Diesel Emissions: How Clean Should They Be?

Quelle: P.M. Magazine 08/2003
http://www.pm-magazin.de/de/heftartikel/ganzer_artikel.asp?artikelid=727


Automotive technology.
How verschandeln does a diesel engine need to be?


Diesel soot has long been suspected of causing cancer. However, researchers are now saying that this is not the case.

The message of the Peugeot advertisement is clear: Not a single speck of black soot remains on a pristine white handkerchief when it's held up to the exhaust pipe of the running engine. The diesel engine of the Peugeot idles quietly, but emits no dirty exhaust fumes – the handkerchief stays clean, and the environment remains pure. This is because the car has a particulate filter.

Diesel vehicles equipped with particulate filters from Peugeot and Citroen are selling very well. However, German automotive companies do not see these "particulate killers" as a universal solution for harmful combustion byproducts. A fierce competition has emerged among engine manufacturers regarding how to neutralize pollutants from the exhaust of increasingly popular diesel engines, or even prevent them from being produced in the first place. The European emission standards serve as guidelines: The current Euro 3 standard will soon expire; from 2005 onwards, all newly registered diesel cars must meet the stricter Euro 4 standard. Five years later, in 2010, the next higher level, Euro 5, will come into effect.

Euro 4 already stipulates a reduction of particulate matter in diesel exhaust by 50 percent compared to the currently applicable standard, down to 0.025 grams per kilometer. For example, on the 1000 kilometers between Hamburg and Bozen, the exhaust pipe is only allowed to emit one teaspoon of soot. These 25 grams are approximately the same amount of dust that brake pads release, and they are just a third of the amount of wear produced by the four tires over this distance.

With the introduction of Euro 4 standards, other pollutants in exhaust emissions are also intended to be further reduced, including a reduction of 22% in carbon monoxide emissions (CO).
"how nitrogen oxides (NOx) are reduced by 50 percent. These political guidelines are referred to as performance standards, which do not specify particular techniques for complying with the emission limits. It is up to the manufacturers to decide which technical solutions they will implement."

The particles emitted by diesel engines are a particularly contentious issue – they consist of combustion residues of various sizes. In older, especially poorly tuned, engines, these particles can be seen as a smoky cloud with the naked eye. They are formed when, despite the typical excess of air in diesel engines, zones with oxygen deficiency develop due to poor mixing of fuel and air. The smoke cloud from the exhaust is particularly dense during rapid acceleration from a standstill at a green light or when trucks struggle uphill: In both cases, the engine is operating at its maximum capacity, meaning that a large amount of fuel is injected, but relatively little air is drawn in.

Modern, well-maintained engines emit significantly fewer soot particles. This is because diesel fuel is injected under much higher pressure, which allows the extremely fine droplets to mix much better with the air and guarantees nearly complete combustion. Nevertheless, even new diesel engines emit invisible "fine particles" measuring 100 nanometers (nm), or 0.01 millimeters, which are released into the atmosphere through the exhaust. How dangerous are they?

The lungs are the entry point for air pollutants. Just as a bellows inflates, we breathe: The diaphragm muscles create negative pressure and draw in approximately half a liter of air, twelve to eighteen times per minute, or about 20,000 times a day. Within 24 hours, we inhale between 10,000 and 20,000 liters of air. It's a long journey from the nose, where the air is pre-cleaned and moistened, through the bronchi and bronchioles, to the approximately 300 to 500 million alveoli (air sacs). The airflow becomes progressively slower, and the deeper a particle enters the body with the inhaled air, the longer it can remain there, settle, and cause damage.

Over the course of evolution, humans have developed mechanisms to filter out floating particles and dust from inhaled air. The largest particles are already trapped in the nose – and can be seen on a tissue after blowing one's nose. This filter is already highly effective, and the more dust accumulates, the better its effect. Deeper inside the body, the dust adheres to the moist surfaces of the bronchi. Together with mucus, it is constantly moved towards the throat by tiny cilia; a warning system with sensitive chemical sensors activates defense mechanisms such as coughing and sneezing, allowing the lungs to clear themselves.

However, some particles are so fine that they pass through the bronchi and travel further. But they don't go unnoticed: Just as police patrol public streets, mobile scavenger cells roam the deep recesses of the lungs, the alveoli, in this area where there are no more cilia. These cells, called macrophages, pounce on the intruders and can digest them – a process known medically as phagocytosis.

"However, if the scavenger cells are overwhelmed by the attack of the particles," says Professor Joachim Bruch from the medical university clinic in Essen, "they start 'acting erratically.' They then release oxygen radicals and nitrogen oxides." These extremely dangerous substances can damage the neighboring DNA and trigger the development of a tumor.

Previously, it was assumed that technical dusts, especially the ultra-fine particles produced by fuel combustion in engines, could bypass the lungs' defense mechanisms and settle in the alveoli. It was believed that these particles could remain there for months, or even years, potentially migrating into blood vessels or damaging the air sacs. However, according to the most recent research by Professor Bruch, which the scientist presented for the first time a few weeks ago at the Vienna Engine Symposium, this assumption is no longer valid.
Our immune system is perfectly capable of defending against the intrusion of these fine particles and successfully destroying them through phagocytes. However, as mentioned earlier, there must be a sufficient number of these cells. This contradicts the common theory that the body's defense system cannot cope with these fine particles. "There is a dose, but no effect," says Bruch. This is because the oxidation catalyst for diesel engines has already converted the carcinogenic pollutants attached to the soot, such as polycyclic aromatic hydrocarbons. The remaining amount of chemically inert soot ("inert" means that the soot does not form any compounds or new substances) is below the Euro 4 limit and so minimal that the threshold for health damage caused by a modern diesel engine is clearly exceeded.

The German automakers feel that the findings of the Bruchs study confirm their views. To date, no studies have proven that diesel soot has carcinogenic properties. Such evidence has only been obtained in rats using doses thousands of times higher than normal exposure levels. Therefore, the domestic car companies see no need for particulate filters like those offered by Peugeot or Citroen, which are known to retain 99 percent of the particles.

Nevertheless, they have agreed among themselves not to cede the field of environmentally friendly diesel engines with particulate filters to the French – German companies want to maintain their technological leadership. At the International Motor Show in September, which is already decided, they will also offer engines with particulate filter systems – almost against their own conviction. To the dismay of the engineers, these will be based on the systems of Peugeot and Citroen.
In the Peugeot version, an additive based on iron, the natural product cerin (derived from beeswax), and a mixture of copper, strontium, and platinum is permanently injected into the fuel. It deposits in the cylinder like a mist and binds to soot, lowering its ignition temperature from normally 600 to approximately 450 degrees – thereby causing the particles to burn. (A similar process is used for cleaning oil furnaces with burner rods). The burning of these particles occurs automatically every 500 or 800 kilometers depending on driving style – at this point, the filter is verschandeln again, and only the ash from the additive remains in it. Disadvantages of this method: fuel consumption increases, and over time, the 5-liter additive tank needs to be refilled repeatedly. Furthermore, after 120,000 kilometers, the amount of ash residue becomes so large that it clogs the filter, requiring replacement – which costs approximately 300 to 600 euros.

German manufacturers aim to avoid this drawback with their filter systems. According to confidential insider information, Audi is working on a system that regenerates the filter without additives, preventing clogging due to residue. This requires temporarily increasing the exhaust gas temperature, as particles are only completely burned and the filter regenerated above 600 degrees Celsius. "By adding a small amount of finely atomized diesel fuel under high pressure to the already burning mixture," says one engine expert, "a cubic millimeter is vaporized but not fully combusted. It is then converted in an oxidation catalyst located upstream of the particulate filter, releasing heat. This chemical conversion process increases the exhaust gas temperature to 650 degrees Celsius – at which point the particles burn within the filter."
The frequency at which such a regeneration phase is necessary for the filter depends on the driving style. In urban areas with short distances, it occurs automatically every 500 to 800 kilometers, depending on how warm the engine is. For long-distance journeys, the intervals can sometimes be much longer. Because if the driver only drives at high speed on the highway for about half an hour, the filter can already regenerate itself without any special preheating. The goal is to avoid cleaning or even replacing the filter.
Within the Volkswagen Group, particulate filters are expected to be offered for vehicles whose engines cannot meet the Euro 4 emissions standards without them, following the expiration of the Euro 3 standard at the end of 2004. These primarily include large, high-performance SUVs and luxury sedans.

To meet the stricter Euro-5 emissions standard, which came into effect around 2010, new combustion processes need to be developed. Engine engineers are therefore working on solutions that allow for a uniform and homogeneously distributed diesel-air mixture within the cylinder. This would significantly increase the contact area between diesel and air particles, resulting in considerably fewer pollutants during combustion. This is to be achieved through new combustion chamber designs, new fuel injectors, and increased injection pressure: Currently, it ranges from 1350 to 2050 bar (equivalent to the pressure of a VW Golf on a thumbnail) – for compliance with the Euro-5 standard, it will be increased to 1800 to 2400 bar. This is still in the future.
Starting in 2005, compliance with the Euro-4 standard is mandatory: a goal that many engines, especially smaller ones, but also those like Audi's powerful V6 engine with 180 hp, already achieve – thanks to sulfur-free diesel fuel, even without particulate filters. When they are launched on the market, manufacturers can receive tax incentives. Those who want to do more for the environment can order a particulate filter as an additional option and demonstrate their commitment to environmental protection by performing a "white handkerchief test" on the exhaust pipe, proving that it produces minimal soot.

Author: Wolfgang Stegers.

More information about the working principle of DPF + Oxicat:
http://www.mdpi.org/ijerph/papers/ijerph2006030038.pdf




Okay, enough quoting!

What's interesting to me is the tissue test. I immediately went to my AXR and performed it. The result: I have a white vest!
It somehow seems that the author views the DPF as positive, but still holds a very critical stance towards it. It's best to control pollutants or particles that don't even form in the first place.
I was not previously aware that an EU4 diesel engine releases more tire wear and brake dust into the environment. However, this statement is very interesting. It's time to start introducing emission standards for tires and brake pads! }


Unfortunately, the article contains some significant errors:
VW plans to offer the DPF only as an optional extra, as the new 4 and 6-cylinder engines already meet the EU4 standards. In the case of large sedans/SUVs, they use the trick of weight-based taxation (over 2.8 tons gross vehicle weight), which means the vehicle falls under the emission regulations for commercial vehicles. The V10 TDI is a real polluter and doesn't even meet EU3 standards (the BMW M5 and Mercedes M Class are no better).
According to SVA and BOSCH, it is unlikely that the EU5 standard can be met with an injection pressure of 1800 bar without a DPF (Diesel Particulate Filter). Common Rail (PD) technology has already reached 2050 bar and achieves EU4 compliance with all optimizations. However, there is still a significant gap to reach EU5 standards.

Oh yes, and also, the diesel particulate filter (DPF) only works with the low-sulfur diesel fuel that is used in the EU.