Comparative Thermal Efficiency – Petrol (SI) Versus Diesel (CI) Engines In general, how does the thermal efficiency of a petrol (spark-ignition) engine compare with that of a diesel (compression-ignition) engine of similar size and technology?

Introduction / Context:
Thermal efficiency indicates how much of the fuel’s chemical energy is converted into useful shaft work. Comparing spark-ignition (SI) and compression-ignition (CI) engines reveals key design trade-offs that drive vehicle choice, fuel economy, and emissions outcomes.

Given Data / Assumptions:

• Similar displacement and technology level (e.g., naturally aspirated or similarly boosted).
• Comparable operating conditions.
• No hybridization or unusual cycles.

Concept / Approach:

Diesel engines typically achieve higher thermal efficiency than petrol engines due to several factors: higher compression ratios enabled by auto-ignition, lean operation which reduces throttling losses at part load, higher ratio of specific heats during expansion, and better effective expansion utilization. Petrol engines are knock-limited and often operate throttled at part load, which increases pumping work and lowers efficiency.

Step-by-Step Solution:

Verification / Alternative check:

Typical brake thermal efficiency ranges: modern light-duty SI about 0.30–0.36; comparable CI about 0.38–0.45. These empirical values support the stated trend.

Why Other Options Are Wrong:

“Same” and “cannot be compared” ignore consistent empirical and theoretical trends. “More” contradicts both cycle analysis and data. Idle is not a meaningful benchmark for efficiency comparison.

Common Pitfalls:

Conflating specific power or transient response with efficiency; they are distinct metrics. Also, turbocharging can raise both engines’ efficiencies but does not reverse the general ranking.

Final Answer:

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