What is 'engine roughness' and what can cause it?

Engine roughness refers to the irregular operation of an engine, which typically manifests as vibrations, unusual noise, and fluctuations in engine performance. This condition results from several potential issues, such as uneven fuel-air mixture delivery, spark plug malfunctions, or problems with engine components like the ignition system.

When an engine is not firing uniformly across all cylinders, it can create a mismatch in power generation, leading to a pronounced sensation of roughness. This irregular operation often indicates that something is misaligned, malfunctioning, or not functioning properly within the engine system, which affects its overall efficiency and can lead to further complications if not addressed.

Other options describe conditions that do not align with the definition of engine roughness. For instance, a smooth-running engine would not exhibit any irregularities or vibrations, and while excessive lubrication or temperature could impact engine health, they are not direct descriptors of the roughness phenomenon. Thus, the correct answer captures the essence of what engine roughness is and highlights the complexity of engine operation.