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North Central US | See the image for additional help here. Correct me if I am wrong.
Diesel emissions actually go back to the 1950s and 1960s, with things such as "amount of visible smoke". There is a "history of Cummins engines" presentation on YouTube that does a good job of explaining that, as many old Cummins engines lines were phased out due to it before the 1980s.
In, for the most part, order of appearance/mandate:
Modern Diesel engines have a few different systems:
EGR: Exhaust Gas Recirculation. This recycles exhaust, it redirects some back into the intake usually via a cooler of some type. EGR's main focus is reducing Nitrogen Oxides (NOx) emissions by lowering combustion temperatures. The downside is when combustion temperatures are lower, diesel does not burn as thoroughly and makes more soot. Not all Diesels have this as some manage to make the regulations via different designs. NOx emissions are usually made under heavy load and acceleration or when the engine is cold.
Catalytic converter: Similar to on a gasoline engine, its goal is to help burn some unburnt fuel, while converting carbon monoxide, some NOx, and soot into CO2, water, and nitrogen.
DPF: Diesel Particulate Filter. This captures most of the soot, the smoke, and traps it in a very fine honeycomb like filter. If the engine is usually kept under load, it usually stays clean from the hot exhaust reburning the soot into ash or gas, but if not this is what requires a "regen" where the engine and system work together to raise the temperature and burn out the soot to clean it. Some systems basically have a fuel injector in front of the DPF to help raise the temperature during regens or even normal operation. DPFs can become plugged and need to either be baked at an extremely high temperature or replaced. Not all diesel have this, some engines designs meet the criteria without it. CNH and Agco are two examples that I know of who have engines without DPFs.
Some new gasoline engines are also receiving "GPF"s, Gasoline Particulate Filters, with the same or similar idea.
SCR: Selective Catalyst Reduction. This is better known as the "DEF systems". SCR again reduces NOx emissions by spraying Diesel Exhaust Fluid into the exhasut stream before a catalyst, similar to a catalytic converter on a gasoline engine, where it reacts to turn the NOx emissions into nitrogen, water, and CO2. Most engines have this, up to a certain horsepower, around 750 HP. Above thay SCR/DEF is usually not required.
2027 will see another mandatory reduction in particulate matter(smoke, soot), by 50%. How this will be accomplished is yet to be seen and universally agreed upon. Cummins, for example*, will add a dedicated 48 volt alternator to their engines and have a 48 volt heater in their new DPFs to raise temperatures in efforts to trap or destroy more particulates to hopefully meet the new emissions requirements. How others will do it is yet to be seen for sure.
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This is my understanding of these systems I largely do not deal with at all. Feel free to correct me, but as per the other discussion, quite a few people were claiming DEF was curing the smoke from engines, when it has very little to do with it as most if not all of the smoke is cured by the DPF on most diesel engines or by the engine design.
Edited by GS2 1/31/2026 08:47
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How modern Diesel Emissions work, paging Silverluv75 and others
