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 quaternized nitrogen 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 (DISI) engine and port fuel injector (PFI) engine; reduce and/or prevent deposits in the injection system, preferably internal diesel injector deposits and/or valve sticking in direct injection diesel engines, preferably in common-rail injection system, where the fuel is diesel fuel, biodiesel fuel, gasoline and 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 is non-toxic and/or non-carcinogenic agent (including e.g. dimethyl sulfate and benzyl halides) and residue-free incinerating agent (including 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 major amount of a fuel or lubricant and a portion of the quaternized nitrogen compound.