Internal Combustion EnginesIntroductionInternal Combustion Engine, a heat
Internal Combustion EnginesIntroductionInternal Combustion Engine, a heat engine in which the fuel is burned (that is, united with oxygen ) within the confining space of the engine itself.
This burning process releases large amounts of energy, which are transformedinto work through the mechanism of the engine. This type of engine differentfrom the steam engine, which process with an external combustion engine thatfuel burned apart from the engine. The principal types of internal combustionengine are : reciprocating engine such as Otto-engine, and Diesel engines ; androtary engines, such as the Wankel engine and the Gas-turbine engine.In general, the internal combustion engine has become the means ofpropulsion in the transportation field, with the exception of large shipsrequiring over 4,000 shaft horsepower ( hp).In stationary applications, size of unit and local factor oftendetermine the choice between the use of steam and diesel engine.
Diesel powerplants have a distinct economic advantage over steam engine when size of theplant is under about 1,000 hp. However there are many diesel engine plants muchlarge than this. Internal combustion engines are particularly appropriate forseasonal industries, because of the small standby losses with these enginesduring the shutdown period.HistoryThe first experimental internal combustion engine was made by a Dutchastronomer, Christian Huygens, who, in 1680, applied a principle advanced byJean de Hautefeuille in 1678 for drawing water. This principle was based on thefact that the explosion of a small amount of gunpowder in a closed chamberprovided with escape valves would create a vacuum when the gases of combustioncooled. Huygens, using a cylinder containing a piston, was able to move it inthis manner by the external atmospheric pressure.
The first commercially practical internal combustion engine was built bya French engineer, ( Jean Joseph ) Etienne Lenoir, about 1859-1860. It usedilluminating gas as fuel. Two years later, Alphonse Beau de Rochas enunciatedthe principles of the four-stroke cycle, but Nickolaus August Otto built thefirst successful engine ( 1876 ) operating on this principle.Reciprocating EngineComponents of EnginesThe essential parts of Otto-cycle and diesel engines are the same.
Thecombustion chamber consists of a cylinder, usually fixed, which is closed at oneend and in which a close-fitting piston slides. The in-and-out motion of thepiston varies the volume of the chamber between the inner face of the piston andthe closed end of the cylinder. The outer face of the piston is attached to acrankshaft by a connecting rod. The crankshaft transforms the reciprocatingmotion of the piston into rotary motion. In multi-cylindered engines thecrankshaft has one offset portion, called a crankpin, for each connecting rod,so that the power from each cylinder is applied to the crankshaft at theappropriate point in its rotation.
Crankshafts have heavy flywheels andcounterweights, which by their inertia minimize irregularity in the motion ofthe shaft. An engine may have from 1 to as many as 28 cylinders.The fuel supply system of an internal-combustion engine consists of atank, a fuel pump, and a device for vaporizing or atomizing the liquid fuel. InOtto-cycle engines this device is a carburetor.
The vaporized fuel in mostmulti-cylindered engines is conveyed to the cylinders through a branched pipecalled the intake manifold and, in many engines, a similar exhaust manifold isprovided to carry off the gases produced by combustion. The fuel is admitted toeach cylinder and the waste gases exhausted through mechanically operated poppetvalves or sleeve valves. The valves are normally held closed by the pressure ofsprings and are opened at the proper time during the operating cycle by cams ona rotating camshaft that is geared to the crankshaft .
By the 1980s moresophisticated fuel-injection systems, also used in diesel engines, had largelyreplaced this traditional method of supplying the proper mix of air and fuel;computer-controlled monitoring systems improved fuel economy and reducedpollution.IgnitionIn all engines some means of igniting the fuel in the cylinder must beprovided. For example, the ignition system of Otto-cycle engines , the mixtureof air and gasoline vapor delivered to the cylinder from the carburetor and nextoperation is that of igniting the charge by causing a spark to jump the gapbetween the electrodes of a spark plug, which projects through the walls of thecylinder. One electrode is insulated by porcelain or mica; the other is groundedthrough the metal of the plug, and both form the part of the secondary circuitof an induction system.
The principal type of high-tension ignition now commonly used is thebattery-and-coil system. The current from the battery flows through the low-tension coil and magnetizes the iron core. When this circuit is opened at thedistributor points by the interrupter cam, a transient high-frequency current isproduced in the primary coil with the assistance of the condenser.
This inducesa transient, high-frequency, high-voltage current in the secondary winding. Thissecondary high voltage is needed to cause the spark to jump the gap in the sparkplug. The spark is directed to the proper cylinder to be fired by thedistributor, which connects the secondary coil to the spark plugs in the severalcylinders in their proper firing sequence. The interrupter cam and distributorare driven from the same shaft, the number of breaking points on the interruptercam being the same as the number of cylinders.
Cooling SystemBecause of the heat of combustion, all engines must be equipped withsome type of cooling system. Some aircraft and automobile engines, smallstationary engines, and outboard motors for boats are cooled by air. In thissystem the outside surfaces of the cylinder are shaped in a series of radiatingfins with a large area of metal to radiate heat from the cylinder. Other enginesare water-cooled and have their cylinders enclosed in an external water jacket.
In automobiles, water is circulated through the jacket by means of a water pumpand cooled by passing through the finned coils of a radiator. Some automobileengines are also air-cooled, and in marine engines sea water is used for cooling.StarterUnlike steam engines and turbines, internal-combustion engines developno torque when starting, and therefore provision must be made for turning thecrankshaft so that the cycle of operation can begin.
Automobile engines arenormally started by means of an electric motor or starter that is geared to thecrankshaft with a clutch that automatically disengages the motor after theengine has started. Small engines are sometimes started manually by turning thecrankshaft with a crank or by pulling a rope wound several times around theflywheel. Methods of starting large engines include the inertia starter, whichconsists of a flywheel that is rotated by hand or by means of an electric motoruntil its kinetic energy is sufficient to turn the crankshaft, and the explosivestarter, which employs the explosion of a blank cartridge to drive a turbinewheel that is coupled to the engine. The inertia and explosive starters arechiefly used to start airplane engines.
Otto-Cycle EnginesThe ordinary Otto-cycle engine is a four-stroke engine; that is, itspistons make four strokes, two toward the head (closed head) of the cylinder andtwo away from the head, in a complete power cycle. During the first stroke ofthe cycle, the piston moves away from the cylinder head while simultaneously theintake valve is opened. The motion of the piston during this stroke sucks aquantity of a fuel and air mixture into the combustion chamber. During the nextstroke the piston moves toward the cylinder head and compresses the fuel mixturein the combustion chamber.
At the moment when the piston reaches the end of thisstroke and the volume of the combustion chamber is at a minimum, the fuelmixture is ignited by the spark plug and burns, expanding and exerting apressure on the piston, which is then driven away from the cylinder head in thethird stroke. At the end of the power stroke the pressure of the burned gases inthe cylinder is 2.8 to 3.5 kg/sq. cm (40 to 50 lb.
/sq. in). During the finalstroke, the exhaust valve is opened and the piston moves toward the cylinderhead, driving the exhaust gases out of the combustion chamber and leaving thecylinder ready to repeat the cycle.The efficiency of a modern Otto-cycle engine is limited by a number offactors, including losses by cooling and by friction. In general the efficiencyof such engines is determined by the compression ratio of the engine.
Thecompression ratio (the ratio between the maximum and minimum volumes of thecombustion chamber) is usually about 8 to 1 or 10 to 1 in most modern Otto-cycleengines. Higher compression ratios, up to about 12 to 1, with a resultingincrease of efficiency, are possible with the use of high-octane antiknock fuels.The efficiencies of good modern Otto-cycle engines range between 20 and 25percent (in other words, only this percentage of the heat energy of the fuel istransformed into mechanical energy).
Diesel EnginesTheoretically the diesel cycle differs from the Otto cycle in thatcombustion takes place at constant volume rather than at constant pressure. Mostdiesels are also four-stroke engines, but operate differently than the four-stroke Otto-cycle engines. The first or suction stroke draws air, but no fuel,into the combustion chamber through an intake valve. On the second orcompression stroke the air is compressed to a small fraction of its formervolume and is heated to approximately 440 C (approximately 820 F) by thiscompression. At the end of the compression stroke vaporized fuel is injectedinto the combustion chamber and burns instantly because of the high temperatureof the air in the chamber. Some diesels have auxiliary electrical ignitionsystems to ignite the fuel when the engine starts, and until it warms up.
Thiscombustion drives the piston back on the third or power stroke of the cycle. Thefourth stroke, as in the Otto-cycle engine, is an exhaust stroke.The efficiency of the diesel engine, which is in general governed by thesame factors that control the efficiency of Otto-cycle engines, is inherentlygreater than that of any Otto-cycle engine and in actual engines today isslightly over 40 percent. Diesels are in general slow-speed engines withcrankshaft speeds of 100 to 750 revolutions per minute (rpm) as compared to 2500to 5000 rpm for typical Otto-cycle engines. Some types of diesel, however, havespeeds up to 2000 rpm.
Because diesels use compression ratios of 14 or more to 1,they are generally more heavily built than Otto-cycle engines, but thisdisadvantage is counterbalanced by their greater efficiency and the fact thatthey can be operated on less expensive fuel oils.Two-Stroke EnginesBy suitable design it is possible to operate an Otto-cycle or diesel asa two-stroke or two-cycle engine with a power stroke every other stroke of thepiston instead of once every four strokes. The efficiency of such engines isless than that of four-stroke engines, and therefore the power of a two-strokeengine is always less then half that of a four-stroke engine of comparable size.The general principle of the two-stroke engine is to shorten the periodsin which fuel is introduced to the combustion chamber and in which the spentgases are exhausted to a small fraction of the duration of a stroke instead ofallowing each of these operations to occupy a full stroke. In the simplest typeof two-stroke engine, the poppet valves are replaced by sleeve valves or ports(openings in the cylinder wall that are uncovered by the piston at the end ofits outward travel). In the two-stroke cycle the fuel mixture or air isintroduced through the intake port when the piston is fully withdrawn from thecylinder. The compression stroke follows and the charge is ignited when thepiston reaches the end of this stroke.
The piston then moves outward on thepower stroke, uncovering the exhaust port and permitting the gases to escapefrom the combustion chamber.Rotary EngineWankel EnginesIn the 1950s the German engineer Felix Wankel developed his concept ofan internal-combustion engine of a radically new design, in which the piston andcylinder were replaced by a three-cornered rotor turning in a roughly ovalchamber. The fuel-air mixture is drawn in through an intake port and trappedbetween one face of the turning rotor and the wall of the oval chamber.
Theturning of the rotor compresses the mixture, which is ignited by a spark plug.The exhaust gases are then expelled through an exhaust port through the actionof the turning rotor. The cycle takes place alternately at each face of therotor, giving three power strokes for each turn of the rotor. The Wankelengine’s compact size and consequent lesser weight as compared with the pistonengine gave it increasing value and importance with the rise in gasoline pricesof the 1970s and ’80s. In addition, it offers practically vibration-freeoperation, and its mechanical simplicity provides low manufacturing costs.
Cooling requirement s are low, and its low center of gravity contributes todriving safety.Gas TurbineAlso called as combustion turbine, engine that employs gas flow as theworking medium by which heat energy is transformed into mechanical energy. Gasis produced in the engine by the combustion of certain fuels. Stationary nozzlesdischarge jets of this gas against the blades of a turbine wheel. The impulseforce of the jets causes the shaft to turn.
A simple-cycle gas turbine includesa compressor that pumps compressed air into a combustion chamber. Fuel ingaseous or liquid-spray form is also injected into this chamber, and combustiontakes place there. The combustion products pass from the chamber through thenozzles to the turbine wheel. The spinning wheel drives the compressor and theexternal load, such as an electrical generator.In a turbine or compressor, a row of fixed blades and a correspondingrow of moving blades attached to a rotor is called a stage. Large machinesemploy multistage axial-flow compressors and turbines. In multi-shaftarrangements, the initial turbine stage (or stages) powers the compressor on oneshaft while the later turbine stage (or stages) powers the external load on aseparate shaft.
The efficiency of the gas-turbine cycle is limited by the need forcontinuous operation at high temperatures in the combustion chamber and earlyturbine stages. A small, simple-cycle gas turbine may have a relatively lowthermodynamic efficiency, comparable to a conventional gasoline engine. Advancesin heat-resistant materials, protective coatings, and cooling arrangements havemade possible large units with simple-cycle efficiencies of 34 percent or higher.The efficiency of gas-turbine cycles can be enhanced by the use ofauxiliary equipment such as inter-coolers, regenerators, and reheaters. Thesedevices are expensive, however, and economic considerations usually precludetheir use.In a combined-cycle power plant, the considerable heat remaining in thegas turbine exhaust is directed to a boiler called a heat-recovery steamgenerator.
The heat so recovered is used to raise steam for an associated steamturbine. The combined output is approximately 50 percent greater than that ofthe gas turbine alone. Combined cycles with thermal efficiency of 52 percent andhigher are being put into service. Gas turbines have been applied to thepropulsion of ships and railroad locomotives. A modified form of gas turbine,the turbojet, is used for airplane propulsion. Heavy-duty gas turbines in bothsimple and combined cycles have become important for large-scale generation ofelectricity. Unit ratings in excess of 200 megawatts (MW) are available.
Thecombined-cycle output can exceed 300 MW.The usual fuels used in gas turbines are natural gas and liquids such askerosene and diesel oil. Coal can be used after conversion to gas in a separategasifier.Internal-Combustion Engines and Air PollutionAir pollution from automobile engines ( smog ) was first detected about1942 in Los Angeles, CA.
Smog arises from sunlight-induced photochemicalreactions between nitrogen dioxide and the several hundred hydrocarbons in theatmosphere. Undesirable products of the reactions include ozone, aldehydes, andperoxyacylnitrates ( PAN ). These are highly oxidizing in nature and cause eyeand throat irritation. Visibility-decreasing nitrogen dioxide and aerosols arealso formed.
Five categories of air pollutants and percent contribution from alltransportation source and the highway vehicle subset are show in Table -1.Virtually all of the transportation CO, about half the hydrocarbons, and aboutone-third of the nitrogen oxides come from gasoline engines. Diesel enginesaccount for the particulate.
Emissions from internal-combustion engines include those from blowby,evaporation, and exhaust. These can vary considerably in amount and compositiondepending on engine type, design, and condition, fuel-system type, fuelvolatility, and engine operating point. For an automobile without emissioncontrol it is estimated that of the hydrocarbon emission, 20 to 25 percent arisefrom blowby, 60 percent from the exhaust, and the balance from evaporativelosses primarily from the fuel tank and to a lesser extent from the carburetor.All other non-hydrocarbon emissions emanate from the exhaust.
At least 200 hydrocarbon (HC) compounds have been identified in exhaust.Some such as the olefin compounds react products. These are termed reactivehydrocarbons.
Others such as the paraffin are virtually unreactive.Special DevelopmentsThe Stratified-Charge Engine a modification of the conventional spark-ignition piston engine, the stratified charge engine is designed to reduceemissions without the need for an exhaust-gas recirculation system or catalyticconverter. Its key feature is a dual combustion chamber for each cylinder, witha prechamber that receives a rich fuel-air mixture while the main chamber ischarged with a very lean mixture. The spark ignites the rich mixture that inturn ignites the lean main mixture.
The resulting peak temperature is low enoughto inhibit the formation of nitrogen oxides, and the mean temperature issufficiently high to limit emissions of carbon monoxide and hydrocarbon.Two rather distinct means for accomplishing the stratified chargecondition are under consideration :1. A single combustion chamber with a well-controlled rotating airmotion. This arrangement is illustrated (Fig.6) by the Texaco Combustion Process(TCP), patented in 1949.2.
A prechamber or two-chamber system. This is illustrated by Fig.7,which shows the general arrangement of the Honda Compound-vortex controlled-combustion (CVCC) system.For both systems, very careful development has proved to be necessary toobtain complete combustion of the fuel under the wide range of speed and loadconditions required of an automotive engine.