Removes unneccesary word

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docs/documentation/libraries/engine/index.md

-# Introduction {#mainpage}
-PENIS
\ No newline at end of file
+# The `engine` Library in UNICADO
+
+The `_engine_` library serves as the core analysis tool for engine data within UNICADO. It provides access to engine data. The data can be fixed for an engine or at a given operating point. The data output depends on various factors such as the scale factor and power and bleed offtakes from the engine. The primary objective is to establish a **single source of truth** for engine data retrieval.
+
+## Role in `propulsionDesign`
+Within the `propulsionDesign` module:
+- Engines for the aircraft are selected, and their respective files are copied to the engine directory.
+- The **scale factor** is calculated, determining how the engine's thrust is adjusted to meet aircraft requirements (refer to the `propulsionDesign` documentation).
+
+The `engine` library applies this scale factor, ensuring that aircraft parameters can be accessed without further manual adjustments.
+
+## Engine Data Formats
+The engine data is stored in:
+- `engine.xml` — Contains data **independent** of the operating point.
+- CSV files — Store values **dependent** on:
+  - **Mach number**
+  - **Altitude**
+  - **Engine power setting**  
+
+> **Note:** The data in these files is **raw and unscaled**. The only modification made in `propulsionDesign` is to the fuel flow CSV file, reflecting user-defined efficiency improvements.
+
+## Functionality of the `engine` Library
+The library is responsible for:
+- **Reading engine data**
+- **Applying scaling factors to the data**
+- **Modifying values based on performance-influencing factors like bleed and power offtakes**
+
+### Factors Affecting Engine Performance
+The `engine` library incorporates the following factors, either by default or as optional parameters:
+- **Scale factor** from `propulsionDesign`
+- **Temperature variations** (non-ISA standard conditions)
+- **Engine derating**
+- **Bleed air extraction** (for turbofan engines)
+- **Spool shaft offtake** (for turbofan engines)
+
+## How the Library Retrieves Data
+- If data is **not dependent** on the operating point → Uses `engine.xml` in a simple readout.
+- If data is **dependent** on the operating point → Uses CSV files and requires:
+  - Mach number
+  - Altitude
+  - Engine power setting (e.g., N1 for turbofan engines)
+
+A **linear interpolation** is performed between existing operating points when retrieving values from CSV files.
+