According to the principle of energy conservation, when a piston in an automobile engine compresses the gas in a cylinder, which of the following must occur ?

According to the first law of thermodynamics, which is the principle of energy conservation, work done on a gas during compression increases its internal energy. In an automobile engine, as the piston moves to compress the gas, mechanical work is performed on the system. For an ideal gas, the internal energy is directly proportional to its absolute temperature, which is a macroscopic manifestation of the average kinetic energy of the gas molecules [4]. As the gas is squeezed into a smaller volume, the molecules collide more frequently and with greater force, leading to an increase in their kinetic energy and a subsequent rise in temperature [3]. While this process changes the state of the gas (pressure, volume, and temperature), the most fundamental energy conservation result is the increase in internal kinetic energy [4]. Chemical changes occur later during the combustion stroke, not primarily due to compression itself [1].

Sources

1. [4] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 22: Vertical Distribution of Temperature > A Parcel of Rising Air > p. 297
2. [3] https://pkel015.connect.amazon.auckland.ac.nz/SolidMechanicsBooks/Part_III/Chapter_4_Thermomechanics/Thermomech_01_Classical_Thermo_First.pdf
3. [1] https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/first-law-conservation-of-energy/