Use of a gas delivery system for cleaning a vehicle's internal combustion engine operating in idle mode; wherein the gas delivery system is adapted to deliver hydrogen gas, and optionally oxygen gas, into the air intake duct of the internal combustion engine when the engine operates idle; wherein the hydrogen gas, and optionally the oxygen gas, is continuously produced by electrolysis of water; wherein the gas delivery system is adapted to deliver hydrogen gas in a predefined amount in response to user input about the internal combustion engine's engine displacement, said amount being: i) 15-30 liters hydrogen gas per minute for a period of 10-90 minutes when the internal combustion engine has an engine displacement of 1-3 liters; ii) 30-50 liters hydrogen gas per minute for a period of 10-90 minutes when the internal combustion engine has an engine displacement of 4-6 liters; iii) 45-70 liters hydrogen gas per minute for a period of 10-90 minutes when the internal combustion engine has an engine displacement of 7-9 liters; iv) 60-90 liters hydrogen gas per minute for a period of 10-90 minutes when the internal combustion engine has an engine displacement of 10-12 liters; v) 75-110 liters hydrogen gas per minute for a period of 10-90 minutes when the internal combustion engine has an engine displacement of 13-15 liters; vi) 90-130 liters hydrogen gas per minute for a period of 10-90 minutes when the internal combustion engine has an engine displacement of 16-18 liters; vii) 105-150 liters hydrogen gas per minute for a period of 10-90 minutes when the internal combustion engine has an engine displacement of 19-21 liters; viii) 120-170 liters hydrogen gas per minute for a period of 10-90 minutes when the internal combustion engine has an engine displacement of 22-24 liters; ix) 135-190 liters hydrogen gas per minute for a period of 10-90 minutes when the internal combustion engine has an engine displacement of 25-27 liters; and x) 150-210 liters hydrogen gas per minute for a period of 10-90 minutes when the internal combustion engine has an engine displacement of 28-30 liters. i) 15-30 liters hydrogen gas per minute for a period of 10-90 minutes when the internal combustion engine has an engine displacement of 1-3 liters; ii) 30-50 liters hydrogen gas per minute for a period of 10-90 minutes when the internal combustion engine has an engine displacement of 4-6 liters; iii) 45-70 liters hydrogen gas per minute for a period of 10-90 minutes when the internal combustion engine has an engine displacement of 7-9 liters; iv) 60-90 liters hydrogen gas per minute for a period of 10-90 minutes when the internal combustion engine has an engine displacement of 10-12 liters; v) 75-110 liters hydrogen gas per minute for a period of 10-90 minutes when the internal combustion engine has an engine displacement of 13-15 liters; vi) 90-130 liters hydrogen gas per minute for a period of 10-90 minutes when the internal combustion engine has an engine displacement of 16-18 liters; vii) 105-150 liters hydrogen gas per minute for a period of 10-90 minutes when the internal combustion engine has an engine displacement of 19-21 liters; viii) 120-170 liters hydrogen gas per minute for a period of 10-90 minutes when the internal combustion engine has an engine displacement of 22-24 liters; ix) 135-190 liters hydrogen gas per minute for a period of 10-90 minutes when the internal combustion engine has an engine displacement of 25-27 liters; and x) 150-210 liters hydrogen gas per minute for a period of 10-90 minutes when the internal combustion engine has an engine displacement of 28-30 liters.