🔍 Key Concepts

• Relationship between pressure and volume during compression and combustion in a diesel engine cycle • What a vertical line (almost constant volume) versus a horizontal line (almost constant pressure) means on a P–V diagram • Sequence of events around top dead center (TDC): end of compression, start of injection, combustion, and exhaust opening

💭 Think About

• On a P–V diagram, if the curve goes nearly straight up or down, is volume changing much, or is pressure changing much? How does that relate to combustion type? • Where on this diagram would you expect the start of fuel injection to occur: during low pressure near the bottom of the stroke, or near the top when air is fully compressed? • At what part of the cycle do the exhaust valves open in relation to the major pressure changes shown by the diagram?

✅ Before You Answer

• Identify whether curve A–d shows mainly a change in pressure or volume • Decide whether this part of the curve is before, during, or after TDC, and what engine event normally occurs there • Eliminate choices that would occur much later in the power stroke, after most of the expansion has already taken place