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INTRODUCTION: -What is Stratified Charge Engine?The stratified charge engine is a type of internal-combustion engine which runs on gasoline. It is very much similar to the Diesel cycle. The name refers to the layering of the charge inside the cylinder. The stratified charge engine is designed to reduce the emissions from the engine cylinder without the use of exhaust gas recirculation systems, which is also known as the EGR or catalytic converters.Stratified charge combustion engines utilize a method of distributing fuel that successively builds layers of fuel in the combustion chamber. The initial charge of fuel is directly injected into a small concentrated area of the combustion chamber where it ignites quickly. As the combustion process continues, it travels across the top of the piston to a lean area of the chamber, where cooler temperatures reduce the formation of harmful NOx emissions. Subsequent additional small injections of fuel can be introduced to propagate the flame front and manage piston knock. This arrangement works well in slow constant speed applications, but has proven difficult to manage across the wide range of speed and load incurred in automotive uses. Examples: Honda has used a stratified charge design in many of its "lean burn" Civic models.PRINCIPLE:- The principle of the stratified charge engine is to deliver a mixture that is sufficiently rich for combustion in the immediate vicinity of the spark plug and in the remainder of the cylinder, a very lean mixture that is so low in fuel that it could not be used in a traditional engine. On an engine with stratified charge, the delivered power is no longer controlled by the quantity of admitted air, but by the quantity of petrol injected, as with a diesel engine. Figure -1 Fresh air + EGR Air + petrol Stratified charge HOW DOES IT WORK?One approach consists in dividing the combustion chamber so as to create a pre-combustion chamber where the spark plug is located. The head of the piston is also modified. It contains a spheroid cavity that imparts a swirling movement to the air contained by the cylinder during compression. As a result, during injection, the fuel is only sprayed in the vicinity of the spark plug. But other strategies are possible. For example, it is also possible to exploit the shape of the admission circuit and use artifices, like “swirl” or “tumble” stages that create turbulent flows at their level. All the subtlety of engine operation in stratified mode occurs at level of injection. This comprises two principal modes: a lean mode, which corresponds to operation at very low engine load, therefore when there is less call on it, and a “normal” mode, when it runs at full charge and delivers maximum power. In the first mode, injection takes place at the end of the compression stroke. Because of the swirl effect that the piston cavity creates, the fuel sprayed by the injector is confined near the spark plug. As there is very high pressure in the cylinder at this moment, the injector spray is also quite concentrated. The “directivity” of the spray encourages even greater concentration of the mixture. A very small quantity of fuel is thus enough to obtain optimum mixture richness in the zone close to the spark plug, whereas the remainder of the cylinder contains only very lean mixture. The stratification of air in the cylinder means that even with partial charge it is also possible to obtain a core of mixture surrounded by layers of air and residual gases which limit the transfer of heat to the cylinder walls. This drop in temperature causes the quantity of air in the cylinder to increase by reducing its dilation, delivering the engine additional power. When idling, this process makes it possible to reduce consumption by almost 40% compared to a traditional engine. And this is not the only gain. Functioning with stratified charge also makes it possible to lower the temperature at which the fuel is sprayed. All this leads to a reduction in fuel consumption which is of course reflected by a reduction of engine exhaust emissions. When engine power is required, injection takes place in normal mode, during the admission phase. This makes it possible to achieve a homogeneous mix, as it is the case with traditional injection. Here, contrary to the previous example, when the injection takes place, the pressure in the cylinder is still low. The spray of fuel from the injector is therefore highly divergent, which encourages a homogeneous mix to form.THEORY:-In a stratified charge engine, the fuel is injected into the cylinder just before ignition. This allows for higher compression ratios without "knock," and leaner air/fuel mixtures than in conventional internal combustion engines. Conventionally, a four-stroke (petrol or gasoline) Otto cycle engine is fuelled by drawing a mixture of air and fuel into the combustion chamber during the intake stroke. This produces a homogeneous charge: a homogeneous mixture of air and fuel, which is ignited by a spark plug at a predetermined moment near the top of the compression stroke.]In a homogeneous charge system, the air/fuel ratio is kept very close to stoichometric. A stoichometric mixture contains the exact amount of air necessary for a complete combustion of the fuel. This gives stable combustion, but places an upper limit on the engine's efficiency: any attempt to improve fuel economy by running a lean mixture with a homogeneous charge results in unstable combustion; this impacts on power and emissions, notably of nitrogen oxides or NOx. If the Otto cycle is abandoned, however, and fuel is injected directly into the combustion-chamber during the compression stroke, the petrol engine is liberated from a number of its limitations. First, a higher mechanical compression ratio (or, with supercharged engines, maximum combustion pressure) may be used for better thermodynamic efficiency. Since fuel is not present in the combustion chamber until virtually the point at which combustion is required to begin, there is no risk of pre-ignition or engine knock. The engine may also run on a much leaner overall air/fuel ratio, using stratified charge. Combustion can be problematic if a lean mixture is present at the spark-plug. However, fueling a petrol engine directly allows more fuel to be directed towards the spark-plug than elsewhere in the combustion-chamber. This results in a stratified charge: one in which the air/fuel ratio is not homogeneous throughout the combustion-chamber, but varies in a controlled (and potentially quite complex) way across the volume of the cylinder. A relatively rich air/fuel mixture is directed to the spark-plug using multi-hole injectors. This mixture is sparked, giving a strong, even and predictable flame-front. This in turn results in high-quality combustion of the much weaker mixture elsewhere in the cylinder. Direct fuelling of petrol engines is rapidly becoming the norm, as it offers considerable advantages over port-fuelling (in which the fuel injectors are placed in the intake ports, giving homogeneous charge), with no real drawbacks. Powerful electronic management systems mean that there is not even a significant cost penalty. With the further impetus of tightening emissions legislation, the motor industry in Europe and North America has now switched completely to direct fuelling for the new petrol engines it is introducing.It is worth comparing contemporary directly-fuelled petrol engines with direct-injection diesels. Petrol can burn faster than diesel fuel, allowing higher maximum engine speeds and thus greater maximum power for sporting engines. Diesel fuel, on the other hand, has a higher energy density, and in combination with higher combustion pressures can deliver very strong torque and high thermodynamic efficiency for more 'normal' road vehicles.HISTORYThe principle of injecting fuel directly into the combustion-chamber at the moment at which combustion is required to start was invented by Rudolf Diesel, but it has been used to good effect in petrol engines for a long time. The Mercedes 300SL 'Gull wing' of 1952 used direct fuelling, though Mercedes-Benz subsequently switched to port fuelling for other models.Honda's CVCC engine, released in the early 1970s models of Civic, then Accord and City later in the decade, is a form of stratified charge engine that had wide market acceptance for considerable time. The CVCC system had conventional inlet and exhaust valves and a third, supplementary, inlet valve that charged an area around the spark plug. The spark plug and CVCC inlet was isolated from the main cylinder by a perforated metal plate. At ignition a series of flame fronts shot into the very lean main charge, through the perforations, ensuring complete ignition. In the Honda City Turbo such engines produced a high power-to-weight ratio at engine speeds of 7,000 rpm and above. Jaguar Cars in the 1980s developed the Jaguar V12 engine, H.E. (so called High Efficiency) version, which fit in the Jaguar XJ12 and Jaguar XJS models and used a stratified charge design called the 'May Fireball' in order to reduce the engine's very heavy fuel consumption.Stratified Charge Engine with Two-Stage Combustion:- Figure-2Two-stage combustion mechanism in twin swirl combustion (1, zone containing pure air; 2, spark plug; 3, turbulizer; and 4, zone containing the fuel-rich mixture). Stratified Charge Engine With Two-Stage Combustion Mechanism Shows 17% Reduction in Fuel Consumption Without Direct Injection. Two-stage combustion mechanism in twin swirl combustion (1, zone containing pure air; 2, spark plug; 3, turbulizer; and 4, zone containing the fuel-rich mixture). A team of researchers from Istanbul Technical University(ITU) in Turkey has presented a 1.6-liter stratified charge gasoline engine featuring a twin swirl combustion chamber operating with a two-stage combustion mechanism and experimentally shown that it can deliver a 17% reduction in fuel consumption with a 7% increase in power compared to a conventional 1.6-liter port-injected engine..The two-stage combustion mechanism was originally proposed by a team comprising researchers from Azerbaijan Technical University (AzTU), Warsaw Technical University (WTU), ITU, and Middle East Technical University (METU). In conventional gasoline engines, every part of the cylinder contains a mixture having an excess air ratio (λ) of approximately 1. Stratified charge engines have frequently stoichometric mixture (λ = 1) only near the spark plug and lean mixture in the cylinder, globally. For the special case of stratified charge engines operating with a two-stage combustion mechanism, there is a lean mixture in the cylinder globally as well; however, there is a fuel-rich mixture in the vicinity of the spark plug. The non homogenous mixture in stratified charge engines is obtained usually with the modification of the piston geometry. The geometry of the intake manifold can also be modified. Because there is a lean mixture in the combustion chamber globally, stratified charge engines have a lower knock tendency than the conventional gasoline engines. Because of this fact, the compression ratio (ε) of a stratified charge engine can be higher than the compression ratio of a conventional gasoline engine; ε ≥ 12 is possible. A higher compression ratio leads to a higher efficiency. The absence of throttle losses in part load operation in combination with the ability to use higher compression ratios leads to lower fuel consumption.The proposed combustion chamber looks like a figure “8” and is separated into two zones. The spark plug mounted part of the combustion chamber contains a fuel-rich mixture with an excess air ratio of 0.6-0.8, while the other part contains pure air. The fuel is injected into the intake manifold and fed into the zone containing the fuel-rich mixture. The intake manifold is designed for the two-stage combustion mechanism, such that it increases the swirl effect and volumetric efficiency. The counter-rotating swirling motion—which occurs during the intake and compression cycles of the engine—does not allow the mixing of the two zones until ignition time. This allows stratification of the air-fuel mixture across the load range. Because the swirl motion occurs with the start of the intake cycle, the air-fuel mixture can be prepared in the intake manifold (outside of cylinders). Therefore, current electronic injection systems or carburetor engines can be used with this method. In other words, special and expensive direct-injection systems are not required, such as in gasoline direct injection (GDI) engines, where the injection of fuel into the cylinder reduces the time available for evaporation and mixing.The two-stage combustion mechanism can also reduce emissions of criteria pollutants. Because the liquid phase of the gasoline does not contact the cold wall of the cylinders, and because the counter-rotating swirling motion reduces the contact of the flame with the piston, the stratified charge engines with the twin swirl combustion chamber produce lower hydrocarbon (HC) emissions. Incomplete combustion products (CO and H2 produced during the combustion of the rich mixture (λ = 0.6-0.8) at the first stage can be burned in the second stage of combustion with the effect of the swirl motion. The lack of oxygen in the rich mixture and low combustion temperature at the first stage of combustion do not allow NOx formation.Direct Petrol Injection.The differences between Petrol and Diesel.It is commonly known that a Diesel engine of the same capacity as its Petrol counterpart is more fuel efficient (approximately 10%). The main reasons why a diesel returns better economy is because of its ability to run very lean Air Fuel Ratios, better thermal efficiency aided by its higher Compression Ratio (CR) and significantly less pumping losses at part load due to the lack of a throttle valve.Diesel engines are not that fussy about the measures of fuel they receive, as long as they get some, they’ll burn it and produce useable power. Petrol on the other hand is far more choosy. If the Air Fuel Ratio (give or take a few ratios) isn’t around the stoichometric value then it really doesn’t want to burn (Stoichometric is the term that identifies the Air Fuel Ratio that offers the most complete burn resulting in the lowest emissions for the hottest flame. For unleaded petrol, it is 14.67:1, which is commonly rounded to 14.7:1. The stoichometric value for other fuels varies with their energy content.) Trying to run a petrol engine any leaner results in partially burnt fuel, unstable combustion and high Hydro Carbon (HC) and Carbon Monoxide (CO) emissions. Getting better economy from a Petrol Engine.Engineers for years have tried to combine the economy of a Diesel engine with the power of a Petrol Engine. There are two main ways of achieving better economy with a petrol engine. The first one is to get the engine to burn very lean mixtures (lean burn engine) and the other is to create a localized stoichometric cloud of mixture at the spark plug (stratified charge engine). The goal of the stratified engine is to run at Wide Open Throttle (WOT) and control the power in much the same as a Diesel by introducing varying amounts of fuel. Under light load conditions it is possible to run AFR’s as high as 60:1.The stratified charge is not a new concept, Ricardo were experimenting with the technology back in 1922. Early stratified engines used traditional carburetors along with a separate mixing chamber to mix the chemically correct AFR mixture which was then introduced into the ‘Clean air’ in the combustion chamber before ignition.Types of GD-i Engines.There are two types of stratified engine, and these differ in the way the air enters the combustion chamber. The swirl method is similar to a Diesel concept in that air rushes into the combustion chamber in an axial motion. This motion centralizes the chemically correct cloud of mixture towards the centre of the chamber in the vicinity of the spark plug. The other method uses what is called reverse tumble. The air entering the combustion chamber from the intake valve is deflected in a circular motion in the opposite plane to the swirl motion. The air hits the cylinder wall adjacent to the intake valve and then down towards the piston. These engines use special ‘Ski jump’ shaped pistons to guide the air and fuel towards the spark plug.Reverse tumble is probably the most suitable stratified charge delivery system as this has already been successfully demonstrated on Mitsubishi’s GD-i range of vehicles.Limitations of GD-i.Even using modern injection technology, it is still not possible to run in stratified mode throughout the rev and load range of the engine. Thus, it is only possible to run in stratified mode at part load. The engine switches over to homogenous mode (early injection) at high speed conditions because there is insufficient time to inject the fuel late into the compression stroke and get the fuel to adequately mix into a cloud of combustible mixture. Injecting the fuel too early when the piston is near Bottom Dead Centre (BDC) results in the fuel missing the ‘ski jump’ on the piston. High load conditions are not possible in stratified mode either as injecting such a large quantity of fuel will result in an ultra rich cloud of mixture at the spark plug that wont burn. Attempting to continue Injecting fuel very late into the compression stroke results in the cloud of mixture hitting the piston when it is near to Top Dead Centre (TDC) that results in the cloud of mixture overshooting the spark plug. Petrol engines also have an optimum timing window when the ignition should ignite the mixture. Too early and the engine will produce too many Oxides of Nitrogen (Nox) and advanced even earlier will begin to ‘Knock’, too late and you only get partial combustion and very high exhaust temperatures. The perfect ignition timing is the Minimum advance for Best Torque (MBT).Stratified charge engine make the timing of the ignition even more critical as the AFR at the spark plug changes as the cloud of chemically correct mixture passes through it. Careful consideration has to be given to the shape of the ramp on the piston as well as the injection angle, pressure and timing in order to coincide with optimum ignition timing. Sometimes throttling is needed at certain engine speeds in order to create the necessary air velocity to adequately mix the air and fuel. OTTO CYCLE Figure-3 P-V DiagramFigure-4T-S diagramThe idealized diagrams of a four-stroke Otto cycle Both diagrams?:the intake(A) stroke is performed by an isobaric expansion, followed by an adiabatic compression(B) stroke. Through the combustion of fuel, heat is added in an isochoric process, followed by an adiabatic expansion process, characterizing the power(C) stroke. The cycle is closed by the exhaust (D) stroke, characterized by isochoric cooling and isobaric compression processes.An Otto cycle is an idealized thermodynamic cycle which describes the functioning of a typical reciprocating piston engine. This thermodynamic cycle is most commonly found in automobiles. The Otto cycle is constructed out of:TOP and BOTTOM of the loop: a pair of quasi-parallel adiabatic processes LEFT and RIGHT sides of the loop: a pair of parallel isochoric processes The adiabatic processes are impermeable to heat: heat flows into the loop through the left pressurizing process and some of it flows back out through the right depressurizing process, and the heat which remains does the work.The processes are described by:Process 1-2 is an isentropic compression of the air as the piston moves from bottom dead center to top dead center. Process 2-3 is a constant-volume heat transfer to the air from an external source while the piston is at top dead center. This process is intended to represent the ignition of the fuel-air mixture and the subsequent rapid burning. Process 3-4 is an isentropic expansion (power stroke). Process 4-1 completes the cycle by a constant-volume process in which heat is rejected from the air while the piston is a bottom dead center. The Otto cycle consists of adiabatic compression, heat addition at constant volume, adiabatic expansion, and rejection of heat at constant volume. In the case of a four-stroke Otto cycle, technically there are two additional processes: one for the exhaust of waste heat and combustion products (by isobaric compression), and one for the intake of cool oxygen-rich air (by isobaric expansion); however, these are often omitted in a simplified analysis. Even though these two processes are critical to the functioning of a real engine, wherein the details of heat transfer and combustion chemistry are relevant, for the simplified analysis of the thermodynamic cycle, it is simpler and more convenient to assume that all of the waste-heat is removed during a single volume change.HistoryThe four-stroke engine was first patented by Alphonse Beau de Rochas in 1861. Before, in about 1854–57, two Italians (Eugenio Barsanti and Felice Matteucci) invented an engine that was rumored to be very similar, but the patent was lost."The request bears the no. 700 of Volume VII of the Patent Office of the Reign of Piedmont. We do not have the text of the patent request, only a photo of the table which contains a drawing of the engine. We do not even know if it was a new patent or an extension of the patent granted three days earlier, on December 30, 1857, at Turin."The first person to build a working four stroke engine, a stationary engine using a coal gas-air mixture for fuel (a gas engine), was German engineer Nicolas Otto. This is why the four-stroke principle today is commonly known as the Otto cycle and four-stroke engines using spark plugs often are called Otto engines.ProcessesProcess 1-2 (B on diagrams): Piston moves from crank end (bottom dead center BDC) to cover end (top dead center TDC) and an ideal gas with initial state 1 is compressed isentropically to state point 2, through compression ratio (V1 / V2). Mechanically this is the adiabatic compression of the air/fuel mixture in the cylinder, also known as the compression stroke. Generally the compression ratio is around 9-10:1 (V1:V2) for a typical automobile. Process 2-3 (C on diagrams):The piston is momentarily at rest at BDC and heat is added to the working fluid at constant volume from an external heat source which is brought into contact with the cylinder head. The pressure rises and the ratio (P3 / P4) is called the "explosion ratio". At this instant the air/fuel mixture is compressed at the top of the compression stroke with the volume essentially held constant, also know as ignition phase.Process 3-4 (D on diagrams):The increased high pressure exerts a greater amount of force on the piston and pushes it towards the BDC. Expansion of working fluid takes place isentropically and work is done by the system. The volume ratio (V4 / V3) is called "isentropic expansion ratio". Mechanically this is the adiabatic expansion of the hot gaseous mixture in the cylinder head, also known as expansion (power) stroke.Process 4-1 (A on diagrams)The piston is momentarily at rest at BDC and heat is rejected to the external sink by bringing it in contact with the cylinder head. The process is so controlled that ultimately the working fluid comes to its initial state 1 and the cycle is completed. Many petrol and gas engines work on a cycle which is a slight modification of Otto cycle. This cycle is called "constant volume cycle" because the heat is supplied to air at constant volume.Exhaust and Intake StrokesExhaust Stroke-Ejection of the gaseous mixture via an exhaust valve through the cylinder head. Induction Stroke-Intake of the next air charge into the cylinder. The volume of the exhaust gasses is the same as the air charge. Cycle AnalysisProcesses 1-2 and 3-4 do work on the system but no heat transfer occurs during adiabatic expansion and compression. Processes 2-3 and 4-1 are isochoric therefore heat transfer occurs but no work is done. No work is done during a isochoric (constant volume) because work requires movement; when the piston volume does not change no shaft work is produced by the system. Four different equations can be derived by neglecting kinetic and potential energy and considering the first law of thermodynamics (energy conservation). Assuming these conditions the first law is rewritten as.ΔE = ΔU = Qin ? WoutEFFICIENCY THE STRATIFIED CHARGE COMBUSTION CONCEPTAs development of the engine proceeded at Curtiss- Wright, it was felt that if the engine could operate unthrottled as does the diesel engine, and if the fuel could be injected directly into the compressed air charge at or near 'TDC', rather than introduced as a fuel air mixture, engine fuel consumption could be improved and emissions decreased. The improved fuel consumption would come from a combination of the unthrottled intake system, and the introduction of fuel only as required rather than trying to 'fill the entire combustion chamber' with an ignitable fuel/air mixture. Furthermore, if the fuel which was introduced could be ignited by an ignition source, rather than relying on the self ignition characteristics of the fuel (as in diesel-compression ignition) then the engine would have a wider tolerance to fuel characteristics. Successful development of such a combustion system would potentially enable the engine to operate as a true "multi-fuel" engine. Early efforts at Curtiss-Wright to develop the stratified combustion concept involved the use of a multi hole nozzle with one of its sprays directed toward the spark plug. This engine, as did most efforts to stratify reciprocating "diesel" engines resulted in engines that ran well under rather narrow operating conditions of speed and load. (The primary difficulty was one of maintaining the correct fuel/air ratio in the vicinity of the spark plug. Under specific operating conditions, the 'fuzz' from the injector spray would be 'just right', and the engine ran fine, having the smoothness, and fuel economy desired. Once the spray penetration characteristics changed as the amount of fuel injected either increased or decreased, the optimum conditions for ignition of the fuel spray that existed by the spark plug changed, resulting in poorer combustion characteristics.) Various schemes of the single nozzle stratified charged combustion concept were evaluated with varying degrees of success. As the Curtiss-Wright engineers worked with the combustion system, they developed the two nozzle stratified charged concept. This concept involves a single orifice 'pilot' nozzle which essentially injects a constant quantity of fuel optimized for producing an ignitable mixture in the vicinity of the spark plug. A second nozzle which incorporates multiple orifices, serves as the main fuel source for the combu inject fuel as it is required for controlled combustion. The quantity of fuel injected is determined by the load requirements on the engine. This dual nozzle stratified combustion system has lived up to its expectations. It has successfully demonstrated its ability to run on gasoline, methanol, diesel fuel, and jet fuel without any engine adjustments. Further details of the performance results obtained during the development program on the Stratified Charged Combustion System. The turbocharged rotary engine uses a rotor with three combustion laces. These laces (which are equivalent to pistons in a reciprocating engine) provide for a power impulse (stroke) during each crank revolution. The rotor Ills closely around the crank eccentric, but turns al 'I, of Its speed. Producing rotary motion directly eliminates all those parts needed In a reciprocating engine to convert the up and down motion to rotary motion. Engine knockingKnocking (also called knock, detonation, spark knock, pinging or pinking) in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front. The fuel-air charge is meant to be ignited by the spark plug only, and at a precise time in the piston's stroke cycle. The peak of the combustion process no longer occurs at the optimum moment for the four-stroke cycle. The shock wave creates the characteristic metallic "pinging" sound, and cylinder pressure increases dramatically. Effects of engine knocking range from inconsequential to completely destructive. It should not be confused with pre-ignition (also discussed in this article). Normal combustion Under ideal conditions the common internal combustion engine burns the fuel/air mixture in the cylinder in an orderly and controlled fashion. The combustion is started by the spark plug some 10 to 40 crankshaft degrees prior to top dead center (TDC), depending on many factors including engine speed and load. This ignition advance allows time for the combustion process to develop peak pressure at the ideal time for maximum recovery of work from the expanding gases. The spark across the spark plug's electrodes forms a small kernel of flame approximately the size of the spark plug gap. As it grows in size its heat output increases allowing it to grow at an accelerating rate, expanding rapidly through the combustion chamber. This growth is due to the travel of the flame front through the combustible fuel air mix itself and due to turbulence rapidly stretching the burning zone into a complex of fingers of burning gas that have a much greater surface area than a simple spherical ball of flame would have. In normal combustion, this flame front moves throughout the fuel/air mixture at a rate characteristic for the fuel/air mixture. Pressure rises smoothly to a peak, as nearly all the available fuel is consumed, then pressure falls as the piston descends. Maximum cylinder pressure is achieved a few crankshaft degrees after the piston passes TDC, so that the increasing pressure can give the piston a hard push when its speed and mechanical advantage on the crank shaft gives the best recovery of force from the expanding gases. Abnormal combustionWhen unburned fuel/air mixture beyond the boundary of the flame front is subjected to a combination of heat and pressure for certain duration (beyond the delay period of the fuel used), detonation may occur. Detonation is characterized by an instantaneous, explosive ignition of at least one pocket of fuel/air mixture outside of the flame front. A local shockwave is created around each pocket and the cylinder pressure may rise sharply beyond its design limits. If detonation is allowed to persist under extreme conditions or over many engine cycles, engine parts can be damaged or destroyed. The simplest deleterious effects are typically particle wear caused by moderate knocking, which may further ensue through the engine's oil system and cause wear on other parts before being trapped by the oil filter. Severe knocking can lead to catastrophic failure in the form of physical holes punched through the piston or head (i.e., rupture of the combustion chamber), either of which depressurizes the affected cylinder andintroduces large metal fragments, fuel, and combustion products into the oil system. Hypereutectic pistons are known to break easily from such shock waves. Detonation can be prevented by any or all of the following techniques: the use of a fuel with high octane rating, which increases the combustion temperature of the fuel and reduces the proclivity to detonate; enriching the fuel/air ratio, which adds extra fuel to the mixture and increases the cooling effect when the fuel vaporizes in the cylinder; reducing peak cylinder pressure by increasing the engine revolutions (e.g., shifting to a lower gear, there is also evidence that knock occurs more easily at low rpm than high regardless of other factors); increasing mixture turbulence or swirl by increasing engine revolutions or by increasing "squish" turbulence from the combustion chamber design; decreasing the manifold pressure by reducing the throttle opening; or reducing the load on the engine. Because pressure and temperature are strongly linked, knock can also be attenuated by controlling peak combustion chamber temperatures by compression ratio reduction, exhaust gas recirculation, appropriate calibration of the engine's ignition timing schedule, and careful design of the engine's combustion chambers and cooling system as well as controlling the initial air intake temperature. Knock is less common in cold climates. As an aftermarket solution, a water injection system can be employed to reduce combustion chamber peak temperatures and thus suppress detonation. Interestingly the addition of certain materials such as lead and thallium will suppress detonation extremely well when certain fuels are used. The addition of tetra-ethyl lead (TEL), a soluble salt added to gasoline was common until it was discontinued for reasons of toxic pollution. Lead dust added to the intake charge will also reduce knock with various hydrocarbon fuels. Manganese compounds are also used to reduce knock with petrol fuel. Steam (water vapor) will suppress knock even though no added cooling is supplied. Certain chemical changes must first occur for knock to happen, hence fuels with certain structures tend to knock easier than others. Branched chain paraffin tend to resist knock while straight chain paraffin knock easily. It has been theorized that lead, steam, and the like interfere with some of the various oxidative changes that occur during combustion and hence the reduction in knock. Turbulence as stated has a very important effect on knock. Engines with good turbulence tend to knock less than engines with poor turbulence. Turbulence occurs not only while the engine is inhaling but also when the mixture is compressed and burned. During compression/expansion "squish" turbulence is used to violently mix the air/fuel together as it is ignited and burned which reduces knock greatly by speeding up burning and cooling the unburnt mixture. One excellent example of this is all modern side valve or flathead engines. A considerable portion of the head space is made to come in close proximity of the piston crown, making for much turbulence near T.D.C. In the early days of side valve heads this was not done and a much lower compression ratio had to be used for any given fuel. Also such engines were sensitive to ignition advance and had less powerKnocking is more or less unavoidable in diesel engines, where fuel is injected into highly compressed air towards the end of the compression stroke. There is a short lag between the fuel being injected and combustion starting. By this time there is already a quantity of fuel in the combustion chamber which will ignite first in areas of greater oxygen density prior to the combustion of the complete charge. This sudden increase in pressure and temperature causes the distinctive diesel 'knock' or 'clatter', some of which must be allowed for in the engine design. Careful design of the injector pump, fuel injector, combustion chamber, piston crown and cylinder head can reduce knocking greatly, and modern engines using electronic common rail injection have very low levels of knock. Engines using indirect injection generally have lower levels of knock than direct injection engine, due to the greater dispersal of oxygen in the combustion chamber and lower injection pressures providing a more complete mixing of fuel and air. Diesels actually don't suffer exactly the same "knock" as gas engines since the cause is known to be only the very fast rate of pressure rise, not unstable combustion. Diesel fuels are actually very prone to knock in gas engines but in the diesel engine there is no time for knock to occur because the fuel is only oxidized during the expansion cycle. In the gas engine the fuel is slowly oxidizing all the while it is being compressed before the spark. This allows for changes to occur in the structure/makeup of the molecules before the very critical period of high temp/pressure. An unconventional engine that makes use of detonation to improve efficiency and decrease pollutants is the Bourke engine.Pre-ignitionPre-ignition (or preignition) in a spark-ignition engine is a technically different phenomenon from engine knocking, and describes the event wherein the air/fuel mixture in the cylinder ignites before the spark plug fires. Pre-ignition is initiated by an ignition source other than the spark, such as hot spots in the combustion chamber, a spark plug that runs too hot for the application, or carbonaceous deposits in the combustion chamber heated to incandescence by previous engine combustion events.The phenomenon is also referred to as after-run, or run-on when it causes the engine to carry on running after the ignition is shut off, or sometimes dieseling. This effect is more readily achieved on carbureted gasoline engines, because the fuel supply to the carburetor is typically regulated by a passive mechanical float valve and fuel delivery can feasibly continue until fuel line pressure has been relieved, provided the fuel can be somehow drawn past the throttle plate. The occurrence is rare in modern engines with throttle-body or electronic fuel injection, because the injectors will not be permitted to continue delivering fuel after the engine is shut off, and any occurrence may indicate the presence of a leaking (failed) injector. In the case of highly supercharged or high compression multi-cylinder engines particularly ones that use methanol (or other fuels prone to preignition) preignition can quickly melt or burn pistons since the power generated by other still functioning pistons will force the overheated ones along no matter how early the mix preignites. Many engines have suffered such failure where improper fuel delivery is present. Often one injector may clog while the others carry on normally allowing mild detonation in one cylinder that leads to serious detonation, then preignition. The challenges associated with pre-ignition have increased in recent years with the development of highly supercharged and "down speeded" spark ignition engines. The reduced engine speeds allow more time for auto ignition chemistry to complete thus promoting the possibility of pre-ignition and so called "mega-knock". Under these circumstances, there is still significant debate as to the sources of the pre-ignition event. Preignition and engine knock both sharply increase combustion chamber temperatures. Consequently, both effect increases the likelihood of the other effect occurring, and both can produce similar effects from the operator's perspective, such as rough engine operation or loss of performance due to operational intervention by a powertrain-management computer. For reasons like these, a person not familiarized with the distinction might describe one by the name of the other. Given proper combustion chamber design, preignition can generally be eliminated by proper spark plug selection, proper fuel/air mixture adjustment, and periodic cleaning of the combustion chambers. Causes of pre-ignitionCauses of pre-ignition include the following: Carbon deposits form a heat barrier and can be a contributing factor to preignition. Other causes include: An overheated spark plug (too hot a heat range for the application). Glowing carbon deposits on a hot exhaust valve (which may mean the valve is running too hot because of poor seating, a weak valve spring or insufficient valve lash). A sharp edge in the combustion chamber or on top of a piston (rounding sharp edges with a grinder can eliminate this cause). Sharp edges on valves that were reground improperly (not enough margin left on the edges). A lean fuel mixture. Low coolant level, slipping fan clutch, inoperative electric cooling fan or other cooling system problem that causes the engine to run hotter than normal. Auto-ignition of engine oil droplets. KNOCK DETECTIONDue to the large variation in fuel quality, a large number of engines now contain mechanisms to detect knocking and adjust timing or boost pressure accordingly in order to offer improved performance on high octane fuels while reducing the risk of engine damage caused by knock while running on low octane fuels.An early example of this is in turbo charged Saab H engines, where a system called Automatic Performance Control was used to reduce boost pressure if it caused the engine to knock. ADVANTAGES OF STRATIFIED CHARGE ENGINE:-1. Compact, light weight design &good fuel economy.2. Good part load efficiency.3. Exhibit multifuel capability.4. The rich mixture near spark-plug &lean mixture near the piston surface provides cushing to the exploit combustion.5. Resist the knocking & provides smooth resulting in smooth & quite engine operation over the entire speed & load range.6. Low level of exhaust emissions, Nox is reduced considerably.7. Usually no starting problem.8. Can be manufactured by the existing technology.DISADVATAGES;-1. for a given engine size, charge charge stratification results in reduced.2. These engine create high noise level at low load conditions.3. More complex design to supply rich & lean mixture & quantity is varied with load on the engine.4. Higher weight than of a conventional engine.5. Unthrotlled stratified charge emits high percentage of HC due to either incomplete combustion of lean charge or occasional misfire of the charge at low load conditions.6. Reliability is yet to be well established.7. Higher manufacturing cost. REFERENCES:Jack Erjavec (2005). Automotive technology: a systems approach. Cengage Learning. p.?630. ISBN?1401848311. .? H.N. Gupta (2006). Fundamentals of Internal Combustion Engines. PHI Learning. pp.?169–173. ISBN?812032854X. .? Daniel Hall (2007). Automotive Engineering. Global Media. p.?32. ISBN?8190457500. .? Barry Hollenbeck (2004). Automotive fuels & emissions. Cengage Learning. p.?165 .? "Advanced simulation technologies". cmcl innovationsAuto mobile engineering (R.K. RAJPUT) ................
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