Fuel composition comprising detergent and quaternary ammonium salt additive

A method of reducing deposits in a diesel engine, the method comprising:

• combusting in the engine a diesel fuel composition comprising a detergent additive, which is not a quaternary ammonium salt or a Mannich reaction product, and a quaternary ammonium salt additive comprising the reaction product of nitrogen containing species having at least one tertiary amine group and an ester quaternizing agent;
• wherein the nitrogen containing species is the reaction product of a hydrocarbyl-substituted acylating agent and a compound comprising at least one tertiary amine group and a primary amine, secondary amine or alcohol group;
• wherein the method provides improved performance in modern diesel engines having a high pressure fuel system and in traditional diesel engines;
• and
• wherein the detergent additive is the reaction product of a carboxylic acid derived acylating agent and an amine.