Chemistry and Combustion Chapter 17

Chemistry and CombustionChapter 17

OBJECTIVES • Understand basic chemistry and its application to fuel systems. • Define elements, mixtures, and compounds. • Describe a simple chemical reaction and chemical bonding. • Outline the structure of an atom. • Define the states of matter and the conditions that predetermine them. • Describe the properties of common elements, mixtures, and compounds.

OBJECTIVES (Cont.) • Outline the dynamics of combustion in an engine cylinder. • Define the conditions required for a stoichiometric reaction. • Calculate air-fuel ratio. Describe the stages of combustion in a diesel engine cylinder. • Explain how burning 1 gallon of diesel fuel produces over 20 pounds of CO2. • Describe the dynamics of detonation.

INTRODUCTION • It is important for any mechanical technician to have a fundamental understanding of chemistry for purposes of explaining fuel composition and combustion dynamics in engines, and for developing an ability to work with electricity and electronics. • Some knowledge of basic chemistry can be an especially useful diagnostic tool for the technician specializing in engines, fuel systems, and emission controls.

BASIC CHEMISTRY • The building blocks of all matter are atoms. • All atoms are electrical. • Electrical charge is a component of all atomic matter.

ELEMENTS • An element is any one of more than 100 substances, most naturally occurring, that cannot be chemically resolved into simpler substances. • The atoms of any one element are exactly alike and possess identical mass (quantity of matter a substance contains: weight). • The name of an element is always a single word, such as hydrogen or oxygen .

MIXTURES • A mixture is composed of two or more elements and/or compounds, all of which retain their own characteristics and identity. • Air is an example of a mixture. It is composed of 23% oxygen and 76% nitrogen by mass (plus about 1% of inert gases).

CHEMICAL BONDING • Chemical bondings are interactions that account for the association of atoms into molecules, ions, and crystals. • When atoms approach one another, their nuclei and electrons interact and distribute themselves in such a way that their combined energy is lower than it would be in an alternative arrangement.

Valency • The number of bonds an atom can form is called its valency (or valence number). • The valency of an atom is simply the number of unpaired electrons in its valence shell.

COMPOUNDS • A compound is a substance composed of two or more elements combined in definite proportions and held together with chemical force. • A compound is composed of identical molecules made up of atoms of two or more elements.

MOLECULES • A molecule is the smallest particle of a compound that can exist in a free state and take part in a chemical reaction. • When an H2O molecule is chemically reduced, it ceases to have the properties of H2O and possesses those of elemental oxygen and hydrogen.

ATOMIC STRUCTURE • Atomic theory seeks to explain both the composition of matter and the laws that apply to chemical combination. • It was conceived in 1801 by a chemist, John Dalton. • Dalton’s atomic theory suggested: • Each element is made up of minute, indivisible particles called atoms. • Atoms cannot be created or destroyed. • The atoms that make up a specific element are all identical. • Atoms of different elements are different. • Atoms of one element may combine with other elements to form compounds. • Dalton’s atomic theory was developed into our current notions of the structure of an atom by the scientists Thomson, Rutherford, and Chadwick in the early part of the twentieth century.

ATOMIC STRUCTURE (Cont.) • Thomson, Rutherford, and Chadwick identified the following subatomic particles: • The electron—symbol e–. Discovered by J. J. Thompson in 1897. • Electrons carry a negative charge and orbit in the shells around the atom’s nucleus. • An electron has 1/1,837 of the mass of a proton. The proton—symbol p+. Discovered by Ernest Rutherford in 1911. • Protons carry a positive charge and are located in the atom’s nucleus. • A proton has 1,837 times the mass of an electron. • The neutron—symbol n°. Discovered by James Chadwick in 1932. • Neutrons are electrically neutral and are located in the atom’s nucleus. • A neutron has slightly more mass than a proton.

ATOMIC STRUCTURE (Cont.) • In 1911, Ernest Rutherford described the nuclear model of the atom and asserted: • An atom consists mostly of empty space. • Each atom has a minute, extremely dense nucleus. • The nucleus is surrounded by a large volume of nearly empty space. • Most of the mass and all of the positive charge is located in the nucleus. • The nearly empty space surrounding the nucleus is sparsely occupied by electrons that possess a fraction of the mass but a negative charge that balances the positive charge of the protons

Balanced Atoms • A hydrogen atom consists of a single proton in the nucleus and one electron in its orbital shell. • The electron orbits the nucleus in much the same manner Earth orbits our sun.

Balanced Atoms

STATES OF MATTER • In physics, matter is defined as anything that has mass and occupies space. • Matter can be generally classified into one of three states or phases within the earth’s atmosphere: solid, liquid, or gas. • Water is the only substance that is familiar in all three states, which we describe as ice, water, and steam.

Determining State • On Earth’s surface, under a given set of temperature and pressure conditions, a substance will be in one of these states. • Most substances can exist in any of the three states.

Changes of State • Kinetic molecular theory also explains the change of state from that of a liquid to a gas known as vaporization. • Dry ice (CO2) sublimates (goes directly from a solid to a gas) at atmospheric pressure and temperature. • Snow can also be sublimated.

PLASMA AND OTHER STATES OF MATTER • At very high temperatures, atoms may collide with each other with a force that results in some electrons being jolted free of the nuclei. • A mixture containing these positive and negative ions (an ion is an atom that has lost or gained one or more electrons) is not defined as a gas but as plasma.

STATES OF MATTER—CONCLUSION • As far as the technician is concerned, a basic understanding of what we know as the three states of matter should suffice.

CHEMICAL REACTIONS • Combustion is an oxidation reaction. • The reactant in the engine cylinder is whatever oxygen is present at the time of ignition.

PROPERTIES OF SOME COMMON ELEMENTS • Each element has a special identity and set of characteristics that make it unique in terms of both its behavior and appearance. • HYDROGEN (H) • Hydrogen is a colorless, odorless, tasteless flammable gas. • Atomic No.: 1 • Melting point: –259.2°C (–435°F) • Boiling point: –252.8°C (–423°F) • CARBON (C) • Carbon is a nonmetallic element that exists in a number of forms and combines to form compounds more readily than any other element. • Atomic No.: 6 • Melting point: 3,550°C (6,420°F) • Boiling point: 4,827°C (8,721°F) • OXYGEN (O) • Oxygen is a colorless, odorless, tasteless gas. • Oxygen is the most plentiful element in the earth’s crust. • Atomic No.: 8 • Melting point: –218°C (–361°F) • Boiling point: –183°C (–297°F)

PROPERTIES OF SOME MIXTURES AND COMPOUNDS • The way in which mixtures and compounds behave when they undergo chemical reactions can be explained by the constituent elements of which they are composed. • The following section describes some of the mixtures and compounds that occur in or result from combustion reactions in an internal combustion engine.

PROPERTIES OF SOMEMIXTURES AND COMPOUNDS • AIR • Air is a mixture of oxygen and nitrogen. • CARBON DIOXIDE (CO2) • CO2 is a compound. It is a colorless gas with a sharp odor and a sour taste. • WATER • Water is a compound. It can be readily observed in its three physical states: ice, water, and steam. • CARBON MONOXIDE (CO) • Carbon monoxide is a compound of carbon and oxygen.

COMBUSTION • Combustion: • Despite the fact that human society has used fire in one form or another since prehistoric times, no person was able to explain it in appropriate chemical terms until Antoine Lavoisier in 1783. • When a fuel is heated to its ignition temperature in the presence of oxygen, a chemical reaction takes place in which the heat energy contained in the fuel is liberated, resulting in a large volume of hot gases. • Combustion with ambient air: • Combustion in an engine cylinder uses the oxygen available in the ambient air mixture, so proportionally the largest ingredient of the reaction is always the element nitrogen.

COMBUSTION IN AN ENGINE CYLINDER • Compression pressures in diesel engines range generally from 27 bar/400 psi (2,750 kPa) to 48 bar/700 psi (4,862 kPa). • In most current direct injection (DI) engines, actual pressures typically would be close to the middle to high end of the above range. • Peak cylinder pressures are managed (by the fuel system) to occur somewhere between 10 to 20 degrees after top dead center (ATDC).

CYLINDER GAS DYNAMICS • Piston crown design, valve configuration, manifold boost, and the engine breathing manifolds all play a role in determining the cylinder gas dynamics; that is, how injected fuel is dispersed, mixed, and subsequently combusted in the cylinder.

STOICHIOMETRY • The term stoichiometric is derived from the Greek school of philosophy called the Stoics, whom among other things, preached the avoidance of excess. • In engine technology, managing a stoichiometric burn ratio means controlling fueling so that the air in the engine cylinder is precisely that which is required to oxidize the fuel completely .

SOME FACTS • In combustion: • 1 lb carbon combines with oxygen and releases 14,540 Btu. • 1 lb hydrogen combines with oxygen and releases 62,000 Btu.

THE ACTUAL COMBUSTIONCYCLE IN A DIESEL ENGINE • Harry Ricardo was the first diesel engine designer to focus on emissions. • His research helped reduce visible smoke from diesel engines, but he also produced detailed analyses of diesel combustion that we still reference today. • The following briefly outlines Ricardo’s description of the diesel engine combustion cycle, which we have updated a little where necessary.

IGNITION DELAY OR IGNITION LAG • Ignition delay, or ignition lag, occurs between the events of the start of injection (injector nozzle opening) and the moment ignition occurs. • In an SI, gasoline-fueled engine, the moment of ignition is controlled by a spark. • Things are not so precise in a diesel engine. Injecting fuel merely begins a sequence of events we know as ignition delay.

IGNITION DELAY OR IGNITION LAG • The duration of the ignition delay is defined by the evaporation rate of the fuel, which is itself factored by the ignition quality of the fuel and the actual temperature in the cylinder, which rises during injection and combustion.

PERIOD OF RAPID COMBUSTION • In this phase, the fuel that evaporated and mixed during the ignition delay period is burned, so the rate and duration of rapid combustion are closely associated with the length of the delay period.

THIRD PHASE OF COMBUSTION • The third phase of the combustion cycle begins at the moment of peak cylinder pressure (wherever that happens to occur) and ends when combustion is measurably complete; that is, the available fuel has been oxidized.

AFTERBURN PHASE • Afterburn in the diesel combustion cycle is a period in which any unburned fuel in the cylinder may find oxygen and burn.

DOSING INJECTION • Some of the latest multipulse diesel injection systems are capable of up to seven injection events per cycle. • The dosing shot of fuel is injected with the intention that it is discharged into the exhaust system as raw fuel to be combusted in exhaust gas aftertreatment systems.

DETONATION • Detonation describes the phenomenon that the diesel technician describes as “diesel knock,” and which a driver of any car knows as “ping.” • The heat and compression may cause unburned portions of the charge to ignite before the arrival of the primary flame front.

SUMMARY • An element is any one of more than a hundred substances that cannot be chemically resolved into simpler substances. • Elements consist of minute particles known as atoms.

SUMMARY (Cont.) • A mixture is composed of two or more elements and/or compounds, all of which retain their own characteristics and identity. • A compound is composed of two or more elements combined in definite proportions and held together by chemical force.

SUMMARY (Cont.) • A molecule is the smallest particle of a compound that can exist in a free state and take part in a chemical reaction. • Electrons carry a negative charge and orbit in shells around the atom’s nucleus. • Protons carry a positive charge and are located in the atom’s nucleus.

SUMMARY (Cont.) • Neutrons are electrically neutral and are located in the atom’s nucleus. • Matter can be classified into three states: solid, liquid, or gas. • Water is the only substance that is familiar in all three states: ice, water, and steam.

SUMMARY (Cont.) • Hydrogen is the simplest of the chemical elements and one of the most reactive. • Carbon exists in a number of forms and combines to form compounds more readily than any other element. • Most fuels are elementally composed of carbon and hydrogen.

SUMMARY (Cont.) • The products of combustion of an HC fuel are water and carbon dioxide. • When nitrogen is oxidized in the combustion process it forms several compounds known collectively as NOx.

SUMMARY (Cont.) • A stoichiometric combustion reaction occurs when the exact proportions of the reactants (fuel and air) are present. • Stoichiometric ratios for typical diesel fuels and gasoline range between 14.5:1 and 15:1.

SUMMARY (Cont.) • The moment of ignition in an engine cylinder occurs when there is visible flame or measurable pressure rise. • Diesel knock is a detonation condition.

Any Questions ? • Thank You !

Chemistry and Combustion Chapter 17