Typical fuel injection timing — four-stroke diesel engine: Select the most appropriate generic timing for the injector (fuel valve) opening and closing in a four-stroke diesel engine relative to top dead centre (TDC).

Introduction / Context:
Injection timing in compression-ignition engines is critical for efficient combustion, controlled pressure rise, noise, and emissions. Generic textbook values describe injection beginning slightly before TDC and ending shortly after TDC in many conventional diesel engines, though exact timing varies with design and operating condition.

Given Data / Assumptions:

• Four-stroke diesel engine with direct injection.
• Moderate speed and load; no advanced multi-pulse strategies considered.
• Seeking representative crank-angle windows rather than manufacturer-specific maps.

Concept / Approach:
Because ignition delay exists between start of injection (SOI) and start of combustion, fuel is injected slightly before TDC so that effective combustion and peak pressure occur just after TDC, maximizing work on the piston while avoiding excessive negative compression work or harsh pressure rise rates. Hence, typical guidance places SOI around 10–20° before TDC and end of injection shortly after TDC.

Step-by-Step Solution:

Verification / Alternative check:
Engine indicator diagrams and calibration literature frequently cite SOI values in the 10–20° BTDC range at nominal conditions, with end of injection shortly after TDC for main injection events.

Why Other Options Are Wrong:

• Opens after TDC or closes before BDC: Not representative for main injection; combustion would be late, harming efficiency.
• Opens at TDC and closes at BDC: Far too long and late; not realistic for diesel combustion phasing.
• May open anywhere: Injection timing is carefully controlled, not arbitrary.

Common Pitfalls:
Confusing start of injection with start of combustion; they are separated by ignition delay, which calibration must anticipate.

Final Answer:
opens at 15° before top dead centre and closes at 20° after top dead centre