According to the principle of energy conservation, the kinetic energy of gas must increase when a piston in an automobile engine compresses the gas in a cylinder. This is because as the piston moves, it compresses the gas, increasing its pressure and temperature. The compression of the gas increases its kinetic energy because the gas molecules are forced to move closer together and collide more frequently. These collisions between the gas molecules result in an increase in the kinetic energy of the gas.

Option 2, stating that the gas must undergo a change of state, is not necessarily true. While the compression of the gas may lead to a change in its temperature and pressure, it does not necessarily result in a change of state. The gas can remain in the same state, either as a gas or a mixture of gases, even after compression.

Option 3, stating that the gas must undergo a chemical change, is also not necessarily true. The compression of the gas does not involve any chemical reactions or changes in the chemical composition of the gas.

Therefore, the correct answer is option 1, as the compression of the gas in the cylinder increases its kinetic energy.