Introduction:
In internal combustion (I.C.) engines, the efficiency of the engine is directly influenced by the temperature of the intake air. The intake air temperature affects the combustion process and the overall performance of the engine. In this case, the increase in intake air temperature leads to a decrease in engine efficiency.

Explanation:
- Effect on Air Density: The increase in intake air temperature causes a decrease in air density. This is because, as the temperature of air increases, its molecular motion also increases, leading to the expansion of air molecules. As a result, the mass of air per unit volume decreases, resulting in decreased air density. The decrease in air density reduces the amount of air available for combustion, resulting in incomplete combustion and decreased engine efficiency.

- Effect on Combustion: The increase in intake air temperature affects the combustion process within the engine. Higher temperature air has a lower oxygen concentration, which in turn affects the combustion reaction. With less oxygen available, the fuel-air mixture burns less efficiently, leading to incomplete combustion and reduced engine efficiency.

- Effect on Heat Transfer: The increase in intake air temperature also affects the heat transfer within the engine. Higher temperature air entering the combustion chamber increases the temperature difference between the combustion gases and the engine components. This leads to increased heat transfer from the combustion gases to the engine components, resulting in higher heat losses and reduced engine efficiency.

- Effect on Specific Heat Capacity: The increase in intake air temperature affects the specific heat capacity of the air. The specific heat capacity is the amount of heat energy required to raise the temperature of a given mass of a substance by a certain amount. As the temperature of air increases, its specific heat capacity decreases. This means that the air has a reduced capacity to absorb heat energy during the combustion process, resulting in decreased engine efficiency.

Conclusion:
In conclusion, the increase in intake air temperature in an I.C. engine leads to a decrease in engine efficiency. This is due to the decrease in air density, incomplete combustion, increased heat transfer, and reduced specific heat capacity of the air. It is important to consider and control the intake air temperature in order to optimize the performance and efficiency of the engine.