Why are branched hydrocarbons used in petrol

The examples in Figure 1 are all "straight chain" alkanes, where the central carbons form a single linear chain. Alkanes may also be branched [3] or cyclic [4] , and can contain any number of carbon atoms.

Similar in structure to alkanes, alkenes are hydrocarbons where the central carbon chain contains at least one double bond between carbon atoms, and alkynes contain at least one triple bond. Also commonly seen are hydrocarbon derivatives , where atoms other than carbon or hydrogen are present.

Hydrocarbons and their derivatives are the main constituents of fossil fuels , and release energy through combustion. Besides their fuel applications, hydrocarbons are also used in chemical synthesis and are major components of lubricating oils, greases, solvents, fuels, wax, asphalts, cosmetics, and plastics. Several empirical equations have been developed by using the physical properties of the fuel since the engine tests are very laborious and costly in determining the cetane number.

These methods, which measure the fuel propensity to ignite, are called cetane index, aniline point, or diesel index. Aniline is an aromatic compound which is very easily mixed with compounds of its group even at low temperatures, while it is more difficult to form mixtures with alkanes paraffins. Therefore, hexadecane C 16 H 34 which is an alkane group and has a high ignition tendency has a high mixing temperature with the aniline.

The mixture of the sample fuel with the same amount of aniline is heated to find the diesel index. Then, all of the aniline is dissolved in the fuel. After that the mixture is cooled for allowing to aniline to separate from the fuel. This temperature, where the aniline is separated from the fuel, is called the aniline point. The diesel index is calculated with the aniline point and API grade specified in Eq. The higher the diesel index value, the fuel is more alkane in paraffinic structure , and it has the higher ignition tendency.

Increasing volatility in diesel fuels causes acceleration of fuel evaporation and decrease in viscosity. This is generally undesirable since the fuel causes a reduction in the cetane number [ 1 , 2 , 4 ]. Some fuels commonly used in engines are presented in Table 3.

Common fuels and their properties [ 4 ]. The cetane index can be calculated from Eq. This formula is related to the number of cetane, unless cetane-enhancing additives are added to the fuel.

Otherwise, the cetane number of doped fuels can be measured by engine test experiments. Another method used to calculate the cetane index is the empirical equation given in Eq. The semiempirical expression that predicts ID duration based on cetane number and other operating parameters is as follows:. ID is calculated by the formula given in Eq.

It is expressed in milliseconds for an engine at n rpm [ 4 ]:. The low cetane number of diesel engines leads to an increase in ID time, which in turn reduces the time required for combustion and CA. An increased TG time leads to accumulate more fuel in the combustion chamber than required. Thus, this excess fuel causes sudden and high-pressure increases during the onset of combustion. These sudden pressures increase cause mechanical stresses and hard engine operation, which is known as diesel knocking [ 2 , 4 ].

In brief, the number of cetane and the number of octane refer to the spontaneous combustion of fuels. A higher cetane number indicates that diesel fuel burns suddenly and easily. The high octane number defines the resistance of gasoline to sudden ignition.

Generally, if the cetane number is high, the octane number is low. There is an inverse relationship between these two properties, so that the cetane number is low if the octane number is high [ 5 ]. Natural gas is a gas mixture containing methane, ethane, propane, pentane, and hexane in a lighter content than air, without color, smell, and taste. However, it contains a small amount 0—0. The natural gas used in the market is refined and separated from other gases and used as almost pure methane gas CH 4 [ 5 ].

Natural gas can be stored by these methods and generally used as compressed natural gas CNG in internal combustion engines with a single-point spray system.

The single-point spraying system allows for the most efficient use of natural gas as it provides a longer mixing time than required for natural gas [ 4 ].

Table 4 shows the compounds that form natural gas and boiling points. Compounds and boiling points in natural gas [ 5 ]. There are dual fuel diesel engines in which natural gas and diesel fuels work together. Natural gas is supplied to the combustion chamber at approximately sound speed. This leads to high turbulence and high flame speeds. Natural gas has lower combustion temperatures than diesel fuel, and with late spraying, the combustion chamber temperature can be further reduced.

Decrease in combustion chamber temperature significantly reduces NO x formation. However, the low carbon content in natural gas leads to less CO 2 emissions and much less solid particulate matter [ 4 ]. Dump gas engines, converting methane gas into energy, are one of the most common natural gas applications.

In addition to methane, these landfill gases contain highly polluting and variable quality gases such as fluorine, chlorine, silicon, and solid particles. Especially due to the corrosive and abrasive effect of these gases, special piston and valve materials must be used in the engines. The thermal value of natural gas is between CO 2 , H 2 O, and kJ of energy are obtained when 1 mol of methane gas is fully combusted.

The combustion equation of 1 mol of methane is as described in Eq. The high flame velocity and octane number of the natural gas enable the natural gas to operate at high compression rates.

This ensures that natural gas is a good gasoline engine fuel. Furthermore, natural gas has low exhaust emissions. In addition, the most important advantage of natural gas fuel is that natural gas can be produced from a source such as coal that has a lot of reserves all over the world. However, since the low-energy capacity of natural gas is in the form of gas, its low volumetric efficiency leads to reductions in engine performance. The disadvantages of this fuel are that natural gas requires high-pressure fuel storage tanks; refueling takes time and has variable fuel components in the content of natural gas [ 4 ].

Table 5 presents the properties of natural gas and its comparison with other fuels as thermal values. The properties of natural gas and its comparison with other fuels [ 11 ]. LPG, a liquefied petroleum gas, is produced as a by-product from natural gas production processes or during the distillation of oil in refineries.

These propane and butane gas ratios in LPG may vary according to the regions and areas of use [ 5 ]. LPG is the most preferred fuel type after gasoline and diesel fuels, since LPG is much easier to store and transport than natural gas [ 1 , 4 ].

LPG is a colorless, odorless, nontoxic, and easily flammable gas. LPG is a mixture of propane and butane gas, which is gas at normal pressures and temperatures. However, LPG is a liquid at moderate pressure. Besides, it is two times heavier than air and half weight of water. Therefore, LPG leaks to the floor in case of leakage. LPG in liquid state expands to approximately times its liquid volume. This is called sudden expansion and cooling of the sudden temperature drop with the very rapid evaporation of the liquid fuel as it passes into the gaseous state.

Since this can cause cold burns, the gas should not be touched with bare hands. Although LPG is a noncorrosive gas, it can melt paint and oil and also inflate natural rubber materials, causing them to lose their properties. LPG system is widely used in gasoline vehicles. With respect to this, the comparison of physical and chemical properties of propane and butane gases which are components of LPG and the gasoline fuel is given in Table 6.

Properties of LPG and gasoline [ 1 ]. Fossil-based fuels such as diesel, gasoline, natural gas, and LPG have been commonly used in engines as a fuel. However, the internal combustion engines show differences in the fuel types depending on the thermodynamic cycles. The residue is coke, which is used both as a fuel and as a source of carbon for the production of steel. The major component in coal gas is methane. Coal tar is an incredible mixture of compounds, mostly hydrocarbons, a substantial number of which are arenes.

Coal and coal tar can be utilized to produce alkanes, but the technology involved is more complex and costly than petroleum refining. It seems inevitable that the cost of hydrocarbon fuel will continue to rise as supply problems become more difficult. And there is yet no answer to what will happen when the world's limited quantities of petroleum and coal are exhausted. Create a free Team What is Teams? Learn more. Why are branched alkanes used as fuels instead of straight chain alkanes?

Ask Question. Asked 1 year, 2 months ago. Active 5 months ago. Viewed 1k times. Improve this question. Jayadithya Jayadithya 1, 5 5 silver badges 23 23 bronze badges. Add a comment. Active Oldest Votes. Improve this answer. The Wikipedia page for Octane explains this quite well: The octane rating was originally determined by mixing fuels from only heptane and 2,2,4-trimethylpentane a highly branched octane , and assigning anti-knock ratings of zero for pure heptane and for pure 2,2,4-trimethylpentane.

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