Phenomenon Of Knocking 2r116b
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C. I. (COMPRESSION IGNITION)
ENGINES Prof. K. K. SHARMA © Prof. K. K. Sharma
PHENOMENON OF KNOCKING IN C.I ENGINES Definition:Knocking is violent gas vibration and audible sound produced by extreme pressure differentials leading to the very rapid rise in pressure during the early part of uncontrolled second phase of combustion. © Prof. K. K. Sharma
KNOCKING IN C.I ENGINES If the ignition delay is longer, the actual burning of the first few droplets is delayed and a greater quantity of fuel droplets gets accumulated in the chamber. If the ignition delay is quite long, so much fuel can accumulate that the rate of pressure rise is almost instantaneous
© Prof. K. K. Sharma
KNOCKING IN C.I ENGINES This rapid rate of pressure rise causes pulsating combustion and creates heavy noise. This type of abnormal combustion is known as detonation. Such, a situation produces extreme pressure differentials and violent gas vibration known as knocking (diesel knock), and is evidenced by audible knock. © Prof. K. K. Sharma
KNOCKING IN C.I ENGINES
© Prof. K. K. Sharma
COMBUSTION IN C.I ENGINES
© Prof. K. K. Sharma
EFFECT OF KNOCKING IN C.I ENGINES
© Prof. K. K. Sharma
KNOCKING IN C.I ENGINES
Delay period is directly related to Knocking in CI engine. Factors affecting delay period:1) Inferior fuels 2) Combustion chamber design 3) Initial condition of air 4) Self ignition temperature of fuel. If the factors mentioned above can be controlled then knocking can also be avoided.
© Prof. K. K. Sharma
METHODS OF KNOCK CONTROL
1) Using a better fuel:Higher CN fuel has lower delay period and reduces knocking tendency.
2) Controlling the Rate of Fuel Supply:By injecting less fuel in the beginning and then more fuel amount in the combustion chamber detonation can be controlled to a certain extent. Cam shape of suitable profile can be designed for this purpose. © Prof. K. K. Sharma
METHODS OF KNOCK CONTROL 3) Knock reducing fuel injector:This can be done by arranging the injector so that only small amount of fuel is injected first. This can be achieved by using two or more injectors arranging in, out of phase.
4) By using Ignition accelerators:C N number can be increased by adding chemical called dopes. The two chemical dopes are used are ethylnitrate and amyle–nitrate in concentration of 8.8 gm/ Litre and 7.7 gm/ Litre. But these two increase the NOx emissions © Prof. K. K. Sharma
Comparison Of Knocking in S.I. & C.I. Engines 1) In SI engines knocking occurs at the end of Combustion. 2)In CI engines it occurs at the Beginning of combustion.
© Prof. K. K. Sharma
Comparison Of Knocking in S.I. & C.I. Engines 3) In SI engines the rate of pressure rise is very high 4) In CI engines the rate of pressure rise is lower than is SI engines. 5) In SI engines knocking and non knocking operation can be easily distinguished. 6) In CI engines knocking and non knocking operation can not be easily distinguished. © Prof. K. K. Sharma
Comparison Of Knocking in S.I. & C.I. Engines 7) In SI engines knocking can be avoided by increasing the ignition lag. 8) In CI engines, if ignition lag is increased then knocking will also increase. Note:- The conditions which reduce the knock tendency in SI engines will increase the knock tendency in CI engines.
© Prof. K. K. Sharma
Rating OF C.I Engine Fuels CI Engine fuels are generally rated by their Cetane number. The property of ignition lag (delay period) is generally measured in of Cetane number.
Higher the Cetane rating of the fuel lesser is the propensity for diesel knock. The Cetane number of a diesel fuel is a measure of its ignition quality. © Prof. K. K. Sharma
Rating OF C.I Engine Fuels The Cetane number of a fuel is the percentage by volume of Cetane in a mixture of Cetane [C16H34] and α-methylnaphthalene [C10H7CH3] that has same performance in the standard test engine as that of the fuel. Cetane is arbitrarily assigned a number 100 and originally α-methylnaphthalene was given a number 0. Cetane number 40 means a mixture containing 40 % Cetane and 60 % of α-methylnaphthalene by volume which gives same ignition delay as tested fuel. © Prof. K. K. Sharma
Rating OF C.I Engine Fuels Good SI engine fuel is bad CI engine fuel. High octane number implies low Cetane number and vice versa.
The Straight chain paraffin are the most suitable fuels for CI engine.
© Prof. K. K. Sharma
Combustion Chamber Design Direct-injection – have a single open combustion chamber into which fuel is injected directly
Indirect-injection – chamber is divided into two regions and the fuel is injected into the “pre- chamber” which is connected to the main chamber via a nozzle, or one or more orifices.
© Prof. K. K. Sharma
Combustion Chamber Design Indirect injection
Direct injection
© Prof. K. K. Sharma
Combustion Chamber Design Direct-injection Combustion Chamber:-
1. Shallow depth Chamber 2. Hemispherical Chamber 3. Cylindrical Chamber 4. Toroidal Chamber © Prof. K. K. Sharma
1. Shallow depth Chamber In shallow depth chamber the depth of the cavity provided in the piston is quite small. Since the cavity diameter is very large, the squish is negligible This chamber is usually adopted for large engines running at low speeds. © Prof. K. K. Sharma
2. Hemispherical Chamber It has a hemispherical Cavity in the piston The squish in this case is better than the previous. The depth to diameter ratio for a cylindrical chamber can be varied to give any desired squish to give better performance. © Prof. K. K. Sharma
3. Cylindrical Chamber This design was attempted in recent diesel engines. Cavity is in the form of a truncated cone with base angle of 30° Much better squish than previous two Used in Heavy vehicles.
Leyland © Prof. K. K. Sharma
4. Toroidal Chamber This type of chamber provides powerful squish It provides better combustion and reduces delay period as better mixing is provided. There is better utilization of oxygen The cone angle of spray for this type of chamber is 150° to 160°.
© Prof. K. K. Sharma
Combustion Chamber Design Indirect-injection Combustion Chamber:-
1. Pre-combustion Chamber 2.Turbulent Chamber/Swirl Chamber
© Prof. K. K. Sharma
1. Pre-combustion Chamber These chambers are divided in two parts, one between the piston and the cylinder head and the other in the cylinder head A small age is provided to connect these two chambers. The small age is orifice. © Prof. K. K. Sharma
1. Pre-combustion Chamber The fuel is injected into the pre-combustion chamber. In this arrangement the precombustion chamber contains nearly 25 to 30% of the total air charge. The delay period is reduced. © Prof. K. K. Sharma
2.Turbulent Chamber/Swirl Chamber Very high swirl is created in this type of chamber. The arrangement is same as that of the Pre-combustion chamber
The shape of prechamber is spherical. 90-95% of air is pushed in the Prechamber. © Prof. K. K. Sharma
2.Turbulent Chamber/Swirl Chamber This type of combustion chamber finds its application where fuel quality is difficult to control Reliability is more important than fuel economy.
© Prof. K. K. Sharma
ENGINES Prof. K. K. SHARMA © Prof. K. K. Sharma
PHENOMENON OF KNOCKING IN C.I ENGINES Definition:Knocking is violent gas vibration and audible sound produced by extreme pressure differentials leading to the very rapid rise in pressure during the early part of uncontrolled second phase of combustion. © Prof. K. K. Sharma
KNOCKING IN C.I ENGINES If the ignition delay is longer, the actual burning of the first few droplets is delayed and a greater quantity of fuel droplets gets accumulated in the chamber. If the ignition delay is quite long, so much fuel can accumulate that the rate of pressure rise is almost instantaneous
© Prof. K. K. Sharma
KNOCKING IN C.I ENGINES This rapid rate of pressure rise causes pulsating combustion and creates heavy noise. This type of abnormal combustion is known as detonation. Such, a situation produces extreme pressure differentials and violent gas vibration known as knocking (diesel knock), and is evidenced by audible knock. © Prof. K. K. Sharma
KNOCKING IN C.I ENGINES
© Prof. K. K. Sharma
COMBUSTION IN C.I ENGINES
© Prof. K. K. Sharma
EFFECT OF KNOCKING IN C.I ENGINES
© Prof. K. K. Sharma
KNOCKING IN C.I ENGINES
Delay period is directly related to Knocking in CI engine. Factors affecting delay period:1) Inferior fuels 2) Combustion chamber design 3) Initial condition of air 4) Self ignition temperature of fuel. If the factors mentioned above can be controlled then knocking can also be avoided.
© Prof. K. K. Sharma
METHODS OF KNOCK CONTROL
1) Using a better fuel:Higher CN fuel has lower delay period and reduces knocking tendency.
2) Controlling the Rate of Fuel Supply:By injecting less fuel in the beginning and then more fuel amount in the combustion chamber detonation can be controlled to a certain extent. Cam shape of suitable profile can be designed for this purpose. © Prof. K. K. Sharma
METHODS OF KNOCK CONTROL 3) Knock reducing fuel injector:This can be done by arranging the injector so that only small amount of fuel is injected first. This can be achieved by using two or more injectors arranging in, out of phase.
4) By using Ignition accelerators:C N number can be increased by adding chemical called dopes. The two chemical dopes are used are ethylnitrate and amyle–nitrate in concentration of 8.8 gm/ Litre and 7.7 gm/ Litre. But these two increase the NOx emissions © Prof. K. K. Sharma
Comparison Of Knocking in S.I. & C.I. Engines 1) In SI engines knocking occurs at the end of Combustion. 2)In CI engines it occurs at the Beginning of combustion.
© Prof. K. K. Sharma
Comparison Of Knocking in S.I. & C.I. Engines 3) In SI engines the rate of pressure rise is very high 4) In CI engines the rate of pressure rise is lower than is SI engines. 5) In SI engines knocking and non knocking operation can be easily distinguished. 6) In CI engines knocking and non knocking operation can not be easily distinguished. © Prof. K. K. Sharma
Comparison Of Knocking in S.I. & C.I. Engines 7) In SI engines knocking can be avoided by increasing the ignition lag. 8) In CI engines, if ignition lag is increased then knocking will also increase. Note:- The conditions which reduce the knock tendency in SI engines will increase the knock tendency in CI engines.
© Prof. K. K. Sharma
Rating OF C.I Engine Fuels CI Engine fuels are generally rated by their Cetane number. The property of ignition lag (delay period) is generally measured in of Cetane number.
Higher the Cetane rating of the fuel lesser is the propensity for diesel knock. The Cetane number of a diesel fuel is a measure of its ignition quality. © Prof. K. K. Sharma
Rating OF C.I Engine Fuels The Cetane number of a fuel is the percentage by volume of Cetane in a mixture of Cetane [C16H34] and α-methylnaphthalene [C10H7CH3] that has same performance in the standard test engine as that of the fuel. Cetane is arbitrarily assigned a number 100 and originally α-methylnaphthalene was given a number 0. Cetane number 40 means a mixture containing 40 % Cetane and 60 % of α-methylnaphthalene by volume which gives same ignition delay as tested fuel. © Prof. K. K. Sharma
Rating OF C.I Engine Fuels Good SI engine fuel is bad CI engine fuel. High octane number implies low Cetane number and vice versa.
The Straight chain paraffin are the most suitable fuels for CI engine.
© Prof. K. K. Sharma
Combustion Chamber Design Direct-injection – have a single open combustion chamber into which fuel is injected directly
Indirect-injection – chamber is divided into two regions and the fuel is injected into the “pre- chamber” which is connected to the main chamber via a nozzle, or one or more orifices.
© Prof. K. K. Sharma
Combustion Chamber Design Indirect injection
Direct injection
© Prof. K. K. Sharma
Combustion Chamber Design Direct-injection Combustion Chamber:-
1. Shallow depth Chamber 2. Hemispherical Chamber 3. Cylindrical Chamber 4. Toroidal Chamber © Prof. K. K. Sharma
1. Shallow depth Chamber In shallow depth chamber the depth of the cavity provided in the piston is quite small. Since the cavity diameter is very large, the squish is negligible This chamber is usually adopted for large engines running at low speeds. © Prof. K. K. Sharma
2. Hemispherical Chamber It has a hemispherical Cavity in the piston The squish in this case is better than the previous. The depth to diameter ratio for a cylindrical chamber can be varied to give any desired squish to give better performance. © Prof. K. K. Sharma
3. Cylindrical Chamber This design was attempted in recent diesel engines. Cavity is in the form of a truncated cone with base angle of 30° Much better squish than previous two Used in Heavy vehicles.
Leyland © Prof. K. K. Sharma
4. Toroidal Chamber This type of chamber provides powerful squish It provides better combustion and reduces delay period as better mixing is provided. There is better utilization of oxygen The cone angle of spray for this type of chamber is 150° to 160°.
© Prof. K. K. Sharma
Combustion Chamber Design Indirect-injection Combustion Chamber:-
1. Pre-combustion Chamber 2.Turbulent Chamber/Swirl Chamber
© Prof. K. K. Sharma
1. Pre-combustion Chamber These chambers are divided in two parts, one between the piston and the cylinder head and the other in the cylinder head A small age is provided to connect these two chambers. The small age is orifice. © Prof. K. K. Sharma
1. Pre-combustion Chamber The fuel is injected into the pre-combustion chamber. In this arrangement the precombustion chamber contains nearly 25 to 30% of the total air charge. The delay period is reduced. © Prof. K. K. Sharma
2.Turbulent Chamber/Swirl Chamber Very high swirl is created in this type of chamber. The arrangement is same as that of the Pre-combustion chamber
The shape of prechamber is spherical. 90-95% of air is pushed in the Prechamber. © Prof. K. K. Sharma
2.Turbulent Chamber/Swirl Chamber This type of combustion chamber finds its application where fuel quality is difficult to control Reliability is more important than fuel economy.
© Prof. K. K. Sharma
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