Overview
This lecture introduces the five main gas power cycles used in internal combustion (IC) engines, focusing in detail on the Carnot cycle, its processes, diagrams, efficiency, and limitations.
Types of Gas Power Cycles in IC Engines
- Five main gas power cycles: Carnot, Otto, Diesel, Dual, and Brayton cycles.
- Each cycle models different thermodynamic processes for engines.
Carnot Cycle: Description and Processes
- Carnot cycle is a reversible cycle that gives the maximum efficiency among all cycles.
- All Carnot cycle processes are reversible and idealized.
- Four main processes:
- 1→2: Reversible adiabatic (isentropic) compression.
- 2→3: Reversible isothermal heat addition.
- 3→4: Reversible adiabatic (isentropic) expansion.
- 4→1: Reversible isothermal heat rejection.
- Combines isentropic (no entropy change) and isothermal (constant temperature) processes.
Carnot Cycle Diagrams (PV and TS)
- PV (Pressure-Volume) diagram illustrates compression and expansion with corresponding volume and pressure changes.
- TS (Temperature-Entropy) diagram shows entropy constant during isentropic processes and temperature constant during isothermal processes.
- Work is input during compression, heat is added during isothermal process, work is output during expansion, and heat is rejected during isothermal heat rejection.
Practical Aspects and Limitations of Carnot Cycle
- Carnot cycle assumes air as the working medium for theoretical analysis.
- During adiabatic (isentropic) processes, the piston must move very quickly to prevent heat exchange.
- During isothermal processes, the piston must move very slowly to maintain constant temperature.
- Sudden changes in piston speed for different processes are not feasible in real engines.
- Carnot cycle is only a theoretical concept, not practical for real engines.
Efficiency of Carnot Engine
- Efficiency formula: (Maximum temperature – Minimum temperature) / Maximum temperature.
Key Terms & Definitions
- Isentropic process — Thermodynamic process with no change in entropy (reversible adiabatic).
- Isothermal process — Process occurring at constant temperature.
- Reversible process — Idealized process that can be reversed without net energy change.
- Adiabatic process — No heat transfer into or out of the system.
Action Items / Next Steps
- Review the diagrams and processes for all five gas power cycles.
- Study the limitations and idealized nature of the Carnot cycle.
- Prepare for comparison with other cycles in upcoming lectures.