Which factor primarily affects an engine's power output at high altitude?

The primary factor affecting an engine's power output at high altitude is air density. As altitude increases, the density of the air decreases. This reduction in air density means there is less oxygen available for combustion in the engine's cylinders. Since engines, particularly naturally aspirated ones, rely on the amount of oxygen in the air to produce power, a decrease in air density directly translates to a reduction in power output.

At higher altitudes, even though the fuel quality or engine temperature might remain constant, the lack of sufficient oxygen means the engine cannot efficiently produce the same amount of power as it would at lower elevations where air density is greater. Therefore, the ability of the engine to breathe properly diminishes, impacting its overall performance and power generation capabilities.