Question 10 3AE01 - Third Assistant Engineer

The Correct Answer is C ### Explanation for C (Correct Option) Fuel combustion in a diesel engine is a complex process that does not happen instantaneously. It involves several distinct phases: ignition delay, rapid pressure rise (premixed burn), mixing controlled combustion (main burn), and the **afterburning period**. The afterburning period is the final phase where the remaining fuel droplets and partially oxidized products continue to burn and release heat, often due to slower mixing and lower temperatures/pressures as the piston moves downward (on the power stroke). While the main heat release occurs around Top Dead Center (TDC) and immediately afterward, combustion—the chemical reaction producing heat—does not strictly stop until all viable fuel has been consumed. Therefore, the combustion process effectively continues (and should continue for optimal efficiency and complete burning) through the expansion stroke, encompassing this necessary afterburning phase. ### Explanations for Incorrect Options **A) end when fuel injection has been completed:** Fuel injection typically ends well before combustion is chemically complete. Since diesel combustion is mixing-controlled, the time required for the injected fuel to fully vaporize, mix with air, and burn completely always extends past the physical duration of the injection event. **B) end at bottom dead center:** While the power stroke ends at Bottom Dead Center (BDC), combustion must be largely completed much earlier in the stroke. By the time the piston reaches BDC, temperatures and pressures are too low for effective combustion. Any combustion still occurring at or near BDC would be wasted energy (as work extraction is minimal) and would likely lead to incomplete combustion and excessive soot. **D) be completed exactly at top dead center:** This is highly impractical and undesirable. Combustion takes time (a finite duration). If combustion were completed exactly at TDC, it would imply a very rapid burn (possibly leading to excessive cylinder pressure and noise, known as "diesel knock") and would prevent the controlled, continuous heat release needed to push the piston effectively down the power stroke. Maximum pressure is typically designed to occur shortly *after* TDC, allowing the heat release to push the piston efficiently.