Effect of higher intake-air temperature on thermal efficiency of an internal-combustion engine If the temperature of the intake air supplied to an IC engine increases (all else being similar), how is the engine's efficiency affected?

Introduction / Context:
Intake conditions strongly influence both the thermodynamic cycle and volumetric efficiency of internal-combustion engines. Warmer intake air changes charge density, mixture preparation, knock margin, and ultimately brake-specific fuel consumption and efficiency.

Given Data / Assumptions:

• Same engine geometry and compression ratio.
• No supercharging interstage cooling change; only intake temperature is increased.
• Comparable loads and speeds within the efficient operating range.

Concept / Approach:
Raising intake temperature lowers air density at a given manifold pressure, reducing the trapped mass of air per cycle. For SI engines, this reduces volumetric efficiency and can also reduce knock margin, requiring spark retard. For CI engines, higher intake temperature may shorten ignition delay but typically still reduces volumetric efficiency. Lower trapped oxygen mass usually reduces peak indicated efficiency because to maintain torque the throttle (SI) or fueling (CI) must adjust, and cycle-average losses rise relative to useful work.

Step-by-Step Solution:
Higher T_in → lower density (rho ∝ 1/T for given p) → less air per cycle.Less air mass → lower potential for complete, efficient combustion at the same fuelling → more throttling (SI) or different injection strategy (CI).Cycle phasing changes (e.g., spark retard in SI) → reduced thermal efficiency.Therefore, efficiency generally decreases as intake air temperature increases.

Verification / Alternative check:
Dynamometer data commonly show reduced brake power and higher BSFC at elevated intake temperatures absent compensating measures like intercooling or boost increase.

Why Other Options Are Wrong:
“Remain the same” ignores volumetric efficiency. “Increase” contradicts the density argument. “Depends only on compression ratio” is incomplete; intake conditions also matter.

Common Pitfalls:
Assuming a small temperature rise is negligible. Even modest increases (e.g., heat-soaked intake) can measurably affect performance.

Final Answer:
Decrease