Emissions
Heat of evaporation of butanol is lower than that of ethanol, but higher than that of gasoline. Increased heat of vaporization is desirable particularly for direct-injection engines according to Wallner et al. 2012. Cooling effect can reduce in-cylinder temperatures, and consequently, NOx emissions and knock propensity.
In IEA-AMF Task 35-2, Rosenblatt et al. (2014) found that the use of low- to mid- level alcohol blends (E10, E15, E20, iB16) with GDI engines/vehicles gave mixed results; with some studies noting decreases in particles and others showing increases.
Tests with cars (post-catalyst)
Very limited data is available on the exhaust emissions with butanol containing gasoline. BP (2006) studied emissions with cars by using n-butanol as gasoline component. This study concluded that CO, HC or NOx emissions did not change significantly when 10 vol-% of n-butanol blend was compared to gasoline with the standard FTP cycle. In the highway cycle, n-butanol showed a slight reduction in CO emission.
Aakko-Saksa et al. (2011) observed that isobutanol containing gasoline decreased CO emission, but increased NOx emission with conventional and direct-injection gasoline cars at -7 °C, whereas an opposite result was obtained with the FFV car. Lower particulate matter emission was observed for butanol-containing fuels than for conventional gasoline with the direct-injection gasoline car. 1,3-Butadiene emission was higher in many cases for isobutanol or n-butanol containing fuels than for non-oxygenated gasoline. Butanol-containing fuels increased formaldehyde, acrolein, butyraldehyde, methacrolein, and propionaldehyde emissions. Sum of analyzed aldehydes was in most cases higher for butanol-containing fuels than for ethanol containing fuels. Emissions with n-butanol were higher than those with isobutanol. Aakko-Saksa et al. (2011) reported that butanol may reduce particulate matter associated PAHs from direct-injection car, but not necessarily Ames mutagenicity. For gasoline cars with indirect injection technology, particulate matter emission levels were low and no significant effect of butanol was seen.
In a direct-injection, spark ignition engine at varying speeds and loads, no significant difference was seen in CO and HC emissions between ethanol blend, n-butanol blend and baseline gasoline. E10 showed the highest, and 10 vol-% n-butanol blend the lowest NOx emissions. The ethanol blend produced the highest peak specific NOx due to the high octane rating of ethanol and effective anti-knock characteristics. (Wallner et al. 2009). Yang et al. (2009) studied blends containing up to 35 vol-% of butanol in gasoline. Engine raw HC and CO emissions were reduced, but NOx emission increased with increasing butanol content.
Cooney et al. (2009) reported of the results with a direct-injection gasoline engine by using n-butanol, isobutanol, and ethanol in blending ratios up to 85 vol-% of the oxygenated fuel. Engine combustion strategy was not changed, closed-loop lambda feedback maintained stoichiometric operation. CO and HC emissions remained unchanged with butanol blends, but decreased when ethanol was compared to gasoline. n-Butanol showed slight increase in NOx emission at low engine loads, but NOx results at high loads were dependent on the exhaust gas recirculation (EGR) valve lift. Duration of injection is longer for alcohols than for gasoline to achieve same loads, and this changes EGR valve lift.
Wallner et al. (2010) used a direct-injected gasoline engine, from which EGR was disabled. In this study both regulated and unregulated emissions were measured. Ethanol, n-butanol and isobutanol were used as blending agents in gasoline. The following results were achieved: NOx emissions decreased with increasing alcohol content; formaldehyde and acetaldehyde emissions increased with n-butanol and isobutanol; a reduction in aromatic hydrocarbon emissions was observed with increased alcohol content; butanol increased propene, 1,3-butadiene, and acetylene emissions. Thewas et al. (2011) observed reduced NOx emissions with ethanol, 1-butanol, and 2-butanol due to lower combustion temperature in homogenous combustion with direct-injection engine. In general, HC emissions and particulate matter emissions were higher at high loads indicating a worse mixture formation. Alcohol fuels lead to higher oil dilution under cold condition than regular gasoline. In this study, tetrahydro-2-methylfuran was tested, as well.