Key Points to remember
- Internal combustion is the most widely used prime mover which converts heat energy in the fuel into mechanical energy in the form of rotation. An internal combustion engine consists of several parts, the machine design deals with applying the basic principles to key parts such as Cylinder and cylinder liners,Piston,piston rings,piston pin, connecting rod, big and small end bearings,crankshafts etc.
- The function of cylinder is to hold the working medium for combustion, and also to guide the piston. The small sized engines are air cooled whereas the large sized engines are water cooled, the water cooled engines have pump and jackets around the cylinder for cooling.
- In small engines the cylinder and frame is made in one piece casting, but in case of large engines frame,jackets and liners are made separate. Cylinder liners are the replacable cylindrical parts which are fitted in the main engine block, the liners actually come in contact with piston. When worn out liners are easy and economical to replace rather than discarding total cylinder.
- Cylinders are mostly made up of gray cast iron or Nickel chromium cast iron . The cylinder and liner materials must have good wear resistance.
- The main function of piston is to transmit the force generated due to high pressure in cylinder to crankshaft through connecting rod. Its another function is to perform the compression of the working medium before it is ignited. It also dissipates large amount of heat from combustion chamber to cylinder wall.
- Pistons are commonly made from cast iron or aluminium alloy. Piston is provided with number of ribs inside to strengthen the piston head from bending failure, as well as to transmit heat.
- Piston rings consist of compression rings and oil scraper rings. The function of compression rings is to maintain seal between the cylinder wall and piston to avoid leakage of working medium. Oil scraper rings allow sufficient quantity of lubrcting oil to move up during the upward movement and at the same time they scrap the excess oil from inner surface of cylinder back to crank case.
- Piston pin is a link which connects the piston with the connecting rod. It is also called 'Gudgeon' pin.
- Connecting rod is an important link in ic engine which transmits force from piston to the crank. The connecting rod is made by drop forging process. It has one big end connected to the crank and one small end which is connected to the piston.
- Connecting rod is subjected to alternate tension and compression, further due to its length it may fail in bucking also.
- At big end connecting rod is connected to crank through big end bearing. This bearing is made in two parts and are connected to each other through bolts.
- Crank shaft converts reciprocating motion of the piston into rotary motion. Crank shafts can be of two types, side crankshaft ( having one end free) or centre crankshaft (having both ends in bearings). Crankshafts are made up of plain carbon steels like 40C8,45C8 or nickel chromium steels.
Question and answers on IC engine components design :
Q.1 ) What are the advantages of Cylinder liners? What are dry and wet cylinder liners? State merits and demerits of dry and wet liners.
Ans : The barrel or bore in which an engine piston moves may be an integral part of the cylinder block, or it may be a separate sleeve called 'Liner'.
Cylinders walls can become very worn or damaged due to prolonged use.One solution is to bore the cylinder to oversize and use oversized piston.But this solution has limitation in terms of number of times the cylinder can be rebored. To avoid this a thin cylindrical sleeve is fitted inside the cylinder which actually comes in contact with piston. It is economical to replace the liner or sleeve when it gets sufficiently worn. So using the liners offers following advantages,
- Liners can be easily replaced after wear and tear, and is economical than reboring or replacing whole cylinder.
- Since liner is undergoing wear we can use cheper grade material for the cylinder body since it need not to be high wear resistant.
- Liners can take longitudinal thermal expansions also.
Dry and Wet Liners
Dry Liners do not have direct contact with the jacket water. The jacket water is in contact wit the cyliner but not with liner.
Advantages of dry liner :
- The replacement of liners is easy .
- there is no danger of leakage of water
Limitations of dry liner :
- The cylinder blocks become complicated and difficult to manufacutre
- Heat dissipation is not very effective because there are more barriers.
Wet liner : A liner which comes in direct contact with the cooling jacket water is called wet liner,
Advantages of wet liner are
- Good and quick heat transfer, since water is in direct contact with the liner.
- Cylinder blocks are easy ot manufacutre since there are no hidden jackets.
- No much thermal expansion of liner due to effective cooling, this relieves cylinder block of stress due to thermal expansion
Limitations of wet liners
- Replacement of liners becomes difficult.
- Liners can also get corroded due to direct contact with water.
- There can be leakage of water from jacket. Proper sealing arrangement is required.
Q.2 ) Draw a neat sketch of piston showing its various elements and also state function of each element.
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Q.3 ) Stat the advantages and disadvantages of aluminium piston over CI piston.
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Q.4 ) Write the formulas for design of pistion crown thickness on the basis of strength as well as heat dissipation?
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Q.5 ) Why pistion rings are provided on pistons? state function of compression and scraper rings.
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Q.6 ) Why connecting rods are made of I sections ?
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Q.7 ) Write the procedure to design the crosssection of connecting rod .
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Q.8 ) Draw a neat sketch of connecting rod and explain its constructional details
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Q.9 ) Explain whipping stress in connecting rod
Ans :
Numerical Problems
A) CYLIDER HEAD AND LINER
1)Design a cylinder, cylinder head and cylinder head studs for a four stroke
C.I engine with the following data :
Brake power = 5 KW
Engine speed = 1200 rpm
Indicated mean effective pressure = 0.35 N/mm 2
Maximum gas pressure = 3.5 N/mm 2
Mechanical efficiency = 80 %
Compression ratio = 12
Reboring factor C1 = 4.0 mm
Cylinder head thickness constant k 1 = 0.35
Take Allowable stresses as,
2) The cylinder of a 4 stroke diesel engine has the following specifications
Brake power = 7.5 kW
Speed = 1400 RPM
IMEP =0.35 Mpa
Mechanical efficiency = 80 %
Maximum gas pressure = 3.5 MPa
The cylinder liner and head are made of grey cast iron with Sut=250 MPa and u=0.25. The studs are made of plain carbon steel with Syt=380 MPa. Factor of safety for all parts is 6.
Calculate
Bore and length of cylinder liner
Thickness of cylinder liner
Thickness of cylinder head
Size,number and pitch of stud
3)The cylinder of a four stroke diesel engine has the following specifications.
Brake power = 7.5 KW
Speed = 1400 rpm
Indicated mean effective pressure = 0.35 MPa.
Mechanical efficiency = 80%
Maximum gas pressure = 3.5 MPa.
The cylinder liner and head are made of gray cast iron FG 260 (Sut =
260 N/mm 2 and μ = 0.25). The studs are made of plain carbon steel (Syt = 380 N/mm ^{2} ). The factor of safety for all parts is 6. Use table 1 for reboring allowance.
Determine
i) bore and length of cylinder liner
ii) thickness of the cylinder liner
iii)thickness of the cylinder head
iv) size and number of studs
Table 1. Reboring allowance for cylinders (mm)
D | 100 | 150 | 200 |
C | 2.4 | 4.0 | 6.3 |
D and C are in mm.
4) The cylinder of a four stroke diesel engine produces 5 kW power at 600 rpm. IMEP is 0.5 MPa, assuming 80% mechanical efficiency and ratio of stroke to length as 1.5, Determine
Bore and length of cylinder liner
Thickness of cylinder liner
thickness of cylinder head
Size and number of studs
take allowable tensile stress for liner material as 30 MPa and for studs 50 MPa.
5) The cylinder of a four stroke diesel engine has the details,
Brake power = 7.5 kW
Speed = 1400 rpm
Mech efficiency = 85%
Max gas pressure =3.5 MPa
IMEP = 0.35 MPa
The cylinder liner and head are made up of gray cast iron having ultimate tensile stress as 260 MPa and poissons ratio 0.3. The studs are made up of carbon steel having yield stress 380 MPa. Take factor of safety as 6 for all components, Calculate
Bore an length of cylinder liner
Thickness of cylinder liner
Thickness of cylinder head
Size, number and pitch of studs
B) Piston, piston rings and Piston pin
1) The following data is given for a four- stroke diesel engine :
Cylinder bore = 250 mm
Length of stroke = 300 mm
Speed = 600 rpm
Indicated mean effective pressure = 0.6 MPa.
Mechanical efficiency = 80 %
Maximum gas pressure = 4 MPa
Fuel consumption = 0.25 kg per BP per hr.
Higher calorific value of fuel = 44000 KJ/kg
Assume that 5% of total heat developed in the cylinder is
transmitted by the piston. The piston is made of gray C.I FG 200
(Sut = 200 N/mm 2 and K = 46.6 W/m/ o C) and the factor of safety
is 5. The temperature difference between the center and the edge
of the piston head is 220 o C.
i) Determine the thickness of piston head by strength consideration and thermal consideration.
ii) State whether the ribs are required, IF so calculate the number and thickness of ribs.
iii) State whether a cup is required in the top of piston head, If so calculate the radius of the cup.
2) Following data is given for a single cylinder four stroke diesel engine:
Cylinder bore = 100mm
Length of stroke = 125 mm
Speed = 2000 rpm
Brake mean effective pressure = 0.65MPa
Maximum gas pressure = 5 MPa
Fuel consumption = 0.25 kg per BP per h
Higher calorific value of fuel = 42,000 kJ/kg
Assume that piston transmits 5% of total heat developed in cylinder. Permissible
stress of piston material is 37.5 N/mm 2 (k = 46.6 W/m/oC). Temperature
difference between center and the edge of piston head is 220oC.
i) Calculate thickness of piston head by strength consideration
ii) Calculate thickness of piston head by thermal consideration
iii) Decide on the criteria that decides piston head thickness
iv) Decide if ribs are required
v) If yes, calculate number and thickness of piston ribs
vi) Decide whether a cup is required at the top of piston head
vii) If yes, calculate radius of cup.
3)The following data is given for a four-stroke diesel engine:
Cylinder bore = 250 mm
Length of stroke = 300 mm
Speed = 600 rpm
Indicated mean effective pressure = 0.6 MPa
Mechanical efficiency = 80%
Maximum gas pressure = 4 MPa
Fuel consumption = 0.25 kg/kW-hr.
Higher calorific value of fuel = 44000 kJ/kg
Assume that 5% of total heat developed in the cylinder is transmitted by the piston. The piston is made of gray C.I FG 200 (Sut = 200 N/mm 2 and K = 46.6 W/moC) and the factor of safety is 5. The temperature difference between the center and the edge of the piston head is 220^{o}C.
i)Determine the thickness of piston head by strength consideration and thermal consideration.
ii)State whether the ribs are required, if so calculate the number and thickness of ribs.
iii)State whether a cup is required in the top of piston head, if so calculate the radius of the cup.
4)Following data is given for piston of four stroke diesel engine,
Cylinder bore- 250 mm
Maximum gas pressure - 4 N/mm 2
Bearing pressure at small end of connecting rod - 15 N/mm 2
Length of piston pin in bush of small end - 0.45D
Ratio of inner to outer diameter of piston pin 0.6
Mean diameter of piston boss - 1.4 x Outer diameter of piston pin
Allowable bending stress of piston pin - 84 N/mm 2
Calculate,
i) Outer diameter of piston Pin
ii) Inner diameter of piston Pin
iii) Mean diameter of piston boss and
iv) Check design for bending stress
5)Design a cast iron piston for a single acting four stroke engine with following data
Power : 75 kW per cylinder when running at 600 r.p.m.,
Maximum gas pressure = 4.8 N/mm 2 ,
Indicated MEP = 0.65 N/mm 2,
Mechanical eff. = 95%,
Crank radius = 110 mm,
Fuel consumption = 0.3 kg/BP/hr,
CV of fuel (higher) = 44 × 10 3 kJ/kg,
Diff of temperatures at the centre and edges of the piston head = 200^{o}C,
Allowable stress for the material of the piston rings and gudgeon pin = 80 MPa,
Allowable stress for piston material = 33.5 MPa,
allowable bearing pressure on the gudgeon pin = 17 N/mm^{ 2}
Allowable bearing pressure on the piston barrel = 0.4 N/mm^{ 2}
6)Design a piston for a four stroke diesel engine with following data :
Fuel consumption = 0.3 kg of fuel per kW of power per hour,
BMEP= 0.7 N/mm 2 .
The maximum gas pressure inside the cylinder is = 5 N/mm^{ 2} at a speed of 3500 r.p.m.
The cylinder diameter is required to be 300 mm with stroke 1.5 times the diameter.
The piston may have 4 compression rings and an oil ring.
Higher calorific value of fuel = 46 × 10^{ 3} kJ/kg,
Temperature at the piston centre = 700 K,
Temperature at the piston edge = 475 K,
Heat conductivity factor = 46.6 W/m/K,
Heat conducted through top =6% of heat produced,
Permissible tensile strength for the material of piston = 27 MPa,
Permissible Pressure on piston barrel = 0.4 N/mm 2 ,
Pressure between rings and piston = 0.04 N/mm 2 ,
Permissible stress in piston pin = 85 N/mm 2
Permissible tensile stress in rings = 80 N/mm 2 ,
Permissible pressure on piston pin = 15 N/mm 2 .
C ) Connecting Rod Problems
1) The following data is given for the connecting rod of a diesel
engine
Cylinder bore = 85 mm
Length of connecting rod = 350mm
Maximum gas pressure = 3 MPa
Factor of safety against buckling failure = 5
(l/d) ratio for piston pin bearing = (1.5)
(l/d) ratio for crank pin bearing = (1.25)
Allowable bearing pressure for piston pin bearing = 13MPa
Allowable bearing pressure for crank pin bearing = 11 MPa.
length of stroke = 140 mm
Mass of reciprocating parts = 1.5 kg
Engine speed = 2000 rpm
Thickness of bearing bush = 3 mm
Material of cap = 40 C8 (S yt = 380 N/mm 2 )
Material of bolts=Alloy steel (S yt =450N/mm 2 )
Factor of safety for cap and bolts = 4 and 5 respectively.
Density of connecting rod = 7800 kg/m 3
Determine :
i) Dimensions of the cross-section of connection rod.
ii) Dimensions of small and big and of bearings.
iii) Nominal diameters of bolts for the cap
iv) Thickness of cap; and
v) Magnitude of whipping stress.
2)Determine the cross section of I section of connecting rod for single cylinder IC engine. Use the following data for engine :
Piston diameter = 100 mm
Mass of reciprocating parts= 2.25 Kg
Length of connecting rod= 300 mm
Stroke length= 125 mm
Speed = 1500 rpm
Maximum explosion pressure = 3.5 N/mm 2
Factor of safety = 7
Density of rod material = 8000 Kg/m 3
Yield stress in compression = 330 MPa
Assume width of section as 4 × t and depth as 5 × t where t is the web
thickness of I section.
3)The following data is given for the cap and bolts of the big end of connecting rod:
Engine speed = 1800 rpm,
Length of connecting rod = 350mm
Length of stroke = 175mm, Mass of reciprocating parts = 2.5kg
Length of crank pin = 76mm, Diameter of crank pin = 58mm,
Thickness of bearing bush = 3mm,
Permissible tensile stress for bolts = 60N/mm 2 ,
Permissible bending stress for cap = 80N/mm 2 ,
Calculate the nominal diameter of bolts and thickness of cap for the big end.
4)Determine the dimensions of the cross section of the connecting rod for a diesel engine with following data:
Cylinder bore = 95 mm σ c = 298 N/mm 2
Length of connecting rod = 350 mm
Maximum gas pressure = 4 MPa
Factor of safety = 6
5)The following data is given for the cap and bolts of the big end of the connecting rod:
Engine speed = 1500 rpm
Length of stroke = 0.140 m
Length of connecting rod = 0.320 m
Permissible tensile stress for bolts = 120MN/m 2
Diameter of crank pin = 38 mm
Length of crank pin = 54 mm
Mass of reciprocating parts = 1.75Kg
Calculate the nominal diameter of bolts and thickness of cap for the big end.
6)Design a connecting rod for a high speed I.C. engine using the following data.
Cylinder bore = 125 mm
Length of connecting rod = 300 mm
Maximum gas pressure = 3.5 MPa
Length of stroke = 125 mm
Mass of reciprocating parts = 1.6 kg
Engine speed = 2200 rpm
Assume suitable data and state the assumptions you make.
7)Design a connecting rod I section for an engine with following details:
a) Cylinder bore:150mm.
b) Length of Connecting -rod:450mm.
c) Maximum gas pressure:6MPa.
d) Overall width to height ratio of I cross section as 4t to 5t where tis
the uniform thickness of cross section.
e) Compressive yield stress=330N/mm 2 .
f) Factor of safety=6.
8)Determine the dimensions of the cross section of the connecting rod for a diesel engine with following data:
Cylinder bore = 95mm
Length of connecting rod= 350mm
Maximum gas pressure = 4 MPa
Factor of safety = 5
9)Determine the dimension of the cross section of the connecting rod for a diesel engine with following data :
σ c = 275 N/mm^{2}
Cylinder bore = 100mm
Maximum gas pressure = 4 MPa
Length of connecting rod = 350mm
Factor of safety = 5
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