Chapter 2 Design of Engine Engine type based on bore/stroke ratio The diameter of the cylinder bore is divided by the length of the piston stroke to give the ratio

Chapter 2
Design of Engine
Engine type based on bore/stroke ratio
The diameter of the cylinder bore is divided by the length of the piston stroke to give the ratio.
Under square
over square

Under square: An engine is described as under square or long-stroke if its cylinders have a smaller bore (width, diameter) than its stroke (length of piston travel) – giving a ratio value of less than 1:1.

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Over square: An engine is described as over square or short-stroke if its cylinders have a greater bore diameter than its stroke length, giving a bore/stroke ratio greater than 1:1.

Square: A square engine has equal bore and stroke dimensions, giving a bore/stroke value of exactly 1:1.

For si engine
The design of combustion chamber has an important influence upon the engine performance and its knock properties. The design of combustion chamber involves the shape of the combustion chamber, the location of the sparking plug and the disposition of inlet and exhaust valves
Basic requirements of a good combustion chamber
high power output
High thermal efficiency and low specific fuel consumption
Smooth engine operation
Reduce exhaust pollutants
Different types of combustion chamber
T- Head combustion chamber – It require two cam shafts (for actuating the inlet value and exhaust value separately) by two cams mounted on the two cam shafts
L- Head combustion chamber – It is a modification of the T-head type of combustion chamber. It provides the two valves on the same side of the cylinder , and the valves are operated through tappet by a single cam shaft
I – Head combustion chamber – These type of combustion chamber has both inlet value and the exhaust value located in the cylinder head
F- Head combustion chamber – In such a combustion chamber one value is in head and other in the block

For CI engine
The design of combustion chamber for CI engines must also take consideration of injection system and nozzles to be used.
Types of CI combustion chamber
Open injection type- In these type the entire volume of combustion chamber is located in the main cylinder and the fuel is injected in to this volume
In direct injection type – The combustion space is divided in to two parts one part in the main cylinder and other part is in cylinder head. The fuel- injection is effected usually in to the part of chamber located in the cylinder head.
Piston is widely used in the diesel engine, gasoline engine and air compressor. Engine piston structure consists of three parts, piston top, piston head and piston skirt
Piston top
Piston top has three different types. Flat piston top, concave top and convex top.

Flat piston top. Flat piston top with an flat surface is simple and easy to produce. It is widely used in the gasoline piston and rarely used in the diesel engine piston.

Concave piston top. Concave top engine piston has an concave top surface, which is conducive to the burning of combustible mixture. The shapes of concave are double vortex shapes, ball shapes and U shapes. Concave top engine piston is commonly used in the diesel engine.

Convex piston top. Convex top engine piston is in raised ball shape, which has high strength. It can improve gas exchange process and act as the guide role. Convex top engine piston is widely used in the two-stroke gasoline engines of motorcycle.

Piston head
Piston head refers to the location from bottom line of piston top to the bottom line of first oil ring groove.

Piston head is mainly used for piston ring installation. piston ring groove has two to five lines. Quantity of piston ring on the gasoline piston is less than the diesel piston. Piston ring has compression ring and oil-control ring two types.

Compression ring groove uses with compression ring to seal cylinder and prevent combustible mixture from entering the crankcase. Oil-controlling ring groove uses with oil-control ring to scrape extra engine oil off cylinder.

Another function of piston head is transfer the heat to cylinder wall through piston ring.

S.I(4-STROKE) 96.8 86. 8.6
S.I(4-STROKE) 84.5 88 8.5
B= Bore
S= stroke length
Cc= cubic centimeter
Th= thickness of piston head
a=radial wall thickness of ring
d=nominal dia of ring
h=axial width of ring
S1=closed gap of ring
t3=thickness of piston barrel at the top end
t4=thickness of barrel at open end
L1=length of piston pin
Di=inner dia of piston pin
CR=compression ratio
Petrol engine cc
Compression of petrol engine=9
Ultimate tensile strength for aluminum alloy=485
Factor of safety=8
500= ?/4*B2*90
Bore=84 mm
Piston head calculations
Th = D?316*p??b=Sut/fs
= 485/8
= 60.625
Th=84?316*460.25 =9.34 mm
Cup dimensions
The radius of cup=l/d?1.5 (cup required)
l/d?1.5 (no cup required)
radius of cup=0.7*D
=58.8 mm
Piston ring
a1=3.6 mm
d1=85 mm
Top land=Th
Th=thickness of piston=9.34 mm
Width of ring groove (piston ring to piston ring gap)
=1.5 to 2 mm
Piston barrel
=11.05 mm
T4=(0.25t3) to (0.35t3)
=0.25*11 to 0.35*11.05
= 2.76 to 3.86
Piston pin
=37.8 mm
Thickness of cylinder wall =0.045*D+1.6
=5.38 mm
Thickness of dry liner =0.03*D
= 0.03*84
=2.52 mm
Thickness of water jacket =0.032*D+1.6
=4.288 mm
Water space between outer cylinder wall & inner jacket wall
= 0.08*D+6.5
= 0.08*84+6.5
= 13.22 mm
Thickness of cylinder head
D = 84
Cylinder head aluminum material Sut=210 mpa
Factor safetyfs = 6
Th=92?0.162*435 = 12.51 mm
Component Material Process
Cylinder block Ductile
Cast iron
30c8(low carbon steel) Casting
After heat transfer
Cylinder head Aluminium alloys Casting
Piston Aluminium alloy 4625 Casting
piston rings Cast iron of fine grain
High elastic material Pot casting method
connecting rod Low carbon steel 30c8 Forging
After that heat treatment
crank shaft 37c15 alloy steel Casting
fly wheel Cast iron Casting
poppet valves Phosphorous bronze
Monel metal manifold Aluminium alloy-4600 gudgeon pin Plain carbon steel rocker arm Medium carbon steel cam shaft Plain carbon steel 10c4 Grinding
Case hardening