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Copyright © 2009 Delmar, Cengage Learning Medium/Heavy Duty Truck Engines, Fuel & Computerized Management Systems, 3E Chapter 16 Chemistry & Combustion
Copyright © 2009 Delmar, Cengage Learning Introduction Knowledge of chemistry important to: Understand fuel composition & combustion dynamics Develop ability to work with electricity & electronics
Copyright © 2009 Delmar, Cengage Learning Basic Chemistry Building blocks of all matter are atoms All atoms are electrical Electrical charge is a component of all atomic matter
Copyright © 2009 Delmar, Cengage Learning Elements An element is any one of more than 100 substances that cannot be chemically resolved into simpler substances Elements consist of minute particles known as atoms Examples: Hydrogen Atom Oxygen Atom
Copyright © 2009 Delmar, Cengage Learning Common Elements Metallic ElementsAtomic #Non-Metallic ElementsAtomic # Iron – Fe26Hydrogen – H1 Sodium – NA11Carbon – C6 Magnesium – Mg13Helium – HE2 Aluminum – Al13Sulfur – S16 Nickel – N28Silicon – Sl14 Rhodium – Rh35Selenium – Se34 Silver – Ag47Oxygen – O8 Zinc – Zn30Nitrogen – N7 Gold – Au79Argon – Ar18 Platinum - Pt78Radon - Rn86 Atomic # identifies number of protons in an atom of the element
Copyright © 2009 Delmar, Cengage Learning Mixtures AA mixture is composed of two or more elements and/or compounds FFor example: AAir = 23% oxygen + 76% nitrogen + 1% inert gases BBoth oxygen & nitrogen: Retain their own identity Retain their own characteristics Can take part in reactions independently of each other MMixture properties depend on the substances in it
Copyright © 2009 Delmar, Cengage Learning Chemical Bondings IInteractions accounting for the association of atoms into molecules, ions, crystals WWhen atoms approach each other: Their nuclei & electrons interact Distribute themselves Their combined energy is lower than in the alternative arrangement VValency Number: Number of bonds an atom can form Valency of an atom is simply the number of unpaired electrons in its valence shell
Copyright © 2009 Delmar, Cengage Learning Compounds A compound is composed of: Two or more elements Combined in definite proportions Held together by a chemical force Can be broken down into their elements by chemical reactions Carbon atoms are unique Have ability to form covalent bonds With each other With other elements Covalent bonding occurs when two electrons are shared by two atoms. Pure compounds can be obtained by physical separation processes such as filtration & distillation
Copyright © 2009 Delmar, Cengage Learning Molecules A molecule is: Smallest particle of a compound Can exist in a free state Can take part in a chemical reaction A water molecule Note: Shared electrons An oxygen molecule
Copyright © 2009 Delmar, Cengage Learning Atomic Structure Electron Carries negative charge Orbit in shells around atom’s nucleus Proton Carries positive charge Located in atom’s nucleus Neutron Electrically neutral Located in atom’s nucleus
Copyright © 2009 Delmar, Cengage Learning Balanced Atoms Electrically balanced atoms have an equal number of electrons & protons An atom with either a deficit or surplus of electrons is called an ion
Copyright © 2009 Delmar, Cengage Learning Balanced Atoms Electrons are arranged in circular orbits around the nucleus Electrical force attracting the electron to the positive charge of the nucleus is offset by the mechanical force acting outwards on the rotating electrons keeping them in their orbits.
Copyright © 2009 Delmar, Cengage Learning States of Matter Generally classified into one of three states or phases Solid Liquid Gas Water is the only substance that is familiar with all three states: Ice (solid) Water (liquid) Steam (gas)
Copyright © 2009 Delmar, Cengage Learning Determining State Difference between solids, liquids & gases can be explained in terms of kinetic molecular theory Kinetic = motion As temperature increases, so does molecular motion Vaporization: heat applied to liquid, converts to gaseous state Condensation: reverse vaporization
Copyright © 2009 Delmar, Cengage Learning States of Matter - Conclusion Typical injector pulse: Fuel directly injected to diesel engine cylinder is atomized (liquid state) Exposed to heat of compression (vaporizes) Gases condensing in exhaust observed as white smoke
Copyright © 2009 Delmar, Cengage Learning Properties of Mixtures & Compounds Each element has: A special identity A set of characteristics that make it unique Chemical Reactions: Explained by their constituent elements Combustion is an oxidation reaction Reactant in engine cylinder is whatever oxygen present at time of ignition
Copyright © 2009 Delmar, Cengage Learning Properties of Common Elements ElementStateAtomic #Properties/Characteristics HydrogenGas1Simplest element, one of the most reactive CarbonVaries6Combines to form compounds more readily than other elements OxygenGas8Most common element in earth’s crust NitrogenGas7When oxidized in the combustion process, it forms several compounds collectively known as Nox SulfurSolid16Appears prominently in residual oil IronSolid26Used extensively in vehicle technology, mostly as steel AluminumSolid13Excellent conductor of heat & electricity Most fuels are elementally composed of carbon & hydrogen
Copyright © 2009 Delmar, Cengage Learning Combustion Reactions Involved Products & Byproducts: Air (a mixture) NitrogenN % OxygenO % ArgonAr00.934% NeonNe % HeliumHe % MethaneCH % KryptonKr % HydrogenH % Nitrous oxideN 2 O % ZenonXe %
Copyright © 2009 Delmar, Cengage Learning Combustion Reactions Involved Products & Byproducts: Water vaporH 2 O0 – 7% OzoneO % Carbon DioxideCO – 0.1% Carbon MonoxideCO Sulfur OxidesSOx Oxides of NitrogenNOx Byproducts of combustion
Copyright © 2009 Delmar, Cengage Learning Unburned Hydrocarbons UUHCs consist of any emitted unburned fuel fractions IInclude: Paraffins Olefins Aromatics LLeast volatile elements of a fuel more likely to result in UHC emissions CClassified as potentially harmful
Copyright © 2009 Delmar, Cengage Learning Partially Burned Hydrocarbons PHCs are a result of low-temperature combustion Include: Aldehydes Ketones Carboxylic acids Can result from extinguishing the flame front before a molecule is completely combusted.
Copyright © 2009 Delmar, Cengage Learning Particulate Matter Any liquid or solid matter emitted from exhaust stack Can be detected in light extinction test apparatus (i.e. smoke opacimeter) Classified as particulate matter (PM) The term PM is more appropriately applied to emitted ash & carbon spots in the solid state
Copyright © 2009 Delmar, Cengage Learning Combustion FuelOxygen Energy Ignition Fuel + Oxygen + Heat = Chemical Reaction HeatHeat To ignition temperature! The reaction causes the energy in the fuel to be liberated resulting in a large volume of hot gases!
Copyright © 2009 Delmar, Cengage Learning Combustion with Ambient Air Combustion in an engine cylinder uses the oxygen available in the ambient air mixture Proportionally the largest ingredient of the reaction is always nitrogen Ideally nitrogen should remain inert, unaffected by the oxidation of the fuel When nitrogen is oxidized, NOx are produced Noxious emissions
Copyright © 2009 Delmar, Cengage Learning Combustion in an Engine Cylinder Pressure volume curve in a diesel engine. The large volume of hot gases produced as a result of the combustion reaction creates this pressure.
Copyright © 2009 Delmar, Cengage Learning Cylinder Gas Dynamics Injected fuel is: Dispersed Mixed Combusted in the cylinder Intent is to create cyclonic turbulence in the cylinder as the piston is driven upwards Behavior governs: Engine’s performance efficiency Noxious emissions “Swirl”
Copyright © 2009 Delmar, Cengage Learning Stoichiometry Actual ratio of the reactants in any reaction to the exact ratios required to complete the reaction Stoichiometric ratio or lambda ( ) factor is dependent on actual chemical composition of the fuel to be burned > = greater than, < = less than = Actual air supplied Stoichiometric requirement > 1 lean burn < 1 rich burn = 1 stoichiometric AFR
Copyright © 2009 Delmar, Cengage Learning Calculating Air-Fuel Ratio Petroleum contains by mass: Carbon 84 – 87 % Hydrogen 11 – 15 % Sulfur % For oxidization: 1 Kg. of carbon (C) requires 2.66 KG. of Oxygen (O) 1 Kg. of hydrogen (H) requires 8.0 Kg. of Oxygen (O) 1 Kg. of Sulfur (S) requires 1.0 Kg. of Oxygen (O) Air contains approximately 23% by mass 1 Kg. of air would contain.23 Kg. of Oxygen 1 Kg. of oxygen is contained in 4.35 Kg. of air Calculating an Air-Fuel Ratio With a hypothetical diesel fuel containing by mass; 86% carbon, 13% hydrogen & 1% sulfur the oxygen required to completely oxidize 1 Kg. of the fuel would be: Carbon (2.66 X.86) + Hydrogen (8 X.13) + Sulfur (1 X.01) = 14.5 Kg. The air fuel ratio for this example would be 14.5:1
Copyright © 2009 Delmar, Cengage Learning Combustion Cycle 1.Ignition delay or ignition lag Occurs between start of ignition & the moment ignition occurs 2.Period of rapid combustion Fuel that evaporated & mixed during ignition delay period is burned, the rate & duration of rapid combustion are closely associated with the length of the delay period 3.Third phase of combustion Begins at the moment of peak cylinder pressure & ends when combustion is measurably complete Available fuel is oxidized. injector nozzle opening.
Copyright © 2009 Delmar, Cengage Learning Combustion Cycle 4.Afterburn phase A period in which any unburned fuel in the cylinder may find oxygen & burn 5.Dosing Injection Final shot of fuel into the cylinder, not intended to be combusted in the cylinder. Shot is injected with intention of discharging into the exhaust system as raw fuel to be combusted in exhaust gas aftertreatment systems
Copyright © 2009 Delmar, Cengage Learning Combustion Cycle 6.Detonation Multiple flame front condition that causes an abnormally high rate of combustion & resultant pressure rise in the cylinder block “Diesel knock” “Ping”
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