A few objections to the current emission standards for Diesel engines

For a long time, Diesel engines have been considered a good option due to their enhanced efficiency over spark-ignition (or older hot-bulb ignition) engines. Initially not so suitable for vehicles due to a poor power-to-weight ratio, further developments reduced this disadvantage in heavy-duty trucks, marine and agricultural machinery.

Since the 70's Diesel engines have been playing an important role in the automotive market, even leading some markets, mostly due to a demand for fuel savings after the oil crisys, but lately its adaptability to alternative fuels without performance changes or compromises to the efficiency while using heavier fuels such as straight vegetable oil in the days when biodiesel was still not taken so seriously.

In the medium-duty truck segment, the availability of spark-ignited engines became hard to justify, altough it still had some supporters in the American market for cultural reasons and easier cold-starts, and in Brazil due to the successful ethanol program. However, in the heavy-duty segments, Diesel prevailed, and it will hardly lose its leadership in a medium to long term...

Some old irons from that era, such as the early Volkswagen Passat (known in the U.S.A. as Dasher) and the Peugeot 504, are often deemed among the best options for experiences with SVO. The higher operation temperatures in the old-school indirect injection layout and the lower combustion speeds lead to a more complete combustion, reducing issues related to glycerin glazing inside the engines...

However, the same higher temperatures quoted as beneficial to an accurate combustion process are usually a reason to blame for the higher nitrogen oxides (NOx) emissions in Diesel engines, then it's quite predictable to see the direct injection as the predominant setup into newer vehicles such as the Peugeot 3008. There is also another opposition raising against the Diesel engines, with the increased popularity of gasoline-electric hybrids such as the Toyota Prius, raised on marketing to be perceived as a "cleaner" alternative in spite of the higher manufacturing footprint mostly due to the electric auxiliary driveline.

In spite of the combustion process in Diesels providing a higher thermal efficiency and adaptability to alternative fuels, including ethanol for direct injection engines, the current emission standards are often disconsidering these advantages, and actually decreasing it with the enforcement of some emission control devices such as the EGR (or the SCR as either an alternative or an add-on measure to reduce NOx emissions) and the DPF. Considering the increased fuel consumption from the usage of EGR and DPF, due to both the lowered combustion efficiency with the EGR recirculating exhaust gases inside the combustion chambers and the requirement of a small amount of Diesel fuel to the DPF "regeneration" (burning-off the trapped soot) process, the energy expense with more petroleum being refined and having its byproducts hauled to the refuelling stations, the tailpipe emisions reduction doesn't seem to match...

That tank between the fuel tank and the air filter is for the DEF

With the SCR/DEF/BlueTec system, which uses a solution composed of 67.5% distilled water with 32.5% industrial-grade urea, more prevalent in medium-duty and heavy-duty trucks and buses, another point to consider is related to the higher complexity to the logistic process due to another fluid required to the proper operation of the vehicle, which absence can lead to failure. No wonder military fleets have been granted an exemption from this kind of stuff.

Alternatives such as water injection at the intake, usually blended with some alcohol (either methanol or ethanol) up to 50% to avoid freezing, are easy to implement and effective in matter of NOx reduction due to the lower exhaust gas temperatures, and instead of just increasing the purchase cost of a new Diesel-powered vehicle or machinery it would be actually providing a pay-off due to the lower fuel consumption. Even pure ethanol can be sprayed at the intake for the same effect, reducing the volume required. Also, due to the enhancement of the combustion process, it decreases the amount of particulate matter and the fuel consumption, making more sense than wasting both the extra fuel required in an engine fitted with EGR or the urea used in the SCR, and keeping the intake manifold cleaner from oily residues brought from the crankcase by its closed vents. The energy input required for industrial processes to meet an increased demand for Diesel fuel and the urea fluid would become lower, and also due to the combustion enhancement some alternative fuels such as pure vegetable oil can be used in a safer way. The DPF is often also deemed to actually have a prejudicial effect regarding respiratory diseases, since the particulate matter oxidized into smaller particles can go deeper into the lungs. Ironically, water+methanol injection was already experienced by Rudolf Diesel himself, altough not being so widespread among OEMs.

Beyond its current relevance for the private vehicle market, Diesel engines also have been prevalent in emergency vehicles such as ambulances, and stationary appliances such as backup power gensets. While the efforts to reduce emissions can be considered relevant, the approaches for that matter have been quite unaccurate due to their side-effects, which not just decrease the efficiency but also raise the purchase cost due to the highly-sophisticated aftertreatment setups required for a compliant operation.