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should have a minimum closed flashpoint of also that the oil in storage should not be heated above 52°C.

In special cases where high viscosity oils are used and high degrees of heating are required to produce atomisation, etc., it is allowable to heat the oil to within 20°C of the closed flashpoint. Great care should always be taken regarding the control of heat to heaters situated on the suction side of the fuels pumps so as not to cause oil vaporisation and the possibility of explosive vapour formation.

(4) Calorific Value

Is the heating value from the complete combustion of unit

mass of fuel, etc.

Approximate heat energy values of fuels are:

Coal 34

Fuel Oil 42

Diesel Oil 45

Pure Hydrocarbon 50 (85% C,

The value quoted is the higher calorific value in every case in preference to the lower value. The higher calorific value includes the heat in water vapour formed from water as the products of combustion are cooled, vapours condensed, and hence latent heat becomes re-available for heat utilisation.

The lower calorific value is more realistic, from the boiler engineers viewpoint, being the actual heat available for boiler water evaporation, but this does not detract from the fact that this is a fault of utilisation and the higher calorific value is the actual heat available and is therefore the preferred value for quotation. Fuels always exhibit a fall of calorific value to some extent during storage.

There are numerous makes of bomb calorimeters but the differences are only slight. The test as conducted is very closely detailed and only a brief synopsis is outlined here. For further close details, if desired, the reader is referred to the relevant B.S. specifications. Consider Fig. 2.7:

The oxygen supply is to give an internal pressure of 26 bar and should not be less than times the theoretical oxygen required. The interior of the bomb must be resistant to condensed acidic vapours from combustion. The thermometer used can be read by means of a lens to and the temperature of the

58 REED'S GENERAL ENGINEERING KNOWLEDGE

hp oxygen valve to supply

born b

supports

furl

THE BOMB CALORIMETER Fig. 2.7

enclosing water, of amount 15-20 should be maintained steady up to the test.

A small specimen is fired by electric charge under conditions

of oxygen and the temperature rise of apparatus and

coolant is noted. 0.01 kg of distilled water are in the bomb to sulphuric and nitric acid vapours (from sulphur trioxide

and nitrogen). Mass of fuel W.E. of apparatus

complete

x

its temperature rise. The above calculation, using masses in kg and temperatures in gives the hcv of the fuel

or The water equivalent (W.E.) of the

apparatus is determined by a test using benzoic acid. This is the calorific value reference fuel, hcv 26.5 showing relatively no deterioration of calorific value during storage. Correction factors are now applied for acids formed under bomb conditions only, radiation cooling effect, etc. The temperature of test is based on 15°C approx. It should be noted that under the bomb's combustion conditions (high excess air and pressure) sulphuric and nitric acids are formed. Whereas

FUEL TECHNOLOGY

under furnace combustion conditions sulphur is burned mainly to sulphur dioxide, with no acid formation, thereby no trioxide, and nitrogen would pass off in the free state.

(5) Pour Point

This is a determination of the lowest temperature value at which oil will pour or flow under the prescribed test conditions. This value is an important consideration for lubricating oils working under low temperature conditions refrigeration machine lubricants, telemotors, etc.

Referring to Fig. 2.8:

Various mixtures are used in the bath, for very low temperatures solid carbon dioxide and acetone are used. At 11°C above the expected pour point the test begins. At temperature intervals of 3°C the test jar is removed, checked for surface oil tilt and replaced in a time interval of 3 s maximum.

thermometers

test jar

I

bath

POUR POINT APPARATUS Fig. 2.8

REED'S GENERAL ENGINEERING KNOWLEDGE

When surface of oil will not tilt, for a time interval of 5 s, note temperature, add 3°C and this is the pour point. The oil is heated to 46°C before the test and is cooled in progressive stages of about 17°C in different cooling agent baths, in each case the jar must be transferred to another bath when the oil reaches a temperature of 28°C above the bath temperature.

(6) Carbon Residue (Conradson method)

This test indicates the relative carbon forming propensity of an oil. The test is a means of determining the residual carbon, etc., left when an oil is burned under specified conditions. This test has been used much more in recent times in line with the use of high viscosity fuels in Z.C. engines.

The mass of the sample placed in the silica crucible must not exceed 0.01 kg. flame gauge

I

sheet block sheet

CONRADSON CARBON TEST APPARATUS Fig. 2.9

FUEL TECHNOLOGY 61

Initial heating period minutes vapour burn off

period 13 minutes 1, further heating for exactly 7 minutes, total heating period 30 minutes 2. The covers must be a loose fit to allow vapours to escape.

The heating and test method are closely controlled. After removal of sample and weighing, the result is expressed as 'Carbon Residue (Conradson)' as a percentage of the original sample mass. The test is usually repeated a number of times to obtain a uniformity of results. (see Fig. 2.9).

(7) Water in Oil

A quick test for presence of water in a substance is to add a sample to white copper sulphate

which turns to blue copper

sulphate in

the presence of water. The following test is

glass water cooled

condenser

more suitable for oil:

Referring to Fig. 2.10:

T h e t e s t u s u a l l y graduated

conducted is t h e Z.P. standard method. 100 ml of sample is mixed completely with 100 ml of special high g r a d e g a s o l i n e having standard properties. Steady heat is applied for about one hour. Water vapours are carried over with the distilled gasoline and are

500

condensed in the condenser and measured in the lower part of the receiver. The result being expressed as say 1 Water, P. Method.

Note. This sketch is very much simplified. The actual

a p p a r a t u s m u s t b e WATER TEST I P METHOD

62 REED'S GENERAL ENGINEERING KNOWLEDGE

exact B.S. dimensions which are highly detailed. The test must also be carried out under closely controlled conditions.

(8) Fire Point

This is the temperature at which the volatile vapours given off from a heated oil sample are ignitable by flame application and will burn continuously. The firepoint temperature can be anything up to about 40°C higher than the closed flashpoint temperature for most fuel oils.

(9) Acidity (or alkalinity)

This is indicated by the neutralisation (or saponification) number. This number is the mass, in milligrammes, of an alkali which is often potassium hydroxide, needed to neutralise the acid in one of sample. The oil is often alkaline, in this case the acid to neutralise it is in turn neutralised by the alkali and the result is then expressed as base neutralisation number. Alternatively the quantities can be expressed in ppm for 1 ml of oil sample (usually dissolved in industrial methylated spirits). Phenolphthalein can be used as the indicator. Total Base Number (TBN) is often used for alkalinity indication for lubricating oils.

(10) Ash

A sample of oil (250 ml minimum) is cautiously and slowly evaporated to dryness and ignition continued until all traces of carbon have disappeared. The ash is then expressed as a mass percentage of the original sample. Ash consists usually of hard abrasive mineral particles such as quartz, silicates, iron and aluminium oxides, sand, etc. A residue test by volume after heating to 350°C) is sometimes used.

(11) Other Tests

These are numerous, examples being: asphaltenes, sediment, suspended solids, oxidation, emulsion number, cloud point, setting point, precipitation number, etc.

These are more complex laboratory tests whose description is difficult to simplify and therefore are not considered further. Three other tests however, not mentioned previously, are regarded as of extreme importance in Z.C. engine practice. In view of this these tests namely, octane number, cetane number and crankcase oil dilution, will now be considered.

FUEL TECHNOLOGY 63

(12) Octane Number

Is indicative of the knock rating. Knocking or pinking are characteristic of some I.C. engine fuels, particularly in spark ignition engines, this can cause pre-ignition, overheat and damage.

Normally on spark initiation the flame front proceeds through the mixture at a speed of about 18 If, due to engine conditions or type of fuel used, the mixture in front of the flame front has its temperature and pressure raised above the spontaneous ignition point then auto ignition occurs. This means that by the time the last gas charge is reached the flame front speeds can reach 2.2 and detonation, temperature rise and heavy shock waves occur. Knocking tendency is dependent on many variables such as compression ratio, turbulence, mixture strength, etc.

Test

Iso-octane has very good anti knock properties and is taken as upper limit 100. Normal heptane has very poor anti knock properties and is taken as lower limit zero. Therefore octane number is the percentage by volume of iso-octane in a mixture of iso-octane and normal heptane which has the same knock characteristics as the chosen fuel. The test is conducted under fixed conditions on a standard engine which usually has electronic detonation detection. Modern fuels, for aviation, etc., have octane numbers over 100 and for these the term Performance Number is used. In this case tetraethyl lead (T.E.L.) is usually added in specified proportions to the octane, this chemical has a very high anti knock characteristic and is in fact often used as a fuel additive.

(13) Cetane Number

Is an indication of the ignition quality of the fuel. In a compression ignition engine, commonly called Diesel engine, cold starting is required. Also the time interval between fuel injection and firing, called ignition delay, must not be too long otherwise collected fuel will generate high pressures when it does ignite and Diesel knock results. Paraffin hydrocarbons have the best ignition quality and are thus most suitable. Speed and cetane number can be correlated, for high speed engines (above 13.3 a cetane number of 48 may be regarded as a minimum, whilst for very slow running engines (below 1.7 a cetane number of 15 is a minimum.

64 REED'S GENERAL ENGINEERING KNOWLEDGE

A Diesel fuel used in a hot petrol engine would cause detonation, it has a low octane number.

Test

Cetane is a paraffin hydrocarbon, hexadecane being

its correct designation, of high ignition quality and is taken as the upper limit of 100. Alpha-methyl-napthalene is of low ignition quality and is taken as the lower limit of zero. Thus cetane number is numerically the percentage by volume of cetane in

a

mixture of cetane and alpha-methyl-napthalene that matches the chosen fuel in ignition quality.

There are a number of tests, one is by measurement of the delay period when running, by use of a cathode ray tube on a standard engine. Another, which is probably the best, is t o use a standard engine running under fixed conditions with a variable compression ratio to give a standard delay, and using the compression ratio as an indication of cetane number.

An alternative method called Diesel index can be used but it is not as reliable as cetane number. Density is often indicative of cetane number especially in the middle ranges, density 850 kg/m3_{, cetane number about 61, density 950 cetane}

number about 37. Some success has been achieved by the use of additives such as acetone peroxide.

(14) Crankcase Oil Dilution

Is the percentage of fuel oil contamination of lubricating oil occurring in Z.C. engines. The lubricating oil sample is mixed with water and heated, fuel volatiles are carried over with the steam vapour formed. By condensation of these vapours and separation, the fuel content can be measured and can be expressed as a percentage of the original lubricating oil sample by mass.

It is also important t o check the lubricating oil for water con- tamination, for this purpose a similar separation test by heating is satisfactory. Severe corrosion of crankshafts has been caused by sulphur products from fuel oil mixing with any water in the lubricating oil to form sulphuric acids which are carried round the lubricating oil system.

Analysis of Fuel Oils (Typical)

It is not practice to assume a trend with one variable will apply to another. As a generalisation the 'heavier' the oil the higher the viscosity and flashpoint and the lower the calorific value. This would indicate extra heating, purification, etc., systems;

FUEL TECHNOLOGY 65

Constituent or Property Petrol Kerosene Diesel Residual

Carbon 85.5 86.3 86.3 86.1

Hydrogen 14.4 13.6 12.8 11.8

Sulphur 0.1 0.1 0.9 2.1

Density, 733 739 870 950

Higher calorific value,

47.0 46.7 46.0 44.0

Lower calorific value

43.7 43.6 43.3 41.4

Viscosity, at 50°C 1.5 1.6 5 350

Closed Flashpoint 0 50 85 90

TABLE 2.1

reduced storage volume for a given bunkering mass; increased fuel demand for a given power.

Proposed Specification for Marine Fuels

Table 2.2 is a draft specification with the objective of providing an agreed standard internationally.

COMBUSTION OF FUEL

The combustible elements in a fuel are carbon hydrogen ( HZ) and sulphur These combustibles when supplied with

oxygen ( 0 2 ) from atmospheric air combust and liberate heat. Exothermic reactions are those involving heat evolution, as are most combustion reactions, but some are endothermic and require heat supplied externally.

Combustion of Carbon

Relative Masses 12

+

(16

x

2 ) 44 + 2 3

Thus 1 kg of carbon requires 23 of oxygen and forms 33 kg of

In document 3 Introducción al convertidor de frecuencia de bajos armónicos 11 (página 150-157)