🔍 Key Concepts

• Relationship between starting air valve opening and piston position near top dead center (TDC) • Shape of a cam that must hold a valve open for a relatively long period around TDC versus a cam for quick events like fuel injection • How symmetry of the cam lobe around TDC relates to when the valve opens and closes

💭 Think About

• For starting an engine, when should compressed air begin to enter the cylinder relative to TDC, and how long must it continue to act to turn the crank? • Which cam profile would keep the follower lifted (valve open) for the broadest, most centered interval around TDC, rather than a short, sharply timed event? • Compare each cam: if the crank is at TDC now, where is the highest part of the lobe relative to the vertical axis, and what does that say about valve timing?

✅ Before You Answer

• Identify which cam has its maximum lift centered on TDC (vertical axis) instead of clearly before or after it • Check which cam provides the widest high-lift area (a dwell) around TDC, suitable for sustained starting air admission • Eliminate any cam shapes that look like they would cause only a brief, sharply timed lift just before TDC (typical of fuel injection, not starting air)