Question 5 MODE02 - Assistant MODU Engineer

The Correct Answer is D **Why option D ("Completion of injection") is correct:** The combustion process in a diesel engine is typically divided into several phases: ignition delay, rapid combustion (premixed burn), steady combustion (diffusion burn), and afterburning. The **steady combustion period** (or diffusion combustion period) is characterized by the burning of fuel as quickly as it is injected, governed primarily by the rate of fuel-air mixing (diffusion). This period defines the main heat release phase. When the fuel **injection is completed**, the primary source of new fuel entering the combustion chamber stops. While residual fuel already present continues to burn (afterburning), the process transitions away from the high, stable heat release rate defined by the controlled injection event. Therefore, the **completion of injection** marks the triggering event for the end of the steady, controlled combustion period. **Why the other options are incorrect:** * **A) Intake valve or port closing:** This event occurs much earlier, during the compression stroke (or slightly before/after BDC in the intake stroke). It sets up the conditions for combustion (compression) but is not directly related to the *end* of the active, steady burning phase which happens near and after TDC. * **B) Piston reaching top dead center (TDC):** Fuel injection and ignition typically begin *before* TDC, and the steady combustion phase extends well *past* TDC (often 10–20 degrees ATDC) to produce torque. TDC is a positional landmark, not the thermodynamic trigger for the end of combustion. * **C) Piston reaching bottom dead center (BDC):** BDC occurs during the intake stroke (at the start of compression) and again at the end of the power stroke. Combustion is long finished by the time the piston reaches BDC on the power stroke, and thus BDC cannot trigger the end of the steady combustion phase.