DieselShip

Engineering terms

The system of measurement has been outlined with an introduction to SI units. Some of the common terms used in engineering measurement will now be described.

Mass

Mass is the quantity of matter in a body and is proportional to the product of volume and density. The unit is the kilogram and the abbreviation used is 'kg'. Large quantities are often expressed in tonnes (t) where 1 tonne = 10s kg.

Force

Acceleration or retardation of a mass results from an applied force. When unit mass is given unit acceleration then a unit of force has been applied. The unit of force is the newton (N).

force = mass X acceleration

N kg m/s2

Masses are attracted to the earth by a gravitational force which is the product of their mass and acceleration due to gravity (g). The value of'g' is 9.81 m/s2. The product of mass and 'g' is known as the weight of a body and for a mass 'w'kg would be w x g=9.81 <u newtons.

Work

When a force applied to a body causes it to move then work has been done. When unit mass is moved unit distance then a unit of work has been done. The unit of work is the joule (J),

work = (mass x g) x distance

J N m

Power

This is the quantity of work done in a given time or the rate of doing work. When unit work is done in unit time then a unit of power has been used. The unit of power is the watt (W).

Energy

This is the stored ability to do work and is measured in units of work done, i.e. joules.

Pressure

The intensity of force or force per unit area is known as pressure, A unit of pressure exists where unit force acts on unit area. The unit of pressure is the newton per square metre and has the special name pascal (Pa).

Another term often used by engineers is the bar where 1 bar is equal to 105 Pa.

The datum or zero for pressure measurements must be carefullyconsidered. The complete, absence of pressure is a vacuum and this is therefore the absolute zero of pressure measurements. However, acting

upon the earth's surface at all times is what is known as 'atmospheric pressure'. The pressure gauge, which is the usual means of pressure measurement, also accepts this atmospheric pressure and considers it the zero of pressure measurements. Thus:

absolute pressure = gauge pressure 4- atmospheric pressure

Readings of pressure are considered to be absolute unless followed by the word 'gauge' indicating a pressure gauge value. The actual value of atmospheric pressure is usually read from a barometer in millimetres of mercury:

atmospheric pressure = mm of mercury X 13.6 x 9.81 Pa

A standard value of 1 atmosphere is often used where the actual value is unknown,

1 atmosphere = 10 1300 Pa

= 1.013 bar

Volume

The amount of physical space occupied by a body is called volume. The unit of volume is the cubic metre (m ). Other units are also in use, such as litre (1) and cubic centimetre (cm3), i.e.

1m3 = 10001 = 10000000cm3

Temperature

The degree of hotness or coldness of a body related to some zero value is known as temperature. The Celsius scale measure in °C simply relates to the freezing and boiling points of water dividing the distance shown on a thermometer into 100 equal divisions. An absolute scale has been devised based on a point 273. 16 kelvin (0.0 1°C) which is the triple point of water. At the triple point the three phases of water can exist, i.e. ice, water and water vapour. The unit of the absolute scale is the kelvin. The unit values in the kelvin and Celsius scales are equal and the measurements of temperature are related, as follows:

x°C = (x°C + 273)K

or y K= (yK- 273)°C

Heat

Heat is energy in motion between a system and its surroundings as a consequence of a temperature difference between them. The unit, as with other forms of energy, is the joule (J).

Power measurement

The burning of fuel in an engine cylinder will result in the production of power at the output shaft. Some of the power produced in the cylinder will be used to drive the rotating masses of the engine. The power

produced in the cylinder can be measured by. an engine indicator mechanism as described in Chapter 2. This power is often referred to as Indicated power'. The power output of the engine is known as 'shaft' or

'brake power'. On smaller engines it could be measured by applying a type of brake to the shaft, hence the name.

Indicated power

Typical indicator diagrams for a two-stroke and four-stroke engine are shown in Figure A. 1. The area within the diagram represents the work

done within the measured cylinder in one cycle. The area can be measured by an instrument known as a 'planimeter* or by the use of the mid-ordinate rule. The area is then divided by the length of the diagram

in order to obtain a mean height. This mean height, when multiplied by the spring scale of the indicator mechanism, gives the indicated mean effective pressure for the cylinder. The mean effective or 'average'

pressure can now be used to determine the work done in the cylinder.

Work done in = mean effective x area of piston X length of

1 cycle pressure (A) piston stroke

(Pm) (L)

To obtain a measure of power it is necessary to determine the rate of doing work, i.e. multiply by the number of power strokes in one second. For a four-stroke-cycle engine this will be rev/sec -r 2 and for a

two-stroke-cycle engine simply rev/sec.

power developed in one cylinder

= mean effective area of length of piston no. of power

pressure (Pm) piston (A) stroke (L) strokes/sec (N)

= Pm L A N

For a multi-cylinder engine it would be necessary to multiply by the number of cylinders.

Example

An indicator diagram taken from a six cylinder, two-stroke engine is shown in Figure A.2. The spring constant for the indicator mechanism is

65kPa mm. The engine stroke and bore are 1100mm and 410mm respectively and it operates at 120rev/min. What is the indicated power of the engine?

The diagram is divided into 10 equal parts and within each a mid-ordinate is positioned.

Shaft power

A torsionmeter is usually used to measure the torque on the engine shaft (see Chapter 15). This torque, together with the rotational speed, will give the shaft power of the engine.

shaft power = torque in shaft x rotational speed of shaft in radians per second

Example

The torque on an engine shaft is found to be 320 kNm when it is rotating at 1 lOrev/min. Determine the shaft power of the engine.

shaft power = shaft torque x 2n x revolutions per second

Mechanical efficiency

The power lost as a result of friction between the moving parts of the engine results in the difference between shaft and indicated power. The ratio of shaft power to indicated power for an engine is known as the 'mechanical efficiency'.

indicated power

Power utilisation

The engine shaft power is transmitted to the propeller with only minor transmission losses. The operation of the propeller results in a forward thrust on the thrust block and the propulsion of the ship at some particular speed. The propeller efficiency is a measure of effectiveness of the power conversion by the propeller.

The power conversion achieved by the propeller is a result of its rotating action and the geometry of the blades. The principal geometrical feature is the pitch. This is the distance that a blade would move forward in one revolution if it did not slip with respect to the water. The pitch will vary at various points along the blade out to its tip but an average value is used in calculations. The slip of the propeller is measured as a ratio or percentage as follows:

 

The theoretical speed is a product of pitch and the number of revolutions turned in a unit time. The actual speed is the ship speed. It is possible to have a negative value of slip if, for example, a strong current or wind were assisting the ship's forward motion.

Example

A ship on a voyage between two ports travels 2400 nautical miles in eight days. On the voyage the engine has made 820000 revolutions. The propeller pitch is 6 m. Calculate the percentage propeller slip.

Power estimation

The power developed by a ship's machinery is used to overcome the ship's resistance and propel it at some speed. The power required to propel a ship of a known displacement at some speed can be approximately determined using the Admiralty coefficient method. The total resistance of a ship, Rt can be expressed as follows;

Total resistance Rt = pS V"

where p is density (kg/m3)

S is wetted surface area (m2)

V is speed (knots)

now Wetted surface area <* (Length)2

Displacement, A « (Length)3

thus Wetted surface area x (Displacement, A)2/3

Most merchant ships will be slow or medium speed and the index 'n' may therefore be taken as 2. The density, p, is considered as a constant term since all ships will be in sea water.

This constant is known as the 'Admiralty coefficient'.

Example

A ship of 15000 tonnes displacement has a speed of 14 knots. If the Admiralty Coefficient is 410, calculate the power developed by the machinery.

Fuel estimation

The fuel consumption of an engine depends upon the power developed. The power estimation method described previously can therefore be modified to provide fuel consumption values. The rate of fuel consumption is the amount of fuel used in a unit time, e.g. tonne/day. The specific fuel consumption is the amount of fuel used in unit time to produce unit power, e.g. kg/kW hr.

Engineering drawing

Most engineering items defy description in words alone. To effectively communicate details of engineering equipment a drawing is usually used. Even the simplest of sketches must conform to certain rules or standards to ensure a 'language' that can be readily understood.

Some of these basic rules will now be described with the intention of enabling the production of a simple drawing for manufacturing or explanation purposes. A drawing produced as a piece of information or communication should stand alone, that is, no further explanation should be necessary. All necessary dimensions should be provided on the drawing and the materials to be used should be specified.

A drawing is made up of different types of lines, as shown in Figure A.3. The continuous thick line is used for outlining the drawing. The continuous thin line is used for dimension lines, to indicate sectioning, etc. A series of short dashes represents a hidden detail or edge and a chain dotted line is used for centre lines.

To represent a three dimensional item in two dimensions a means of projecting the different views is necessary. Two systems of projection are in use, First Angle and Third Angle. The First Angle system will be described with reference to the object shown in Figure A.4. Three views are drawn by looking at the object in the directions 1, 2 and 3. The views

seen are then drawn out, as shown in Figure A.5. View 1 is called the 'front elevation'. View 2 is the 'end elevation' and is located to the right of the front elevation. View 3 is the 'plan' and is positioned below the front elevation.

Sections are used to show the internal details of a part or an assembly as full lines. Section lines or hatching are used to indicate the different items which have in effect been cut. Each different item will have section lines at a different angle with 45° and 60° being most usual. Examples of

a sectioning can be seen for the internal threads shown in Figure A.3. The plane of section is always given and is shown on other views of the item.

Dimension lines are essential for the manufacture of an item. The dimension line is a thin continuous line with an arrowhead at each end and the dimension is placed above it at right angles. Where possible

projection lines are used to allow the dimension line to be clear of the drawing. The projection line begins a small distance clear of the drawing outline. Leader lines are used to indicate information to the appropriate part of the drawing and an arrowhead is used at the end of the line.

Scales are used to reduce drawings to reasonable sizes while retaining the correct proportions. Standard scale reductions are 1:1; 1:2; 1:5; 1:10 etc., where for example 1:10 means one-tenth full size. The in-between scale sizes are not normally used. Special scale rules are available to simplify drawing in any one of the above scales.

Standard representations are used for common engineering items such as nuts, bolts, studs, internal and external threads, etc. These are shown in Figure A.3. The proportions used for drawing nuts and bolts should be remembered and used whenever necessary.

A final word on the subject of information is necessary. A drawing should enable the item to be manufactured or at least identified so that a replacement can be obtained. Apart from the actual drawing there should be a block of information giving the item name, any material to be used, the drawing scale, stating the projection and possibly the date and the name of the person who made the drawing.