HEAT ENGINE
Heat engine - Heat engine is the machine which converts heat energy into mechanical energy. Heat engine are of two types.
a. External combustion engine - In External combustion engine fuel is burnt or ignited outside the engine cylinder. So it is called as external combustion engine e.g. Steam engine.
b. Internal combustion engine - In internal combustion engine fuel is burnt or ignited inside the engine cylinder so it is called internal combustion engine e.g. Petrol engine, diesel engine.
They are of two types
a. Spark ignition engine - In this type of engine fuel is injected along with air in the cylinder and after compression this mixture is given electric spark due to which fuel starts burning and produces power. e.g. Petrol engine
b. Compression ignition engine - It is an engine in which the fuel is ignited in to charge of compressed air and ignited spontaneously by the high temperature of the air induced by the heat of compression. e.g. Diesel engine.
Combustion - Burning of fuel with air is called combustion.
Piston stroke - The movement of piston from top dead centre to bottom dead center is called the piston stroke.
Dead centre - The place beyond which the piston cannot move further in a cylinder is called dead center.
Top dead centre - The end towards the head of the cylinder is called top dead center.
Bottom dead centre - The end towards the base of the cylinder is called top dead center.
Total Volume - The area between piston and cylinder head when the piston is at BDC is called total volume.
Swept Volume – When the piston travel during one stroke then the area covered by it, is called swept volume.
Clearance Volume – The area above piston when it is at TDC is called Clearance Volume.
Compression ratio - The ratio of total volume and clearance volume is called compression ratio.
Compression Ratio = Total volume Clearance
Volume
Total volume = Swept volume + clearance Volume
Diesel cycle - In the compression ignition engine to convert heat energy into mechanical energy the following actions takes place in fixed sequence, when this action is completed once is called Diesel cycle.
a) Suction – In this atmospheric air enters in to the cylinder.
b) Compression - The air inside the cylinder is compressed with the help of piston due to which pressure increases resulting in rise in temperature.
c) Fuel injection - Injection of fuel in atomized form at right time when air is at high temperature.
d) Power - When fuel burns inside the cylinder the heat energy is discharged leads piston to move downwards.
e) Exhaust – The gasses produced in cylinder is exhausted to atmosphere.
Depending upon the cycle diesel engine is classified into two types.
1) Two stroke Engine - In this engine one cycle is completed by two piston strokes. All four actions (Suction, compression, power and Exhaust) are completed in one revolution ( 3600 ) of the crankshaft In this cycle piston starts from BDC and every downward stroke is power stroke, to complete one stroke the crankshaft rotates 900 only.
a) Suction - when piston is at BDC fresh air or mixture (petrol and air) is admitted into the cylinder from ports specially provided in cylinder liner.
b) Compression - when piston travel in upward and cover the ports, compression starts and completes when piston reaches TDC.
c) Power - When the piston reaches TDC at this time fuel is injected into the cylinder in atomized form, exhaust gases are produce and push the piston downwards. In this stroke piston travels from TDC to BDC.
d) Exhaust - When piston travel half distance during downward stroke either exhaust valve or exhaust port open and exhaust gases goes out. This stroke completes when piston reaches BDC.
2) Four stroke Engine - In this type of engine the cycle completes in two revolution of the crankshaft (7200 )
a) Suction stroke - In this stroke piston moves from TDC to BDC during this time inlet valve is open and exhaust valve remains closed and the air from the V gallery enters into the cylinder through inlet valve.
b) Compression stroke - In this stroke the piston moves from BDC to TDC both inlet and exhaust valve will remains close. Air in the cylinder is compressed due to which the temperature and pressure increases. Before the piston reaches at TDC the fuel is injected into the cylinder through injector in atomization form, fuel starts burning.
c) Power stroke - when the fuel burnt gases is created this tends the piston move from TDC to BDC. In this stroke both exhaust and inlet valve close. Before reaching to the BDC, Exhaust valve opens.
d) Exhaust stroke - In this stroke piston travels from BDC to TDC this time inlet valve remains closed but exhaust valves are opened and gases goes out.
Scavenging: It means kick out the residual exhaust gasses from cylinder with the help of fresh charge air. It is essential in super charged engine that the cylinder be adequately scavenged before the fresh air charge is compressed otherwise such charge is contaminated by the residual exhaust gases from the previous cycle.
Different between two stroke and four stroke engine
