Crankshaft And Connecting Rod And Piston Assembly Engineering Essay

As shown in the figure1.1 above one end of the connecting rod is connected to the crank and other end is connected to the piston with the help of gudgeon pin. Figure 1.2 illustrates the operation of 4 stroke engine. These four strokes are known as inlet stroke, compression stroke, and power stroke and exhaust stroke. During inlet stroke piston moves from top dead centre to bottom dead centre. This creates vacuum at the top resulting into opening of inlet valve. As inlet valve opens, fresh amount of air is fed into the combustion chamber of cylinder. During compression stroke air and fuel mixture is compressed by the upward movement of the piston as it reciprocates inside the cylinder. At the end of compression stroke a charge is ignited to burn the mixture. High temperature and pressurised gases are generated. These gases push piston down during power stroke and reciprocating motion of piston is converted into the rotary motion of a crankshaft. The heat energy generated is used for mechanical work. Exhaust valve opens at the end of power stroke and exhaust gases exit from port when piston travels again from bottom dead centre to top dead centre. Stroke is defined as the distance travelled by piston from TDC to BDC.[6]

Fig 1.2 Working of four stroke engine [2]

The only difference between 2 stroke and 4 stroke engine is that, 2 stroke engine has ports instead of valves and it performs twice as many of power strokes per cylinder per revolution.[2]

FUNCTION OF PISTON ASSEMBLY, CONNECTING ROD AND CRANKSHAFT

2.1 PISTON ASSEMBLY

Piston assembly consists of a piston, piston rings and piston pin known as gudgeon pin. Functions of piston assembly are given below:

It transmits gas force generated during combustion to the connecting rod and crankshaft.

As piston is connected to small end of the connecting rod with the help of gudgeon pin. It guides the connecting rod when gas force is transmitted.

Air/fuel mixture is compressed by piston.

Piston rings restrict the flow of hot gases into the crankcase. This is also referred as blow by control.

Piston rings lubricate the cylinder wall.

Piston rings restrict the amount of oil from crankcase into the gap between piston and cylinder head.

Heat is transferred to the cylinder wall from piston by piston rings. [4]

Fig 2.1 piston construction [2]

Fig 2.1 illustrates the construction of a piston. Piston top is the part of combustion chamber. Top portion can be a flat or bowl is provided as per the combustion requirement. Grooves are provided in the ring belt area to accommodate piston rings. The heat flows from crown, out of the ring belt. Heat is transferred to the cylinder walls through piston rings and then small portion is transferred to the skirt. Some amount of heat is also taken by the lubricant. [1]

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Piston rings

There are two types of piston rings: compression and oil control ring. Piston rings are generally made up of rectangular section. To prevent sticking of ring into the groove, their inner and outer edges are chamfered. Compression rings are subjected directly to gas pressure. [2]

Gudgeon Pin or piston pin

Gudgeon pin is the main load carrying part in piston assembly. Small end of the connecting rod is connected to piston with the help of gudgeon pin as shown in fig 2.2.It consists of aluminum pads are placed at the ends; these will not damage the cylinder. In order to maintain a correct fit between piston and connection rod, external surfaces of gudgeon pins are finished to a high degree of accuracy. [1]

2.2 CONNECTING ROD

The main function of a connecting rod is to form a link between a piston and crankshaft. A small end of the connecting rod is connected to the piston with gudgeon pin and big end is separated into two parts for ease of assembly with crankpin. The two parts of the big end are bearing cap and big end housing. Both are being bolted together. This is done for ease of assembly of connecting rod with crankpin. To supply oil to the big end, the oil hole is drilled from big end. A typical arrangement of connecting rod is given below:[1]

Fig 2.2 Construction of a connecting rod [2]

2.3 CRANKSHAFT

Crankshaft is the final link in the slider crank mechanism. The linear motion of a piston is converted into rotary motion of a crankshaft. The energy is generated into the combustion chamber by transforming the reciprocating motion of a piston into rotary motion of a crankshaft. Crankshaft consists of two main bearings at the other side of the it, made up in to two half liners. Crankshaft, crankshaft oil seals, main bearings, pulley and vibration damper are parts of crankshaft assembly. [4]

Fig 2.3: Crankshaft and main bearings [2]

IN SERVICE CONDITIONS OF PISTON ASSEMBLY, CONNECTING ROD AND CRANKSHAFT

3.1 PISTON ASSEMBLY

Pistons are subjected to mechanical load, thermal load and side load. During combustion stroke high pressurised gases are formed. Mechanical loading is due to these high pressurised gases and the reaction from the pin and cylinder wall. Thermal loading is due to the temperature and heat transfer conditions in the cylinder. Heat flux is a measure of thermal loading. Articulation of the connecting rod results in a side loading.

Different operating conditions of a piston are given below:

Max operating pressure: Up to 80 to 110 bars in case of naturally aspirated engines, 130 to 180 bar in case of supercharged diesel engines.

Max operating temperature: Peak gas temperature is between 300 to 400°C

Pistons are also subjected to mechanical and thermal stresses. Temperature distribution on the top of the piston is uneven. These loads and stresses are repeated every cycle and can vary with speed. This induces fatigue stress into piston and gudgeon pin.

Piston rings are subjected to inertia force, gas pressure from all sides of the ring and friction force as ring rub against cylinder walls. Fig 3.1 represents the pressure distribution and forces acting on piston ring. [4]

Fig3.1Temperature distribution across piston [7] Fig3.2 Forces acting on a piston ring [5]

Gudgeon pin is subjected to inertia force of the piston and gas pressure. It is aslo subjected with bending stress and shear stress due to the vertical component and normal component of force acting on the piston. Because of the temperature variations pin may deform. [4]

3.2 CONNECTING ROD

Connecting rod is subjected to;

Load due to gas pressure on a piston.

Inertia load due to reciprocating parts and piston.

Inertia load due to connecting rod weight itself.

Force due to friction of piston rings and piston.

Force due to friction of gudgeon pin bearing and crankpin bearing which increase the compressive stress on the connecting rod.

Bending stress and tensile stress is developed due to inertia load.

Small end bearings may subject to corrosion because of the degradation of engine oil at elevated temperatures. That weakens the bearing structure.

3.3 CRANKSHAFT

Crankshaft is subjected to;

Gas forces and mass action.

Centrifugal force of rotating parts of connecting rod mass acts on a crank pin.

Bending stress.

Shear stress produced due to torsional moment on the shaft.

Fracture because of repeatative bending and torsional stresses.

Direct compressive stress due to radial force acting on it.

May subject to gyroscopic effect produced because of misalignment in main bearing, vibration damper lever arm and weight force.

Fig 3.3 Load distribution on connecting rod and crankshaft [7]

OPERATING REQUIREMENTS OF PISTON ASSEMBLY, CONNECTING ROD AND CRANKSHAFT

4.1 PISTON ASSEMBLY

a) As inertia force is increases with the square of the engine speed, it is desirable to keep weight of piston, piston pin and piston rings as low as possible in high speed engines.

b) The heat of combustion should disperse quickly to the cylinder walls.

c) Reciprocating action should be without noise.

d) Gas and oil sealing must be effective.

4.2 CONNECTING ROD

MATERIAL CHARACTERISTICS OF PISTON ASSEMBLY, CONNECTING ROD AND CRANKSHAFT

5.1 PISTON ASSEMBLY

Material of a piston should ,

Have higher coefficent of thermal expansion.

Have high UTS and fatigue strength.

Have high thermal conductivity.

Have resistant to wear.

Have resistance to corrosion.

Sustain temperatures between 300°C to 400°C.

Material of Piston rings should,

Have resistant to wear.

Have scuff resistance coatings.

Material of gudgeon pin should,

Have maximum stiffness.

Have lower density.

Have ductility and tensile strength.

Have resistant to wear.

5.2 CONNECTING ROD

Material of a piston should,

Have maximum stiffness.