Final answer:
The Atkinson cycle aims to improve efficiency through greater expansion, especially at idle, the pressure may drop below atmospheric, but this is not inherently indicative of wasted energy as the cycle is designed to utilize the full potential of the expansion stroke.
Step-by-step explanation:
When considering the efficiency of the Atkinson cycle, especially at idle, it is important to understand that this cycle is designed to optimize efficiency at the cost of power. The additional expansion phase in the Atkinson cycle allows the engine to expand the combusted gases over a greater volume than the conventional Otto cycle, which theoretically improves efficiency by extracting more work. However, at idle, the efficiency gain may not be as pronounced due to lower power demands and the engine operating at lower speeds. It is possible that the pressure may drop below atmospheric during this phase, but this does not necessarily indicate wasted energy, as the cycle's design aims to utilize the expansion stroke to its full potential, even if the resultant pressure at the end of the expansion is low.