14-02-2013, 03:43 PM
Role of Fuel AND Combustion Method in Rate of Heat Release
[attachment=50927]
Model Constants
The values of a1 and a2 in the two Wiebe approximation ranged from 10 to 2000 and 3 to 37 respectively.
The values of the m constants also changed between the two types of approximations.
The values of m1 and m2 ranged from 2 to 9 and 2 to 8 instead of about 3.
The value of x, the scale factor, ranges from 0.5 to 0.9.
Think From the Beginning
Sadi Carnot identified several fundamental ideas for internal combustion engines:
1. He noted that air compressed by a ratio of 15 to 1 would be hot enough (200°C) to ignite dry wood.
2. He recommended compressing the air before combustion. Fuel could then be added by "an easily invented injector".
3. Carnot realized that the cylinder walls would require cooling to permit continuous operation.
4. He noted that usable heat would be available in the exhaust, and recommended passing it under a water boiler.
Type of Fuel Vs Combustion Strategy
Highly volatile with High self Ignition Temperature: Spark Ignition. Ignition after thorough mixing of air and fuel.
Less Volatile with low self Ignition Temperature: Compression Ignition , Almost simultaneous mixing & Ignition.
Patterns of Fuel behavior in CI Engines
A. Fuel injection across the chamber with substantial momentum.
Mixing proceeds immediately as fuel enters the chamber and is little affected by combustion.
B. Fuel deposition on the combustion chamber walls.
Negligible mixing during the delay period due to limited evaporation.
After ignition, evaporation becomes rapid and its rate is controlled by access of hot gases to the surface, radial mixing being induced by differential centrifugal forces.
Burning is therefore delayed by the ignition lag.
C. Fuel distributed near the wall.
Mixing proceeds during the delay but at a rate smaller than in mechanism A.
After ignition, mixing is accelerated by the same mechanism as in mechanism B.
[attachment=50927]
Model Constants
The values of a1 and a2 in the two Wiebe approximation ranged from 10 to 2000 and 3 to 37 respectively.
The values of the m constants also changed between the two types of approximations.
The values of m1 and m2 ranged from 2 to 9 and 2 to 8 instead of about 3.
The value of x, the scale factor, ranges from 0.5 to 0.9.
Think From the Beginning
Sadi Carnot identified several fundamental ideas for internal combustion engines:
1. He noted that air compressed by a ratio of 15 to 1 would be hot enough (200°C) to ignite dry wood.
2. He recommended compressing the air before combustion. Fuel could then be added by "an easily invented injector".
3. Carnot realized that the cylinder walls would require cooling to permit continuous operation.
4. He noted that usable heat would be available in the exhaust, and recommended passing it under a water boiler.
Type of Fuel Vs Combustion Strategy
Highly volatile with High self Ignition Temperature: Spark Ignition. Ignition after thorough mixing of air and fuel.
Less Volatile with low self Ignition Temperature: Compression Ignition , Almost simultaneous mixing & Ignition.
Patterns of Fuel behavior in CI Engines
A. Fuel injection across the chamber with substantial momentum.
Mixing proceeds immediately as fuel enters the chamber and is little affected by combustion.
B. Fuel deposition on the combustion chamber walls.
Negligible mixing during the delay period due to limited evaporation.
After ignition, evaporation becomes rapid and its rate is controlled by access of hot gases to the surface, radial mixing being induced by differential centrifugal forces.
Burning is therefore delayed by the ignition lag.
C. Fuel distributed near the wall.
Mixing proceeds during the delay but at a rate smaller than in mechanism A.
After ignition, mixing is accelerated by the same mechanism as in mechanism B.