Which factor can directly affect a diesel engine's efficiency?

The quality of the diesel fuel used in an engine is a crucial factor that directly affects diesel engine efficiency. High-quality diesel fuel contains appropriate levels of energy density, lubrication properties, and suitable combustion characteristics. When the fuel has a higher energy density, it allows for better power output and efficiency during combustion. Additionally, cleaner fuels with fewer impurities contribute to more efficient combustion and help maintain the engine's performance over time. Poor quality fuel can lead to incomplete combustion, increased emissions, and potential damage to engine components, ultimately decreasing overall efficiency.

Other factors, while may play a role in engine performance, do not have the same direct impact on combustion efficiency as fuel quality. The color of engine parts and the frequency of washing primarily relate to maintenance aesthetics or other operational aspects and have little to no direct correlation with fuel efficiency. Similarly, the choice of air filter can affect air intake and engine performance but is more dependent on the filtering efficiency rather than the fundamental combustion process influenced by fuel quality.