How does a ramjet engine compress air without a compressor?

A ramjet engine does not require a mechanical compressor because it relies on the craft's high forward velocity to compress incoming air, a phenomenon known as the ram effect.12

As the vehicle travels at high speeds—typically Mach 2 or greater—the air is forced into a specially shaped intake passage or diffuser. This geometry decelerates the incoming airstream, which converts the kinetic energy of the high-speed air into a significant increase in static pressure. Once the air is sufficiently compressed by this process, it enters the combustion chamber where fuel is injected and ignited. The resulting expansion of hot exhaust gases out of the rear of the engine provides the necessary forward thrust.2314

Because this process depends entirely on the vehicle's forward motion to create pressure, ramjets cannot produce static thrust and require an auxiliary propulsion system to accelerate the craft to a speed where the ram effect becomes efficient.12