Four Stroke Cycle Engines

FOUR STROKE CYCLE ENGINES

Cycle of Operation: There are distinctly four strokes, viz., Suction Stroke, Compression Stroke, Expansion Stroke and Exhaust Stroke for different Operations in a Cycle. Each stroke is identified as per the function.

Classification of 1.C. Engines: The internal combustion engines are classified as Four Stroke and Two Stroke Engines.

Four Stroke Cycle Engine: In the four stroke engine, there is one power stroke in every four strokes or during two revolutions of the crank. The four stroke engines are further classified as Four Stroke Petrol Engine and Four Stroke Diesel Engine according to the type of fuel used in the engine.

 

1. FOUR STROKE PETROL ENGINE

Petrol Engine is also known as Spark Ignition (S.I.) Engine. Four Stroke Petrol Engine requires four strokes of the piston to complete one cycle of operation in the engine cylinder.

See Fig. 2. It consists of a cylinder. Its one end is fitted with a cover and the other end left open. The cover is provided with inlet and exhaust apertures. These apertures are opened and closed by inlet and exhaust valves. A spark plug initiates the ignition of the fuel. The piston reciprocates inside the cylinder. The connecting rod and crank convert the reciprocating motion of the piston into rotary motion.

Power Cycle of Petrol Engine – Otto Cycle

The petrol engine works on the principle of Otto Cycle, also known as Constant Volume Cycle. Fig. 3 shows the Pressure Velocity Diagram of Theoretical Otto Cycle.

1. Suction Stroke: Fig. 2(a)

During suction stroke, the Inlet valve (I) opens and air and fuel (petrol) mixture (charge) is sucked into the cylinder. The piston moves downward from Top Dead Center (TDC) till it reaches Bottom Dead Center (BDC). During suction stroke the Exhaust value (E) is closed.

See Fig. 3. Suction stroke is theoretically represented by the horizontal line 1-2 in the PV Diagram. The drawal of air-fuel mixture is taking place at atmospheric pressure.

2. Compression Stroke: Fig. 2(b)

 During this stroke, both the inlet and exhaust valves are closed. The air-fuel mixture is compressed as the piston moves upwards from BDC to TDC. The compression ratio in petrol engines varies from 7 to 10. As a result of compression, pressure and temperature of the charge are increased to 15-20 bar and 400°C respectively.

See Fig. 3. The process of compression is theoretically represented by the curve 2-3 in the PV Diagram.

Shortly before the piston reaches TDC, the charge is ignited by means of a Spark Plug. It suddenly increases the pressure and temperature of the products of combustion, but the volume remains constant.

During the burning process, the chemical energy of the fuel is converted into heat energy, producing a temperature rise of about 2000° C.

See Fig. 3. This constant volume combustion process is theoretically represented by the vertical line 3-4 in the PV Diagram.

3. Expansion or Power or Working Stroke: Fig. 2(c)

During this stroke, both the valves remain closed. Due to the rise in pressure, piston is pushed down with a great force. The hot burnt gases expand pushing the piston from TDC to BDC. It is also called Working Stroke as work is done by the expansion of hot gases.

See Fig. 3. The expansion stroke is theoretically represented by the curve 4-5 in the PV Diagram.

At or near the end of the expansion stroke, the exhaust valve opens to release the burnt gases to the atmosphere. This suddenly brings down the cylinder to atmospheric pressure.

This drop in pressure at constant volume is theoretically represented by the vertical line 5-2 in the PV Diagram as shown in Fig. 3.

4. Exhaust Stroke: Fig. 2(d)

During this stroke, the exhaust valve opens, as piston moves from BDC to TDC. This movement of the piston pushes out the exhaust gases from the cylinder. The exhaust gases are exhausted through the exhaust valve into the atmosphere.

See Fig. 3.

The exhaust stroke is theoretically represented by the horizontal line 2-1 in the PV Diagram.

Uses: Four stroke petrol engines have higher load carrying capacities than two stroke petrol engines. Hence, they are used in high power – high speed motor cycles and passenger cars.

 

2. FOUR STROKE DIESEL ENGINE

The basic construction of a four stroke cycle diesel engine is the same as that of four stroke cycle petrol engine, except that instead of a spark plug, a fuel injector in mounted in its place. A fuel pump supplies diesel to the injector at higher pressure.

Dr. Rudalf Diesel invented the Diesel Engine. It is also known as Compression Ignition (C.I.) Engine, since ignition takes place due to the high temperature produced during the compression of air in engine cylinder. Liquid fuel, i.e., diesel, which cannot be vapour-ized, is injected into the cylinder in the form of fine spray using fuel pump and injector.

Power Cycle of Diesel Engine - Diesel Cycle

 Diesel engine works on the principle of Theoretical Diesel Cycle, also known as Constant Pressure Heat Addition Cycle. Fig. 5 shows the Pressure Velocity Diagram of the same. The ideal sequence of operation for the four stroke C.I. engine is explained as follows:

1. Suction Stroke: Fig. 4(a)

During suction stroke, inlet valve (I) opens and exhaust valve (E) remains closed. The piston travels downwards from TDC. Air is drawn in, from outside to fill the cylinder through the inlet valve till the piston reaches BDC. The air taken in is at atmospheric pressure.

Suction stroke is theoretically represented by the horizontal line AB in the PV Diagram in Fig. 5.

2. Compression Stroke: Fig. 4(b)

 At the end of the suction stroke, both the inlet and the exhaust valves remain closed. The piston moves upwards from BDC to TDC. The air sucked in during suction stroke is compressed to a high pressure (35 – 40 bar) and temperature with a decrease in volume. These two strokes, viz., suction stroke and compression stroke complete one revolution of the crankshaft.

The compression stroke is theoretically represented by the curve BC in Fig. 5.

3. Expansion or Power or Working Stroke: Fig. 4(c)

Just before the beginning of this stroke, fuel (diesel) is injected in the form of fine spray into the cylinder through the Fuel Injector. At this moment, the fuel is ignited by the temperature of the hot compressed air and it starts burning at constant pressure.

Due to the high compression ratio of 16 to 20, the temperature at the end of compression stroke is more than 550°C. This temperature is sufficient to ignite the fuel, injected into the combustion chamber. The fuel is continuously injected for 20% of the expansion stroke.

The ignited air-fuel mixture expands and forces the piston downwards from TDC to BDC. During this constant pressure expansion stroke, both the valves remain closed.

See Fig. 5. This constant pressure expansion with simultaneous combustion is theoretically represented by the horizontal line CD in the PV Diagram.

The piston is forced further during the remaining part of the expansion stroke due to the expansion of the burnt gases. [The linear motion of the piston causes the piston to produce the mechanical work during this stroke.]

As the piston moves, the pressure of the hot gases gradually decreases. The expansion of the burnt gases is theoretically represented by the curve DE in the PV Diagram as in Fig. 5.

At the end of the outstroke, the exhaust valve opens. Some of the burnt gases escape into the atmosphere from the cylinder through the exhaust outlet at constant volume. This is theoretically represented by the vertical line EB

4. Exhaust Stroke: Fig. 4(d)

During the exhaust stroke, the inlet valve is closed and the exhaust valve is opened. The piston is on its upstroke from BDC to TDC, forcing the burnt gases out of the cylinder through the exhaust valve.

See Fig. 5.

The exhaust stroke is theoretically represented by the horizontal line BA. Expansion and exhaust stroke complete one revolution of the crankshaft. This completes the cycle and the engine cylinder is ready to suck the fresh air once again.

Uses: They are used in heavy-duty transport vehicles such as trucks, tractors, bulldozers, etc., power generation, industrial and marine applications.