Module 6. I. C. engine
Lesson 14
PARTS OF I.C. ENGINE
14.1 Parts and Construction of an I.C Engine
Until now after studying the heat engines, we know that work means movement of piston from one end to other end in the cylinder of engine. It means the work is available in the form of a straight movement of piston and that too only during the expansion or working stroke. Out of this work some work has to be spent in the suction, compression & exhaust stroke. Thus work available at the piston is not continuous. On the other hand, practical requirement from a heat engine is continuous supply of mechanical work in the form of a moving wheel. Thus the actual I.C engine is not merely a cylinder/piston arrangement but there are some other parts and mechanism also which make it a complete machine producing power by burning of fuel. Practically there are hundreds of different parts of an I.C engine and it is practically impossible to explain each in the limited contents. However the main components which form an integral part of engine are explained here and also shown in the Fig 14.1 of a single cylinder air cooled I.C. engine.
For more animations of I. C. engine please visit www.animatedengines.com
14.1.1 Cylinder block
It is the major part of an I.C engine which is casted of hard grade cast iron or aluminum alloy. It has one or more cylindrical barrels/cylinders which guide the piston, depending on single cylinder or multi-cylinder engine. The bore and length of cylinder depending on stroke length are the two main design parameters of an I.C engine. Most of the time, separate cylindrical sleeves of superior quality material i.e. steel or aluminum alloy are fitted in the cylinder blocks to act as the cylinder. On wear and tear these can be replaced in the engine block during over hauling of engine. In air cooled cylinder, fins are provided around the cylinder to increase/facilitate dissipation of heat to air. In water cooled cylinder water jackets are there around the cylinder through which cooling water flows. The manufacturing quality of the cylinder or sleeve contributes mainly towards the life of engine as it has to bear a lot of mechanical & thermal stresses.
14.1.2 Cylinder head
It covers the cylinder block & so the cylinder from top end is closed. It is filled tightly with cylinder block with the help of studs and nuts. A gasket is also used between cylinder head and cylinder to avoid any leakage of gases. It has a complicated design as it incorporates in it the hemispherical shape combustion chamber, inlet and exhaust ports/ passage, valve & valve seat and also valve guide mechanism. The spark plug for S.I engine or Fuel injector for C.I engine is also screwed in at the appropriate place in cylinder head. In case of water cooled engine, the cooling water galleries are also incorporated in the cylinder head which meet the corresponding water galleries in cylindrical block. Thus it has a complicated design. It is also casted of high grade C.I or aluminum alloy.
14.1.3 Piston
It is a moving component of cylindrical shape which remains slide fitted in the cylinder and can reciprocate in the cylinder. Its outer diameter is within the limits and fits of inner diameter of cylinder. The number of piston used is same as number of cylinders i.e. for each cylinder there is one piston. It is made up of aluminum alloy and strong enough to bear the force of expanding fuel gases on one side and transmit it to connecting rod on the other side. It is also light in weight to minimize the energy loss due to inertia forces. It is a semi-hollow cylindrical component with the solid face towards the cylinder head. Over the top solid face it bears the force of burning & expanding gases and in the hollow space below connecting rod is hinged with the help of a gudgeon pin. On its circumference, it has rectangular slots/grooves to fit in various types of piston rings. The O.D of piston is designed in such a way that even after increase in size due to thermal expansion at high working temperature, it should easily slide in the bore of cylinder.
14.1.4 Piston rings
Along with easy sliding, the piston should also give a sealing fit in the cylinder to avoid leakage of gases in between the piston & cylinder. For this purpose, piston is equipped with piston rings. Piston rings are made of fine grain cast iron or spring steel which gives high elasticity even at higher temperature. These are circular in shape and sized as per the groove of piston. They are split at one point for fitting in the grooves. When rings are put on piston and it is slided inside the cylinder walls, their closed end comes close and due to elasticity they try to expand outside and form a sealing fit with the cylinder walls. The split end of each ring is kept at a different point than that of other ring. In case of a 4 stroke engine where more lubricating oil is supplied between the piston and cylinder, a separate ring is there to scrap off all the oil from the cylinder wall, before it is exposed to burning gases to prevent burning of lubricating oil.
14.1.5 Piston pin (gudgeon pin)
It is a hardened steel pin pressed through the piston and small end of connecting rod fitted in between. By piston pin, connecting rod is connected to piston and can swivel on it.
14.1.6 Connecting rod
It is a forged component hinged between the piston and crankshaft. The small end is hinged to the piston with the help of gudgeon pin. Big end is hinged to the crank with the help of another set of split bushes and crank pin. This big end is open able and closed by a rod cap after taking in the crank pin with split bushes in between a shown in fig 14.2. The job of connecting rod is to transmit reciprocating motion of piston to the crank in rotary form. Connecting rod is made up of nickel/chrome vanadium steels. The assembly of piston, piston pin and connecting rod with crank shaft as shown is Fig 14.2.
14.1.7 Crankshaft
It is the main shaft of engine mounted over the main bearings in crankcase and has crank or misaligned/eccentric crankpin hinged with big end of connecting rod. This shape facilitates the conversion of reciprocating motion into rotary motion. The eccentricity between the crankshaft and crank pin is half of the stroke length of piston. Thus during two complete stroke of piston, crank shaft undergoes one complete revolution.
14.1.8 Crank case
Crank Case is made up of cast iron and covers the lower portion of cylinder block. It houses the crankshaft, connecting rod, cam shaft, push rod etc and also stores lubricating oil at its bottom. So, the bottom portion of crank case is called oil sump which can be dismantled separately. At its bottom a drain plug is there to drain the lubricant oil during routine servicing of engine. Lubricating oil pump is also installed inside the crank case.
14.1.9 Engine bearing
Any rotating mechanical part of a machine is supported by some type of bearing. The function of bearing is to allow the required moment of parts and also to support or bear the load or force exerted by moving parts. The bearing at the small end and bigger end of connecting rod are sleeve and split-sleeve bearings. The main bearings in the crankcase on which crank shaft is supported are ball bearings or roller bearing. All the bearings are lubricated continuously with pressurized lubrication system of engine. If wear does take place, it is the bearing or sleeve that wears and it can be replaced instead of whole crank shaft or other expensive engine part. However, due to costly material, bearings are also costly parts but need to be replaced only after a long time running of engine.
14.1.10 Flywheel
A flywheel mounted on the crankshaft outside the crank case has an important part to play in the working of an I.C engine. It is a heavy part and rotates along with the crank shaft. It stores the energy in the form of a heavy rotating mass, when it is available on crankshaft during working stroke and gives it back when it is needed in the idle stroke. So it keep on moving the engine during idle stroke. It also facilitates the starting of engine, overcoming the starting over-load. The weight of fly-wheel depends on the type and size of engine. Therefore lighter flywheel is required for a multi-cylinder engine than that in single-cylinder engine of same displacement. Also lighter flywheel is required for a 2-stroke engine than that in 4-stroke engine. Further lighter flywheel is needed in petrol engine as compared to Diesel Engine.
14.1.11 Governor
Governor is a device used for regulating automatically, the output of engine by regulating the supply of fuel/fuel charge. In an engine running at a normal speed, if the load on output shaft increases, rpm of engine tends to decrease. Conversely if the load decreases, rpm tends to increase. If governor is, fitted on the engine, it senses its rpm and accordingly regulates the fuel supply to the engine. If due to increased load, engine tends to slow down, governor increases the fuel supply and if engine tends to rotate fast due to decreased load, governor decreases the fuel supply and keeps engine rpm at a constant value by adjusting the power produced with power required.
14.1.12 Valves and valve operating mechanism
The various components of valve operating mechanism are
i) Inlet and Exhaust Valves with extended Stem
ii) Valve Seat in the cylinder Head
iii) Valve bush
iv) Valve Spring
v) Rocker Arm or Puppets
vi) Push Rod
vii) Cam Shaft
viii) Timing Gear or Belt
The Inlet & Exhaust Valves of conical disc type are provided in the cylinder Head. Generally one Inlet Valve & One Exhaust Valve is there in each cylinder. But more than one Inlet & Exhaust Valve can also be used. These valves rest on their seat made in the bottom face of head with the help of valve stem, bush, spring retainer etc. Due to spring force, valve remains closed by tightly fitting over its cone shaped seat. It can be made open by pushing valve stem with the help of rocker arm downward. The rocker arm is actuated by means of a push rod and camshaft resting on its bearings in the crankcase. On the camshaft, various cams are there in different angles, one for each valve. The camshaft is rotated by engine crankshaft itself with the help of timing gears. The relative position of crankshaft and camshaft is very important as it ensure the correct timing of operation of valves with the help of already determined and fixed position of cams on camshaft. Valves and rocker arms are fitted in cylinder head and camshaft is in the crankcase. Push rods pass through cylinder block to pass on the actuating or pulsating moments of cams to rocker arms.
14.2 Important Terms Related to an I.C. Engine
(a) Bore
It means the inside diameter of cylinder of Engine.
(b) Stroke
The distance moved by piston along the cylinder axis from one end to other end i.e. BDC to TDC is called stroke.
(c) T.D.C.
The end position of piston in cylinder towards cylinder head side is called top dead centre or TDC.
(d) B.D.C.
The end position of piston in cylinder towards crankcase side is called bottom dead centre or BDC.
(e) Size
Size of engine means the stroke volume of all the cylinders of engine which is measured in cu cm (c.c.) by multiplying the cross-section area of cylinder and stroke of piston. So 800 c.c. of engine means total stroke volume of all cylinders in cu. cm
(f) Clearance Volume
The volume contained in the cylinder above the piston when it is at TDC is called Clearance Volume. It is the least volume to which gases/air can be shrinked to.
(g) Total Volume
It is the volume contained in cylinder above the piston when position is at BDC. It is the maximum volume, the gases can occupy in the cylinder.
(h) Swept Volume
It is the difference of total volume and clearance volume or it is the volume which the piston can sweep in one stroke. It is also called stroke volume and stroke volume of all the cylinder contribute to size of engine.
(i) Compression ratio
It is the ratio of total volume to clearance volume. It’s value in S.I. Engine is 7 to 9 and in C.I. Engine is 15 to 24.
(j) Average piston speed
Due to slider crank mechanism and reciprocating motion of piston, it accelerates and de-accelerates continuously. But its average speed can be calculated as 2LN, where L is the length of stroke and N is rpm of engine.