In a spark ignition engine, spark ignites a fuel-air mixture inside the cylinder via compression. The combustion of the mixture causes gas expansion, which then pushes the piston upward. This expansion process is referred to as the power stroke. What are the signs of the work and internal energy change of the gas at the power stroke? a. + W, +∆U b. + W, −∆U c. −W, −∆U d. −W, +∆U

In a spark ignition engine, spark ignites a fuel-air mixture inside the cylinder via compression. The combustion of the mixture causes gas expansion, which then pushes the piston upward. This expansion process is referred to as the power stroke. What are the signs of the work and internal energy change of the gas at the power stroke? a. + W, +∆U b. + W, −∆U c. −W, −∆U d. −W, +∆U