Crankshaft Connecting Rod And Piston Assembly Engineering Essay

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Internal combustion engine dates back to 1876 when Otto first developed the spark ignition engine & 1982 when Diesel invented the compression-ignition engine. Since then these engines have continued to develop as our knowledge of engine processes has increased, as new technologies became available, as demands of new types of engine arose, and as environmental constraints on engine used changed. Internal combustion engines, and the industries that develop, manufacture and support their use, now play a dominant role in the fields of power, propulsion & energy. The last Twenty-five years or so have witnessed an explosive growth in engine research & development as the issues of air pollution, fuel cost & market competitiveness have become increasingly important.

The purpose of a Combustion Engine is to produce mechanical energy from chemical energy contained in the fuel. In internal combustion engine, as distinct from external combustion engine, the energy is released by burning or oxidation of fuel inside the engine. The air-fuel mixture before combustion & the exhaust gases post combustion are the actual working fluids in the entire working process of an IC engine. The work transfers which provide the desired power output occur directly between the working fluid and the mechanical power transmitting components of the engines. Due to combustion of compressed air-fuel mixture within the cylinder, a large pressure gradient is created which exerts mechanical force on the piston tending it to move in downward direction. Connecting rod, which forms a medium in between the piston and the crankshaft, starts rotating the crankshaft. Hence converting the mechanical force into the torsion on the crankshaft, transferring the chemical energy of the fuel to the mechanical energy at the crankshaft.

Components function

During operation, accurate measure of air-fuel mixture is sucked into the combustion chamber of the engine during the suction stroke of the engine at suction pressure of around 0.7-0.8 bar. This mixture is then compressed in the compression stroke at the maximum pressure (depending upon the compression ratio of the engine). In the power stroke, the compressed mixture is ignited either by external source (in SI engine) or by"self-ignition temperature"of the mixture (in CI engine), thus applying force on the piston head. During the downward movement of piston in the power stroke, the downward linear force on the piston is transformed to torsion by the means of "single-slider mechanism"of the combined system of piston-connecting rod-crankshaft.

Piston:

The force exerted due to the combustion pushes the piston along the cylinder wall. This sliding movement along the cylinder transmits the exerted force to the connecting rod. In order to transmit the force efficiently, the inertia of piston is kept minimal by using aluminium pistons in combination with hollow structure. 2 sets of Piston rings are fitted on the piston to fill up the gap between the piston & cylinder. The upper set of rings is to provide air tight seal to prevent leakage of the burnt gases into the lower portion. The lower rings provide effective seal to prevent leakage of oil in the engine cylinder.

Connecting Rod:

Connecting rod links piston with the crankshaft, whose main function is to transmit force from the piston to the crankshaft. Moreover it converts the sliding motion of the piston into rotational motion of the crankshaft, in the working stroke. The upper (smaller) end of the rod is connected to the piston through gudgeon pin whereas the lower (larger) end of the rod is connected to the crankshaft through crank pin. Special steel alloys or aluminium alloys are used for the manufacture of the rods. A special care has to be taken while designing & manufacturing connecting rod, as it is subjected to alternate tensile & compressive stresses alongwith bending stresses.

Crankshaft:

It considered to be the backbone of IC engine whose function is to convert the reciprocating motion of the piston to the rotational motion with the help of connecting rod. The shaft contains number of eccentric portions called as "crank". The bigger part of connecting rod is connected to the crank by the means of crank pin. Crank pins are the most crucial part in the crankshaft as the force transmitted through the piston is directly applied on to it. Hence careful designing and manufacturing has to be done for crankshaft.

In-service Conditions

Piston:

During operation, piston undergoes thermal & mechanical stresses due to high-pressure temperature conditions. In absence of lubricating oil, there are chances of piston jamming in the cylinder block which causes "engine seizure". Also lack of coolant can cause ineffective heat dissipation which causes thermal expansion of cylinder & piston which causes engine seizing. Due to impulse force acted by the chemical combustion, chances of pitting occur on the piston head surface. Prolong cyclic loads on piston causes bending & shear failure of gudgeon pin. Overheating of engine causes burning of piston rings due to which oil enters the combustion chamber causing white smoke.

Connecting Rod:

Due to continuous cyclic tensile & compressive loads, connecting rod tends to fail in bending moment. External factors like simultaneous acceleration & braking can overload the engine resulting to the failure of connecting rod.

Crankshaft:

Engine crankshafts are subjected to torsional wind-up & vibration at certain speeds due to power impulses. Under accelerating condition the farthest end of crankshaft connected to the flywheel tend to turn first than the flywheel end due to presence of inertial difference at both the ends. Vice-versa in case of engine deceleration. These situations results in repeated shock loads, wear & noise in form of gear clatter imposed on the gear teeth in the transmission system. Thus to overcome torsional vibrations, a torsion damping device is incorporated within the driven plate hub assembly in the transmission system.

Operational Requirements

Piston:

Pistons are subjected to cyclic loads at high temperature- pressure conditions. Hence the piston material should have good thermal conductivity to transfer the heat from piston to the piston rings and eventually through the cylinder block efficiently.

The piston material should have low thermal co-efficient of expansion such that the piston doesn't get jammed up in the cylinder block while operation.

Piston material should have high compressive strength to withstand the impulsive load due to combustion of air-fuel mixture.

The material should have highly stable chemical composition at high temperatures such that it shouldn't chemically react with the air-fuel mixture during combustion.

The piston material should have high melting point to withstand the high temperatures in the combustion process.

Connecting Rod:

Connecting rods should have higher buckling strength in order to withstand buckling loads during operation.

Crankshaft:

Crankshaft should be well-balanced in order to avoid vibrations & noise during operation.

Crankshaft should have optimum torsional stiffness to avoid gear clattering, vibrations & noise during operation.

Crankpins should have higher shear strength to withstand radial load acting from the connecting rod.

Crankshafts should possess sufficient stiffness to resist bending along their length.

Low vibration.

Resistance to wear in the bearing areas.

Material Characteristics

Piston:

Due to application of combined load during the combustion process, the piston material should be able to withstand both mechanical & thermal fatigue. It should have good thermal conductivity to dissipate heat to the cylinder block. The material should have stable chemical composition such that it won't oxidise with the air-fuel mixture. Piston material should have low thermal co-efficient of expansion so that piston don't get jammed at high temperature. Material should be able to withstand high operating temperatures. Also it has to be light in weight in order to increase the mechanical efficiency of engine. Material should be easily forged to withstand the impulsive force on the piston head.

Connecting Rod:

Connecting rods operate under high loads requiring:

High strength in both tension and compression.

High fatigue strength.

Highly efficient engines demand the lowest possible component weight.

Materials with good rigidity / high strength.

Weight consistency to facilitate engine balancing.

Forged steel offers the best combination of strength, stiffness & cost.

Cast iron rods are heavier and sintered powder products are more expensive.

In future the rod and cap will be fracture split to minimise cost for most volume produced connecting rods.

Lower through cost pressures demand:

• Elimination of a heat treatment process

• Improved machinability for high volume production

• Low distortion on fracture splitting

Controlled air cooled steels have largely replaced heat treated steels for connecting rod applications. Higher strength grades are required for the heavier loads typically found in diesel engines and higher performance petrol driven cars. Weight reduction and packaging constraints are also driving up the need for higher strength materials.

Crankshafts:

The manufacturing route for forged steel crankshafts is usually; hot forging, heat treatment, machining and surface treatment. Controlled air cooling after forging is lower cost than the traditional quench and temper and is now the preferred route.

Efficient and cost effective processing requires:

Consistent hardening response.

Good machinability in the hardened condition.

Predictable response to surface modification such as induction hardening, nitriding or fillet rolling.

Controlled hardenability steels ensure repeatability of mechanical properties.

Optimised sulphur content balances the conflicting benefits of low sulphur for fatigue properties and high sulphur for improved machinability.

Controlled carbon content produces consistent response to induction hardening

Controlled chromium and aluminium additions ensure consistent surface hardening through nitriding.

Clean steels provide good fatigue resistance from low overall inclusion content.

Summary

 The Piston rides inside the combustion chamber, around the piston are a series of piston rings that seal the combustion chamber and allow the piston to move up and down. The piston is connected to the crankshaft with a connecting rod. The crankshaft converts the up and down motion of the piston into a circular motion that is ultimately transmitted to the drive wheels.

Care should be taken while designing & selecting materials for these critical components as it adversely affects the engine's performance & efficiency.

Reference

Advanced Vehicle Technology - Heinz Heisler

Internal Combustion Engineering - John H. Weaving

Automotive Technology - Fredrick C. Nash

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