A vehicle internal combustion engine comprising: at least one cylinder defining a combustion chamber; a piston disposed for movement within at least one cylinder to define a swept volume of at least one cylinder being less than or equal to 3.0 litre; a primary fuel supply system configured to supply a primary fuel from a primary fuel tank to the combustion chamber to create a primary fuel-air mix in the combustion chamber; and an ignition system comprising: a capsule defining a pre-chamber; an ignition fuel supply system comprising an ignition fuel tank which is separate to the primary fuel tank and contains an ignition fuel which differs from the primary fuel, the ignition fuel supply system being configured to inject the ignition fuel to the pre-chamber to create an ignition fuel-air mix in the pre-chamber; an ignition surface within the pre-chamber, the ignition surface being defined by an interior surface of the capsule and configured to be contacted by the ignition fuel in the pre-chamber to thereby ignite the ignition fuel by hot surface ignition; and at least one jet nozzle; wherein the ignition fuel is characterised by all of the following characteristics: a carbon content by mass less than 65%; a hot surface ignition temperature less than 500 deg C; and a volumetric energy density greater than 18 MJ/L, wherein the pre-chamber has a volume of less than the swept volume of the at least one cylinder and less than 5.0% of a compressed volume of the at least one cylinder, wherein the ignition fuel supply system is configured to inject the ignition fuel into the pre-chamber of quantity less than 5% equivalent energy content of the primary fuel introduced into the combustion chamber wherein the at least one jet nozzle is configured such that ignition of the ignition fuel by contact with the ignition surface causes at least one of hot gases, partially combusted fuel and flames to leave the pre-chamber and enter the combustion chamber through the at least one jet nozzle to thereby ignite the primary fuel. at least one cylinder defining a combustion chamber; a piston disposed for movement within at least one cylinder to define a swept volume of at least one cylinder being less than or equal to 3.0 litre; a primary fuel supply system configured to supply a primary fuel from a primary fuel tank to the combustion chamber to create a primary fuel-air mix in the combustion chamber; and an ignition system comprising: a capsule defining a pre-chamber; an ignition fuel supply system comprising an ignition fuel tank which is separate to the primary fuel tank and contains an ignition fuel which differs from the primary fuel, the ignition fuel supply system being configured to inject the ignition fuel to the pre-chamber to create an ignition fuel-air mix in the pre-chamber; an ignition surface within the pre-chamber, the ignition surface being defined by an interior surface of the capsule and configured to be contacted by the ignition fuel in the pre-chamber to thereby ignite the ignition fuel by hot surface ignition; and at least one jet nozzle; wherein the ignition fuel is characterised by all of the following characteristics: a carbon content by mass less than 65%; a hot surface ignition temperature less than 500 deg C; and a volumetric energy density greater than 18 MJ/L, a capsule defining a pre-chamber; an ignition fuel supply system comprising an ignition fuel tank which is separate to the primary fuel tank and contains an ignition fuel which differs from the primary fuel, the ignition fuel supply system being configured to inject the ignition fuel to the pre-chamber to create an ignition fuel-air mix in the pre-chamber; an ignition surface within the pre-chamber, the ignition surface being defined by an interior surface of the capsule and configured to be contacted by the ignition fuel in the pre-chamber to thereby ignite the ignition fuel by hot surface ignition; and at least one jet nozzle; wherein the ignition fuel is characterised by all of the following characteristics: a carbon content by mass less than 65%; a hot surface ignition temperature less than 500 deg C; and a volumetric energy density greater than 18 MJ/L, a carbon content by mass less than 65%; a hot surface ignition temperature less than 500 deg C; and a volumetric energy density greater than 18 MJ/L, wherein the pre-chamber has a volume of less than the swept volume of the at least one cylinder and less than 5.0% of a compressed volume of the at least one cylinder, wherein the ignition fuel supply system is configured to inject the ignition fuel into the pre-chamber of quantity less than 5% equivalent energy content of the primary fuel introduced into the combustion chamber wherein the at least one jet nozzle is configured such that ignition of the ignition fuel by contact with the ignition surface causes at least one of hot gases, partially combusted fuel and flames to leave the pre-chamber and enter the combustion chamber through the at least one jet nozzle to thereby ignite the primary fuel.