The technical progress of engines for agricultural use
Similar to those for the automotive sector, endothermic engines for off-road vehicles (thus including those mounted on agricultural tractor machines) also have a impact on the environment. Adoption of the latest anti-pollution devices enables a reduction in harmful emissions, up to more than 90%
It is now well known that modern agriculture cannot be independent from machines in order to guarantee the production levels required by the market. With the aim of increasing production, the number and size of machines operating in the agricultural sector has gradually increased. Over the years, the increase of mechanized vehicles in agriculture (as well as in other sectors) equipped with internal combustion engines has highlighted the problem of gaseous pollutant emissions. In addition to progressively reducing the pollution emissions, among other things, regulating them was intended to contribute to the renewal of the fleet, which in a country like Italy is very obsolete (the average age of tractors operating in Italy is over 20 years).
1996 saw the first step of a change that to all intents and purposes can be defined as "epochal", with the introduction of the first steps of the regulations for the progressive limitation of polluting gases produced by engines mounted on off-road vehicles. Europe and the United States agreed and issued parallel regulations with the same limits, aimed at reducing the amount of pollutants in the exhaust gases of non-automotive vehicles. It was a true "road map" that, in successive steps spread over time, envisaged drastic reductions in the potential for emissions. The increasingly stringent thresholds pushed manufacturers to force the technical progress of the engines, to meet the parameters being imposed. Similarly to what happened for the automotive sector, the regulations were then structured with a series of steps identifiable by numbers and letters: for European regulations with the prefix of "Stage", while in the USA they were identified as "Tier" by the EPA (the Enviromental Protection Agency).
Because of their long service life and high hourly consumption, high-powered agricultural tractors (which usually work with fairly high loads) produce a significant amount of pollutants. In order to reduce the release of greenhouse gases into the atmosphere, specific devices have been developed to reduce the production and/or zero the dispersion of harmful compounds. The most frequently adopted ones are EGR, SCR, DPF and DOC.
Exhaust Gas Recirculation (EGR)
EGR was the most immediate solution put in place, and thus historically the first to be applied to agricultural engines. It consists in the re-introduction of a small percentage of the exhaust gases, about 5-15% by volume, in order to reduce the combustion temperature and consequently the production of NOx. Over the years, the operating logic of the device has been improved, with real-time modulation of the quantity of gas to be reintroduced into the combustion chamber according to actual needs, thanks to a specific electronically controlled valve, and the cooling of the gas by a heat exchanger, in order to increase its density and thus reduce the quantity required. However, although it achieves the desired effect, this solution has as a heavy side effect an increase in HC and PM production, due to the loss of engine efficiency and incomplete combustion of diesel fuel. In addition, lubricating oil is subjected to faster degradation.
Selective Catalytic Reduction (SCR)
This is a solution widely adopted in Europe by diesel engine manufacturers with the aim of reducing NOx emissions, but without intervening in the combustion chamber, as EGR does. In practice, this is a post-treatment of exhaust gases at very high temperature, on which a solution of pure urea 32.5% concentration, known commercially as AdBlue, is sprayed inside a confined environment, transforming NOx into environmentally inert agents such as atmospheric nitrogen and water in the form of steam. The spraying of AdBlue on the exhaust gases is regulated by a sensor connected to a control unit, which measures the concentration of NOx in the fumes in real time and defines the amount of AdBlue to be reacted. As an after-treatment of exhaust fumes, SCR does not lead to a reduction in power and engine efficiency, but it does lead to a consumption of AdBlue, with a certain operating cost and an increase in the space required on board the tractor to house the SCR unit and the solution storage tank. The latter represents a particularly critical aspect, especially for specialized models, which are characterized by their particularly small size.
Diesel Particulate Filter (DPF)
This consists of a physical filter made of refractory material with a honeycomb structure and a very dense mesh to be able to retain larger particulate matter (PM). Due to its constitution, the DPF generates a significant back pressure at the expulsion of the fumes, reducing (already with free filter) the engine efficiency. The problem is even more pronounced when the filter mesh begins to clog. Precisely for this reason, the filter must be subjected to periodic cleaning, obtained through a "regeneration", which can be performed manually or automatically, often activated on-site in an active or passive way. In the first case it is a periodic intervention, whose frequency is defined by a pair of sensors located upstream and downstream of the filter and that measure the difference in pressure; when this reaches an excessive level, regeneration is performed, which basically consists of a temporary increase in the flow of diesel fuel, so as to increase the temperature of the exhaust fumes and consequently cause the gasification of the particles that occlude the filter. Passive regeneration, on the other hand, involves coating the structure of the filter with noble metals, which act as catalysts favouring the transformation of the retained carbon particles into CO2 molecules. These filters are placed very close to the engine's exhaust outlet so that their high temperature (300-350°C) can be used to carry out the chemical reaction, without the need for additional fuel.
Diesel Oxidation Catalyst (DOC)
Often installed in addition to or instead of the DPF, it is a device that, in addition to transforming polyaromatic hydrocarbons into CO2 and water (in the presence of oxygen), is also capable of immobilizing PM and non-carbonaceous organic particulate residues within it. The structure of this filter is composed of ceramic honeycomb plates, again coated with noble metals with catalytic function. This device is particularly efficient at trapping even the finest PM, and is also able to prevent the formation of ozone, which is harmful to the troposphere. However, part of its efficiency is compromised if it works on exhaust gases from very sulfur-rich types of diesel fuel.
New frontiers with paired devices
The separate introduction of the individual devices described above has in most cases successfully limited the pollutant emissions of engines within the limits up to Stage 4/Tier 4 final. However, due to subsequent increasingly stringent thresholds, the adoption of individual and separate devices is no longer sufficient to meet the requirements of the regulations. On the other hand, the most effective solution to reduce polluting emissions into the atmosphere has been, and still is, a careful design and/or tuning of engines, investing above all in combustion optimization. An eloquent "historical" example of this was the introduction of common-rail injection, which, thanks to the considerable increase in working pressures, was able to generate a much higher level of fuel atomization, which was able to mix much more efficiently with the comburent (oxygen contained in the air), to the benefit of an increase in engine efficiency and a considerable reduction in emissions. Various combinations of anti-pollution devices have been developed and then adopted, in order to meet the most stringent requirements of the regulations, with the important complementary objective of containing the overall dimensions, useful to better manage this problem in increasingly "crowded" engine compartments. The combinations most frequently adopted at the moment, to simultaneously reduce the emission of both NOx and PM are: EGR+DOC, ERG+DPF, SCR+DPF. However, it is not uncommon to find cases where 3 or 4 anti-pollution devices are applied at the same time on an engine, such as EGR+SCR+DPF, SCR+SCR+DPF or DOC+DPF+SCR+AOC (Ammonia Oxidation Catalysis).
The main pollutants of diesel engines
The main greenhouse gases generated by internal combustion engines are carbon monoxide (CO), unburned hydrocarbons (HC), nitrogen oxides (NOx), and particulate matter (PM). Engines for agricultural use are almost exclusively fueled by diesel, the pollutants of greatest impact are NOx, due to high combustion temperatures, and PM, which differs according to the diameter of the carbon particles generated by the incomplete combustion of diesel.