Introduction to Internal Combustion Engine

Internal combustion Engine

An engine is a device that converts the chemical energy of fuel into mechanical work with the help of a piston connecting rod and crank. For the production of heat, the burning of fuel is a necessary condition. In some of the engines, the burning of fuel takes place inside the engine or outside the engine. if the fuel is burnt inside the engine is called Internal Combustion Engines and if the fuel is burnt outside the engine is known as the external combustion engine. 

The most common applications of I C engines are in automobile engines, lawn cutter engines, diesel generators, boat engines, and many more.

The thermodynamic cycle of an engine consists of four processes like suction, compression, power or expansion, and exhaust. In an engine, this process is completed by the piston movement. The movement of the piston from TDC to BDC is known as a stroke and the process strokes are known as suction stroke, compression stroke, power stroke, and exhaust stroke.

Based on stroke, the engine is defined as 2-Stroke engines and 4-Stroke engines. In 2-stroke engines, all four processes are completed in one revolution of crank whereas in the 4-stroke engine all four processes are completed in 2 revolutions of crank.

Figure: Cross-section of petrol the engine.

The above figure shows the parts of an I C engine. the description of the parts are as follows

• The cylinder is the hollow cylindrical part of an engine in which all the four processes take place, the material of cylinder block is cast iron and aluminum alloy. The aluminum alloy cylinder is light in weight as compared to cast iron piston. 

• The piston is the reciprocating member of the engine, it is also called the heart of an engine, all the four processes are done with the help of piston reciprocating movement. The piston is also made by aluminum alloy.

• Piston rings are mounted near to the top of the piston as shown in the above figure by a yellow color, the first ring is called the compression ring, which prevents the leakage of charge from the combustion chamber to crankcase and second ring is known as the oil ring, which is used to remove the engine oil from the cylinder surface. There are three piston rings are used instead of two rings shown in the figure. The first two top rings of the piston are called compression rings and the third ring is called oil rings. The piston rings are made of cast iron. it also provides safety against wear of the piston from cylinder wall during reciprocation movement.  

• The top dead center is extreme top the position of piston and bottom dead center is the extreme bottom position of the piston.

• Exhaust valve allows the combustible gases to move out of the engine during the exhaust stroke.

• The exhaust manifold is the special design passage in which exhaust gases come from the engines that are collected and sends it to the atmosphere through the pipe consists of the catalytic converter and muffler. 

• A spark plug is a device that helps to ignite the fuel-air mixture inside the cylinder.

• Inlet manifold is a passage in which a fresh mixture of air and fuel comes from carburetor to engine.

• The Inlet valve allows the fresh mixture of air and fuel to come inside the cylinder.

• The carburetor is a device which mixes the fuel and air at a required proportion for the proper working of the engine at different revolutions

• After burning of air-fuel mixture large amount of heat is released, some heat is absorbed by the cylinder wall, this absorbed heat is removed by a water jacket to provide safety against metallurgical failure of the engine cylinder.

• The piston and connecting rod are connected by a pin are called the Gudgeon pin.

• The connecting rod is connecting member and it oscillates between piston and crank. The force acting on the piston is transferred by the connecting rod to the crankshaft. 

• Crankpin is the joint between the crank and the connecting rod.

• The oscillating movement of the connecting rod is converted into rotary motion by the crank. The heat energy of the fuel is converted into the reciprocating movement of the piston then; it converted into the oscillating moment of connecting rod and then converted into rotary motion of crank, which is used for further useful work. 

• The Crankcase the space in which engine oil is filled for cooling purposes.

• The rotation of the crank is transfer to the shaft. On this shaft flywheel of an engine or generator is mounted outside the engine. Here the shaft is used to transmit the mechanical work outside the engine, which is produced inside the engine.

 Figure: Cross-section of diesel engine

In the above figure, the cross-section of the diesel engine is shown, all the parts are the same except the fuel injector which is used to spray the diesel over the hot compressed air instead of spark plug and only air is injected during suction stroke instead of the air-fuel mixture.

The different types of cylinder arrangements are used for a different purpose. Some following arrangements are here

Figure: Cylinder arrangement

from the above figure, we have seen that in the In-line cylinder, the number of the piston is mounted on a single crankshaft. 

In the U-type arrangement, two crankshafts are used for two-piston arranged in a parallel manner. 

In the V-type arrangement, two pistons are mounted on a single crankshaft. the angle between the two pistons is 60 degrees or 90 degrees. 

In the X-type arrangement, four pistons are mounted on a single crankshaft. 

In the radial arrangement, more than four pistons are arranged on the circumference of a circle and all the pistons are mounted on the single crankshaft. Radial engines are found in airplanes. 

In the H-type, on a single crankshaft, two pistons are mounted. Four pistons are mounted on a two crankshaft as shown in the figure. 

In the opposed cylinder arrangement, a single crankshaft is used to connect two pistons as shown in the figure.

In the opposed piston arrangement, two crankshafts is used to connect two pistons as shown in the figure.

In the delta arrangement, three crankshafts are used and six pistons are arranged in such a way to form a delta symbol. 

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Direct and Indirect Injection combustion Chamber

Hi there, today I am going to discuss about Direct injection (DI) and Indirect injection (IDI) engines combustion chamber.

What is the combustion chamber?
The combustion chamber is a place, where the mixture of air and fuel burnt, produces heat.

And what is Injection?
Injection is a process, in which pressurized fuel is injected or sprayed on the high temperature and the high-pressure air at the end of the compression stroke.

And what is air swirl:  air swirl is a controlled turbulence motion of air inside the cylinder and is useful for better ignition.

The design of CI engine combustion chambers is a very important and challenging task because the mixing of air and fuel takes place inside the cylinder in a very short period is about 250-350 crank angle at the end of the compression stroke.
The combustion chamber design plays a very important role in the proper mixing of fuel in this short period of time. The moment of air inside the cylinder during the injection of fuel is important to achieve proper mixing and better combustion of fuel within a shorter period. The moment of air is called “air swirl”.

There are three types of air swirl on the basis of how they are produced.
First is Induction swirl – it is achieved by directing the flow of air during its entry to the cylinder, known as induction swirl.
Different inlet manifold like deflector wall, helical wall ramp, masking on inlet valve, shrouding on inlet valve, etc.  are used to generate swirl in the cylinder in suction stroke. By using the above systems Intake air tangentially enters into the cylinder.

Second is compressed swirl, in this system, an auxiliary chamber is used with the main chamber, during compression stroke; the air is forced to enter in auxiliary chamber tangentially to generate swirls motion. The auxiliary chambers known as swirl chambers.
The third is induced swirl, in an auxiliary chamber called pre-combustion chamber is used where swirl is induced by pre-combustion of fuel. Pre-combustion leads to pressure rise which rushes the gases in the main chamber for further combustion.
Next is injection systems, there are two types of injection systems are used in CI engines, first is direct injection second is indirect injection.
In Direct injection fuel is directly injected in the cylinder with high-pressure fuel jet, the jet pressure is sufficient to generate air swirl for mixing of air and fuel, and start the ignition.  It is used in large cylinder engines where mixture requirements are least stringent. The shape of the combustion chamber is shallow in the piston crown. For small engines bowl in the chamber is used to achieve a fast mixing rate.

In an indirect injection system, fuel is injected in an auxiliary chamber instead of the main chamber, it is used in small automobile engines which requires a high rate of swirl motion for fast mixing of fuel and air.
High turbulent motion is generated by compression swirl or induced swirl in the pre-combustion chamber or divided chamber. The compressed air is forced from the main chamber to the auxiliary chamber, through the nozzle or orifice. The fuel is injected in a divided chamber at lower injection pressure than the DI engine. The ignition starts in the pre-chamber, pressure rises, and flame rushes to the main chamber through nozzle or orifice, and mix with the main chamber air.

Diesel engine knock

What is engine knock?

Engine knock is the noise produce by the engine due to abnormal combustion. So what are the reasons for knock in a diesel engine?

Diesel Knock

ž If the delay period is long, a large amount of fuel will be injected and accumulated in the chamber.

ž The auto-ignition of this large amount of fuel may cause a high rate of pressure rise.

ž This high rate pressure rise and high maximum pressure leads to a knocking. 

The comparison of SI and CI engine knock

                                                                                  

Figure 1 Knocking in CI Engine

Figure 2 Knocking in SI  Engine

As we have seen from the figure the SI engine knock starts at the end of the combustion because of auto-ignition temperature whereas in diesel engines the knocking starts at the start of combustion because of longer ignition delay. 

Ignition delay is a time taken by the fuel mixture from point of injection to the first occurrence of the auto-ignition on the P-θ diagram. 

We also have seen from figure in SI engine the pressure is very high at the time of detonation whereas in CI engine pressure rise is lower. 

Pre-ignition is the main cause of detonation in the SI engine whereas diesel is injected at the end of compression stroke so there is no chance for pre-ignition of diesel. 

The combustion on the CI engine is based on the auto-ignition of the fuel mixture. If fuel mixture is not uniform over the area of cylinder there is a chance of knock

There are some factors which affect the engine knock in SI and CI engine are follows:

Self-ignition temperature

The self-ignition temperature in the Si engine is high to avoid the pre-ignition tendency whereas in diesel engines the self-ignition temperature is low to achieve auto-ignition as soon as possible to reduce the knock.

Time lag of delay period

The time lag of delay period, in SI engine time lag is high because, after the spark, flame spread slowly and gain strength for further ignition whereas in diesel engine delay period is low because of the self-ignition temperature of diesel is low to reduce the knock.

Compression ratio

In SI engine compression ratio should be low to avoid the pre-ignition of fuel, whereas in diesel engine the combustion starts with the help of self-ignition, so the compression ratio is high to achieve the self-ignition of the mixture as soon as possible to avoid sudden pressure rise as well as engine knock.

Inlet temperature low high

In SI engine the inlet temperature of the wall should be low to avoid pre-ignition which leads the detonation whereas in diesel engine the inlet wall temperature is high to increase the inlet air temperature which the help to attain the self-ignition of fuel as soon as possible and reduce the ignition delay to avoid engine knock.

Inlet pressure

Inlet pressure in SI engine is low to avoid the engine knock, for same compression ratio, the pressure and temperature at the end of the compression stroke is high for high inlet pressure with leads pre-ignition of fuel and engine knock whereas in diesel engine inlet pressure should be high to achieve the self-ignition temperature which reduces ignition delay to avoid engine knock.  

Combustion chamber wall temperature low high

If the wall temperature of a combustion chamber is high in SI engine it causes pre-ignition of fuel which lead engine knock, so it is recommended to maintain the low temperature of the cylinder wall in Si engine whereas high wall temperature in CI engine helps to reduce ignition delay and reduce the engine knock.

Introduction to combustion in CI engine

Combustion in CI engine    

Click above title for video

So, what is Combustion?

Combustion is a chemical process, in which fuel is burning in the presence of oxygen, and produces heat and light.

In the CI engine, the only air is supplied during suction stroke. This air is compressed to attain high pressure and high temperature at the end of the compression stroke, The high-pressure diesel is injected in the cylinder. Injection of diesel at the high pressure helps in the disintegration of fuel jet to achieve better combustion of fuel. Better the atomization, assure better fuel combustion because of large surface area available to absorb heat from hot air and start the ignition as soon as possible.

In the cylinder, air and diesel form a heterogeneous mixture for combustion.

After injecting the diesel in the engine, the droplet of fuel absorbs the latent heat from the hot air and gets evaporate, and form a combustible mixture to start the ignition.

Once ignition has taken place, the heat released by combustion of the first fuel droplet is helped to further evaporation and combustion.

As we have seen from the figure air swirl help to disintegrate the fuel jet into small droplets and form a combustible mixture. Air swirls ensure that the sufficient air is available to support the flame travel in the engine to achieve the complete combustion of fuel.

What happens if there is no swirl or very little swirl present?

The air swirl plays a very important role in the combustion of diesel fuel, air swirl means a controlled turbulence of compressed air which helps to form a combustible mixture and complete ignition of all the fuel inside the cylinder.

 If there is no air swirl inside the cylinder it decreases the air-fuel mixture quality, which leads to incomplete combustion. 

In a figure red circle shows a fuel droplet, it will absorb heat from surrounding air shown by a yellow ring, fuel droplet need more heat to achieve self-ignition temperature, because of no swirl the yellow ring form a barrier to transfer heat from the black ring which is at a higher temperature, to fuel droplet. Because of this barrier ignition delay increases. Here swirl helps to displace the yellow ring and provide heat to fuel droplets to achieve self-ignition temperature.

 High turbulence is also affecting the ignition; high turbulence may extinguish the flame, which also leads to incomplete ignition.

To achieve the controlled turbulence or swirl, intake manifold design plays an important role here.

In the below figure four stages of combustion are shown.

The total injection time is shown here. Injection starts at about 19⁰ BTDC and end of injection is about 10⁰ ATDC, the total injection timing is about 29⁰.

First stage is ignition delay

Second stage is Rapid or uncontrolled combustion

Third stage is controlled combustion

Fourth stage is after burning

ignition delay

It is counted from the start of injection to the point where the p-θ curve separates from the pure air compression curve (motoring curve).

The ignition delay period is divided in two parts

                1. Physical delay

                2. Chemical delay

1. Physical delay

ž  The time measured between the start of injection to the attainment of chemical reaction condition, is called Physical delay period.

ž  The physical delay period consists of the atomization, vaporization, mixing with air and rising in temperature.

2. Chemical delay

ž  Pre-flame reactions start slowly and then accelerate until local inflammation or ignition takes place.

ž  In most of CI engines, the ignition delay period is shorter than the duration of injection.

Rapid or Uncontrolled Combustion

The fuel accumulated due to ignition delay burns rapidly resulting in sudden pressure rise.

ž  It is measured between the ends of the delay period to the point of maximum pressure.

ž  In this stage pressure rise is rapid.

ž  During the delay period, the mixture of fuel and air spread over the wide area.

ž  The mixture of fuel and air come across the high-temperature fresh air, rapid ignition takes place.

Controlled Combustion

In this stage the injected fuel burns directly because of the high temperature and pressure achieved in the second stage.  

ž  In this phase, the pressure rise is controlled by the rate of injection of fuel.

ž  Some fuel is injected during the end of burning, increase the pressure.

ž  This pressure is controlled by the injection rate of fuel.

After Burning

ž  Ideally, in the first three phases, the combustion process is completed.

ž  After burning is present because of the poor mixing/ distribution of fuel particles.

The factor affecting the ignition delay is as follows

S. No.	Variable increases	Effect on delay period	Reason
1	Cetane Number	Reduces	Reduction in self-ignition temp.
2	Injection Pressure	Reduces	Reduces physical delay because of smaller droplet size.
3	Injection advance angle	Increases	Temperature and pressure is low when injection start.
4	Compression Ratio	Reduces	Air pressure and temperature increases, and reduces the auto-ignition temperature.
5	Intake Temp.	Reduces	Increase in air temperature
6	Jacket water temp	Reduces	Increase wall and air temperature
7	Fuel temperature	Reduces	Better vaporization and increases chemical reaction
8	Intake pressure (supercharging)	Reduces	Reduces auto ignition temp.
9	load	Decreases	Operating temp increases