Automotive Technology. Objectives. Objectives

Transcription

1 Automotive Technology FIFTH EDITION CHAPTER 66 Gasoline Objectives Explain the chemical composition of gasoline and the process of refining gasoline. Discuss how volatility affects driveability. Explain the process of gasoline combustion and the means of avoiding abnormal combustion. Objectives Discuss the benefits of using gasoline additives, reformulating gasoline, and blending gasoline. Discuss safety precautions when working with gasoline. 1

2 Gasoline Chemical Composition Combination of hydrocarbon molecules having between 5 12 carbon atoms Names based on number of carbon atoms Methane one carbon atom Ethane two carbon atoms Gasoline Propane three carbon atoms Butane four carbon atoms Pentane five carbon atoms Hexane six carbon atoms Heptane seven carbon atoms Octane eight carbon atoms Refining Types of Crude Oil Thin crude oil: high API gravity, called high-gravity crude Contains more natural gasoline Lower sulfur and nitrogen content makes it easier to refine 2

3 Refining Thick crude oil: called low-gravity crude Low-sulfur crude oil: also called sweet crude High-sulfur crude oil: also called sour crude Refining Distillation Late 1800s, crude separated into different products by boiling (distillation) Worked because crude oil composed of hydrocarbons with many boiling points Distillation column: products with different boiling points drawn off at different heights Refining Cracking Hydrocarbons with higher boiling points broken down (cracked) into lowerboiling hydrocarbons Very high temperature process (thermal cracking) 3

4 Refining Today, cracking performed using catalyst (catalytic cracking) Produces higher-quality gasoline than thermal cracking Hydrocracking similar to catalytic cracking uses a catalyst Catalyst is in a hydrogen atmosphere Refining Can break down hydrocarbons resistant to catalytic cracking alone Used to produce diesel fuel rather than gasoline Refining Other types of refining processes: Reforming Alkylation Isomerization Hydrotreating Desulfurization 4

5 Volatility Driveability Index Index created to predict cold-weather driveability (abbreviated DI) Developed using temperature for evaporated percentage of 10% (T10), 50% (T50), and 90% (T90) DI = 1.5 T T50 + T90 Total DI is temperature; usually ranges from 1,000 F to 1,200 F Volatility Driveability Index Lower values of DI generally result in good cold-start and warm-up performance High DI number less volatile than low DI number Gasoline Combustion Process Chemical Reactions Chemical combination of oxygen (O2) from air with hydrogen and carbon from fuel Spark starts process; takes about 3 ms (0.003 sec) to complete 5

6 Gasoline Combustion Process Heat Energy Heat produced by combustion measured in British thermal units (BTUs) One BTU is amount of heat required to raise one pound of water one Fahrenheit degree Gasoline About 130,000 BTUs per gallon Gasoline Combustion Process Air-Fuel Ratios Fuel burns best when turned into a fine spray and mixed with air Direct relationship between engine airflow and fuel requirements (air-fuel ratio) Proportion by weight of air and gasoline mixed as needed for engine combustion Mixtures, with which engine can operate without stalling, range from 8 to 1 to 18.5 to 1 Gasoline Combustion Process Stoichiometric Air-Fuel Ratio Ideal mixture at which all fuel combines with all oxygen and burns completely In theory, ratio for gasoline is air-fuel mixture of 14.7 to 1 6

7 Normal and Abnormal Combustion Gasoline Grades and Octane Number Octane rating of gasoline is measure of its antiknock properties Engine knock is metallic noise resulting from abnormal or uncontrolled combustion Normal combustion occurs smoothly Progresses across combustion chamber from point of ignition at about mph ( km/h) Normal and Abnormal Combustion Gasoline Grades and Octane Number During periods of spark knock (detonation), combustion speed increases by up to 10 times Increased speed causes increased temperatures and pressures Can damage pistons, gaskets, and cylinder heads Gasoline Additives Dye Usually added to help identify grade and/or brand of fuel Octane Improver Additives Aromatic hydrocarbons (hydrocarbons containing benzene ring) such as xylene and toluene 7

8 Gasoline Additives Octane Improver Additives Alcohols such as ethanol (ethyl alcohol), methanol (methyl alcohol), and tertiary butyl alcohol (TBA) Metallic compounds such as methylcyclopentadienyl manganese tricarbonyl (MMT) Gasoline Additives Oxygenated Fuel Additives Oxygenated fuels contain oxygen in molecule of fuel itself Used in high-altitude areas to reduce carbon monoxide (CO) emissions Helps ensure enough oxygen to convert all CO into CO2 during combustion Gasoline Additives Oxygenated Fuel Additives Additives include: Methanol, Ethanol, Methyl tertiary butyl ether (MTBE), Tertiary-amyl methyl ether (TAME), Ethyl tertiary butyl ether (ETBE) 8

9 Gasoline Blending Three methods to blend ethanol with gasoline to create E10 (10% ethanol, 90% gasoline) In-line blending Most likely to produce a well-mixed blend of ethanol and gasoline Sequential blending Splash blending Least accurate blending Reformulated Gasoline RFG manufactured to help reduce emissions Use additives containing at least 2% oxygen by weight; reducing additive benzene to maximum 1% by volume Reduce light compounds Reduce heavy compounds General Gasoline Recommendations Purchase fuel from a busy station to help ensure that it is fresh Keep the fuel tank above one-quarter full Do not purchase fuel with a higher octane rating than is necessary 9

10 General Gasoline Recommendations Avoid using gasoline with alcohol in warm weather Do not purchase fuel from retail outlet when tanker filling underground tanks Do not overfill the gas tank Be careful when filling gasoline containers Summary One BTU is amount of heat required to raise one pound of water one Fahrenheit degree. Octane rating of gasoline is measure of its antiknock properties. Purchase fuel from a busy station to help ensure that it is fresh. 10