Diesel (CI) vs. petrol (SI) engines at rated load A diesel engine is ________ in efficiency compared to a petrol engine when both run at rated load.

Introduction / Context:
Comparing thermodynamic efficiencies of compression-ignition (diesel) and spark-ignition (petrol) engines is a staple topic in automotive engineering. Diesels typically achieve higher brake thermal efficiency under rated load conditions.

Given Data / Assumptions:

• Both engines operate at rated load and similar displacement class.
• Standard fuels: diesel vs. petrol.
• Focus on brake thermal efficiency, not transient or idle behavior.

Concept / Approach:
Diesel engines run at higher compression ratios and avoid throttling the intake air, reducing pumping losses. The diesel cycle’s ideal efficiency rises with compression ratio, and practical combustion with lean mixtures further improves fuel economy. Consequently, diesels generally consume less fuel energy per unit output at rated load than petrol engines.

Step-by-Step Solution:
Higher compression ratio → higher theoretical efficiency.Unthrottled intake → lower pumping work at load.Lean operation and high energy conversion → higher brake thermal efficiency.Therefore, select “more efficient.”

Verification / Alternative check:
BSFC charts show diesel engines typically in the 190–230 g/kWh range at rated points, versus higher values for comparable SI engines.

Why Other Options Are Wrong:

• Equally efficient: contradicts empirical data.
• Less efficient: opposite of standard results.
• Random or “only at idle” are not meaningful or accurate comparisons.

Common Pitfalls:
Mixing operating cost with capital cost or emissions constraints; the question targets efficiency at rated load.

Final Answer:
more efficient