The chamber is pressurized with pure oxygen, and the pressure inside the chamber will slowly rise to about two and a half times normal atmospheric pressure.

CNN: Gangrene

The EPA pulled off the feat by sharply increasing the amount of oxygen-poor exhaust that is recirculated into the combustion chamber and increasing the pressure of the fuel being forced into the cylinder.

FORBES: EPA, Inc.

The oxygen that remains is burned with fuel and water in a chamber modeled after a rocket engine.

FORBES

To simulate the atmosphere, they have built a special cloud chamber of their own, with the air manufactured from scratch, using liquid nitrogen and oxygen together with precise amounts of trace compounds, including sulphuric acid and ammonia.

ECONOMIST: Climate science (II)

In this configuration, the HTP is passed over a catalyst pack to decompose it into steam and oxygen, but instead of igniting a fuel grain, this superhot gas is allowed simply to vent through the chamber nozzle.

BBC: Science & Environment