Poor physical properties of diesel fuel will affect the reliability of diesel engines. Diesel fuel is intended to cool and lubricate the fuel injection system. Poor lubricity will result in excessive wear and premature failure of injectors and fuel pumps. Poor lubricity can be due to inadequate manufacturing of the fuel, using improper solvents, or using low quality additives. Severe processing of fuels can reduce the amount of surface protecting agents within the fuel. Fuels that are a heavier grade (require less refinement) will create combustion chamber deposits. These deposits will reduce the life of cylinder liners and rings. Low cetane numbers will also damage engines over time, resulting in engine knock, trouble starting, and black exhaust smoke.