Heat Engine Cycle
Rectangular cycle on PV — η = W_net / Q_in.
Key Notes
Heat engine: device that converts HEAT into WORK over a cycle.
Takes heat Q_h from a hot reservoir, dumps heat Q_c to a cold reservoir, produces work W = Q_h − Q_c.
Efficiency: η = W/Q_h = 1 − Q_c/Q_h.
Real engines: gasoline (Otto cycle), diesel (Diesel cycle), gas turbine (Brayton), steam (Rankine).
Maximum possible efficiency: Carnot's η = 1 − T_c/T_h.
Heat engines are CYCLIC: return to initial state, ΔU = 0 over cycle, Q_net = W_net.
On PV diagram: clockwise closed loop. Enclosed area = net work output.
Kelvin-Planck statement (2nd law): No engine can convert 100% of heat to work in a cycle.
Formulas
Heat engine efficiency
Fraction of input heat that becomes useful work.
Work output
Conservation of energy over the cycle.
Carnot limit
Upper bound on efficiency for given reservoir temperatures.
Real efficiency factor
Real engines run irreversibly ⇒ less efficient.
Important Points
Heat engine PRODUCES WORK from heat. Refrigerator does the opposite.
Efficiency is ALWAYS < 100% — Kelvin-Planck statement of 2nd law.
Higher T_h or lower T_c ⇒ better Carnot limit ⇒ potentially better real efficiency.
Power plants: typical ~ 35-45% efficient. Gas turbine combined cycle: up to ~60%.
Gasoline car: ~25-35%. Diesel: ~35-45%.
Real engines waste heat as 'cold reservoir' exhaust — usually atmosphere.
Heat Engine Cycle 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.