2.4 Working of IC Engines
The four-stroke-cycle principle can be described as follows:
- During the intake stroke, the piston travels downward in the cylinder from TDC to BDC, the volume of the cylinder chamber is enlarged. This enlargement of the cylinder chamber causes a decrease in air pressure which creates a partial vacuum inside the cylinder. Atmospheric pressure forces air through the carburetor, where it mixes with gasoline and enters the cylinder through the open intake valve. The exhaust valve remains closed during this stroke and the crankshaft turns through half a revolution.
- The intake valve closes, the piston moves up from BDC to TDC, and the gas in the cylinder is compressed in the combustion chamber. This stroke is called the compression stroke and, depending on the type of engine, it usually ranges between 850 kPa and 1000 kPa. Atmospheric pressure is 100 kPa.
- The fuel is now ready to be ignited. This is accomplished by an electric spark at the spark plug. Combustion immediately takes place, and the gas, as it burns, heats and expands instantly. The rapid expansion of the gas greatly increases the pressure in the cylinder. This pressure increase in approximately five times greater than the compression pressure, being between 4200 kPa and 4900 kPa and forces the piston down from TDC to BDC, causing the crankshaft to turn. This is known as the power stroke. Both valves remain closed during this stroke.
- The crankshaft has now rotated 1.5 revolutions, and the cylinder has become filled with burnt gases that must be removed. The exhaust valve opens, and the piston moves from BDC to TDC, forcing the burnt gases out of the cylinder. This known as the exhaust stroke. During this stroke the intake valve remains closed. Now the crankshaft has completed two revolutions. The piston is at top dead centre and the engine is ready to repeat the cycle of operations.
2.4.1 Working of four Stroke Cycle Petrol Engine
Working cycle compiled in four stroke (suction stroke, compression stroke, expansion & exhaust) or two revolution of the crank.
2.4.2 Working of Four Stroke Diesel Engine
- Suction Stroke – It is similar to the suction stroke S1 engine. Inlet valve open and exhaust valve is closed position during this stroke (piston moves from TDC to BDC in 180° crank rotation). Only air inducted in to the cylinder head.
- Compression stroke – both inlet and exit valve is closed position during this stroke. (180° to 360° of crank rotation). At the end of this stroke the temperature and pressure of air increased about 600°c and 60 bar respectively.
- Expansion Stroke – In this stroke the inlet and exhaust valve remain closed. The injector inject the fuel to the cylinders with very high pressure. Due the high temperature of air inside the cylinder sufficient to ignite the fuel. The combustion of fuel started and continued at constant pressure. So there is sudden rise of pressure and volume inside the cylinder pushes the piston downward.
- Exhaust stroke – during this stroke inlet valve closed and exhaust valve remain open. The piston moves up in the cylinder and pushes out burned gas. The piston reaches the TDC completely the inlet valve open and cycle starts again.
2.4.3 Working of Two Stroke Cycle IC Engine
Engine working on two-stroke cycle complete in one revolution of the crankshaft or two strokes of the piston. The following section explains the working of two strokes SI and CI engines.
2.4.4 Working of Two Stroke Petrol Engine (SI Engine)
The piston moves up from bottom dead center (BDC) to top dead center (TDC). Both the transfer and exhaust ports covered by the piston. Air fuel mixture from carburetor is transferred already into engine cylinder is compressed by the moving piston. The pressure and temperature increases at the end of compression.
The piston reaches the TDC the charge ignited by means of electric spark provided by the spark plug. At the same time the inlet the piston uncovers port and fresh charge enters the crankcase the inlet port as shown in Figure
The burning gases expanded in the cylinder and due to this the piston moves down, thus useful work obtained. During this movement charge in the crankcase is partially compressed and this is known as crank case compression.
At the end of expansion, exhaust port is uncovered. The burnt gases escape to atmosphere, during this time the transfer port is also open. The partially compressed charge from crankcase enters to the cylinder through transfer port. The special shape provides the cycle reduce the escape of fresh charge along with the burned product.
Again the piston moves up, the transfer port is first closed and then exhaust port is closed. The compression of charge takes place inside the cylinder. When it reaching TDC the inlet port uncovered and low pressure is created to bring fresh charge to crank case. After compression, the ignition of charge take place and the cycle is repeated. All the events are completed in two strokes of the piston and the crankshaft makes one resolution.
2.4.5 Working of Two Stroke Diesel Engine (CI Engine)
The operation of the diesel engine same as explained above example only air enters to the crank case and fuel injected at the end of compression stroke. The fuel mixes with hot air and burns and moves as explained above
The compressed charge passes through the transfer port into the engine cylinder flushing the products of combustion. This process is called scavenging. The top of the piston and the ports are usually shaped in such a way that the fresh air is directed towards the top of the cylinder before flowing towards the exhaust ports. This is for the purpose of scavenging the upper part of the cylinder of the combustion products and also to minimize the flow of the fresh fuel-air mixture directly through the exhaust ports. The projection on the piston is called deflector.
Comparison between four strokes and two-stroke cycle engine
Comparison between Petrol and Diesel Engine