Emissions from high-level ethanol/gasoline blends (E85)

Evaporative emissions, CO, and HC

Fuel evaporative emissions are lower for E85 than for gasoline due to the E85 fuel’s low vapor pressure, potential reduction being approximately 30% (Yanowitz and McCormic 2009, Westerholm 2008, CRFA 2003). Nearly all FFVs have onboard vapor recovery system (E85 Handbook 2013, Martini et al. 2012). Haskew and Liberty (2006) observed lower permeation emissions for E85 fuel tested in the flexible-fuel vehicle system than for the low ethanol-content fuel. This is also seen in Figure 1.

Figure 1. The effect of ethanol on permeation (Stahl et al. 1992, Kassel 2006).

In the AMF Task 44 (Fan & Donglian 2016), carbon monoxide (CO) reduced, while total hydrocarbons (HC) increased by switching to E85 from E10. In the earlier studies, CO and non-methane hydrocarbon emissions (NMHC) have often been lower, or not significantly changed, when comparing E85 with gasoline at normal test temperature (Yanowitz and McCormic 2009, Graham et al. 2008, West et al. 2007). However, at -7 °C higher CO and HC emissions have been observed for E85 than for gasoline (De Serves 2005, Westerholm et al. 2008, Aakko and Nylund 2003).

In the AMF Task 36, Sandström-Dahl et al. (2010) observed that the HC emission results for E85 depend on the calculation methodology used. The flame ionization detector (FID) detects all carbon-containing compounds, also oxygenates, and not only hydrocarbons. This is taken into account in the calculation methodology used in the US for non-methane hydrocarbons (NMHC = HCFID – 1.04 x CH₄ – 0.66 x ROH) and non-methane organic gases (NMOG = ΣNMHC + ΣROH + ΣRHO). European emissions regulations does not recognize this behavior of FID. Furthermore, a higher exhaust gas density of 0.932 g/dm³ is used for for E85 (C₁H_2.74O_0.385) than for gasoline (0.619 g/dm³ C₁H_1.85,) in Europe, whereas in the US calculation method density of 0.619 g/dm³ is used for both fuels. An example of the effect of calculation methods on the HC results is given in Table 2 for an FFV using the E85 fuel. The results obtained by the European calculation method for E85 are close to the NMOG results obtained by the US calculation method.

Table 2. HC emissions from FFV car using E85 fuel with different calculation methods (Aakko-Saksa et al. 2011).

Aldehydes and ethanol

In AMF Task 44 (Fan & Donglian 2016), higher ethanol, formaldehyde and acetaldehyde emissions were observed for E85 than for E10 during the first minutes after the cold-start of car, while emissions were very when the TWC catalyst was warm. The formaldehyde and acetaldehyde emissions were generally from 2 to 20 times higher with the use of E85 compared to E0 or E10 when tested at 22-23°C, and respective increases were even more pronounced at cold testing temperature. Elevated acetaldehyde and formaldehyde emissions for E85 have also been observed in the earlier studies, particularly at low sub-zero test temperatures (Yanowitz and McCormic 2009, Graham 2008 and West et al. 2007, Westerholm et al. 2008, Clairotte et al. 2013). Over the hot-start tests, acetaldehyde emissions have been low for E85 and E5 regardless of the test temperature (Task 44 Fan & Donglian 2016, De Serves 2005, West et al. 2007). Warm catalyst effectively reduces acetaldehyde emission, while formaldehyde emission may remain at relatively high level even with warmed-up catalyst (Aakko-Saksa et al. 2014).

Ethanol emissions are higher for E85 fuel than for gasoline, particularly at low test temperatures, but not necessarily in the hot-start test (Task 44 Fan & Donglian 2016, Yanowitz and McCormic 2009, Westerholm et al. 2008 and West et al. 2007). At cold temperatures, even 2.5% of the ethanol that is fed into the engine may release unburned (Laurikko et al. 2013). Ethanol may also transform to acetaldehyde in atmospheric reactions (Clairotte et al. 2013).

Chiba et al. (2010) observed that ethanol, formaldehyde, and acetaldehyde emissions represent a major part of NMOG emissions during engine cold starts with E85 (Figures 2 and 3). The latent heat of vaporization of ethanol is higher than that of gasoline, leading to poor cold-startability and high organic gas emissions during cold starts. NMOG emissions increased by about 50% in cold starts when E85 was compared with gasoline, and ethanol represented the highest share of NMOG. The evaporative characteristics of E85 may also lead to the condensation of unburnt alcohol in the combustion chamber (Chiba et al. 2010).

Figure 2. Ethanol, formaldehyde, acetaldehyde, and total hydrocarbons in cold starts with E85 fuel (Chiba et al. 2010).

Figure 3. The effect of fuel ethanol content on NMOG emissions (Chiba et al. 2010).

Methane, 1,3-Butadiene, benzene, and toluene

Higher methane emissions have been observed for the E85 fuel than for gasoline at normal and at -7 °C, while 1,3-butadiene, benzene and toluene emissions are generally lower for E85 than for gasoline (Timonen et al. 2017, Clairotte et al. 2013, Yanowitz and McCormic 2009, Westerholm et al. 2008). In AMF Task 44, Fan & Donglian 2016 found that the dominating hydrocarbons for E85 were methane, ethene, xylenes and acetylene. Emissions of aromatic compounds (BTEX) were from 50% to 84% lower with use of E85 compared to E0/E10 at 22°C and at -7°C. Clairotte et al. (2013).observed that the exhaust gas contains more water when using E85 instead of E5, which may lead to decreased conversion of hydrocarbons in three-way catalyst.

Nitrogen oxides and ammonia

The NO_x emissions from FFVs running on E85 are generally lower than or at the same level as those from gasoline-fuelled cars (Yanowitz and McCormic 2009, Graham et al. 2008 and Westerholm et al. 2008). Yanowitz et al. (2013) noticed with nine in-use FFVs (Tier 1 and Tier 2) on average -25% reduction in the NO_x emission when  E76 was compared with E10. De Serves (2005) found lower NO_x for E85 than for E5 both in the cold-start and hot-start tests. NO_x consisted almost totally of NO indicating low NO₂ emissions. In AMF Task 44, Fan & Donglian 2016 found relatively low NO_x emissions for E85 and and only small differences between E85 and E10.

Ammonia emission from the TWC catalyst equipped cars have been observed at normal and cold test temperatures (AMF Task 44 Fan & Donglian 2016, Westerholm et al. 2008, Aakko-Saksa et al. 2014). Ammonia is formed in the TWC catalyst, and it is not primarily fuel-related emission (Mejia-Centeno et al. 2007). However, Clairotte et al. (2013) observed lower ammonia emission associated to E75–E85 than to E5, which could be due to leaner air-fuel ratio for E85 than for gasoline, or due to high water content of exhaust gas with E85.

Ozone forming potential

Yanowitz and McCormick (2009) reviewed studies of the ozone-forming potential (OFP) for the E85 fuel. With Tier 1 vehicles, despite of lower ozone reactivity of the exhaust gases for E85 than for reformulated gasoline, the OFP for E85 was higher. Cold-start emissions seemed to dominate the result, and studies did not consider atmospheric chemistry, nor the effect of E85 fuel on NO_x emissions. In the study by Graham et al. (2008), OFP was lower for E85 than for gasoline-fuelled FFV cars. Aakko-Saksa et al. (2011) observed higher OFP for E85 than for gasoline due to increased ethanol, ethene and acetaldehyde emissions, and also Clairotte et al. observed higher OFP for E75-E85 than for E5 with Euro 4 and Euro 5a cars at -7 °C.

Jacobson (2007) studied the effect of E85 on cancer and mortality in the US, Los Angeles basin. He concluded that the E85 fuel may increase ozone-related mortality, hospitalization, and asthma when compared to gasoline. The ratio between VOC and NO_x is critical in estimating OFP. Ozone may increase for example in locations where the baseline ratio of reactive organic gases to NO_x is below 8:1. Generally, both NO_x and VOC emissions from vehicles are decreasing with tightening emission limits.

Millet et al. (2012) pointed out that the atmospheric impacts of increased fuel ethanol use will be minimal, because significant sources of atmospheric acetaldehyde already exist. In addition, the potency-weighted toxicity will be reduced with E85 use.

Particles, PAH, and mutagenicity

An FFV using E85 emits lower particulate matter (PM) emissions than gasoline-fuelled cars, which was pointed out for example in Task 54 (Rosenblatt et al. 2020), Task 35-2 (Rosenblatt et al. 2014) and AMF Task 44 (Fan & Donglian 2016), and also in other studies (Yanowitz and McCormic 2009, De Serves 2005). In Task 54, direct and indirect observations (tailpipe measurements and secondary formation in the smog chamber) suggest a favorable effect on PM emissions with alcohol fuel blends, particularly in the case of GDI vehicles. Minor effects of alcohol content (E21 and iBu12) on PM emissions were observed in the Unites States study on start-stop operation, although these were not consistent in all cases. For one Euro-5 GDI vehicle, ethanol blending gave substantial reductions in genotoxic potential of the eight carcinogenic PAH compounds, most pronounced in the hot WLTC with reductions of 77% and 84% for E10 and E85 respectively.

At cold test temperature of -7 °C, PM may be elevated for E85 probably due to its poor cold-start behavior (Westerholm et al. 2008). The PM emissions are generally low for the port fuel injected (PFI) cars at normal temperature (below 1 mg/km) (Westerholm et al. 2008). In AMF Task 44, Fan & Donglian (2016) observed that the direct-injection spark-ignited (DISI) car using E85 achieved as low PM as the PFI gasoline car.

In AMF Task 44, Fan & Donglian 2016 studied also number of particles (PN) besides PM emission. Number of “wet” and “dry” particles were low for E85. The use of E85 decreased dry PN emissions rate by 78% to 90% compared to E0. A shift to a lower primary peak diameter (from 70-80nm to 34nm at 22°C) was observed due to the use of E85.

Under the AMF Task 35-2, Rosenblatt et al. (2014) investigated the particle size distributions and PN emission rates of DISI vehicles using E85 fuel and operated over different drive cycles and at different ambient temperatures The rate of PN emissions was reduced by 70-90% between E85 and E0, and the distribution peak occurred at a smaller particle size (Figure 4). Results were consistent between different laboratories indicating the potential of E85 to mitigate particle emissions from DISI engines under a variety of operating scenarios.

Figure 4. Average particle number size distributions for a FFV operated using E0, E10 and E85 fuels over the FTP-75 (EPA Federal Test Procedure) and NEDC (New European Driving Cycle) at different ambient temperatures. (Rosenblatt et al. 2014, AMF Task 35-2).

In the Task 35-2, Rosenblatt et al. (2014) draw the following key conclusions (Figures 4 and 5):

• The use of low- to mid- level alcohol blends (E10, E15, E20, iB16) with DISI engines/vehicles gave mixed results; with some studies noting decreases in particles and others showing increases. In contrast to the low level ethanol blends the E85 studies did yield consistent results indicating the potential to mitigate particulate emissions from DISI engines.
• E85 can reduce particle emissions from DISI engines under varying operating conditions and ambient temperatures.
• Number of particles was roughly an order of magnitude lower with E85 as compared to E10 and resulted in reductions in the range of 70 -90%.

Figure 5.  Particle number concentration for gasoline (no oxygen), ethanol and i-butanol (E10 and iB16, same oxygen content) and E85. (Rosenblatt et al. 2014, AMF Task 35-2).

Also Szybist et al. (2011) observed low PN emissions from DISI car when using E85 fuel, comparable to emissions from PFI. E85 enabled achieving the efficiency and power advantages of DI without generating the increase in PN emissions.

Timonen et al. (2017) studied E10, E85 and E100 in relation to primary emissions and subsequent secondary aerosol formation from a Euro 5 FFV. As the ethanol content of the fuel increased, the average primary PM decreased significantly. Similarly, a clear decrease in secondary aerosol formation potential was observed with a larger contribution of ethanol in the fuel. The secondary-to-primary PM ratios were 13.4 and 1.5 for E10 and E85, respectively. For all fuel blends, the formed secondary aerosol consisted mostly of organic compounds.

Particulate and semi volatile associated polyaromatic hydrocarbons (PAHs) together with cancer potency have been reported to be lower or at the same level for E85 fuel as for E5 at normal test temperature, while at -7 °C contradictory results have been obtained (Westerholm et al. 2008, Aakko-Saksa et al. 2014).

Summary

High-concentration ethanol fuels can be used in flexible-fuel vehicles (FFV). E85 is the most common fuel for FFVs today, although high concentration methanol blends are also used for example in China. Fuel injectors of FFVs are designed for higher fuel flows than those in conventional gasoline cars compensating the low heating value of E85. Energy efficiency of cars can be better when using E85 than when using gasoline, though volumetric fuel consumption is higher for E85. The energy efficiency of an FFV engine could be further improved by using an elevated compression ratio to utilize ethanol’s high octane rating, but engines still today represent a compromise as compared to dedicated ethanol cars. Direct injection technology provides improved fuel economy for spark-ignited cars, however, at cost of increased particle emissions. The use of E85  can mitigate this PM increase.

Compared with gasoline, E85 fuel reduces CO, NO_x, benzene, toluene and 1,3-butadiene emissions, and also fuel evaporative emissions. In the opposite, acetaldehyde, ethanol, formaldehyde and methane emissions tend to be elevated for E85 when running with a cold engine. Particular strengths for the E85 fuel are low PM and PAH emissions, as well as low cancer potency compared with gasoline at normal test temperatures. The ozone-forming potential of E85 fuel may be higher than that of gasoline, although this issue is complex involving reactions of a number of compounds in relation to regionally varying atmospheric conditions. Many emission species from FFVs are high after cold-start, particularly at low ambient temperatures, as excess ethanol injection is needed before the car warms up. When the engine and catalyst are fully warmed-up, the differences in exhaust emissions between E85 and gasoline are small. Summary results of Graham (2008) are shown in Figure 5, and examples of other studies in Table 3.

Figure 6. The effect of E85 on exhaust emissions (Graham 2008).

Table 3. Examples of changes in emissions when E85 is compared to gasoline (negative values = reduction in emissions, positive values = increase in emissions).

*) Number of studies reviewed in parentheses. ns = not significant
^aJacobson 2007   ^bGraham 2008   ^cYanowitz and McCormic (2009) ^dWesterholm et al. (2008)  ^eKarlsson 2008  ^fAakko-Saksa (2011), Clairotte et al. (2013)  ^gTimonen et al. (2017)

Emissions from intermediate blends (E30-E60)

Intermediate-level ethanol fuels are common as a result of mixed re-fuelling of E10 and E85. In addition, blender pumps are available in the US. Haskew and Liberty (2011) studied emissions with E32 and E59 fuels in comparison to E10 and E85 over three driving cycles. NMHC emission decreased with increasing ethanol content over the US06 high speed/load test, whereas such trend was not seen over the cold start FTP test. CO and NO_x did not indicate a trend with ethanol level. Fuel economy decreased with increasing ethanol content. The higher ethanol blends (E59 and E85) resulted in higher diurnal emission levels (different conclusion from CRC E-65-3), which was not in line with expectation of lower permeation of ethanol molecules for E85 than for E10. Acetaldehyde emission increased with increasing ethanol content with all three driving cycles. Formaldehyde emission increased with increasing ethanol content over two cycles, but not over the US06. The average Carter Reactivity of the exhaust decreased with increasing ethanol content of the fuels on the cold start FTP, but the results were not consistent for the US06 and Unified Cycle tests.

Yanowitz et al. (2013) determined the fuel economy and tailpipe emissions impact of operation on E40 on nine in-use FFVs (Tier 1 and Tier 2). Testing was conducted after a fuel change to study how the engine adapts the new ethanol concentration. The intermediate blends were not commonly available when older FFVs were built, and failure to rapidly adapt to a new fuel results in a non-optimal operation. Evidence was found of incomplete adaptation during the hot test immediately after refueling with E40, however, on average adaption to midrange blends was successful with average emissions falling between those of E10 and E76. Generally, the increase in emissions between E10, E40, and E76 was small compared to the measured differences between the different vehicle models.

In a study with low-oxygen fuels, E30 and E85 at -7 °C a reduction in NO_x emission was observed with increasing fuel oxygen content for the Euro 4 emission level FFV (Aakko-Saksa et al. 2014). E30 and lower ethanol concentrations resulted in lower acetaldehyde, formaldehyde, ethanol, methane, ethene, and acetylene emissions when compared to E85. The emission level of 1,3-butadiene was very low in all cases. Acetaldehyde and ethanol emissions increased with increasing ethanol content of the fuel non-linearly: when changing from E30 to E85, acetaldehyde emission increased by 7.6 times and ethanol emission by 27 times. PM and PAH emissions were low and changes seemed not to be fuel-related with FFV. The indirect mutagenicity of PM extracts were lower for FFV than for the DISI car, but higher than for the MPFI car.

Emissions from low-level ethanol/gasoline blends (E10)

Generally, low-level ethanol/gasoline blends tend to increase evaporative emissions (see Figure 1). However, the results from different studies are not consistent. With “splash blended”, the vapor pressure is higher for low concentration ethanol blend than for gasoline. With “tailored” fuels, vapor pressure of ethanol blends are adjusted by using low volatility baseline gasoline (Vapor pressure). In some areas, legislation allows higher vapor pressure limit for ethanol containing gasoline.

Variation between the evaporative emission results with different cars is thought to be due to factors such as design and volume of carbon canister and vapor system. The cars with small carbon canisters compared to tank volume struggle with capacity, whereas larger canisters cope better with increased vapor pressure. (Australia 2008). Martini (2007) reported that a reduction in canister working capacity due to ethanol may increase evaporative emission. Studies in 70’s and 80’s addressed higher binding efficiency of ethanol to the activated carbons, as well as tendency of hygroscopic ethanol to carry water in carbon canister (Croes et al. 1999). Martini (2007) reported that heavy hydrocarbons and ethanol are hard to purge from active carbon in the canister. A trace-effect of ethanol may be seen after the tests with ethanol-containing fuels. Some carbon types preferentially absorb ethanol whilst others do not. Even 8 years old vehicles may emit artifacts, remnants of the solvents and adhesives, which is challenging for experimental work. (Australian study 2008).

Examples of studies, which do not show an increase in evaporative emissions with low-level ethanol blend

• Graham et al. (2008) did not find any statistically significant differences in diurnal and hot soak losses between E0, E10, E20 or splash-blended E10 fuel. Ethanol concentration of evaporative emissions followed the ethanol concentration of fuel. The MPFI and GDI cars, model years from 1998 to 2003 were tested +20 and -10 °C.
• Martini (2007) and Concawe (2006) reported that evaporative emissions from cars were dependent on vapor pressure of fuel, not on the ethanol content of fuel. Ethanol blends with vapour pressure around 75 kPa showed higher evaporative emissions than the fuels with vapour pressure in the range of 60-70 kPa, whether or not they contained ethanol. Conditioning carbon canisters of cars between the tests was challenging. Martini (2007) observed that evaporative emissions contained relatively high levels of light hydrocarbons (C4-C6), low levels of ethanol and significant concentrations of heavier hydrocarbons, like aromatics. The main sources of the light hydrocarbons are canister bleed emissions and breathing losses, while heavier hydrocarbons may originate through fuel permeation.

Examples of studies, which show an increase in evaporative emissions with low-level ethanol blends

• A 20-80% increase in evaporative emissions was observed with splash-blended E10 (AFDC 2009)
• At least double hot soak and diurnal losses with splash-blended E5 when compared to baseline gasoline. Benzene and aromatic emissions followed similar patterns as total evaporative hydrocarbon emissions. Alcohol emissions followed alcohol content of fuel, but not with all cars. (Australian study 2008)
• Tailored low-level ethanol blends did not increase refueling loss emissions, whereas running loss emissions with ethanol increased more than with gasoline. This was thought to be due to faster increase in vapor pressure with increasing temperature for ethanol than that for gasoline (link). In another referred study ethanol increased diurnal breathing losses likely due to differences in permeation abilities of fuels. (Wallace et al. 2009 references).
• Ethanol blends increased diurnal permeation rates compared to E0 gasoline, but no significant differences in ethanol or aromatics contents was observed (Haskew et al. 2006). Despite of higher evaporative emissions with ethanol blends, the average specific reactivities of the permeates were lower for the ethanol blends than for E0. The diurnal permeation rate was on average from 177 mg/day for E0, 484 mg/day for E10 and only 36 to 64 mg/day for “Zero Fuel Evaporative Emission” car. Study included E0, E6, E10 and E20 fuels and model year 2000, 2001 and 2004 vehicles. 
• In the in-use program in Sweden 20 of 50 cars tested exceeded the 2 grams limit of evaporative emissions, which was likely due to 5% ethanol content of gasoline in Sweden. (Åsman 2006).
• The level of evaporated HCs was around 200 ppm with 63 kPa gasoline, and 340 ppm with E10 blended in the 70 kPa gasoline. Butane was used to adjust vapor pressure of gasoline, and it was the main component that vaporized during the tests. The evaporation of ethanol followed the similar trend as evaporated HCs. (Egebäck 2005). 
•  Tailored E10 decreased diurnal, but increased hot soak evaporative emissions ending up to a slight increase of evaporative emissions. Higher hot soak emissions were explained by distillation characteristics. (Environment Australia 2002 references).

Nitrogen oxides

Larsen et al. (2009) reviewed literature on the exhaust emissions with ethanol blends, and noticed that results regarding NO_x emissions are contradictory. In the old studied, differences in NO_x emissions with low-level ethanol blends ranged from 5% decrease to 5% increase when compared to base-line gasoline (CRFA 2003). A study with six cars from 90's showed an increase in NO_x emission at higher ethanol concentrations than 12 vol-% (Environment Australia 2002). An Australian study (2008) did not find a clear trend for the NO_x emissions.

Graham et al. (2008) observed that NO_x emission increased as ethanol content increased mainly during the cold-start of test cycle and during aggressive driving conditions. There was no significant difference in NO_x emissions during stabilized driving. Increase in NO_x emission was substantial for E20 fuel at -20 °C.

Durbin et al. (2006) observed that the effect of ethanol content on NO_x emissions may be dependent on the distillation characteristics. NO_x emissions increased with increasing ethanol content at the low T50 (50 vol-% evaporated temperature), but no effect was seen at the higher T50 level.

The behaviour of the NO_x emission with ethanol-containing fuels was car-dependent in a study by Aakko-Saksa et al. (2011).  NO_x emission decreased with increasing ethanol content for the FFV, whereas it increased for the MPFI and direct-injection gasoline cars at -7 °C.

Flame temperatures for alcohols are lower than for e.g., aromatics, which could lead to lower NO_x emissions for alcohol fuels than for gasoline. However, this is not the case in old cars without close-loop control due to the leaning effect that raises combustion temperature. With modern cars, an increase in NO_x emissions with ethanol may indicate lean mixture in some driving conditions with an influence on the performance of the exhaust catalyst.

Nitrous oxide

Graham et al. (2008) reported that the nitrous oxide emission (N₂O), strong greenhouse gas, tends to increase with increasing ethanol content. Formation of N₂O depends on the performance of the exhaust catalyst, which may generate N₂O at certain driving conditions. Old cars from 70’s and 80’s did not show significant influence of 10% ethanol blend on N₂O emission (Environment Australia 2002).

Carbon monoxide and hydrocarbons

Larsen et al. (2009) reviewed literature on the exhaust emissions with ethanol blends. They reported that tailpipe CO and HC emissions generally reduced with increasing ethanol content. This was a conclusion also in the reviews by AFDC (2009), Niven (2005) and Karman (2003).

With old cars, enleanment of air to fuel ratio with ethanol leads to a decrease in CO and HC emission compared to baseline gasoline. For instance, in one study E10 fuel resulted in a 25-30% reduction in CO emission, and about 7% reduction in HC emission when compared to baseline gasoline. (CRFA 2003).

Australian study (2008) reported that CO and HC emissions decreased with E5 and E10 fuels compared to baseline gasoline, but the trends for new cars varied more and absolute differences were smaller than for older cars. For cars with closed-loop systems and catalysts, benefits in CO and HC emissions with ethanol are gained mainly at cold start or heavy acceleration (Graham 2008, Environment Australia 2002).

Graham et al. (2008) reported that tailored E10 showed lower CO emissions than baseline gasoline at normal and at -10 °C temperatures, whereas splash-blended E10 resulted in 35-50% higher CO emissions than tailored E10 at cold test temperature. At 20 °C, there was no statistical difference in NMHC or NMOG emissions between the fuels.
Durbin et al (2006) studied the effect of ethanol (0, 5.7, and 10 vol-%) and volatility parameters (mid-range and back-end volatility) on the exhaust emissions. CO emission decreased by 6-18% when ethanol content of fuel increased from 0 to 10%, depending on T50 temperature. NMHC emissions increased with increasing ethanol content, if distillation T90 temperature was high.

1,3-Butadiene, benzene and methane

Generally, tailpipe benzene and 1,3-butadiene emissions reduce with increasing ethanol content (Larsen et al. 2009, Niven 2005 and Karman 2003, Graham 2008, Aakko-Saksa et al. 2011). If ethanol is splash-blended gasoline, dilution effect leads to lower content of aromatics and olefins in gasoline resulting in lower benzene and 1,3-butadiene emissions. For non-consistent results on emissions of benzene and 1,3-butadiene emissions with ethanol blended fuels has been thought to be due to the performance of the exhaust catalyst in the cold-start (Environment Australia 2008). Durbin et al. (2006) noticed that in addition to ethanol content also distillation characteristics have an impact on the benzene and 1,3-butadiene emissions.

Larsen et al. (2009) reported that methane emissions increase when ethanol is added into gasoline, whereas Graham et al. (2008) reported that methane emissions seemed to decrease with increasing ethanol content of fuel.

Aldehydes and ethanol

Acetaldehyde emission increases substantially when ethanol containing fuels are compared to gasoline. Formaldehyde emission increases also when ethanol is used as low level blending component for gasoline. (Environment Australia 2008, Graham 2008, Durbin 2006, Aakko-Saksa et al. 2011).

In the study by Durbin et al. (2006), addition of 10% ethanol into gasoline increased acetaldehyde emissions by 73%. In this study, an interaction between ethanol content and the T50 temperature was observed. Graham et al. (2008) observed increase in acetaldehyde with both splash-blended and tailored E10.

An increase in acetaldehyde emissions have been observed mainly during cold-start and aggressive driving, not during stabilized driving (Graham et al. 2008), and especially at cold test temperatures (Aakko-Saksa et al. 2011). Acetaldehyde emissions are efficiently removed by the catalyst, which reduces risk associated acetaldehyde emissions (CRFA 2003).

Ethanol emissions are high for ethanol containing fuels at cold-start, and especially at cold temperatures (Aakko-Saksa et al. 2011). However, with warmed-up engine ethanol emissions are typically below detection limit with E10 and E20 fuels (Graham et al. 2008). Ethanol may hung-up in vehicle system leading to carry-over effect (ethanol found even with E0 fuel).

Particulate matter, PAH and mutagenicity

Particulate matter emission (PM) is not considered as significant emission from conventional gasoline fuelled cars, whereas PM from direct-injection gasoline cars may be substantial. Generally, ethanol reduces PM emissions compared to gasoline. In the study from Australia (2008) the PM emissions were 35% lower for E10 fuel than for gasoline with model year 2006 or newer cars.

Indication of lower particulate PAH emissions and mutagenicity of particulates with ethanol when compared to MTBE containing reformulated gasoline has been reported (Pentikäinen 2004, Aakko 2003). Aakko-Saksa et al. (2011) reported that ethanol may reduce particulate matter associated PAHs from direct-injection car. The same trend was not seen for Ames mutagenicity. For gasoline cars with indirect injection technology, particulate matter emission levels were low and fuel related differences were not observed.

Ozone forming potential

AFDC (2009) summarized that when tailpipe and evaporative emissions with E10 are considered, ozone forming potential of exhaust is increasing when compared to gasoline. This is due to increased evaporative emissions, acetaldehyde emissions and NO_x emissions. However, Graham et al. (2008) reported that E10 and E20 blends did not affect the ozone forming potential for the MPFI gasoline cars, andt decreased ozone forming potential for a direct-injection gasoline car. Aakko-Saksa et al. (2011) did not observe significant effect of low-level ethanol blends on ozone forming potential.

Australian study (2008) modeled photochemical smog using Sydney as a sample area. In these conditions, an increase in peak ozone concentration was seen in E5/E10 scenarios. Reduction in tailpipe HC and CO emissions with ethanol did not compensate the increase in volatile organic emissions.

Summary

Car technology has developed drastically in 10-20 years. Many of the studies with low concentration ethanol blends have been carried out with cars that do not represent today’s technology. Emission level from new cars tends to be low, and thus the effect of fuel on the exhaust emissions is small at normal temperature. Differences in emissions between fuels are mainly seen at cold-start, heavy-driving conditions and at cold temperatures.

The major drawback of adding low-level ethanol into gasoline is an increase in volatile organic compounds and acetaldehyde emissions. Exhaust catalyst can efficiently remove aldehyde emissions, but it is not operating properly e.g. during cold-start. The same applies to ethanol emissions.

Ethanol has generally a positive impact on tailpipe CO and HC emissions. For NO_x emissions, contradictory results are reported. Benzene and 1,3-butadiene emissions seem not to be influenced by ethanol content of gasoline, if olefins and aromatics contents of fuel do not change. Particulate matter emissions generally decrease with ethanol containing gasoline.

Ozone forming potential may increase with ethanol/gasoline blends due to the increased evaporative and acetaldehyde emissions. However, this phenomenon is complex process dependent on many factors, such as NO_x emissions and climatic conditions.

Summary of emission studies is shown in Figure 7 and Table 4.

Figure 7. The effect of E10 on exhaust emissions (Graham 2008).

Table 4. Changes in emissions when E10 is compared to gasoline (negative values = reduction in emissions, positive values = increase in emissions).

Emissions from ethanol in diesel engines

In IEA-AMF Task 46 (Nylund et al. 2016), new commercial ignition improvers for alcohol application in diesel engine (Scania concept) are now available, and two of these were studied together with a well-known ignition improver. The project showed that neat alcohols can be applied at very high compression ratios in diesel engines with the same performance as conventional diesel fuel. The performance was achieved by applying different ignition improvers in the form of new as well as a well known additive. Blends of alcohols and diesel showed a reduction in particulate emissions from a compression ignition engine with lower compression ratios.These new ignition improvers worked well with ethanol in a Scania compression ignition engine with a very high compression ratio (28:1). For methanol, the fuel system of the engine needs to be modified in order to manage the higher amounts of fuel flow needed due to the lower calorific value of the fuel. The relative specific fuel consumption measurements are shown in Figure 8. The injection timing needs to be optimized for the individual fuels (alcohol + ignition improver), since the rate of heat release was found to differ from one fuel to the other. Also the effect of the changes in fuel water content on the engine performance was studied. Decreased fuel water content increased both CO and NO_x emissions, whereas adding water reduced both these emissions marginally in comparison with the commercial hydrous fuel. Applying the selective catalytic reduction (SCR) for NO_x control would diminish the role water for NO_x. In addition, different blends of diesel and alcohols, methanol, ethanol and butanol, were tested in another Scania heavy-duty diesel engine with a lower compression ratio (16:1). The addition of alcohols to diesel was found to decrease particulate emissions in general, and the gaseous emissions were unchanged, except at idle operation at which aldehyde, carbon monoxide, and hydrocarbon emissions increased.

Figure 8. Specific fuel consumptions (IEA-AMF Task 46 Nylund et al. 2016).

One part of the IEA-AMF Task 46 reported by Nylund et al. (2016) explored possibilities to reduce the additive dosing by injecting part of the fuel into the intake manifold. Using intake manifold injection, 25 % of the standard additive dosing was sufficient to achieve normal combustion (ignition delay and rate of heat release), however, at the cost of increased fuel consumption. Further testing to optimise, e.g. amount of pilot fuel and timing of main fuel injection, is needed to really show the potential of the concept. One way to take advantage of this concept would be to keep the dosing of additive as it is, but to reduce the compression ratio, thus lowering both mechanical stresses and costs for the engine. The Scania ethanol engine used for the testing was equipped with unit injectors. A common-rail fuel system would enable pre-injection without any additional hardware. A significant reduction of additive dosing might jeopardise cold starting.

Earlier generations of Scania's ethanol buses showed exhaust emissions similar to advanced diesel buses equipped with an oxidation catalyst or particle filters, but higher emissions than those of sophisticated CNG buses (particularly the NO_x emission). The smoke emission from ethanol buses was almost negligible (likewise buses on gaseous fuels).  Acetaldehyde emissions were high for ethanol buses, but the cancer risk index was low (similar EPA factors used as for E85). (Ahlvik 2001).

Nylund et al. (2011) studied two trucks and one bus equipped with Scania's new generation ethanol engines; all with the 8.9-litre 270 hp ethanol engine. As reference, three diesel trucks (Euro V), three diesel buses and two CNG buses (stoichiometric and lean-burn) were studied. In general, the NO_x emissions from ethanol vehicles were average, but PM emissions lower (buses) or significantly lower (trucks) compared to diesel vehicles without particulate filters (even 75% reduction in PM). The stoichiometric CNG bus showed lowest, whereas the lean-burn CNG bus the highest NO_x values. PM emissions were higher for ethanol than for CNG average (Figure 9). Energy consumption of the ethanol bus was some 8% higher, and for ethanol trucks marginally lower, than the average diesel. It was noticeable that for the CNG buses energy increase was close to 40%, respectively. The conclusions seems to be quite similar with the new and older generation of ethanol vehicles: exhaust emissions from ethanol vehicles are close to clean diesel vehicles equipped with NO_x and PM reducing technologies, whereas the sophisticated stoichiometric CNG vehicles are the cleanest. 

Figure 9. NO_x vs. PM emissions for buses. The results with ethanol bus is marked with red star. (Nylund et al. 2011).