What is the purpose of combustion?

• A. Increase velocity and temperature
• B. Increase pressure only
• C. Cool the gas
• D. Decrease mass flow

Answer: (A)

Combustion is about releasing chemical energy by burning fuel with an oxidizer, which heats the working gas and adds energy to it. That heat makes the gas expand and move faster as it loses energy through the turbine or nozzle, so the hot, fast-moving gas produces more thrust or power. So the main effect of combustion is to raise the temperature and the velocity of the gas, providing the energy needed to do work in the engine or propulsion system.

That’s why increasing both velocity and temperature is the best description. The other options don’t fit as the primary purpose: just increasing pressure isn’t the main goal (pressure can rise as a byproduct, but the engine relies on higher temperature and subsequent expansion to gain speed and thrust); cooling the gas is the opposite of what combustion does; and decreasing mass flow isn’t the aim—combustion adds energy to the existing flow, not reduce it.