The Engine
The engine is used as a prime mover to get the power needed for a motor vehicle. And also Various fuels are used as the energy source.
The main function of an engine is to convert Thermal energy into kinetic energy when the fuel supplied to the engine is burned.
Various types of engines are used for automobiles. Of those Reciprocating piston engines are commonly used. Therefore, in this article, we will be studying the Reciprocating piston engines that are used as the prime mover of motor vehicles.
The image below shows the structure of an engine with reciprocating motion.
In a cylindrical cube, a piston is inserted to allow systematic upper and lower movement. This motion of the piston is known as reciprocating motion. The piston is connected to the crankshaft by the connecting rod. Depending on the specific shape of the crankshaft, the crankshaft will rotate around its axis as the piston goes up and down linearly.
The cylindrical hole with the piston is called the engine cylinder. The combustion chamber is the closed space confined to the top of the piston by which the fuel is burned by the engine. The volume of the combustion chamber changes with the motion of the piston.
There is always space between the top of the piston and the valve face of the cylinder. Therefore, the face and the face are not touching each other in the Reciprocating motion of the piston. The maximum position the piston reaches in its motion is called the Top Dead Centre – TDC. The minimum point at which face reaches is also known as Bottom Dead Centre-BDC. Therefore the motion of the piston is always confined to the TDC and the BDC. Thus the piston moves from TDC to BDC or BDC to TDC is called a stroke. The volume of the cylinder section between the TDC and the BDC is called the Swept Volume. The closed face of the cylinder above the TDC is called the Clearance Volume.
The compression ratio is the ratio of the total volume of the cylinder to the Clearance volume.
Compression Ratio=(Swept Volume+Clearance Volume )/(Clearance Volume)
There are also two ports attached to the top of the engine cylinder as shown in the figure above. One is to suck the air into the engine for combustion, and the other is to release the remaining combustion gas out the engine after the fuel is burned. Valves are used to open or close the ports as needed. When the valves are open, the combustion chamber and the external environment are connected.
Inlet / Intake / Suction port is used to allow air in the external environment to flow into the engine. The valve attached to it is also called the input / Intake/suction valve. Also, the port used to release combustion gas from the engine is called the exhaust port. The associated valve is also called the Exhaust Valve.
The piston engine, which has a Reciprocating motion, can be classified into two main parts according to their operating cycle. That is,
1. Four Stroke Engine
2. Two Stroke EngineFour Stroke Engine
A four-stroke engine has a four-stroke performance. That is,
- suction / Intake stroke
- compression stroke
- Power Stroke
- Exhaust Stroke
The diagram below shows the engine layout for each of these strokes.
 |
| Image Source: https://www.britannica.com |
1. suction / Intake stroke
During the suction stroke, the piston moves from TDC to BDC. Meanwhile, the exhaust valve is closing and the suction valve opens. The volume of the combustion chamber increases gradually as the piston moves down to the BDC. Then the air pressure inside the cylinder becomes less than the atmospheric pressure. As a result, external atmospheric air flows through the suction port into the engine cylinder. Then the piston moves from TDC to BDC is calling suction / Intake stroke.
2. compression stroke
Once the suction stroke is over, the compression stroke begins. In compression stroke, the piston moves from BDC to TDC. In the meantime, the valves are all closed. As the piston moves to TDC, the pressure in the cylinder increases as the volume of the combustion chamber gradually decreases. Moving the piston from BDC to TDC by increasing the pressure of the air mixture in the cylinder is known as compression stroke.
3. Power Stroke
Once the compression stroke is over, the Power Stroke begins. In power Stroke, the piston moves from TDC to BDC. In the meantime, the valves are all closed. Fuel combustion of the engine cylinder during a power stroke. The heat released here expands the air mixture in the cylinder. This will Be Using a thrust force On the piston and push it towards the BDC. It transmits power to the crankshaft through the connecting rods generated power by the combustion of fuel during the power stroke. thus The motion of the piston from TDC to BDC is called the power stroke. The power stroke is the only time the engine generates power. In all other strokes, a portion of that energy is spent to keep the crankshaft movement steady.
4. Exhaust Stroke
Once the power stroke is over, the Exhaust Stroke begins. During Exhaust stroke, the piston moves from BDC to TDC. In the meantime, the exhaust valve is open and the intake valve is closed. Due to this motion of the piston, the combustion gas is pushed out of the engine cylinder through the exhaust valve and released to the external environment. The movement of the piston from the BDC to the TDC is called the Exhaust Stroke, thus removing the combustion gas from the engine cylinder.
At the end of the Exhaust Stroke the suction strokes again and the cycle continues. The process of completing the above four strokes is known as the operating cycle of the four-stroke engine. For this crankshaft needs to rotate two Rounds (720° ).
Two Stroke Engine
As we mentioned above, a four-stroke engine needs two crankshaft rotating cycles to complete one operating cycle. But to complete a one operating cycle of operation on a two-stroke engine, only one Rotation of crankshaft required. Moving the piston from TDC to BDC in that 360 rotation range is known as the downstroke. The piston then moves from BDC to TDC, also known as the upstroke. Also, in these two strokes, the external air intake the engine, compressing them, Fuel burn, and exhausting the exhaust air from the engine, All four processes are performed.because it requires only two strokes to complete one operating cycle, These engines are called two-stroke engines.
The following image shows a cross-section of a two-stroke engine.
 |
| Image Source: https://www.cycleworld.com/ |
The layout of a two-stroke engine shows significant differences over the layout of a four-stroke engine. A suction port for the intake of external air into the engine is attached to the engine cylinder itself. The port is opened or closed according to its position in the piston's reciprocating motion. Also, the exhaust port used to remove combustion air is attached to the top of the cylinder while a valve is used to open or close it as needed.
Down Stroke
During the downstroke period, when the piston is moving from TDC to BDC, the flow of external air into the engine (suction process) and the combustion air mixture are pushed out of the engine (exhaust process) Both processes occur. firstly the exhaust valve opens and then the combustion air flows out.after the piston gradually moves to down, then and the suction window opens, allowing external air to flow into the engine. In this case, a blower is attached to the suction port to pump out Burned gas in the combustion chamber and to get more air into the engine.
Upstroke
Both compression and fuel combustion processes occur when the piston moves from BDC to TDC (During upstroke). The valves and ports are closed during this period. Because of this, compresses the air mixture in the engine during the beginning part of the up-stroke. The combustion process begins as the piston approaches the TDC.
Also, the fuel combustion process of the engine can also be classified as follows.
1. Spark Ignition Engines
2. Compression-Ignition Engines
In the next article, we will talk about the performance of the aforementioned engine types.
0 coment�rios: