-: Flame Speed :-
A property of fuel which determines its ability to undergo controlled combustion without detonation is known as flame speed.
The flame speed is the measured rate of expansion of the flame front in a combustion reaction .
-: Factors affecting / Influencing the Flame speed :-
The factors which can affect the flame speed are given as -
1- Turbulence
2- Fuel-Air Ratio
3- Temperature and Pressure
4- Compression Ratio
5- Engine output
6- Engine Speed
7- Engine Size
1- Turbulence :-
The motion of air-fuel mixture inside the combustion chamber is known as turbulence . The flame speed is quite low in non-turbulent mixture and increases with increasing turbulence.
The turbulence in the incoming mixture is generated during the admission of fuel-air mixture through comparatively narrow section of the intake pipe , valve openings etc,in the suction stroke . A suitable design of the combustion chamber which involves the geometry of cylinder head and piston crown increases the turbulence during the compression stroke .
The increase of flame speed due to turbulence reduces the combustion duration and hence minimizes the tendency of abnormal combustion .
2- Fuel-Air Ratio :-
The fuel-air ratio has a very significant influence on the flame speed . The higher flame velocities are obtained with somewhat richer mixture . When the mixture is made leaner or richer , the flame speed decreases . Very rich mixture lead to incomplete combustion which result again in the release of less thermal energy .
3- Temperature and Pressure :-
Flame speed increases with an increase in intake temperature and pressure . A higher initial pressure and temperature may help to form a better homogeneous air-vapour mixture which helps in increasing the flame speed .
4- Compression Ratio :-
A higher compression ratio increases the pressure and temperature of the working mixture which reduces the initial preparation phase of combustion and hence less ignition advances is needed . High pressure and temperature of the compressed mixture also speed up the second phase of combustion.
This increases the peak pressure and temperature and the total combustion duration is reduced . Thus engines having higher compression ratios have higher flame speed .
5- Engine Output :-
The cycle pressure increases when the engine output is increased . With the increased throttle opening the cylinder gets filled to a higher density . This results in increased flame speed .
6- Engine Speed :-
The flame speed increases almost linearly with engine speed since the increase in engine speed increases the turbulence inside the cylinder .
7- Engine Size :-
The size of the engine does not have much effect on the rate of flame propagation . In large engines the time required for complete combustion is more because the flame has to travel a longer distance .
A property of fuel which determines its ability to undergo controlled combustion without detonation is known as flame speed.
The flame speed is the measured rate of expansion of the flame front in a combustion reaction .
 |
| Image Source : Google |
-: Factors affecting / Influencing the Flame speed :-
The factors which can affect the flame speed are given as -
1- Turbulence
2- Fuel-Air Ratio
3- Temperature and Pressure
4- Compression Ratio
5- Engine output
6- Engine Speed
7- Engine Size
1- Turbulence :-
The motion of air-fuel mixture inside the combustion chamber is known as turbulence . The flame speed is quite low in non-turbulent mixture and increases with increasing turbulence.
The turbulence in the incoming mixture is generated during the admission of fuel-air mixture through comparatively narrow section of the intake pipe , valve openings etc,in the suction stroke . A suitable design of the combustion chamber which involves the geometry of cylinder head and piston crown increases the turbulence during the compression stroke .
The increase of flame speed due to turbulence reduces the combustion duration and hence minimizes the tendency of abnormal combustion .
2- Fuel-Air Ratio :-
The fuel-air ratio has a very significant influence on the flame speed . The higher flame velocities are obtained with somewhat richer mixture . When the mixture is made leaner or richer , the flame speed decreases . Very rich mixture lead to incomplete combustion which result again in the release of less thermal energy .
3- Temperature and Pressure :-
Flame speed increases with an increase in intake temperature and pressure . A higher initial pressure and temperature may help to form a better homogeneous air-vapour mixture which helps in increasing the flame speed .
4- Compression Ratio :-
A higher compression ratio increases the pressure and temperature of the working mixture which reduces the initial preparation phase of combustion and hence less ignition advances is needed . High pressure and temperature of the compressed mixture also speed up the second phase of combustion.
This increases the peak pressure and temperature and the total combustion duration is reduced . Thus engines having higher compression ratios have higher flame speed .
5- Engine Output :-
The cycle pressure increases when the engine output is increased . With the increased throttle opening the cylinder gets filled to a higher density . This results in increased flame speed .
6- Engine Speed :-
The flame speed increases almost linearly with engine speed since the increase in engine speed increases the turbulence inside the cylinder .
7- Engine Size :-
The size of the engine does not have much effect on the rate of flame propagation . In large engines the time required for complete combustion is more because the flame has to travel a longer distance .
No comments:
Post a Comment