I’m curious how combustion temperatures are lowered like you say - is it a pv=nrt relationship hinging on changes in compression ratio or something like injector technology?
Yes, the latest generation diesel engines have lower compression ration than engines from a couple decades ago. Maybe something like 16:1 instead of 19:1. But also exhaust gas recirculation: https://en.wikipedia.org/wiki/Exhaust_gas_recirculation