Adding the hydrogen directly into the cylinder during the compression stroke itself seems to be the most promising concept as far as we have come today with the development of the engine types we have. By doing it this way, you can more or less  prevent unwanted self-ignitions or back-ignitions that occur in the PFI engine system. Because the fuel injection time can be regulated as desired. You could wait to inject it so late that both valves are closed, which minimizes the exposure of hot parts.

The problem you have with hydrogen is that it takes up a lot of space when it is injected via the intake manifold and then also takes the place of the air supply. This in turn contributes to power loss that you also get rid of. This is because only the air has already been sucked into the cylinder via the inlet valve, and the fuel is then supplied into the cylinder when the inlet valve is closed. However, when injecting the fuel in the compression stroke, you have to have a higher injection pressure from the injector due to the higher pressure that prevails inside a cylinder than what is required from an injector for the PFI system. In a PFI system, there is no excess pressure at all, but instead it is mostly a negative pressure instead. It has been shown that this system, which works with such high pressures, can almost be compared to the high efficiency of diesel. The system is often abbreviated with HPDI, which stands for High Pressure Direct Ignition. There are many advantages to this system, it can be made more flexible in many different situations, it can be fine-tuned in a completely different way than a PFI system. You can adjust the injection pressure, you can adjust the injection timing, you can also adjust the injection design, all this to be able to optimize the engine power to the optimum. There are a few different ways of igniting the fuel, and you can read about that further on.[1]


[1] Ibid. s. 9