Valve Timing Diagram Of 2 Stroke Ci Engine
A four-stroke engine is an Internal combustion engine, where four successive strokes (i.e. Suction-Compression-Power-Exhaust) completes in two revolutions of the crankshaft. Therefore, the engine is called a Four-stroke engine.
In recent days the majority of automobile runs on a four-stroke cycle.
Basic some terms used in this article:
Top dead center (TDC):
It is the extreme position of the piston at the head end of the cylinder.
At TDC the crank angle is 0*
Bottom dead center (BDC):
The extreme position of the piston at the crank end side.
At BDC, the crank angle is a 180*
Cylinder bore:
It is the internal diameter of the cylinder.
Stroke:
It is the distance through which the piston moves in the cylinder during 1 stroke.
NOTE: * = Degree
Four-Stroke engine's parts:
A Four-stroke engine consist of the following parts:
- Cylinder: It is the heart of the engine. The piston reciprocates in the cylinder.
- Cylinder head: It is the top cover of the cylinder, towards TDC, which is called the cylinder head.
- Piston: It is the reciprocating member of the engine. It reciprocates in the cylinder.
- Piston rings: Two or three piston rings are provided on the piston. It seals the gap between the cylinder liner and piston.
- Crank: It is a rotating member. It makes a circular motion inside the crankcase.
- Crankcase: It is the housing of crank and other engine parts. It is also used as the sump of lubricating oil.
- Connecting rod: It is used to convert the reciprocating motion of the piston to rotary motion of the crankshaft.
- Crankshaft: It is the rotating member, which connects the crank.
- Cooling fins or Water jackets: It is used for cooling purposes.
- Cam and Camshaft: It is provided to operate the opening and closing of the Inlet and Exhaust valve and also operate the fuel injection pump in the Diesel engine.
- Inlet valve: This valve controlled the admission of charge or air inside the engine cylinder.
- Exhaust valve: This valve controls the removal of burnt gas after combustion.
- Intake manifold: This is a passage that carries the fresh charge or air.
- Exhaust manifold: This a passage through which the exhaust gas goes out of the engine cylinder.
- Spark plug: It is used in a Petrol engine or SI engine to ignite the fuel.
- Fuel injector: It is used in a Diesel engine or CI engine to sprayed the fuel inside the engine cylinder.
- Carburetor: It is used in a Petrol engine to mix the air-fuel properly.
- Flywheel: It is mounted on the crankshaft and is made of cast iron. It stores energy in the form of inertia.
Classification of 4-stroke engine:
The four-stroke engine generally classified in 2 types:
- The four-stroke cycle petrol engine
- Four-stroke cycle diesel engine
Four-stroke Petrol engine:
This type of engine based on the Otto cycle. The various strokes in the operation of a four-stroke cycle petrol engine are described below.
Construction:
A four-stroke petrol engine consists of:
- A cylinder
- Cylinder head attaches with spark plug
- Piston attach with piston ring
- Connecting rod
- Crank
- Crankshaft
- Valves ETC.
In four-stroke engines, Valves are used instead of Ports.
There are two valves:
- Suction valve
- Exhaust valve
And these valves are operated by means of Camshaft. It is rotated at half the speed of a crankshaft.
The working principle of the Four-stroke petrol engine:
The travel of the piston from one dead center to another is called piston stroke and a four-stroke cycle consists of four strokes:
- Suction Stroke
- Compression Stroke
- Power or Expansion Stroke
- Exhaust Stroke
Let me introduce these:
- Suction stroke:
In this stroke the suction valve opens, exhaust valves remain in close condition. The piston moves from the top dead center to the bottom dead center and the charge (Air+Fuel) enters the engine cylinder due to the partial vacuum developed inside the cylinder.
When the piston reaches the BDC, the inlet valve closes and the flywheel makes a half revolution.
- Compression Stroke:
In this stroke, the piston moves from the bottom dead center (BDC) to the top dead center (TDC), and both the suction and exhaust valves are in closed condition. The mixture, which was sucked into the cylinder during the suction stroke, is compressed into the combustion chamber. The heat produced during the compression stroke and vaporize the mixture. As the piston approaches the top dead center, the compression stroke completes, and then the spark plug gives the spark.
- Power Stroke or Expansion stroke:
Again both the inlet and exhaust valves are closed in this stroke. The compressed charge ignited by the high intensity of the spark and expands adiabatically, therefore, it pushes the piston downward i.e. towards BDC then the stroke is completed.
- Exhaust Stroke:
During the exhaust stroke, the inlet valve is in a closed condition and the exhaust valve is opened. The piston rises up from BDC to TDC. The stroke is completed when the piston reaches the Top Dead Center. And then again stokes are recycled.
In the exhaust stroke in one situation both the valves are in an open condition for some seconds so that fresh charge entered to engine cylinder and pushes the burnt gases out of the cylinder. This process is called Scavenging.
This is how a Four-stroke Petrol engine works.
Four-stroke Petrol engine diagram:
Valve-timing diagram for four-stroke Petrol engine:
Theoretical:
- The Inlet valve opens when the piston at TDC
- The Inlet valve closes when the piston reaches BDC
- Spark produces when the piston reaches TDC
- The exhaust valve opens when the piston at BDC
- Exhaust valve close when the piston at TDC
Actual valve-timing diagram:
- The Inlet valve opens about 10*-20* before TDC
- Inlet valve close 30*-40* after BDC to take advantage of rapidly moving gas
- The spark occurs 20*-35* before TDC
- The exhaust valve opens about 30*-50* before BDC
- Exhaust valve close about 10*-15* after TDC
Applications of Four-stroke Petrol engine:
It has various types of usages like:
- In automobiles
- Motorcycles
- Cars
- Buses
- Trucks
- Airplanes
- Small pumping sets
- Mobile electric generator
Four-stroke Diesel engine:
Construction:
A four-stroke Diesel engine consists of:
- A fuel injector
- Fuel pump
- Cylinder
- Cylinder head
- Inlet and exhaust valves
- Piston
- Piston ring
- Connecting rod
- Camshaft
- Crankshaft
- Cam ETC.
Working principle of Four-stroke Diesel engine:
The cycles of the Four-stroke diesel engine are the same as the Petrol engine.
- Suction Stroke
- Compression Stroke
- Power or Expansion Stroke
- Exhaust Stroke
- Suction Stroke:
In a suction stroke, the inlet valve is in the opened condition and the exhaust valve remains closed. The atmospheric air is drawn into the cylinder as the piston moves from TDC to the BDC due to the partial vacuum air is sucked inside the cylinder.
- Compression Stroke:
In this stroke Piston moves from BDC to TDC, as a result, the air inside the cylinder is compressed and heated.
In this stroke both the valves are in closed condition.
The air temperature is around 800*C
At the end of the compression stroke, the fuel injector inject the fuel at high pressure into the compressed hot air, thus the ignition takes place inside the cylinder.
- Power or Expansion Stroke:
During this stroke, both the valves are in closed condition. The piston at the TDC is pushed downward by the expansion of burned gas.
Therefore a Power stroke is produced.
- Exhaust stroke:
In this stroke, the exhaust valve is opened and the inlet valve is in the closed condition. The piston travels from BDC to TDC and pushed the burned gas out of the cylinder.
And here is also the scavenging process that happened to remove the burnt gas completely.
Valve-timing diagram for four-stroke Diesel engine:
Actual valve-timing diagram:
- Inlet valve open at 10*-30* before TDC
- The Inlet valve closes at 20*-50* after BDC
- The exhaust valve opens approximately 40* before BDC
- Exhaust valve close 15* after TDC
- Fuel injection starts 5*-15* before TDC
- Fuel injection stops 15*-25* after TDC
Applications of Four-stroke Diesel engine:
The four-stroke Diesel engine is one of the most popular prime overs. It has various types of applications like:
- Small pumping sets
- In the automobile industry
- Air compressor
- Drilling jigs
- Boats and Ships
- Power saw
- Bulldozers
- Tanks
- Diesel-electric locomotives ETC.
2-Stroke engine vs 4-Stroke engine
Petrol Engine (SI) vs Diesel Engine (CI)
Advantages and Disadvantages of Four-Stroke engine:
Advantages of 4-Stroke engine:
- More torque
- More fuel economy than a 2-stroke engine
- Less consumption of lubrication oil
- Less pollution because of proper burning of fuel
- More durability
- Well managed cooling system, so that longer engine life
- Less noisy
- Scavenging happens really well, therefore Thermal efficiency and HP of the engine is high
Let's talk about some disadvantages:
- There are many moving parts and hence there is more friction, therefore the engine has less mechanical efficiency.
- The design is complicated
- The weight of the engine is more because of heavier flywheel construction.
- Due to more parts, the maintenance required quite often.
Fuel supply system in Four-stroke engine:
The petrol engine fuel supply system consists of:
- Fuel feed system
- Gravity system
- Vacuum system
- Pressure system
- Pump system
- Fuel injection system
- Fuel tank
- Fuel lines
- Fuel filter
- Fuel pump
- Carburetor
At first, an electrical petrol pump or mechanical petrol pump sucked the fuel from the fuel tank via a fuel filter as shown in figure then the fuel through the fuel lines goes to the carburetor where air+fuel mixed with proper ratio and enters into the engine cylinder for combustion.
To know more, checkout the Fuel supply system in petrol engines
Diesel engines fuel supply system consists of:
- A Diesel tank
- A feed pump
- A filter
- An injection pump
- An injector
- Connecting lines
There are two types of the fuel injection system in a Diesel engine:
- Air injection system or Indirect injection
- Solid injection system or Direct injection
And there are two types of solid injection system:
- Common rail fuel injection system
- Individual pump fuel injection system.
The lubrication system in Four-stroke engine:
The supply of lubricating oil between two moving parts of the vehicle is called lubrication.
Why we need a lubrication system?
The primary objectives of lubrication are:
- To reduce friction between moving parts to its minimum so that power loss is less
- To reduce wearing out of moving parts
- To provide a cooling effect
- To provide the cleaning action
4-stroke engine lubrication system consists of:
- Oil pump
- Oil cooler
- Oil strainer
- Oil filter
- Oil pressure gauge
- Crankcase dilution
Parts to be lubricated:
The following parts in the engine require lubrication.
- Crankshaft bearings
- Crankpins
- The big end of connecting rod bearing
- The small end of the connecting rod bearing
- Bushes of gudgeon pin or Wrist pin
- The inner wall of the cylinder
- Valve operating mechanism
- Timing gears
- Camshaft bearing
The lubrication system in engines:
- Petrol system
- Splash system
- Pressure system
- Dry-sump system
- Combined splash and pressure-feed system
More on the Lubrication System.
The cooling system in the Four-Stroke engine:
All the heat produced by the combustion of fuel is not totally converted into power at the crankshaft.
Only about 20 percent of the heat is used as power at the crankshaft. And 35 percent of the heat is transferred to the cylinder wall which cause pre-ignition of the charge and also lubricant burned away due to the heat, therefore the engine maybe seized.
Keeping above factor in mind we need to install a proper cooling system.
In a Four-stroke engine their are two types cooling method.
- Cooling by air
- Cooling by water
More on Engine Cooling System.
Some FAQ:
What is Scavenging?
In the exhaust stroke in one situation both the valves are in opened condition for some seconds so that fresh charge entered to engine cylinder and pushes the burnt gases out of the cylinder. This process is called Scavenging.
What is the Four-Stroke Engine?
A four-stroke engine is an Internal combustion engine, where four successive strokes (i.e. Suction-Compression-Power-Exhaust) completes in two revolutions of the crankshaft. Therefore, the engine is called a Four-stroke engine.
What are the Parts of Four-stroke Engine?
A four-stroke petrol engine consists of:
1. A cylinder
2. Cylinder head attaches with spark plug
3. Piston attach with piston ring
4. Connecting rod
5. Crank
6. Crankshaft
7. Valves ETC.
So this is the complete overview on Four Stroke Engine, I hope I am able to explain it well, for further more doubts do let me know in the comment section. And below is your PDF downloadable link.
P.S.: I reply every comment posted here!
You may have interest to read about 2-Stroke Engine also.
RESOURCES:
- Function of Diesel Fuel Injection
- Injection pump
- Wikipedia
Posted by: sharensharenconaghane0272543.blogspot.com
Source: https://learnmechanical.com/four-stroke-engine/
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