🔍 Key Concepts

• How air start systems work on medium-speed and slow-speed diesel engines • Relationship between camshafts, valve gear, and timing for fuel and starting air • Difference between control components (that set timing) and flow components (that just pass air)

💭 Think About

• On a large diesel, what mechanical part usually determines when any valve (fuel or air) opens and closes relative to crank angle? • Which of the listed items actually has a shaped motion profile synchronized to the crankshaft, rather than just opening when pressure is applied? • Think about what would have to be changed if you wanted to adjust the timing of the starting air to each cylinder—what component would you work on?

✅ Before You Answer

• Identify which options are simply passive flow devices (they open from pressure or simple movement but do not set timing). • Ask: which option is positively driven by the engine’s rotation and is designed to control the sequence and duration of valve events? • Eliminate any choice that cannot be adjusted to advance or retard timing relative to crank angle.