Besides the renewability of raw materials used for their production, alcohol fuels are reported to be advantageous over petroleum derived ones thanks to their better environmental characteristics. The oxygen contained in alcohol molecules is supposed to affect combustion process and cause soot and particulate reduction; some studies show that there is the potential for reduction of NOx emissions. While there was much information collected about the use and combustion behaviour of lower-molecular weight alcohols, such as methanol and ethanol, substantially less effort was yet put to the research of the properties of butanol (especially n-butanol as a product of fermentation during ABE process) upon their use in internal combustion engines.

For the evaluation of emission characteristics, it is very important to study combustion processes at different air/fuel ratio and thermodynamic conditions. The combustion of neat butanol as well as its mixtures with other fuels or chemicals was studied (Agathou & Kyritis, 2011; Broustail et al., 2011; Dagaut & Togbe, 2008; Sarathy et al., 2009) to obtain combustion velocities and kinetic data for modelling processes of butanol oxidation at the conditions of engine cylinder. However, it must be noted that real-world emissions level is affected by the interaction between fuel itself and the engine used, mainly its fuel injection system and engine control unit together with emission control systems — catalytic converters, particulate filters, exhaust recirculation etc.

Although butanol properties (boiling point, viscosity, octane number) predetermine it for the use in spark ignition engines as a partial substitute for conventional gasoline, a number of studies were carried out using butanol/diesel fuel mixtures in compression ignition engines. The addition of butanol (or other alcohols) significantly increases volatility and decrease lubricity of diesel fuel, which requires additional measurements for their use in today’s diesel engines. Yao (Yao et al., 2010) studied emission characteristics of CI engine using diesel fuel containing 0 % to 15 % v/v n-butanol. By varying exhaust gas recirculation rates, they kept NOx emissions constant, while CO and PM (particulate matter) emissions significantly decreased with the concentration of n-butanol in the fuel. Rakopoulos et al. (Rakopoulos et al., 2010a) compared conventional diesel fuel, diesel fuel with 30% biodiesel (FAME), and biodiesel with 25 % n-butanol in turbo-charged CI truck engine; the experiments were focused on transient regimes causing temporary increase of pollutant emissions. Both FAME and butanol helped to improve the particulate emissions in the transient engine regimes, but in both cases the emissions of NOx increased. In stationary regimes at different engine speed and load, the authors (Rakopoulos et al., 2010b) determined emissions of all regulated pollutants. In all cases, the positive effect of butanol in diesel fuel was found on the emissions of particulates, NOx, and carbon dioxide, whereas hydrocarbon emissions slightly increased.

Much greater potential and possibility of utilization without necessity to solve technical problems has butanol used as a partial substitute of motor gasoline. The total miscibility with hydrocarbons, boiling point, flash point and other properties allow mixing butanol with gasoline in wide range of concentrations and combustion in common spark ignition engines. In comparison to other alcohols in the range of Q to C5 mixed to gasoline in concentrations matching fuel oxygen content, butanol does not differ significantly in its effect on the emissions of regulated pollutants (Yacoub et al., 1998). The emissions of total hydrocarbons decrease, while significant increase takes place in the emissions of aldehydes, whose main constituent was formaldehyde.

One of the substantial drawbacks connected with the use of alcohols in SI engines is the problem of cold starts especially in winter conditions. Difficulties caused mainly by high heat of vaporization have to be eliminated by greater enrichment of air/fuel mixture in the period in which the engine heats up. This, on the other hand, can bring an increase in emissions of unburned or partially burned fuel due to near zero efficiency of catalytic converter in the early period after engine start. Irimescu (2010) modelled the situation for gasoline/butanol mixtures at different ambient temperatures and successfully verified the results with those obtained in experiments with a port injection engine.

The effect of butanol (or other alcohols) use in spark ignition engines depends also on the technique of fuel injection before its ignition in engine cylinder. Conventional way to prepare air/fuel mixture is the injection of fuel into the engine intake manifold, where it evaporates and the mixture is drawn to the cylinder in the suction cycle. Some engine manufacturers offer engines equipped with direct injection of fuel into the cylinder. Such engines allow the use of advanced techniques of emission control, such as lean (stratified) mixture combustion connected with the use of sequential injection. The direct injection engine was used by Cooney (Cooney et al., 2006), who investigated the effect of ethanol and butanol in blends with gasoline used in a series of engine tests conducted at varied loads. They reported the increase in engine efficiency at higher engine loads by a 4% with either 85 % n-butanol or 85 % ethanol. The efficiency is reported to be affected by lower octane number of n-butanol, even though knock combustion was not observed, and, on the other hand, by the higher flame speed of alcohols. Faster combustion can increase the efficiency if combustion timing was adjusted, while lower octane number should decrease it.

In contrast to modern engines of current passenger cars, there are still applications where carburetted engines or engines with open-loop control of fuel injection are used, without the ability to compensate for air-fuel ratio of specific fuels. In such cases, butanol blends result in approximately 50% enleanment connected with oxygen content in fuel, compared to ethanol. The authors evaluated the effect of the use of butanol-gasoline mixtures on pollutants emission of four different passenger cars equipped with spark ignition engines — from older Euro 2 vehicle to modern multipoint injection turbocharged one. As a baseline, unleaded gasoline with addition of 4 % ethanol was used. Mixtures containing butanol were prepared by addition of 10 %, 20 %, and 30 % pure synthetic n-butanol to the same gasoline. The properties of the mixtures were modified with small amounts of isooctane, toluene, and petroleum ether to keep their octane number and vapour pressure, which deteriorated by the addition of butanol. Four test vehicles manufactured by Skoda were used with different engine displacement, power, and technology level (see Table 5).

The emission tests were performed on a vehicle dynamometer according to ECE 83 emission test with the determination of CO, HC, and NOx emissions during two driving cycles. In addition to the measurement of regulated emissions, samples were taken during both phases of ECE 83 test for determination of individual hydrocarbons and aldehydes. Basic
engine parameters were monitored during the tests using an engine diagnostic unit to detect possible abnormal operation states of engine control unit.

The addition of butanol to the fuels used caused only little change in regulated emissions (Fig. 6) measured in ECE 83 test. Although more significant changes were found in emission levels determined in individual ECE 83 test phases, with regard to regulated pollutants, total values show only the increase in NOx emissions for all three vehicles. As expected, the use of butanol caused also small increase in emissions of aldehydes, whose main constituent was formaldehyde.

Fig. 6. Effect of butanol in gasoline fuel on emissions of regulated pollutants (CO, HC, NOx) and aldehydes in ECE 83.03 emission test

4. Conclusion

The significance of the presented fermentation data lies in several fields:

• methodologically — fluorescence staining and flow cytometry proved to be very useful tools for nearly on-line evaluation of physiological state of clostridial population during the fermentation. Both method of discrimination of acidogenic/solventogenic status of individual cells based on fluorescence alternative to Gram staining and vitality staining by bisoxonol were never applied on bacteria of the genus Clostridium.

• the greatest attention was concentrated on the strain C. pasteurianum NRRL B-598 which was never studied before in such detail. The comparison of three types of fermentation arrangements, batch, fed-batch and continuous represents the unique set of data not usually available for the tested butanol producers. As the strain had somewhat distinct physiology from type C. pasteurianum strains and flow cytometry analysis displayed very short acidogenic metabolic phase and presumable overlapping of acidogenic and solventogenic phases, the strain itself and its behaviour is worth further investigation. Moreover, the strain can also be regarded the very promising hydrogen producer

• the best fermentation parameters, yield of ABE 37% and ABE productivity 0.40 g. LAh-1, were achieved using sugar beet juice as the feedstock and C. beijerinckii CCM 6182 as the microbial agent. In Europe and especially in the Czech Republic, the sugar beet has a potential to become significant source of sugar utilizable for non-food purposes. The abilities and the fermentation characteristics of the strain C. beijerinckii CCM 6182 (and neither its analog C. beijerinckii ATCC 17795) has not been studied intensively although the strain behaved like C. pasteurianum NRRL B-598 i. e. favourable butanol production kinetics consisting in onset of butanol formation during exponential growth phase was its typical feature.

• the preliminary experiments dealing with gas stripping as potential concentration and/or separation method for solvents from the fermented media confirmed feasibility of this solution under certain assumptions. The gas stripping must not affect adversely the fermentation and cost of the solvents transition from the gas into liquid phases must be minimized. However further ideally pilot experiments are necessary for full evaluation of gas stripping role in the butanol production.

With reference to the use of biobutanol as a fuel for transportation purposes, it can be

concluded:

• in comparison with other bio-components used for blending automobile fuels, especially bioethanol, biobutanol exhibits very attractive properties — high energy content, low water solubility, total miscibility with gasoline hydrocarbons, and appropriate boiling point and vapour pressure

• the use of gasoline containing high concentrations (10 % to 30 % v/v) of butanol did not negatively affect operational parameters of common spark ignition engines used in passenger cars representing current European vehicle fleet. Only slightly increased emissions of NOx emissions and production of aldehydes was found out during standard ECE 83 emission tests

5. Acknowledgement

This research could be performed thanks to financial support of projects No. QH81323/2008

of the Ministry of Agriculture of the Czech Republic, TIP No. FR-TI1 / 218 of the Ministry of

Industry and Trade of the Czech Republic, No. MSM6046137305 and No. MSM 6046137304 of the Ministry of Education, Youth and Sport of the Czech Republic.