Design of IC Engine Cylinder

1. DESIGN OF ENGINE CYLINDER In designing of Engine cylinder, following parameters have to be determined 1. Bore (D) and length of cylinder (Lc ) 2. Thickness of cylinder liner (t) 3. Thickness of cylinder head (t h ) 4. Size (d) , number (z) of studs 1.1 Bore (D) and length of cylinder (𝐋𝐋𝐜𝐜 ): The dimensions of cylinder are calculated by using the following equations: 𝐁𝐁. 𝐏𝐏 𝛈𝛈𝐦𝐦 = 𝐈𝐈. 𝐏𝐏 Where

𝛈𝛈𝐦𝐦 = Mechanical Efficiency;

𝐈𝐈. 𝐏𝐏 =

𝐩𝐩𝐦𝐦 𝐋𝐋𝐋𝐋𝐋𝐋 𝟔𝟔𝟔𝟔

B.P = Brake Power i.e. power available at crankshaft I.P = Indicated Power i.e. power produced inside the cylinder; 𝐩𝐩𝐦𝐦 = Mean effective pressure; L = stroke length; N = Engine speed in rpm n = no. of working stroke per minute = N for 2 stroke engine = N/2 for 4 stroke engine

Note: • If mechanical efficiency is not given, assume as 80 %. • •

𝐋𝐋

For an IC Engine, stroke to bore ratio ( ) lies between 0.9 to1.9. (From Mahadevan 𝐃𝐃 DDB, page no. 364, table 18.2) In design problem its avg. value can be assumed i.e. 1.4. Length of cylinder is taken as 15 % more than the stroke length due to clearance provided on both sides i.e.

Lc = 1.15 L

1.2 Thickness of cylinder liner (t): The engine cylinder liner is assumed as thin cylinder and its thickness is calculated by the following equation

t=

p max D 2σc

+ C

C = Reboring allowance (to compensate for reboring at intervals during the lifetime of cylinder) σc = Permissible Hoop stress = Allowable tensile stress for the cylinder material (𝜎𝜎𝑡𝑡 ) pmax = Maximum gas pressure inside the cylinder

Note: • If pmax is not given, it is assumed as 10 times of mean effective pressure. • The cylinder liner is made of brittle material (generally Grey cast Iron). If FOS and material is specified then permissible hoop stress is calculated as below

𝐒𝐒𝐮𝐮𝐮𝐮 𝐅𝐅𝐅𝐅𝐅𝐅 Otherwise select suitable value for allowable tensile stress from Design data book (page no 359, after equation number 18.14) 1.3 Thickness of cylinder head (t h ): A separate cylinder head (or cover) is provided in most of the IC engines to accommodate following parts: • Inlet and exhaust valves • Spark plug / injector • Air and gas ports The thickness of cylinder head is obtained it by the following relation assuming it as flat circular plate 𝛔𝛔𝐜𝐜 (𝐨𝐨𝐨𝐨 𝛔𝛔𝐭𝐭 ) =

𝐭𝐭 𝐡𝐡 = 𝟎𝟎. 𝟑𝟑𝟑𝟑 𝐃𝐃 �

𝐩𝐩𝐦𝐦𝐦𝐦𝐦𝐦 𝛔𝛔𝐭𝐭

( From DDB , page no 359, equation no 18.14)

Note: • The cylinder head is made of brittle material. If FOS and material is specified then Permissible tensile stress (𝜎𝜎𝑡𝑡 ) is calculated as below 𝐒𝐒𝐮𝐮𝐮𝐮 𝛔𝛔𝐭𝐭 = 𝐅𝐅𝐅𝐅𝐅𝐅 Otherwise select suitable value for allowable tensile stress from Design data book (page no 359, after equation number 18.14) 1.4 Design of Studs: Studs are used to make the leak proof assembly of cylinder, cylinder head and gasket. In design of studs, following parameters are important • Number of studs (Z): Z = 0.015 D + 4 (From Design Data book, page no 359, equation no 18.15) • Size of studs: Let the core diameter = dc Nominal diameter = d • Core (or root ) diameter is obtained by equating the gas force on cylinder head to the resisting force offered by all studs. 𝜋𝜋 𝜋𝜋 Gas Force = 𝐷𝐷 2 𝑝𝑝𝑚𝑚𝑚𝑚𝑚𝑚 ; Resisting force = 𝑑𝑑𝑐𝑐2 𝜎𝜎𝑡𝑡 𝑍𝑍 4

4

After equating the above two expressions

•

𝑑𝑑𝑐𝑐 = 𝐷𝐷�

𝑝𝑝 𝑚𝑚𝑚𝑚𝑚𝑚 𝑧𝑧𝜎𝜎𝑡𝑡

(From DDB, page no 358, equation no 18.8)

• Relation between core diameter and nominal diameter is 𝐝𝐝𝐜𝐜 = 𝟎𝟎. 𝟖𝟖 𝐝𝐝 Pitch of studs: Pitch circle diameter Dp is expressed as [𝑫𝑫𝒑𝒑 = D + 3d] Pitch of studs =

𝑷𝑷𝑷𝑷𝑷𝑷𝑷𝑷𝑷𝑷 𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄𝒄 𝒅𝒅𝒅𝒅𝒅𝒅𝒅𝒅𝒅𝒅𝒅𝒅𝒅𝒅𝒅𝒅 𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝑵 𝒐𝒐𝒐𝒐 𝑺𝑺𝑺𝑺𝑺𝑺𝑺𝑺𝑺𝑺

=

𝑫𝑫𝒑𝒑 𝒁𝒁

Note: • The studs are made of ductile material. If FOS and material is specified then Permissible tensile stress (𝜎𝜎𝑡𝑡 ) is calculated as below 𝐒𝐒𝐲𝐲𝐲𝐲 𝛔𝛔𝐭𝐭 = 𝐅𝐅𝐅𝐅𝐅𝐅 Otherwise it is taken as 35 to 70 MPa. Its avg. value 50 MPa (approximate) can be used in design problems.