Use and method

1 . A method of reducing the impact of deposits in a post combustion system of a diesel engine, the method comprising combusting in the engine a diesel fuel composition comprising as an additive one or more nitrogen containing detergent, wherein the method reduces the formation of deposits in the post combustion system of a diesel engine; and wherein

the one or more nitrogen containing detergents are selected from:

(a) quaternary ammonium salt additives;

(b) the reaction product of a carboxylic acid-derived acylating agent and an amine;

(c) the product of a Mannich reaction between an aldehyde, an amine and an optionally substituted phenol; and

(d) mixtures thereof.

2 . The method of claim 1 , wherein the diesel fuel composition comprises (a) a quaternary ammonium salt additive.

3 . The method of claim 1 , wherein the diesel fuel composition comprises (b) the reaction product of a hydrocarbyl substituted acylated agent and an amine.

4 . The method of claim 1 , wherein the diesel fuel composition comprises (c) the product of a Mannich reaction between an aldehyde, an amine and an optionally substituted phenol.

5 . The method of claim 1 , wherein component (a) comprises a quaternary ammonium salt formed by reacting methyl salicylate, dimethyl oxalate or propylene oxide (optionally in combination with an acid) with the reaction product of a polyisobutylene-substituted succinic anhydride having a PIB molecular weight (Mn) of 700 to 1300 and dimethylaminopropylamine.

6 . The method of claim 1 , wherein component (a) comprises a quaternary ammonium salt formed by reacting methyl salicylate, dimethyl oxalate or propylene oxide (optionally in combination with an acid) with the reaction product of a fatty acid of formula RCOOH in which R is an alkyl or alkenyl group having 12 to 24 carbon atoms, and a compound of formula (II), wherein both R 2 and R 3 are C1 to C 4 alkyl groups and X is alkylene group having from 2 to 5 carbon atoms.

7 . The method of claim 1 , wherein component (a) comprises a quaternary ammonium salt formed by reacting methyl salicylate, dimethyl oxalate or propylene oxide (optionally in combination with an acid) with a tertiary amine of formula R 5 R 6 R 7 N, wherein each of R 5 , R 6 and R 7 is independently an alkyl group or a hydroxy alkyl group.

8 . The method of claim 1 , wherein component (b) comprises the reaction product of a polyisobutene-substituted succinic acid or succinic anhydride and a polyethylene polyamine selected from ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, hexaethylene-heptamine and mixtures and isomers thereof; wherein polyisobutene substituent has a number average molecular weight of between 500 and 2000.

9 . The method of claim 1 , wherein component (c) comprises the reaction product of formaldehyde, a polyethylene polyamine and a para-substituted monoalkyl phenol.

10 . The method of claim 1 , wherein the diesel fuel composition comprises from 1 to 1000 ppm of (a) a quaternary ammonium salt additive; and/or from 1 to 1000 ppm of (b) the reaction product of a carboxylic acid-derived acylating agent and an amine; and/or from 1 to 1000 ppm of (c) the product of a Mannich reaction between an aldehyde, an amine and an optionally substituted phenol.

11 . The method of claim 1 , further comprising reducing formation of deposits on the turbocharger of the post combustion system.

12 . The method of claim 1 , further comprising reducing formation of deposits on the diesel oxidation catalyst of the post combustion system.

13 . The method of claim 1 , further comprising reducing formation of deposits on the diesel particulate filter of the post combustion system.

14 . The method of claim 1 , further comprising reducing accumulation of soot in the diesel particulate filter.

15 . The method of claim 1 , further comprising reducing formation of deposits on the selective catalytic reduction unit of the post combustion system.

16 . The method of claim 1 , further comprising reducing formation of deposits on the ammonia oxidation catalyst of the post combustion system.

17 . The method of claim 1 , further comprising reducing formation of deposits on sensors within the post combustion system.

18 . The method of claim 1 , further comprising reducing formation of deposits the formation of deposits in one more components of the post combustion system by at least 5%.

19 . The method of claim 1 , wherein the diesel fuel composition comprises one or more nitrogen containing detergents.

20 . The method of claim 1 , wherein the diesel engine is an off road engine.

21 . The method of claim 1 , further comprising providing one or more benefits selected from: an increase in power generation; an increase in torque; an increase in fuel economy; a reduction in emissions; a reduction in combustion chamber deposits; an acceleration improvement; driveability improvements; a reduction in cold start issues; lower soot formation; mitigation of lubricant degradation and/or performance loss; a reduction in diesel exhaust fluid and consumption; reduction in wear on all post combustion components increased longevity of exhaust components; an increase in the maintenance period for the engine and/or post combustion components; and the protection of intake components downstream of the EGR.

22 . The method of claim 1 , wherein the one or more nitrogen containing detergent selected is from:

(a) a quaternary ammonium salt additive formed by reacting a quaternising agent with the reaction product of a hydrocarbyl substituted succinic acid derived acylating agent and a compound able to react with said acylating agent and which includes a tertiary amine group;

(b) the reaction product of a polyisobutene-substituted succinic acid-derived acylating agent and a polyethylene polyamine; or

(c) the product of a Mannich reaction between formaldehyde, a polyethylene polyamine and a para-substituted monoalkyl phenol.