STM317 said:
This is a complex topic, so I apologize for the lengthy response. "Emissions" is a vague term that covers anything created during combustion. Could be invisible gaseous stuff like NOx, CO2, or Hydrocarbons and it could be particulates that you can see. The NOx and particulates are the ones that are particularly bad for human health, while CO2 is the primary focus these days from an environmental perspective.
NOx causes smog and makes breathing difficult. It's formed by high temps in the combustion chamber. Therefore, lean combustion increases NOx. This is why diesels in particular have high NOx emissions compared to gasoline cars. There are generally three ways to cool combustion: reducing compression ratio, altering fuel timing/quantity, and adding EGR. You might notice that there's a trade off here, where you can improve fuel economy and reduce hydrocarbon emissions, but that creates more harmful NOx. You can increase compression and improve power, but that increases NOx emissions. You can use less EGR and get more of both, but that creates more NOx.
Hydrocarbons (HC) are basically fuel. Burning more fuel cools combustion, which reduces NOx but hurts fuel economy and increases particulates and HC emissions. The earliest diesel emissions standards focused on reducing NOx and resulted in loss of fuel economy, and implementation of particulate filters in the exhaust as companies just added more fuel to the combustion process or in some cases power was reduced via decreasing compression too. Basically, they tried to meet emissions standards by cleaning up the combustion process, and it worked until standards tightened.
Particulates come from burning more fuel. This results in cooler combustion temps, and lower NOx, but increased HCs. They're nasty because they burrow deep into lungs and like to cause cancer.
Over time, emissions standards tightened and manufacturers added additional tech to remain compliant. Now, modern diesel uses an aftertreatment system composed of different parts. There's a catalyst that converts hydrocarbons, a particulate filter that catches soot, and an SCR that neutralizes NOx by spraying DEF into the exhaust. There's also EGR and variable turbos that work on the engine to impact emissions. Together, these systems are good enough that a manufacturer can run a dirtier combustion process, and rely on the aftertreatments to clean them up enough to meet today's standards. In fact, the combustion process and aggressive fuel timing has allowed dirty enough combustion that we now have 1000ft-lbs trucks sold to the general public with full warranties.
So, it's entirely possible that a 2019 3/4 ton truck with deleted or modified emissions devices is emitting more than a 2001 3/4 ton truck that had no emissions rules at all. But without knowing exactly what was modified or deleted completely, and how the ECU was tuned after the fact, it's hard to say exactly how the claimed "emissions" increases break down. Generally, diesels struggle with NOx and Particulates, so I'd guess that the article stipulating "emissions" is referring to NOx emissions, but it could also be HCs or CO2. It's also possible (perhaps even likely) that all of them are higher than current standards at the same time.
Some clarification-
Diesel doesn't have high NOx because it runs lean vs. gas- it has high NOx because is it SO very hard to convert it in an oxygen rich environment. On an engine out basis, diesel is cleaner than gas. But for gas cars, converting NOx into N2 is pretty easy to do.
There's almost no HC in diesel.
PM does come from more fuel to burn, but in today's direct injection gas cars, even running stoich you can get PM- thanks to how the fuel is injected. For diesel, the mechanism is actually the same, as the actual PM limit that we all have to meet is not visible. If you can see it, it's REALLY bad.
As a check for gas cars- just removing the catalyst will result in about 100x more emissions for most cars these days. So 32 is probably lowballing the number. Especially for PM. The current PM standard is 10mg/mi, which is really low. Once you see the smoke, I'm pretty sure its 10-100x that amount, even if you drive for 100 miles.
Lastly, CO2 came off the regulated emissions recently.... I'm betting it won't be long before it goes back on.
