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1. CHAPTER 4 Fuel for IC Engines

2. 4 .1 Introduction In an internal combustion engine the heat energy is released by burning fuel in the engine cylinder. The chemical reactions which permit the release of heat energy are quite fast but the time taken in preparing a proper mixture of fuel and air depends mainly upon the nature of the fuel and the method of introducing it into the combustion chamber . In other words, the fuels used in the internal combustion engines are designed to satisfy the performance requirements of the engine system in which they are used.

3. The required properties of fuels Thus, fuel must have certain physical, chemical, and combustion properties, such as: 1. High energy density 2. Good combustion qualities 3. High thermal stability 4. Low deposit forming tendencies 5. Compatibility with the engine hardware 6. Good fire safety 7. Low toxicity 8. Low pollution 9. Easy transferability and onboard vehicle storage

4. The basic requirements The basic requirements for an internal combustion engine fuel are: The combustion process in the cylinder must take as little time as possible A maximum amount of heat energy must be released during this period.

5. Source of fuels The natural petroleum oil is the largest single source of internal combustion engine fuels. The boiling range of petrol is 30°C to 200°C and that of diesel oil 200°C to 375°C. Ordinary petroleum contains or can be very easily converted into these fractions.

6. The constituents of crude petroleum The constituents of crude petroleum are classified in four main groups. There are: paraffins having the general formula C n H 2n+2 , olefins having the general formula C n H 2n , naphthene having the general formula C n H 2n , and aromatics having the general formula C n H 2n-6

7. (a) Paraffin series (alkanes) The normal or straight chain paraffin hydrocarbons consists of a straight chain molecular structure. The general chemical formula for this series is CnH2n+2 where n is the number of carbon atoms. The name of each member ends in ‘ ane ’ as in methane, propane, hexane, etc. n-Propane means normal propane. The valency of carbon is fully utilized by single bonds, hence straight chain paraffins are saturated compounds and are characteristically very stable.

8. Branch chain or iso-paraffins Branch chain or iso-paraffins have an open chain structure which is branched. Iso-butane and iso-octane are examples of this type. The general formula for iso-paraffins is again CnH2n +2. Iso-paraffins are also stable compounds and are highly knock-resistant when used as SI engine fuels.

9. (b) Olefin Series (Alkenes) Olefins are compounds with one or more double-bonded carbon atoms in a straight chain. Due to double bonds these are capable of uniting with other materials without displacing hydrogen, i.e. they are unsaturated compounds. The general formulae are CnH2n for mono-olefins (one double-bond) and CnH2n-2 for the diolefins (two double-bonds).

10. (c) Naphthene series (cyclo-paraffins) The naphthene series has the same chemical formula as the olefins series, CnH2n, but have greatly different properties. The naphthene's are ring or cyclic compounds whereas the olefins are straight-chain compounds with a double-bond connecting two carbon atoms. The naphthene's are saturated compounds whereas olefins are unsaturated .

11. (d) Aromatic series Aromatics are ring structure compounds having the benzene molecule (C 6 H 6 ) as their central structure. Various aromatic compounds are formed by replacing one or more of the hydrogen atoms of the benzene molecule with an organic radical such as paraffins, naphthene and olefins. The three double-bonds make aromatics very active and so they are highly unsaturated compounds. Both benzene and toluene are present to a modest extent in most unaltered gasolines.

12. Refining Process The crude oil, as it comes out from the oil wells, contains impurities such as water, solids and gases consisting mainly of ethane and methane. By simple atmospheric pressure distillation , utilizing the fact that the boiling points of various hydrocarbons increase with an increase in molecular weight, the crude oil is separated into gasoline , kerosene , fuel oil or reduced crude oil. First of all the lightest fractions such as butane, propane, ethane , etc., which were either standing above the liquid in earth or dissolved in the crude, are obtained at low temperature and form natural gas.

13. Products of Refining Process Natural Gas: Natural gas is found dissolved in petroleum or in huge amounts under earth surface in oil and gas bearing areas. Natural gas is made up mainly of the paraffinic compound methane , a small amount of propane , ethane , butane and other light hydrocarbons plus some nitrogen and oxygen. Liquified Petroleum Gas (LPG): Propane and butane and some other light hydrocarbons after separation from natural gas, if stored under pressure , form liquid and this is called liquified petroleum gas (LPG). LPG is supplied in containers under pressure.

14. Con….. Gasoline: Gasoline is the lightest liquid petroleum fraction. All material boiling up to 200°C is generally considered as gasoline. This is mixture of a number of hydrocarbons (more than 40). The composition depends upon the crude oil and refining process. Gasoline lies in specific gravity range 0.70 to 0.78. This covers most of fuels used for spark-ignition engines.

15. Con….. Kerosene: The kerosene is a fraction heavier than gasoline . Its boiling range is 150 to 300°C and the specific gravity range is 0.78 to 0.85. Kerosene is used in aviation gas turbines , as jet fuel and in lamps and stoves .

16. Con…….. Diesel Oils: Diesel oils are fuels which lie between kerosene and lubricating oils . These cover a wide range of specific gravity and boiling point. Boiling range is 200 to 370°C . These form the fuels for compression- ignition engines. Fuel Oils: Fuel oils are similar to diesel fuel in specific gravity and distillation range but their composition varies in a range wider than those of diesel fuels. These are used as industrial fuels.

17. Con…….. Lubricating Oils: Lubricating oils are made up of heavy distillate of petroleum and residual oil. These are used for lubricating purposes. Tar and Asphalt: Tar and asphalt are solid or semi-solid undistilled products of petroleum. Petroleum Coke: Petroleum coke is used as solid industrial fuel.

18. 4.2 Fuels for Spark-ignition Engines Gasoline is the major product of the petroleum industry and forms most of the SI engine fuel. It is a mixture of various hydrocarbons such as paraffins , olefins , naphthene , and aromatics. The composition of gasoline varies with the source of crude oil and the nature of refining process.

19. Requirement of Gasoline The following are the requirements of an ideal gasoline: It should mix readily with air It must be knock-resistant. It should not pre-ignite easily. It should be easy to handle. It must be cheap and should be available everywhere. It must burn clean and produce no corrosion on engine parts. It must have a high calorific value. It should not form gum and varnish.

20. Gum Content All petroleum motor fuels oxidize slowly in presence of air. The oxidation of unsaturated hydrocarbons results in formation of resinous materials called gum. Oxidized gasoline shows a loss of antiknock quality. High gum content fuels may clog carburetor jets , promote sticking of automatic chokes , sticking of the intake valves , piston rings , and promote formation of manifold deposits.

21. Sulphur Control Almost all gasoline's have some sulphur. Sulphur troubles in three ways: corrosion , odor and poor explosion characteristics of gasoline fuels . High sulphur causes corrosion in the muffler and tail pipe of the exhaust system.

22. Knock Rating of SI Engine Fuels Octane Number In octane number tests the performance of unknown gasoline is compared with that of a series of reference fuels consisting of mixtures of iso-octane and n-heptane . Iso-octane is a low boiling point branched chain compound and has a very slight tendency to knock and, hence, arbitrarily assigned an octane number of 100 . n-heptane detonates rapidly and hence, has been assigned an octane number of zero.

23. Con……… These fuels are known as the Primary Reference Fuels (PRF) and the octane number of an unknown fuel is defined as the percentage of iso-octane in the Primary Reference Fuel that gives the same knock intensity. Thus, a gasoline that gives the same knock intensity as 90 PRF (90% vol. iso-octane, 10% vol. n-heptane) in a test is said to have an octane number of 90. The engine used for this test if CFR (Cooperative Fuel Research) variable compression ratio engine. In order to find the octane number of certain fuel the fuel is tested in the CFR engine until the condition of detonation is reached.

24. The advantages of high-octane fuel The advantages of high-octane fuel are summarized below: The engine can be operated at high compression ratio and, therefore, with high efficiency without detonation. The engine can be supercharged to high output without detonation. Optimum spark advance may be employed raising both power and efficiency.

25. 4.3 Diesel Fuels Diesel Fuels Diesel fuels are petroleum fractions that lie between kerosene and the lubricating oils. Diesel fuel properties are influenced by the crude source and the method of refining. The properties important for a good diesel fuel can be discussed under three main headings: Satisfactory handling and storage Smooth and efficient burning Continued cleanliness during use

26. Handling and Storage The properties of diesel fuel which control safety and convenience in handling are flash and fire points , viscosity , cloud and pour points . Flash point: Flash point is the temperature at which a flammable liquid will produce. Fire point: Fire point is the temperature at which the flash will sustain itself as a steady flame for at least five seconds. The flash and fire points indicate the temperature below which oil can be handled without danger of fire, otherwise they are of little consequence.

27. Con…….. Viscosity Viscosity of a fuel is a measure of its resistance to flow. Liquid fuel is injected into combustion space of the diesel cylinder by a high pressure pump and this pump must overcome the resistance of the liquid to flow and the friction which resists the motion of the pump. Very high viscosity can render starting from cold difficult and produce undesirable high pressure on pump and fuel lines. Too low viscosity can cause undue wear of pump and increased maintenance.

28. Con…….. Cloud point: The temperature below which the wax content of the petroleum oil separates out in the form of a solid is called cloud point. Such waxy solid can clog fuel lines and fuel filters . Pourpoint: Pour point is the temperature below which the entire mass of the fuel freeze and thus cause flow of fuel impossible. Pour point is usually 5 to 10°C below the cloud point.

29. Smooth and Efficient Burning In order to burn well a diesel fuel should atomise properly, ignite quickly, and burn completely. Volatility , cetane number , aromatic contents are important properties which affect ignition quality of a diesel fuel.

30. Cleanliness This is very important for a diesel fuel. Even a small amount of dust can affect engine operation as the plunger of the injection pump is lapped to tolerance of 0.00025 mm. The cleanliness of an engine depends to a large extent on the completeness with which the fuel burns, which in turn depends partly on the design and service conditions of the engine.

31. Knock Rating of CI Engine Fuels Cetane number The cetane rating of a diesel fuel is a measure of its ability to auto ignite quickly when it is injected into the compressed and heated air in the engine. The cetane number is a numerical measure of the influence the diesel fuel has in determining the ignition delay. Higher the cetane rating of the fuel lesser is the propensity for diesel knock.

32. Con…… The cetane number of a fuel is the percentage by volume of cetane in a mixture of cetane and methyl naphthalene (C10H7CH3) that has the same performance in the standard test engine as that of the fuel. Cetane (C16H34) is arbitrarily given a number 100 and originally α -methyl naphthalene was given a number . Diesel fuels have typical cetane rating of 40 to 60

33. 4.4 Alternative Fuels The reasons for alternate fuel development are: Increased use and the depletion of fossil fuels. The concern over the emission problems. A large percentage of crude oil must be imported from other countries.

34. Possible Alternatives (1) Alcohol Alcohols are an attractive alternate fuel because they can be obtained from both natural and manufactured sources. Methanol (methyl alcohol) and ethanol (ethyl alcohol) are two kinds of alcohols that seem most promising. The advantages of alcohol as a fuel are: It is a high octane fuel so it can run more efficiently by using higher compression ratios. It produces less overall emissions compared to gasoline Alcohols have low Sulphur content in the fuel.

35. Con…….. The disadvantages of alcohol as a fuel are: Alcohols have a low energy content It has poor cold weather starting characteristics due to low vapor pressure and evaporation. Alcohols have almost invisible flames Many people find the strong odor of alcohol very offensive. Headaches and dizziness have been experienced when refueling an automobile.

36. Con…….. (2) Vegetable Oil Vegetable oil is considered as one of the alternative fuels for diesel engines However, the viscosity of vegetable oil is higher compared to diesel. Therefore, it must be lowered to allow for proper atomization in engines designed to burn diesel fuel.

37. Con…….. (3) Biodiesel Biodiesel refers to a vegetable oil or animal fat-based diesel engine fuel consisting of long-chain alkyl (ethyl, methyl, or propyl) esters. Much of the world uses a system known as the ”B” factor to state the amount of biodiesel in any fuel mix. 20% biodiesel, 80% petro-diesel is labeled B20 Blends, less than 20% biodiesel can be used in diesel engines without any or with very minor modifications of the engine.

38. Con…….. (4) Hydrogen A number of automobile manufacturers have built with prototype or modified engines which operate on hydrogen fuel . The advantages of hydrogen as an IC engine fuel include: Low emissions Fuel availability Fuel leakage to environment is not a pollutant High energy content per volume when stored as a liquid

39. Con…….. (5) Biogas Biogas is another alternate fuel tried in diesels. Biogas can be produced by anaerobic digestion of organic matter . Potential raw materials available on a large scale are cow dung, municipal wastes, and plants. It consists mainly of methane and carbon dioxide. Its calorific value is low but it’s knock resistance (octane number) is high.

40. Thank you!