Diesel engines on weak (lean) mixtures: effect on thermal efficiency For compression-ignition (diesel) engines operating with weak or lean mixtures (excess air), how does the thermal efficiency change? Choose the correct option.

Given data

• Engine type: compression-ignition (diesel), typically unthrottled and operating lean with excess air.

Concept/Approach
Diesel engines admit air first and inject fuel later. Running lean reduces pumping losses and often reduces heat transfer per unit fuel, bringing operation closer to the idealized cycle. Hence, thermal efficiency generally increases on leaner operation compared with richer diesel combustion.

Notes
Real engines have multiple influences (injection timing, combustion phasing, EGR), but foundational MCQ theory states that leaner (weak) mixtures in diesels indicate higher efficiency.

Final Answer
Higher