Car appreciation... the aesthetics, the engineering, etc

For optimum fuel consumption you neeed a high combustion temperature but that creates Nitrous Oxides, older diesels used EGR Exhaust Gas Recirculation to reduce combustion temperature but that causes soot, hence the needs for DPF Diesel Particulate Filters but they don't sit well with short Journey's.
Ad Blue is a urea mixture injected direct into the exhaust prior to a catalytic converter that reacts to break down the nitrous oxides to harmless gases, so higher combustion temperatures can be maintained.

https://www.stratstone.com/blog/adblue-everthing-you-need-to-know/#:~:text=How%20does%20AdBlue%20work%3F,carbon%20dioxide%20(CO2).