Effect of intake air temperature on engine efficiency In internal combustion engines operating at fixed hardware (same compression ratio and calibration), increasing the intake air temperature will generally __________ the overall efficiency.

Introduction / Context:
Intake air temperature influences charge density, knock tendency, and pumping losses. Knowing the direction of its effect on efficiency helps interpret intercooling benefits and hot-weather performance changes.

Given Data / Assumptions:

• Hardware unchanged (same compression ratio and combustion system).
• Throttle and calibration adjust to maintain load.
• Comparison over typical operating conditions (SI focus, but principles also affect CI engines).

Concept / Approach:

Hotter intake air is less dense, so less oxygen mass enters for a given manifold pressure. To achieve the same torque, the throttle must open further in SI engines, but the lower charge density typically reduces volumetric efficiency and can increase propensity to knock (forcing spark retard), both of which reduce thermal efficiency. In turbocharged engines, intercooling (cooling the intake air) commonly increases efficiency by improving charge density and allowing more optimal spark or fueling.

Step-by-Step Solution:

Verification / Alternative check:

Practical data: intercoolers in turbo/supercharged engines measurably improve brake specific fuel consumption by cooling the intake; the inverse (heating) degrades it.

Why Other Options Are Wrong:

“Increase” contradicts general observations; “no effect” would only hold in an ideal Otto analysis where efficiency depends solely on compression ratio, ignoring real-world volumetric, timing, and knock constraints.

Common Pitfalls:

Confusing ideal-cycle dependence (efficiency vs compression ratio only) with real-engine behavior where volumetric efficiency and knock play crucial roles.

Final Answer:

decrease