Action areas and the need for research in biofuels

Fuel 268 (2020) 117227

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Review article

Action areas and the need for research in biofuels a,⁎

b

c

T d

c

e

Martin Unglert , Dieter Bockey , Christine Bofinger , Bert Buchholz , Georg Fisch , Rolf Luther , Martin Müllerf, Kevin Schaperc, Jennifer Schmittc, Olaf Schrödera, Ulrike Schümanne, Helmut Tschökeg, Edgar Remmeleh, Richard Wichti, Markus Winklerj, Jürgen Krahlk a

Coburg University of Applied Sciences and Arts, Germany Union for the Promotion of Oilseeds and Protein Plants (UFOP), Berlin, Germany c SGS Germany GmbH – Oil, Gas & Chemicals, Speyer, Germany d LKV – Chair of Piston Machines and Internal Combustion Engines, University of Rostock, Germany e Fuchs Schmierstoffe GmbH, Mannheim, Germany f ERC Additiv GmbH, Buchholz, Germany g Institute of Mobile Systems, University of Magdeburg, Germany h Technology and Support Centre in the Centre of Excellence for Renewable Ressources (TFZ), Straubing, Germany i AGQM – The Association Quality Management Biodiesel e. V., Berlin, Germany j DEUTZ AG, Cologne, Germany k Technische Hochschule Ostwestfalen-Lippe, University of Applied Sciences and Arts, Lemgo, Germany b

A R T I C LE I N FO

A B S T R A C T

Keywords: Biofuel Technical challenges Polarity effects Future requirements Engine oil

The combustion of chemicals in engines is much more than a transformation of chemical energy into kinetic energy. Fuel, engine and exhaust gas treatment form a unit with mutual dependencies and optimisation potentials. Above all, the creation of diesel fuels with different biodiesel proportions or biocomponents (Hydrotreated Vegetable Oil, HVO) and raw material provenances is becoming one of the greatest challenges for the petroleum and vehicle industry from a global point of view. But also offers the potential of a timely, based on existing infrastructure, positive climate change. New regenerative fuels entail properties such as polarity, soot mitigation potential and strong solution properties that require optimal formulations, suitable combustion parameters, and chemical resistance of fuel-bearing components. The paper summarizes the most important findings on biofuels and describes the need for action and research, as well as future challenges for biofuels as a pure fuel and blend component from the point of view of business and science.

1. Introduction

breakthrough of biodiesel around the mid-1990s, which has been endorsed from the beginning by the Union for the Promotion of Oilseeds and Protein Plants (Union zur Förderung von Oel- und Proteinpflanzen e.V. - UFOP). Since 2005, UFOP has drawn a portion of its expertise from the technical commission “Biokraftstoffe und Nachwachsende Rohstoffe (Biofuels and Renewable Resources)“, in which 22 representatives from the motor, mineral oil, biofuel, additive and automotive industries, alongside scientists from universities and research institutions and association representatives, debate the potentials and challenges for biofuels in the future, as well as discuss strategies and initiate project proposals. When possible, junior scientists are funded specifically. Between 2016 and 2018, the future challenges for biofuels as a pure

The combustion of chemicals in engines is much more than a transformation of chemical energy into kinetic energy. Fuel, engine and exhaust gas treatment form a unit with mutual dependencies and optimisation potentials [1–4]. While Rudolf Diesel was already using biofuels and vegetable oils in his engines, the breakthrough of biofuels in Germany and in the European Union only occurred at the turn of the last century. Modern high-tech units today require bespoke fuels, which must also harmonise with exhaust gas treatment and fulfil sustainability criteria. In Germany, the story of alternative fuels began with the ⁎

Corresponding author. Tel.: +49 9561 317 481. E-mail address: [email protected] (M. Unglert).

https://doi.org/10.1016/j.fuel.2020.117227 Received 29 August 2019; Received in revised form 15 January 2020; Accepted 27 January 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.

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List of abbreviations

LCV LNG LPG MTBE MVEG NEDC NIR NOx OECD OME PHEV PM PME PtG PtL RED RME SME TBN TME UBA UCO UCOME UDC UDDS UFOP WHTC WWFC

BtL Biomass-to-Liquid CEN European Committee for Standardization CENELEC European Committee for Electrotechnical Standardization CNG Compressed Natural Gas CO Carbon monoxide DBFZ German Biomass Research Centre DDFT Diesel Deposit Formation Test DF Diesel fuel DPF Particulate filters DME Dimethyl ether ESC European Stationary Cycle ETBE Ethyl tert-butyl ether ETC European Transient Cycle EUDC Extra-Urban driving cycle EV Electric Vehicle FAME Fatty acid methyl ester FT fuels Fischer-Tropsch fuels FTP-75 Federal Test Procedure GHG Greenhouse Gases GtL Gas-to-Liquid HC Hydrocarbons HFFA Hydrogenated free fatty acids HHDDT Heavy Heavy-Duty Diesel Truck HVO Hydrotreated Vegetable Oil HWY Highway Fuel Economy Test Cycle IDID Internal Diesel Injector Deposits ITF International Transport Forum

Light commercial vehicle Liquefied natural gas Liquefied Petroleum Gas Methyl tert-butyl ether Motor Vehicle Emissions Group European driving cycle Near infrared spectroscopy Nitric oxide emissions Organisation for Economic Co-operation and Development Polyoxymethylene ethers Plug-in Hybrid Particulate mass Palm oil methyl ester Power-to-Gas Power-to-Liquid European Renewable Energy Directive Rapeseed oil methyl ester Soya oil methyl ester Total Base Number Tallow methyl ester Federal Environment Agency (Umweltbundesamt) Used cooking oil Used cooking oils methyl ester Urban driving cycle Urban Dynamometer Driving Schedule Union for the Promotion of Oilseeds and Protein Plants World Harmonized Transient Cycle Worldwide Fuel Charter

Change Conference (COP22) in Marrakesh in November 2016 [9]. In their coalition agreement, the Federal Government announced that it would finalise the binding sectoral greenhouse gas targets set out in the climate action plan with the climate protection law. In light of these challenges, the natural question is how to design the transformation process, i.e. the changeover to efficient and affordable greenhouse gasneutral alternative fuels and powertrains. Politicians and the industries involved are both under intense pressure to act and innovate. Transition processes, which also affect the usual demand behaviour of consumers as a benchmark for success, are time-consuming and must therefore be organised in a way that is free of ideology and open to technology with regard to the various challenges in the different action areas. The set deadline of 2050 makes it clear that in order to reach this target, further development of existing technology as well as the testing and use of innovative technology is required for the decarbonisation of global transport. Globally, the combustion engine is and will remain the most important drive in this process. The global process for the defossilisation of fuels, and thus of transportation, begins with biofuels. This process is driven regionally by the potential of resource availability and by legally prescribed national blending quotas. Here, the general principle must be that only greenhouse gas-optimised and certified sustainable biomass raw materials or biofuels are used. In principle, the European Renewable Energy Directive (2009/28/EC) and the strengthened subsequent directive (RED II: 2018/2001/EC) stipulate these requirements as prerequisites for access to the market in the EU. Therefore, in the case of third-country imports (biomass raw materials or biofuels), also from outside the EU, these statutory requirements must not only be implemented but also ambitiously further developed and monitored in unison. Above all, the creation of diesel fuels with different biodiesel proportions or biocomponents (Hydrotreated Vegetable Oil, HVO) and raw material provenances is becoming one of the greatest challenges for the petroleum and vehicle industry from a global point of view. Due to ever-increasing and dated legal emissions requirements, engine

fuel and a blending component from an economic and scientific perspective were discussed. This paper is a result of these discussions and summarises, from the perspective of the UFOP commission, the research and action required for ensuring research funding for this important economic sector in the coming years, including the acquisition of young scientists. 2. Motivation and background By signing the Paris climate agreement, the states involved agreed to the internationally binding target of limiting the rise in global warming by 2050 to a maximum of two degrees Celsius. The aim should be to limit this increase to a maximum of 1.5° if possible [5]. These objectives mean that the total global “budget” of all greenhouse gases permitted to be emitted into the atmosphere before the deadline is around 750 gigatonnes (for the two-degree target). In 2018, the global total of greenhouse gases emitted annually is around 60 gigatonnes [6]. So time is running out. The fundamental goal is to limit the consequences of climate change that are already visible. It is therefore the burden and the particular responsibility of developed countries to speed up this transformation process with ambitious measures and innovative technologies. Considering the global population will likely increase to more than nine billion people by 2050, a key challenge is increased global traffic, which is growing particularly in emerging markets [7]. Climate scientists are sounding the alarm and calling for an urgent transport revolution. At present, transport accounts for more than 14 percent of greenhouse gas emissions globally. In Germany, this figure is higher at 19 percent, some 170.6 million tonnes [8]. According to the International Transport Forum (ITF), CO2 emissions related to mobility amount to three tonnes per inhabitant in OECD countries (Organisation for Economic Co-operation and Development). In light of this, the signatory countries of the climate agreement pledged to submit binding national climate action plans by 2019/2020 at the latest. Germany presented its 2050 national climate protection plan at the Climate 2

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safety and social aspects. Reliable engine technologies and climate-efficient fuels contribute to this significantly. Premature vehicle bans may be politically opportune in certain circles. For many directly and indirectly affected parties (commuters, companies, customers), the consequences cannot be foreseen. In any case, subjective criticism of the future viability of combustion engines means that young people do not want to study this comprehensive subject. This means that the industry, in the medium term, is lacking junior engineers, who are desperately needed to safeguard the future of Germany as an industry hub. The authors of this paper are motivated by the question of the feasibility of the combustion engine in the future with the aid of sustainable fuels. In the following, the focus will be on biofuels and identifying the future challenges that exist for them, so that they can contribute to sustainable mobility.

development and increasingly more complex systems for exhaust gas treatment are the main trendsetters here [10,11]. At the same time, reduced-consumption, climate-friendly engines are being demanded, not only by statutory requirements, but also by customers. With its 2050 national climate protection plan, Germany has concretely specified the greenhouse gas reduction target for 2030 in the transport sector. The sector’s targets have been legally and bindingly enshrined as a result of coalition negotiations. With the CO2 limit values for new vehicles prescribed at EU level from 2021 (passenger vehicles: 95 g CO2/km and light commercial vehicles: 147 g CO2/km), in combination with further (already dated) decreases, the introduction of electrification via hybridisation and purely electric powertrains is a given [12]. At EU level, a 15% reduction in CO2 by 2025 and a 30% reduction by 2030 were agreed for heavy commercial vehicles. The reduction percentage refers to CO2 emissions in 2019. Failure to reach these targets will otherwise lead to financial penalties to be paid to the EU Commission, amounting to billions which will thus no longer be available on a national level for necessary investments. Not least this threat of penalties drives innovation developments in an economic sector which is of such importance for the German national economy. In order to reach the maximum achievable CO2 savings potential by 2030, a responsible and proactive environmental policy must be able to differentiate between the technological challenges and what is feasible within the time frame. The diesel scandal has wrongfully discredited combustion engines as technology carriers. A blend of publicly driven discussion about manipulated exhaust gas treatment systems, pollution in inner cities, driving bans etc. has led to the key question about the outlook for combustion engines and even to discussions about banning them. The fact that legal emissions requirements must be fulfilled remains undisputed. Therefore, the future of combustion engines will be determined by which processes are needed to make the engines more efficient, even lower in emissions and, depending on the development of alternative fuels, as greenhouse-gas neutral as possible. This is where the potential of mutual optimisation of engine, fuel and exhaust gas treatment assumes an important role. The broad variety of competences needed for this can only be successful as part of a wide interdisciplinary approach. Inner city emission loads must be evaluated against the background of their health effects, but also especially with respect to the different measuring scenarios in Europe [13,14]. Ideological or technophobic approaches should be rejected. The protection of health and of the climate must be evaluated in view of resource efficiency, workplace

2.1. Availability of fossil resources The Federal Institute for Geosciences and Natural Resources (Bundesanstalt für Geowissenschaften und Rohstoffe - BGR) has formulated the following core statements, among others, regarding the fossil energy raw materials crude oil and natural gas, which are important for fuels: [15]

• At approximately 31%, crude oil remains the most significant energy source worldwide for the foreseeable future (Fig. 1) • Supply is guaranteed even with a moderate increase in demand, • • • • • •

reserves and resources are increasing due to non-conventional crude oil (shale oil and heavy oil reserves). The conventional reserves important for mineral oil supply remain almost constant. Newly-found conventional crude oil deposits are declining significantly (Fig. 2). A functioning world crude oil market is essential in ensuring supply. Natural gas is the third most important energy source. Natural gas will be available in sufficient quantities for many decades, even with the increased demand expected in the medium term. Europe is one of the largest importers of crude oil and natural gas.

An insufficient supply of conventional fossil fuels is not predicted from a geological perspective and is therefore not a decisive reason to develop alternative fuels [16,17]. To this end, the country-specific, European and international CO2 targets, the required reduction in

Fig. 1. International primary energy consumption from 1980 to 2017 and proportion of all energy and energy sources consumed in 1980 and 2017 [18,19]. 3

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Fig. 2. New finds, oil extraction and reserves [15].

powered by combustion engines in 2030 will be approx. 80%; in total, combustion engines will still be part of approx. 90% of powertrains, Fig. 3. Growth will take place predominantly in developing and emerging countries [20,21]. Electrically powered vehicles, including hybrids, will make up a proportion of around 15% of the total number of vehicles globally in 2040. Freight transportation is increasing dramatically worldwide. In Germany, an increase of 30–35% is expected between 2000 and 2040, mainly on the roads, see Fig. 4. Therefore, the energy required for all transportation services increases overall. This is especially dramatic for commercial freight transportation via roads but also in the sea, rail and air transport sectors. From this data we can see that with electromobility, even if the

pollution and a lower dependency on imports are decisive. Renewable energy reaches a consumption percentage of almost 18%, with the classic renewable energy sources such as biogenic energy sources and hydropower dominating in the sector at over 90%. Wind and solar energy meet just under 10% of demand. Biofuels for the transport sector contribute to global energy consumption only slightly at just 0.8%. Production has, however, quadrupled since 2004 from 30 billion litres to 135 billion litres and is continuing to increase [15]. 2.2. Expected development of mobility The number of vehicles worldwide (passenger vehicles and light commercial vehicles [ < 6 t]) will double in the next 20 to 25 years from around 1 billion to around 2 billion. In terms of global production, the proportion of vehicles purely or predominantly (48 V hybrid)

Fig. 3. Number of vehicles worldwide and vehicle production (passenger vehicles and LCV) [20,21]. 4

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Fig. 4. left. According to this prognosis, 85% of the fuel used for this is still crude oil, see Fig. 5, right. This Is, however, incompatible with the set CO2 targets [22,20].

environment. There is no “either/or”, but rather only an optimal powertrain that is adapted to needs and regional circumstances. While an infrastructure for battery charging is, to some extent, economically and logistically conceivable in cities and heavily-populated regions such as Europe, this is still far from possible in emerging and developing countries [24,25]. Here, a readily-available, easy to use, economical and low emission fuel must ensure mobility for people and goods, also in the long term. Initially, these will be not only crude oilbased fuels - petrol, diesel and natural gas (CNG, LNG) - but also biogenic liquid and gaseous fuels of the so-called first and second generation. In the medium term, it is likely that new synthetic fuels and socalled e-fuels will also come onto the market.

electrical energy required is obtained from renewable sources, CO2neutral mobility is not achievable by 2050. In principle, each drive concept must be evaluated in terms of its climate efficiency from production through to operation and, finally, recycling as part of a comprehensive Life Cycle Assessment. The current exclusive CO2 assessment of the operating phase is not sufficient for a comparison of different concepts.

2.3. Powertrains and fuels of the future The move to electric powertrains is – depending on the need for mobility and the local air quality - a necessity for a pollution-free

Fig. 5. Growth of freight transportation internationally and in Germany [22,23]. 5

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synthetically produced fuels. This category predominantly includes fuels which are produced using a Fischer-Tropsch process [28]. In a first step, synthesis gas, which then reacts further to paraffinic hydrocarbons with a defined chain length, is produced from a carbonaceous raw material. Alongside coal and natural gas, among others, biomass, agricultural waste and wood can be used as raw materials. The resulting fuels are predominantly labelled as Biomass-to-Liquid (BtL) and (Bio)Gas-to-Liquid (GtL) or marked with the generic name XtL fuels or Fischer-Tropsch fuels. Fischer-Tropsch fuels (FT fuels) are currently mainly implemented as so-called drop-in fuels for diesel fuels or kerosene, although use as petrol is also possible in principle [29]. As well as these FT fuels, so-called e-fuels also belong to the group of synthetically produced fuels. Carbon dioxide and hydrogen obtained from regenerative electricity are used in the production of e-fuels, in order to synthesise gaseous or liquid combustibles. The process is correspondingly called Power-to-Gas (PtG) or Power-to-Liquid (PtL), or PtX in general [29,30]. Oxygenous fuels such as dimethyl ether (DME) and polyoxymethylene ethers (OME) are also synthesised in this way. In these fuels, the carbon chain is interrupted by oxygen atoms. The third group encompasses biofuels obtained from non-agricultural, biological resources, which are also marked as third generation fuels. Biokerosene, biogas and biodiesel from algae in particular, which, depending on the definition, also count as second generation fuels, must be named here. Despite the classification detailed above, biofuels of all generations exhibit significant commonalities. All biofuels must meet strict sustainability criteria in order to be recognised and used as biofuels [31,32]. This has had the effect, among others, that not only biofuels themselves must be certified, but also the raw materials used for the biofuels. The potential of greenhouse gas savings has also more than doubled in recent years to over 80%, measured by the statutory minimum obligation (35%, Renewable Energy Directive 2009/28/EC) (See BLE Evaluations- und Erfahrungsbericht 2017 [33]). Special attention is also paid to the fact that modern biofuels can be used in existing applications or with only slight adjustments. This shortens the development time and the financial expense for the use of biofuels with high greenhouse gas savings in existing systems. In order to guarantee safe use, all biofuels must meet the high quality requirements of the fuel industry and of users. In order to be able to guarantee the quality and operational capability of biofuels and fuel mixtures, requirements and properties are determined by standardisation. The parameters and limit values for this are based on scientific results as well as field experience and observations. The standards are usually developed at a European level as part of a multi-layered process with consensual integration of all affected parties and are subsequently translated into national standards or legislation by the member states. The most significant standards for the fuel sector are currently EN 590 for the description of test methods and requirements for diesel fuels and EN 228 for the description of test methods and requirements of petrol. Since the initial introduction of EN 590 in Germany in 1993, the standard has been subject to multiple changes, such as reduction in sulphur content or the introduction of biofuel components. Currently, EN 590 permits an addition of maximum 7% (V/V) FAME, as well as, under compliance with the requirements of the standard, the addition of further (non petroleum-based) paraffinic hydrocarbons from GtL or BtL processes and HVO in any quantity. A limiting factor here is the lower density limit value, so that approx. 26% (V/V) paraffinic diesel fuel (HVO) and 7% (V/V) biodiesel can be mixed with fossil diesel within the framework of EN 590. A fuel with this composition was developed and tested under the name “Diesel R33” and its use would already be possible nationwide today [34]. The biofuels used for admixture are again subject to their own requirements standards. EN 14214 specifies the properties and test methods for FAME (biodiesel) as a pure fuel and a blending component.

Due to the long-term availability of a large number of these fuels, infrastructure conditions and, above all, economic aspects, the combustion engine will ensure mobility worldwide for decades to come. Pollutant emissions, specifically nitrogen oxides, can today already be reduced to extremely low levels in petrol and diesel engines [26]. The problem of particle emissions has been solved with the widespread introduction of particulate filters in diesel engines around 2003/2004 and the upcoming implementation of these in petrol engines with direct injection. Due to the strong growth in mobility, in the next five to ten years, more combustion engines will be produced per year globally than ever before. Assuming that the electrification of powertrains mainly takes place via the hybrid drive, passenger vehicles with petrol engines will first be “electrified”. Diesel engines will continue to dominate the freight transportation market segment (truck, sea, rail) and the off-road sector (construction and agriculture machinery and industrial engines) but will be withdrawn from the small and medium-sized passenger vehicle sector for reasons of cost. In principle however, the powertrain concepts will be orientated more than ever towards mobility needs. Wherever heavy goods are to be transported for long distances and the operating costs exceed the investment costs, the diesel engine will still be the first choice. It is the best compromise in terms of cost, CO2, durability, flexibility, availability and pollution. 3. Biofuels Regenerative fuels are commonly referred to as being of different “generations” (fuels of the first and second generations) and certain fuels are considered “advanced”. When assigning fuels to generations it is often assumed that subsequent generations are superior to the previous generation and are therefore “more advanced”. As no universal definition for the terminology exists, different evaluation criteria, which ultimately produce partly varying classification results, are often used when classifying fuels. Examples of the evaluation criteria used include:

• The sustainability and ethical safety of raw materials used during •

fuel extraction (food production competition, spatial efficiency, cultivated biomass versus residual material or excess electricity, direct and indirect land use changes, greenhouse gas emissions etc.) Specific fuel properties (e.g. tailor-made synthetic fuels for innovative combustion processes)

The umbrella of conventional biofuels, i.e. first generation biofuels, encompasses cultivated biomass-based fuels such as vegetable oils, biodiesel from vegetable oils, hydrogenated vegetable oils and bioethanol, which are in competition with food production. Biodiesel or fatty acid methyl ester (FAME) are produced from the corresponding vegetable oil by transesterification with methanol. Rapeseed oil methyl ester (RME), soya oil methyl ester (SME) and palm oil methyl ester (PME), for example, are formed in this process. Glycerine occurs as a by-product of this process. Hydrogenated vegetable oils (HVO) are produced by hydrogenating vegetable oils with hydrogen, where propane (bioLPG) is also produced. Bioethanol is obtained from raw materials containing starch or sugar such as wheat, corn, sugar beet or sugar cane via a fermentation process. Advanced biofuels are split into three sub-groups. Biofuels from existing and residual materials, such as biodiesel from used cooking oils (UCOME) or animal fats (TME) are already used to a great extent in Germany and Europe [27]. These also include bioethanol from lignocellulose as well as hydrogenated used cooking oils and hydrogenated free fatty acids (HFFA). Biomethane extracted during anaerobic decomposition of manure, straw and residual materials also falls into this category. The second group of advanced biofuels encompasses all 6

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Due to varying requirements for higher FAME-diesel fuel blends, the specifications for B10 (EN 16734 - Automotive diesel fuel with a proportion of maximum 10% (V/V) FAME) and B20/B30 (EN 16709 Automotive diesel fuel with a proportion of maximum 20% (V/V) or 30% (V/V) FAME) are recorded in their own standards. With the revision of ISO 8217:2017, FAME may also be implemented in the ship fuel sector, in the so-called DF grades up to a maximum of 7% (V/V). The requirements for all paraffinic fuels produced by synthesis or hydrogenation processes (BtL, GtL, HEFA) and used as diesel fuels are defined in EN 15940. Paraffinic fuels are, however, most interesting because of their use as kerosene, since other biofuels, such as biodiesel and bioethanol, cannot be used in this way. Paraffinic fuels (HEFA) and FT fuels are already permitted to be used as aircraft fuel according to ASTM D1655-19 and Def Stan 91-91 (2019) [35]. Biogenic components are also permitted in the heating oil sector. DIN SPEC 51603-6 describes the corresponding requirements for these fuels. As soon as new fuels and biofuels, such as oxymethylene ether or pyrolysis oils, are available, these are checked in terms of their technical and safe applicability. In the second step, the limits and parameters necessary for the technical safety of the application must be specified. Efforts are currently being made to describe OME and pyrolysis oils by way of a requirements standard. EN 228 specifies similar conditions in the petroleum sector. However, the resulting petrol is classified by octane number here. Several paraffinic fuels (XtL and PtX), as well as different oxygenates (ethanol, ethyl tert-butyl ether (ETBE), methyl tert-butyl ether (MTBE), methanol etc.) are suitable as biogenic fuels. The admixture of bioethanol is of major importance for petroleum fuels. Blends of petrol with a maximum of 5% (V/V) (E5) and a maximum of 10% (V/V) (E10) bioethanol are covered by EN 228. EN 51625 (EN 15293 in future) describes the standard requirements of an E85 fuel with a particularly high ethanol content (max. 85% (V/V) ethanol). The ethanol used is in turn described in EN 15376. In the agricultural sector, steps are being made towards self-generated energy supply for vehicles using vegetable oil fuels, which the agriculture can produce itself and which are especially suitable for use in off-road areas, in terms of soil and water protection. Vegetable oil can be used in engines which are suitable for vegetable oils. In Germany, DIN 51623 defines the requirements for vegetable oil fuel and the requirements for rapeseed oil fuel for use in suitable engines are described in DIN 51605. Hydrogen is not only used as fuel, but is also converted into electrical energy in vehicles using fuel cell technology. Therefore, for standardisation, there is a community body of CEN (European Committee for Standardization) and CENELEC (European Committee for Electrotechnical Standardization) which deals with characteristics and requirements.

Table 2 Ethanol and biofuel mandates (volumetric percentage; worldwide - as of 2018) [60,61]. Country

Ethanol

Biodiesel

Country

Ethanol

Biodiesel

Angola Ethiopia Kenya * Malawi Mozambique Nigeria ** South Africa Sudan Zimbabwe Australia * China * Fiji ** India Indonesia Malaysia Philippines South Korea Republic of China

10.0 5.0 10.0 10.0 10.0 10.0 2.0 5.0 5.0–15.0 3.0/7.0/10.0 10.0 10.0 5.0 3.0 – 10.0 – –

– – – – – – 5.0 – – –/2.0 – 5.0 – 20.0 7.0 2.0 2.5 1.0

Thailand EU Norway Canada * Costa Rica Jamaica Mexico Panama USA * Argentina Brazil Chile ** Colombia * Ecuador Paraguay Peru Uruguay

– 5.0–10.0 – 5.0–8.5 7.0 10.0 5.8 10.0 See Fig. 8 12.0 27.0 5.0 6/8 – 25.0 7.8 9.0–10.0

7.0 7.0 3.5/5.0/7.0 2.0–4.0 20.0 – – – See Fig. 8 10.0 9.0 5.0 8–9 5.0 1.0 2.0 6.0

* Dependent on state/province/region. ** (voluntary) targets.

The standards for various fuels and biofuels generate a framework which guarantees the quality and safety of these in accordance with the current state of the art (Table 1). The standards also simultaneously describe the existing technical possibilities for the use of biofuels. An implementation of this operational capability is governed by corresponding political and economic framework conditions Table 2. Further biogenic fuel pathways are possible in principle, although some of these are grouped under the cited standards, such as DIN EN 15940 (for BtL) or EN 590 (Diesel R33).

3.1. Overview of selected biofuels 3.1.1. Biodiesel - fatty acid methyl ester (FAME) As it is relatively easy to obtain and is highly compatible with traditional diesel fuels (DF), biodiesel is the biofuel that has been in use for the longest time. Biodiesels are simple conversion products of natural fats and oils (triglycerides = esters from fatty acids with glycerine as trivalent alcohol), which are preferably derived from oil plants from temperate climates, whose main component is therefore oleic acid [37]. The glycerine is replaced in what is called a transesterification reaction by the monohydric alcohol methanol, so that a fat molecule yields three molecules of fatty acid methyl ester (FAME); glycerine is released in this

Table 1 Standardisation status [36]. Standard Number

Title

Edition

DIN DIN DIN DIN DIN DIN DIN DIN DIN DIN DIN DIN DIN DIN DIN

Fuels for vegetable oil compatible combustion engines - Fuel from rapeseed oil - Requirements and test methods Fuels for vegetable oil compatible combustion engines - Fuel from vegetable oil - Requirements and test methods Automotive fuels - Ethanol Fuel - Requirements and test methods Liquid fuels – Fuel oils - Part 6: Heating oil EL A, Minimum requirements Automotive fuels - Unleaded petrol - Requirements and test methods (up to E10) Automotive fuels - LPG - Requirements and test methods Automotive fuels – Diesel - Requirements and test methods (up to B7) Automotive fuels - Ethanol as a blending component for petrol - Requirements and test methods Liquid petroleum products - Fatty acid methyl esters (FAME) for use in diesel engines and heating applications - Requirements and test methods Automotive fuels - Paraffinic diesel fuel from synthesis or hydrotreatment – Requirements and test methods (HVO, GtL, CtL, BtL, PtL) Automotive fuels - High FAME diesel fuel (B20 and B30) - Requirements and test methods Automotive fuels - Automotive B10 diesel fuel - Requirements and test methods Automotive fuels - Automotive ethanol (E85) fuel - Requirements and test methods Natural gas and biomethane for use in transport and biomethane for injection in the natural gas network - Part 2: Automotive fuels specification Petroleum products - Fuels (class F) -Specifications of marine fuels

2016–01 2015–12 2008–08 2017–03 2017–08 2019–03 2017–10 2014–12 2019–05 2018–08 2019–02 2019–02 2018–10 2017–10 2018–10

51605 51623 51625 SPEC 51603-6 EN 228 EN 589 EN 590 EN 15376 EN 14214 EN 15940 EN 16709 EN 16734 EN 15293 EN 16723-2 ISO 8217

7

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The influence of OME, particularly blends of OME and conventional diesel fuels, on lubrication has not yet been investigated in detail. The key physical data of OME suggests that a similar risk as known for biodiesel, i.e. the permanent dilution of the engine oil through OME accumulation, should be considered.

reaction. Biodiesel is similar to diesel fuel in terms of its resulting properties and is suited for use as a blending component with diesel fuel due to its ability to be mixed as required [38,39]. Standardised in EN DIN 14214, a consistently good quality is guaranteed for this alternative fuel. In Central Europe, rapeseed naturally provides the basis for transesterification (RME). However, sunflower oil, soybean oil, palm oil (imports), used cooking oil (then designated as UCOME = Used Cooking Oil Methyl Ester) and animal fats are also used - partly as mixtures - in order to comply with the limits of the standard. According to DIN EN 14214, biodiesel is also used as a “B100” pure fuel and as a “Bnn” blending component in nn% admixture to diesel fuel according to DIN EN 590. In this case, FAME, as a blending component, must also comply with DIN EN 14214. In most cases, fossil methanol is used for the transesterification of fats when producing biodiesel. If the proportion of methanol, which is derived from fossil sources, is also considered, a renewable proportion of approx. 95% remains [46]. In the literature, [40–45] there are a number of studies which account for all the processing steps in the production of biodiesel in terms of CO2. The CO2 savings potential thus amounts to between 50% and 80%. When producing used cooking oil methyl ester, even higher reduction potentials can be achieved [46].

3.1.3. Hydrogenated vegetable oil (HVO) Fully saturated, broadly paraffinic hydrocarbons can be produced from vegetable oils using full hydrogenation and implementation of special catalysts (hydrotreating with metal catalysts at temperatures around 400 °C and hydrogen pressure up to 150 bar) [54,55]. Their properties are similar to those of petrochemical hydrocarbons. Their vaporisation behaviour is particularly comparable. Thus, there are hardly any or only insignificant influences on the engine oil, its functions and its life cycle. The most famous technology was developed by the Finnish company Neste Oil (NExBTL). Palm oil or residual materials are preferable as feedstock here [56,57].

3.1.4. Pure vegetable oils Due to their high biodegradability and low ecotoxicity, as fuels, pure vegetable oils can be viewed as a niche market in environmentallysensitive sectors, like agriculture and forestry. Due to the excellent dedication of TFZ Bavaria [58], ASG, Deutz AG, and John Deere etc., in 2006, a major step was taken to further expand on this alternative by creating a pre-standard “Fuels for vegetable oil compatible combustion engines - Fuel from rapeseed oil - Requirements and test methods” (DIN 51605).

3.1.2. Oxymethylene dimethyl ether (OME) (Poly) Oxymethylene dimethyl ethers are oxygenous oligomers with the chemical structure H3C-O-(CH2O)n-CH3. Components suitable for diesel fuels are the result with repetition factors n between 3 and 5 [47–49]. OME can, depending on its proportion in the diesel fuel, contribute to the reduction of soot emissions from diesel engines [50–51]. OME can be produced using the key substance methanol both petrochemically from synthesis gas, as well as from renewable raw materials via a methanol synthesis [49]. For example, methanol can be produced from methane by an enzyme catalytic process with methanotrophic bacteria. Furthermore, methanol can be produced using carbon dioxide and water by supplying an electric current as a reversal of the reaction in fuel cells [52]. This production method is described as an example of “Power-to-Liquid”, with which surplus electricity generated by regenerative energy sources (solar, wind) can be used or stored. The energy balance of this process is, however, still not attractive. Perspectively, some studies view the costs for the production of OME as potentially comparable with those involved in the production of diesel fuel [53].

3.1.5. Alcohols Alcohols can not only be mixed with gasoline as a biofuel. A mixture without miscibility gap is possible from butanol. For alcohols applies, that with increasing length of the carbon chain, the physicochemical properties are increasingly like diesel fuel. As a result, with increasing chain length, the potential as a blend component for diesel admixture increases due to the higher energy density, higher cetane number, better mixing stability and less hygroscopic nature. The admixture of suitable alcohols such as e.g. Butanol, pentanol or octanol provides the potential to adjust the fuel properties to requirements. Higher alcohols rank among the second generation of biofuels made from ligno-cellulosic biomass without reliance on food crops [59].

Fig. 6. Biofuel mandates (worldwide - as of 2018) [60,61]. 8

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waste animal fats (16%). For ethanol, the beginnings of production using residual materials have become apparent since 2015. The state also provides tax breaks, which do make palm and castor oil worse options, but still better than fossil fuel alternatives [70]. For China, ranking in ninth place as biodiesel manufacturers and fourth place as ethanol manufacturers, there are major differences between the different provinces. In principle, however, there is a national mandate for E10 until 2020. In contrast, biodiesel has only played a minor role thus far. China is also setting itself major goals with its “13th Five-Year Plan (FYP) (2016)”. The production of ethanol and biodiesel is to be significantly increased again by 2020. There are also efforts to raise waste material-based ethanol production to a commercial level by 2025. The current production share fluctuates around 10%. Only used cooking oil (UCO) is used for FAME production [71]. The USA is the largest ethanol producer and the second largest biodiesel producer. As regulations are greatly dependent on the individual states, no coherent national data is available. E85 and/or E85 and E15 are common in several states. In biodiesel fuel mixtures, almost the entire spectrum of possible admixtures can be found (Figs. 7 and 8).

3.2. Inventory In 2018, both the production and the consumption of biofuels (ethanol and biodiesel) are not only supported by many countries, but are also legally required by mandates for admixture quotas and partly by tax incentives. An increase in mandates can be seen in biomassproducing countries in particular. Especially active here are North and South America, large parts of Asia and also Australia and Europe, though the strategic focus can vary significantly. Various countries in Africa have also already implemented corresponding measures. In contrast with this trend, Russia, a country with a great deal of agricultural land and one of the largest producers of fossil fuels and combustibles, is currently showing only a marginal interest in this and can therefore be considered negligible in this regard (Fig. 6; Table 3) [60,61]. At around 6%, in Germany and the European Union biofuels have been the only comprehensively notable alternative to fossil fuels to date. Access to the market and the maximum admixture level, as well as the renewable energy mix (funding of e-mobility), are regulated by law very differently among the EU member states and account for the greatest differences between the 28 EU member countries. This diversification is the consequence of national authorisations to implement this directive as part of the compromise negotiations of the EU’s Energy Council, i.e. between the member states and the standpoint of the European Parliament (co-decision procedure). This means that the proportion of renewable energy in transport in Sweden is 38.6 percent, whereas in countries such as Greece and Spain this proportion is only 4% and 5.9% (2018) [62,63]. The use of biodiesel and bioethanol is “capped” by the respective fuel standards for B7, E5, and E10, while a B30 standard has been specially designed for the use of these fuels in closed commercial vehicle fleets. Additionally, releases for B100 for Euro VI vehicles are individually granted if the fleet operator makes this certified release a condition of purchase. This means that even the most modern vehicle engines could be operated using B100, under certain possible adjustments to engine management, seals and decreased engine oil replacement intervals. In addition, a higher clogging of the injectors and an increase in NOx emissions of 10–30% relative to diesel fuel must be taken into account [64–67]. Further extensive data about the German and European market can be found in the report from the Deutsches Biomasseforschungszentrum (German Biomass Research Centre - DBFZ), “Monitoring Biokraftstoffe” (Biofuel Monitoring), 4th edition [68]. In conjunction with an increasingly global trend, where the raw materials mix of field crops used for the production of biofuels is gradually changed to residual and waste materials, the recast of the renewable energy directive (2018/2001/EC) prescribes mandatory minimum quotas in the EU for biofuels made from residual materials e.g. straw. This means that their share of total energy consumption in transport must reach a minimum of 3.5% by 2030. However, this quota can be reached through double allocations for energy content. As a consequence, the physical demand of the biofuel halves. With this measure, the target of a 14% proportion of renewable energies specified in this directive is achieved only virtually. For climate protection, however, a considerably lower contribution is achieved. In the absence of existing production capacities for biofuels from residual materials, it is clear that multiple allocations should trigger investments. In the case that this obligatory quota, which must be demonstrated by the mineral oil companies concerned, is not complied with, corresponding severe fines are a possibility. As the second largest ethanol producer and third largest biodiesel producer, Brazil has increased in 2019 the minimum biodiesel content to 11% (B11). According to a test phase for B15, realised between 2015 and 2018, Brazil revised biodiesel specifications to include blends up to 15% (B15) [69]. For ethanol, the mandate for E27 will remain unchanged until at least 2026. Biodiesel production has increased continually in recent years and is based primarily on soybean oil (70%) and

3.3. Sustainability and acceptance Biofuels are traded globally and are the subject of an acceptance and availability discussion. In light of this, it should essentially be welcomed that the EU, with the Renewable Energy Directive [2009/28/EC] and the subsequent directive [2018/2001/EC], has introduced and tightened minimum requirements for sustainability, dated acreage origins and greenhouse gas reductions (at least 50%) as a prerequisite for access to the market. The subsequent directive also includes synthetic Fischer-Tropsch fuels (e-fuels). These statutory regulations are the driving force for an increase in the efficiency of process technology that is open to technology and driven by competition, but also for increased sustainability requirements, independent of biomass usage, as part of the bioeconomy and defossilisation strategy for the material use of sustainable raw materials. At the same time, the transparent fulfilment of these criteria is a prerequisite for public and political acceptance. In light of its diversity, sustainably produced biomass is one of or the most important source of energy and revenue in many countries across the world. Today, this resource is “controllable”, thanks to satellite technology. Sustainable production for the use of these resources (particularly cultivated biomass) as alternative fuels is necessary because this biomass potential can be supplied to the food market, depending on the respective supply situation. However, the international agriculture commodity markets have exhibited structural supply surpluses and thus pricing pressure for the agricultural sector for years. An analogous challenging discussion about power generation from wind power and raw material extraction for battery production has also long been under way. 4. (Bio) fuels and electromobility Electromobility takes precedence over mobility with combustion engines in current political discussion. For example, in Germany the CO2 emissions for these vehicles is set at 0 g/km. Thus, it is ostensibly Table 3 Change in viscosity of engine oils during operating (Increase ↑ Decrease ↓) [126]. Change in viscosity of engine oil due to Sheer stress Structural viscosity Soot, foreign bodies Evaporation loss Base oil oxidation Fuel (DF, biodiesel)

9

↓ ↓ ↑ ↑ ↑ ↓

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Fig. 7. USA – States (E15 and E85) [72].

Fig. 8. USA – States (Fuel mixtures with proportion of biodiesel) [72].

fired power stations, as the feed-in law gives priority to electricity from renewable resources and this energy is already fully fed-in and consumed. However, even if the average energy mix were to be considered, CO2 emissions are still in the same ballpark as emissions caused by combustion engines (Umweltbundesamt UBA, 2016) [74]. For the future, it should be assumed that emissions from electricity generation will decrease significantly, especially due to the development of solar energy. With the development of renewable energies, however, other problems will arise. The power grid is currently still designed for electricity generation in large power plants. Renewable energies, in contrast, are not centrally generated and the amount of electricity that they provide

suggested that this form of mobility will be the ideal way forward for the future of mobility. However, two crucial points are neglected here: On one hand, the emissions generated by the production of electricity, and on the other, the provision of energy for consumers (vehicles) [73,74]. The electricity mix in Germany consists of regenerative sources (wind, solar, water) and fossil sources (gas, oil, coal, uranium) in variable proportions. The regenerative proportion can thus amount to up to 80 percent in sunny and windy weather. On the other hand, when there is no wind, the regenerative proportion can fall to under 10 percent at night. If electromobility were to be dramatically increased immediately, the energy required would need to be provided by coal-

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fluctuates considerably. The capacity of the power grid during distribution is an additional issue. This means that quick-charging stations cannot be offered in certain supply areas due to the infrastructure. Therefore, significant research activity into these problems are undertaken. It is neither expected that the grid expansion will progress quickly enough by 2030 nor that storage facilities for the fluctuating electricity supply will be installed in sufficient quantities [75]. A third problem is battery technology. Currently, both the storage densities (volume- and mass-related) are too low and the charging times are too long to be able to accommodate the mobility needs of today’s users. These disadvantages are the biggest obstacles on the consumer side. One possible solution is the generation of power fuels from CO2 and electrical excess energy (PtL, Power-to-Liquid fuels), which is also being intensely researched. However, efficiency losses must be expected here in comparison with battery operation. The advantage is that e-fuels are compatible with existing vehicle fleets and with the infrastructure, and they provide the possibility of the energy generated being stored for a long time [29,30,53]. The research and introduction of e-fuels is primarily a “project” of industrialised countries, which have access to the necessary financial resources to finance relevant development concepts. This is especially true for the construction of large-scale plants, where overcoming “first mover” problems plays an important role.

been summarised by Hoeckman and Robbins [83]. The oil type that is used to produce biodiesel also has an influence on emissions. For example, coconut oil methyl ester with short molecular chains and a very low number of double bonds produces lower emissions than biodiesel varieties whose fatty acid chains are longer and which have a higher number of double bonds [84]. The use of hydrogenated vegetable oil (HVO) displays a similar emission trend to biodiesel. Fig. 10 depicts the emissions of HVO blends, as well as pure HVO, in relation to diesel fuel. The content of HVO in the fuel increases on the x-axis, Fig. 10. Limited emissions were measured both on light passenger vehicles in the new European driving cycle (NEDC) as well as on commercial vehicle engines. For the comparison of emissions with fossil diesel literature values of different test cycles were evaluated. The test cycles include heavy-duty (Heavy Heavy-Duty Diesel Truck (HHDDT), Urban Dynamometer Driving Schedule (UDDS), European Transient Cycle (ETC), European Stationary Cycle (ESC), Braunschweig transient cycle, World Harmonized Transient Cycle (WHTC)) and light-duty (European driving cycle (NEDC), Urban driving cycle (UDC), Extra-Urban driving cycle (EUDC)) testing. The use of HVO reduces emissions of hydrocarbons (HC), carbon monoxide (CO) and particulate mass (PM) noticeably, relative to diesel fuel. This decrease correlates with the HVO content in the fuel. The more HVO added in, the greater the observed effect, Fig. 10. By contrast, the nitric oxide emissions (NOX) exhibit a non-uniform trend. In the NEDC for passenger vehicles, emissions increase by approx. 10% through the use of HVO (Fig. 10, NEDC). In contrast, the NOX emissions from commercial vehicles increase by approx. 10% here (Fig. 10, all except NEDC, UDC and EUDC). Due to the increased use of exhaust gas aftertreatment systems, which react actively to higher NOX concentrations of pollutants, for example, the influence of fuel on the emission behaviour of the engine will decrease somewhat in the future.

5. Technical challenges 5.1. Biofuel emissions Biofuels, in contrast to fossil fuels, have the potential of reducing emissions locally. Fig. 9 depicts an overview of the relative emissions of biodiesel in comparison to fossil diesel fuel [76,77]. Here, only sources in which biodiesel fuels were compared with fossil diesel fuels in a standardised test procedure were evaluated. For the general comparison of biodiesel with fossil diesel, light and heavy test cycles were used. Detailed information of the respective test cycles can be found in the literature (ECE R49, ESC, FTP-75, JC08, UDDS, C1 Cycle, JE05, JMEC, MVEG-A [78]; ESC tests over 1000 h [79]; ECE R83 [80]; CFR T40 P86 SpN [81]). On average, a reduction of emissions from hydrocarbons (−36%), carbon monoxide (−25%) and particulate mass (−31%) can be observed. Only an increase in emissions of nitrous gases is shown (13%). The reason for this is an increase in the combustion temperature due to biodiesel, which increases the amount of nitrous oxide [82]. The reasons for the increase in nitrous oxide discussed in the literature have

5.2. Physical and chemical properties of biodiesel The use of biofuels like biodiesel or vegetable oil fuels leads to specific loads on engine lubrication, potentially with the consequence of shortened oil change intervals. This chapter aims to address the challenges presented to engine lubrication by the use of biodiesel (FAME), vegetable oil fuel, and fully hydrogenated vegetable oil (HVO). Biogenic petroleum fuels, such as ethanol in the form of blends E5 and E85 are not discussed here.

relative to diesel fuel / % 300

1000 h ECE R49 ESC

250

FTP-75 HWY

200

JC08 ECE R83

150

UDDS C1 Cycle

100

CFR T40 P86 SpN 50

JE05 JMEC

0

HC

CO

NOx

PM

MVEG-A

Fig. 9. Limited emissions of biodiesel compared to diesel fuel [76,79–81,85–103]. 11

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Fig. 10. Relative change of limited emissions with different admixtures of HVO into fossil diesel fuel [104–110].

temperature required for the soot to “burn off”. This process does however inevitably lead to a higher ingress of partly burned fuel into the engine oil. 3. Due to the higher boiling temperature (boiling curve) of FAME as opposed to diesel fuel, the proportion of FAME that has leaked into the engine oil remains in the lubricant and leads to a permanent dilution. Fig. 12 shows the boiling curves of different diesel fuels [82,112]. Engine oil can reach 300 °C and more in the range of piston rings and cylinder range [113]. In the case of fossil diesel fuel, 11% of the undiluted portion is still present at 325 °C, whereas biodiesel does not boil above 325 °C. Accordingly, 100% of the registered biodiesel remains in the engine oil. The diagram shows, for example, that 100 ml of FAME and 99 ml of DF remain as the nonevaporated residue from a litre of B10 at 325 °C - i.e. almost a B50 can be found in engine oil. By adapting the engine control, these

5.2.1. Oil dilution with FAME Oil dilution by fuel ingress is, of course, also observed in DF operation. In Fig. 11, the mixture viscosity of an engine oil with fuel is depicted - there is hardly any difference between diesel and FAME. The specific “FAME problem” is in the accumulation of fuel without evaporation, i.e. the permanent dilution of the lubricant. There are three essential causes of this: 1. The creep capability of ester oils is - in contrast to equiviscous hydrocarbons - generally very pronounced. This leads to a greater ingress of unburned FAME into the engine oil. 2. Modern diesel engines with particulate filters (DPF) often have postinjection systems to regenerate the filter. The filter injects a small amount of fuel from time to time during the combustion cycle. This causes the emissions temperature to rise, thus reaching the

Fig. 11. Mixture viscosity of an engine oil 10W-40 with diesel fuel or RME [111]. 12

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Fig. 12. Boiling curves of different diesel fuels.

that thickening of the lubricating oil occurs more frequently in directinjection engines than in pre-chamber or swirl-chamber engines. Engine oil additives do not play an active role in the formation of oil thickening, but do however affect the service life of the engine oil. Field tests have shown that, under the protection of laboratory tests, relatively long oil change intervals can be achieved, even in vegetable oil operation. However, the dangers of a “stuck” engine should not be underestimated. The monitoring devices designed for operation with diesel fuel, such as oil level and oil pressure sensors, usually respond too slowly to warn of lubricating oil thickening in good enough time. Under certain conditions, individual engine oils have advantages over others in terms of ageing. However, changing conditions may also result in disadvantages for the same engine oil. This shows that the risk of a lubricating oil problem in vegetable oil engines can currently only be minimised through a targeted combination of several measures. Among these are constructive measures for the engine, such as:

effects can be reduced, for example, by the application of multiple injections [114,115]. It is known from many studies that at the end of “conventional” oil change intervals, up to 20% FAME can be found in engine oil, but generally this proportion is between 5 and 10%; this also depends on the age of the engine [116,117]. The oil dilution results in a decrease in viscosity with the risk of operating conditions conducive to wear. A mostly neglected influence on the lubricant is, under certain circumstances, the dilution of the additives in engine oil, which can impair their function. An Example is the anti-wear additive dialkyldithiophosphate (ZDDP). ZDDP is chemically designed to be attracted to polar metal surfaces to form a protective layer. But if the engine oil is diluted with polar biodiesel the biodiesel competes with the additive over the metal surface and the effectiveness of the additive is reduced [118].

5.2.2. Oil dilution with vegetable oil fuel The fuel-related ageing mechanisms with vegetable oil fuel are similar to those with biodiesel. However, an oil dilution is hardly observed (viscosity of FAME approx. 4 mm2/s, of rapeseed oil approx. 36 mm2/s at 40 °C). The use of vegetable oil in suitable engines causes typical operational failures [119]. The triglyceride structure causes ageing mechanisms which can lead to a relatively strong, spontaneous thickening of contaminated engine oil (polymerisation). This occurs during operation with a warm engine, but more often during the cool-down phase after shutting down the engine. Thickened engine oil often leads to the failure of engine lubrication; serious engine damage such as piston seizure and bearing failures are the result. In vegetable oil-powered engines, this oil thickening is preceded by an ingress of vegetable oil into the lubricant. Reasons for this are poor mixture preparation in the combustion chamber, sticking of the piston rings, leaking nozzles or defective engine oil-lubricated injection pumps. Cold starts and partial load operation additionally increase the ingressed fuel quantity. Practical cases with vegetable oil fuels show

- Improved conversion systems which minimise the amount of fuel that ingresses into the lubricating oil - Better sealing between the combustion chamber and crankcase (reduction of blow-by gases) - Greater amount of recirculating oil - Integration of a system for continuous oil purification [120,121] favourable operating conditions, such as: - Avoidance of weak load operation and frequent cold starts - Avoidance of very high temperatures (with heavy engine load) careful maintenance, such as: - Compliance with the prescribed oil changes - Regular visual checks of the engine oil, oil level checks and used oil analysis - Observation of automatic warning displays for oil pressure, oil level

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additionally accelerate polymerisation. As a result, sludge deposits can form, i.e. the compounding of oil components or oil ageing products with solid impurities, water and acids, often accompanied by a phase separation in inviscid and thickened components [123,125].

etc. the use of engine oils with highly effective additives to slow down the ageing reactions and neutralise acidic ageing products, and the integration of a reliable lubricating oil monitoring system in vegetable oil-powered engines, such as sensors for continuous monitoring of oil quality (in development). In terms of engine oil, it is imperative that only fuel that conforms to the quality standard DIN 51605 is used. Without this restriction, no legitimate assertions about “oil change times” should be made. The DIN 51605 standard expressly defines the use of rapeseed oil as fuel, as earlier tests showed the decisive influence of the type of vegetable oil. As early as 1937, Gaupp [122] was investigating the effect of the operation of various vegetable oils (palm oil, peanut oil, sesame oil, soybean oil) on the lubricating oil. The results indicate that vegetable oil fuels with a high number of double bonds (e.g. soybean oil) in engine oil cause more severe ageing than vegetable oils with fewer double bonds (e.g. rapeseed oil) [123].

5.2.5. Oil change intervals when using FAME The described effects “Oil dilution with FAME” and “Oil thickening due to oil ageing” unfortunately do not mutually compensate. Table 3 qualitatively evaluates the effects of use on the viscosity of an engine oil. For biodiesel blends, the problems described are only proportionate. Previous studies into B7 fuels have not revealed any serious lubricant changes, even in critical driving conditions. At higher levels, selective biodiesel enrichments in engine oil must be considered. The described influences on the engine oil thus result in restrictions for the oil change intervals, at least under neat FAME operation. In order to avoid a critical decrease in viscosity, the oil change intervals recommended for vehicle use are often simply halved. In individual cases, the conventional oil change interval can be transferred to biodiesel operation. However, a postponing the recommended oil change should only be decided on the basis of a laboratory test of the used oil. As a rule, without an oil test, the engine manufacturer will continue to refer to shortened oil change intervals when operating with B100. In the case of a FAME proportion over 6% in the engine oil, an oil change is already recommended. As an alternative it is repeatedly discussed whether ester-based engine oil is generally more “compatible” with biogenic fuels, according to the old principle: Similia similibus solvuntur - Like dissolves like. In fact, there are indicators of this better compatibility, which can, however, only properly work in a corresponding additive environment. There are, of course, engine oils which are “compatible” and “less compatible” with biodiesel. However, the most important prerequisite for compatibility is compliance with the biodiesel standard [123].

5.2.3. Interactions between FAME fuel and engine components When using FAME, possible incompatibilities, especially with polymeric materials, should be considered. This affects, above all, seals and plastic hoses, as well as plastic housings and coatings, if applicable. Years of experience have meant that such problems hardly occur today due to the use of biodiesel-resistant materials. Also with new fuels, attention must be paid to material compatibility in both new vehicles as well as in the existing fleet. This means that OME, for example, displays a corrosive character compared to many polymers, and can therefore not be introduced as a drop-in fuel in the short term into the fuel market [124]. 5.2.4. Engine oil ageing when using FAME During use, the engine oil is subject to a variety of stresses such as high temperatures, shearing forces, blow-by gas ingress and fuel. In the case of vegetable oil fuel, the latter contributes significantly to oil ageing due to low long-term stability. As a result, viscosity increases and liquid and solid reaction products form. A distinction is made between oxidative ageing, i.e. reactions with atmospheric oxygen, in which organic, oil-soluble, often short-term and therefore relatively strong acids are formed, and thermal ageing, which leads to a substantial increase in viscosity and even resin formation due to the formation of high-molecular structures (polymerisation). In practice, both processes occur together. The oxidation of oils can be avoided or minimised through so-called antioxidants; however, ingressed FAME fuel is especially susceptible to oxidative, thermal and even hydrolytic ageing. The polymerisation begins with oxidation reactions, an initially rather slow process, as only a few radicals and sufficient inhibitors (antioxidants) are present in the oil. This mechanism increasingly attacks the double bonds, meaning that unsaturated fatty acids play an important role. Thus, there is a direct connection between oil ageing and the proportion of FAME here. Solid reaction products from thermo-oxidative reactions or combustion residues are suspended in the oil by means of dispersants to avoid sedimentation at engine standstill. To avoid residue formation of high-molecular, organic, low hydrogen and oil-insoluble compounds, detergents are used. A counter measure to the acidification of the lubricant is the “basic setting” of engine oils through so-called TBN (Total Base Number) carriers. This describes the buffer capacity of an oil. Furthermore, the effects of acidification are counteracted by the addition of corrosion inhibitors. Over the course of oil ageing, on one hand, the presence of reaction centres increases, while on the other hand, the concentration of inhibitors decreases, so that the oil ageing progresses faster and faster. Certain oxidation products also have an auto-catalytic effect and

6. Polarity effects The polarity of fuels and of ageing products which are formed in the thermo-oxidative ageing of fuels have a range of influences on miscibility, solubility and toxicity [127–129].

6.1. B20 effect The oxidation of fuels, both fossil as well as synthetic, causes an increase in polarity. With advancing ageing, polar deposition products form [128]. The amount of ageing products which can be diluted in the fuel matrix depends on their polarity. Fang and McCormick [130] reported that the formation of deposits is non-linear if the biodiesel proportion in fossil diesel fuel is increased gradually. A maximum deposit formation was found at a proportion of approx. 20% biodiesel in fossil fuel (B20). Depending on the composition of the fossil components, the deposit formation maximum shifts. Eskiner et al. reported a maximum in the deposit formation with B15 [131]. With an increasing proportion of biodiesel, the polarity of the fuel matrix increases, so that the formed polar ageing products are suspended in solution. With smaller biodiesel content, the amount of formed polar ageing products is smaller. Corresponding to the amount of deposits, Schröder et al. [132] reported a significant increase in the antigenicity of emissions in diesel fuel-biodiesel blends, which demonstrates a maximum at B20. Analogous to the formation of deposits, the mutagenicity decreases from a B20 composition up to neat RME and with neat RME reaches a lower value than with diesel fuel. Overall, the similarities of the results suggest that there is a correlation between the formation of deposits and the increased mutagenicity of emissions [92]. 14

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Vegetable Oil (HVO) and longer-term electricity-based renewable fuels (Power-to-Liquid) is rising [143]. The fact that the dwell times of fuel mixtures in the vehicle tank are extending as a result of increasing hybridisation of the powertrains is challenging, and polar ageing products increase the polarity of fuels. A comparable increase in polarity is also caused by the synthetic PtX fuel, OME. OME creates challenges for miscibility through its higher polarity compared to fossil fuels. Large differences in polarity between HVO and OME result in a miscibility gap in the case of an admixture of 30% OME to 70% HVO. An admixture of biodiesel plays a central role in this context, as it guarantees lubricity and can also be used advantageously as a solubiliser in fuels. The role of the solubiliser is based on the polarity between HVO and OME and the amphiphilic structure of the FAME. In light of this, systematic research would need to be proactively intensified in order to check the functionality of the various fuel blends, ideally while the vehicle is running, and/or to combine them in the best possible way during production.

6.2. Internal diesel injector deposits (IDID) The introduction of stricter emissions laws requires continuous development of the combustion process within diesel engines and simultaneously, development of the fuels used. Deposits on hot metal surfaces in injectors or other components caused by fuel components are not a new phenomenon [133–135]. Due to the increased sensitivity of modern injectors, the increasing variety of fuels and the use of different blended fuels in diesel engines, the deposition problem must be given more attention in future. Internal Diesel Injector Deposits (IDID) can have several causes. They are caused by polar fuel ageing products, certain additives (dodecenyl succinic acid) and impurities (saponification agents), among others. Higher injection pressure, in particular, as well as the related higher system temperatures present high demands on the thermal-oxidative fuel stability and additive resistance. For varying fuel polarities, sufficient solubility of all fuel components and the additives used must be ensured [136]. Essential influencing factors for the minimisation of deposition formation are an optimal fuel polarity (e.g. adjustable using different FAME components) and a possibly reduced amount of aromatic nitrogen, oxygen and sulphur compounds. To be able to reliably and cost-effectively detect critical fuel compositions in advance, the development of new methods of analysis and, where applicable, the inclusion of additional fuel parameters in the fuel specifications is required. For example, a “pre-screening” of fuels can be made possible by a newly developed “Diesel Deposit Formation Test“ (DDFT) for testing the tendency of deposit formation of diesel fuels depending on the temperature. Additionally, the test offers potential for stability, compatibility and efficacy tests of fuel additives [137]. Furthermore, it should be examined how useful or meaningful a parameter “polar components” would be for diesel fuels and whether this correlates with the formation of deposits in injection components.

6.4. Combustion optimisation using fuel identification The variety of fuels and fuel compositions present new challenges for automobile manufacturers to optimally adjust the tuning parameters of the engines. This will become even more important given future stricter emissions specifications. Fuel composition identification offers the possibility of best optimising engine management to the respective fuel available. Therefore, sensors which can detect various fuel parameters through different measuring processes are subsequently presented. 6.4.1. Dielectric relaxation spectroscopy In plug-in hybrid vehicles, a dwell time of the fuel in the tank that is longer than was previously the case with pure combustion engine vehicles is assumed for purely electrical applications. The ageing process can lead to the formation of polar high-molecular ageing products. The dielectric relaxation spectroscopy can highlight a change in polarity and detect polarisation losses, caused by oligomers, in the imaginary part of the complex permittivity by measuring the relative permittivity. This way, the degree of ageing of a fuel can be determined. Fig. 13 shows the increase in relative permittivity for UCOME as an example of ageing detection. By measuring the permittivity, a warning of aged fuel is only possible after the induction time has elapsed and aging products are already present. The relative permittivity shows a linear correlation between polarity and degree of ageing.

6.3. Polarity of synthetic fuels In order to reach the 2030 climate protection goal for transport, various strategies are being pursued and promoted. In addition to efficiency increases and the use of fully and partly electrical drives (hybridisation), CO2-neutral biofuels, in particular, are currently determining the strategy for defossilising transport in the short term. Due to the increasingly non-polar composition of fuels and longer service lives of fuel mixtures in fuel tanks, an intensification of systematic research is required in this field to avoid negative interaction effects. Measured in terms of global significance, biodiesel in particular, as a polar component in different admixture proportions, poses particular challenges to fuel quality development and guarantee [138–142]. However, the share of non-polar biofuels such as Hydrogenated

6.4.2. Near infrared spectroscopy Near infrared spectroscopy (NIR) covers the wave length range from 750 to 2500 nm and captures the overtones and combinational

Fig. 13. Real part (left) and imaginary part (right) of thermo-oxidatively aged UCOME at 100 kHz and 25 °C (ageing in Rancimat at 110 °C). 15

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The fuel properties required in the respective standards and in the “Worldwide Fuel Charter” (WWFC) [45] must, however, be guaranteed in all cases here. The requirement of maintaining compatibility with existing (or expected) fuel qualities in wide concentration ranges as a “Drop-in Fuel” must be fulfilled if a significant range of applications is to be achieved in the fuel mix. However, these types of challenges are nothing new when it comes to fuel development. Over the last 100 years, the health-related, environmental and technical properties of fuels have changed continuously. By adding property-modifying additives, it was always possible to (re)establish the suitability of the fuels. Starting with the combustibility correctors and additives to guarantee the knock resistance, 100 years ago, new additives - antioxidants, stabilisers, lubricant correctors, deposit reducers, corrosion protection and many other active substances - were continuously developed. In concentrations of only a few ppm, these were able to guarantee the desired characteristics in the fuel [148]. The regenerative fuel qualities must also guarantee the requirements of technical suitability and also of miscibility with each other, and have been heavily researched in this regard [149]. Many research projects are concentrating on these aspects of fuel development and have already developed numerous strategies for adapting the products [150]. Technical application problems can usually be avoided by using suitable additive concepts. Diesel R33 is an example of how blends of completely unpolarisable substances (HVO) with highly polar biodiesel can be combined in mineral oil-based diesel fuel, together with suitable additives, to make a stable “performance fuel”, and even to complement each other with their respective positive properties.

vibrations of the molecular groups of IR-active molecules. Thus, fuels can be categorised by the functional groups of their individual components. It is possible, for example, to detect the aromatics content, ester content and degree of isomerisation, in order to adapt the engine management accordingly [144,145]. Further parameters such as density, heating value and viscosity, which influence the engine combustion, can also be determined [146]. 6.4.3. Fluorescence spectroscopy Using excitation with light, valence electrons from occupied molecular orbitals of non-binding and π electrons are energetically excited into excited singlet states. By initially non-radiative transitions from higher vibration states, the relaxation of the electrons, with the emission of fluorescent light, takes place from the first excited singlet state into the ground state. The potential of fluorescence spectroscopy lies in the detection of minute admixtures. Thus, fluorescent additives in the ppm range can be identified. An evaluation supported by a database enables the differentiation of fuels from different manufacturers (Fig. 14). The consumption of antioxidants during the induction phase and the formation of ageing products also allows to track the degree of ageing by fluorescence. Overall, these three sensor principles offer a partly redundant option of fuel detection. Coburg University TAC is working on the implementation of such sensors currently. 6.5. Additives and possible effects on quality In addition to criteria such as sustainability, emission reduction potential or production costs, the chemical-physical behaviour of modern fuels takes on an essential role in terms of their technical properties [147]. The biofuels presented cover a broad spectrum (the broadest to date) in terms of their solution behaviour (polarity/polarisability), their low-temperature performance, their miscibility with each other, their stability, lubricity and many other chemical-physical properties. Thus, in the middle distillate range, a highly polarised OME faces the non-polar paraffinic HVO, for example; biodiesel and mineral oilbased diesel occupy an intermediate position between these two extremes (with respect to their polarity). The behaviour is similar with petrol – with the extremes of paraffinic XtL fractions and the polar ethanol, as well as the intermediate mineral-oil-derived petrol.

7. Demands relating to climate policy In view of the 2030 or 2050 climate change policy target, there is a great time pressure to fulfil greenhouse gas reduction targets. Sustainable and low-greenhouse-gas biofuels from cultivated biomass are globally available today, marking the beginning of a medium to long-term development strategy to replace fossil fuels and biofuels from cultivated biomass with other alternatives such as biofuels from waste and residual materials, e-mobility and synthetic renewable fuels. For the evolutionary development of these options, a period of time is needed, which cannot yet be predicted in terms of future market

Fig. 14. Fluorescence spectra of different fossil and biofuels (λExcitation = 405 nm). 16

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significance. These alternatives have for years not been competitive and require an unpredictable amount of public funding for infrastructure construction (charging columns, distribution networks), investment promotion, conversion or investment incentives etc. The discussion today is broadly still about research needs and project development when it comes to these alternatives. In order to guarantee the climate protection targets in 2030 or 2050, policy must, however, make distinctions between the action areas for funding, as described above. In terms of fulfilling the climate change targets, politicians cannot afford to remove existing options from the market, if it is not yet possible to predict what progress can be made with an approach that is open to technology. In order to achieve the targets in 2030, efforts must be aligned with what is currently possible. Right now, engine and fuel development must not be neglected. In contrast however, with the hybridisation of drives comes the increase of electrification, which physically reaches its limits with heavy commercial vehicles and off-road machines (construction and agriculture). An infrastructure for creating overhead lines across the country and/or across Europe is, judging by the time frame (planning and construction) likely not achievable. Additionally, shipping and air traffic rely on liquid renewable fuels. Therefore, the focus must also be on consumption reduction. As a result of the development of engine technology, potential conservation reserves must be enhanced in order to reduce the existing biogenic fuel substitution potential, as measured by the ever increasing total fuel demand. With the recast of the Renewable Energy Directive (REDII), the greenhouse gas reduction specifications are being increased further as a prerequisite for access to the market. The GHG reduction efficiency of biofuels (competition open to raw material) rises in the short term and could be strengthened if a mandatory greenhouse gas reduction requirement (in accordance with the German model, with specified quotas) would be introduced across the EU. This would boost this competition without the “substitution effect”. In terms of environmental policy, it makes little sense if waste oil is exclusively used in the EU’s most profitable funding areas and if these raw materials are even imported from third countries which themselves need this raw material. As a consequence of the GHG reduction requirement, in Germany, the reduction in GHG in biofuels increased to an average of around 81 percent compared to fossil fuels [151]. Competition that is based on technology and raw materials would also promote the site-adapted and most efficient technology for the production of alternative fuels or for the use of renewable electricity. European policy is required to monitor development through corresponding funding policy measures, which must simultaneously aim to expedite infrastructure development (charging stations), especially in the EU’s poorer countries.

This approach will lead to more public acceptance. For example, the use of rapeseed oil fuel in diesel engines was subject to many tests in the past and this successful implementation was able to be demonstrated in conjunction with various modifications made to fuel and engine systems. In conjunction with new diesel engines, which are characterised by modern high-pressure injection and exhaust gas treatment systems, even less research has been conducted with rapeseed oil fuel in comparison with fossil diesel fuel or FAME. Therefore, more in-depth information on behaviour in fuel systems, in mixture preparation during injection, on ignition and combustion behaviour, on behaviour in exhaust gas treatment systems, on emission behaviour (even in real operation) and on durability is necessary for the optimisation of engines that are suitable for rapeseed and other vegetable oils. Finally, for type approval, the necessary and increasingly strict statutory requirements in the emissions law must be fulfilled. Analogous requirements and research questions also arise with biodiesel with different fatty acid compositions, chain lengths and admixture proportions of fossil or even renewable aliphatic fuels. Possible interaction effects may impede versatility. The wide variety of potential synthetic biofuels makes it possible to research optimum admixture quotas in order to guarantee the best possible synergy of the respective advantages of individual biofuels within a multi-component mixture. The differences between the polarities of biofuels requires research into miscibility and the use of amphiphilic fuels such as biodiesel, for example, as a solubiliser. In this context, the biofuel mixtures as well as the clean fuels must be examined in terms of their specific properties in order to be able to take advantage of other specific fuel properties in a targeted manner, as well as replacing fossil fuels. Examples here are the use of biologically degradable fuels in environmentally sensitive sectors and the use of fuels with good low-temperature performance in aviation. Additionally, the question is raised as to whether and how biodiesel can be chemically optimised in terms of improved miscibility with fossil and synthetic fuels. The aim is the improvement of the boiling curve by optimising the chain length using metathesis, for example. At this point, it should also be examined as to whether a separation of fatty acids according to the chain length by distillation represents another approach along these lines. For example, natural fatty acid distributions can be separated by distillation after transesterification to the monohydric alcohol methanol. Thus, fractions of different chain lengths can be represented. If FAME with one, two or three double bonds can be separated using distillation, there is the possibility of taking advantage of different solubility properties in a targeted manner and of thus avoiding miscibility gaps when combining with synthetic fuels. This corresponds to the commercial distillation of pure fatty acids after fat splitting. This idea could be conceivably expanded by a “chemistry based on methyl esters”. A wide variety of modifications to the fatty acid chain have been described in recent decades, for example selective hydrogenation, dimerisation, co-oligomerisation, Friedel-Crafts alkylation or acylation, ene reaction, radical addition, epoxidation, oxidative splitting/ozonolysis, cyclopropanation, metathesis and more. It must be checked as to which of these reactions is also applicable to the methyl esters of these fatty acids. The resulting reaction products, modified fatty acid methyl esters, can be finally interesterified into other alcohols. This way, base oils, for example high performance lubricants, can be synthesised [152]. In this way, today’s plants, optimised exclusively for continuous FAME production, would be further developed into bio-refineries with a diverse range of products. As well as the production of fuels, this concept also offers the possibility of substantial use of renewable raw materials. These new products, but primarily new synthetic fuels or mixtures, have functional groups which do not occur in fossil fuel. Through the ether group, fuels such as dimethyl ether or OME clearly show other solubility properties, which requires considerable research into the

8. Future requirements concerning biofuel research In 2018, both international production and the consumption of biofuels (ethanol and biodiesel) are not only ideally supported by many countries, but are also legally required by mandates for admixture quotas and partly by tax incentives. In this context, while there is a global trend of expanding biofuel production to residual and waste materials, traditional raw materials such as vegetable oils, sugar and starch are by far the most important source of raw materials for the onset of global transport defossilisation. The research projects, conducted primarily in Germany, in the area of biofuels are therefore an important scientific source of information for the international exchange of expertise and for the avoidance of duplicating research. However, there is no strategic promotion in the sense of proactive accompanying research. The goal must be to promote existing raw material options such as biodiesel from vegetable oils and also vegetable oil fuels from decentralised processing, because this will also create regional added value. 17

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material compatibility of fuel-carrying components. There is also the fact of the functionality test of the fuel additives. With regard to an increasing number of plug-in hybrid vehicles, which combine in their powertrain the advantages of combustion engine and electrical drive, the longer dwell time of fuels in the tank must be taken into account and new additives to improve the oxidation stability of, in particular, biogenic and synthetic fuels must be researched. Engines, exhaust gas aftertreatment systems and engine components for biofuels and biofuel blends must be tested in operation under permanent loads as a basis for market introduction (e.g. studies of deposit formation in injectors and measures to avoid this). Therefore, the engines and the exhaust gas treatment system for biofuels and biofuel blends must be optimised with the goal of improving efficiency and the output of harmful substances, in order to fulfil increasing requirements of emission standards in future. For various reasons, the ingress of biogenic fuels into lubricant often leads to a reduction of the recommended oil change intervals. To avoid unnecessary early oil changes, oil condition sensors would be necessary. These would continuously detect the specific engine oil load during operation, forward essential information on the oil condition to the operator and send warnings punctually before important limits are exceeded. Oil change intervals can then be set according to need. It should be noted that the overwhelming majority of the reported lubricant problems when using biogenic fuels are due to the fuel quality. Thus it is clear that the (further) standardisation of “new” fuels is of particular importance. Regarding a future tightening of emissions guidelines in the Euro 7, solutions must be found early on in order to reduce emissions further. The degree of effectiveness of modern engines is already near the upper limit of the physically feasible. Optimisation potential exists in the interaction of engine and fuel. If fuel parameters can be determined via sensors, combustion can be optimised using suitable engine management. Not only does the development of sensors play an important role here, but also the adaptation of engine parameters specifically to the fuels. Component detection plays a particular role in the increase of biocomponent quotas in fuels, as this can only be realised through a variety of different raw material sources and, therefore, of different chemical compositions. Lastly, alongside the scientific research, it is also necessary not to neglect communication with society in order to systematically establish causes for the lack of biofuel acceptance (especially of first generation biofuels) and to be able to take measures against these reservations for all generations of biofuels. New technical developments must be brought into the public sphere in an ideology-free, understandable and explanatory manner, so that both society and politics are able to understand the change and so that they do not actively obstruct it. This is where scientific dialogue takes on an especially important role. The protection of both the climate and resources is a topic that is still part of positive public discourse and can be accompanied by questions about emissions, in particular in the case of biofuels. The rollout of sustainable inner city fuels such as diesel R33 shows clearly the great potential of scientific dialogue. This must now be urgently expanded so that the combustion engine, in conjunction with sustainable fuels, can find technical and societal acceptance and a place in the future.

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