Brazil

Historically, Brazil has the world’s most mature market for bioethanol in road transport fuels. Although ethanol-gasoline blending was taking place in Brazil on a significant scale in the 1930s, it was the OPEC oil crisis of 1973 which prompted the large-scale introduction of ethanol made from sugar cane as part of a national alcohol programme (‘ProAlcool’) in 1975 (Soccol et al., 2005). The evolution of alcohol-fuelled vehicles in Brazil is well summarized by Kramer and Anderson (2012). The extent of the penetration of ethanol in the Brazilian fuel pool is such that it is not possible to purchase gasoline which does not contain bioethanol. The level of ethanol in gasoline (to form ‘gasohol’) is currently allowed to vary between 18 and 25 vol%, depending on the state of the global sugar market. Hydrous ethanol is also sold, consisting of at least 94.5 vol% ethanol, with the balance being a permitted mix of components consisting of water (mainly), hydrocarbons and other alcohols.

After initially blending ethanol with gasoline at around 20 vol%, dedicated E100 vehicles were introduced in 1979 and, by 1985, these vehicles represented 80% of light-duty vehicle production, assisted by ethanol prices which were sufficiently lower than the gasoline price to easily offset the volumetric energy density differential. Subsequent fluctuations in the global sugar market led to gasoline being cheaper than ethanol and the demand for dedicated E100 vehicles virtually disappeared by the mid-1990s. The introduction of flexible-fuel vehicles (FFVs), capable of using anything from gasohol to E100 (actually hydrous ethanol about E94 with 6 vol% water), around 2002 rejuvenated ethanol sales and enabled customers to exploit the price and tax advantages of E100 over gasoline whilst protecting them from the volatility of the sugar price by enabling operation on lower ethanol blends when desirable.

United States

In the US, the oil crisis of the 1970s and the drive to improve air quality in states such as California gave rise to an interest in methanol. Abundant availability of indigenous feedstocks (principally coal and natural gas) and low production costs drove the introduction of gaoline-M85 FFVs (the first ‘modern’ FFVs), capable of operating on any mixture ranging from conventional gasoline to a mixture of 85 vol% methanol/15% gasoline. A national M85 standard (ASTM D5797 – covering mixtures containing between 70 and 85% methanol in gasoline) was put in place. Political factors, combined with the drop in the oil price and a methanol shortage brought about by the sale of methanol stocks to make methyl-tert-butyl ether (MTBE) for use as an oxygenate in reformulated gasoline, required by the Clean Air Act Amendments (EPA, 2012), led to the reduction of interest in methanol in the late 1990s and the rise of bioethanol production. The use of reformulated gasoline is mandated in some areas of the US in order to reduce emissions of usnburned hydrocarbons from older vehicles and to help reduce smog (ground-level ozone) formation. MTBE had been used in gasoline at low levels since 1979 as an octane enhancer when tetra-ethyl lead was banned. MTBE itself was subsequently banned in many states and its role as an oxygenate component in gasoline has now been completely replaced by ethanol.

The role of ethanol as an oxygenate source in reformulated gasoline began to rise in the early 2000s when the use of MTBE as an oxygenate additive was phased out, as shown in Fig. 13.5(a). Ethanol consumption has risen strongly since 2005 due to the federal policies which encouraged its use, reaching over 7.3 billion gallons gasoline-equivalent (gge) in 2009 and representing over 90% of US alternative fuel energy demand in 2009. The rapid growth in consumption has been matched by the rapid growth in production, as shown in Fig. 13.6 which reveals that the US is now by far the largest producer of ethanol, its output having risen to 62% of the total of 22.4 billion gallons produced globally in 2011. Together, the US and Brazil accounted for 87% of global production in that year.

Figure 13.5(b) focuses on the lower demand fuels in the US market; they are all dwarfed by the quantities of ethanol used as an oxygenate component (see Fig. 13.5(a)). The rise, decline, and recent huge jump in biodiesel consumption can be seen, together with the gradual fall of liquid petroleum gas (LPG) consumption. Natural gas consumption (compressed (C)NG and liquefied (L)NG in the figure) has shown a steady rise, and it might be postulated that the current low cost of natural gas resulting from the exploitation of shale gas reserves could lead to further significant increases. An alternative scenario is the conversion of natural gas to methanol for transport use (Turner et al., 2012a). It is also clear that electricity and hydrogen have no significant demand up to 2009, comprising 0.06% and less than 0.002% of US alternative fuel energy demand in 2009, respectively.

The original Renewable Fuels Standard (RFS) (EPA, 2007) was created under the Energy Policy Act of 2005 and required 7.5 billion US gallons of renewable fuel to be blended into gasoline by 2012. As a result of the Energy Independence and Security Act of 2007 (US Congress, 2007) the Renewable Fuel Standard was expanded (RFS2) (EPA, 2010a) to include diesel as well as gasoline, mandated an increase of renewable fuel from 9 billion gallons in 2008 to 36 billion gallons in 2022, and established new categories of renewable fuels, setting separate targets for volume and greenhouse gas (GHG) reduction for each fuel. Figure 13.7 shows the ramp up in fuel volumes required to meet the RSF2 stipulation. The target for ‘biomass-based diesel’ is a minimum of 1 billion gallons from 2012 to 2022 (with the exact target to be set by future rulemaking) whilst that for ‘cellulosic biofuel’ was set to rise from 0.5 billion gallons in 2012 to 16 billion gallons in 2022, most of which is expected to be in the form of cellulosic ethanol (EPA, 2010a). In total, 21 billion gallons of ‘advanced biofuels’ (including cellulosic biofuels) are required in 2022, leaving 15 billion gallons to be supplied by first generation fuels, mostly in the form of corn ethanol. Many problems have been encountered meeting the targets for cellulosic biofuels and the 2012 target has recently been revised downward to 8.65 million gallons – this will represent less than 0.006% of US fuel usage that year (more precisely, the figure of 0.006% represents cellulosic biofuels as a fraction of non-renewable gasoline and diesel use), compared with 9.23% overall for renewable fuels, comprised mostly of ‘corn ethanol’ (EPA, 2011a).

Anderson et al. (2012a) provide an excellent account of the introduction of ethanol into the gasoline fuel pool in the US, including details of the changes in the gasoline blend stock, namely the blend stock for oxygenate blending (BOB), into which the ethanol is blended. Between 2000 and 2010, US ethanol consumption grew from 1.6 billion gallons/year to 13 billion gallons/year; the latter figure represents a hypothetical nationwide uniform ethanol-gasoline blend level of almost 10 vol% (Anderson et al., 2012a). In fact, virtually all the ethanol used in US transport is used in the form of E10 (a mixture of 10 vol% of ethanol in ‘gasoline’) which has been available in the US since the 1980s but is now widespread due to the political and environmental developments described above.

A ‘blend wall’ has arisen due to all the low-level ethanol-gasoline blends, often referred to as ‘gasohol’, being close to the maximum ethanol content (10 vol%) which vehicle manufacturers will allow for a user to remain within their warranty provision. In order to address this issue, the EPA has granted two partial waivers (EPA, 2010b, 2011b) which allow, but do not mandate, the introduction of E15 for use in light-duty vehicles of model year 2001 and later. The approval process is inherently slow since it involves extensive testing and is open to challenge by vehicle manufacturers.

Anderson et al. (2012) examine various scenarios for ethanol introduction, including the most optimistic, where the RFS2 targets are met. This latter scenario would lead to notional uniform ethanol blend levels of E24 by 2022 and of E29 by 2035 (assuming a 2% annual growth post-2022). In the absence of significant growth in E85 sales, this could not be implemented with new waiver approvals since these levels of ethanol are well beyond the tolerance capability level of current conventional vehicles. New vehicle and engine specifications would be required, together with the maintenance of a protection grade fuel (E10 or E15) for existing vehicles. A discussion of octane targets for these blends is also included.

Flexible-fuel (or flex-fuel) vehicles (FFVs) are capable of using ethanol in concentration levels of up to 85 vol% (E85). However, despite the registration of over 9 million such cars at the end of 2011, incentivized by the rating of FFVs under CAFE legislation (EPA, 2010c), representing 4% of the US LDV fleet (Anderson et al., 2012a), only 1% of the total ethanol use has been in the form of E85 sales, as shown in Fig. 13.5(a) and (b). This has led to a reappraisal of the Alternative Motor Fuels Act (AMFA) credits given to FFVs in future CAFE and EPA fuel economy and emissions legislation (EPA, 2011c). The fuel economy rating of an FFV is calculated by taking the harmonic mean of the fuel economy on gasoline (or diesel) and the fuel economy of the alternative fuel divided by 0.15. Thus, a vehicle which achieves 25 miles/US gallon using gasoline and 15 miles/US gallon using E85 would be rated having a fuel economy of (2/((1/25) + (0.15/15)) = 40 miles/US gallon. This assumes that the FFV operates on the alternative fuel 50% of the time. US Congress has extended the FFV incentive (called the ‘dual-fuelled’ vehicle incentive) to model year (MY) 2019 but has provided for its phase-out between MY 2015 and MY 2019 by gradually reducing the allowed limit of the maximum fleet fuel economy increase for a manufacturer due to this credit from 1.2 miles/gallon. (MYs 1993–2014) to 0 miles/gallon after MY 2019 in 0.2 miles/gallon decrements over that period (EPA, 2011c). FFV credits for MY 2020 and beyond will reflect the ‘real-world’ percentage of usage of the alternative fuels (EPA, 2011c).

Ethanol in high-concentration form, E85 (now defined by ASTM D5798 as 51–83 vol% ethanol in gasoline), has suffered both from limited availability and uncompetitive pricing on an energy basis in the US. There are fewer E85 pumps in the US than there are EV charging stations (about 2,500 versus about 6,750 in 2012). The requirement for FFVs to run on any concentration of ethanol in gasoline, from 0% to 85%, also means that the vehicles are generally not configured to be capable of exploiting the high octane numbers of the higher level ethanol blends – this prevents such vehicles being able to offset the reduction in volumetric energy content of the fuel by increasing the thermal efficiency of the engine. Conversely, if FFVs were to be optimized to have high compression ratios in order to reduce their volumetric fuel consumption on E85 (as discussed in Section 13.4.2), they would give an unattractive increase in fuel consumption when operating on fuel with lower ethanol concentration such as E10 if the octane level of the BOB is maintained (Anderson et al., 2012a). Although this may be viewed as a mechanism to incentivize the FFV customer to use E85, the proposition is only reasonable if there are sufficient fuel stations offering the fuel.

European Union

Whilst the European gasoline specification EN228 allows up to 3% methanol in gasoline, there has never been any specification for high concentration of methanol in Europe analogous to the ASTM 5797 standard. In the EU, by the end of 2020, the Renewable Energy Directive (RED) and the Fuel Quality Directive (FQD) (EC, 2009a, 2009c) together require that 10% of transport energy be supplied in renewable form and that the overall GHG intensity of fuels should be reduced by 6%. With diesel penetration at approximately 50% across the EU, it is possible that, due to lack of sufficient supplies of sustainable vegetable oils for biodiesel manufacture and some issues of achieving emission compliance of modern vehicles using more than 7 vol% of biodiesel, the RED and FQD targets may need to be met by supplying base fuel in the form of E20, when the lower volumetric energy density of ethanol is considered (Cooper, 2011). However, recent attempts to introduce E10 into the German market did not go well due to some customer confusion (Kramer and Anderson, 2012).

In contrast to the CAFE regulations which make it attractive to manufacture FFVs in the US, the fiscal penalties for GHG emissions from cars sold in the EU is based only on tailpipe CO₂ emissions. This gives vehicle manufacturers no incentive to spend even the small extra amount required in order to produce an FFV (ca. €100/vehicle). It has recently been suggested by a major transport energy supplier (Cooper, 2011) that attributing some CO₂ benefit to manufacturers will provide a more compelling reason for OEMs to make FFVs and thus produce a greater outlet for ethanol as an automotive fuel.

Despite the lack of apparent incentive, manufacturers such as Saab, Ford, Volvo, Renault, and VW have introduced FFVs into their vehicle range. The EU vehicle tailpipe CO₂ penalty system does, however, presently allow a 5% reduction in tailpipe CO₂ to be claimed for any FFV that an OEM sells, provided one-third of the fuel stations in the country in which the vehicle is sold has at least one E85 refuelling pump (EC, 2009b). In Sweden there has been co-ordinated activity to install E85 pumps so that by 2009 50% of the network was covered (Bergström et al., 2007a), rising to 59% in 2011 (Kramer and Anderson, 2012), and in 2008, 22% of all new car sales were FFVs (Kramer and Anderson, 2012), driven by government fuel tax relief, which made E85 cheaper than gasoline on an equal energy basis, and vehicle use initiatives. Kramer and Anderson (2012) show that this fuel tax benefit has been variable since 2005. They also show that the drop in FFV sales which occurred in 2009 coincided with a period when E85 had a cost disadvantage of 30% relative to gasoline.

The benefits of liquid fuels compared with their gaseous counterparts are again highlighted by the 2006 Swedish legislation requiring that all fuel stations above a certain size offer at least one alternative fuel: most stations covered by the law installed E85 pumps and storage tanks which, at €40,000–45,000, offered a ten-fold lower installation cost compared with biogas storage and dispensing equipment (Kramer and Anderson, 2012).

In the rest of Europe, legislation and incentives for high concentration ethanol use are largely absent and thus fuel pump availability for E85 and FFV sales remain sparse. For Germany in 2010 FFVs represented 0.05% of the 2.9 million new vehicle sales (Kramer and Anderson, 2012). It is, however, perhaps worth noting that for a vehicle at the 2011 EU average of 135.7 g CO₂/km, and at the highest proposed fine rate in 2015 of €95/(gCO₂/km), this represents a saving to the manufacturer of €541 per car (with the benefit limited to 5.7 g CO₂/km in this instance since the target would be achieved), which the authors contend is significantly greater than the additional costs of producing a vehicle which is flex-fuel capable E85 (Turner et al., 2012a).

China

Ethanol blends in gasoline have been used in five Chinese provinces since 2004; however, the use of methanol is favoured in order to avoid conflicts with food demand. Whilst China produced only 3% of the ethanol used in road transport globally in 2009 (US DoE, 2012a), it has dominated the production and use of methanol in this sector. The consumption of methanol in China, mainly in the form of M15, is around 3 million tonnes per annum (Niu and Shi, 2011) and is motivated by China’s large coal reserve which offers the potential of greater energy independence. Processes to convert coal to ethanol are also being investigated (Pang, 2011).

Volumetric energy density and stoichiometry

The impact of alcohol concentration on the volumetric energy density of gasoline blends with ethanol, methanol, and butanol is shown in Fig. 13.8. For alcohol concentration of 10 vol% in gasoline, the reduction in volumetric energy density relative to the gasoline is 1.5%, 3.3% and 5%, for butanol, ethanol and methanol, respectively, whereas for 85% alcohol concentration the respective reductions in energy density are 12.8%, 28% and 42.5% for these alcohols. With vehicles which use engines not optimized for use with alcohol fuels (which is the current situation, even with FFVs), the consequence of this reduced volumetric energy content is an approximately proportional increase in volumetric fuel consumption for operating cycles which are not sufficiently aggressive to exploit the higher octane numbers of the resulting fuel blends. A consequence of this is that it is difficult to achieve significant market penetration of fuels with high alcohol concentrations with a volumetric-based fuel taxation system, rather than one based on the energetic content of the fuel, or the non-renewable carbon component (Turner and Pearson, 2008).

As a corollary to the presence of the oxygen atom in alcohol fuels having an impact on reducing the volumetric energy content which reduces as the length of the carbon chain increases, Fig. 13.9 shows how the stoichiometric air–fuel ratios of gasoline-butanol mixtures are closer to that of gasoline than those of gasoline-ethanol and gasoline-methanol mixtures. This is one of the reasons that gasoline standards have oxygenate limits. The impact of the alcohol concentration on the oxygen mass fraction is shown in Fig. 13.9. The European EN228 standard for gasoline currently limits the mass fraction of oxygenates in the fuel to 2.7% – this limit corresponds to about 7 vol% of ethanol in the blend (E7) and 11 vol% of butanol (Bu16). Methanol is limited as a specific component to 3 vol%. The increase in this level to 3.7% specified in the Fuel Quality Directive (EC, 2009c) allows E10 and Bu15 blends. Methanol content is again limited to 3 vol%. For comparison, a 3.7% oxygenate limit allows approximately 22 vol% MTBE or ETBE in gasoline – these fuels can be made from methanol and ethanol, respectively, with a process energy overhead.

Vapour pressure

Vapour pressure is an important property of automotive fuels, influencing the start quality of an engine in cold ambient temperatures (if the vapour pressure is too low), and affecting the evaporative emissions from the vehicle (adversely if the vapour pressure is too high). Gasoline specifications stipulate allowable vapour pressures dependent on the season, geographical location and alcohol content – these are extensively reviewed by Andersen et al. (2010). Figure 13.10 shows how vapour pressures (calculated as Reid vapour pressure, or RVP) of methanol, ethanol and iso-butanol vary as a function of the volumetric concentration of the alcohol in a blend with a reference gasoline fuel (RF-02-03 (Turner et al., 2012b; Pearson et al., 2012b)). Both ethanol and methanol cause a peak in measured RVP at concentration levels between 5 and 10 vol%, increasing the value relative to the base gasoline by about 8 kPa and 23 kPa, respectively. This requires an adjustment in the vapour pressure of the BOB in order for the blend to remain below normal specified limits. The EU Fuel Quality Directive (EC, 2009c) allows a derogation from the maximum summer vapour pressure for low-level ethanol blends.

Figure 13.10 shows that, at high concentrations, the vapour pressure of the methanol and ethanol blends reduces below that of the gasoline blend stock: this occurs at about 80 and 45 vol% for methanol and ethanol, respectively, with the gasoline used in this case. At high concentrations this can cause low temperature cold-start difficulties in unmodified engines, but this issue is easily overcome in FFVs, particularly with direct fuel injection technology where injection into the hot compressed air is found close to top-dead-centre. Siewart and Groff (1987), Kapus et al. (2007), Marriott et al. (2008) and Hadler et al. (2011) discuss the successful use of fuel injection strategies to augment the quality of the engine start. For port-fuel-injected engines, measures such as heating the fuel rail can enable acceptable cold-start performance down to − 25 °C. Bergström et al. (2007a) report acceptable cold starts down to − 25 °C in the absence of additional technology on an engine with port-injection (PI) of fuel using a Swedish winter grade bioethanol (E75 with a Reid vapour pressure of 50). In the US, the ASTM D5798 standard for E85 can allow the ethanol concentration to be as low as 51% by volume ethanol – this provides a significantly easier blend for low-temperature starting.

Octane numbers

Table 13.1 shows the low-carbon number alcohols have a number of physico-chemical characteristics which are synergistic with modern engine designs, and in particular highly pressure-charge downsized engines. In particular, the high research octane numbers (RONs) of methanol, ethanol and iso-butanol are facilitators for improved combustion phasing and lower component protection over-fuelling (to control exhaust gas temperatures). The motor octane numbers (MONs) of these fuels are proportionally less high and thus their sensitivity (RON-MON) is very high relative to gasoline; nevertheless, these fuels would not be considered as having a low MON relative to most gasoline sold throughout the world.

Moran and Taylor (1995) posited that, because the intake manifold temperature is not controlled during the RON test, the high heat of vaporization of the low-carbon number alcohols would cause a physical cooling effect which would have a direct bearing on the knock-limited compression ratio. In contrast, the MON test uses a heater to try to control the intake manifold temperature to 149 °C, which is well above the respective boiling points of these alcohols and is thus not affected by this physical cooling effect.

Stein et al. (2012) showed that both denatured ethanol (97% ethanol, 1% water, 2% gasoline) and hydrous ethanol (94.2% ethanol, 5.8% water) are extremely knock resistant, allowing very high engine loads to be achieved at low speeds in an engine with a 14.0:1 compression ratio.

Performance

Nakata et al. (2006) used a high compression ratio (13:1) naturally aspirated port-fuel injected spark-ignition engine and found that torque increased by 5% and 20% using E100 compared with the operation on 100 RON and 92 RON gasoline, respectively. The full improvements in torque due to being able to run MBT ignition timing were apparent for E50.

Marriott et al. (2008) show significant performance benefits for E85 (RON measured as 107.7) compared with a 104 RON gasoline when used in a naturally aspirated direct-injection gasoline engine. Peak torque generated by the engine increased by 5% and peak power by about 4% at the same enriched air–fuel ratio. An increase in volumetric efficiency of about 3% was measured at the peak torque operating point. Smaller but still significant performance benefits were available from operation at stoichiometric conditions when using E85 fuel. The majority of the combustion-related benefit in performance using E85 was determined to come from a reduction in the cumulative heat energy rejected to the engine coolant.

Korte et al. (2011) and OudeNijeweme et al. (2011) show significant synergies between the high octane properties of alcohol fuels and pressure-charged down-sized engines. Alcohol blends were made up using a 95 RON blend stock so that the E10 blend had a RON of 99 and the 16 vol% iso-butanol blend (Bu16) had a RON of 98. Note that this value is higher than that of a typical commercial E10 blend, even within Europe, due to the use of a 95 RON gasoline blend stock. Octane numbers of alcohol fuels with various blend stocks are discussed by Anderson et al. (2012a, 2012b).

Other blends used were E22, which matched the 102 RON of the high-octane forecourt specification gasoline included in the study, Bu68 (RON 104) and E85 (RON 106). Korte et al. (2011) conclude that all the high octane fuels used show a synergy with down-sizing under high-load conditions and that such fuels enable either higher levels of downsizing or increased compression ratios to be used, leading to additional CO₂ improvements. In related work using substantially the same blends, Stansfield et al. (2012) showed that it is possible to use high-alcohol blends in an unmodified production vehicle which has not been sold as having flex-fuel capability. Here E85 gave appreciably the most power (approximately 20% more than 95 RON gasoline), with the other blends forming a fairly tight band lying between the two.

Efficiency

Using E100, a full-load thermal efficiency at 2,800 rev/min. of 39.6% was reported by Nakata et al. (2006), compared with 37.9% and 31.7% using the high- and low-octane gasoline, respectively. A thermal efficiency improvement of 3% was achieved using E100 over the 100 RON gasoline at a typical part-load operating point, where the engine was far from the area where knock becomes a limiting factor. The efficiency benefit using ethanol in the part-load region was attributed to the lower heat losses due to the reduction in combustion temperatures. At high loads, when the engine is knock-limited, spark timing must be retarded and this leads to higher exhaust gas temperatures. Typically a pre-turbine temperature limit is imposed. High-octane fuels enable more advanced combustion phasing which in turn requires lower levels of over-fuelling in order to remain within the temperature constraints.

Bergström et al. (2007a, 2007b) found that, using a production turbocharged ethanol-gasoline flex-fuel engine with port-fuel injection, the high knock resistance and concomitant lower exhaust gas temperatures experienced when running on E85 allowed the fuel enrichment level at full-load to be reduced to the extent that, for the same limiting peak pressures as those tolerated using gasoline fuel, stoichiometric operation across the engine speed range is possible. Kapus et al. (2007) found that for identical engine performance, the more favourable combustion phasing when operating on E85 at full load leads to less requirement for fuel enrichment giving a 24% improvement in efficiency compared with operation on 95 RON gasoline. Thermal efficiency improvements at full load of over 35% relative to 95 RON gasoline have been found using E100 in a direct-injection, turbocharged spark-ignition engine (Brewster, 2007) operating at high BMEP levels.

Korte et al. (2011) show detailed fuel flow rate maps for an engine with a peak BMEP of over 30 bar using the fuels described in Section 13.3.2. It was found that whilst an increase in fuel flow was required with the alcohol fuels in order to obtain the same load for part-load operation, when the load was above 20 bar BMEP, operation on E22 (rated at 102 RON) required a lower fuel flow rate than when 95 RON gasoline was used. In order to protect engine components from over-heating, ‘over-fuelling’ is applied. Around the peak power point, operation on E85 required approximately the same fuel flow rate as the 95 RON gasoline whilst the fuel consumption of E22 was about 20% lower. At the peak power point these fuel flow rates correspond to increases of thermal efficiency, relative to the 95 RON gasoline, of 35% and 38% for E22 and E85, respectively. The full-load torque curve of the engine could be achieved at stoichiometric air–fuel ratio using E85.

Dedicated alcohol engines

The greater dilution tolerance of methanol and ethanol was exploited by Brusstar et al. (2002) who converted a base 1.9-litre direct-injection turbocharged diesel engine to run on M100 and E100 by replacing the diesel injectors with spark plugs and fitting a low-pressure alcohol fuel injection system in the intake manifold. Running at the 19.5:1 compression ratio of the base diesel engine, the PI methanol variant increased the peak brake thermal efficiency from 40% to 42%, while parity with the diesel was achieved using ethanol. Cooled exhaust gas recirculation (EGR) enabled the engine to achieve close-to-optimum ignition timing at high loads, while high levels of EGR dilution were used to spread the high efficiency regions to extensive areas of the part-load operating map. This was possible due to the lower molar air–fuel ratios and higher flame speeds of the fuels providing higher combustion tolerance to cooled EGR.

Vancoille et al. (2012) have confirmed the effectiveness of this approach for methanol, investigating the viability of throttle-less load control using EGR or excess air. Experiments performed on a single-cylinder engine showed that the EGR and excess air dilution tolerances of methanol are significantly higher than for gasoline. Using a turbocharged multi-cylinder engine with a 19:1 compression ratio, similar to that used by Brusstar et al. (2002), they found it was possible to use EGR levels of up to 30 mass% at a range of engine speeds and loads with limited cycle-to-cycle combustion variation. A peak thermal efficiency of 42% was obtained. The engine operating maps from this work were used by Naganuma et al. (2012) to model the potential benefits of a dedicated methanol-fuelled vehicle. The predictions indicated that a vehicle using an engine optimized for operation on M100 would increase the average engine efficiency on the NEDC from 22.8% for operation on baseline gasoline to 26% for M100, reducing CO₂ emissions from 167 g/km to 132 g/km. Still larger benefits are predicted for other vehicles (Naganuma et al., 2012).

Pollutant emissions, deposits, and lubricant dilution

Bergström et al. (2007b) show that, using a vehicle with a port-fuel-injected LEV2 emissions-capable engine on the US FTP 75 cycle, NO_x emissions levels are reduced for all the ethanol blends tested without secondary air injection (SAI). CO emissions levels were increased by up to 50% for the highest concentration ethanol blends tested (E64 and E85) and unburned hydrocarbon emissions (uHC) were increased by about 75% using the E85 blend. They also show that when SAI is used, the uHC and CO emissions are significantly reduced. For uHC emissions, the levels for E85 were reduced by more than a factor of three and became independent of ethanol concentration. Bergström et al. (2007a) show that a similar technology engine calibrated to EU IV emissions compliance using gasoline would also be compliant using E85. Turner et al. (2012c) report emissions results for an EU V vehicle with a boosted DI engine, using a bespoke E85 blend and the iso-stoichiometric ternary blends of gasoline, ethanol and methanol described in Section 13.5.3. While emissions analysers are known to have a lower response to oxygenated species than to pure hydrocarbons, the level of reduction in uHC shown by Turner et al. (2012c) for their blends is of the order that, correcting using a ratio of 1.154 from the earlier work of OudeNijeweme et al. (2011) (measured by them for E85), still resulted in uHC emissions no worse than for gasoline.

Of the various types of uHC emissions, those of the aldehydes in particular are a potential concern, and especially those of formaldehyde. This is because in the metabolism of formaldehyde the crucial step is that from formic acid to carbon dioxide and water, which depends on folic acid and gives rise to a wide variation in fatal dose depending on the victim’s age, body mass and, in the case of women, whether they are pregnant or not. With alcohol combustion, aldehydes are intermediate species of the oxidation of the fuel: methanol primarily yields formaldehyde and ethanol acetaldehyde, together with lower amounts of formaldehyde. Early work, presumably with older engine and emissions control technology than is commonly used today, suggested that generally some changes to catalyst formulation may be necessary to ensure long-term catalyst durability with methanol (Nichols et al., 1988), but this is not expected to be an issue with present-day technology: flex-fuel vehicles have been shown to be capable of meeting limits for formaldehyde when operated on E85 (West et al., 2007) and are expected to be able to do so for other alcohols such as n-butanol (Gingrich et al., 2009). It is known that aldehyde emissions can be successfully neutralized by the type of three-way catalysts typically used to control emissions of modern spark-ignition engines (Menrad et al., 1988; Wagner and Wyszyński, 1996; Shenghua et al., 2007). Gasoline may actually yield greater challenges with respect to aldehydes on legislated drive cycles in the future: Benson et al. (1995) reported acetaldehyde emissions being three times higher in a flex-fuel vehicle when operated on gasoline than E85, and in another study West et al. (2007) reported that gasoline had higher aldehyde emissions than E85 in a vehicle operated on the US06 drive cycle (which requires higher driving loads than is typically the case for other drive cycles). Furthermore, some potential technologies to improve the knock limit of down-sized engines such as cooled EGR have been found to increase aldehyde emissions from gasoline (Gingrich et al., 2009). Pearson and Turner (2012) discuss the potential health effects arising from aldehyde emissions from the combustion of the alcohols as well as giving an overview of other safety aspects of the fuels.

Cairns et al. (2009) found, using a boosted DI engine, that E22 blends produced the highest levels of smoke emissions in their study of ethanol and butanol blends with gasoline. Negligible smoke emissions were found using E85 fuel. Additionally they found that fuel injector deposits were highest using an E10 blend whereas the use of E85 at part load produced almost no deposits. Bergström et al. (2007a) found significant deposits accumulating on a port injector using E85 due to the presence of polyisobutylene components in the gasoline.

Price et al. (2007) measured particulate matter (PM) number concentrations for alcohol-gasoline blends in a direct injection engine. They found that PM number was lowest for E85, with 95 RON gasoline, E30 and M30 blends giving similar PM number concentrations, and M85 giving the highest level. They suggested that with the high-alcohol blends, the potential for PM to form at high load is due to the fact that the local AFR is above the saturation point of the mixture, leading to droplet burning which is only partially offset by the oxygen bound in the fuel. Later work by Chen et al. (2011) showed that under stoichiometric conditions high-ethanol-blend fuels gave higher PM emissions than low-blend ones when operated in a DI engine under homogeneous stoichiometric conditions. Under rich conditions, the PM output from an E10 ethanol blend was much lower, however, and they suggested that this is what gives much lower PM from vehicles operating on alcohol blends when they are operated on transient drive cycles. This suggests that any stratification in the combustion chamber will have some effect on PM formation with high-ethanol-blend fuels. Conversely, Stansfield et al. (2012) reported data from vehicle tests showing that the two high-alcohol blends that they tested, E85 and Bu68, both failed the EU V particulate mass limit, although it should be noted that the production vehicle that they were using had not been sold as being flex-fuel capable.

Bergström et al. (2007a, 2007b) and Hadler et al. (2011) describe the potential for high levels of oil dilution in the engine lubricant using ethanol, particularly if the engine is repeatedly cold-started in a cold climate and run for a short time. Bergström et al. (2007a) state that the build-up of fuel is not harmful as long as the engine is given time to reach normal operating conditions, preferably within ten cold start events.

Recycling CO₂

Synthetic carbon-neutral liquid fuels mimic the overall function of photosynthesis by reducing water and CO₂ to make hydrocarbon-based products. The energy input to the process can either be provided directly in the form of sunlight (to produce ‘solar fuels’) or may be off-peak renewable energy (see Section 13.6.3). By combining hydrogen with CO₂ it can be chemically liquefied into a high energy density hydrocarbon fuel. Clearly, if the captured CO₂ stems from the combustion of fossil energy resources, this approach is not renewable and will still result in an increase in atmospheric CO₂ concentration. Rather than a recycling process, it amounts to CO₂ re-use and offers the potential of a notional reduction in emissions of approximately 50% (Graves et al., 2011). In a closed-cycle fuel production process, ideally there is no net release of non-renewable CO₂. If the hydrogen generation process is via the electrolysis of water, this represents by far the greatest energy input to the process. For this reason the fuels produced in this way may be referred to as ‘electrofuels’, as they are essentially vectors for the storage and distribution of electricity generated from renewable energy. When the feedstocks are water and CO₂ from the atmosphere, the fuel production and use cycle is materially closed and therefore sustainable. Such a cycle offers security of feedstock supply on a par with that of the ‘hydrogen economy’, as the timescale for mixing of CO₂ in the atmosphere is sufficiently short to ensure a homogeneous distribution. With access to sufficient water and renewable energy, the process has the potential to provide fuel from indigenous resources for most nations.

CO₂ and water are the end products of any combustion process involving materials containing carbon and hydrogen. Further reactions to form carbonates are exothermic processes. The capture of CO₂ in inorganic carbonates and other media is a burgeoning area of research and some of the literature is described in detail in Graves et al. (2011) and Pearson et al. (2012a).

Whereas the adoption of battery electric or hydrogen fuel cell vehicles requires paradigm shifts in the costs of the vehicles themselves or their fuel distribution infrastructure, or both (Pearson et al., 2012a), the development of carbon-neutral liquid fuels enables a contiguous transition to sustainable transport. Drop-in fuels such as gasoline, diesel and kerosene can be produced from CO₂ (via CO) and H₂ via Fischer–Tropsch (FT) synthesis, but the simplest and most efficient liquid fuel to make is methanol (Pearson and Turner, 2012). Indeed the option to make gasoline is retained even if methanol is produced initially since the former can be made via the Exxon-Mobil methanol-to-gasoline process. In addition to being the simplest fuel to synthesize from CO₂ and water feedstocks, methanol provides much greater biomass feedstock diversity, as it can be made from anything which is (or ever was) a plant.

Fuel synthesis

Once hydrogen and CO₂ are available, the simplest and most direct route to producing a high-quality liquid fuel is the catalytic hydrogenation of CO₂ to methanol via the reaction:

${\mathrm{CO}}_2+3{\mathrm{H}}_2\to {\mathrm{CH}}_3\mathrm{OH}+{\mathrm{H}}_2\mathrm{O}\mathrm{\Delta }{\mathit{H}}_{298}^0=-49.9\mathrm{kJ}/\left(\mathrm{mol}.\mathrm{methanol}\right)$

During the production of methanol via direct hydrogenation of CO₂, by far the largest component of the process energy requirement is the hydrogen production (Pearson and Tuner, 2012; Pearson et al., 2012a). This is true of any electrofuel using hydrogen as an intermediate or final energy carrier. Assuming an electrolyser efficiency of 80% and a CO₂ extraction energy of 250 kJ/mol.CO₂ (representing about a 10% rational thermodynamic efficiency relative to the minimum thermodynamic work requirement of 20 kJ/mol.CO₂) gives a higher heating value (HHV) ‘electricity-to-liquid’ efficiency of about 45%, including multi-pass synthesis of the methanol and re-compression of the unconverted reactants (Pearson and Turner, 2012; Pearson et al., 2012a). In the late 1990s, Specht et al. (1998a, 1998b) measured total process CO₂ capture energy levels of 430 kJ/mol, in a demonstration plant using an electrodialysis process to recover the absorbed CO₂, representing a rational efficiency of less than 5%. Despite this low CO₂ capture and concentration efficiency, the measured overall fuel production efficiency was 38%.

Without policy intervention, the intermittent use of alkaline electrolysers, due to their limited current densities, is likely to be too expensive (Graves et al., 2011) to produce fuel under present market economics. Improvements to this technology are at an advanced stage of development (Graves et al., 2011; Ganley, 2009) and other promising technologies are emerging. Graves et al. (2011) describe the use of high temperature co-electrolysis of CO₂ and H₂O giving close to 100% electricity-to-syngas efficiency for use in conventional FT reactors. This ultra-efficient high temperature electrolysis process using solid oxide cells combined with a claimed CO₂ capture energy (from atmospheric air) as low as 50 kJ/mol (Lackner, 2009) leads to a prediction of an electricity-to-liquid efficiency of 70% (HHV basis). With a constant power supply this high overall efficiency enables the production of synthetic gasoline at $2/gallon ($0.53/litre) using electricity available at around $0.03/kWh (Graves et al., 2011). Doty et al. (2010) state that off-peak wind energy in areas of high wind penetration in the US averaged $0.0164/kWh in 2009 and the lowest 6 hours of the day averaged $0.0071/kWh. With more pessimistic values for the cost of CO₂ capture such as the $1,000/tonne quoted by House et al. (2011), the gasoline cost component due to the supply of the carbon feedstock alone might be as high as $7.5/gallon (about €1.30/litre). With 20% electrolyser capacity the cost of fuel synthesis could be as high as $4/gallon at $0.03/kWh electricity (higher current density electrolysers could reduce this to $2.2/gallon) (Graves et al., 2011). For perspective, a cost of $11.5/gallon is around €2.05/litre. Currently gasoline retail costs in the EU range from €1.14/litre to €1.67/litre including duties and taxes (which can be as high as €0.6/litre).

In a reconfigured system which bases fuel duty and taxation on non-renewable life cycle carbon intensity, a fuel made from air-extracted CO₂ and water might be commercially attractive in the medium term, i.e. in advance of the point where sequestration of air-captured CO₂ becomes economically feasible. Recycling of the CO₂, rather than sequestrating it after it has been removed from the atmosphere, cannot result in any net greenhouse gas (GHG) reduction. Its inclusion in a closed carbon cycle to make transport fuels, however, can potentially have the effect of rendering carbon-neutral the fastest growing GHG emissions sector as well as providing a spur to the development of air capture technology for CO₂ which may subsequently be used for sequestration purposes.