Game Engines

Purpose
Game engines are used to build video games. Their purposes are that they provide a suite of development tools. For simplified and rapid development, the tools are usually provided in an IDE (Integrated Development Environment). Game engine developers try to develop robust software which includes many elements that are required to build a game. Most game engines include graphics, sound, physics and AI functions. Sometimes these engines are called "middleware" because of the flexible and reusable software they provide the required core functionality. This helps with the critical factors in the video game industry. They feature tools such as renderers, animation and scripting, and programming languages such as Java and C++. A game engine's purpose is to give the developer(s) a wide range of tools for production

Examples of game engines 
Some examples of game engines are:
Unreal Engine
Construct 2
Frostbite

The constraints
Unreal Engine
 Polygon count - The polygon count for Unreal Engine depends on how many models are in the game at once. If you don't have many characters in your game, the recommended polygon count is around 30k, whilst that would be too much if you have over 50 characters.
 Real time rendering - Unreal Studio has a tool called Sequencer. It is used to create cinematics and videos with the game.
 Hardware - Unreal Engine started off as an engine for PC, until Unreal Tournament for PS2 and Dreamcast. UE4 had a rough start on PS4 and Xbox One, however now it works well. Epic Games has began to focus on UE4 for mobile and Switch too.
 LOD's - LOD (Level of Detail) is used to limit the detail of the graphics depending on how far the camera is.
 File size - UE4's file size is 15GB.
 Collision detection - Every model with collision has an 'object type', letting the creator choose whether it can affect, be affected, or none.

Construct 2
 Polygon count - Having a certain number of sprites in a game can cause it to lag and crash.
 Real time rendering - Since Construct 2 is a 2D game creator, it is unable to create real time cinematics.
 Hardware - Construct 2 games can run on PCs through HTML5 and mobile.
 LOD's - The LODs for sprites cannot be altered in Construct 2.
 File size - Unknown
 Collision detection - The sprites have many options for collision.

Frostbite

 Polygon count - Unknown
 Real time rendering - Frostbite Engine uses real time rendering for cinematics.
 Hardware - Frostbite Engine is compatible with PC, PS4 and Xbox One. It's also rumoured that it's getting support for Switch.
 LOD's - Frostbite switches from using LOD to not using LOD for different games.
 File size - Frostibite is currently only available for EA.
 Collision detection - The engine doesn't stop players from walking through the floor/wall.