Qualities Of Engine Fuels Engineering Essay
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Published: Mon, 5 Dec 2016
Depleting petroleum reserves and increasing cost of the petroleum products demand an intensive search for new alternative fuels. Biofuels are proved to be the best substitutes for the existing petrofuels. But they require little engine modification or fuel modification.
Generally biofuels are the oils obtained from the living plant sources. These oils may be obtained from resin and plant seeds. Plant oils are renewable and have low sulphur in nature. As biofuels are more expensive than fossil fuels, the widespread use of biofuel was restrained from its use in I.C. engines (Mayer-Pitroff, 1995; Choi et al., 1997). The use of vegetable oil in diesel engine was identified well before the exploration of the other promising alternative fuel alcohol. But the problems associated with vegetable oil are the high viscosity and low volatility. These properties have an adverse effect on fuel injection system and may lead to heavy carbon deposits in the engine combustion chamber (Choi et al., 1997; Masjuki et al., 1997; Husna et al., 1995). In the present work, turpentine is used as an alternate fuel to diesel. Turpentine is also a biofuel. It is the volatile fraction of resin extracted from pine tree. The pine tree comes under the plant class of conifers. Most of the conifers will exude resin if wounded or naturally from branches. The distillation of pinus resin yields two products} turpentine and rosin. Turpentine was used in early engines without any modification. The abundant availability of petrofuels had stopped the usage of turpentine in I.C. engines. But the increasing cost of petrofuel prevailing today reopens the utility of turpentine in I.C. engine.
Turpentine oil has low cetane number; it could be used in direct injection diesel engines in the form of turpentine-diesel blends (Rickeard and Thompson, 1993) and dual fuel (DF) mode. The specific objectives of this study are to analyse the performance and emission characteristics of turpentine in direct injection diesel engine and to analyse its feasibility as a fuel in a D.I diesel engine. From the authors’ earlier study (Karthikeyan and Mahalakshmi, 2005), it was identified that 20% turpentine and 80% diesel is the optimum blend in terms of performance and emission characteristics.
In the present work, a normal diesel engine was modified to work in a DF mode with turpentine and diesel as primary and pilot fuels, respectively. The resulting homogeneous mixture was compressed to a temperature below the self-ignition point. The pilot fuel was injected through the standard injection system and initiated the combustion in the primary fuel air mixture. The primary fuel supplied most of the heat energy. It has been reported that the DF operation at lighter load is less efficient than its diesel counterpart. However, beyond half load, the efficiency of DF operation is improved sufficiently and can even become better than that of diesel engine. However, at higher loads due to the occurrence of knock, a poor performance was recorded. This can be solved by the addition of water diluent into the cylinder at the time of knocking. Water, a very good diluent, is inducted along with the primary fuel during the knocking. The inducted water will evaporate by absorbing the heat from the combustion chamber and is mixed into the charge for keeping the mixture below self-ignition temperature. The important advantages of this method of knock suppression are restoration of performance at full load, maintenance of the same pilot quantity throughout the load range and reduction in the fuel consumption at full load. From the results, it was found that all performance and emission parameters of turpentine except volumetric efficiency are better than those of diesel fuel.
Important qualities of engine fuels
The fuel must have certain physical, chemical and combustion properties in general which are enumerated below:-
High energy density.
Good combustion qualities.
High thermal stability.
Low deposit forming tendencies.
Compatibility with the engine hardware.
Good fire safety.
Easy transferability and onboard vehicle storage.
These properties are elaborated by dividing the fuels for SI and CI engines. Fuels used in ic engines should possess certain basic qualities which are important for smooth running of engines. In this section the important qualities of fuels for both SI and CI engines are shown.
SI Engine Fuels
Gasoline which is mostly used in the present day SI engines is usually a blend of several low boiling paraffins, napthalenes and aromatics in varying proportions. Some of the important qualities of gasoline are discussed below.
Volatility:- Volatility is one of the main characterstic properties of gasoline which determines its suitability for use in an SI engine. Since gasoline is a mixture of different hydrocarbons, volatility depends on the fractional compositon of the fuel. The usual property of measuring the fuel volatility is the distillation of the fuel in special device at atm pr. And the presence of its own vapour. The fraction that boils off at a definite temp. is measured. The characterstic ponts are the temperature at which 10, 40, 50 and 90% of the volume evaporates as well as temperature at which boiling of the fuel terminates. The method of measuring volatility is standardized by American Society of Testing Materials(ASTM) and the graphical representation of the tests is generally termed as the ASTM distillation curve. The more important aspects of volatility related to engine fuels are discussed in the following bits.
Starting and warming up:- A certain part of gasoline should vapourize at room temperature for easy starting of the engine. Hence the portion of the distillation curve between 0 and 10% boiled off have relatively low boiling temperatures. As the engine warms up, the temperature will gradually increase to the operating temperature. Low distillation temperatures are desireable throughout the range of the distillation curve for best warm-up.
Operating range performance:- In order to obtain good vapourisation of the gasoline, low distillation temperatures are preferable in the engine operating range. Better vapourisation tends to produce both more uniform distribution of fuel to the cylinders as well as better acceleration characterstics by reducing the quantity of liquid droplets in the intake manifold.
Crankcase dilution:- Liquid fuel in the cylinder causes loss of lubricating oil( by washing away oil from the cylinder walls ) which deteriorates the quality of lubrication and tends to cause damage to the engine through increased friction. The liquid gasoline may also dilute the lubricating oil and weaken the oil film between rubbing surfaces. To prevent this situation, the upper portion of the distillation curve should exhibit sufficiently low distillation temperatures to ensure that all gasoline in the cylinder is vapourized by the time the combustion starts.
Vapour lock characterstics:- High rate of vapourisation of fuel can upset the carburetor metering or even stop the fuel flow to the engine by setting up a vapour lock in the fuel passages. This characterstic demands the presence of relatively high boiling temperature through out the distillation range.
Antiknock quality:- Abnormal burning or detonation in an SI engine combustion chamber causes a very high rate of energy release, excessive temperature and pressure inside the cylinder adversely effects its thermal efficiency. Therefore, the characterstic of fuel should be such that it reduces the tendency to detonation and this property is called its antiknock property. The antiknock property of a fuel depends on the self-ignition characterstics of its mixture and vary largely with the chemical composition and molecular structure of fuel. In general, the best SI engine fuel will be that having the highest antiknock property, since this permits the use of higher compression ratios and thus the engine thermal efficiency and the power output can be greatly increased.
Gum deposits:- Reactive hydrocarbons and the impurities in the fuel have a tendency to oxidize and form liquid and solid gummy substances. Unsaturated hydrocarbons are more prone to form gum deposits. Gum deposits may lead to clogging of carburetor jets and enlarging of the valve stems, cylinders and pistons.
Sulphur content:- Hydrocarbon fuels may contain free sulphur, hydrogen sulphide and other sulphur compounds which are objectionable for several reasons. The sulphur is the corrosive element of the fuel that can corrode fuel lines, carburetors and injection pumps and it will unite with oxygen to form sulphur dioxide that, in presence of water at low temperatures, may form sulphurous acid. Since sulphur has a low ignition temperature, the presence of sulphur can reduce the self-ignition temperature, then promoting knock in the SI engine.
CI Engine Fuels
Knock characteristics:- Knock in the CI engine occurs because of an ignition lag in the combustion of the fuel between the time of injection and the time of actual burning. As the ignition lag increases, the amount of fuel accumulated in the combustion chamber increases and when combustion actually takes place, abnormal amount of energy is suddenly released causes an excessive rate of pressure rise which results in an audible knock. Hence, a good CI engine fuel should have a short ignition lag and will ignite more readily. Furthermore, ignition lag affects the starting, warm up, and leads to the production of exhaust smoke in CI engine. The present day measure in the cetane rating, the best fuel in general, will have a cetane rating sufficiently high to avoid objectionable knock.
Volatility:- The fuel should be sufficiently volatile in the operating range of temperature to produce good mixing and combustion.
Starting Characteristics:- The fuel should help in starting the engine easily. This requirement demands high enough volatility to form a combustible mixture readily and a high cetane rating in order that the self-ignition temperature is low.
Smoking and odor:- The fuel should not promote either smoke or odour in the engine exhaust. Generally, good volatility is the first prerequisite to ensure good mixing and therefore complete combustion.
Viscosity:- CI engine fuel should be able to flow through the fuel system and the strainers under the lowest operating temperatures to which the engine is subjected to.
Corrosion and Wear:- The fuel should not cause corrosion and wear of the engine components before or after combustion. These requirements are directly related to the presence of sulphur, ash and residue in the fuel.
Handling Ease:- The fuel should be a liquid that will readily flow under all conditions that are encountered in actual case. 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
Rating of fuels:-
Rating of fuels is normally done for their antiknock qualities. The rating of fuels is done by defining two parameters cetane number and octane number for diesel and gasoline respectively. Here the detailed description of the rating is given.
Rating of CI engine fuels:-
The knock resistance depends on chemical properties as well as on the operating and design conditions of the engine. So the knock rating of a diesel fuel is found by comparing the fuel at a specific condition with primary reference fuels. The reference fuels are normal cetane C16H34, which has been assigned a cetane number of 100 and alpha methyl naphthalene, C11H10, with a cetane number of 0.
Def. Cetane number of a fuel is defined as the percentage by volume of normal cetane in a mixture of normal cetane and alpha methyl naphthalene which has the same ignition characteristics (ignition delay) as the test fuel when combustion is carried out in a standard engine under specified operating conditions.
The knock should be directly related to the ignition delay as it is the major factor in controlling of the autoignition in the CI engine. Knock resistance property of a diesel oil can be improved by adding small quantities of compounds like amyl nitrate, ethyl nitrate or ether.
Rating of SI engine fuels:-
The knock resistance is the most important characteristic of the fuel for SI engine. The fuels differ widely in their ability to resist knock depending on their chemical composition. In addition to the chemical properties of the hydrocarbons in the fuel other operating parameters such as fuel-air ratio, ignition timing, dilution, engine speed, shape of combustion chamber, ambient conditions, compression ratio etc. affect the tendency to knock in the engine cylinder. Therefore, in order to determine the knock resistance characteristic of the fuel, the engine and its operating variables must be fixed at standard values.
Here also there are two reference fuels viz. iso-octane (C8H18) chemically being a very god antiknock fuel, has been assigned an octane number of 100 and normal heptane (C7H16), it has very poor antiknock qualities and is assigned an octane number of 0.
Def. The octane number of a fuel is defined as the percentage, by volume, of iso-octane in a mixture of iso-octane and normal heptanes, which exactly matches the knocking intensity of the fuel in standard engine under a set of standard operating conditions.
The octane number at the higher range of scale will produce greater antiknock effect compared to the same unit at the lower end of the scale e.g. octane number increase from 90 to 91 produces greater antiknock effect than a similar increase from 30 to 31. The addition of some chemicals like tetra ethyl lead(TEL) to iso-octane produces fuels of greater antiknock qualities.
Some additives are used to improve the combustion in the IC engines which are discussed in the next section.
Some compounds called additives or dopes are used to improve the combustion properties of fuels. The main combustion problems that arise when the operating conditions become severe are knocking and surface ignition. That can be tackled by a lot of ways of which one is using additives.
For an additive to be acceptable, it must satisfy some basic requirements. These are as follows:-
It must be affective in desired reaction that is knock resistance or surface ignition or both.
It should be soluble in fuel under all conditions.
It should be stable in storage and have no adverse effect on fuel stability.
It should be in the liquid phase at normal temperature, and volatile to give rapid vaporization in the manifold.
It must not produce harmful deposits.
Its water solubility must be minimum to minimize handling loses.
Alternative fuels for SI and CI engines:-
There are three types of fuels viz. solid, liquid and gaseous fuels. Mainly liquid fuels are used in ic engines. Nowadays gaseous fuels such as LPG and CNG are also in use as automobile fuels. In early periods even solid fuels like charcoal, coal and slurry were also tried.
They are not used nowadays, but when Rudolf was designing the engine he used coal dust mixed with water. He used very fine coal particles thoroughly mixed with water and injected in the engine. As coal is abundantly available it becomes an attractive fuel, but there are problems in using it. Major problems are abrasiveness due to solid particles which leads to wear of injectors and the piston rings.
Liquid fuels are preferred due to their high calorific value and they can be easily stored. Moreover the problem of wear is also overcome by using liquid fuels. The most common liquid alternative is alcohol. Alcohol has both advantages and disadvantages as a fuel which is discussed below.
It can be manufactured and even obtained from natural sources.
It has a high octane number even greater than 100, so a large compression ratio can be employed.
It has higher flame speed.
Overall emissions produced by alcohol are less than gasoline.
It provides higher pressure and more power in the expansion stroke.
Sulphur content is less in alcohols.
The calorific value of alcohol is very less, almost half of the general fuels used in ic engines. That means the fuel quantity required to produce a certain amount of power is doubled if we use alcohol, which inturn means that a vehicle can travel only half the distance with full fuel tank as it would have travelled if gasoline was used.
It is more corrosive than gasoline on metal and plastic parts. So use of alcohol puts restrictions on the design of the engine. All the parts like piston rings gaskets, etc. get worn out by long term alcohol use.
Its combustion produces aldehydes in the exhaust which is not acceptable.
They have poor ignition characteristics in general.
Its use leads to poor starting characteristics in cold weather.
Air can enter the storage tank due to low vapor pressure of alcohol and can form combustible mixture.
Mainly methanol and ethanol are used as fuel in ic engines.
Since physical delay is almost zero for gaseous fuels, they are suited for use in ic engines. Since the gas displaces the equal amount of air, the volumetric efficiency of the engine decreases. The major gaseous fuels are as follows:-
Advantages of hydrogen:-
Since there is no carbon in the fuel so the emissions are devoid of CO or HC. The exhaust mainly consists of H2O, N2 and NOx.
It is easily available. It can be manufactured by a number of ways including electrolysis of water.
If incase it is leaked to environment, it doesn’t act as a pollutant.
It has high energy content per unit volume. So for a given tank size, a larger distance can be traversed.
Disadvantages of hydrogen as a fuel:-
It is difficult to refuel and the possibility of knocking is more.
The volumetric efficiency decreases by the use of hydrogen as a fuel.
The flame temperature is very high so the NOx emissions increase.
Its operation is costlier than gasoline.
It has a lot of storage problems. In liquid state, it requires a thermally insulated fuel tank. In gas phase, it will require high pressure vessel with limited capacity.
Hydrogen can be used in diesel engines in 2 ways:-
By using in a dual fuel mode, in which hydrogen is inducted along with air and then the mixture of air and hydrogen is compressed in the cylinder. At the end of the compression stroke diesel is injected and the combustible mixture is burned. But hydrogen should be put in certain limits as it can lead to high pressure rise.
By surface ignition. Hydrogen is sprayed at the end of the compression stroke directly inside the cylinder. But the self ignition temperature of hydrogen is high, so it is sprayed on the hot glow plug in the combustion chamber which leads to the burning of hydrogen. This is known as surface ignition.
Hydrogen is a very reactive fuel, so a lot of care is to be taken in handling it. A flame arrester should be used to stop any possible back flash to the storage tank from the engine cylinder.
Natural gas is very easily available and is present at a number of locations. It can be easily obtained by process of drilling wells. When natural gas is obtained from drilling wells, it is known as casing head gas. It is generally treated for obtaining gasoline. When gasoline is taken out from natural gas, it is known as dry gas. Natural gas mainly consists of methane (60-95%) and other hydrocarbons. It also contains various amounts of N2, CO2, He and traces of other gases. When sulphur content is low, it is called sweet or else sour. It can be stored in two ways that is as compressed natural gas (CNG) and liquefied natural gas (LNG). In CNG, pressure of 16-25 bar is maintained and in LNG 70 to 210 bar at a temperature around -160oc. Now the advantages and disadvantages of the natural gas are discussed below.
Advantages of natural gas:-
Octane number of natural gas is very high about 110. So, it has a very high flame speed and thus provides a higher compression ratio.
Emissions are comparatively less. The aldehyde content in the emission is considerably less than methanol.
Natural gas is abundantly available in the world.
Disadvantages of natural gas as fuel:-
Volumetric efficiency of the engine decreases as it is a gaseous flow so the amount of air intake by the engine decreases.
Energy density is low which leads to low engine performance.
Fuel properties are inconsistent.
Refueling is a slow process.
Large pressurized fuel tank is required for its storage.
Methane is used with diesel in CI engine. Methane becomes the major component (90% of methane in the mixture). Methane is introduced in the engine cylinder with the help of pressurized pipes.
Compressed natural gas (CNG) is nowadays commonly used in big cities like Delhi, where the emissions from automobiles have crossed the limits as the emissions from burning of CNG are considerably less as compared to the emissions produced by a gasoline engine. CO emission is almost nullified by the use of CNG.
Liquefied petroleum gas:-
Propane and butane are mainly used as LPG. Both are obtained from the drilling well process. Sometimes they are used alone and sometimes combination of the two is used in the engine. These gases are compressed and cooled and stored under pressure in tanks in liquid form which are sealed.
Advantages of LPG as a fuel:-
It is obtained by distillation of resin obtained from trees, mainly pine trees. It is combination of ¡-pinene (65-70)% and ¢-pinene (30)%. It is mainly used as thinner for paints
Properties of turpentine oil:-
colourless, characterstic odor and taste.
Soluble in 5 vol of alcohol.
When perfectly pure, it exclusively consists of carbon and hydrogen.
boiling point-(153-175 )c.
Solubility:-Insoluble in water, soluble in benzene, chloroform, ether, carbon disulphide, petroleum oils.
Advantages of turpentine oil over diesel:-
1) It is a renewable fuel and biofuel, obtained from pine tree.
2) Self-ignition temperature-close to diesel fuel.
3) Boiling point-almost equal to diesel fuel.
4) Calorific value-slightly higher than diesel fuel.
5) Viscosity-almost equal to diesel fuel.
6) Offers 11-15% higher calorific value compared to other biofuels (biodiesel and
neat vegetable oil).
Pure acetylene is a colorless, highly flammable gas with an agreeable ethereal (ether-like) odor, but the odor of the commercial purity grade is distinctively garlic-like. Acetylene can be safely stored and used in cylinders filled with a porous material and containing a solvent (acetone) into which the acetylene has been dissolved.
Acetylene, when not dissolved in a solvent (free acetylene), can begin to dissociate (decompose) at pressures above 15 pounds per square inch gauge (psig). The products of dissociation are carbon, in the form of lampblack, and hydrogen. Considerable amounts of heat are generated by dissociation, which may produce explosions of great violence. Steel and wrought iron are recommended for use in acetylene piping. Rolled, forged, or cast steel, or malleable iron fittings may be used. Cast iron is not permissible for fittings. Unalloyed copper, silver, or mercury should never be used in direct contact with acetylene since there is the possibility of forming explosive acetylides. Wet acetylene will produce explosive acetylides on copper, 70-30 brass, and aluminum-bronze. Weight (not pressure) is used to determine the amount of acetylene in a cylinder. The tare weight is subtracted from the actual weight, and the difference is multiplied by 14.7 to determine the amount of gas in standard cubic feet. The molecular symbol for acetylene
Physical and Chemical Properties
APPEARANCE: Colorless gas
ODOR: Acetylene of 100% purity is odorless, but commercial acetylene has a distinctive, garlic-like odor.
PHYSICAL STATE: Gas at normal temperature and pressure
SUBLIMATION POINT at 1 atm: -118°F (-83.3°C)
MELTING POINT at 10 psig (170 kPa abs): -116°F (-82.2°C)
BOILING POINT at 10 psig (170 kPa abs): -103.4°F (-75.2°C)
FLASH POINT: -0°F (-17.8°C)
FLAMMABLE LIMITS IN AIR, % by volume: LOWER: 2.5% UPPER: 100%
VAPOR PRESSURE at 70°F (21.1°C): 649.6 psia (4479 kPa abs)*
VAPOR DENSITY at 32°F (0°C) and 1 atm: 0.07314 lb/ft3 (1.1716 kg/m3)
SPECIFIC GRAVITY (Air = 1) at 32°F (0°C) and 1 atm: 0.906
SOLUBILITY IN WATER vol/vol at 32°F (0°C): 1.7
AUTOIGNITION TEMPERATURE: 581°F (305°C) at 1 atm
PERCENT VOLATILES BY VOLUME: 100
MOLECULAR WEIGHT: 26.04
MOLECULAR FORMULA: C2H2
Uptil now the properties of the two fuels to be mixed are and the properties of the ideal fuels for SI and CI engines are studied. Now the next step will be comparing these properties and designing of an apparatus to mix the two fuels viz. turpentine oil and acetylene.
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