Compositions and methods and uses relating thereto

A method of improving the performance of an engine, the method comprising combusting in the engine a diesel fuel composition comprising as an additive a quaternary ammonium compound of formula (I):

• wherein R0, R1, R2 and R3 is each independently an optionally substituted hydrocarbyl group, X is a linking group, R4 is an optionally substituted alkylene group, R5 is hydrogen or an optionally substituted alkyl, alkenyl or aryl group, and n is 0 or a positive integer, provided that n is not 0 when R5 is hydrogen;
• wherein X is CH2CHR or CHRCH2 wherein R is an alkyl or alkenyl group having 20 to 24 carbon atoms.

A method of preparing a fuel composition, the method comprising preparing a quaternary ammonium salt by reacting (a) a tertiary amine of formula R1R2R3N with (b) an acid-activated alkylating agent in the presence of (c) a compound of formula HOOCXCOO—(R4O)n—R5, and adding the quaternary ammonium salt into the fuel;

• wherein R4 is an optionally substituted alkylene group, R5 is hydrogen or an optionally substituted alkyl, alkenyl or aryl group, and n is 0 or a positive integer, provided that n is not 0 when R5 is hydrogen;
• and
• wherein n is a positive integer and/or X is CH2CHR or CHRCH2 wherein R is an alkyl or alkenyl group having 20 to 24 carbon atoms.