. It is not until the ports are covered and exhaust valves are closed that the first stroke begins with the piston moving up the cylinder.
Compression takes place during this stroke.
POWER
Once the piston reaches TDC, fuel is injected into the cylinder and is ignited by the heat from compression. The rapidly expanding gases react against the piston forcing down the cylinder. Once the intake ports are uncovered and exhaust valves are opened, the cycle begins again. The two-stroke engine has completed both cycles in one revolution.

Question 16

15. Describe the operating principles of two-stroke cycle diesel engines without blowers

Two-stroke diesel engine without blowers
Not all two-stroke diesels use exhaust valves or blowers. The piston on the power stroke acts as a compressor on the back side. As the piston moves down, it compresses the air in the crankcase. As the piston moves farther along the cylinder, exhaust ports are uncovered allowing spent fuel to escape. At this point the intake ports are uncovered and the slightly

Answer 16

Two-stroke diesel engine without blowers
Not all two-stroke diesels use exhaust valves or blowers. The piston on the power stroke acts as a compressor on the back side. As the piston moves down, it compresses the air in the crankcase. As the piston moves farther along the cylinder, exhaust ports are uncovered allowing spent fuel to escape. At this point the intake ports are uncovered and the slightly

Question 17

Describe the purpose and operating principles of the following components:
- crankshaft and journal bearings

Answer 17

The crankshaft has two main functions. The fIrst is to support the pistons and relating parts. The second function is the most important-it transfers energy of motion. For example, in generators, trucks, and cars, the crankshaft takes reciprocating motion and changes it to rotary motion. The crankshaft can also transfer the energy of reciprocating motion from the engine, to the energy of reciprocating motion for a compressor.
The main journal bearings in the crankshaft along with the bearing caps have two functions:
• hold the crankshaft in the block
• provide a lubrication surface for the crankshaft to turn in.

Question 18

Describe the purpose and operating principles of the following components:
- flywheel

Answer 18

The crankshaft carries the flywheel which has three uses:
• It provides inertia that carries the crankshaft through periods of no power stroke and also smooths out the power stroke.
• In automobile or truck use, the flywheel acts as a mounting surface for the clutch.
• It provides an ideal surface for a ring gear in which to mount a starter for starting the engine.

Question 19

Describe the purpose and operating principles of the following components:
- connecting rod and piston assembly

Answer 19

connecting rod and piston assembly

The connecting rod connects between the crankshaft throw and the piston. The piston houses the piston rings which seal against the cylinder wall. The first two are compression rings which are designed to stop leakage of air or hot gases into the crankcase. The third ring on the piston is an oil ring which wipes oil off the cylinder walls so that no crankcase oil is burned and wasted. The cylinder liner (sleeve) is a removable chamber which acts as a guide for the piston and as a combustion chamber wall for the power stroke.

Question 20

Describe the purpose and operating principles of the following components:
- cylinder head and valve assembly

Answer 20

cylinder head and valve assembly
The cylinder head is the top end of the compression-combustion chamber. The cylinder head assembly consists of the valves, valve springs, rocker arms, and rocker arm shaft. In some applications, it includes the camshaft. The cylinder head also houses the intake and exhaust ports of four-stroke engines, and the exhaust ports of the two-stroke engine.

Question 21

Describe the purpose and operating principles of the following components:
- camshaft with lifters and push rods

Answer 21

camshaft with lifters and push rods
The camshaft is a shaft with eccentric machined lobes called cams. It works
as follows:
1. As the camshaft rotates the lobe lifts a lifter which in turn pushes on the
push rod. The push rod then moves the push rod which in turn opens the valve.
2. As the cam lobe turns the valve spring forces the valve to seat and seal
the compression-combustion chamber (cylinder).

Question 22

Describe the purpose and operating principles of the following components:
- engine block

Answer 22

The engine block is a stationary piece which holds other things in place. It is also called the cylinder block. The block houses or supports all major and most auxiliary parts. The basic block is most often cast iron or cast aluminum. It is machined to accept camshafts. the crankshaft. cylinder sleeves. bearings. and all other pieces of the engine. Engine blocks are webbed for strength and heat dissipation.

Question 23

Describe the purpose and operating principles of the following components:
- turbochargers

Answer 23

Turbochargers are blowers that are powered by the exhaust waste gases of the engine. They are basically air pumps (compressors).
The turbocharger is not connected by gears, chains or other mechanical means to the engine’s moving parts. Figure 18 shows the path followed by air as it flows through the turbocharger into the cylinder and back out through the exhaust.
Turbochargers can run at speeds from 10 000 rpm to 100 000 rpm. Turbocharger controls adjust the flow of air or of air and fuel. depending on the engine type.

Question 24

Compare the construction and components of two-stroke gasoline engines to two-stroke diesel engines

Answer 24

Each of these engines has a:

• crankshaft with main and crank throw bearings
• connecting rod and piston assembly
• head assembly.

The gasoline engine has only a spark plug in the centre of the head and the diesel engine has only the fuel injector. In diesel engines, each cylinder head has a fuel injector.

Question 26

18. Compare superchargers to turbochargers

Answer 26

that turbochargers are driven by exhaust gases. Superchargers are driven mechanically either from the engine or from a separate source such as an electric motor. Another difference is that superchargers are rotary lobe as opposed to turbine wheel.Note that this blower is more complex than a turbocharger.
However, rotary blowers are very positive when full torque over a wide range of speed is needed. In other words, regardless of the rpm, the blower
always delivers the same amount of air to the cylinders.

Question 27

19. What procedures are followed for preventive maintenance with internal combustion engines?

Answer 27

Always follow the recommendations and maintenance procedures specified by the manufacturer in the service manual. Always keep records of tasks performed so that there is less chance of missing something later on.

Routine maintenance requires:

• regular oil particle tests
• regular oil changes
• regular filter changes (for air, oil, and fuel)
• daily checks for leaks and missing bolts.

Question 28

Describe the design characteristics of the following DC motors: Series-connected Field motors

Answer 28

The series motor is one in which the field windings are connected in series with the armature. The same current which flows through the armature conductors also establishes the magnetic field.

Question 29

What are the operating principles of electric motors?

Answer 29

The principle of electric motor is when a current carrying conductor is placed in a magnetic field it experiences a force. In case of dc motor we have field windings on stator and armature on the rotor. The field and the armature are connected to the dc supply. The field winding generates a constant dc flux.

Question 30

How are electric motor frames constructed?

Answer 30

The frame of an electric motor is made of laminated silicon steel and is often
enclosed in a housing. The purpose of the frame is to support the windlings and to provide the magnetic circuit. There are three types of frames:

• The open frame has end bells so that air can circulate.
• The semi-closed has a screen in one end bell to prevent debris from being sucked into the motor.
• Enclosed frames are often used in explosion-proof motors.

Question 31

How are frame sizes classed and why is this important to a millwright?

Answer 31

Motors of the same horsepower may have different frame sizes. Frame sizes are classified by diameter. Within each diameter series, there are a few standard lengths. Frame sizes are used to obtain frame dimensions such as the overall length and diameter of the motor, and distances between mounting bolts. They also indicate shaft length, diameter, and height. This information is used by the millwright when installing and aligning motors.

Question 32

Describe DC motor construction including field poles

Answer 32

The field poles are usually made of laminated silicon steel. They support the field windings and act as a magnetic path betwecn the frame and armature.

Question 33

Describe DC motor construction including armature assembly

Answer 33

The armature is made of laminated steel which is part of the magnetic circuit. It supports the armature windings in slots. The speed and voltage of the machine determine the size of the wire and the number of windings.

Question 34

Describe DC motor construction including commutator

Answer 34

The commutator is made from copper bars that surround the shaft, upon which the brushes ride. The bars are insulated from the shaft and from each other with mica or same other insulating compound. The function of the commutator is to provide a means to feed the external electrical supply into the rotating coils.

Question 35

Describe DC motor construction including brush holders, tension springs, brushes

Answer 35

Brush holders support the carbon brushes and the tension springs provide pressure on the brushes so as to provide an electrical connection between the
commutator and the brush.

Question 36

Describe DC motor construction including bearing

Answer 36

Bearings support the weight of the armature and provide a surface of minimal resistance to rotating motion.

Question 37

Describe the design characteristics of the following DC motors:

(a) series-connected field motors

Answer 37

The series motor is one in which the field windings are connected in series
with the armature. The same current which flows through the armature conductors also establishes the magnetic field. Speed of the motor can be controlied by changing the applied voltage or by
increasing or decreasing the resistance of the field winding. Therefore, as the load is increased the speed of the motor decreases. This gives the motor a very high starting torque with a high no-load speed. The advantage of this DC motor’s high starting torque is that it allows battery operation, which makes it portable.

Question 38

Describe the design characteristics of the following DC motors:

(b) shunt-connected field motors

Answer 38

Unlike the series motor, this motor type has its field windings connected in parallel (shunt) with the armature winding. This motor has low starting torque but has good speed regulation. Shunt motors are used widely in applications requiring adjustable-speed control. Speed can be varied in one of two ways. First, the speed can be varied by changing the magnitude of the field current. From no load to full load, motor speed does not change a great deal which results in good speed regulation.

Question 39

Describe the design characteristics of the following DC motors:

compound field motors

Answer 39

The compound field motors combine the characteristics of the shunt-motor and series motor by incorporating both series and parallel windings. These types of motors are variable speed. They work well with loads that require constant speed but have high peaks needing high torque. Applications include reciprocating equipment such as pumps and compressors.

Question 40

Describe the design characteristics of the following DC motors:

permanent-magnet motors

Answer 40

These motors use pennanent magnets to produce the necessary magnetic field. As a result, these motors have similar torque-speed characteristics to a shunt-connected field motor. Permanent magnet motors usually havc ceramic magnets which resist demagnetization and are much smaller than conventional magnets. These motors have a wide variety of uses from small toys to aerospace applications.

Question 42

Describe the design characteristics of the following three-phase AC motors:

squirrel cage induction motors

Answer 42

In a squirrel cage induction motor, there are no physical connections between the rotor and the stator. Hence the magnetic field must be induced into the rotor. Its construction is the simplest of the AC motors.
A squirrel cage induction motor has:
• a frame that provides a magnetic circuit and supports the stator windings
• stator windings
• a squirrel cage rotor
• end bells.
Depending upon their design, these motors can have a starting torque from normal to very high. These wide variances in starting torque allow for extensive variation in applications. Therefore, when combining the range of starting torques, efficiency, and economic considerations, these motors are very useful for industry.

Question 43

Describe the design characteristics of the following three-phase AC motors:

wound rotor motor

Answer 43

The difference between wound rotor motors and squirrel cage motors is in their rotor design. The wound rotor has insulated windings.
Like squirrel cage motors, these motors are used widely throughout industry to drive fans, compressors, machine tools and process pumps.

Question 44

Describe the design characteristics of the following three-phase AC motors:

synchronous

Answer 44

In these motors, speed is synchrouized between the rotor and the rotating magnetic field. That is, the speed of the motor is directly proportional to the frequency of the operating current. Because these motors require assistance in starting. squirrel cage windings are placed into the pole faces. The poles stick out toward the rotor and are connected to slip rings on the shaft. These motors find applications where there are heavy loads that run for long periods, such as compressors.

Question 45

Describe the design characteristics of the following single-phase AC motors:

split-phase motors

Answer 45

Split-phase motors use an auxiliary winding on the starter to help start the motor. These motors are fractional hp units. They can use either wound or squirrel-cage rotors, although squirrel-cage is the most widely used. These motors are found in small pumps. These motors consist of a housing, an armature, a stator, and a centrifugal switch inside the housing. A centrifugal switch opens at a preset rotational speed. The rotor most often used has a squirrel cage design. A squirrel cage rotor is a configuration of bars which resemble a cage.

Question 46

Describe the design characteristics of the following single-phase AC motors:

capacitor-start motors

Answer 46

These motors are either fractional horsepower or integral horsepower as high as 15 hp. This motor has a similar construction to the split-phase except that there is a capacitor on the starter winding. The capacitor acts to provide a high starting torque. Capacitor-started motors utilize the capacitor only while starting. It is disconnected once the motor reaches around three-quarter speed. This is done by means of a centrifugal switch which opens to bypass the capacitor in the circuit. These motors are used in applications where a high starting torque is required, such as conveyors, pumps, and reciprocating compressors. These motors are also available with normal starting torques and are used for such things as centrifugal pumps and fans.

Question 47

Describe the design characteristics of the following single-phase AC motors:

permanent-split capacitor motors

Answer 47

Permanent-split capacitor molors keep the capacitor operating at all times. The permanent-split capacitor does not have a centrifugal switch. These motors have low starting torque and a high rated torque. These motors are ideal for grinders and sanders.

Question 48

Describe the design characteristics of the following single-phase AC motors:

repulsion motors

Answer 48

Repulsion motors can be divided into several groups, such as repulsion-start, induction-run motors; repulsion motors; and repulsion-start induction motors. Despite the diversity of these repulsion motors their basic construction is similar. For example, their brushes are supported either on the rotor shaft or the end plate and their starter is similar to the winding on a split-phase motor.

Question 49

List the information on the nameplate and state its significance.

Answer 49

Every electric motor has a name plate. This provides basic, important information about the motor. When performing maintenance on a motor, it is important to know how to get information from the name plate. Information on the name plate includes horsepower rating. phase, frame size, volts, amps, rpm, bearing numbers for both ends, cycle, model, code. class of insulation, temperature rating, and service factor.

Question 50

What are the advantages and disadvantages of series-connected field motors?

Answer 50

advantages

suited to heavy starting duty where severe loads are expected
• small size for comparable hp motors,
makes them ideal for high-torque
loads

disadvantages

requires load to limit speed

speed adjustment is limited

limited application to traction and lifting.

Question 51

What are the advantages and disadvantages of squirrel cage rotor motors?

Answer 51

advantages

starting and operating torques
can be changed by using
different rotor designs
• very efficient

disadvantages

• rotor must always turn slower than the synchronous speed or there will be zero Induced current and no torque.
• requires 3-phase power supply

Question 52

What are the advantages and disadvantages of repulsion and repulsion-start motors?

Answer 52

advantages

•Different classes of repulsion
motors allow for versatility

-high starting torque and
constant speed characteristic
- high starting torque and
variable speed

disadvantages

• brushes ride on armature all the time in repulsion motors and repulsion-start-induction motors
•repulsion-start induction motors
develop problems:
-in commutator brushes
- in centrifugal switch
- too little or too much tension on throw-out centrifugal spring
- opens, short or grounds in the rotor and starter windings.