Storing hydrogen is one of the major issues concerning its utilization on board vehicles. A promising solution is storing hydrogen in the form of ammonia that contains almost 18% hydrogen by mass and is liquid at roughly 9 bar at environmental temperature. As a matter of fact, liquid ammonia contains 1.7 times as much hydrogen as liquid hydrogen itself, thus involving relatively small volumes and light and low-cost tanks.It is well known that ammonia can be burned directly in I.C. engines, however a combustion promoter is necessary to support and speed up combustion especially in the case of high-speed S.I. engines.The best promoter is hydrogen, due to its opposed and complementary characteristics to those of ammonia, Hydrogen has high combustion velocity, low ignition energy and wide flammability range, whereas ammonia has low flame speed, narrow flammability range, high ignition energy and high self-ignition temperature. Another important point is the possibility to obtain hydrogen on board from ammonia, by means of a catalytic reactor.In a previous research the possibility to run a small 4-stroke twin-cylinder SI engine of 505 cm3 with ammonia plus a small amount of hydrogen was demonstrated. Apart, the suitability of a custom-designed catalytic reactor to produce hydrogen from ammonia with the required flow rate was verified as well.This paper shows experimental results obtained coupling the catalytic reactor to the engine and using the heat of the exhaust gases as energy source to keep the reactor at its operative temperature. The device necessary to control the ammonia reactor is described too.