Agricultural engines, technological evolution and alternative fuel
While waiting for the complete electrification of agricultural machinery, for several years, the internal combustion engine will still be widely used. To limit the environmental impact, in addition to the application of anti-pollution devices, more sustainable solutions addressing alternative fuels are actually taken into account, also concerning their production
For its performance characteristics, the diesel engine has established itself as a major player in global agricultural mechanization, basically because of its great robustness and excellent thermodynamic efficiency. However, it has long been pointed out (and not only in the "non-road" sphere but also for road implementations) as one of the main defendants of air pollution, although not all experts in the field agree on this hypothesis, particularly after the progress of the last 30 years that, thanks to the equipment of several specific devices, has recorded a huge decrease in the emission of polluting compounds (for diesel, especially particulate matter and nitrogen oxides). Consider that compared to an identical so-called "non-emitting" agricultural engine, a current one pollutes almost 100 times less. In other words, it would be like saying that a single 30-year-old tractor produces as much polluting gas as 100 modern tractors of the same power rating. Moreover, it must be taken into account that the problem is far from being finally solved if we consider that, for example, in Italy, the tractor fleet has an average age of 27 years.
However, it is precisely the fueling of the diesel engine with diesel, which is a fossil fuel, that produces a not inconsiderable amount of pollutant emissions. Therefore, manufacturers have long been moving toward the development of powertrains, including Otto-cycle engines, suitable for running on alternative fuels, either blended or pure, that could limit the production of harmful gases.
Furthermore, while waiting for the massive advent and ultimate electrification of the industry (for which we will have to wait a little longer, given the current incomplete maturation of battery technology), so-called multi-fuel engines are appearing on the market, i.e., capable of operating normally with different fuels, in a mixture or even pure.
Biodiesel, HVO and oils as such. The possibility of powering the diesel engine with fuels from renewable sources is now a widely established reality. Biodiesel has long since been accepted for use in whole form, i.e., 100 percent, although its production may pose sustainability problems. More interesting then is the use of HVO, which, although still made from oil-based waste and by-products (see box), undergoes a different process. If, in fact, biodiesel is the result of the transesterification of vegetable oils combined with methanol, HVO is instead produced by hydrotreatment, with several advantages, particularly of a microbiological nature.
Except in rare cases, today's diesel engines (now almost all common-rail type) are not designed to run on pure vegetable oil, which would risk causing serious damage and leading to the voiding of the warranty. The major problem is the significantly higher viscosity of vegetable oil compared to diesel fuel, which becomes more pronounced at low temperatures. Furthermore, its use runs the risk of damaging the injection system and creating significant fouling of unburned material on the inner surfaces of pistons and cylinders. The most suitable seed oil for fueling specially prepared diesel engines is rapeseed oil due to its lower viscosity than other oils, which is still significantly higher than that of diesel fuel. It is still possible to fuel diesel engines, even modern ones, with vegetable oil, albeit with necessary modifications, minor or medium. Indirect-injection models can be fueled with significant percentages of oil mixed (up to 50 percent) with diesel fuel by changing the pre-ignition glow plugs, injectors and the oil heating circuit, which, moreover, can be used pure up to about 10°C ambient temperature and together with diesel fuel for lower temperatures. Conversely, common rail direct injection engines require the installation of a real secondary tank for diesel fuel, used to start, warm up and shut down the engine, which can run on oil alone when hot.
Biogas, methane and biomethane. Primarily interested in this group of fuels are endothermic Otto-cycle engines. It is, therefore, a relatively new strand for the agricultural field, dominated as is known by the diesel cycle engine. Direct biogas power is an established feature of the large CHP (Combined Heat and Power) engines that are a key part of generator sets coupled to anaerobic digesters. To be able to handle a rather critical fuel such as biogas adequately, these engines provide for a continuous type of operation (h 24/7, thus avoiding the problematic initial warm-up phase altogether), a constant operating speed of about 1,500 rpm for optimized efficiency, the equipment of a two-stage turbocharger, a purged pre-combustion chamber in a low-pressure central position and with amplified spark plug ignition energy, and finally a highly accurate electronically controlled gas metering.
Considerably less critical in Otto-cycle engines is the use of methane, which in agriculture originates precisely from properly purified biogas, thus obtaining bio-methane, which has recently become an interesting fuel also for tractor drives. For this purpose, New Holland has developed and perfected the 175-hp T6.180 Methane Power, a model that mounts the 6-cylinder, 6,700-cfm NEF engine specially tuned for agricultural implementation by FPT Industrial. In this case, overall pollutant emissions are up to 80 percent lower than with diesel (and reduced by as much as 98 percent for particulate matter alone), while exhaust after-treatment makes use of a simple 3-way catalytic converter. What makes the tractor stand out is the conspicuous additional fuel container placed as a front ballast, which allows for an increased storage capacity of 270 liters in addition to the 185 liters of the main tank.
Ethanol and bioethanol. Ethanol fuel has long been adopted for automotive and other niche uses, such as on model airplanes. Nevertheless, its "bio" version, that is, obtained from products or, rather, by-products and biomasses of agricultural origin, is of interest to the tractor world. Mixed diesel-ethanol fuel experiments have already been carried out in the past, but John Deere's launch of a prototype 9,000-cfm engine compatible with 100 percent ethanol fuel is recent. Although it has a lower calorific value, the main benefit is, again, a tangible reduction in particulate matter production.
Dual-fuel kit. Although rather demanding modifications of the fuel establishment are required, many diesel engines (with both electronically controlled and mechanical fuel injection) can be modified with special "dual fuel" kits so that they can also run on CNG, LPG and other biofuels assimilated to them. The add-on kits include a dedicated tank, from which gas is taken to the pressure reducer (250 to 1.5 to 2.5 bar for CNG and 30 to 0.6 to 1.5 bar for LPG) and then to the injectors that feed the gas into the intake manifold. Despite these modifications, unlike Otto-cycle engines, a small initial percentage of diesel fuel must still be injected at each cycle to ignite combustion. The additional gas metering injectors use a specific electronic control unit, which adjusts their opening and closing times, as well as the amount of alternative fuel, a function of power demand and a range of other data inherent to combustion. Thanks to these kits, engine performance improves, with an increase of up to 30 percent in maximum power, 10 percent in maximum torque, and a reduction in pollutant emissions between 20 and 60 percent. Moreover, the increase in combustion temperature results in less frequent regeneration routines for the particulate filter on engines that fit it.
Emphasis on sustainability
Clearly, it makes little sense to recharge a battery with electricity from a coal-fired power plant, whereas it is much more sustainable if photovoltaic panels produce the energy. Thus, for a proper and comprehensive analysis of the sustainability of the production of alternative fuels to diesel, it is also necessary to address the aspect of raw materials, whether they compete with food supply (human or livestock farm animals) and/or land consumption, which likewise could be allocated to more profitable crops for direct human consumption.
An illustrative example in this regard concerns the production of (bio)ethanol, which in South America, and particularly in Brazil, has for many years been produced from sugarcane for use in fueling the eight-cycle engines of the national vehicle fleet. Among other things, this activity has long been the subject of considerable controversy, not least because of the excesses of cultivation, which result in the unbridled deforestation of the Amazon, that is, the main lung of our planet.
For some time now, technologies that are capable of solving both problems have been developed. In fact, the so-called "second-generation" production of bioethanol involves the exploitation of lignocellulosic matrices (obtained as a by-product of other processing): initially destructured by cellulolytic enzymes to get simple sugars, they are then converted into ethanol by fermentation, after distillation to separate it from lignin. The residue is finally subjected to combustion in a boiler to allow for efficient energy integration of the entire process.
However, this solution still involves land occupation. The production of "third generation" ethanol also solves this problem, coming from so-called blue-green algae, grown in specially prepared bio-reactors, and then extracting the starch component, where a specially selected group of cyanobacteria produces bioethanol. In addition to their high photosynthetic efficiency, the algae are grown in special ponds, thus not taking away land for food production.
HVO (Hydrotreated Vegetable Oil)
It is a range of paraffinic fuels, low in sulfur and aromatic hydrocarbons, obtained from waste feedstocks, e.g., cooking oil, but also vegetable residues and oils from crops not competing with the food chain. HVO has a lower density of about 7 percent but a higher cetane value (which is an indication of the propensity for self-ignition) than diesel fuel. Even if it results from raw materials similar to those used for biodiesel, HVO is, however, produced by hydrotreating and not by transesterification and thus is not subject to bacterial growth, which is typical of biodiesel. Compared to diesel fuel, it has a very few percent lower energy content, so engines are able to deliver quite similar power. On the other hand, its combustion results in a significant reduction in particulate matter (PM) emissions, with negligible changes in NOx emissions. As a result of this, no special changes in hardware or software are required to fuel the latest generation of diesel engines as long as the use of HVO is expressly permitted.
In Italy, ENI has decided to promote from Oct. 1 to Dec. 31, 2023, the use of this automotive fuel, making it available at 50 refueling stations (which will increase in the near future) at a promotional price, a few cents lower than that of conventional diesel fuel. As expected, engine manufacturers in the agricultural field have also welcomed the use of HVO. Among others, one example is the initiative of Claas, which, as of Oct. 1, 2023, has officially declared all its self-propelled machines to be HVO-enabled. Again, for promotional purposes, all new units delivered after that date will be filled with this biofuel.
Hydrogen
For more than a few insiders, this would be the breakthrough fuel, although, for the time being, this gas seems to remain in the background of alternatives to the classic diesel-powered engine. The hydrogen molecule has a very high energy density, but studies aimed at its direct exploitation as a fuel in Otto-cycle engines have not yielded extremely encouraging results. Conversely, it proves to be the ideal solution for powering fuel cells since the latter, coupled with an electric motor, provides practically twice the efficiency of using hydrogen as such in endothermic engines. However, it should not be forgotten that the environmental benefit of using hydrogen is real only when it comes from sustainable production, for example, gasification of woody biomass or electrochemical dissociation of water (however, strictly carried out with electricity from renewable sources, and exploited during periods of energy surplus). However, unfortunately, all of these processes are much more energy-unfriendly than production via fuel cells and subsequent storage in batteries. Fendt has chosen this solution and, as part of the specific H2 Agrar project developed jointly with the Green H2 Hub-Haren initiative in Lower Saxony, has recently developed an orchard-vineyard model equipped with fuel cells that recharge a battery pack intended to drive the tractor electrically.
