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Reducing emissions from diesel engines

The exhaust pollutant emissions from endothermic diesel engines have a high environmental impact, however, the modern anti-pollution systems can narrow reduce it by over 90%

by Domenico Pessina e Lavinia Eleonora Galli
July - September 2019 | Back

Ever since the distant 1996, the world of endothermic engines has been upset by a historic event: the introduction of rules, actually extended worldwide, aimed at drastically cutting polluting emissions into the atmosphere. A sort of wide-ranging road map has been drawn up, which has resulted in almost eliminating the detrimental impact of exhaust gases on the delicate global ecosystem.

To fully understand the scope of these measures, just think that: 1) both the European Union and the United States have agreed at the same time on times and methods of the various passages concerning the progressive limitation of emissions; 2) for once, it has happened that a piece of legislation has prompted (sometimes extremely decisively) the technical progress and not the other way round (as it almost always happens), that is to say, that technical developments anticipate legislation. In this way, the law only conforms itself (sometimes with considerable delay) to what has already been acquired. The developed legislation was structured in some passages (“steps”) identified by numbers corresponding to certain emission limits, which in Europe were defined by the European Commission as "Stage" and in the USA by the EPA (Environmental Protection Agency) as "Tier".

Also, the so-called "off-road" vehicles sector, where agricultural machinery belongs, was obviously involved in this epochal change. Therefore, the engines mounted on the tractors and self-propelled operating machines also are following the same trend, even if with less stringent timeframes compared to other sectors. Especially tractors produce a significant rate of pollutants, both because of the typical considerable duration of these vehicles, and therefore to the continued operability of machines with several tens of years of service, but also because of the high hourly consumption of the models more powerful in carrying out the most demanding jobs, such as deep working of the soil.

Manufacturers were therefore forced (it is appropriate to say...) to develop new technologies to limit the environmental impact of polluting gaseous emissions.


The endothermic engines main pollutants

Environmental pollution is mainly caused by the greenhouse effect that these gases cause; the main components are carbon monoxide (CO), unburned hydrocarbons (HC), nitrogen oxides (NOx) and particulate matter (Particulate Matter, PM).

Diesel engines, which represent the type almost exclusively adopted in the agricultural sector, are critical for the production of nitrogen oxides (NOx), due to the high temperature generated during combustion, and of particulate in its different dimensional classifications (PM 10, PM 5 and PM 2.5), due to the incomplete combustion of diesel fuel.

In order to reduce the emission of these two pollutants into the atmosphere, some devices capable of decreasing and/or neutralizing (in whole or part) the dispersion have been studied, validated and marketed as they are.


Exhaust Gas Recirculation (EGR)

It is a solution based on the re-introduction of a part of the exhaust gas (about 5-15% by volume) into the combustion chamber, thus lowering the combustion temperature and consequently reducing the production of oxides of nitrogen. This process is modernly carried out externally, thanks to the interception of a variable part of the exhaust gases by a dedicated electronically managed valve, subsequently cooled by a heat exchanger, in order to reduce the temperature and therefore increase the density, and then returned to the combustion chamber together with the diesel fuel and air.

Although it is effective for reducing NOx, this solution (the first in order of time adopted by the manufacturers) inevitably increases the production of PM and HC, besides causing a loss of efficiency, substantially due to the combustion that occurs at lower temperatures and is, therefore, less complete. Furthermore, the lubricating oil is subjected to faster degradation.


Selective Catalytic Reduction (SCR)

This is the solution preferred by European diesel engine manufacturers aiming at reducing the NOx resulted from combustion through post-treatment (i.e. after the generation of the fumes, but before they are released into the atmosphere), thanks to a chemical reaction located in a special environment, in which an aqueous solution of pure urea with a concentration of 32.5%, known commercially as AdBlue, is nebulized on very high-temperature exhaust gases.

The resulting reaction produces components harmless to the environment, i.e. atmospheric nitrogen and water in the form of steam.

The AdBlue nebulization is regulated by a sensor positioned inside the exchanger and connected to a control unit, which monitors and consequently doses the correct amount of urea to be released as required.

Therefore, the SCR does not involve power reductions, but the increased operating cost for the purchase of AdBlue and the additional burdens on board the machine, both of the reagent tank and above all of the container where the described chemical reaction takes place.

Precisely this last aspect highlights a heavy problem about its adoption on specialized tractors, where the chronic lack of space under the hood does not allow its installation in any way.


Diesel Particulate Filter (DPF)

Most famous in Italy as FAP (Diesel Particulate Filter), it consists precisely in a physical type filter, which retains the larger particulate particles (PM). It is made of refractory material with a honeycomb type structure, with extremely dense links able to effectively intercept the PM particles. The DPF inevitably creates a back pressure for the exhaust fumes expulsion, thus generating a resistance affecting the performance of the engine even with a clean filter, but above all with an occluded filter.

Therefore, to ensure correct functioning, the filter must undergo a constant cleaning, carried out through the so-called "regeneration", which can be active or passive, and performed automatically or also manually (see box).


Diesel Oxidation Catalyst (DOC)

Set in addition to the DPF, or even as a replacement, beyond processing the polyaromatic hydrocarbons (converting them into CO2 and water in the presence of oxygen), it manages to internally immobilize the PM and the non-carbonaceous organic particles. It is made up of ceramic plates with a honeycomb structure, covered with noble metals acting as catalysts.

It is particularly efficient for retaining the smallest fraction of the PM and avoiding the ozone formation, harmful to the troposphere. On the other hand, its efficiency is reduced if diesel with high sulfur content is used.


Where technical progress is heading

The application, often single and as an alternative, of the described devices has so far allowed compliance with the limits imposed by the regulations up to Stage 4/Tier 4 final. However, the progressive tightening of the emission limits imposed by the most recent steps has forced manufacturers to change their intervention strategy.

There is no doubt that the best solution turned out to be to design and develop new engines with less polluting emissions, focusing above all on improving combustion.

From this point of view, an extraordinary contribution was offered by the evolution of the common rail, which reached very high injection pressures (above 2.000 bar) and by an increasingly accurate study of the combustion chamber conformation (to favor an optimal mixing between fuel and combustion material), in addition of course to the more precise management, obviously electronic, of the injection itself (for example with the technology of multiple partial injections).

On engines intended for medium-small tractors, even for the aforementioned lack of space under the hood, only DOC or DPF are now set (or a combination of the two devices); however, for more powerful engines this is not enough, so it is often necessary to apply two or more different devices at the same time, which simultaneously eliminate PM and NOx. The most commonly used combinations are EGR + DOC, EGR + DPF, or SCR + DPF. In order to be able to fall within the most restrictive emission limits, in some cases (particularly complex) it is also possible to find combinations consisting, from upstream to downstream, of DOC, DPF, SCR and AOC (Ammonia Oxidation Catalyst).

AdBlue in agriculture

With the introduction of the SCR (Selective Catalytic Reduction) on vehicles of the automotive sector, the availability of AdBlue (i.e. the commercial name of the fluid necessary for the conversion of nitrogen oxides into components that are harmless to the environment) has become widespread, so much so that today it is easy to buy it at many petrol stations, and also in some chains of specialized shops and supermarkets.   The supply of AdBlue is carried out directly through a special pump (very similar to the diesel version, even if with a lower flow rate) in the petrol stations, while it is available in 5-15-liter jugs from authorized retailers and supermarkets. 

In order to ensure adequate autonomy (and to reduce costs) in the agricultural sector, AdBlue is managed differently.   In fact, typically agricultural holdings buy this fluid in a cubic polyethylene tank, often protected by a metal grid, of varying capacities (but usually 1.000 liters) already equipped with an autonomous 12 V electric pump and a nozzle to easily refuel at the company center.

Regeneration of DPF devices

This is a necessary procedure for the DPF (Diesel Particulate Filter): in fact, since this is a physical type filter, it is inevitably subject to a progressive clogging which affects its performance.   It is, therefore, necessary to restore its efficiency through an extraordinary maintenance operation, which can be performed by removing the component and cleaning it with solvents or, more rarely, with the complete replacement of the filter (a rather expensive practice). 

Conversely, the so-called “regeneration” can take place "on-site", and be essentially of two types, active o passive. In the first case, the process is periodic and managed by a control unit, controlling the pressure difference between the DPF entry and exit through dedicated sensors: if a value exceeding a set threshold is detected, the cleaning process is automatically triggered.  In practice, for a limited period (usually around 10-15 minutes) the flow of diesel in the combustion chamber is increased, so as to increase the temperature of the exhaust fumes to allow the gasification of the carbon particles occluding the links of the filter. 

It is an effective solution to reduce the PM emitted into the atmosphere, which however highlights some disadvantages, such as a limited reduction in the motor power generated, due to the resistance that the progressively clogged filter opposes to the expulsion of the exhaust gases and the greater emission of NOx due to the increase in temperature in the combustion chamber when regeneration is performed.  Automatic regeneration handling can be postponed (usually for a maximum of 2-3 times) if potentially dangerous conditions occur, such for example, the risk of fire because of the dry vegetation under the tractor, which could be subject to combustion due to the high temperatures to which the entire engine block is subjected during regeneration.  In this case, manual intervention is then necessary, with the so-called manual or "parking" regeneration.   In case also this procedure is neglected, usually even if only once, regeneration can only be carried out in the workshop. 

Finally, it must be considered that the periodic regeneration of the DPF has an impact, although limited, on the overall consumption of the engine. This is a figure not easy to quantify, but recent and authoritative studies have made it possible to ascertain that the burden on the tractor's diesel consumption amounts to an average of about 1% of the total.

On the other hand, the so-called passive regeneration is spreading more and more. It involves the use of filters (DPF coated) always with a honeycomb type structure, but in which the surface is covered by a thin layer of noble metals (often platinum) which acts as a catalyst for the continuous chemical transformation of carbon particles into CO2 molecules.

In practice, this type of filter (which is sometimes combined with the DOC, Diesel Oxidation Filter) is placed very close to the exhaust gas output so as to exploit the still high temperature (around 300-350 °C), facilitating in real-time the specified chemical reaction, with no additional injection of diesel fuel required for the temporary increase in the temperature of the exhaust gases.


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