CONVENTIONAL FUELS
5.6 RATING OF FUELS
Volume evaporated (%)
0 20 40 60 80 100 150
200 250 300 350 400
Temperature ( C)o
Fig. 5.3 Typical distillation curve for diesel
(vii) Handling Ease : The fuel should be a liquid that will readily flow under all conditions that are encountered in actual use. This requirement is measured by the pour point and the viscosity of the fuel. The fuel should also have a high flash point and a high fire point.
very poor antiknock qualities and is given a rating of 0 octane number. The Octane number fuel is defined as the percentage, by volume, of iso-octane in a mixture of iso-octane and normal heptane, which exactly matches the knocking intensity of the fuel in a standard engine under a set of standard operating conditions. The addition of certain compounds (e.g. tetraethyl lead) to iso-octane produces fuels of greater antiknock quality (above 100 octane number). The antiknock effectiveness of tetraethyl lead, for the same quantity of lead added, decreases as the total content of lead in the fuel increases. Further, each octane number at the higher range of the octane scale will produce greater antiknock effect compared to the same unit at the lower end of the scale. For instance, octane number increase from 92 to 93 produce greater antiknock effect than a similar increase from 32 to 33 octane number. Because of this non-linear variation, a new scale was derived which expresses the approximate relative engine performance and the units of this scale are known as the Performance Numbers, P N. Octane numbers, ON above 100 can be computed by
ON(>100) = 100 + 28.28A
1.0 + 0.736A+√
1.0 + 0.736A−0.035216A2 whereAis TEL in ml/gal of fuel, or from the performance number,P N,
Octane Number = 100 +P N −100 3
Laboratory Method: The engine is run at specified conditions with a defi- nite compression ratio and a definite blend of reference fuels. The intensity of knock at these standard conditions is called standard knock. The knock meter is adjusted to give a particular reading under these conditions. The test fuel is now used in the engine and air-fuel ratio is adjusted to give maximum knock intensity. The compression ratio of the engine is gradually changed until the knock meter reading is the same as in the previous run (standard knock).
The compression ratio is now fixed and known blends of reference fuels are used in the engine. The blend of reference fuels which gives a knock meter reading equal to the standard value will match the knocking characteristics of the test fuel. Percentage by volume of iso-octane in the particular blend gives the octane number.
5.6.2 Rating of CI Engine Fuels
In compression-ignition engines, the knock resistance depends on chemical characteristics as well as on the operating and design conditions of the en- gine. Therefore, the knock rating of a diesel fuel is found by comparing the fuel under prescribed conditions of operation in a special engine with pri- mary reference fuels. The reference fuels are normal cetane, C16H34, which is arbitrarily assigned a cetane number of 100 and alpha methyl naphthalene, C11H10, with an assigned cetane number of 0. Cetane number of a fuel is defined as the percentage by volume of normal cetane in a mixture of normal cetane andα-methyl naphthalene which has the same ignition characteristics (ignition delay) as the test fuel when combustion is carried out in a stan- dard engine under specified operating conditions. Since ignition delay is the
primary factor in controlling the initial autoignition in the CI engine, it is reasonable to conclude that knock should be directly related to the ignition delay of the fuel. Knock resistance property of diesel oil can be improved by adding small quantities of compounds like amyl nitrate, ethyl nitrate or ether.
Laboratory Method: The test is carried out in a standard single cylinder engine like the CFR diesel engine or Ricardo single cylinder variable compres- sion engine under the conditions shown in Table 5.3. The test fuel is first used in the engine operating at the specified conditions. The fuel pump delivery is adjusted to give a particular fuel-air ratio. The injection timing is also ad- justed to give an injection advance of 13 degrees. By varying the compression ratio the ignition delay can be increased or decreased until a position is found where combustion begins atT DC. When this position is found, the test fuel undergoes a 13 degree ignition delay. The cetane number of the unknown fuel
Table 5.3 Conditions for ignition quality test on diesel fuels
Engine speed 900 rpm
Jacket water temperature 100◦C Inlet air temperature 65.5◦C
Injection advance Constant at 13◦bT DC
Ignition delay 13◦
can be estimated by noting the compression ratio for 13 degree delay and then referring to a prepared chart showing the relationship between cetane number and compression ratio. However, for accuracy two reference fuel blends differ- ing by not more than 5 cetane numbers are selected to bracket the unknown sample. The compression ratio is varied for each reference blend to reach the standard ignition delay (13 degrees) and, by interpolation of the compression ratios, the cetane rating of the unknown fuel is determined.
Review Questions
5.1 What are the different kinds of fuels used in an IC engine?
5.2 Briefly explain the chemical structure of petroleum.
5.3 Give the general chemical formula of the following fuels: (i) paraffin (ii) olefin (iii) diolefin (iv) naphthene(v) aromatic Also state their molecular arrangements and mention whether they are saturated or unsaturated.
5.4 Briefly explain the petroleum refining process.
5.5 Discuss the significance of distillation curves.
5.6 Discuss the important qualities of an SI and CI engine fuel.
5.7 What is the effect of high sulphur content on the performance of SI and CI engines?
5.8 How are SI and CI engine fuels rated?
Multiple Choice Questions (choose the most appropriate answer) 1. Advantage of gaseous fuel is that
(a) it can be stored easily (b) it can mix easily with air
(c) it can displace more air from the engine (d) all of the above
2. Paraffins are in general represented by (a) CnHn
(b) CnH2n
(c) CnH2n+2 (d) CnH2n−6
3. Paraffins have molecular structure of (a) chain saturated
(b) chain unsaturated (c) ring saturated (d) ring unsaturated
4. Olefins are represented by the formula (a) CnH2n
(b) CnH2n+2
(c) CnH2n−4
(d) CnH2n−6
5. Hydrocarbons are decomposed into smaller hydrocarbons by (a) reforming
(b) refining (c) cracking (d) polymerization
6. The molecular structure of the straight-run gasoline is changed by (a) cracking
(b) reforming (c) refining (d) boiling
7. For SI engines fuels most preferred are (a) aromatics
(b) paraffins (c) olefins (d) napthenes
8. For CI engine fuels most preferred are (a) napthenes
(b) paraffins (c) olefins (d) aromatics
9. Octane number of iso-octane is (a) 0
(b) 30 (c) 60 (d) 100
10. Ignition quality of diesel fuel is indicated by its (a) octane number
(b) cetane number (c) flash point (d) fire point 11. Iso-octane has
(a) straight chain structure with 8 carbon atoms (b) ring chain structure with 8 carbon atoms (c) branched chain structure with 8 carbon atoms (d) none of the above
12. An effective method to prevent detonation in SI engines is (a) heating of the charge
(b) cooling of the charge
(c) increasing the charge pressure (d) none of the above
13. The major constituent of natural gas (a) butane
(b) ethane (c) methane (d) propane
14. Addition of tetraethyl lead in gasoline is being discontinued as (a) it has bad odour
(b) it is costly
(c) decreases engine efficiency (d) blocks the catalytic converter
15. Crude oil is separated into gasoline, kerosene and fuel oil by (a) cracking
(b) heating
(c) fractional distillation (d) reforming
16. Abnormal combustion in a SI engine causes (a) high ratio of energy decrease
(b) excessive rise in pressure and temperature (c) reduction in thermal efficiency
(d) all of the above
17. A good CI engine fuel should have (a) high octane number
(b) very high cetane number (c) a short ignition lag (d) all of the above
18. Blending of fuel is the process of (a) just mixing two fuels
(b) obtaining a product of desired quality (c) mixing of fuel and air for combustion (d) none of the above
Ans: 1. – (b) 2. – (c) 3. – (a) 4. – (a) 5. – (c) 6. – (b) 7. – (a) 8. – (b) 9. – (d) 10. – (b) 11. – (c) 12. – (b) 13. – (c) 14. – (d) 15. – (c) 16. – (d) 17. – (c) 18. – (b)