Steam engine control methods: throttle governing Throttle governing of steam engines is a method of controlling the engine output by varying the ________ of the intake steam (at the valve/throttle upstream of the cylinder), while keeping valve timing essentially unchanged.

Introduction / Context:
Steam engines are governed to maintain speed and power under varying loads. Two classical methods are throttle governing and cut-off governing. This question focuses on throttle governing, which alters the conditions of steam before admission without changing the valve timing significantly, and contrasts it with approaches that change the quantity admitted by valve-gear action.

Given Data / Assumptions:

• Simple reciprocating steam engine with a throttle (or governor-controlled valve) placed in the steam line ahead of the inlet valve.
• Valve timing and cut-off point remain essentially fixed.
• Load disturbances require quick power adjustments.

Concept / Approach:
In throttle governing, the governor modulates a throttle valve to reduce or increase the absolute pressure of steam at admission. Lowering intake pressure reduces the indicated mean effective pressure and hence power; raising it does the opposite. The mass flow per cycle changes primarily because the inlet pressure is changed, not because the cut-off position is varied.

Step-by-Step Solution:
Engine at constant cut-off: mass of steam admitted per cycle depends on inlet pressure and specific volume.Governor senses speed error and moves the throttle.Throttle action changes inlet steam pressure p_in to the cylinder ports.Indicated power ∝ mean effective pressure * piston area * stroke * rpm, so changing p_in changes power.

Verification / Alternative check:
Indicator diagrams show reduced card height (pressure) with throttling, while card width (cut-off fraction) is nearly unchanged. This is the hallmark of throttle governing versus cut-off governing, where the width changes markedly.

Why Other Options Are Wrong:
Varying volume or temperature alone is not the governing variable; both respond secondarily to pressure and superheat but are not the direct control knob. “All of these” and “degree of superheat only” misstate the control principle.

Common Pitfalls:
Confusing throttle governing with variable cut-off (which changes admission duration) and assuming temperature control is primary (superheat usually follows pressure and boiler conditions).

Final Answer:
pressure of intake steam