An exhaust system of an internal combustion engine; the exhaust system comprises: an exhaust duct, which originates from an exhaust manifold of the internal combustion engine and ends with a silencer, from which exhaust gases are released into the atmosphere; an exhaust gas treatment device, which is arranged along the exhaust duct; a heating device, which is connected to the exhaust duct upstream of the treatment device by means of an outlet duct coming out of the exhaust duct and is designed to generate, by burning fuel, a hot air flow; a temperature sensor (37, 38), which is arranged along the outlet duct or along the exhaust duct downstream of the point in which the outlet duct branches off; and a control unit, which adjusts the combustion in the heating device also depending on the measure provided by the temperature sensor (37, 38); wherein the heating device comprises: a tubular body, where a combustion chamber is obtained on the inside; a fuel injector, which is mounted through a base wall of the tubular body so as to inject fuel into the combustion chamber; at least one inlet opening, which can be connected to a fan so as to receive an air flow, which is directed to the combustion chamber and gets mixed with the fuel; a feeding channel, which is entirely contained inside the tubular body, receives air from the inlet opening, surrounds an end portion of the fuel injector and ends with a nozzle, which is arranged around a spray tip of the fuel injector; and a spark plug, which is mounted through a side wall of the tubular body so as to trigger the combustion of a mixture of air and fuel; wherein an axial distance, namely measured along a longitudinal axis of the tubular body, between the spray tip of the fuel injector and a longitudinal axis of the spark plug ranges from 33% to 100% of an inner diameter of the tubular body; wherein the fuel injector is configured to spray at least part of the fuel against an inner surface of the feeding channel. an exhaust duct, which originates from an exhaust manifold of the internal combustion engine and ends with a silencer, from which exhaust gases are released into the atmosphere; an exhaust gas treatment device, which is arranged along the exhaust duct; a heating device, which is connected to the exhaust duct upstream of the treatment device by means of an outlet duct coming out of the exhaust duct and is designed to generate, by burning fuel, a hot air flow; a temperature sensor (37, 38), which is arranged along the outlet duct or along the exhaust duct downstream of the point in which the outlet duct branches off; and a control unit, which adjusts the combustion in the heating device also depending on the measure provided by the temperature sensor (37, 38); wherein the heating device comprises: a tubular body, where a combustion chamber is obtained on the inside; a fuel injector, which is mounted through a base wall of the tubular body so as to inject fuel into the combustion chamber; at least one inlet opening, which can be connected to a fan so as to receive an air flow, which is directed to the combustion chamber and gets mixed with the fuel; a feeding channel, which is entirely contained inside the tubular body, receives air from the inlet opening, surrounds an end portion of the fuel injector and ends with a nozzle, which is arranged around a spray tip of the fuel injector; and a spark plug, which is mounted through a side wall of the tubular body so as to trigger the combustion of a mixture of air and fuel; wherein an axial distance, namely measured along a longitudinal axis of the tubular body, between the spray tip of the fuel injector and a longitudinal axis of the spark plug ranges from 33% to 100% of an inner diameter of the tubular body; wherein the fuel injector is configured to spray at least part of the fuel against an inner surface of the feeding channel. a tubular body, where a combustion chamber is obtained on the inside; a fuel injector, which is mounted through a base wall of the tubular body so as to inject fuel into the combustion chamber; at least one inlet opening, which can be connected to a fan so as to receive an air flow, which is directed to the combustion chamber and gets mixed with the fuel; a feeding channel, which is entirely contained inside the tubular body, receives air from the inlet opening, surrounds an end portion of the fuel injector and ends with a nozzle, which is arranged around a spray tip of the fuel injector; and a spark plug, which is mounted through a side wall of the tubular body so as to trigger the combustion of a mixture of air and fuel; wherein an axial distance, namely measured along a longitudinal axis of the tubular body, between the spray tip of the fuel injector and a longitudinal axis of the spark plug ranges from 33% to 100% of an inner diameter of the tubular body; wherein the fuel injector is configured to spray at least part of the fuel against an inner surface of the feeding channel.