Injection into the inlet port in the intake manifold. Which means that the fuel is injected outside the inlet valve, i.e. not directly into the combustion chamber. Only when the intake valve opens and the piston moves down the  fuel is sucked into the cylinder due to the negative pressure that is then created, the intake stroke. This type of injection is the most common on Otto engines. Converting this system to hydrogen operation is relatively "simple", as we previously covered. The injectors need to be replaced to be able to handle hydrogen instead of petrol, and the ignition system needs to be modified, also mentioned earlier. But there are risks with this system that we also mentioned earlier, namely premature self-ignition because the fuel enters so early. Another problem with this type of injection system is that when the fuel is injected into the intake port it pushes out the air and the engine's power is then limited. A simple description of different types of how the combustion can take place, as well as slightly different types of ignition of the fuel, follows below.[1]


[1](Yip et al., 2019, s. 7)