Diesel engines tend to be larger/bigger/heavier than petrol engines due to higher compression/pressure inside. They warm slower. Cold engine needs a rich mixture and that causes polution with unburnt fuel (mostly carbon). These devices are designed to filter that.
The downside is that engines used for short trips at low power (city driving) rarely get to high enough temperature to burn the carbon in the filters. Filters/catalytic converters get clogged and cause engine warnings / errors and bad performance.
Depends on what the owner wants. The ones that are rolling coal are removing all the filters then deliberately running their engine rich. The black smoke is literally unburnt diesel. Obviously when you're spraying huge clouds of diesel, your fuel efficiency is going to go down.
After Dieselgate, Volkswagen brought their diesels into compliance by software patch. People did dyno runs of all VWs before and after the patch, and in many cases (e.g. the 180HP Passat) got something like 10% less horsepower and drastically worse torque curves (max torque at 1900 rpm instead of at 1500 rpm) after the patch. Fuel economy also went down by around 10% because of the patch.
Makes sense from a thermodynamic perspective: your engine runs with more power/torque and more efficient if you run it hotter. But running a diesel hotter makes much more NOx.
Some parts of the emissions control systems (I was informed by competent technicians, but not yet adequately "educated") work through burning extra gasoline and fuel - they are active, they use energy.
The extra intake may seem pretty dramatic - though of course full data for comparison should be in order... It should be possible to compare euro-4 vs euro-5 model consumption through spritmonitor.de (through selecting by year) - I have no time right now but may do it later, and possibly report.
Multiple things. EGR, the scr, and particulate filters are not super reliable, and when they fail, either your truck goes into limp mode or if your EGR cooler failed and your engine sucked in coolant, you're buying a new engine. They also give you slightly better fuel economy and more torque/horsepower.
Then there's the real jackasses (like my cousins) that delete all that stuff along with the muffler and deliberately blow clouds of black smoke because deep down they are deeply insecure people and suffer from undiagnosed depression and anxiety.
Given that diesels emit particulates and nox as well as co2, I'll note it's theoretically possible to tune them in a way that increases efficiency at the cost of other pollutants. I've no idea what the actual devices sold do, though.
