Taylor D.A. Introduction to marine engineering

10

Diesel engines

Figure

2.2

Four-stroke timing diagram

gases

and

then

to

fill

or

charge

the

space

with

fresh air. Instead

of

val*"js

holes, known

as

'ports',

are

used

which

are

opened

and

closed

by the

sides

of the

piston

as it

moves.

Consider

the

piston

at the top of its

stroke where

fuel

injection

and

combustion have

just

taken place (Figure

2.3(a)).

The

piston

is

forced

down

on its

working stroke until

it

uncovers

the

exhaust

port

(Figure

2.3(b)).

The

burnt gases then begin

to

exhaust

and the

piston continues

down

until

it

opens

the

inlet

or

scavenge

port

(Figure

2.3(c)).

Pressurised

air

then enters

and

drives

out the

remaining exhaust gas.

The

piston,

on

its

return

stroke,

closes

the

inlet

and

exhaust

ports.

The air is

then

compressed

as the

piston moves

to the top of its

stroke

to

complete

the

cycle

(Figure

2.3(d)).

A

timing diagram

for a

two-stroke engine

is

shown

in

Figure 2.4.

The

opposed

piston

cycle

of

operations

is a

special

case

of

the

two-stroke

cycle.

Beginning

at the

moment

of

fuel

injection, both pistons

Diesel engines

11

Piston

Fuel

injector

Cylinder

Exhaust

port

_

Connecting

rodi

Crank

Scavenge

port

Exhaust

(b)

Rotation

Compression

(d)

Figure

2.3

Two-stroke

cycle

are

forced

apart—one

up, one

down—by

the

expanding

gases

(Figure

2.5{a)).

The

upper piston opens

the

exhaust

ports

as it

reaches

the end

of

its

travel (Figure

2.5(b)).

The

lower piston,

a

moment

or two

later,

opens

the

scavenge ports

to

charge

the

cylinder

with

fresh

air and

remove

the final

traces

of

exhaust

gas

(Figure

2.5(c)).

Once

the

pistons

reach their extreme points they both begin

to

move inward.

This

closes

off

the

scavenge

and

exhaust ports

for the

compression stroke

to

take

place

prior

to

fuel

injection

and

combustion (Figure

2.5(d)).

This cycle

is

used

in the

Doxford engine,

which

is no

longer manufactured although

many

are

still

in

operation.

12

Diesel

engines

Fuel

injection

begins

Fuel

injection

ends

Exhaust

ports

close

Exhaust

ports

open

Figure

2.4

Two-stroke timing diagram

I

"

i-

f

k

Upper

/piston

Fuel

^injector

\Lower

piston

,

1

r

t

T_

J

.

^

I

n

r

'

,t

h-Exhaust

port

•-Scavenge

port

1_

_r

ITO

i—

r

•

4 r

1

f

-

-4

L-t

J

Injection

Exhaust Scavenging

Compressioi

(a)

(b)

(c)

(d)

Figure

2.5

Opposed

piston engine cycle

Diesel

engines

i 3

The

four-stroke

engine

A

cross-section

of a

four-stroke cycle engine

is

shown

in

Figure 2.6.

The

engine

is

made

up of a

piston

which

moves

up and

down

in a

cylinder

which

is

covered

at the top by a

cylinder head.

The

fuel

injector, through

which

fuel

enters

the

cylinder,

is

located

in the

cylinder

head.

The

inlet

and

exhaust

valves

are

also housed

in the

cylinder head

and

held shut

by

springs.

The

piston

is

joined

to the

connecting

rod by a

gudgeon pin.

The

bottom

end or big end of the

connecting

rod is

joined

to the

crankpin

which

forms

part

of the

crankshaft.

With

this

assembly

the

Rocker

arm

Cylinder

head

Crankpin

Bottom

end

bearing

Crankcase

Figure

2.6

Cross-section

of a

four-stroke

diesel

engine

14

Diesel

engines

linear up-and-down movement

of the

piston

is

converted

into rotary

movement

of the

crankshaft.

The

crankshaft

is

arranged

to

drive

through

gears

the

camshaft, which

either

directly

or

through

pushrods

operates

rocker

arms which

open

the

inlet

and

exhaust valves.

The

camshaft

is

'timed'

to

open

the

valves

at the

correct

point

in the

cycle.

The

crankshaft

is

surrounded

by the

crankcase

and the

engine

framework

which

supports

the

cylinders

and

houses

the

crankshaft

bearings.

The

cylinder

and

cylinder

head

are

arranged

with

water-cooling

passages

around

them.

The

two-stroke

engine

A

cross-section

of a

two-stroke cycle

engine

is

shown

in

Figure 2.7.

The

piston

is

solidly

connected

to a

piston

rod

whkh

is

attached

to a

crosshead

bearing

at the

other end.

The top end of the

connecting

rod is

Exhaust

manifold

Turboblower

Air

inlet

ports

Crosshead

Bottom

end

bearing

Crankshaft

Connecting

rod

A-frame

Bedplate

Figure

2.7

Cross-section

of a

two-stroke

diesel

engine

Diesel

engines

15

also

joined

to the

crosshead bearing. Ports

are

arranged

in the

cylinder

liner

for air

inlet

and a

valve

in the

cylinder head enables

the

release

of

exhaust

gases.

The

incoming

air is

pressurised

by a

turbo-blower which

is

driven

by the

outgoing exhaust gases.

The

crankshaft

is

supported

within

the

engine bedplate

by the

main bearings.

A-frames

are

mounted

on

the

bedplate

and

house guides

in

which

the

crosshead

travels

up and

down.

The

entablature

is

mounted above

the

frames

and is

made

up of

the

cylinders,

cylinder heads

and the

scavenge trunking.

Comparison

of

two-stroke

and

four-stroke

cycles

The

main difference between

the two

cycles

is the

power developed.

The

two-stroke

cycle

engine,

with

one

working

or

power stroke every

revolution,

will,

theoretically, develop

twice

the

power

of a

four-stroke

engine

of the

same swept volume.

Inefficient

scavenging however

and

other

losses,

reduce

the

power advantage

to

about

1.8.

For a

particular

engine power

the

two-stroke engine

will

be

considerably

lighter—an

important consideration

for

ships.

Nor

does

the

two-stroke engine

require

the

complicated

valve

operating mechanism

of the

four-stroke.

The

four-stroke engine however

can

operate

efficiently

at

high

speeds

which

offsets

its

power disadvantage;

it

also consumes less lubricating

oil.

Each

type

of

engine

has its

applications

which

on

board

ship have

resulted

in the

slow speed (i.e.

80—

100

rev/min)

main propulsion diesel

operating

on the

two-stroke cycle.

At

this

low

speed

the

engine requires

no

reduction gearbox between

it and the

propeller.

The

four-stroke

engine (usually rotating

at

medium

speed,

between

250 and

750

rev/

min)

is

used

for

auxiliaries such

as

alternators

and

sometimes

for

main

propulsion

with

a

gearbox

to

provide

a

propeller

speed

of

between

80

and

100

rev/min.

There

are two

possible measurements

of

engine power:

the

indicated

power

and the

shaft

power.

The

indicated power

is the

power developed

within

the

engine cylinder

and can be

measured

by an

engine indicator.

The

shaft

power

is the

power available

at the

output

shaft

of the

engine

and can be

measured using

a

torsionmeter

or

with

a

brake.

The

engine

indicator

An

engine indicator

is

shown

in

Figure 2.8.

It is

made

up of a

small

piston

of

known

size

which

operates

in a

cylinder against

a

specially

16

Diesel engines

Drum

Piston

rod

Calibrated

spring

Linkage

to

provide

straight

line

movement

of

stylus

Piston

Cylinder

Indicator

piston

Section

showing

indicator

piston

^vindicator

cord

Figure

2.8

Engine

indicator

—I

Coupling

*—'

nut

to

fasten

onto

indicator

cock

calibrated spring.

A

magnifying

linkage transfers

the

piston

movement

to

a

drum

on

which

is

mounted

a

piece

of

paper

or

card.

The

drum

oscillates

(moves

backwards

and

forwards)

under

the

pull

of the

cord.

The

cord

is

moved

by a

reciprocating

(up and

down) mechanism which

is

proportional

to the

engine piston movement

in the

cylinder.

The

stylus

draws

out an

indicator diagram which represents

the gas

pressure

on

the

engine piston

at

different

points

of the

stroke,

and the

area

of the

indicator diagram

produced

represents

the

power developed

in

the

particular cylinder.

The

cylinder power

can be

measured

if the

scaling

factors, spring calibration

and

some basic engine details

are

known.

The

procedure

is

described

in the

Appendix.

The

cylinder power values

are

compared,

and for

balanced loading should

all be the

same.

Adjustments

may

then

be

made

to the

fuel

supply

in

order

to

balance

the

cylinder

loads.

Torsionmeter

If

the

torque transmitted

by a

shaft

is

known,

together

with

the

angular

velocity,

then

the

power

can be

measured, i.e.

shaft

power

=

torque

x

angular

velocity

The

torque

on a

shaft

can be

found

by

measuring

the

shear stress

or

angle

of

twist

with

a

torsionmeter.

A

number

of

different types

of

torsionmeter

are

described

in

Chapter

15.

Diesel

engines

17

The gas

exchange process

A

basic part

of the

cycle

of an

internal combustion engine

is the

supply

of

fresh

air and

removal

of

exhaust gases.

This

is the gas

exchange

process.

Scavenging

is the

removal

of

exhaust gases

by

blowing

in

fresh

air.

Charging

is the filling of the

engine cylinder

with

a

supply

or

charge

of

fresh

air

ready

for

compression.

With

supercharging

a

large mass

of air

is

supplied

to the

cylinder

by

blowing

it in

under

pressure.

Older

engines

were

'naturally

aspirated'—taking

fresh

air

only

at

atmospheric

pressure. Modern engines make

use of

exhaust

gas

driven

turbo-

chargers

to

supply

pressurised

fresh

air

for

scavenging

and

supercharg-

ing.

Both four-stroke

and

two-stroke

cycle

engines

may be

pressure

charged.

On

two-stroke diesels

an

electrically driven

auxiliary

blower

is

usually

provided

because

the

exhaust

gas

driven turboblower cannot

provide

enough

air at low

engine speeds,

and the

pressurised

air is

usually

cooled

to

increase

the

charge

air

density.

An

exhaust

gas

driven

turbochargmg

arrangement

for a

slow-speed two-stroke

cycle

diesel

is

shown

in

Figure

2.9(a).

A

turboblower

or

turbocharger

is an air

compressor driven

by

exhaust

gas

(Figure

2.9(b)).

The

single

shaft

has an

exhaust

gas

turbine

on one

end

and the air

compressor

on the

other.

Suitable casing design

and

shaft

seals ensure that

the two

gases

do not

mix.

Air is

drawn

from

the

machinery

space through

a filter and

then compressed before passing

to

the

scavenge space.

The

exhaust

gas may

enter

the

turbine directly

from

the

engine

or

from

a

constant-pressure chamber.

Each

of the

shaft

bearings

has its own

independent lubrication

system,

and the

exhaust

gas end of the

casing

is

usually

water-cooled.

Scavenging

Efficient

scavenging

is

essential

to

ensure

a

sufficient

supply

of

fresh

air

for

combustion.

In the

four-stroke cycle engine there

is an

adequate

overlap

between

the air

inlet

valve

opening

and the

exhaust

valve

closing.

With

two-stroke

cycle engines

this

overlap

is

limited

and

some

slight

mixing

of

exhaust gases

and

incoming

air

does

occur.

A

number

of

different scavenging methods

are in use in

slow-speed

two-stroke

engines.

In

each

the

fresh

air

enters

as the

inlet

port

is

opened

by the

downward movement

of the

piston

and

continues until

the

port

is

closed

by the

upward moving

piston.

The flow

path

of

the

scavenge

air is

decided

by the

engine port shape

and

design

and the

exhaust

arrangements.

Three

basic systems

are in

use:

the

cross

flow, the

loop

and the

uniflow.

All

modern slow-speed diesel engines

now use the

uniflow

scavenging

system

with

a

cylinder-head exhaust

valve.

18

Diese! engines

Exhaust

gas

,

outlet

Compressor

Turboblower

-*f

Auxiliary

_^.

blower

Air

intake

Air

in

Compressor

Exhaust

gas

in

Turbine

rotor

Figure

2.9 (a)

Exhaust

gas

turbocharging

arrangement,

(b) A

turbocharger

Diesel

engines

19

In

cross scavenging

the

incoming

air is

directed upwards, pushing

the

exhaust gases before

it. The

exhaust gases then travel down

and out of

the

exhaust ports. Figure

2.10(a)

illustrates

the

process.

In

loop scavenging

the

incoming

air

passes over

the

piston crown then

rises

towards

the

cylinder head.

The

exhaust gases

are

forced before

the

air

passing down

and out of

exhaust ports located just above

the

inlet

ports.

The

process

is

shown

in

Figure

2.10(b).

With

uniflow

scavenging

the

incoming

air

enters

at the

lower

end of

the

cylinder

and

leaves

at the

top.

The

outlet

at the top of the

cylinder

may

be

ports

or a

large

valve.

The

process

is

shown

in

Figure

2.10(c).

Each

of the

systems

has

various advantages

and

disadvantages. Cross

scavenging

requires

the

fitting

of a

piston skirt

to

prevent

air or

exhaust

gas

escape when

the

piston

is at the top of the

stroke. Loop scavenge

Scavenge

air

in

Opposed piston

Tinir

Exhaust

valve

Figure 2.10 Scavenging

methods,

(a)

Cross-flow

scavenging,

(b)

loop scavenging,

(c)

uniflow

scavenging

Taylor D.A. Introduction to marine engineering

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