How does two stroke engine works ?

After understanding about the four stroke engine from my previous post.This time we will learn about the second type of engine i.e 2 stroke engine. The 2 stroke engine is a type of Internal combustion engine where one power cycle is completed with two strokes of the piston during only one rotation of crankshaft.

The 2 stroke engine performs all the same steps which are performed by 4 stroke engine i.e intake stroke, compression stroke, power stroke & exhaust stroke; but the 2 stroke engine performs all the 4 process(stroke) in only 2 stroke.

The first commercial two-stroke engine involving in-cylinder compression is attributed to Scottish engineer Dugald Clerk, who patented his design in 1881

Application : Two-stroke engines are generally less expensive to build compared to four-stroke engines, and they are lighter and can produce a higher power-to-weight ratio. For these reasons, two-stroke engines are ideal in applications such as chainsaws, weed trimmers, outboard motors, off-road and motorcycles.Two-stroke engines are also easier to start in cold temperatures, making them ideal for use in snowmobiles

Components of two stroke engine

Main Components of Two stroke engine : The above diagram shows components of two stroke engine 

Spark plug : A spark plug delivers electric current to the combustion chamber which ignites fuel.The spark produced is used to burn the fuel in the cylinder.

Piston : Piston is the reciprocating part of the engine. It is used for transferring the energy of the expansion of gases to crankshaft using connecting rods

Connecting rod : It connects the piston and crankshaft. It transfer the power from the piston to the crankshaft.

Crank case : It acts as the base of the engine. It supports the crankshaft and camshaft in suitable bearings and provides the arms for supporting the engine on the frame.

Crank shaft : It is that part of the engine which is used to convert the reciprocating motion of the engine into the rotating motion with the help of a connecting rod.

Inlet port & Exhaust port : The fresh charge enters into the cylinder through inlet port and transfer the burnt gases out of the engine through exhaust port.

Counterweight : Counterweight on crankshaft are used to reduce the vibrations due to imbalances in the rotating assembly 

Counterweight are used to prevent bending of crankshaft by offsetting the weights of rod and pistons. For a crankshaft to be balanced, at any point in the assembly’s rotation, the sum of all of the forces are roughly equal to zero. If the crankshaft is not balanced, it will deform especially at pressure points and eventually fail.

In two stroke engines, there is no suction and exhaust strokes. There are only two remaining strokes the compression stroke and power stroke. These are usually called the upward stroke and downward stroke. Also, instead of valves, there are inlet and exhaust ports in two stroke cycle engines.

Working principle of two stroke engine is explained in as  shown in diagram below :

1. Upward Stroke

2. Downward Stroke

1. Upward stroke:

During upward stroke, the piston moves from BDC to TDC and compresses the charge (air-fuel mixture) in the combustion chamber of the cylinder.

Because of the upward movement of the piston a partial vacuum is created in the crankcase and this allows the entry of the fresh charge into the crankcase through uncovered inlet port.

The exhaust port and the inlet port remains covered when the piston at the TDC.

The ignition of the fresh charge is takes place by the spark plug.

Two Stroke working principle

2. Downward stroke:

As soon as the combustion of the fresh charge takes place, a large amount of the pressure created due to combustion of fuel exerts a very high pressure force on the top of the piston. Due to this high pressure force, the piston moves downward and rotates the crankshaft

During this stroke the inlet port is covered by the piston and the new charge is compressed in the crankcase.

Further downward movement of the piston uncovers first the exhaust port and the transfer port and the exhaust starts through the exhaust port.

As soon as the transfer port opens, the charge through it is forced into the cylinder.

The charge strikes the deflector on the piston crown, rises to the top of the cylinder and pushes out most of the exhaust gases.

The piston is now at BDC position. The cylinder is completely filled with the fresh charge but it is somewhat diluted with the exhaust gases.

Finally the cycle event is then repeated. We get two strokes for the single revolution of the crankshaft.

The below mentioned upward and downward stroke in two stroke engine :

Animation depicting upward and downward stroke

How does four stroke engine works ?

After understanding about the classification of I.C Engines lets try understand the "How does four stroke engine works" ?
These are the basic engines used in Automotive Industry, in this post we would be learning about Four Stroke Gasoline or Petrol Engine

The name itself says that piston is going to complete four stroke while rotating the crankshaft twice, so one operating cycle requires 720° rotation of crankshaft, so what is stroke ?
Basically stroke is movement of piston in either direction from TDC(Top Dead Centre) to BDC(Bottom Dead Centre) or vice versa, the below pictorial represents TDC, BDC and Stroke.
On the engine head we can see Spark Plug for Gasoline Engine or Fuel Injector for Diesel Engine

Diagram denoting Engine Cylinder Parts

One operating cycle gets completed when all the four strokes gets completed, this also known as Otto cycle since it was demonstrated by Nikolaus Otto in 1876.

Piston  – In an engine, piston is used to transfer the expanding force of gases to mechanical rotation of crankshaft via a connecting rod.

Crankshaft  – A crankshaft is a part which is able to convert the reciprocating motion of piston  to rotational motion.

Connecting rod  – A connecting rod transfers motion from a piston to crankshaft

Flywheel – Flywheel is a rotating mechanical device which is used to store energy.
The functions of a flywheel are the following :

1. It stores up energy to help the engine over idle strokes of the piston i.e. suction,compression and exhaust.
2. It dampens out speed fluctuations of the crankshaft due to the varying effect of the firing impulses during the engine cycle.
3. It provides a convenient mounting point for the clutch and starter ring gear.

Spark Plug  – A spark plug delivers electric current to the combustion chamber which ignites the air-fuel mixture coming from Carburetor

Carburetor -  Its main function is to mix the fuel with air in a perfect ratio. It depends upon the engine’s speed and load conditions. Theoretically, the ideal mixture contains 15 parts of air and 1 part of gasoline fuel

Intake Stroke :

The intake is when the air-fuel mixture from carburetor  is introduced to fill the combustion chamber. The intake event occurs when the piston moves from TDC to BDC and the intake valve is open. The movement of the piston toward BDC creates a low pressure in the cylinder. Basically due to pressure difference the air-fuel mixture through the open intake valve moves into the cylinder to fill the low pressure area created by the piston movement. The intake valve then closes and the air-fuel mixture is sucked inside the cylinder is also called Suction Stroke

The below diagram depicting Intake Stroke, with the direction of piston movement along with valve position

Intake Stroke

Main Points :

1.Intake valve - Open, Exhaust valve - Closed
2.Crankshaft rotation - 180°
3.Piston Movement - from TDC to BDC

Compression Stroke :

The compression stroke is when the trapped air-fuel mixture is compressed inside the cylinder. Compressing the air-fuel mixture allows more energy to be released when the charge is ignited. Intake and exhaust valves must be closed to ensure that the cylinder is sealed to provide compression. Compression is the process of reducing or squeezing a charge from a large volume to a smaller volume in the combustion chamber. The flywheel helps to maintain the momentum necessary to compress the charge. Due to reduce in the volume of charge (Air fuel mixture), temperature in the chamber increases which would help the Ignition.

The below diagram depicting Compression Stroke, with the direction of piston movement along with valve position

Compression Stroke

 Main Points :

1.Intake valve - Closed, Exhaust valve - Closed
2.Crankshaft rotation - 180° ( Total 360°)
3.Piston Movement - from BDC to TDC


Power Stroke:

The ignition (combustion) event occurs when the charge is ignited with the help of Spark Plug, It (Charge) gets rapidly oxidized through a chemical reaction to release heat energy. The spark at the spark plug initiates combustion at approximately 20° of crankshaft rotation before TDC.

The power stroke is an engine operation Stroke in which hot expanding gases forces the piston head away from the TDC or top cylinder head. Piston force reciprocated motion is transferred through the connecting rod to apply torque to the crankshaft. This torque makes crankshaft rotate. The amount of torque produced or rotation of piston  depends upon by the pressure on the piston head, the size of the piston head, and other factors. During the power Stroke, both  Intake and Exhaust valves are closed.


The below diagram depicting Power Stroke, with the direction of piston movement along with valve position

Power Stroke

Main Points :
1.Intake valve - Closed, Exhaust valve - Closed
2.Crankshaft rotation - 180°( Total 540°)
3.Piston Movement - from TDC to BDC


Exhaust Stroke

The exhaust stroke occurs after the Power Stroke when exhaust gases (NOx,CO,SOx etc) are generated after the combustion needs to be expelled from the combustion chamber. The exhaust stroke is the final stroke and occurs when the exhaust valve is open and the intake valve is closed. Piston movement evacuates exhaust gases to the exhaust management system where lot of treatment strategies applied on the gases is done before expelling it to atmosphere.
The exhaust valve opens, and inertia of the flywheel and other moving parts push the piston back to TDC, forcing the exhaust gases out through the open exhaust valve. At the end of the exhaust stroke, the piston is at TDC and one operating cycle has been completed.


The below diagram depicting Exhaust Stroke, with the direction of piston movement along with valve position.

Exhaust Stroke

Main Points :
1.Intake valve - Closed, Exhaust valve - Opened
2.Crankshaft rotation - 180°( Total 720°)
3.Piston Movement - from BDC to TDC

Now after looking in to all the stroke individually, the animation shows Four stroke in proper sequence

Four Stroke

Classification of IC Enignes

After understanding External Combustion Engine and Internal Combustion Engine in last post lets understand what are the parameters on which IC Engines could be classified.

1. Number of Strokes - On the basis of number of stroke, the types of engine are :

• Four Stroke Engine: It is an engine in which the piston moves four times i.e.2 upward (from BDC to TDC) and 2 downward (from TDC to BDC) movement in one cycle of power stroke is called four stroke engines. [ TDC - Top Dead Centre and BDC - Bottom Dead Centre]
  
  
                                                                                                          GIF  Credit
• Two Stroke Engine: The engine in which the piston does two times motion i.e. one from TDC to BDC and other from BDC to TDC to produce a power stroke is called two stroke engines. 

 

GIF Credit

                                                                                                                                           

     2. Types of Fuel Used - On the basis fuel used :  

• Gasoline engine: The engine which uses petrol for its working is called petrol engine.

• Diesel engine: The engine which uses diesel for its working is called diesel engine.

• Gas engine: An engine using gas fuel for the working is called gas engine. 

3. Type of Ignition - On the basis of ignition, the engines are classified as:

• Spark ignition engine (S.I. engine): In spark ignition engine there is a spark plug which is fitted at the engine head. The spark plug produces spark after the compression of the fuel and ignites the air fuel mixture for the combustion. The gasoline engines are spark ignition engine

• Compression ignition engine (C.I. engine): In Compression ignition engine there is no spark plug at the cylinder head. The fuel is ignited by the heat of the compressed air. The diesel engines are compression ignition engine.

The below diagram depicts the basic difference between SI Engine and CI Engine, left one being S.I Engine ( Which has Spark Plug at the engine head ) and right one being C.I Engine ( having Injector at the engine head)

Difference between S.I and C.I Engine

 

   4. Number of Cylinders - On the basis of number of cylinders present in the engine :

• Single cylinder engine: An engine which consists of single cylinder is called single cylinder engine. Generally the single cylinder engines are used in motorcycles, scooter, etc. 

• Double cylinder engine: The engine which consists of two cylinders is called double cylinder  engine.  

• Multi cylinder engine: An engine which consists of more than two cylinders is called multi cylinder engine. The multi cylinder engine may have three, four, six, eight, twelve and sixteen cylinder.

The below image shows the number of cylinders 6 and 4 :

ImageRef 1

                                                                                                                                                             

   5. Arrangement of Cylinders - On the basis of arrangement of cylinders the engine classification:

• Vertical engine: in vertical engines, the cylinders are arranged in vertical position as shown in the diagram

• Horizontal engine: In horizontal engines, the cylinders are placed horizontal position as shown in the diagram given below.

• Radial engine:  The radial engine is reciprocating type internal combustion engine configuration in which the cylinders radiate outward from a central crankcase like the spokes of a wheel. When it is viewed from the front, it resembles a stylized star and is called a ‘star’ engine. Before the gas turbine engine is not become predominant, it is commonly used for aircraft engines.

• V-engine: In v types of engine, the cylinders are placed in two banks having some angle between them. The angle between the two banks is keep as small as possible to prevent vibration and balancing problem.

• W type engine: In w type engines, the cylinders are arranged in three rows such that it forms W type arrangement. W type engine is made when 12 cylinder and 16 cylinder engines are produced.

• Opposed cylinder engine: In opposed cylinder engine, the cylinders are place opposite to each other. The piston and the connecting rod show identical movement. It is runs smoothly and has more balancing. The size of the opposed cylinder engine increase because of its arrangement. 

 The diagram below depicts arrangement of Cylinders :

Arrangement of Cylinders

    6.  Types of cooling

• Air cooled engines: In these engines, the air is used to cool the engines. In air cooled engines the cylinder barrels are separated and metal fins are used which provides radiating surface area that increase cooling. The air cooled engines are generally used in motorcycles and scooters.

• Water cooled engines: In water cooled engines, the water is used for the cooling of engine. Water cooled engines are used in cars, buses, trucks and other four wheeled vehicles, heavy duty motor vehicles. An anti-freezing agent is added in the water to prevent it from freezing during cold weather. Every water cooled engines has radiator for the cooling of hot water from the engine 

7. Types of Design

• Reciprocating engine: In reciprocating engine, there is a piston and cylinder, the piston does reciprocating (to and Fro) motion within the cylinder. Due to the reciprocating motion of the piston, it is called reciprocating engine. 2 stroke and four stroke engines are the common examples of reciprocating engine.

• Rotary engine: In rotary engine, the rotor does rotary motion to produce power. There is no reciprocating motion. A rotor is present in the chamber which does rotary motion inside a chamber. Wankel rotary engine , turbine engines are the rotary types of engine.

Rotatory Engine

Reciprocating Engine

Understanding Engine

What is Engine ?

Any device which can convert chemical energy of fuel in to mechanical energy is called an Engine
Engine is heart of automobile in which combustion of fuel takes place inside the cylinder or outside the cylinder

Classification based on place where the combustion takes place :

• External Combustion Engine ( EC Engines ) : An engine in which combustion of fuel take place outside of the cylinder is known as EC engine. The Chemical energy of fuel (Solid, liquid or gas state) is converted in to heat energy which in turn is used to low boiling temperature liquid into steam like water, which rotates the turbine i.e basically converting in to Electrical Energy. Application : ships, locomotive, generation of electric power etc.

          For more information of  Application,Advantages, Disadvantages refer this link

          External Combustion Engines | Applications, Advantages, Disadvantages

The below GIF explains about one of the application of electricity generation :

GIF Reference : https://commons.wikimedia.org/wiki/File:BoilingWaterReactor.gif

• Internal Combustion (I.C.) Engine : An engine in which combustion of fuel take place inside the Cylinder. When the fuel burns inside the engine cylinder, it generates a high temperature and pressure. This high pressure force is exerted on the piston (A device which free to moves inside the cylinder and transmit the pressure force to crankshaft by use of connecting rod), which used to rotate the wheels of vehicle with the help of differential. In these type engines we can use only gases and high volatile fuel like petrol, diesel. These engines are generally used in automotive industries. In this blog we will be focusing on IC Engine  

          The below is gif for four stroke (4 Cylinder Engine),example of IC Engine :                

GIF Reference : https://www.pinterest.com/pin/34881793361869782

In the next post we will learn about I C Engine types on the basis of Stroke ( 2 or 4 Stroke)