Diesel fuel compositions

Abstract

Use of a Fischer-Tropsch derived gas oil, and/or of a fuel composition containing it in operating a diesel engine and/or a vehicle which is driven by a diesel engine is provided, to clean a diesel engine. Method for assessing a diesel fuel composition, involving measurement of engine fouling levels is also provided. Use of a Fischer-Tropsch derived gas oil in a diesel fuel composition, optionally together with a detergent, to reduce and/or clean up engine fouling is also described.

1 . (canceled)  
     
     
         2 . (canceled)  
     
     
         3 . (canceled)  
     
     
         4 . A method of operating a diesel engine, and/or a vehicle which is driven by a diesel engine, said method comprising introducing into a combustion chamber of the engine a diesel fuel composition incorporating a Fischer-Tropsch derived gas oil, thereby reducing subsequent combustion related deposits in the engine and/or removing previously incurred combustion related deposits in the engine.  
     
     
         5 . The method of  claim 4  wherein the diesel fuel composition further comprises a detergent.  
     
     
         6 . (canceled)  
     
     
         7 . A method for assessing the performance of a candidate diesel fuel composition, comprising the steps of: 
 1) measuring the level of combustion related deposits in a diesel engine running on a standard diesel fuel composition, which standard fuel composition contains no, or less than 1% w/w of, Fischer-Tropsch derived gas oils;    2) subjecting the engine to a first test cycle running on the standard fuel composition;    3) measuring the level of combustion related deposits in the engine after the first test cycle;    4) calculating the increase in deposits during the first test cycle;    5) subjecting the engine to a second test cycle running on the candidate diesel fuel composition;    6) measuring the level of combustion related deposits in the engine after the second test cycle;    7) calculating the increase in deposits, if any, during the second test cycle; and    8) if applicable, calculating the extent of removal of deposits during the second test cycle.    
     
     
         8 . A diesel fuel composition which, when used as the candidate fuel composition in the method of  claim 7 , leads to removal of at least 5% of the combustion related deposits accumulated in the engine prior to step 5 of the test, when the duration of the second test cycle is the same as or less than that of the first test cycle.  
     
     
         9 . A diesel fuel composition comprising a major proportion of a fuel or fuel blend for an internal combustion engine of the compression ignition type, wherein the fuel or fuel blend comprises at least 30% w/w of a Fischer-Tropsch derived gas oil.  
     
     
         10 . The diesel fuel composition of  claim 9  further comprising a detergent.  
     
     
         11 . The diesel fuel composition of  claim 9  wherein the amount of the Fischer-Tropsch derived gas oil used in the fuel composition is 10% w/w or greater