Thermochemistry and Fuels

Thermochemistry and Fuels

Combustion Reactions

Internal combustion engines obtain their energy from the combustion of hydrocarbon fuel with air. The chemical energy stored in the fuel is converted to energy that the engine can use in the through hot gases within the chamber. The combustion process involves the chemical reaction of hydrocarbon fuel with oxygen to produce water vapor and CO2. The maximum amount of chemical energy from the hydrocarbon fuel is when it reacts with stoichiometric oxygen. The meaning of stoichiometric oxygen is de ned as the amount of oxygen that is needed to convert all of the carbon in the fuel to CO2 and all of the hydrogen to H2O. The simplest chemical reaction using the simplest hydrocarbon with stoichiometric oxygen is:

CH4 + 2O2  --------CO2 + 2H2O 

For this reaction to be complete it would take two moles of oxygen to react with one mole of methane to produce one mole of carbon dioxide and two moles of water vapor. The hydrocarbon fuel used in engines is not a simple fuel like methane but rather consists of isooctane and various additives. The chemical reaction involving isooctane and oxygen is:

C8H18 + 12:5O2 > 8CO2 + 9H2O

The above two chemical reactions involve the reaction of a hydrocarbon with oxygen. Since it would be extremely expensive to use pure oxygen the atmosphere is used as a rich source of oxygen. The hydrocarbon reacts with air which is composed of many substances. Nitrogen and oxygen are the two most found substances in air with a nitrogen composition of 78%, by mole, and oxygen composition of 21%.

Combustion can occur with an either lean or rich mixture. If the mixture is for example 150% of stoichiometric then there will be an excess amount of air and the products will involve excess oxygen. This is called a lean mixture since there is a de ciency is fuel. If on the other hand the mixture is 80% of stoichiometric then there will be excess fuel and carbon monoxide (CO) will be in the end product. This is a rich mixture since the misture has excess of fuel. Carbon monoxide is a colorless, odorless, poisonous gas which can be further burned to form CO2. If there is a further de ciency in oxygen then more CO will go into the atmosphere as pollution. 

Hydrocarbon Fuels

In SI engines the fuel used is gasoline. Gasoline on the other hand is composed of a mixture of many hydrocarbon fuels, which all come from crude oil. Crude oil is composed mainly of 85% carbon and 13% hydrogen. The carbon and hydrogen molecules combine to form thousands of hydrocarbon mixtures. Crude oil is seperated into components by cracking and or distillation using thermal or catalytic methods at oil re neries. The process of cracking involves the breaking up of large molecular components into smaller molecular components, which are then used for processing. It is important to break up the large strands because the smaller the molecular weight of the component the lower the boiling temperature will be. Fuels need to have components with low boiling points so that they can be readily vaporized. Crude oil in the U.S. is basically divided into Pennsylvania and Western crude. Pennsylvania crude has a high concentration of para ns and Western crude has a high concentration of asphalt. Figure 1 shows the temperature vs. percent of fuel evaporated.

This gure shows the importance of having fuel consisting of a mixture of hydrocarbons. Figure 1 represents the typical gasoline mixture for SI engines. The mixture is composed of low and high molecular weight compounds. The low molecular weight compounds aid in the cold starting of the engine while the high molecular weight compounds increase the e ciency by not vaporizing until late into the compression stroke. The low molecular weight compound is de ned as front-end volatility while high-end volatility corresponds to high molecular weight compounds. One of the problems with front-end volatility is that the e ciency of the engine will be reduced if fuel vapor replaces air too early in the intake system. Another problem is vapor lock. Vapor lock occurs when fuel vaporizes in the fuel supply lines or carburettor. If this happens fuel is cut o and the engine will stop. One problem with high-end volatility is when too much fuel is supplied to the engine and not all the fuel is burnt during combustion. The unburnt fuel will then end up as pollution in the environment. Following are some basic hydrocarbon components.

Para ns

The para n family, also called alkanes, are molecules with a carbon-hydrogen combination of CnH2n+2. The most stable para n is methane(CH4), which is the main component of natural gas. The molecule structure Other para ns include propane(C3H8), butane(C4H10), and isobutane(C4H10). The di erence between the formulas for butane and isobutane is the chemical structure. Isooctane best matches the structure and thermodynamic properties of gasoline. The structure for isooctane(C8H18) 

Ole ns

The ole n family is made up of one double carbon-carbon bond. The structure of ole ns is of the form CnH2n. Some ole ns are ethene (C2H4), butene-1 (C4H8), and butene-2 (C4H8). Once again butene-1 and butene-2 have the same chemical formula but di erent chemical structure. These are called isomers.

Others 

Cyclopara ns have a single-bond ring and a chemical formula of CnH2n. Some cyclopara ns are cyclobutane (C4H8) and cyclopentane (C5H10). Cyclopara ns are good gasoline components. Aromatics have double carboncarbon bonds, and a chemical structure of CnH2n6. The basic structure is the benzene ring. Aromatics usually are good gasoline components. They have high densities in the liquid state and therefore have high energy content per unit volume. Some other hydrocarbon components are alcohols, diole ns, and acetylene.

Octane Number

The octane number of a fuel describes how well it will or will not self ignite. The numerical scale is set by testing fuels. The fuel at question is compared to other fuels that have set standards. One fuel that is used for the test is isooctane (2,2,4 trimethylpentane), which is given the octane number (ON) of 100. The other fuel used for the test is n-heptane, which is given the ON of 0. The higher the octane number of the fuel the less likely it will self-ignite. In SI engines self-ignition will occur when the fuel ignites before the use of the spark due to high tempeartures. When self-ignition occurs in SI engines pressure pulses are genereted. This high pressure causes damage to the engine. This activity of self-ignition is called knock. Engines with low compression ratios can use low octane fuels since the temperatures and pressures are lower. High compression engines must use high octane fuel to avoid self-ignition and knock. Figure 4 shows the relationship between the Fuels that were used earlier had low octane numbers so therefore engines with low compression ratios were used. As technology advanced the engine design advanced. Engines were designed with higher compression ratios so higher pressures and temperatures were attained. fuel had to be manufactured to have higher octane numbers. The structure of the fuel depicts the value of the octane number. For example hydrocarbon components that have long chains have low ON. On the other hand components with more side chains have higher ON. Also fuel components with ring molecules have high ON. One additive that was used to raise the ON of fuels was TEL, (C2H5)4P b. A few milliliters of TEL into several liters of fuel and the ON would rise several points in a very predictable manner. When TEL was rst used it was an additive that was manually put into the fuel tank at the gas station. The turbulance created by the pouring was enough to create the mixture. Handling of the TEL by people at gas stations was harmful due to the toxic vapors and the harm that TEL could do the skin. Because of its harm to people, TEL was blended into the gasoline at re neries. This however created a need for more pumps and storage facilities for the new gasoline which was now divided into high-octane and low-octane gasoline. Figure 5 shows the relationship between the ON and the TEL added to the the gasoline.

The problem with TEL is the lead content that ends up in the exhaust. Lead is a very toxic engine emission, and its pollution to the atmosphere ended in the early 1990's. The elimination of leaded gasoline created problems for older cars and other older engines. When lead is introduced into combustion one of the results is lead deposited into the walls of the combustion chamber. This lead reacts with the hot walls and forms a very hard surface. When older engines were manufactured they were engineered to have softer steels in the walls, heads, and valve seats. When the engines were operated using leaded fuels the idea was that these parts would become heat treated and hardened during use. When these engines are operated with unleaded fuel the hardening process is not there and the parts wear through use. There are now TEL substitutes for older vehicles such as alcohols and organomanganese compounds. 

Cetane Number

Diesel fuels are usually characterized by their molecular weight. There are low and high molecular weight fuels, each with di erent characteristics. Usually the greater the re ning done on the fuel the less viscous, lower molecular weight, and higher cost of the fuel. The less re ning done on the diesel fuel the more viscous, higher molecular weight, and lower cost of the fuel. Numerical scales exist that denote whether a diesel fuel is high or low in 

molecular weight. The scale ranges from one (1) to ve (5) or six (6), with subcategories using alphabetical letters (e.g. 3B, 2D). The lower the number the lower the molecualr weight of the diesel fuel. Fuels with lower numbers are typically used in CI engines, while the high numbered diesel fuels are used in large, massive heating units. Diesel fuels can be divided into two extreme categories; light and heavy diesel fuel. Light diesel fuel has a molecular weight of about 170 and is approximated by the chemical formula C12:3H22:2. Heavy diesel fuel has a molecular weight of about 200 with approximately a chemical formula of C14:6H24:8. 

In CI engines combustion starts with the self ignition of the air and fuel mixture. There are di erent fuels which have di erent ignition characteristics. Ignition delay is a property of CI engines that is dependant on the fuel used. The cetane number (CN) is a quanti able number that gives a fuel the property of wether it will self ignite early or late. The higher the CN the shorter the ignition delay. On the other hand the lower the CN the longer the ignition delay. The CN ratings are established through testing. The two fuels used for the test are n-cetane (hexadecane), C16H34, and heptamethylnonane (HMN), C12H34. The n-cetane is given the cetane number of 100, while HMN is given the number of 15. The CN is then determined using equation 6.4

CN = (percent of n-cetane) + (0:15)(percent of HMN)

The degree of CN of a fuel gives certain characteristics to the engine. Normal cetane numbers range on the order of 40 to 60. For a given engine, if the CN is too low then ignition delay will be too long. If ignition delay is prolongated then extra fuel \will be injected into th ecylinder" before the rst fuel is ignited \causing a very large, fast pressure rise at the start of combustion." (Pulkrabek, 149) This fast pressure rise will cause low thermal e ciency. If the CN of the fuel is too high then combustion will start too soon in the compression stroke. Early combustion will cause a pressure rise before top dead center, and more work will be required.