Thermal Analysis of Combustion Chamber of Two Stroke SI Engine

Thermal Analysis of Combustion Chamber of Two Stroke SI Engine

Manish S. Lande1 Roshan D.Bhagat2

Assistant Professor Assistant Professor

Mechanical Engg.Department Mechanical Engg.Department

College of Engg.And Tech.Akola College of Engg.And Tech.Akola

S.G.B.A.University Amravati. S.G.B.A.University Amravati.

ABSTRACT:-

Cylinder is the heart of internal combustion engine as combustion takes place inside the cylinder large amount of heat is produce inside the cylinder due to that heat distortion of cylinder wall May takes place. Due to inadequate heat transfer through the engine cylinder block the engine cylinder gets overheated, lead to knocking and some time result into structural failure. This also causes an increase in the thermal stresses in the liner wall which ultimately affects the strength of liner wall. The main objective of this project is to carry out thermal analysis of combustion chamber (Liner, piston,) in Ansys10 to predict temperature distribution across the combustion chamber of scavenged engine. For the analysis purpose firstly we modeling the liner and piston in Pro/E wildfire 4.0 and analyzed the temperature distribution in Ansys 10.

1. 1INTRODUCTION

Heat transfer is a very wide field used in analysis of internal combustion engine heat transfer effect parameter such as performance, emission and also efficiency. It is said that for a given mass of fuel higher the heat transfer to the combustion wall will reduce the average combustion pressure and temperature, this indirectly reduces the work done by the piston per cycle and these effects the specific power.

Temperature rise of the engine parts may cause a serious durability of the engine. The shape of isothermal lines and high temperature regions become more important in these studies. The experimental way will find these regions are costly and time consuming; Analytical methods are almost equally good for fast conformation of this region by using finite elements

Measuring the actual dimension of various components of two-stroke S.I engine (Bajaj bravo, 150cc). modeling of piston, liner along with combustion chamber are done using Pro/E wildfire4.0,then by using Ansys 10 we analyzed the temperature distribution and thermal stresses on above component, compare that thermal stresses with theoretically calculated thermal stresses.

1. 1 AIM AND OBJECTIVE:-

   Aim: –

   The main objective of this project is to carry out thermal analysis of combustion chamber in Ansys to predict temperature distribution across the liner wall and piston of two strokes SI engine also find out the thermal stresses.

   Objective:-

   • Modeling of liner and piston in pro/E; wildfire 4.0

   • Performance evaluation of 150cc, two stroke SI engine with and without scavenging process.

   • Find out the theoretical thermal stresses in liner and compare which found out by software.

   • Analyze the heat distribution across the assembly of piston and liner considering both temperatures with and without scavenging with the help of ANSYS 10 software.

2. FACTOR AFFECTING HEAT TRANSFER IN ENGINE

It may be noted that the engine heat transfer depended upon parameter. Unless the effect of this parameter is known, the design of a proper cooling system will be difficult.

• Fuel-air ratio:-

  A change in fuel-air ratio will change the temperature of the cylinder gases and affect the flame speed. The maximum gas temperature will occurs at an equivalence ratio of about 1.12 i.e., at a fuel-air ratio about

  0.075. At this a fuel-air ratio Twill be a maximum .However, from experimental observations the maximum heat rejection is found to occur for a maximum, slightly leaner than this value.

• Compression ratio:-

  An increase in compression ratio causes only a slight increase in gas temperature near the top dead centre; but, because of greater expansion of the gases, there will be a considerable reduction in gas temperature near bottom dead centre where a large cylinder wall is exposed. The exhaust gas temperature will also be much lower because of greater expansion so that heat rejected during blow down will be less. In general, as compression ratio increase their tend to be a marginal reduction in heat rejection.

• Spark advance:-

  A spark advances more than the optimum as well as less than the optimum will result in increased heat rejection to the cooling system. This is mainly due to the fact that the spark timing other then MBT value (minimum spark advance for best torque) will reduce the power output and thereby more heat is rejected.

• Engine output:-

  Engines which are designed for high mean effective pressure or high piston speeds, heat rejection will be less. Less heat will be lost for the same indicated power in large engine.

• Speeds and loads:-

  Prediction of spark ignition engine heat transfer as a function of speed and load. The cycle heat transfer is expressed as a percent of fuels chemical energy. The relative importance is of heat losses per cycle decrease as speed and load increase: the average heat transfer per unit time, however, increased as speed and load increase.

• Spark timing:-

  Retarding the spark timing in as SI engine decreases the heat flux. The burned gas temperature is decreased as timing is retarded because combustion occurs latter when the cylinder volume is larger. Temperature trends vary component. Piston and spark plug electrode temperature change most with timing variation; exhaust wall temperature increases as timing as retarded due to higher exhausting gas temperature.

• Inlet temperature:-

  The heat flux increases linearly with increases inlet temperature. The gas temperature throughout the cycle is increased. An increase of 100K gives a 13 percent increases in heat flux.

• Cylinder gas temperature:-

The average cylinder gas temperature is much higher in comparison to the cylinder wall temperature. Hence, any marginal change in cylinder gas temperature will have very little effect on the temperature difference and thus on heat rejection.

.

2.1 MAXIMUM TEMPERATURE INSIDE COMBUSTION CHAMBER

Given Data:-

Compression ratio (r) = 8 , Room Temp. (T1) = 300 k., Ratio of specific heat () = 1.4 r = (T3 /T1 )1/2.(-1)

T3 / T1 = (r)2.(-1)

T3 = T1 × (r)2.(-1)

= 300 × (8)2(1.4-1)

= 300 × (5.27)

= 1581k.

T3 = 1308c.

It is the highest temperature inside the engine.

From the above derivation we find out the maximum temperature inside the combustion chamber, at speed 5500 rpm. Here we consider the compression ratio 8 and at room temperature 300k.

1. 2 EXPEIMENTAL SETUP AND CALCULATION Engine Specification under Study

   • Type 2-stroke 5-ports single cylinder S.I engine

   • Cooling Air cooled

   • Bore ¢ 57 mm

   • Stroke 54 mm

   • Displacement 145.5cc

   • Compression ratio 8

   • Connecting rod length 105mm

   • Maximum engine output 5.9kw@5500rpm

Fig. 2.1 Experimental setup.

LEGENDS

1. Engine Frame, 2-Crank Case, 3-Caburetor, 4-Air Box, 5-Fuel Tank, 6-Burrete, 7-Exhaust Of Engine, 8-PUC Machine Setup, 9-Rope Brake Dynamometer, 10-Air Box Stand, 11-Machine Stand, 12-Hole Provided For Direct Air Injection, 13-Tachometer

   • Test Procedures

     1. Start the ngine by cranking the kick provided for cranking.

     2. Adjust the burret level up to 25ml.

     3. Then close the tank fuel valve and open burette valve & measure the fuel consumption for 10 sec with the help of stopwatch. .

     4. Note down the speed of engine by the tachometer.

     5. Note down the manometric reading from u-tube manometer.

   • Reading:-

     Sr.no.	Speed (r.p.m.)	Manometric reading(H) (cm)	Mass of fuel(ml)	Time(sec.)	Velocity (m/s)
     1	1160	8	0.9	10	36.62
     2	1300	9	1	10	38.84
     3	1600	13	1.2	10	46.49
     4	2000	16	1.5	10	51.79
     5	2750	20	1.7	10	57.91

     • Table 2.1 Readings with scavenging

       Sr.no	Speed (r.p.m.)	Manometric reading(H) (mm)	Mass of fuel(ml)	Time(sec.)	Velocity (m/s)
       1	1000	6	1	10	10.03
       2	1550	8	1.4	10	11.58
       3	1700	12	1.9	10	14.18
       4	2100	20	2.3	10	18.31
       5	2400	28	2.8	10	21.66

   • Where,

   • Table 2.2 Readings without scavenging

     • V= 2gh

   • V= velocity of air, g = acceleration due to gravity, h= manometric head

   • When considering the readings with direct injection of air and without direct injection of air, there is a difference in fuel consumption rate. The velocity of air is increased in direct air injection mean there is better burning of fuel are take place due to the better burning of fuel temperature inside the combustion chamber has been increased. We considering the 50c increased in temperature with direct injection of air

     3] CALCULATION

     3.1] Theoretical Thermal Stresses.

     Whenever there is some increase or decrease in the temperature of a body, it causes the body to expand or contract. A little consideration will show that if the body is allowed to expand or contract freely, with the rise or fall of the temperature, no stresses are induced in the body. But, if the deformation of the body is prevented, some stresses are induced in the body. Such stresses are known as thermal stresses.

     Let

     l = Original length of the body, t = Rise or fall of temperature, and = Coefficient of thermal expansion, Increase or decrease in length,

     l = l. . t

     If the ends of the body are fixed to rigid supports, so that its expansion is prevented, then compressive strain induced in the body,

     Therefore

     c = l / l

     = l. . t/ l

     = . t

     Thermal stress, th = c .E

     = . t. E

   • Thermal Stresses For Without Scavenging:-

     Given data;-

     l = 0.124m, t = 12810k, = 23.3×10-6 /0c, E = 2×1011 N/m2

     Increase or decrease in length,

     l = l. . t

     = 0.124× 23.3×10-6 × 1281

     = 0.00365 m

     c = l / l

     = 0.00365/0.124

     = 0.0294

     Thermal stress, th = c .E

     = 0.02796 × 2×1011

     = 0.58 × 1010 N/m2

   • Thermal Stresses For With Scavenging:-

Given data;- l = 0.124m, t = 13310k, = 23.3×10-6 /0c , E = 2×1011 N/m2 Increase or decrease in length,

l = l. . t

= 0.124 × 23.3×10-6 × 1331

= 0.00384 m

c = l / l

= 0.00384/0.124

= 0.0310

Thermal stress, th = c E

= 0.0291 × 2×1011

= 0.62×1010 N/m2.

3.2 Thermal Stresses from Analysis

Fig. 3.1 Thermal Stresses on color scale.

Fig. 3.1 Thermal Stresses on color scale by zoomed view.

The theoretical thermal stresses and by using software ansys10 are nearly equal.

1. 3 Material Properties:

   SR.NO	PART	MATERIAL	THERMAL COMDUCTIVITY	DENSITY(kg/m³)
   1	PISTON	CAST IRON	60 (W/m °k)	7200
   2	PISTON RING	HIGH SPEED STEEL	54 (W/m °k)	7750
   3	LINER	CAST IRON	60 (W/m °k)	7200
   4	PISTON	ALUMINIUM	225( W/m °k)	2750

   Table 3.1: Material Properties

2. GEOMETRY DEFINITION OF ASSEMBLY

A 3D model of engine (liner, piston, and combustion chamber) has been drawn using CAD software pro/E; wildfire 3.0 as shown in the figure.

Fig 4.1 Assembly in ansys window

This model have very complicated shape, therefore it always difficult to mesh and there is lot of geometry loss take place when importing it to the analysis software ANSYS from different CAD software i.e. As an IGES file format. Our results do not match with realistic model due to element shape quality therefore it is necessary to simplify the model in ANSYS for maintaining the element shape quality as well as controlling the number of element for reducing the time for analysis figure shows the simplified (liner, piston, and combustion chamber) model in ANSYS.

4. 1 BOUNDARY CONDITION AT THE COMBUSTION CHAMBER

A variety of thermal boundary conditions is necessary to complete the application of FEM models for the prediction of temperature and heat flux distribution on engine structure. Since the application of these conditions introduce a factor of uncertainty on to the final results, a detailed knowledge of physical mechanisms become essential. For our model we used three boundary conditions, first condition is applied inside the combustion chamber and top of the piston, when the position of piston is 8 after TDC.The temperature applied at that condition is 1581 k for without direct injection which is calculated in. earlier. And for direct injection we assume temperature increased 50c .The second boundary condition is applied at piston skirt, the temperature applied at that condition which is to be assume is 423k .And third boundary condition is applied at bottom of liner which is to be at 300 k. Also on piston ring 1000k temperature is applied. There are some assumption are made like, Effect of piston motion on the heat transfer is neglected. The ring does not twist.

1. 2 TEMPERATURE DISTRIBUTION IN ASSEMBLY

   Result from the ANSYS program gives us a temperature distribution of engine liner. To review this result general post processor has been used. Post processor enables to review results at one time step over the entire model.

   • Temperature Distribution Considering Without Scavenging For C.I

     Fig. 4.2 Temperature distribution for CI

     The above figure shows temperature distribution from higher temperature to lower temperature ranging between 1581 to 3000k. The distribution of temperature on assembly shown by various color. Various ports affect the temperature distribution on assembly.

   • Temperature Distriution Considering With Scavenging For C.I

     Fig .4.3 Temperature distribution for CI

   • Temperature Distribution Considering Without Scavenging For Aluminium

     Fig.4. 4 Temperature distribution for Al

   • Temperature Distribution Considering With Scavenging For Aluminium

Fig. 4.5 Temperature distribution in Al

From the analysis we find out that the heat distribution in aluminium piston is better than cast iron due to its high thermal conductivity. Hence it increases the heat transfer rate and it can reduce the tendency of knocking.

4. 3 NODAL TEMPERATURE DISTRIBUTION

TEMPERATURE DISTRIBUTION ON EACH NODE WITHOUT SCAVENGING ON PISTON FOR CI

TEMPERATURE DISTRIBUTION ON EACH NODE WITHOUT SCAVENGING ON PISTON FOR ALUMINIUM

TEMPERATURE DISTRIBUTION WITH SCAVENGING ON PISTON FOR C.I.

TEMPERATURE DISTRIBUTION WITH SCAVENGING ON PISTON FOR ALUMINIUM

CONCLUSION

The thermal stresses which are found in Ansys software are nearly equal to theoretical thermal stresses. The value of thermal stresses is th = 0.62×1010 N/m2.and according to material property of engines its safe. The ports present on the liner affect the temperature distribution across the liner body due to the reduction of area. The nodal temperature distribution table shows there is proportionality variation in nodal temperature for with and without scavenging. The temperature distribution in case of cast iron and aluminum piston show the variation due to the thermal conductivity difference, means aluminum piston are able to better heat distribution so we can say that aluminum piston gives better heat transfer than cast iron piston so it can reduced tendency of knocking.

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