Real vs Ideal Engine
Compare side-by-side — losses in real systems.
Key Notes
Ideal (Carnot) engine: reversible, infinitely slow, max efficiency.
Real engines: irreversible, finite speed, lower efficiency.
Sources of inefficiency: friction, turbulence, heat leakage, fast non-quasi-static processes, incomplete combustion.
Real engine efficiency = η_real < η_Carnot = 1 − T_c/T_h.
Carnot's theorem: NO engine can exceed Carnot efficiency; only reversible engines reach it.
Practical efficiencies: gasoline ~25-35%, diesel ~35-45%, gas turbine ~30-40%, combined cycle ~50-60%.
Higher T_hot (more efficient) requires advanced materials — engineering trade-off.
Engine output power = efficiency × heat input rate.
Formulas
Real efficiency
Measured directly from engine operation.
Carnot bound
Theoretical upper limit.
Reduction factor
Typical for real engines (depends on quality).
Important Points
Carnot efficiency is THEORETICAL UPPER BOUND.
Real engines lose to friction, finite speeds, heat leaks, non-equilibrium combustion.
η_real / η_Carnot ≈ 0.5-0.8 for well-designed engines.
Increasing T_h is key to higher η — modern gas turbines push T_h to ~1500°C using ceramic blades and cooling.
Diesel engines achieve higher η than gasoline because higher T_h (compression-ignition vs spark).
Carnot bound is INDEPENDENT of working fluid — only T_h and T_c matter.
Real vs Ideal Engine notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 11 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.