During which phase is the fuel injected into a diesel engine?

In a diesel engine, fuel is injected during the ignition and combustion phase. This is distinctive to diesel engines as they rely on compression to heat the air to a temperature that ignites the fuel. After the air is compressed in the cylinder, it reaches a temperature high enough for the fuel to ignite spontaneously upon injection. This process occurs typically at the end of the compression stroke and is crucial for the engine's operation, as it directly correlates with the efficiency and power output of the engine.

During the intake phase, air enters the cylinder but no fuel is injected yet. The compression phase involves compressing that air, raising its temperature before the fuel injection. The expansion phase comes after combustion, where the hot gases push the piston down to produce power. Thus, the injection and ignition of fuel is a key process that occurs precisely during the combustion phase, making it the correct answer.