In the perspective of a reduction of emissions and a rapid decarbonisation, especially for compression ignition engines, hydrogen plays a decisive role. The dual fuel technology is perfectly suited to the use of hydrogen, a fuel characterized by great energy potential. In fact, replacing, at the same energy content, the fossil fuel with a totally carbon free one, a significant reduction of the greenhouse gases, like carbon dioxide and total hydrocarbon, as well as of the particulate matter can be obtained. The dual fuel with indirect injection of gaseous fuel in the intake manifold, involves the problem of hydrogen autoignition. In order to avoid this difficulty, the optimal conditions for the injection of the incoming mixture into the cylinder were experimentally investigated. All combustion processes are carried out on a research engine with optical access. The engine speed has is set at 1500 rpm, while the EGR valve is deactivated. The purpose of this work is to research the minimum amount of diesel fuel, which allows efficient and controlled hydrogen ignition. Starting from the dual fuel conditions investigated in previous works with two injections per cycle, one of the diesel injections was removed. Subsequently, the shift of the start of injection and the reduction of the energizing time of the diesel injection as well as the increase in the delivered mass of hydrogen are analysed. The final aim is to obtain an indicated mean effective pressure equal to the one previously analysed avoiding backfiring phenomena in the manifolds or abnormal engine operation. All the analysed tests are in ultra lean combustion conditions with premixed ratio higher than 95% and equivalence ratio higher than 0.32. From the investigated cases, it can be found that the best combustion efficiency is determined with a diesel start of injection around 10 before top dead centre, while the lowest amount of diesel corresponds to an energizing time of the injector equal to 209μs. Regarding the hydrogen injection in the intake manifold, a dependency on the intake valve timing is highlighted. Hydrogen was prevented from being thrown into the exhaust by starting its supply after the valve crossing; on the other hand, to avoid backfiring phenomena, it is noted that the hydrogen injection has to end prior to the compression phase commences. This information is of particular interest to fulfil engine decarbonisation optimizing the use of hydrogen in compression ignition engines and facilitating CFD analysis of hydrogen combustion in ultra lean conditions.