A jet engine performs by converting fuel into thrust. How well it performs is an indication of what proportion of its fuel goes to waste. It transfers heat from burning fuel to air passing through the engine. In doing so it produces thrust work when propelling a vehicle but a lot of the fuel is wasted and only appears as heat. Propulsion engineers aim to minimize the degradation of fuel energy into unusable thermal energy. Increased emphasis on performance improvements for commercial airliners came in the 1970s from the rising cost of fuel. The meaning of jet engine performance has been phrased as 'the end product that a jet engine company sells' and, as such, criteria include thrust and fuel consumption, life, weight, emissions, diameter and cost. Performance criteria reflect the level of technology used in the design of an engine and the technology has been advancing continuously since the jet engine entered service in the 1940s. Categories of performance include performance improvement, performance deterioration, performance retention, bare engine performance (uninstalled) and performance when part of an aircraft powerplant (installed). Jet engine performance (thrust and fuel consumption) for a pilot is displayed in the cockpit as engine pressure ratio (EPR) and exhaust gas temperature (EGT) or fan speed (N1) and EGT. EPR and N1 are indicators for thrust. EGT is an indicator for fuel flow but more importantly is a health monitor as it rises progressively with engine use over thousands of hours, as parts wear, until it reaches a limiting value. The performance of an engine is calculated using a thermodynamic analysis of the engine cycle. It works out what happens inside the engine. The conditions inside the engine, together with the fuel used and thrust produced, may be shown in a convenient tabular form summarising the analysis.