🔍 Key Concepts

• camshaft and valve train operation in a four-stroke diesel engine • the role of a valve spring versus the role of the cam lobe/tappet • difference between opening the valve and keeping it closed/seated when the cam is not acting

💭 Think About

• Look at how the cam lobe, push rod/tappet, valve, and valve spring interact in the illustration. When the cam lobe is NOT pushing up, what must the spring be doing? • Ask yourself: which part actually forces the valve to lift (open) and which part simply returns the valve to its normal position? • Consider what would happen to the valve and its sealing surface if the spring were removed while the engine was running.

✅ Before You Answer

• Identify in the illustration which component directly contacts the cam lobe and which component directly contacts the valve spring. • Decide whether a spring can control the timing of when something happens, or only how it moves when another part releases it. • Eliminate any options that confuse the job of the bushings (guides) with the job of the spring.