In the power stage of the Otto cycle, what is closed and what is ignited?

• A. Intake closed, exhaust open
• B. Mixture ignited, both valves closed
• C. Exhaust valve closed, mixture ignited
• D. Both valves open, fuel ignited

Answer: (B)

During the power stage, the goal is to convert the fuel’s energy into work by burning the air–fuel mixture inside a sealed cylinder. To do that, both the intake and exhaust valves are closed, trapping the mixture and the resulting combustion products. A spark plug ignites the mixture, causing a rapid rise in pressure that expands the gases and pushes the piston downward, delivering work to the crankshaft. If a valve were open, the high-pressure gases would escape instead of doing work, which is why the other scenarios don’t fit this stage.