Reduction in fossil fuels, contributing to greenhouse gases, and improvement of air quality from vehicle emissions is of growing concern worldwide. This has led to the introduction of several binding and non-binding agreements, such as the Renewable Energy Directive to increase the renewable content of fuel for transportation, the carbon dioxide emissions stand- ards to limit the amount of carbon dioxide emissions from vehicles and the Euro Standards to limit the amount of emissions harmful to human health in the exhaust. However, the in uence of the fuel composition on hazardous exhaust emissions is a complex, and often contradictory, relationship between factors such as the fuel properties, combustion charac- teristics and engine load. Therefore policy implemented to improve one aspect, such as a reduction in carbon dioxide, can have a detrimental e ect on another such as increased NO x emissions. This paper analyses, in a holistic manner, the impact on carbon dioxide and harmful emissions from transient compression ignition engines when increasing the renewable con- tent of the fuel to meet the renewable energy targets. The analysis is based on a model developed from a rigorous Design of Experiment methodology used to determine the com- plex relationship between renewable fuel content and exhaust emissions (carbon monoxide, carbon dioxide and nitrogen oxides). Unlike other studies, the results were collected from a transient engine cycle, the World Harmonised Light vehicle Test Procedure, rather than steady state conditions, thus the results are more applicable to the real world. The results generally show that as the amount of ethanol is increased then the NO xand CO emissions decrease compared to current pump diesel. Increasing the biodiesel content 1 generally increases the CO and CO 2emissions from the engine. For practical reasons a tern- ary blend is required to minimise the diesel engine emissions whilst meeting the UK's future renewable content target. A blend of B2.4E10 was found to be the optimum compromise between renewable content and engine emissions. However, for this to be achieved the UK will have to invest in second and third generation ethanol. Keywords: biodiesel, ethanol, emissions, NO x, CO 2 1. Introduction Transportation is a signi cant source of greenhouse gases and harmful emissions. Con- sequently to address this a range of legislation is in place aimed at reducing carbon dioxide CO2emissions, reducing harmful emissions and increasing renewable content. Each of these will be discussed in turn. Passenger cars account for around 12 % of CO 2emitted in the 5 European Union (EU) [1]. EU regulation 333/2014 [2] limits the carbon dioxide eet average emissions from passenger vehicles. Currently the target emissions level for cars sold in the EU is 130 g/km. However, this will be reduced to 95 g/km by 2020 [2]. The CO 2limit e ectively places a fuel consumption constraint on the vehicle. Manufacturers are ned an 'excess emissions premium' for each car registered that exceeds the target [3]. 10 In addition to greenhouse gases, the exhaust contains harmful emissions such as nitro- gen oxides (NO x) and carbon monoxide (CO) which are harmful to both the environment and human health. Emissions of nitrogen dioxide alone cause an equivalent of over 23,000 deaths per year [4] and cost the UK $2.7 billion through the impact on productivity [5]. Additional regulations have been introduced to limit the emission of harmful gases. The 15 European emission (Euro) standards state the acceptable limits for exhaust emissions for all new vehicles to be sold in the EU, currently Euro 6d [6]. Table 1 states the permissible emissions for passenger vehicles with at least four wheels (Category M). The regulation was updated to include the measurement of particulate matter by number as well as by mass Corresponding author Email address: adriaan.vanniekerk@uwe.ac.uk (A.S. van Niekerk) Preprint submitted to Fuel 16th December 2019 [7]. Consequently, in order to curb on-road emissions a number of, sometimes contradictory, 20 legislations have been implemented. Table 1: Summary of Euro 6 permitted emissions for passenger cars (Category M) [6]. Emissions Diesel Gasoline Carbon Monoxide (g/km) 0.50 1.00 Total Hydrocarbons (g/km) - 0.10 Nitrogen Oxides (g/km) 0.08 0.06 Particulate Matter (g/km) 0.005 Particulate Number (# /km) 6 1011 To reduce the impact of fossil fuel CO 2emissions, increase energy security and strengthen it's competitiveness the EU has introduced further targets. These targets can be summarised as follows [8]: 20 % reduction in EU greenhouse gases from 1990 levels. 25 20 % improvements in EU's energy eciency. 20 % of EU energy from renewable energy. The nal target is enforced by the EU by giving all the member countries binding targets to raise their share of renewable energy in their energy consumption by 2020. These targets are determined based on the individual country's use of renewable energy and the potential 30 to increase their production. The UK has a target of 15 % [8]. Ultimately, all the individual targets will ensure the EU meets its target of 20 % by 2020. Included in the individual targets of each country, a 10 % share of renewable energy in the transport sector is required. This can be achieved using a combination of biofuels, hydrogen or 'green' electricity [8]. The UK government hopes to reach it's 2020 target of 10 % renewable energy in transport 35 with the promotion of ultra low emission vehicles (ULEV) as well as increasing its use of renewable fuels in the transport sector [9]. 3 A regulation [10] published in April 2018 by the Department for Transport stated that the Renewable Transport Fuel Obligation biofuel targets are to increase from the current value of 4.75 %, to 9.75 % in 2020 and 12.4 % in 2032. In addition to the total biofuel 40 targets, additional targets and caps have been introduced to ensure a sustainable supply of the biofuel. Advanced waste derived renewable fuels have a target of 2.8 % by 2032 whilst the biofuels from crops must reduce to 2 % by 2032 to ensure that the supply of crops for consumption is not compromised. So far the state of play concerning targets and legislation governing the emissions from 45 transportation and the renewable content of fuel has been presented. The rest of this section will summarise the complex relationships between the renewable fuel blends, engine loads and the impact on emissions. Biofuels, which include biodiesel and ethanol, are a renewable source of energy in the transport industry [11{13]. Currently renewable fuels, such as biodiesel, can be used as a 50 'drop in' fuel together with diesel up to a maximum of 7 % at pump stations [12]. Studies have been conducted on biodiesel and ethanol blends ranging from pure fuels (e.g. B100) to binary blends with petroleum diesel (e.g. B20, E10, etc.) and ternary blends (e.g. B20E2, B40E5, etc.) Table 2 shows a summary of the e ects of binary blends of diesel and biodiesel and diesel 55 and ethanol and their e ects on harmful emissions, for di erent loads, based on the steady state studies in the literature. The metric used for engine load is the Brake Mean E ective Pressure (BMEP) since this allows comparison across engines with di erent displacements. Part load represents BMEP values between 0 bar and 6 bar and full load represents BMEP values greater than 6 bar. Additionally, a '+' represents a minor increase and ' ' represents 60 a minor decrease (a minor change is de ned as 5 %). The ' ' represents an insigni cant change ( 1 %) and double '++' and ' ' represent a signi cant change ( 5 %). The 'N/A' label indicate that for the considered literature, the speci c engine emission was not considered. From Table 2 it is evident that the bene ts of using renewable fuels to reduce harmful emissions are dependent on the fuel blend composition as well as on engine oper- 65 ation. For example, generally, the amount of carbon monoxide and hydrocarbons increase 4 at part load but reduce at full load for ethanol mixtures. The impact of engine load on emissions is signi cant, since in real world driving the engine load is varied transiently (with regards to time) throughout the journey and not kept constant as per the tests in Table 2. Table 2: Summary of e ects of binary blends on harmful emissions [11, 14{18]. Harmful emissions Biodiesel Ethanol Part load Full load Part load Full load NO x - - - - CO  - - ++ - - HC - - ++ - CO2 N/A + - - PM - - - - - - - Therefore, ternary blends of biodiesel, ethanol and diesel are being investigated to help 70 mitigate the increase of some harmful emissions for some of the binary blends. However, the chemistry of combustion becomes even more complex with ternary blends compared to binary blends with transient engine loads and speeds, as shown in Table 3. Table 3 presents a summary of steady state studies of the impact of ternary blends on exhaust emissions. It is clear from Table 3, by examining each pollutant in turn that the interaction between 75 the pollutant and the many variables is complex. With such complex interactions between engine conditions and ternary fuel blends on the emissions, results from studies based on steady state conditions have limited value and cannot be easily translated to the real world, where the engine speed and load is varied based on driver behaviour and the local environ- ment. 80 To try and understand how the biofuel composition is likely to impact the emissions in the real world, transient behaviour needs to be considered. Figure 1 shows the World Har- monised Light vehicle Test Procedure (WLTP) that is used, as the standard test, to ensure that new vehicles are compliant with the legislation, e.g. the Euro standards. Total test time of the WLTP is 1477 seconds with an average velocity (stops excluded) of 26 km/h in the 85 low phase, 44 km/h in the medium phase and 57 km/h in the high phase. Figure 2 compares 5 Table 3: Summary of e ects of ternary blends on harmful emissions. Hulwan and Joshi [19] Zhu et al. [20] Khoobbakht et al. [21] Yilmaz et al. [14] % Biodiesel 10 10 10 15 15 20 49 43 % Ethanol 20 20 20 15 15 10 3 15 BMEP (bar) 2 4 6 2 7 11.5 3.7 3.7 Speed (rpm) 1600 1600 1600 1800 1800 1900 3000 3000 NO x    - - + - - CO +   +  -  + HC N/A N/A N/A -  - - - FC + + + ++ + N/A N/A N/A CO2 + + + N/A N/A + N/A N/A PM N/A N/A N/A - - N/A N/A N/A 0 500 1000 1500 Time (s)0102030405060708090Vehicle speed (km/h) Figure 1: World Harmonised Light Test Procedure 6 the variation of engine load with engine speed for the steady state engine points cited in the literature in this paper and the current transient drive cycle (WLTP) implemented on the engine used in this study. Figure 2 shows that, despite the wide range of steady state tests it does not cover the whole range of engine loads and speeds experienced in the real world as 90 de ned by the WLTP, in particular engine speeds over 2500 rpm and under 1500 rpm have not been extensively covered. Consequently, it is clear that real world driving behaviour as well as the e ect of ternary blends on real world exhaust emissions cannot be accurately translated from steady-state emission tests. 500 1000 1500 2000 2500 3000 3500 4000 Engine speed (rpm)051015BMEP (bar) Drive cycle From literature Figure 2: Comparison of engine operating points as discussed in literature compared to the operating points in the WLTP drive cycle. The aim of this paper is to address these shortcomings and provide a holistic view of the 95 complex relationship between UK policy, biofuel composition, transient engine loads and engine emissions and propose an optimal mixture suitable for compression engines, using results in previous research [22]. 7 2. Methodology 2.1. Experimental set-up 100 A 2.4 L Euro 4 compression ignition (CI) engine with a programmable after-market ECU was used as the test engine to collect the data. Although the research in this paper was conducted on a Euro 4 engine, which was manufactured in 2008, this research is still highly relevant to the current UK eet. According to the Department for Transport's statistics [23] approximately 14 % of the current diesel eet is of this engine type. Additionally the 105 conclusions for this paper are qualitatively relevant to more modern Euro 5 and Euro 6 diesel engines [24{27]. Figure 3 shows a schematic of the CI engine testing facility that was used for studying the engine emissions. The engine, whose speci cations are listed in Table 4, was connected to a Froude FO271 dynamometer which is capable to absorbing a maximum of a 1000 kW and 4000 Nm. Two gas analysers were used; one (NOVA 7466K) for measuring 110 CO2and NO xemissions and the other (TESTO 350) for measuring CO emissions. Both were located upstream of any exhaust after treatment systems. A summary of the analysers is presented in Table 5. The factory tted mass air ow sensor (part number 6C11-12B579- AA), calibrated with a Super ow SF-120 ow bench, was used to measure the intake mass air ow in kg/s. CA D BEF IG HAir flow path Signal wiring Figure 3: Layout of the engine test cell together with measuring equipment; A: air lter; B:Mass air ow sensor; C: engine; D: Turbo charger; E: gas analyser; F: exhaust outlet; G: load cell; H: dynamometer; I: Data Acquisition System. 115 8 Table 4: Engine parameters used for experimentation. Engine parameter Characteristics Engine code H9FB (Ford Transit) Rated power (kW) 103 Rated Torque (Nm) 375 Bore (mm) 89.9 Stroke (mm) 94.6 Volume (cm3) 2402 Compression ratio (CR) 17.5 Number of cylinders 4 Method of cooling Water cooled (21C,= 3) Table 5: Method and accuracy of the instruments used to measure the engine emissions. Exhaust gas Range Accuracy Method CO (ppm) 0 10000<10 electrochemical CO2(%) 0 20<0:2 infra-red NO (ppm) 0 2000<20 electrochemical NO2(ppm) 0 800<8 electrochemical 9 Table 6: Main fuel properties of neat test fuels. Diesel Biodiesel Ethanol Cetane number 51.7 52.8 7.0 LHV (MJ/kg) 42.8 38.0 26.8 Density at 15C (kg=m3) 831.1 883.2 790.0 Viscosity at 40C (mm3=s) 2.686 4.372 1.200 Oxygen content (%) 0 10.8 34.8 CFPP (C) -26 -6 -38 Flash point (C) 65 179 40 2.2. Experimental Procedure A mixture Design of Experiment (DoE) approach was adopted to explore the individual e ects of diesel, biodiesel and ethanol and their interactions in a blend for di erent engine responses. The test fuels used in the DoE were B0 reference diesel, rapeseed methyl ester biodiesel (RME) and ethanol. B0 diesel was chosen as a reference fuel and benchmarked throughout the DoE. As pump diesel has biodiesel present in the fuel blend, it is necessary to use B0 diesel to make it possible to accurately control the percentage of biodiesel present in the blends used during testing. The fuel supplier provided physiochemical properties for the diesel fuel, RME and ethanol, respectively. The fuel properties of each fuel can be seen in Table 6. DoE is an established statistical tool that is used to identify the underlying complex relationships between variables whilst minimising the number of experiments necessary. The selection of the mixture DoE is appropriate as the sum of the input variables, in this case the blend components, must be unity [28]. A summary of the procedure is presented here. As opposed to a response surface design, the factors in a mixture design is not independent from each other. If x1;x2;:::;x pdenote the proportions of pcomponents of a blend, then 0xi1i= 1;2;:::;p (1) 10 Figure 4: Extreme vertices simplex design plot of the mixture DoE. and x1+x2++xp= 1 (2) For a mixture design with three components, the design space is a triangle with vertices corresponding to formulations that are pure blends (100 % of one blend). Figure 4 shows an extreme vertices design, where upper limits have been set to the amount of biodiesel and ethanol. The upper limits are based on previous research [19, 29] where the maximum addition without engine modi cation was determined: xD+xB+xE= 1xB;xE0:2 (3) wherexDis the fraction component of diesel, xBis the fraction component of biodiesel andxEis the fraction component of ethanol in the blend. The whole mixture design was replicated once and the runs were randomised to ensure experimental errors are independ- ently distributed. The optimisation of the fuel blend is dependent on more than one engine response which include CO emissions, CO 2emissions and NO xemissions. The desirability approach was used for the optimisation of the fuel blend parameters (diesel, biodiesel and ethanol) for the properties of the engine response mentioned above. The software trans- forms each response to a dimensionless desirability value d. The value ranges from d= 0 , which indicates that the response is unacceptable, to d= 1 which shows that the response 11 is more desirable. For minimising engine emissions the desirability of each of the responses was calculated using [30]: di(^Yi) =8 >>>>>< >>>>>:1 if ^Yi(x)<T i ^Yi(x)Ui TiUiifTi^Yi(x)Ui 0 if ^YiUi(4) wheredi(^Yi) is the desirability function of response ^Yi(x),TiandUiare the target and upper values respectively that are desired for response ^Yi(x). The individual desirability functions are combined using the geometric mean, which gives the overall desirability: D= (d1(Y1)d2(Y2))0:5(5) The di erent blends of diesel, biodiesel and ethanol fuel were mixed in batches of 5 l in the determined blend ratios based on the mixture DoE. The blend components of diesel, biodiesel and ethanol were mixed together using lab equipment with an accuracy of 10 ml to make the homogeneous fuel blends. Each blend was then kept in a sealed glass container 120 for a maximum of 24 hours to observe its physical appearance. In order to ensure that the old fuel blend from the previous test in the fuel system does not in uence the next test, the fuel system was ushed with the next test's blend of fuel before formal testing began. It was determined that the engine fuel system needed to be ushed four times to successfully remove all remaining fuel blend from the previous test before the next test was conducted. 125 3. Results and Discussion It is important to state from the outset of the discussion that the following analysis prioritised emissions reduction and meeting the renewable content target. Some argument could be made to accept a compromise on the engine emissions over renewable content, since exhaust after treatment systems are required to meet the Euro standards anyhow. However, 130 it is commonly accepted that reducing the emissions at source (in the engine) has a knock-on positive e ect on the whole vehicle. Since initial emissions reduction will result in a lighter, smaller and cheaper exhaust after treatment system [31]. 12 It should be reiterated here that all the results discussed have been collected based on the transient WLTP drive cycle unlike previous steady state studies or transient studies on the 135 now obsolete New European Driving Cycle (NEDC). Equations used to generate Figure 5 and Figure 6 were obtained from previous research [22]. As discussed previously the addition of ethanol to diesel has a positive impact on the harmful emissions. Figure 5 shows the variation of CO 2, NO xand CO with increasing amounts of ethanol in diesel. The use of binary blends of ethanol between E5 and E15 result 140 in CO emissions being reduced by approximately 36 %, NO xemissions by approximately 11 % and CO 2emissions by approximately 19 %. For blends greater than E15 the emissions tend to increase rapidly compared to pure diesel. The reduction of CO emissions, CO 2 emissions and NO xemissions for binary blends between E5 and E15 can assist the UK in reducing harmful emissions in CI engines as well as achieve the renewable fuel target for 145 2032 of 12.4 % [10]. Therefore, assuming that the renewable biofuel content is ful lled by 0 5 10 15 20 Percentage of blend (%)00.511.522.5CO, NOx (g/km) 050100150200250300 CO2 (g/km) CO NOx CO2 Figure 5: Variation of CO 2, NO xand CO with increasing amounts of ethanol in diesel ethanol alone compared to pump diesel a binary blend of E12.4 would result in a decrease of CO 2, NO xand CO of 26 %, 12 % and 33 % respectively. Note the comparison has been made with pump diesel, which we have assumed to have a 5 % biodiesel content [32], rather than pure diesel. The reason for this is that it is now not possible to purchase pure diesel 150 from a lling station in the UK, and so the comparison should be made with the fuel that 13 is currently available [32]. The addition of ethanol seems positive in terms of both reducing the harmful emissions from vehicles whilst simultaneously increasing the renewable content of the fuel in line with EU targets. However, there are a number of practical issues that nullify these advantages. 155 Firstly when considering alternative fuels for vehicles, the carbon dioxide emissions from the vehicle should not be considered in isolation. All the carbon dioxide emissions along the supply chain need to be considered [33]. Table 7 shows the equivalent carbon dioxide emissions upstream of the vehicle and are often referred to the 'Well-To-Tank' (WTT) emissions. These WTT emissions include 160 everything upstream such as extraction, re ning, puri cation, transportation, etc. The term 'equivalent' refers to processes where there is a Greenhouse Warming Potential and these are then expressed as the equivalent grams of carbon dioxide. The data in Table 7 Table 7: Comparison of equivalent carbon dioxide Well-To-Tank emissions for diesel and renewable fuels. [34] Fuel gCO 2e/Litre Percentage change Pump Diesel 618.46  Ethanol 613.77 1 % Biodiesel (RTFO average) 312.95 49 % shows that the equivalent carbon dioxide emissions for pump diesel are similar compared to ethanol. This means that the net carbon dioxide saving of using ethanol comes from the 165 displaced diesel fuel in the tank and the emissions saving from the combustion process, but there are not any upstream savings. However, a more pressing technical barrier for a binary mixture of ethanol and diesel is that ethanol is immiscible in diesel. Therefore additives (emulsi ers) are required to improve the miscibility of the ethanol in the diesel [35, 36]. Biodiesel can act as an emulsi er for 170 ethanol. The addition of biodiesel also has the added bene t that it has a much lower WTT 14 carbon dioxide emissions compared to ethanol (Table 7). Based on current gures the WTT carbon dioxide emissions are around half that of regular pump diesel and ethanol. Consequently, based on the reduced WTT carbon dioxide emissions it would be best to 175 meet the renewable targets with biodiesel alone. However, the addition of biodiesel tends to increase emissions. Figure 6 shows the variation of CO 2, NO xand CO with increasing amounts of biodiesel in diesel. Figure 6 shows that for biodiesel blends between B5 and B15 the CO and CO 2emissions are at a maximum. If the renewable fuel target of 12.4 % is ful lled with biodiesel, then compared to pump diesel, this would result in an increase of 180 CO and CO 2of 3.3 % and 1.4 % respectively, whilst a decrease in NO xof 1.7 %. The changes are modest compared to pump diesel. The reason for this modest increase is because the comparison is with pump diesel, which already has a biodiesel content. There 0 5 10 15 20 Percentage of blend (%)00.20.40.60.811.21.41.6CO, NOx (g/km) 050100150200250300 CO2 (g/km) CO NOx CO2 Figure 6: Variation of CO 2, NO xand CO with increasing amounts of biodiesel in diesel is an argument that, when considering how best to meet the future renewable targets, the well-to-wheel (WTW) CO 2emissions need to be considered. The WTW emissions consider 185 the carbon dioxide emissions generated from all the upstream processes and the emissions from the vehicle itself. To address the con icting requirements of increasing the WTW CO 2emissions, additional exhaust harmful emissions and practical miscibility aspects of the fuel a ternary blend of diesel, biodiesel and ethanol is required. The proposed criteria for the fuel is: 190 15 A renewable biofuel content of 12.4 % to meet the 2032 UK target. Minimise the well-to-wheels CO 2emissions. Minimise the harmful emissions of carbon monoxide and nitrogen oxides. A homogeneous mixture that will not separate. To minimise the WTW CO 2emissions the maximum amount of biodiesel is required. To 195 minimise the harmful emissions the maximum amount of ethanol is required. Based on previous experience the minimum amount of biodiesel to form a homogeneous mixture of ethanol, biodiesel and diesel is 2 %. A previous mixture of B2E11 resulted in a stable blend [22]. Other research has also found ternary blends of B10E20 to be stable [26]. Therefore a mixture of B2.4E10 is proposed. This mixture satis es the criteria in that: 200 it meets the 12.4 % biofuel requirement, reduces the WTW emissions, has a maximum ethanol content to reduce CO and NO xemissions. has a minimum biodiesel content of 2 % to maintain a stable mixture. The predicted reduction in emissions for CO 2, NO xand CO are 20 %, 9.6 % and 28 %. 205 Table 8 compares the changes in emissions for each mixture compared to pump diesel. The proposed optimum mixture requires a 10 % content of ethanol. Therefore, if the recommended blend were to be adopted, this would potentially impact on the UK food industry. The UK needs to invest in second and third generation ethanol feed stocks (non- edible organic matter and algae lipids) and move away from rst generation feed stocks by 210 2032. The EU requires member states to cap feed stocks from edible sources by 2032 to 2 %. Currently 100 % of the UK ethanol is rst generation. 16 Table 8: Summary of results showing the change in emissions compared to pump diesel Fuel Blend CO 2 NO x CO E12.4 26 % 12 % 33 % B12.4 1 :4 %1:7 % 3:3 % B2.4E10 20 % 9:6 %28 % 4. Conclusion This paper took a holistic view to analyse the complex and contradictory relationship between UK policy, biofuel composition, transient engine loads and engine emissions. The 215 main conclusions are: 1. The optimum mixture to meet the UK biofuel content of 12.4 % in 2032, whist min- imising carbon dioxide and harmful exhaust emissions is B2.4E10. 2. Steady state engine emission tests have limited value, compared to the real world, based on the complex relationship between engine composition and engine loads. 220 3. From a carbon dioxide viewpoint a binary mixture of B12.4 would be best, due to the low well-to-tank emissions of biodiesel. However, increasing the biofuel content in the range B5 to B15 maximises the harmful exhaust emissions. 4. From a harmful emissions viewpoint a binary mixture of E12.4 would be best, due to the minimisation of carbon monoxide and nitrogen oxides in the range of E5 to E15. 225 However, ethanol is immiscible in diesel and for practical reasons biodiesel needs to be added as an emulsi er. 5. To achieve the optimum mixture, investment into second and third generation ethanol is required. 5. Future work 230 Future work will investigate the e ect of engine emissions that did not form part of this research's scope (soot, HC and PM/PN) when the optimum blend of B2.4E10 is used. The 17 desired outcome will conform with future EU and UK renewable targets for 2030 and beyond. Funding This research did not receive any speci c grant from funding agencies in the public, com- 235 mercial, or not-for-pro t sectors. Declaration of interest None