During the compression stroke, which statement is true?

During the compression stroke of an engine, the mass of the mixture remains constant because, according to the law of conservation of mass, the amount of gas present does not change during this process. As the piston moves upwards in the cylinder, the volume of the space is decreased, leading to an increase in pressure and temperature of the gases due to compression. However, this does not affect the mass; it remains the same throughout the stroke.

The other statements, while related to the behavior of gases during the compression process, do not accurately describe what happens during this phase. The temperature of the gases does not stay constant; instead, it increases due to the work done on the gas as it is compressed. The volume of the gases decreases since the piston is moving upward, compressing the gas. Finally, the mass of the mixture cannot decrease during compression unless there is a leak, which is not a characteristic of the ideal compression stroke being considered.