🔍 Key Concepts

• starting air system function on large diesel engines • difference between mechanical resistance (blocked movement) and insufficient driving force (weak turning) • role of scavenge air during starting versus running

💭 Think About

• Think about what would physically happen to engine speed if a cylinder were full of water or had a solid obstruction. Would the engine turn slowly, or would it tend to stop or jam? • Consider which system actually provides the power to turn the engine over during starting. If that system is weak, what symptom would you expect? • Ask yourself: during cranking, does the engine need full scavenge air to spin, or mainly to help it fire once it’s already turning at normal starting speed?

✅ Before You Answer

• Distinguish between a condition that would block or hydraulically lock a cylinder versus one that would just reduce cranking force. • Verify what the starting air system does (turns the engine) versus what the scavenge air system does (supplies air for combustion). • Match the symptom: "turns over very slowly without firing" with the most likely system that is weak, not damaged or blocked.