Heat Engine is a machine which converts the heat energy produced by burning of the fuel into mechanical energy.The power House of automobile is the Engine.
Engines are mainly classifieds as Internal&External combustion engines.If the combustion of the fuel takes place outside of the Cylinder then the engine known as external combustion engines. Ex;Steam Engines and Steam Turbines.If the combustion of the fuel take place inside of the working Cylinder, then the engine known as 'internal combustion engines',Eg: Petrol Engine,Diesel Engine, Gas Engine, Gas Turbines. I.C engine mainly Classified As Two stroke Engine , Four stroke Engine.Two stroke Engine If all the events take place within two stroke .i.e, one revolution of crank shaft, then the engine known as two stroke engine. If all the events take place within four stroke, i.e, two revolution of crank, then the engine known as four stroke cyclic engine.According to the types of engine, they are classifieds in to 1. Four stroke cyclic petrol engine,2.Four stroke cycle diesel engine,3.Two stroke cycle petrol engine,4.two stroke type diesel engine. Basic Engine Nomenclature;
The Inside diameter of the Engine Cylinder is known as 'bore'.Cylinder block:Cylinder block is the main body of the engine. It consists of cylinder in which piston slides up and down, ports or opening for the inlet and exhaust valves and passage cooling water circulation. It is made of gray cast iron or aluminium with steel sleeves.It has supporting ribs for cam shaft and crank shaft. The bottom portion is known as sump. Sump contains lubricating oil. Sump is made of pressed steel sheet.Cylinder head :It is fitted on the top of the cylinder by bolts and nuts. It consists of spark plug. combustion champer and passage for cooling water circulation.It made of gray cast iron or aluminium alloy. A gasket is provided in between cylinder block and cylinder head to prevent leakage of gases. Also it consists of provisions for valve mechanism.Cylinder liners:> The piston constantly moving up and down inside the cylinder results wear out of the cylinder. When the cylinder diameter is increased beyond permissible limit, we may have to discard the entire cylinder block, which is costly item.To avoid cylinder wear ,a separate liner in the form of a barrel, made of special alloy iron is inserted inside the cylinder in which the piston will work. Therefore the wear and tear will take place on the cylinder liner only, which can be replaced when it wears out beyond the permissible limit.Crank case: It is cast iron or aluminium case, which bolds the cylinder and crankshaft. It also serves as the sump for the lubricating oil. It is cast separately and then attached to the cylinder block or sometimes be cast integral with cylinder block.oil pan or oil sump: The bottom portion of cylinder block is known as oil pan or oil sump. Its function is to store the lubrication oil. It is made of pressed sheet steel. Piston The piton is connected to a connecting rod which controls its sliding within the cylinder. The movements of the piston changes power stroke of the cylinder. Piston transmits the power during power stroke of the cycles shaft through connecting rod. It consist of grooves for the provision of piston rings.During operation the piston temperature is very high than the cylinder, because the cylinder is surrounded by air or cooling water.Hence the piston expands more then cylinder. This must be controlled in order to avoid loss of piston clearance. This expansion cause serious engine trouble. the expansion of piston skirt can be controlled by following methods; By
keeping heat away from the lower part of the piston as much posible. By making Heat dam, By cam Ground The piston.This can be done in full skirt piston by cutting horizontal and vertical slot in the piston.Cut the groove near the top of the piston, It reduce the size of the path.The heat can travel from the piston head to the skirt. Therefore the skirt runs cooler and less expand.The piston are made in the form os eliptical(oval) when cold. these pistons are known as cam grounded pistons, If the piston warm up, its shap changes into circular. Thus, the piston after expansion at operating temperature become circular.
The piston materials are cast iron, cast steel, and aluminium alloy. To accommodate the expansion of the piston material, a small clearance is provided in between piston and cylinder which is called as piston clearance. the top of the piston known as head or crown. The bottom of the piston known as skirt. Piston consists of three grooves. two grooves are for accommodation of compression rings and third once for oil ring. The part in- between grooves are called as piston lands. The boss is a place where piston pin is fixed. The skirt consists of slots.Based on piston materials two major type of material can use to make piston.1)LO-EX Alloy piston: It is a light alloy aluminium piston with silicon, nickel, magnesium and copper. In this type, the co-efficient of expansion is very low and good heat resisting qualities and hence wear and tear is less.2)Invar Another alloy is 'invar'
containing nickel and iron used for pistons which has very low coefficient of expansion.
PISTON RINGS are placed in the grooves provided in the piston: They are used to maintain a pressure tight seal between the moving piston and the cylinder wall.
They are also used to remove the heat from piston to cylinder and prevent oil from entering the combustion chamber.Two Types Rings used in one piston for various purpose.The piston rings made of fine grained alloy such as cast iron, and stainless steel.Compression Rings Normally two or three compression rings are fitted at the top of the in their grooves to prevent gas leakages. The many types of compression ring available.plain compression ring, Ridge dodging compression ring, Tapper faced compression ring, Tapper sided compression ring, Internally stepped compression ring, oil control.Oil Rings At least one oil ring is provided at the bottom most groove. It wipes off the excess oil in the cylinder wall. They have slots all round.
Connecting Rod :is made of forged steel. It has small End And Big End. Usually big end is of split type. The small end of the connecting rod is attached to piston by means of piston pin and the big end is connected to crank shaft.It converts the up and down motion of the piston to a rotary motion of the crank shaft. The connecting rod usually has I- beam cross section and is made of forged steel. aluminium ally also. It is carefully connected to assembly to maintain engine balance uniformly. The connecting rod carries the power thrust from piston to the crank pin and hence it must be very strong. rigid and light as possible.
Crankshaft: It receives power from the piston through connecting rod and transmits this power to drive. The crankshaft is held in position by the mean bearing.The counter weights are provided to keep the system in perfect balance. It is made of casting or drop forging. The material used for crank shaft is carbon steel or alloy steel.Front end of the crankshaft is connected to pulley, which drives fan,water pump and dynamo, and rear end is connected to flywheel assembly.
Cam Shaft:It is the shaft having valve operating cams. The camshaft receives the power from crankshaft. the speed of the camshaft is half as crankshaft. It also consists of electric cam for fuel pump operating and helical gear to drive the ignition distributer and oil pump. The opening and closing of valves are controlled by means of cam provided on the camshaft.The material used for cam shaft is carbon steel or alloy steel.
Valves: Valve are used to let the air or air- fuel mixture in to the cylinder and also to let out the burnt gases to atmosphere. There are two types of valves. a) inlet valve,b)outlet valve
Inlet valve: The air flow mixture is admitted to the cylinder through this valve only.Generally inlet valves are larger in size.Than the exchaust valve, because the velocity of incoming air or air- flow mixture is less than the velocity of exhaust gases. It made of nickel chromium alloy steel.Exhaust Valve:The burnt gases escape by the exhaust valve only. It is made of silicon- chromium alloy steel, which is corrosive resistance and hat resistance materials. the poppet Type are used in 4 stroke engine.
Timer gears:The crank shaft receives power from crankshaft through a pair of meshing gears provided on each shaft.The pair of meshing gears are called as timing gears. The camshaft gear rotates camshaft at half the crankshaft speed.The drive from crankshaft to camshaft may be either chain or gear drive.Timing gears are made of alloy steel, pins are made of hardened steel and sprockets are made of aluminium, steel or nylon.
Valve actuating mechanism: The engine valves (inlet and exhaust ) are operated from the camshaft in different ways according to their position. The common valve actuating Mechanism is. Side Valve mechanism and Overhead Valve mechanism.
Side Valve mechanism:This type of valve mechanism the camshaft is driven by crank shaft and the speed of the cam shaft is half the engine speed. The cam lifts as tappet and the valve.During this action the valve spring is compressed , energy is stored and the valve opens. Further rotation of the cam, the valve and tappet move downward direction and valve is closed by the valve spring. The Valve spring is held in position by means of spring retainer. the type of valve mechanism is mainly used in L- head T- head and F -head engines.
Valve tappet clearance:A small clearance valve stem and tappet is known as tappet clearance. When The engine becomes hot , the valve expands. This clearance allows the expansion of the valve. The clearance is adjusted by means of adjusting screw provided on the valve tappet. The valve tappet clearance is given by manufactures. It Depends on,a)Material of the Valve,b)Engine Temperature,c)Length of the valve steam.
Over head Valve Mechanism:This type of valve mechanism when the cam shaft rotates the cam lifts the tappet. This tappet lifts the push rod. The one of the push rod is connected to tappet and other end is connected to rocker arm shaft. the upward movement of the push rod causes one end of the rocker arm pushes down the valve stem and energy is stored in valve spring. Due to this action the valve is opened now. Further rotation of the cam causes the tappet to move downwards. Now the valve is closed by means of valve spring.
Four stroke diesel Engine Working Procedure:The construction of four stroke cyclic disel engine is similiar to four stroke petrol engine, but instead of spark plug, diesel engine consists fuel injector:
Diesel Engine also called as compression ignition(C.I) engine. In a four stroke cyclic disel engine, the four strokes are completed within two revolution for one cycle of operation. this cycle of operation is known as diesel engine cycle.
1.Suction stroke: During this stroke , the piston moves downward from TDC to BDC. Therfore the increases and the pressure decreases. Pressure inside the cylinder is below the atmosphere pressure. So that the fresh air alone from air filter is admitted in to the engine cylinder, through the inlet valve. In this stroke, the inlet valve is opened and the exhaust valve is closed. During this stroke , the crank made half revolution.
2.Compression stroke:During this stroke, the piston moves upward from BDC to TDC . Therefor the volume decreases and the pressure increase during the upward movement of the piston to compress the air alone. The compression ratio for diesel engines varies from 16 to 20. The pressure at the end of the stroke is about 3500kN/m2 to 4000KN/m2 and the temperature is about 600 degree to 700 degree. During this stroke, both inlet and exhaust valves are closed. the crank made second half revolution. the above two strokes complete one revolution of the crank shaft.
3.Working or Power stroke:During this stroke, both the valves remain closed. Shortly before the piston reaches TOP, the fuel (diesel) is injected in the form of fine spray in to the engine cylinder through the fuel injector. At that time, the fuel is ignited by the temperature of hot air inside the engine cylinder. Now the pressure and the temperature are increased. The piston move downward from Top to bottom some force. At the end of combustion the pressure is above 4000 KN/m2 and temperature is above 1000 degree Celsius now the crank made another off rotation.
Exchaust stroke: During this stroke , the exhaust valve opens and the inlet valve is closed. Now the piston moves from BDC to TDC and the exhaust gases are exhausted through the exhaust valve in to atmosphere. Now the crank made 4 th half revolution. These stroke complete on cycle of operation. Again the inlet valve open and the same process are repeated.