Comparative efficiency: The thermal efficiency of a typical two-stroke cycle engine is generally ________ that of a four-stroke cycle engine of similar size and technology.

Introduction / Context:
Two-stroke and four-stroke engines differ in gas exchange, combustion timing, and scavenging. These differences affect indicated and brake thermal efficiencies. Understanding the comparative trends is important for engine selection and application.

Given Data / Assumptions:

• Similar displacement class and technology level.
• Conventional crankcase-scavenged two-stroke versus poppet-valve four-stroke.
• No direct-injection stratified-charge two-stroke improvements assumed.

Concept / Approach:
Traditional two-stroke engines suffer from short-circuiting (fresh charge lost during scavenging), poorer volumetric efficiency control, and higher mixing losses. Four-strokes separate intake and exhaust strokes, enabling better gas exchange and more complete combustion, typically yielding higher thermal efficiency.

Step-by-Step Solution:

Verification / Alternative check:
Brake-specific fuel consumption (BSFC) data commonly show higher BSFC (worse efficiency) for traditional two-strokes compared with comparable four-strokes.

Why Other Options Are Wrong:

• Equal to / greater than: Only in specialized modern two-strokes (e.g., DI with advanced scavenging) might parity be approached; the general case is lower efficiency.

Common Pitfalls:
Confusing power density (two-strokes deliver more frequent power strokes) with efficiency. More frequent power strokes do not guarantee better fuel economy.

Final Answer:
less than