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TE Unit-2 Spectrum

Thermal engineering

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Jayanth Lolla
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42 views28 pages

TE Unit-2 Spectrum

Thermal engineering

Uploaded by

Jayanth Lolla
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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SS THERMAL ENGINEERING-1 (JNTU-HYDgp PART-B NS ESSAY QUESTIONS WITH SOLUTIO NE: 2.1 COMBUSTION IN SI ENG! a 2.4.4 Normal Combustion and Abnormal Combustion In SI Engines ~ Importance on [Model Papert, 4(a) (Ju Normal Combustion Tn S.1 engines, the air ited by means of high intensity spark produced across the electrods gp ‘emperature of the flame and density of the mixture, which is surrounded by the flame At first, «small hollow nucleus of flame in the form of bubble is formed and expands steadily with inerense ig, : She Nhole mass is occupied. But in actual engi s, the mixture is in highly turbulent condition, by aking the flame fgets ste propagation of fame Neil ‘area for radiation of heat Hence, the propagat te vel amet bu doesnt have moch effect on he direction gf ga According to Ricardo, the combustion is assumed to be consisting of two phases. They are, 1. Preparation phase or ignition lag Actual burning phase front thereby presenting a greater st degree of turbulence governs the rate of t Premuei?) AB-Igntien lg BE: Actin! burning phase Crank angie) Figure: Stages of Combustion of SI Engine - ss Pu ¢ growth and development of BP plating ‘nucleus and it depends upon the ta u ‘Sure, proportion of éxhatst gases and Of the teinpetature coefficient of the fuel. It ‘by the curve 48 as shown in figure, During Tali stage-the ‘increased. Therefore, ignition] ‘8s the time lag between the first ignition of fet &hd the ic ‘actual burning phase. . The time interval between 4 and B or the period of igaltion lag 18 very small and lies between 0 0.002 seconds. sor ar mn 2 Actual Burning Phase * ta oh F PE eiany terete ce ave i Hoe fhpeari noc. tis represented by point C represents the attainment pressure’ cORTHLEIGN Of flame travel. It should not be m the point C represents the liberation of heat. Bat, after the" ‘fime, some: hs head burs due to chemical reassociation and is formed outside the cylinder and the combustion takes place inside the cylinder then that mish If the mix , In this mixture, the flame-is spread over the air-fuel mixture which uniformly dst is created in the combustion chamber the spark plug gives up a spark which is spread through out tJ pressure a zone of the flame front separating the burned and unbumed gases move with a velocity cal as a flame. A narrow ‘of the flamie is 40 em/see when the equivalent ratio @ (actual air-fuel ratio to st = The normal be The preferable range of equivatent ratio is 1.1 t0 1.2 for proper combustion of the ratio) is will be oui the flame will be exting is lowered et, 2, combustion in SI and Cl Engines nit? Y —geneous Mixture 2.7 12-50 the temperature in combustion eo ake Place in overall ean Inixture, For rich mixtures, combustion takes place ‘Therefore, the process of combustion in LCengines take erogenous actthe at and fel that depends onthe C engines take place either with the use of homogeneous mixture or heterogen0 type of engine used. Fez. what respect Working Of ah actual engine jer from theoretical cycle. yswer ‘npriuMay-12, Sot-4, 2(0) A theoretical p-O diagram (pressure (p) - crank of the homogeneous mixture of the fuel and ai ‘pressure at constant volume is not possible, The a ce S.1 engine, the angle (0)) is show in figure (1). In an ideal four-strok ‘an actual engine ris raised by compressing it at a constant volume. For tual p-0 diagram is shown in igure (2) 20 ei Pressure (p) as Pressure (@) —> ° 180 fy 0A a0. 2080, 0A 60 8 ‘Crank angle (6) —> Crank angk (0) —> Figure (1): Theoretical p-0 Diagram of S.1 Engine Figure (2 Actual p-0 Diagram of S.1 Engine Sir Ricardo, father of engine research describes three stages of combustion process in S.1 engines. The pressure variation. ‘shorn on ap-0 diagram for practical actual engine as in figure (2). The points 1, 2, 3 refer to asa spark passed, starting point of ‘esr rise and maximum pressure attained and the process 1-2 is referred as stage-I, 2-3 referred as stage-II and 3-4 is referred ssge I. i Stages the ignition lag phase where the flame grows and is developed. This is a chemical process and depends on both aperture and pressure, nature of fuel used for combustion, amount of exhaust gases remains and also depends on temperature tthe rate of reaction. ‘age-II isa physical phase, the pressure increases and the process of combustion departs from that of the compression sunfered by point 2, called the motoring curve. Movement of velocity is practically constant inthis stage (as piston is near the ‘dead centre), Only a small amount of burning mixture comes into contact with the wall ofthe cylinder, so less amount of heat ‘asi takes place from the wall ofthe cylinder. The rate of pressure rise is drectly proportional tothe rate of heat released. Saget is at a point, where the pressure is at a maximum value (ie. at point 2). Piston moves from top dead centre to. tom dead centre lame velocity decreases at this stage. Also there is no pressure rise due to low flame velocity and the rate of ) Temperature and pressure (©) Proportion of the exhaust gas (‘Temperature coefficient of the fuel i: burning ‘ gine combustion? Explain the influence of diferent operating parameters : | sat ENGINEERING! INTU-HYDE 2.8 THER! “ vebair Ratio 1) nds on the following. vir cage in fuel-ai rat he The duration of ignition lag depe inet rated changes which effects the 1. Fuel 2. SaMintpre ay peed, Use of Ie van mixtures reduce era fan Aa hand ch mint oth veh main | ee an ame velo Pe OWN in hon the rate of burning. From figure, ini i stoichiometric value gives A - gives incomplete comm has the sm than stoichiometric ratio. 3, Initial temperature As the ini ignition lag reduces. This ean be achieved by increasing the compression ratio, 4. Turbulen ‘Turbulence doesn’t have much affect on ignitio =) Dtbulence doesn’t have much affect on ignition ng. 4 nich £ | \ influence the flame speed in an S.1. engine, Q29. With the help of graph, explain the factors [Model Paper-Hl, Q4 | May/June-19, (R16), 24(0)) } ure ratio, which gives the maximum temperature ure strengtl that 10% of rich fuel from tat 10% flame speed al temperature and pressure are increased, OR One Explain the various factors that influence the Mixture strength . flame speed. Figure: Effect of Mixture Strength on Rate of Busing panes,» ay-16,(819).088) | (@) Turbulence Factor Inte eee: Turbulence of the mixture has a profound i Inn SI engine, the air-fuel mixture is compressed iP | 4, name speed. With an inerease in surbulenes te the engine cylinder to high temperature and pressure. T0 stat | increases, This is du to the fact that, tule oe Higttabenaieiabach IF reguie. This Hine heat transfer and intermingling of the burning and ides necessary t of jxture, resulting 3 ng Ot Oe ere coabntgaealattnt This perk feiicd just efore | Particles at the aie eee ae — > the piston reaches the TDC position during the compression contact, At less turbulence, the Bote Py: sabe Fete taetrrantinlich, then the tame | tUrenienee of the charge ean be increased by ie thes and revilts in a high combustion rate resulting in} °F combustion chamber i. piston crown and ei Jose heat losses. Thus, that the combustion i S.1 engine depends admitting the mixture through a narrow section “the flame produced by the spark plug for initiating the | © ‘The turbulence can also be increased by ol reaction. The flame provided should be atthe right piston speed. Apart from this, excessive turbulence so as to start the combustion reaction | ‘© ‘engine vibrations due to rapid rise in pressure, Mifiniiience the flame, | engine running Soiey, in compression ratio, the temperature and of the working mixture increaSes and the clearance _which results in exhaust of greater portion rap gases. Due to high temperature and pressure, the Sa esembustion speed up. The reduced clearance Sete density during burning Thisazan a ature, reduces the total ean Tali inreases the Intake temperature =. turbulence Figuro: Effoct of Turbulence and Er f (Intake Temperature and Pressure ‘The flame speed is directly proport temperature and pressure. With an increase in in and pressure, the flame speed increases. High’ ‘and pressure increases the charge density, thereb| heat transfer rate and consequently the flame QB on the TITLE COVER before you guit-2_ Combustion in SI and C1 Engings Engine speed — ‘nd pressure ftfect of Intake Temperature and Pressure on Flame Speed Engine Speed wit increase in engine speed the turbulence of the gue increases resulting in increase in flame speed linear! engine speed. Doubling the engine speed results in reducing Jefame traverse time through the combustion space nearly to thi: But the erank degrees required for the flame propagation weains same at all speeds of rotation. 9 Engine Load ‘The cycle pressure itfcreases with increases in engine ipa and hence increases the flame speed. @ Flame speed > |. ras ‘Engine bad = —> Figure: Effect of Engine Load on Flame Speed © Engine Size For similar engines running at same piston speed, the layer engine have less r.p.m and the smaller engine have a izzerp.m. Due to this, the inlet velocity, degree of turbulence ‘8é flame speed are nearly same irrespective of the size. But ‘fame travel in smaller engines is small and that the larger ‘gies is large. Thus, by doubling the size of the engine, the ‘se fr flame propagation increases. But for larger engines with ‘nailer :p.m., the time for flame, ‘in terms of crank geremains same as that of smafler engine, Ifthe engines are ‘il, the time required for flame propagation will be same imapective ofthe size. 2.9 ‘| ‘30. Explain flame front propagation with suitable a bs sketch, & Anawer i The rates which advance the normal flame front Propagation are of two types, They are 1. Reaction rate 2. Transposition rate. The reaction rate is mainly due to the chem combination. process and the transposition rate is due to the physical movement ofthe flame front with respect to the cylinder walls, jeal Distance of flame travel across the chamber: s o 2 40. 6 8% 100 ‘Time of flame travel across the chamber (%) Figure: Details of Flame Travel ‘Area Lin the figure shows the slow rate of movement of the flame front in the first stage. This movement of flame front is slow due to transposition rate and low turbulence. As the mass of charge burned at the start is very small, the transposition of flame front is very little. Due to the location of spark plug in the quiescent zone, the reaction rate and flame gets speed. ‘Due to the lack of turbulence in the zone. The flame front after Teaying the quiescent zone enters into the turbulent area (area 1) consisting of large mass of mixture, In this zone, the flame front propagates more rapidly at constant rate, ‘At the end of the flame travel, the volume of unburned charge is very less resulting in decreasing the transposition rate nearly negligible, Due to this, the flame speed decreases as well as the reaction rate is low as shown in area IL. 31. Discuss the effect of turbulence and compression ratio on the combustion characteristics in S.1 ‘engine. Answer May-18, (R13), Oday Effect of Turbulence 7 : ‘Turbulence is very helpful in S.1 engines combustion » because of the following reasons, aye 1, Turbulence increases the flame speed by accel chemical reaction by intimate mixing of fuel and ‘The tendency of knocking is also reduced. is created by admission of fuel-air mixture ‘comparatively narrow sections of intake pipe t i io il a Figure: tect of Engin Sie on Fame Speed ‘openings. sa effective considered as y 6 commer = er is general pis, alcoho ints commen hy ‘Speed due to turbu aa Wate yn. Because © tions of water. To. and hence the ignition ye proporti ci ae eee nae consists 50m hen considered as preignition ge of abnormal combustion is mini hecylinder | atways consists ofr Tusbulence incr SHE rae oe ai | cana a the phenomena of a sual thereby taking away the heat from the sp Explain wi tod ontaast valve oe Engine? fect of Compr - . : 3 ne Fe Ricoto rt, $e | amewer + piaeties eee combus fot talh onesie nahn ;. | Detonation oF hows detonation or kno ton Potion delay period esting ig Increasing the knocking tendency ai Toreduce the knocking ine A t0-ignition of, ® Figure aused due to the au commesionntoshadtedeceae: | Fnws hor hay -cur, the last or auto-ignition to oc © before the fame. For autor =a el fon of the charge must have a certain critical te ibang For remainin, ly expi Sort is temperature for some period the stage of combustion is Si | Posten t) eemain at its sr ofthe normal ae Sngines elaborating the flame front propagation. | 7 compression of arnleg Wiigtice era the temper Answer : May-17, (R18), 4) | the expansion of sharge. If this rise in temperature, ensity ofthe end charg : For answer r fer Unit-1 anc and density of re and the end gas Temains a ieee Canin rela | ett we = ‘al Combustion, Pre- i (ignition delay), the Ponting Saree Ree 933. Explain the Phenomenon of pre-ignition. How spontaneous ignition raises the pee er Th Pre-ignition leads to detonation and vice versa? pressure differential created on the cylinder wale relent Pre-ignition be detected? What are | vere pressure wave which strikes Pre-ignition inhibitors? in vibrations. This leads to a pitched metalic pricking Answer : called as knocking or detonation. ; Pre-ignition AD If the ignition of charge occurs before the Passage of e pearty t's termed as Pre-ipnition This Iainly due to over | +// \ Rested spovin sparkplug. exhane vans ot carbon deposits on | (a, the cylinder walls, The above factors acts as ignition source ]} Same as regular spark. nee i ‘These parts ignites the fresh. ‘combustible charge adjacent B to it and i i | | f | Peak presse cain the yc, Ifthe Bit, detonation may lead to over hans rie ane ne tihin | the unburres {um causes pre-ignition of charge. From the above Aiseussion, | period, then pre-ignition leads to detonation and vice-versa. : Pre-ignition reduces the net wotk of the process by autovignition and acs DCE roe increasing the time loss. It increases the heat transfer ot (qual to delay period) certain chemin ea pe i | vera ps Butin Gee mecee ce wns Chances of auto ignites thereby detonation ‘ral e-eylinder engines. But i ie depet TIN thove diocne, cet ns oe ‘one or more cylinder causes the other cylinders is dependent on the Teeuss 'on, itis noticed that the: aig fal pncby sinew ee results to pr Yr not occur if se sf OPE esto a lange exe Det ich is experie i wid ne arora reach its aut in dragging of piston, which is experiencing pe ae the fan v8 08 ieiton leet anagem ‘ \ ne front Bateson se ese N ta 8 fenerally used to | in oraee "Eton oy ‘There are it the unbumed charge 1 oid detonation. aks igh ition, if the en Hon, the fuel should have ignition One so shutoff the place, Second Sane Tow aint longer ignition delay, rat eae a ow oY and the ignition delay of the fuel 8 fires continuously, the the preignition i the sudden Toss of | enough Allow 4 800- ignition, before he pare HEM inca day mechanical malineionng ofthe engine. | gut 8, ton . before the ame ithout any mect power wit Will be a possibility of detonation ok for the SIA GROUP Loco QB onthe TITLE Gi i Look fo ‘OVER ynit-2_ Combustion in SI and Cl Engine. 3s Discuss the desirable char 036. i combustion chamber for ai ‘acteristics of good nSlengine. [Model Pap. 4a) | Nov/Dec. ec.-18, (R16) Qa(a) & “an How can be the possibilit reduced at the design stage in Si seen) Oe ! es. answer t The design of combustion ch n chamber has a profoun roparties. This design mainly involves the shape of combustion Framber, spark plug location and shape, siz god exhaust valVeS. Pe, size and location inlet ‘Thus, for a good combustion cham ame front travel should be minimum, rhs aiden thine of tune font travel to reach the end gas. Effective designing of ‘amombstion chamber results in adequate turbulence of the inixture thereby increasing the speed of the flame front travel ‘his still reduces the knocking tendency of the engine. Therefore, in order to avoid the knocking tendency the {ollowing design characteristics of the combustion chamber are ecessary, Design Characteristics of Combustion Chamber 1 ‘The design of combustion chamber should be such that, it should permit the use of higher compression ratio without detonation, thereby maintaining high power outputs and efficiency. 2 ‘The velocity of the mixture from the inlet valve should be very hi ch The design should provide high degree of turbulence to: mixture during its flow passage, which in tum increases the flame front travel velocity. This can be achieved by inducing squish, providing bow! shape on piston head or ‘with dome shaped cylinder head “Squish” can be defined. as the rapid movement of the gas. trapped in between the piston and cylinder head. ‘The size of the bore should be.as small as possible. 5. The ratio of flame path to bore should be minimum. 6 The location of spark plug should be at the geometric symmetry of the ‘combustion chamber. It should give highest volumetric efficiency. It ean be achieved by providing clearance around the valve ‘heads, proper valve timing, large diameter valves ete: 8. Itshould give maximum thermal efficiency by reducing the heat losses during combustion. Good scavenging of exhaust BaS¢S. "0. Htshould permit smooth engine operation by regulating the rate of pressure rise such that the greater force is applied to the piston as closely after TDC on the power stroke SPECTRUM @LLIN-ONE JOURN “Wi 2.41 Q36, Explain the methods to control the knocking in S.1. engines. ‘Apritiay 41, Set2, 2310) OR ‘ What are the methods to be followed to avo! knocking in SI engine. Answer : ‘They: are different methods adopted to control knocking in S.1 engines, . id saay17, (R15), 0510) the 1, Super charging 2, Turbo charging 3. Decarbonization 1. Super Charging It is defined as a process of supplying air fuel mixture above atmospheric pressure to engine cylinder. Objectives of Super Charging (® —Toreduce the space required for an engine. (ii) To reduce specific fuel consumption. (iii) To provide turbulence effect, which gives complete ‘combustion of mixture. (iv) To get required horse power of an engine: (¥) Teincrease the volumetric efficiency of engine at higher altitudes, where oxygen gets reduced. ‘Advantages (i) Itis done'to increase the output power and torque of an engine. (ii) Itdecreases the knocking in spark ignition engines. (ii) Tt increases fuel consumption by which’ the engine can withstand at higher forces. (iy) _ It increases pressure and density of air-fuel mixture, which increases the weight per stroke for same swept volume of engine cylinder. Super chargers are widely used to control knocking in Slengines are, (@ Centrifugal super charger. (b) Vane type super charger. (©) Roots type super charger. 2. Turbo Charging Itis a process of increasing power air which is entering into the’

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