FOUR-STROKE ENGINE
Hello mechons, in this sparking day we are going to explain to you about what is Four-stroke engine, the parts of a four-stroke engine, what are the types of a four-stroke engine, the working of the four-stroke engine also the advantages and disadvantages of Four-Stroke engine with its applications.
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| (Four-Stroke engine) |
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What is the Four-Stroke Engine?
A four-stroke engine is one of the categories of Internal combustion(IC)engine, where four strokes take place successively. They are the
- Suction stroke
- Compression stroke
- Power stroke or expansion stroke
- Exhaust stroke
which will be completed on two revolutions of the crankshaft. So, these categories of engines are so-called Four-stroke engine.
Nowadays, almost all the automobiles in the world function on the basis of four-stroke engines only.
There are some basic terminologies that one must learn before knowing about the four-stroke engine they are:
It is the uppermost end position of the piston at the head of the cylinder.
At TDC the angle of the crank is supposed to be at 0'.
2. Bottom dead center (BDC):
The lower end position of the piston at the end of the crank is called the bottom dead center. At BDC, the angle of the crank is supposed to be at 180'.
3. Cylinder bore:
The inner diameter of the cylinder is called the cylinder bore.
4. Stroke length:
The distance through which the piston moves within the cylinder of the engine during one stroke is known as stroke length.
Parts(Components) of a Four-Stroke engine:
The Four-stroke is comprised of the following parts. They are:
It is also referred to as the heart of the engine. The piston of the engine moves within the cylinder.
It is the topmost position of the cylinder nearer to Top dead center(TDC), and it is called a cylinder head.
The reciprocating member of the engine which moves within the cylinder is called Piston.
The piston is provided with some ring-like structures in it. So, it is called Piston rings. And it is sealed between the gap cylinder lines and the piston. It is responsible for carrying the lubricants between those parts, i.e. Piston and Cylinder.
The rotating member of the engine which makes circular motion inside the crankcase is known as a crank.
It is one of the parts of the engine which provides shelter for the crank and other engine parts. It also acts as the sump for storing lubricants in it.
The part of the engine which converts the reciprocating motion of the piston into rotary motion of the crankshaft is called the connecting rod.
The rotating member of the that connects the crank and connecting rod is known as the crankshaft.
The purpose of this part is to provide cooling for the engine.
It is provided for operating the function of opening and closing of both Inlet and Exhaust valves and it also operates the fuel injection pump in case of a Diesel engine.
The valve which controls the entry of fuel or air-fuel mixture into the cylinder of the engine is called the Inlet valve.
The valve which controls the process of removing burnt gas after combustion is known as the exhaust valve.
The passage which carrying the fresh fuel or air-fuel mixture is known as the intake manifold.
The passage which is carrying out the exhaust gas from the engine cylinder is known as exhaust manifold.
It is the small part of a Petrol engine or SI engine which is used to produce a spark to burn the fuel.
The part which is only used in a Diesel engine or CI engine for spraying the fuel inside the engine cylinder is called the fuel injector.
It is a part that is used in a Petrol engine to pass the air-fuel mixture into the cylinder and also for maintaining the mixture incorrect ratio.
It is a part that mounted over the crankshaft and made up of cast iron. It is responsible for storing energy in the form of inertia and also for converting it into mechanical work.
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| (Parts of an IC engine) |
Classification of Four-stroke engine:
The Four-stroke engine is basically classified into two types:
- Four-stroke - Petrol engine
- Four-stroke - Diesel engine
Four-Stroke Petrol engine:
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In this picture the numbers 1,2,3, 4 denote the Suction stroke, compression stroke, power or expansion stroke, and exhaust stroke respectively.
Construction of a Four stroke - petrol engine:
A four-stroke petrol engine consists of:
- Cylinder
- Spark plug
- Piston with piston rings
- Connecting rod
- Crank
- Crankshaft
- Valves etc.,
In four-stroke engines, ports are replaced by means of using valves.
There are two valves:
- Suction valve
- Exhaust valve
And those valves are operated with the help of a Camshaft which is rotated at speed of half the speed of a crankshaft.
Working of Four stroke - petrol engine:
The piston moves from the top dead center to the bottom dead center is called the piston stroke and it consists of four strokes:
- Suction Stroke
- Compression Stroke
- Power or Expansion Stroke
- Exhaust Stroke
- In the Suction stroke, the suction valve opens, exhaust valves remain under closed condition. The piston starts moving from TDC to BDC and the air-fuel mixture enters into the engine cylinder due to the partial vacuum pressure produced inside the cylinder.
- At once, the piston reaches the BDC, the inlet valve closes and a half revolution takes place at the flywheel.
- In the compression stroke, the piston starts moving from BDC to TDC, at that time both the suction and exhaust valves are in the closed condition.
- The air-fuel mixture, which was entered into the cylinder during the suction stroke, is now compressed into the combustion chamber where the heat which is produced during the compression stroke and vapourises the mixture.
- As the piston approaches the TDC, the compression stroke ends with a spark created from the spark plug.
- Once again both the inlet and exhaust valves are kept at the closed conditions in this stroke.
- The compressed fuel gets ignited by the spark which has high intensity and expands adiabatically,
- Therefore, the piston starts moving downward i.e. towards BDC then the stroke is completed.
- During the exhaust stroke, the inlet valve is kept in the closed condition and the exhaust valve is kept open. The piston starts moving up from BDC to TDC.
- The stroke is completed at once the piston reaches the TDC.
- And again these strokes are repeated to carry the next cycle.
In the exhaust stroke at a particular time and angle, both the valves are in an open condition for some time period. This time period is called valve overlap and the angle is known as the Angle of overlap. It takes because at this time and angle the fresh air-fuel mixture or fuel is entered to the engine cylinder and pushes out the burnt gases from the engine cylinder. And this process is called Scavenging.
These are the working process of the Four-stroke Petrol engine.
Valve timings of Four stroke petrol engine
- The Inlet valve will open when the piston is at Top Dead Center(TDC).
- The Inlet valve will close when the piston reaches Bottom Dead Center(BDC)
- Spark produces when the piston reaches Top Dead Center.
- The exhaust valve will open when the piston at Bottom Dead Center.
- The exhaust valve will close when the piston reaches the Top Dead Center.
Actual valve-timings:
- The Inlet valve opens at an angle of 10'-20' before TDC
- The Inlet valve closes at an angle of 30'-40' after BDC.
- Spark produces at an angle of occurs 20'-35' before TDC.
- The exhaust valve opens at an angle between 30' and 50' before BDC
- The exhaust valve closes at an angle of 10'-20' after TDC.
Four-stroke petrol engine's application :
It has various types of usages like:
- In automobiles generally on two-wheelers, also in cars, trucks, etc.,
- Small pump sets
- Electrical generator and so on.
Four-Stroke Diesel engine:
Construction of Four-stroke Diesel engine
A Four-stroke - Diesel engine contains the following parts. They are:
- Cam
- Camshaft
- Crankshaft
- Cylinder
- Connecting rods
- Fuel injector
- Fuel pump
- Inlet and exhaust valves
- Piston etc.,
Working of Four-stroke- Diesel engine:
The Four-stroke diesel engine functions as similar to that of the Petrol engine. Here also the four strokes take place successively. They are :
- Suction Stroke
- Compression Stroke
- Power or Expansion Stroke
- Exhaust Stroke
- In a suction stroke, the inlet valve is in the open and the exhaust valve remains in closed condition.
- The air is sucked into the cylinder as the piston moves from TDC to the BDC due to the development of the partial vacuum air is entered inside the cylinder.
- At once, the piston reaches the BDC, the inlet valve closes and a half revolution takes place at the flywheel.
- In this stroke Piston reciprocates from the bottom dead center to the top dead center as a result of this the air which is present inside the engine cylinder is compressed at high pressure and heated.
- In compression stroke, both the inlet and exhaust valves are in the closed condition. A very high temperature is produced during this stroke.
- At the last stage of this compression stroke, the fuel injector injects the fuel at high pressure into the compressed hot air, thus the ignition carried out inside the cylinder.
- During the power stroke, both the inlet and exhaust valves are kept in the closed conditions.
- The piston at TDC is pushed downward by the expansion of burnt gas. Therefore a Power stroke is produced.
- In exhaust stroke, the exhaust valve is opened and the inlet valve is in a closed condition. The piston moves from BDC to TDC and it pushes the burnt gas out of the cylinder.
- And here also the scavenging process happened for the removal of the burnt gas and admittance of fresh fuel.
valve timings of Four stroke Diesel engine
Actual valve-timing:
- The Inlet valve opens at an angle of 10'-30' before TDC
- The Inlet valve closes at an angle of 20'-50' after BDC
- The exhaust valve opens nearly at an angle of 40' before BDC
- The exhaust valve closes at an angle of 15' after TDC
- Fuel injection starts at an angle of 5' - 15' before TDC
- Fuel injection stops at an angle of 15' -24' after TDC.
Four-Stroke Diesel engine Applications :
The four-stroke Diesel engine is one of the most popular varieties of engines over a long time. It has various types of applications like:
- Small pumping sets
- In the automobile industries
- Air compressors
- Drillers
- Boats & Ships
- Power saw
- JCB's
- Tanks
- Diesel-electric locomotives and so on.
Advantages of Four -Stroke engine:
- High torque
- High fuel economy than a two-stroke engine
- Consumption of lubricants is low
- Pollution caused is less due to the proper burning of fuel
- Durability is more
- Proper cooling system, so that the life of the engine is longer.
- The Thermal efficiency and HP of this type of engine is relatively high.
Disadvantages of Four-stroke engine:
- Many moving parts, so there is more friction and hence the engine has less mechanical efficiency.
- The complicated design of the engine.
- The weight of the engine is more because of the heavy construction of flywheel.
- Due to the presence of more parts the maintenance of the engine often required.
Fuel Supply System in Four-Stroke engine:
Fuel supply system in Four-stroke petrol engine:
The petrol engine fuel supply system consists of:
- Fuel feed system
- Fuel injection system
- Fuel tank
- Fuel lines
- Fuel filter
- Fuel pump
- Carburetor
At first, an electric petrol pump or mechanical petrol pump sucks the fuel from the fuel tank via a filter then the fuel through fuel lines passes to the carburetor where the air-fuel mixed with proper ratio and enters into the engine cylinder for combustion.
Fuel supply system in Four-stroke Diesel engine:
Diesel engines fuel supply system consists of:
- Diesel tank
- Feed pump
- Filter
- Injection pump
- Injector
- Connecting lines
There are two categories of the fuel injection system of the Diesel engines. They are:
- Air injection system or Indirect injection
- Solid injection system or Direct injection
Four-stroke engine Lubrication system:
The supply of lubricating oil between two moving parts of the vehicle is called lubrication.
Needs for lubrication in the engine :
The primary objectives of lubrication are:
- To reduce friction between moving the parts to the minimum extent so that power loss is less.
- To reduce wear and tear of moving parts.
- To cool the system when it is overheated.
- It also provides cleaning processes of removing waste from inside the system.
Parts that are needed to be lubricated in an engine :
The following parts in the engine required to lubricated. They are
- Crankshaft bearings
- Both ends of connecting rod bearings
- The inner wall of the cylinder
- Valve openings
- Timing gears
- Camshaft bearings
Cooling system of Four-Stroke engine:
All the heat that is produced by the combustion of the fuel is not completely or totally converted into power at the crankshaft. Its only nearly about 20% of the heat is used as the power for the crankshaft. And nearly 35% of heat is transferred to the cylinder wall that causes pre-ignition of the fuel or air-fuel mixture and also the lubricants were burnt away due to that heat, If the engine was not cooled properly it maybe lead to the seizing of the engine. There are two types of cooling processes available for the engines. They are
1. AIR COOLING SYSTEM
2. LIQUID COOLING SYSTEM
Ways to improve the efficiency of Four-stroke engines
Still, there are many methods that have been implemented to extract the vast amount of waste heat which is expelled at the exhaust of the engine and convert it into some useful work, that decreases the exhaust pollutants in the mean time.
Some of those ways are
1. Usage of Rankine Cycle,
2. Supercharging
3. Turbocharging and thermoelectric generation can be very useful as a waste heat recovery system.
Also read:
2) Two-Stroke engine with its types and working principles
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