Brake Thermal Efficiency – Relative Ranking Across Basic Engine Types Arrange the following in decreasing order of typical brake thermal efficiency (highest first): four-stroke compression-ignition (C.I.) engine, four-stroke spark-ignition (S.I.) engine, two-stroke spark-ignition (S.I.) engine.

Introduction / Context:
Brake thermal efficiency (BTE) reflects how effectively an engine converts fuel energy into shaft power. Different combustion modes and gas exchange strategies lead to characteristic efficiency differences between engine families.

Given Data / Assumptions:

• Steady operation at comparable loads and technology levels.
• Compression ratios: CI > SI; two-stroke SI suffers from scavenging losses.
• No hybridization or waste-heat recovery considered.

Concept / Approach:

Four-stroke CI engines typically achieve the highest BTE due to high compression ratios, lean operation with lower pumping losses, and higher ratio of specific heats during expansion. Four-stroke SI engines rank lower because of throttling (at part load) and knock-limited compression ratios. Two-stroke SI engines generally have the lowest BTE because of short-circuiting/scavenging losses, higher hydrocarbon emissions, and less efficient cylinder filling and combustion control.

Step-by-Step Solution:

Verification / Alternative check:

Empirical BTE ranges: modern light-duty SI ~0.30–0.36, CI ~0.38–0.45, small two-stroke SI often <0.30, aligning with the ranking.

Why Other Options Are Wrong:

Any ordering placing SI ahead of CI or two-stroke SI above four-stroke SI contradicts fundamental thermodynamics and observed scavenging losses.

Common Pitfalls:

Confusing specific power (which can be high in two-strokes) with efficiency; these are different metrics.

Final Answer:

four stroke C.I. engine, four stroke S.I. engine, two stroke S.I. engine