Thermal Efficiency Calculator

Thermal efficiency describes the fraction of supplied heat energy that a heat engine converts into useful work. This calculator uses η = Wout/Qin × 100%, where η is efficiency as a percentage, Wout is net useful work output, and Qin is heat absorbed from the hot source. The remaining energy is rejected as heat: Qout = Qin - Wout. Heat and work must use the same unit, such as joules, kilojoules, or kilowatt-hours, because efficiency is a dimensionless ratio. A heat engine operates cyclically between hot and cold surroundings. It cannot convert all absorbed heat to work while completing a cycle; some energy must be discharged to a lower-temperature sink. This is a consequence of the second law of thermodynamics, not merely imperfect construction. Real engines lose additional useful energy through friction, combustion irreversibility, finite temperature differences, pumping, exhaust, cooling, and auxiliary equipment. Consequently, measured efficiency is always below the relevant ideal limit. Thermal efficiency may be reported on different boundaries and fuel bases. Gross indicated efficiency, brake thermal efficiency, and overall plant efficiency do not necessarily include the same losses. Fuel calculations may use higher or lower heating value, which changes the denominator. Compare systems only when heat input, useful output, operating state, and measurement boundary are defined consistently. For a device producing electrical power, integrate power over the same interval used to determine fuel heat input, or compare steady powers when storage is negligible. The ideal maximum for an engine operating between two constant-temperature reservoirs is the Carnot efficiency, 1 - Tc/Th, with both absolute temperatures in kelvins. The simple energy-ratio calculator does not require temperatures and does not claim Carnot performance. Instead, it evaluates observed or specified energy flows. A result above 100 percent signals inconsistent inputs, mismatched units, an omitted energy source, or an accounting-boundary problem, so this interface rejects work greater than heat input. Use this result for classroom thermodynamics, first-pass engine comparisons, and energy-balance checks. Detailed system assessment should include uncertainty, transient operation, fuel composition, parasitic loads, heat recovery, and the exact definition of useful output. Efficiency is not the same as power: a highly efficient engine can have low power, while a powerful engine can waste a large share of its input energy.