We're not yet completely informed about the PSA-concern's plans for 2018, to reduce the consumption of their petrol engines by 10 percent. Apparently, this concept has already been patented. However, by looking at the above pictures and from the short press-releases, one can interpret the following:

As one can see in the 1st picture, the left-hand cylinder is not connected to an exhaust manifold, but is led into a larger casing, in which, one could surmise, there would be a catalytic converter. In the further course, a threaded opening can be seen. This is presumably for a Lambda-sensor, in this case as a monitoring-probe.

By probably being able to regulate the injection amount for this cylinder separately, e.g., through a certain amount of over-enrichment, a larger than normal hydrogen portion can be created. This over-enrichment does not have a negative effect, because, in this case, we're dealing with exhaust gas which is to be recirculated. Since on the fresh-gas side, all the cylinders are, as before, connected with each other and with the turbo- charger.

So, one cylinder generates the exhaust gas to be recirculated for all of them. This cylinder does not necessarily run lean, but can be thus configured, that through specific injection control, e.g., the hydrogen portion generated by the catalytic converter produces a compression that is higher than the exhaust-gas recirculation would normally have, without causing the dreaded knocking. The possibility of the somewhat uneven performance of the three cylinders compared with the fourth one doesn't seem to bother he crankshaft at all.