🔍 Key Concepts

• combustion efficiency in boilers and diesel engines • relationship between black smoke and fuel/air mixture • how maintenance issues (dirty burners, soot buildup) show up in exhaust appearance

💭 Think About

• When you see black smoke, what does that usually tell you about how completely the fuel is burning? • Which of the listed conditions can cause incomplete combustion, and are any of them closely related or overlapping? • Could more than one of the answer choices reasonably happen at the same time to produce the same visible symptom?

✅ Before You Answer

• For each option, ask: Would this realistically cause black smoke in the stack? • Consider whether any option is just a specific example of another, more general condition listed. • Check if it is common on exams for multiple individual causes of the same symptom to be grouped into a single “all of the above” choice.