Here are four key assumptions commonly made in the thermodynamic analysis of the CI (Compression Ignition) engine combustion process:
🔧 1. Ideal Gas Behavior
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The working fluid (air or air-fuel mixture) is treated as an ideal gas, obeying the ideal gas law:
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This simplifies analysis by ignoring real gas effects.
🔧 2. Combustion is Modeled as a Heat Addition Process
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Combustion is not analyzed in chemical detail; instead, it is modeled as external heat addition to the system (as in air-standard cycle analysis).
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Chemical reactions are not explicitly solved.
🔧 3. Uniform Pressure and Temperature (Quasi-Steady or Lumped System)
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The pressure and temperature inside the combustion chamber are assumed to be uniform at each crank angle.
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This neglects spatial variations and assumes a homogeneous combustion process.
🔧 4. No Heat Loss or Friction (Air-Standard Cycle Assumption)
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The system is often assumed to be adiabatic, with no heat loss to cylinder walls, and no friction.
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This simplifies the first law of thermodynamics and focuses on ideal performance.
✅ Summary Table:
Assumption | Purpose |
---|---|
Ideal gas behavior | Simplifies thermodynamic equations |
Combustion = heat addition | Avoids complex chemical kinetics |
Uniform pressure and temperature | Allows single-zone analysis |
No heat loss or friction | Ideal cycle comparison |
These assumptions help create a theoretical model that provides insights into engine performance, efficiency, and combustion behavior, though actual engine conditions are more complex.
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