AIR-FUEL MIXTURE AND STOICHIOMETRIC RATIO

AIR-FUEL MIXTURE AND STOICHIOMETRIC RATIO

CHEMICAL COMBUSTION OF FUEL

Subhankar Karmakar
Assistant Professor; SGIT
Jindal Nagar; Ghaziabad

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Chemical Combustion is basically a rapid oxidation process of hydro-carbon fuel inside thekjm combustion chamber in the presence of air. The oxidation of fuel is basically a Exothermic or heat liberating chemical process.

In SI engines generally we use volatile hydrocarbon as fuel. The intermixing of fuel with air takes place outside the engine and the device that prepares air-fuel mixture of required mixture strength is called CARBURETION and the device is known as CARBURETTOR.

Estimation of air quantity needed for complete combustion of a given fuel

We know that any chemical reaction can be represented by the corresponding chemical equation like
CH₄ + 2O₂ = CO₂ + 2H₂O
here molecular weight of CH₄
μ_CH4 = 12 + 4x1 = 16
μ_2O2 = 2x16x2 = 64
μ_CO2 = 12 + 2x16 = 44
μ_2H2O = 2x(2+16) = 36

For complete combustion of CH₄
16 kg CH₄ needs 64 kg O₂ therefore,
1 kg of CH₄ needs (64/16) = 4 kg of O₂
For 23 kg of O₂ air needed is 100 kg
hence, for 4 kg of O₂ air needed is (100/23)x4 = 17.39 kg of air.
Air-fuel ratio will be 17.39 : 1

The ratio of air fuel mixture, needed for the complete combustion of the fuel or the chemically correct ratio of air fuel mixture required for complete combustion of the fuel is called " Stoichiometric Air fuel mixture. "

If the amount of air in the air-fuel mixture is less than the chemically correct amount of air, then the mixture is called rich mixture, where as if the quantity of air is more than the chemically corrected amount of air it is called lean mixture.

The strength of air-fuel mixture has a profound influences on the process of combustion. The required mixture strength for different operation conditions are different.

Different Operating Conditions	Required air-fuel Mixture Strength
For Max. Efficiency	17 : 1, 16.4% weak
For Max. Power	12 : 1 17.8% rich
For Starting, Idling, & Low load running	11 : 1 ~ 16 : 1 very rich mixture
For accelerated motion	13 : 1 rich mixture
For Part Load running Cruising Range	17 : 1 Lean Mixture Strength

FUEL USED IN IC ENGINES AND REFINERY PROCCESSES; EME-505

FUEL USED IN IC ENGINES

An article on fossil fuels

Internal Combustion Engines are the generators of the energy mainly used for transportation. Almost more than 90% of the total IC Engines run on fossil fuels or different derivatives of petroleum.

IC Engines are a kind of open cycle heat engine where heat is supplied to the engine by the combustion of working fluids thus releasing huge amount of energy due to the combustion processes of the working fluids. Combustible working fluids are called fuels.

The natural petroleum oil is the largest single source of internal combustion engine fuels. Petrol and Diesel are the most used among them. The boiling point of petrol is 30°C to 200°C and that of diesel oil is from 200°C to 375°C.

Fuels of most of the IC Engines are the derivatives of Petroleum like gasoline, diesel oil, kerosene, jet fuel etc. All of these fuels are produced during the fractional distillation of Petroleum Oil obtained from crude from oil wells.

The fuels used in the IC Engines are designed to satisfy the performance requirements of the engine system in which they are used. As a result the fuels must have certain

• (i) physical,
• (ii) chemical and
• (iii) combustion properties.

Following are the some characteristics a fuel must have in order to produce the desirable output to the engine performance.

1. A fuel must have a large energy density to be capable to release huge amount of energy during its combustion in side the combustion chamber.
2. A fuel must posses a good combustion quality to produce large amount of energy in smooth way.
3. A fuel must have high thermal stability or pre-ignition may occur.
4. A fuel must show a low deposit forming tendency else gum formation and other deposit forming processes will hamper the combustion process.
5. A fuel must be non-toxic, easy to handle and storage.

CRUDE PETROLEUM OIL:

Petroleum or often referred as "Crude Oil" is a naturally occurring inflammable mixtures of liquid and mud and it contains a complex mixture of different hydrocarbons of various molecular weights. It is mainly recovered through a process called "Oil Drilling".

Oil Wells and Gas Wells:

An oil well produces mainly crude oil with some natural gas dissolved in it. In contrast a gas well produces natural gases although it may contain heavier hydrocarbons like pentane, hexane or hepthane in gaseous state due to the extreme pressure and temperature inside the well, but at surface conditions condensation starts and forms "Natural Gas Condensate" or simply known as Condensate.



COMPOSITIONS OF CRUDE WELL:

Basically, crude well is the muddy mixtures of different hydrocarbons of different molecular weights. Alkanes, Cyclo-alkanes or napthenes, aromatics. It contains nitrogen, oxygen, sulfur and phosphorous. It may also contains metallic compounds too.

Four different types of hydrocarbon molecules appear in crude oil. The relative percentages are widely varied from oil to oil. They are:

• i) Paraffins (alkanes,  C_nH_{2n + 2} )
• ii) Olefins (alkenes, C_nH_2n),
• iii) Napthenes (cyclo-alkanes, C_nH_2n ),
• iv) Aromatics (having benzene ring, C_nH_{2n - 6}).

It is then refined by fractional distillation in oil refinery to obtain a large number of consumer products, from petrol or gasoline, diesel to kerosene, heavy oil, fuel oil, asphalt, chemical reagents, plastics etc.

Most of the derivatives of the petroleum have been used as fuel or heating purpose. The major products of a petroleum refinery are:

• (i) Gasoline,
• (ii) Kerosene,
• (iii) Diesel Oil,
• (iv) Fuel oil,
• (v) Heavy Oil,
• (vi) Lubricating Oil,
• (vii) Asphalts

INTRODUCTION: 

As the demands for gasoline, kerosene/ jet fuel and diesel oil are maximum, refineries around the world have started to convert heavy fuels and other higher hydrocarbons into gasoline, kerosene and diesel oil. To perform this, refineries have adopted several thermo-chemical processes those can convert high molecular weight hydrocarbons into lighter ones by breaking them.

GENERAL REFINERY PROCESSES:

Petroleum refining has evolved continuously in response to changing consumer demand for better and different products. The original requirement was to produce kerosene as a cheaper and better source of light than whale oil. The development of the internal combustion engine led to the production of gasoline and diesel fuels. The evolution of the airplane created an initial need for high-octane aviation gasoline and then for jet fuel, a sophisticated form of the original product, kerosene. Present-day refineries produce a variety of products including many required as feedstock for the petrochemical industry.

a) Distillation Processes:

The first refinery, opened in 1861, produced kerosene by simple atmospheric distillation. Its by-products included tar and naphtha. It was soon discovered that distilling petroleum under vacuum could produce high-quality lubricating oils. However, for the next 30 years kerosene was the product consumer wanted. Two significant events changed this situation. The invention of the electric light decreased the demand for kerosene and the invention of the internal combustion engine created a demand for diesel fuel and gasoline (naphtha). 

b) Thermal Cracking Processes:

With the advent of mass production and World War I, the number of gasoline-powered vehicles increased dramatically and the demand for gasoline grew accordingly. However, distillation processes produced only a certain amount of gasoline from crude oil. In 1913, the thermal cracking process was developed, which subjected heavy fuels to both pressure and intense heat, physically breaking the large molecules into smaller ones to produce additional gasoline and distillate fuels. Visbreaking, another form of thermal cracking, was developed in the late 1930's to produce more desirable and valuable products. 

c) Catalytic Processes:

Higher-compression gasoline engines required higher-octane gasoline with better antiknock characteristics. The introduction of catalytic cracking and polymerization processes in the mid- to late 1930's met the demand by providing improved gasoline yields and higher octane numbers.   Alkylation, another catalytic process developed in the early 1940's, produced more high-octane aviation gasoline and petrochemical feedstock for explosives and synthetic rubber. Subsequently, catalytic isomerization was developed to convert hydrocarbons to produce increased quantities of alkylation feedstock. Improved catalysts and process methods such as hydrocracking and reforming were developed throughout the 1960's to increase gasoline yields and improve antiknock characteristics. These catalytic processes also produced hydrocarbon molecules with a double bond (alkenes) and formed the basis of the modern petrochemical industry. 

d) Treatment Processes:

Throughout the history of refining, various treatment methods have been used to remove non-hydrocarbons, impurities, and other constituents that adversely affect the properties of finished products or reduce the efficiency of the conversion processes. Treating can involve chemical reaction and/or physical separation. Typical examples of treating are chemical sweetening, acid treating, clay contacting, caustic washing, hydrotreating, drying, solvent extraction, and solvent dewaxing. Sweetening compounds and acids desulfurize crude oil before processing and treat products during and after processing. 

Following the Second World War, various reforming processes improved gasoline quality and yield and produced higher-quality products. Some of these involved the use of catalysts and/or hydrogen to change molecules and remove sulfur. 

 Basics of Hydrocarbon Chemistry:

Crude oil is a mixture of hydrocarbon molecules, which are organic compounds of carbon and hydrogen atoms that may include from one to 60 carbon atoms. The properties of hydrocarbons depend on the number and arrangement of the carbon and hydrogen atoms in the molecules. The simplest hydrocarbon molecule is one carbon atom linked with four hydrogen atoms: methane. All other variations of petroleum hydrocarbons evolve from this molecule. 

 

Hydrocarbons containing up to four carbon atoms are usually gases, those with 5 to 19 carbon atoms are usually liquids and those with 20 or more are solids. The refining process uses chemicals, catalysts, heat, and pressure to separate and combine the basic types of hydrocarbon molecules naturally found in crude oil into groups of similar molecules. The refining process also rearranges their structures and bonding patterns into different hydrocarbon molecules and compounds. Therefore it is the type of hydrocarbon (paraffinic, naphthenic, or aromatic) rather than its specific chemical compounds that is significant in the refining process. 

Principal Groups of Hydrocarbon

• Paraffins - The paraffinic series of hydrocarbon compounds found in crude oil have the general formula C_nH_2n+2 and can be either straight chains (normal) or branched chains (isomers) of carbon atoms. The lighter, straight chain paraffin molecules are found in gases and paraffin waxes. Examples of straight-chain molecules are methane, ethane, propane, and butane (gases containing from one to four carbon atoms), and pentane and hexane (liquids with five to six carbon atoms). The branched-chain (isomer) paraffins are usually found in heavier fractions of crude oil and have higher octane numbers than normal paraffins. These compounds are saturated hydrocarbons, with all carbon bonds satisfied, that is, the hydrocarbon chain carries the full complement of hydrogen atoms.
  • Example of simplest hydrocarbon molecule: Methane (CH₄), Examples of straight chain paraffin molecule (Butane) and branched paraffin molecule (Isobutane) with same chemical formula (C₄H₁₀)
    
    
    
    
• Aromatics - Aromatics are unsaturated ring-type (cyclic) compounds which react readily because they have carbon atoms that are deficient in hydrogen. All aromatics have at least one benzene ring (a single-ring compound characterized by three double bonds alternating with three single bonds between six carbon atoms) as part of their molecular structure. Naphthalenes are fused double-ring aromatic compounds. The most complex aromatics, polynuclears (three or more fused aromatic rings), are found in heavier fractions of crude oil.
  • Example of simple aromatic compound: Benzene (C₆H₆), Examples of simple double-ring aromatic compound: Naphthalene (C₁₀H₈)
    
    
    
    
• Naphthenes - Naphthenes are saturated hydrocarbon groupings with the general formula C_nH_2n, arranged in the form of closed rings (cyclic) and found in all fractions of crude oil except the very lightest. Single-ring naphthenes (monocycloparaffins) with five and six carbon atoms predominate, with two-ring naphthenes (dicycloparaffins) found in the heavier ends of naphtha.
  • Example of typical single-ring naphthene: Cyclohexane (C₆H₁₂), Examples of naphthene with same chemical formula (C₆H₁₂) but different molecular structure: Methyl cyclopentane (C₆H₁₂)

Other Hydrocarbons

• Alkenes - Alkenes are mono-olefins with the general formula C_nH_2n and contain only one carbon-carbon double bond in the chain. The simplest alkene is ethylene, with two carbon atoms joined by a double bond and four hydrogen atoms. Olefins are usually formed by thermal and catalytic cracking and rarely occur naturally in unprocessed crude oil.
  • Example of simples Alkene: Ethylene (C₂H₄), Typical Alkenes with the same chemical formula (C₄H₈) but different molecular structures: 1-Butene and Isobutene
    
    
    
    
• Dienes and Alkynes - Dienes, also known as diolefins, have two carbon-carbon double bonds. The alkynes, another class of unsaturated hydrocarbons, have a carbon-carbon triple bond within the molecule. Both these series of hydrocarbons have the general formula CnH2n-2. Diolefins such as 1,2-butadiene and 1,3-butadiene, and alkynes such as acetylene,occur in C5 and lighter fractions from cracking. The olefins, diolefins, and alkynes are said to be unsaturated because they contain less than the amount of hydrogen necessary to saturate all the valences of the carbon atoms. These compounds are more reactive than paraffins or naphthenes and readily combine with other elements such as hydrogen, chlorine, and bromine.
  • Example of simplest Alkyne: Acetylene (C₂H₂), Typical Diolefins with the same chemical formula (C₄H₆) but different molecular structures: 1,2-Butadiene and 1,3-Butadiene

Non-hydrocarbons

• Sulfur Compounds -  Sulfur may be present in crude oil as hydrogen sulfide (H₂S), as sulfur compounds such as mercaptans, sulfides, disulfides, thiophenes, etc. or as elemental sulfur. Each crude oil has different amounts and types of sulfur compounds, but as a rule the proportion, stability, and complexity of the compounds are greater in heavier crude-oil fractions. Hydrogen sulfide is a primary contributor to corrosion in refinery processing units. Other corrosive substances are elemental sulfur and mercaptans. Moreover, the corrosive sulfur compounds have an obnoxious odor.  Pyrophoric iron sulfide results from the corrosive action of sulfur compounds on the iron and steel used in refinery process equipment, piping, and tanks. The combustion of petroleum products containing sulfur compounds produces undesirables such as sulfuric acid and sulfur dioxide. Catalytic hydrotreating processes such as hydrodesulfurization remove sulfur compounds from refinery product streams. Sweetening processes either remove the obnoxious sulfur compounds or convert them to odorless disulfides, as in the case of mercaptans.
  
  
• Oxygen Compounds -  Oxygen compounds such as phenols, ketones, and carboxylic acids occur in crude oils in varying amounts. 
  
  
• Nitrogen Compounds -  Nitrogen is found in lighter fractions of crude oil as basic compounds, and more often in heavier fractions of crude oil as nonbasic compounds that may also include trace metals such as copper, vanadium, and/or nickel. Nitrogen oxides can form in process furnaces. The decomposition of nitrogen compounds in catalytic cracking and hydrocracking processes forms ammonia and cyanides that can cause corrosion. 
  
  
• Trace Metals -  Metals, including nickel, iron, and vanadium are often found in crude oils in small quantities and are removed during the refining process. Burning heavy fuel oils in refinery furnaces and boilers can leave deposits of vanadium oxide and nickel oxide in furnace boxes, ducts, and tubes. It is also desirable to remove trace amounts of arsenic, vanadium, and nickel prior to processing as they can poison certain catalysts. 
  
  
• Salts -  Crude oils often contain inorganic salts such as sodium chloride, magnesium chloride, and calcium chloride in suspension or dissolved in entrained water (brine). These salts must be removed or neutralized before processing to prevent catalyst poisoning, equipment corrosion, and fouling. Salt corrosion is caused by the hydrolysis of some metal chlorides to hydrogen chloride (HCl) and the subsequent formation of hydrochloric acid when crude is heated. Hydrogen chloride may also combine with ammonia to form ammonium chloride (NH₄Cl), which causes fouling and corrosion. 
  
  
• Carbon Dioxide -  Carbon dioxide may result from the decomposition of bicarbonates present in or added to crude, or from steam used in the distillation process. 
• Naphthenic Acids -  Some crude oils contain naphthenic (organic) acids, which may become corrosive at temperatures above 450° F when the acid value of the crude is above a certain level.

 Major Refinery Products

• Gasoline. The most important refinery product is motor gasoline, a blend of hydrocarbons with boiling ranges from ambient temperatures to about 400 °F. The important qualities for gasoline are octane number (antiknock), volatility (starting and vapor lock), and vapor pressure (environmental control). Additives are often used to enhance performance and provide protection against oxidation and rust formation.
• Kerosene. Kerosene is a refined middle-distillate petroleum product that finds considerable use as a jet fuel and around the world in cooking and space heating. When used as a jet fuel, some of the critical qualities are freeze point, flash point, and smoke point. Commercial jet fuel has a boiling range of about 375°-525° F, and military jet fuel 130°-550° F. Kerosene, with less-critical specifications, is used for lighting, heating, solvents, and blending into diesel fuel.
• Liquified Petroleum Gas (LPG). LPG, which consists principally of propane and butane, is produced for use as fuel and is an intermediate material in the manufacture of petrochemicals. The important specifications for proper performance include vapor pressure and control of contaminants.
• Distillate Fuels. Diesel fuels and domestic heating oils have boiling ranges of about 400°-700° F. The desirable qualities required for distillate fuels include controlled flash and pour points, clean burning, no deposit formation in storage tanks, and a proper diesel fuel cetane rating for good starting and combustion.
• Residual Fuels. Many marine vessels, power plants, commercial buildings and industrial facilities use residual fuels or combinations of residual and distillate fuels for heating and processing. The two most critical specifications of residual fuels are viscosity and low sulfur content for environmental control.
• Coke and Asphalt. Coke is almost pure carbon with a variety of uses from electrodes to charcoal briquets. Asphalt, used for roads and roofing materials, must be inert to most chemicals and weather conditions.
• Solvents. A variety of products, whose boiling points and hydrocarbon composition are closely controlled, are produced for use as solvents. These include benzene, toluene, and xylene.
• Petrochemicals. Many products derived from crude oil refining, such as ethylene, propylene, butylene, and isobutylene, are primarily intended for use as petrochemical feedstock in the production of plastics, synthetic fibers, synthetic rubbers, and other products.
• Lubricants. Special refining processes produce lubricating oil base stocks. Additives such as demulsifiers, antioxidants, and viscosity improvers are blended into the base stocks to provide the characteristics required for motor oils, industrial greases, lubricants, and cutting oils. The most critical quality for lubricating-oil base stock is a high viscosity index, which provides for greater consistency under varying temperatures.

Common Refinery Chemicals

• Leaded Gasoline Additives: Tetraethyl lead (TEL) and tetramethyl lead (TML) are additives formerly used to improve gasoline octane ratings but are no longer in common use except in aviation gasoline.
• Oxygenates: Ethyl tertiary butyl ether (ETBE), methyl tertiary butyl ether (MTBE), tertiary amyl methyl ether (TAME), and other oxygenates improve gasoline octane ratings and reduce carbon monoxide emissions.
• Caustics: Caustics are added to desalting water to neutralize acids and reduce corrosion. They are also added to desalted crude in order to reduce the amount of corrosive chlorides in the tower overheads. They are used in some refinery treating processes to remove contaminants from hydrocarbon streams.
• Sulfuric Acid and Hydrofluoric Acid: Sulfuric acid and hydrofluoric acid are used primarily as catalysts in alkylation processes. Sulfuric acid is also used in some treatment processes.
  
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INTERNAL COMBUSTION ENGINE (IC ENGINE)

MECHANICAL ENGG : INTERNAL COMBUSTION ENGINE

fig: wankel engine

single cylinder IC engine

IC engine

Definition:

The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber. In an internal combustion engine the expansion of the high temperature and pressure gases, which are produced by the combustion, directly applies force to a movable component of the engine, such as the pistons or turbine blades and by moving it over a distance, generate useful mechanical energy.

Combustion Type:

• Intermittent Combustion:

The term internal combustion engine usually refers to an engine in which combustion is intermittent, such as the more familiar four-stroke and two-stroke piston engines, along with variants, such as the Wankel rotary engine.

• Continuous Combustion:

A second class of internal combustion engines use continuous combustion: gas turbines, jet engines and most rocket engines, each of which are internal combustion engines on the same principle as previously described.

Uses and Applications:

Internal combustion engines are most commonly used for mobile propulsion in vehicles and portable machinery. In mobile equipment, internal combustion is advantageous since it can provide high power-to-weight ratios together with excellent fuel energy density. Generally using fossil fuel (mainly petroleum), these engines have appeared in transport in almost all vehicles (automobiles, trucks, motorcycles, boats, and in a wide variety of aircraft and locomotives).

Internal combustion engines appear in the form of gas turbines as well where a very high power is required, such as in jet aircraft, helicopters, and large ships. They are also frequently used for electric generators and by industry.

Combustion Mechanism:

All internal combustion engines depend on the exothermic chemical process of combustion: the reaction of a fuel, typically with oxygen from the air (though it is possible to inject nitrous oxide in order to do more of the same thing and gain a power boost). The combustion process typically results in the production of a great quantity of heat, as well as the production of steam and carbon dioxide and other chemicals at very high temperature; the temperature reached is determined by the chemical make up of the fuel and oxidisers.

Types of Fuels it uses:

The most common modern fuels are made up of hydrocarbons and are derived mostly from fossil fuels (petroleum). Fossil fuels include diesel fuel, gasoline and petroleum gas, and the rarer use of propane. Except for the fuel delivery components, most internal combustion engines that are designed for gasoline use can run on natural gas or liquefied petroleum gases without major modifications. Large diesels can run with air mixed with gases and a pilot diesel fuel ignition injection. Liquid and gaseous biofuels, such as ethanol and biodiesel (a form of diesel fuel that is produced from crops that yield triglycerides such as soybean oil), can also be used. Some engines with appropriate modifications can also run on hydrogen gas.

Comparison of IC Engine with Steam Engine:

a) Both IC engine and steam engine are basically heat engines used to convert heat energy into mechanical energy.

b) In IC engine, the combustion of fuel (liquid or gas) takes place inside the engine cylinder. Where as, in steam engine combustion occurs outside engine, in a boiler to raise the temperature which in turn is used in the heat engine.

c) The working temperature and pressure inside an IC engine are much higher than that of steam engine. It requires the design be robust and strong temperature and pressure resistant.

d) IC engines are mostly single acting while most of the steam engines are double acting. Hence, no need of stuffing box in IC engines.

e) IC engine produces high efficiency in the range of 35% to 40%, while steam engine can produce work with an efficiency in the range of 10% to 15%.

f) Compared to long starting procedure of a steam engine, an IC engine can be started instantenously.

Classification of IC engines:

IC engines can be classified on different characteristics basis.

a) Type of Ignition process: i) Spark Ignition or SI engine, ii) Compression Ignition or CI engine, iii) Hot spot ignition engine.	b) Type of Fuel used: i) Petrol/Gasoline engine, ii) Diesel engine, iii) Gas engine.	c) Number of Strokes per cycle: i) Four stroke engine, ii) Two stroke engine.
d) Type of Cooling system: i) Air cooled engine, ii) Water cooled engine, iii) Evaporative cooling engine.	e) Cycle of Operation: i) Otto cycle engine, ii) Diesel cycle engine, iii) Dual cycle engine.	f) Method of fuel injection: i) Carburettor engine, ii) Air injection engine, iii) Airless or solid injection engine.
g) Arrangement of Cylinders: i) Vertical engine, ii) Horizontal engine, iii) Radial engine, iv) V engine, v) Opposed cylinder engine, vi) Opposed piston engine.	h) Application fields: i) Stationary engine, ii) Automotive engine, iii) Marine engine, iv) Aircraft engine, v) Locomotive engine.	i) Valve Locations: i) Over-head valve engine, ii) Side valve engine.
j) Speed of the engine: i) Slow speed engine, ii) Medium speed engine, iii) High speed engine.	k) Method of Governing: i) Hit and Miss governed engine, ii) Qualitatively governed engine, iii) Quantatively governed engine.

TERMINOLOGY: IC ENGINE

BORE: The inside diameter of the cylinder is known as bore. It is always measured in mm. 

STROKE: The distance travelled by the piston from one of its dead center positions to the other dead center position. 

DEAD CENTERS: They correspond to the positions occupied by the piston at the end of its stroke where the center lines of the connecting rod and crank are in the same straight line. These conditions arise at two specific positions of the piston. At the start of the journey of stroke and at the end of the stroke are these two specific conditions, which are named as Top Dead Center (TDC) and Bottom Dead Center or BDC for vertical engines and IDC or Inner Dead Center and ODC or Outer Dead Center for horizontal engines. 

TDC: The top most position of the piston towards the cover end side of the cylinder of a vertical engine is called Top Dead Center or TDC. 

BDC: The lowest position of the piston towards the crank end side of the cylinder of a vertical engine is known as BDC. 

CRANK THROW/ CRANK RADIUS: The distance between the center of main shaft and center of crank pin is known as Crank Throw or Crank Radius. This distance will be equal to half the stroke length. 

PISTON DISPLACEMENT/ SWEPT VOLUME: It is the volume through which the piston sweeps for its one stroke. Swept Volume is represented by Vs and it is equal to cross-sectional area of the piston x stroke length. 

Vs = {(π x d²)/4} x stroke length (L)
∴ Vs = (π.d².L)/4
CLEARENCE VOLUME: It is the volume included between the piston and the cylinder head when it is at TDC (for vertical engines) or IDC (for horizonal engine). The piston can never enters this portion of the cylinder during its travel. Clearence volume (Vc) is generally expressed as percentage of the swept volume and is denoted by Vc. 

COMPRESSION RATIO: It is the ratio of the total cylinder volume to the clearance volume. If swept volume is (Vs) and clearance volume is (Vc) then total volume of the cylinder V = Vs + Vc and Compression Ratio will be equals to (Vs + Vc)/Vc. For petrol engine it varies from 5:1 to 9:1 and for diesel engines from 14 : 1 to 22 : 1. 

PISTON SPEED: It is the distance travelled by piston in one minute. If rpm of engine shaft is (N) and length of stroke is (L), then piston speed will be 2LN m/min. 

TERMINOLOGY:

(i) Internal combustion (IC): The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber. In an internal combustion engine the expansion of the high temperature and pressure gases, which are produced by the combustion, directly applies force to a movable component of the engine, such as the pistons or turbine blades and by moving it over a distance, generate useful mechanical energy.

(ii) Spark Ignition(SI): An engine in which the combustion process in each cycle is started by use of a spark plug.

(iii) Compression Ignition(CI): An engine in which the combustion process starts when the air fuel mixture self ignites due to high temperature in the combustion chamber caused by the high compression. CI engines are often called diesel engines especially in the non technical community.

(iv) Top-Dead-Center (TDC): Position of the piston when it stops at the furthest point away from the crankshaft. Top because this position is at the top of most engines (not always) and dead because the piston stops at this point. Because in some engines top-dead-center is not at the top of the engine (e.g., horizontally opposed engines, radial engines, etc.,), some sources call this position Head-End-Dead-Center (HEDC). Some sources call this position Top-Center (TC). When an occurrence in a cycle happens before TDC, it is often abbreviated bTDC or bTC. When the occurrence happens after TDC or a TC. When the piston is at TDC, the volume in the cylinder is a minimum called the clearance volume.

(v) Bottom-Dead-Center (BDC): Position of the piston when it stops at the point closest to the crankshaft. Some sources call this Crank-End-Dead-Center(CEDC) because it is not always at the bottom of the engine. Some sources call this point Bottom-Center(BC). During an engine cycle things happen before Bottom-Dead-Center, bBDC or bBC, and after bottom-deadcenter, aBDC or aBC.

(vi) Direct Injection:Fuel injection into the main combustion chamber of an engine. Engines either have one main combustion chamber (open chamber) or a divided combustion chamber made up of a main chamber and a smaller connected secondary chamber.

(vii) Indirect injection: Fuel injection into the secondary chamber of an engine with a divided combustion chamber.

(viii) Displacement volume: Volume displaced by the piston as it travels through one stroke. Displacement cans b given for one cylinder or for the entire engine (one cylinder time’s number of cylinders). Some literature calls this swept volume.

(ix) Gasoline Direct Injection (GDI): Spark ignition engine with fuel injectors mounted in combustion chambers. Gasoline fuel is injected directly into cylinders during compression stroke.

(x) Homogeneous Charge Compression Ignition (HCCI): Compression-Ignition engine operating with a homogeneous airfuel charge instead of the diffusion combustion mixture normally used in CI engines.

(xi) Smart Engine: either computer controls that regulate operating characteristics such as air fuel ratio, ignition timing, valve timing, exhaust control, intake tuning, etc.Computer inputs come from electronic, mechanical, thermal and chemical sensors located throughout the engine. Computers in some automobiles are even programmed to adjust engine operation for things like valve water and combustion chamber deposit build up as the engine ages. In automobiles, the same computers are used to make smart cars by controlling the steering, brakes, exhaust system, suspension, seats, anti-theft systems, sound-endear analysis navigation entertainment systems, shifting, doors, noise, suppression, environment, comfort,etc.(o) Engine Management System: Computer and electronics used to control smart engines.

(xii) Wide- Open throttle (WOT): Engine operated with throttle valve fully open when maximum power and/or speed is desired.

(xiii) Ignition Delay (ID): Time interval between ignition initiation and the actual start of combustion.

(r) Air Fuel Ratio: Ratio of mass air to mass of fuel input into engine.

(xiv) Fuel-Air ratio: Ratio of mass of fuel to mass of air input into engine.

(xv) Brake Maximum torque: (BMT): Speed at which maximum torque occurs.

(xvi) Overhead Valve (OHV): Valves mounted in engine head.

(xvii) Overhead Cam (OHC): Camshaft mounted in engine head, giving more direct control of valves which are also mounted in engine head.

(xviii) Fuel Injection (FI):

MAIN ENGINE COMPONENTS:

The following is the list of major components found in most reciprocating internal combustion engines.

• Block: Body of engine containing the cylinders made of cast iron or aluminum. In many older engines the valves and the valve ports were contained in the block. The block of water cooled engines includes a water jacket cast around the cylinders. On air cooled engines the exterior surface of the block has cooling fins. 

• Camshaft: Rotating shaft used to push open valves at the proper time in the engine cycle either directly or through mechanical or hydraulic linkage (push rods, rocker arms, and tappets). Most modern automobile engines have one or more camshafts mounted in the engine head (Overhead cam). Older engines had camshafts in the crank case. Crankshafts are generally made of forget steel or cast iron and driven off the crankshaft by means of a belt or chain (Timing chain). To reduce weight, some cams are made from a hollow shaft with the cam lobes press-fit on. In four stroke cycle engines the camshaft rotates at half engine speed.

• Carburetor: Venturi flow device that meters the proper amount of fuel into the air flow by means of pressure
  differential. For many decades it was the basic fuel metering system on all automobile (and other) engines. It is still used on low cost small engines like lawn mowers but is uncommon on new automobiles.

• Catalytic converter: Chamber mounted in exhaust flow containing catalytical material that promotes reduction of emission by chemical reaction.

• Choke: Butterfly valve at carburetor intake, used to create rich fuel-air mixture in intake system for cold weather starting.

• Combustion chamber: The end of the cylinder between the head and the piston face where the combustion occurs. The size of the combustion chamber continuously changes from a minimum volume when the piston is at TDC to a maximum when the piston is at BDC. The term cylinder is sometimes synonymous with combustion chamber (e.g., the engine was firing on all cylinders). Some engines have open combustion chambers which consist of one chamber for each cylinder.

• Other engines have divided chambers which consist of dual chambers on each cylinder connected by an orifice passage.

• CRANK CASE: In IC Engine terminology, Crank Case is the housing of Crank Shaft. It is the largest cavity in engine and is fixed to cylinder. 
  
  
  
  
  

OCTANE AND CETANE NUMBERS

Self ignition temperature (SIT) of a fuel is the temperature at which the fuel ignites on its own without spark. If large amount of mixture in an engine cylinder auto ignites, there will be a rapid rise in pressure causing direct blow on engine structure accompanied by thudding sound. This causes vibrations in the engine. The phenomenon is called knocking.

If however, a small pocket of fuel-air mixture auto ignites, pressure waves are generated which travel with the speed of sound across the cylinder. These pressure waves are of such small duration that indicator diagram mechanism fails to record them. These waves interact within themselves and with the cylinder walls, creating characteristics ping sound. The phenomenon is called pinking.

The engine runs rough, overheats and loses efficiency due to knocking and pinking.

The processes of knocking and pinking are related to the nature of the fuel and relative merits of the fuel are decided on the basis of their anti-pinking and anti-knock property. The merit is measured by octane number such that a fuel of high octane number will be liable to less pink or knock as compared to a fuel of low octane number in the same engine. It is important to note that the same fuel will show same tendency to pink or knock in all engines.

Commonly used fuel in SI engines is a mixture of iso-octane and n-heptane. Iso-octane has minimum tendency to knock and this fuel is arbitrarily assigned an octane number of 100 (ON = 100) where as n-heptane has maximum knocking tendency with ON = 0. The octane number of a given fuel is percentage of iso-octane in the mixture of iso-octane and n-heptane. Thus a fuel other than mixture of iso-octane and n-heptane if assigned an ON of 80, it means, it will knock under standard operating condition similar to the mixture of 80% iso-octane and 20% n-heptane.

The tendency to knock in an engine increases with the increase in compression ratio. The highest compression upto which no knocking occurs in a given engine is called highest useful compression ratio (HUCR).

Certain chemical compounds when added to the fuel successfully suppress the knocking tendency. Tetra-ethyl lead [Pb(C₂ H₅)₄] also commonly called TEL and tetra-methyl lead [Pb(CH₃)₄] also referred to as TML are effective dopes in the automobile fuel to check knocking. They are called as anti-knocking agents. However, because of lead poisoning effects TEL and TML are not being used now-a-days. In stead, some organic auto knocking agents have been developed to check the undesirable effects like knocking.

In CI engine air alone is compressed to a compression ratio of 15 to 20 (commonly). The fuel is injected under a pressure of 120 to 210 bars about 20° to 35° before TDC. As the fuel in the engine starts to evaporate the pressure in the cylinder drops and it delays the ignition process by a small amount. The time between beginning of injection and the beginning of combustion is known as the delay period which consists of time for atomization, vapourization and mixing along with time of chemical reaction prior to auto-ignition. The combustion of fuel continues in the expansion and is called after burning. Increased delay period causes accumulation of atomized fuel in the combustion chamber and as the pressure and temperature continue to rise at one instant, the bulk of fuel auto-ignites. This would result in high forces on the structure of the engine causing vibration and rough running.

The CI engine fuel rating is based on ignition delay and is measured in terms of cetane number. Cetane fuel [C₁₆ H₃₄] has very low delay period and is arbitrarily assigned a cetane number of 100. Another fuel a α-methyl-napthalene [C₁₁ H₁₀] has poor ignition quality and is assigned zero cetane number. The volume percentage of cetane in a mixture of cetane and a-methyl naphthalene is the cetane number of the fuel that produces same delay period as the mixture under specified test conditions. Additives such as methyl nitrate, ethyl thio-nitrate and amyl nitrate increase cetane number of a fuel respectively by 13.5%, 10% and 9% if added to the extent of 0.5%.

Introduction To the Combustion of Fuels

Combustion:

Principle of Combustion:

Combustion is the conversion of a substance called a fuel into chemical compounds
known as products of combustion by combination with an oxidizer. The combustion
process is an exothermic chemical reaction, i.e., a reaction that releases energy as it
occurs.

Thus combustion may be represented symbolically by:
Fuel + Oxidizer = Products of combustion + Energy

Here the fuel and the oxidizer are reactants, i.e., the substances present before the
reaction takes place. This relation indicates that the reactants produce combustion
products and energy. Either the chemical energy released is transferred to the
surroundings as it is produced, or it remains in the combustion products in the form of
elevated internal energy (temperature), or some combination thereof.

Fuels are evaluated, in part, based on the amount of energy or heat that they
release per unit mass or per mole during combustion of the fuel. Such a quantity is
known as the fuel’s heat of reaction or heating value.

Heats of reaction may be measured in a calorimeter, a device in which chemical
energy release is determined by transferring the released heat to a surrounding fluid.
The amount of heat transferred to the fluid in returning the products of combustion to
their initial temperature yields the heat of reaction.

In combustion processes the oxidizer is usually air but could be pure oxygen, an
oxygen mixture, or a substance involving some other oxidizing element such as
fluorine. Here we will limit our attention to combustion of a fuel with air or pure
oxygen.

Chemical fuels exist in gaseous, liquid, or solid form. Natural gas, gasoline, and
coal, perhaps the most widely used examples of these three forms, are each a complex
mixture of reacting and inert compounds. We will consider each more closely later in
the chapter. First let’s review some important fundamentals of mixtures of gases, such
as those involved in combustion reactions.

Therefore, combustion refers to the rapid oxidation of fuel accompanied by the production of heat, or heat and light. Complete combustion of a fuel is possible only in the presence of an adequate supply of oxygen.

Oxygen (O₂) is one of the most common elements on earth making up 20.9% of our air. Rapid fuel oxidation results in large amounts of heat. Solid or liquid fuels must be changed to a gas before they will burn. Usually heat is required to change liquids or solids into gases. Fuel gases will burn in their normal state if enough air is present.

Most of the 79% of air (that is not oxygen) is nitrogen, with traces of other elements. Nitrogen is considered to be a temperature reducing dilutant that must be present to obtain the oxygen required for combustion.

Nitrogen reduces combustion efficiency by absorbing heat from the combustion of fuels and diluting the flue gases. This reduces the heat available for transfer through the heat exchange surfaces. It also increases the volume of combustion by-products, which then have to travel through the heat exchanger and up the stack faster to allow the introduction of additional fuel air mixture.

This nitrogen also can combine with oxygen (particularly at high flame temperatures) to produce oxides of nitrogen (NO_x), which are toxic pollutants.

Carbon, hydrogen and sulphur in the fuel combine with oxygen in the air to form carbon dioxide, water vapour and sulphur dioxide, releasing 8084 kcals, 28922 kcals & 2224 kcals of heat respectively.

Under certain conditions, Carbon may also combine with Oxygen to form Carbon Monoxide, which results in the release of a smaller quantity of heat (2430 kcals/kg of carbon) Carbon burned to CO2 will produce more heat per pound of fuel than when CO or smoke are produced.

C + O₂ → CO₂ + 8084 kCals/kg of Carbon

2C + O₂ → 2 CO + 2430 kCals/kg of Carbon

2H₂ + O₂ → 2H₂O + 28,922 kCals/kg of Hydrogen

S + O₂ → SO₂ + 2,224 kCals/kg of Sulphur

3 T’s of Combustion:

The objective of good combustion is to release all of the heat in the fuel. This is accomplished by controlling the "three T's" of combustion which are

1. Temperature high enough to ignite and maintain ignition of the fuel,
2. Turbulence or intimate mixing of the fuel and oxygen, and
3. Time sufficient for complete combustion.

Commonly used fuels like natural gas and propane generally consist of carbon and hydrogen. Water vapor is a by-product of burning hydrogen. This robs heat from the flue gases, which would otherwise be available for more heat transfer.

Natural gas contains more hydrogen and less carbon per kg than fuel oils and as such produces more water vapor. Consequently, more heat will be carried away by exhaust while firing natural gas.

Too much, or too little fuel with the available combustion air may potentially result in unburned fuel and carbon monoxide generation. A very specific amount of O2 is needed for perfect combustion and some additional (excess) air is required for ensuring complete combustion. However, too much excess air will result in heat and efficiency losses.

Not all of the heat in the fuel are converted to heat and absorbed by the steam generation equipment. Usually all of the hydrogen in the fuel is burned and most boiler fuels, allowable with today's air pollution standards, contain little or no sulfur. So the main challenge in combustion efficiency is directed toward unburned carbon (in the ash or incompletely burned gas), which forms CO instead of CO₂.

                                                                                                                             Subhankar Karmakar

IC ENGINES AND COMBUSTION CHAMBER

IC ENGINES :

IC engines, or internal combustion engines, are engines in which combustion of fuel and air occurs within the engine cylinder, converting the chemical energy of the fuel into mechanical energy to perform work. The combustion chamber is a critical component of an IC engine, as it is the location where combustion occurs.

COMBUSTION CHAMBER:

The combustion chamber is typically located at the top of the cylinder in a reciprocating engine, or in the center of the combustion chamber in a rotary engine. It is designed to confine the fuel and air mixture to a small volume, allowing for efficient and controlled combustion.

The shape and size of the combustion chamber can have a significant impact on the performance and efficiency of the engine. The shape of the combustion chamber can affect the way that the fuel and air mixture is mixed and ignited, as well as the speed at which the flame front propagates through the mixture. The size of the combustion chamber can affect the compression ratio of the engine, which in turn affects the power output and fuel efficiency of the engine.

There are various types of combustion chambers used in IC engines, including the traditional spark ignition chamber and the compression ignition chamber. The spark ignition chamber is typically used in gasoline engines, where a spark plug is used to ignite the fuel and air mixture. The compression ignition chamber is typically used in diesel engines, where the fuel is ignited by the heat generated by compressing the air in the cylinder.

Overall, the design of the combustion chamber is a critical factor in the performance and efficiency of an IC engine, and careful attention must be paid to its design in order to optimize engine performance.

COMPONENTS OF A COMBUSTION CHAMBER:

The combustion chamber in an internal combustion engine is typically composed of several key components that work together to promote efficient combustion of the fuel and air mixture. The following are some of the common components of a combustion chamber:

• Cylinder Head:

The cylinder head is the top part of the engine cylinder that contains the combustion chamber. It is typically bolted onto the engine block and is responsible for sealing the combustion chamber and providing a mounting point for the valves, spark plugs, and fuel injectors.

• Piston:

The piston is a cylindrical component that moves up and down within the engine cylinder. It is responsible for compressing the air/fuel mixture and transmitting the force generated by combustion to the crankshaft.

• Valves:

The valves are located in the cylinder head and are responsible for controlling the flow of air and fuel into the combustion chamber and the flow of exhaust gases out of the engine. There are typically two types of valves: intake valves and exhaust valves.

• Spark Plug:

The spark plug is a small device that is used to ignite the fuel and air mixture in the combustion chamber. It generates an electrical spark that ignites the mixture and initiates the combustion process.

• Fuel Injector:

The fuel injector is responsible for delivering fuel into the combustion chamber in a precise and controlled manner. It typically uses a high-pressure fuel system to inject fuel into the combustion chamber at the correct time and in the correct amount.

• Combustion Chamber Walls:

The walls of the combustion chamber are typically made of high-strength materials such as steel or aluminum. They are designed to withstand the high temperatures and pressures generated by combustion and to provide a seal for the combustion gases.

• Intake and Exhaust Ports:

The intake and exhaust ports are openings in the cylinder head that allow air and fuel to enter the combustion chamber and exhaust gases to exit the engine. Overall, the components of a combustion chamber work together to promote efficient and controlled combustion of the fuel and air mixture, maximizing engine performance and efficiency.

DESIGNING CRITERIA OF A COMBUSTION CHAMBER:

The design of a combustion chamber in an internal combustion engine is a critical factor in determining the performance, efficiency, and emissions of the engine. The following are some of the key criteria that must be considered in the design of a combustion chamber:

• Air/Fuel Mixture:

The combustion chamber must be designed to provide proper mixing of air and fuel. This is necessary to ensure efficient combustion and minimize emissions.

• Flame Propagation:

The combustion chamber must be designed to promote fast and efficient flame propagation. This is necessary to ensure that the fuel is burned completely and to maximize power output.

• Compression Ratio:

The combustion chamber must be designed to achieve the desired compression ratio. This is important for determining the engine's power output and fuel efficiency.

• Combustion Efficiency:

The combustion chamber must be designed to promote complete combustion of the fuel. This is necessary to minimize emissions and maximize fuel efficiency.

• Turbulence:

The combustion chamber must be designed to promote turbulence in the air/fuel mixture. This is important for promoting efficient combustion and minimizing emissions.

• Wall Heat Transfer:

The combustion chamber must be designed to minimize heat transfer to the cylinder walls. This is important for reducing engine heat loss and maximizing power output.

• Knock Resistance:

The combustion chamber must be designed to resist engine knock. This is important for maximizing power output and engine efficiency.

• Emissions:

The combustion chamber must be designed to minimize emissions of pollutants such as nitrogen oxides (NOx), carbon monoxide (CO), and particulate matter (PM). This is important for meeting emissions regulations and minimizing environmental impact. Overall, the design of the combustion chamber is a complex process that requires consideration of multiple factors. Careful attention to these criteria is necessary to optimize engine performance and meet emissions regulations.

FAILURE CRITERIA OF COMBUSTION CHAMBER:

The failure of a combustion chamber in an internal combustion engine can be catastrophic and can result in engine damage, reduced performance, or even complete engine failure. The following are some of the common failure criteria of a combustion chamber:

• Overheating:

One of the most common failure modes of a combustion chamber is overheating. This can be caused by a variety of factors, such as a lean air/fuel mixture, excessive compression, or a malfunctioning cooling system. Overheating can cause cracking or warping of the combustion chamber, leading to leaks or even catastrophic failure.

• Detonation:

Detonation occurs when the fuel/air mixture in the combustion chamber detonates spontaneously instead of burning in a controlled manner. This can be caused by factors such as excessive compression, hot spots in the combustion chamber, or low-quality fuel. Detonation can cause the combustion chamber to deform or crack, leading to reduced engine performance or even complete engine failure.

• Pre-ignition:

Pre-ignition occurs when the fuel in the combustion chamber ignites before the spark plug fires. This can be caused by factors such as hot spots in the combustion chamber, high compression, or low-quality fuel. Pre-ignition can cause damage to the combustion chamber and other engine components, leading to reduced engine performance or even complete engine failure.

• Corrosion:

Corrosion can occur in the combustion chamber due to the corrosive nature of the fuel or the combustion process itself. Corrosion can weaken the walls of the combustion chamber, leading to cracks or other types of damage that can compromise engine performance.

• Mechanical Damage:

Mechanical damage to the combustion chamber can occur due to improper installation, poor maintenance, or external factors such as debris striking the engine. This type of damage can cause leaks or other types of damage that can affect engine performance or even cause complete engine failure. Overall, the failure of a combustion chamber can have severe consequences for engine performance and reliability. Regular maintenance and proper operation of the engine can help to prevent these failure modes and ensure the long-term reliability and performance of the engine.