This system, which is described in English as PFI SI, SI stands for Spark Ignition,  i.e. an ignition of the fuel with the spark from a spark plug. PFI with SI is the most researched system when it comes to internal combustion engines that will burn Hydrogen. This system, if you have a fuel/air mixture that is extremely lean, will be able to get down to very low NOx figures in the exhaust gasses, below 100 ppm without any external treatment.

 It is also desirable to reduce the amount of unwanted residual products in connection with combustion. This can be achieved through Stoichiometric relationships.

Stoichiometric relationships could be described in a simplified way as meaning that the precise correct amount of matter together with another matter of precise correct amount is included in a combustion where everything ignites. This also, eliminates any undesired residue. The use of an EGR-system reduces the NOx even further. EGR stands for Exhaust Gas Recirculation. Which means that a small amount of exhaust gasses is recirculated back in to the combustion process in order to decrease the combustion temperature and thereby reducing the NOx figures which are formed at high temperatures.[1]

A three-way catalytic converter is also used, which further lowers the value of NOx to values as low as close to zero. Instead of EGR, water injection into the intake manifold is sometimes used instead because it has the same effect as EGR, since in the hydrogen combustion there is only water vapor as a residual product. This system has, as we have previously experienced, some problems such as unwanted premature ignition of the mixture but also backfires that leads to engine power losses.


[1] Ibid. s. 8