Internal Combustion (IC) Engine
One Cycle consists of 4 operations:
(1) Suction / Induction / Scavenge
(2) Compression
(3) Power / Expansion / Working / Firing
(4) Exhaust
One complete sequence of 4 OPERATIONS (i.e. One Cycle) is completed in:
- TWO REVOLUTIONS of the Crank Shaft (or) FOUR STROKES of the Piston
= 4 Stroke Cycle Engine
(ii) ONE REVOLUTION of the Crank Shaft (or) TWO STROKES of the Piston
= 2 Stroke Cycle Engine
Cycle of Operations/ Working of 2 Stroke Cycle Diesel Engine
Process of Compression & Firing:
As the piston moves up, the exhaust and scavenge ports are covered by the piston
Fuel valve is kept shut
Air previously taken into the cylinder is compressed to above 40 bar and 5500C. This is called Compression Stroke
Fuel is then injected into the cylinder
Fuel mixes with the air, burns and gives out heat. This is called combustion / firing
High pressure gases that are produced pushes the piston downwards. This is called power stroke
Process of Exhausting and Scavenging:
As the piston continues to move downwards, it uncovers the exhaust ports
Exhaust gases are blown out under own pressure
As the piston continues to move downwards, it uncovers the scavenge ports
Fresh air is blown into the cylinder
Fresh air first scavenge out the leftover exhaust gases which are still present in the cylinder
Fresh air secondly fill the cylinder (i.e. provide charged air) for next compression
Efficient scavenging is essential to ensure a sufficient supply of fresh air for combustion
In a four stroke cycle engine there is a sufficient overlap between the air inlet valve opening and exhaust valve closing
But in a two stroke cycle engine overlap between air inlet port opening and exhaust port closing is limited
This results in slight mixing of exhaust gases with the incoming fresh air
Different methods of Scavenging:
3 methods of scavenging are in use in a two stroke cycle engine
- Cross flow scavenging
- Loop scavenging
- Uniflow scavenging (modern engines use this method – An exhaust valve is fitted on the cylinder head)
Cross flow Scavenging
Scavenge port and exhaust ports are placed on opposite sides of the cylinder
As the piston moves downward, it first uncovers the Exhaust port
Exhaust gases are blown out under own pressure
As the piston continues to move downwards, it uncovers the scavenge ports
Fresh air is blown into the cylinder
Fresh air scavenge out the leftover exhaust gases which are still present in the cylinder
Loop Scavenging
Scavenge port and exhaust ports are placed on the same side of the cylinder
As the piston moves downward, it first uncovers the Exhaust port
Exhaust gases are blown out under own pressure
As the piston continues to move downwards, it uncovers the scavenge ports
Fresh air is blown into the cylinder
Fresh air scavenge out the leftover exhaust gases which are still present in the cylinder
Uniflow Scavenging
An exhaust valve is fitted on the cylinder head
As Piston moves downward to a certain distance, Exhaust valve is opened by a cam mechanism
Exhaust gases are blown out under own pressure
As the piston continues to move downwards, it uncovers the scavenge ports
Fresh air is blown into the cylinder
Fresh air scavenge out the leftover exhaust gases which are still present in the cylinder
Supercharging:
To burn larger mass of fuel in the cylinder, a larger mass of air should be supplied to the cylinder
Mass of air can be increased by increasing the pressure of the air / scavenge air
The process of increasing the pressure of the scavenge air and then supplying this pressurised scavenge air to the engine is called supercharging
This is achieved by using Turbochargers
Turbochargers consist of a gas turbine and a rotary air compressor/blower (both mounted on the same shaft)
Gas turbine is driven by the engine exhaust gas
Rotary air compressor compresses the scavenge air (i.e. it increases the pressure of the scavenge air)
Turbocharger converts the waste energy of exhaust gas into useful work
This pressurised scavenge air is used for supercharging