How does a ramjet engine work?

In a Ramjet engine the air flows through the combustion chamber of the engine at subsonic speeds. The incoming air is compressed using the forward movement of the engine. You can read about the Hypersonic Technology Demonstrator Vehicle (HSTDV) – unmanned scramjet technology of DRDO in the given link.

Further readings:

1. Advanced Technology Vehicle – India, one of the 4 Countries To Develop Scramjet Engine
2. Reusable Launch Vehicle – Technology Demonstrator (RLV – TD)