NOVELTY - Use of a reaction product comprising an quaternized nitrogen compound or its partial fraction obtained via purification, as a fuel-, lubricant- or kerosene-additive, is claimed. The reaction product is obtained by (a) reacting a quaternizable alkylamine containing at least one quaternizable tertiary amino group with a quaternizing agent, where the tertiary amino group is converted into a quaternary ammonium group. The quaternizing agent is a hydrocarbyl epoxide in combination with a free hydrocarbyl-substituted polycarboxylic acid.    USE - The reaction product is useful as a fuel-, lubricant- or kerosene-additive to: reduce the fuel consumption and/or minimize the power loss in direct-injection diesel engines, preferably diesel engines with common rail injection system; reduce deposits in the intake system of a gasoline engine, preferably direct-injection spark-ignition and port fuel injector motors; reduce and/or prevent deposits in the injection system, preferably internal diesel injector deposits and/or valve sticking in direct injection diesel engines, especially in common-rail injection system, where the fuel is diesel fuels, biodiesel fuels, gasoline or alkanol containing petrol (all claimed).    ADVANTAGE - The fuel additive containing the quaternized nitrogen compound, even under small consumption, highly increases the performance of the diesel engines. The quaternizing agent used in the fuel has no toxic and/or carcinogenic agent (e.g. dimethyl sulfate and benzyl halides) and residue-free incinerating agent (e.g. dimethyl sulfate and alkyl halides).    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) the quaternized nitrogen compound;    (2) preparing the quaternized nitrogen compound by performing the step (a) as above per se;    (3) an additive concentrate containing the quaternized nitrogen compound in combination with other diesel or gasoline fuel additives or lubricant additives; and    (4) a fuel or lubricant composition and kerosene composition comprising a major amount of a conventional fuel or lubricant and a portion of the quaternized nitrogen compound.