Question 39 MODE01 - Chief MODU Engineer

The Correct Answer is D **Why option D ("continue through the afterburning period") is correct:** Fuel combustion in a diesel engine cylinder is not instantaneous. It occurs in several distinct phases: the ignition delay period, the rapid or uncontrolled combustion period, the controlled combustion period (during which most of the fuel is injected), and finally, the **afterburning period**. The afterburning period is a relatively slow, diffusion-controlled burning of remaining fuel molecules and soot particles, which continues as the piston moves down the power stroke. Combustion only truly ends when all combustible material has been consumed or the temperatures drop too low, meaning it effectively continues through this final afterburning phase to maximize energy extraction. **Why the other options are incorrect:** * **A) end at bottom dead center:** This is incorrect because most of the useful work must be extracted during the expansion (power) stroke, which means the main combustion process should ideally be completed well before the piston reaches bottom dead center (BDC). While a very small amount of residual combustion might occur near BDC, stating that combustion *should end* there is too late for efficient operation. * **B) be completed exactly at top dead center:** This is physically impossible and undesirable. If combustion were completed exactly at top dead center (TDC), the time available for energy release would be minimal, leading to extremely high, uncontrolled pressure spikes (detonation-like behavior) and very poor power generation during the expansion stroke. Combustion is initiated just before TDC and continues well past it. * **C) end when fuel injection has been completed:** This is incorrect because of the finite time required for the injected fuel to mix, vaporize, and react chemically. Even after the fuel injector stops spraying (end of injection), the fuel already in the cylinder continues to burn through the afterburning phase. Combustion always extends beyond the physical injection event.