Steam engine speed control: cut-off governing versus throttle governing Compared with throttle governing, cut-off governing is __________ and __________ for steam engines operating over varying loads.

Introduction / Context:
Steam engines maintain speed under load changes through governing. Two classical methods are throttle governing (vary inlet pressure by throttling) and cut-off governing (vary admission period while keeping initial pressure high). Understanding which yields better thermal performance and fuel economy is key for historical engine design and modern analogies in control systems.

Given Data / Assumptions:

• Saturated or slightly superheated steam supply.
• Comparable mechanical condition and valve gear.
• Focus on thermodynamic efficiency and specific steam consumption.

Concept / Approach:
Throttle governing lowers inlet pressure, reducing the mean effective pressure but also introducing throttling losses and degrading cycle efficiency. Cut-off governing keeps inlet pressure high, admits steam for a shorter portion of the stroke, and relies on expansion to extract work efficiently, thus improving thermal efficiency and economy.

Step-by-Step Solution:
Throttle governing: reduce pressure across a throttling valve ⇒ irreversibility and lower mean effective pressure.Cut-off governing: keep inlet at near-boiler pressure ⇒ earlier cut-off ⇒ larger expansion ratio and better indicated efficiency.Fuel and steam rates: lower specific steam consumption for cut-off at similar power levels.Hence, cut-off governing is both more efficient and more economical than throttle governing.

Verification / Alternative check:
Indicator diagrams show higher area per unit steam with earlier cut-off at the same speed, confirming improved economy relative to throttling for the same brake power.

Why Other Options Are Wrong:
Any option labeling cut-off as “less efficient” conflicts with reduced throttling irreversibility and better expansion work. The mixed options do not capture the combined thermodynamic and economic benefits.

Common Pitfalls:
Equating speed control smoothness with thermodynamic efficiency; ignoring leakage or valve gear specifics that do not change the basic thermodynamic advantage.

Final Answer:
more efficient and more economical