Which of the following statements best represents how the start air valve, shown in the illustration, operates to admit starting air to the main engine cylinder? See illustration MO-0054.
• Study how control air (CA), opening pilot air (OP), and closing pilot air (CP) are routed in the illustration. • Notice where the start air (SA) passage is in relation to the valve piston and spindle. • Think about the normal function of a spring in a starting air valve: does it usually hold the valve open or keep it shut when no air is applied?
• Look at the top of the valve piston in the drawing: which line (CA, SA, OP, or CP) actually connects to that area? • If the engine is stopped and there is full starting air pressure in the manifold, what prevents the valve from opening by itself? Which force or pressure must change for it to open? • Would the main engine cylinder ever be able to "suck" this valve open against manifold pressure and spring force, or must it be positively actuated?
• Verify which port in the illustration is labeled for opening pilot air (OP) and which for start air (SA), and see which one reaches the top of the piston. • Confirm what the spring is doing in the neutral condition: holding the valve closed or opening it. • Make sure the statement you select describes the active opening force that lifts the valve off its seat, not just a force that keeps it shut.
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