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the cylinder (known as Bottom Dead Centre or BDC) the exhaust valve starts to open. As the piston now moves up the cylinder, the hot gases (consisting mostly of nitrogen, carbon dioxide, water vapour and unused oxygen) are expelled from the cylinder. As the Piston approaches TDC again the inlet valve starts to open and the cycle repeats itself.
Volcanic Gases An erupting volcano will release gases, tephra, and heat into the atmosphere. The largest portion of gases released into the atmosphere is water vapor. Other gases include carbon dioxide (CO2), sulfur dioxide (SO2), hydrochloric acid (HCl), hydrogen fluoride (HF), hydrogen sulfide (H2S), carbon monoxide (CO), hydrogen gas (H2), NH3, methane (CH4), and SiF4.