Why are diesel engines often more fuel-efficient at idle and reduced power than gasoline engines?

The primary reason diesel engines are more fuel-efficient at idle and reduced power compared to gasoline engines is that they do not rely on throttling the intake mixture to control power output.1

In a gasoline engine, a throttle valve regulates the volume of the air-fuel mixture entering the cylinders. At idle or low power, this valve restricts airflow, which creates pumping losses that reduce overall efficiency. Conversely, a diesel engine operates by compressing only air and then injecting fuel directly into the combustion chamber. Because the engine does not use a throttle to restrict the intake air, it avoids these specific pumping losses, resulting in superior efficiency during idling and reduced-power operation.1