5.1 Introduction

Cycle analysis studies the thermodynamic changes of the working fluid (air and products of combustion in most cases) as it flows through the engine. It is divided into two types of analysis: parametric cycle analysis (also called design-point or on-design) and engine performance analysis (also called off-design). Parametric cycle analysis determines the performance of engines at different flight conditions and values of design choice (e.g., compressor pressure ratio) and design limit (e.g., combustor exit temperature) parameters. Engine performance analysis determines the performance of a specific engine at all flight conditions and throttle settings.

In both forms of analysis, the components of an engine are characterized by the change in properties they produce. For example, the compressor is described by a total pressure ratio and efficiency. A certain engine's behavior is determined by its geometry, and a compressor will develop a certain total pressure ratio for a given geometry, speed, and airflow. Because the geometry is not included in parametric cycle analysis, the plots of specific thrust F/ , and thrust specific fuel consumption S vs, say, Mach number or compressor pressure ratio are not portraying the behavior of a specific engine. Each point on such plots represents a different engine. The geometry for each plotted engine will be different, and thus we say that parametric cycle analysis represents a "rubber engine." Parametric cycle analysis is also called