Comparative analysis of the Performance and Emission Characteristics of ethanol-butanol-gasoline blends

Global warming and energy security being the global problems have shifted the focus of researchers on the renewable sources of energy which could replace petroleum products partially or as a whole. Ethanol and butanol are renewable sources of energy which can be produced through fermentation of biomass. A lot of research has already been done to develop suitable ethanol-gasoline blends. In contrast very little literature available on the butanol-gasoline blends. This research focuses on the comparison of ethanol-gasoline fuels with butanol-gasoline fuels with regard to the emission and performance in an SI engine. Experiments were conducted on a variable compression ratio SI engine at 1600 rpm and compression ratio 8. The experiments involved the measurement of carbon monoxide, carbon dioxide, oxides of nitrogen and unburned hydrocarbons emission and among performance parameters brake specific fuel consumption and brake thermal efficiency were recorded at three loads of 2.5kgs (25%), 5kgs (50%) and 7.5kgs (75%). Results show that ethanol and butanol content in gasoline have decreased brake specific fuel consumption, carbon monoxide and unburned hydrocarbon emissions while the brake thermal efficiency and oxides of nitrogen are increased. Results indicate thatbutanol-gasoline blends have improved brake specific fuel consumption, carbon monoxide emissions in an SI engine as compared to ethanol-gasoline blends. The carbon dioxide emissions and brake thermal efficiencies are comparable for ethanol-gasoline blends and butanol-gasoline blends. The butanol content has a more adverse effect on emissions of oxides of nitrogen than ethanol.


Introduction
Alcohols being oxygenated fuels have been developed as a strong alternative to gasoline by researchers. Earlier researches have shown that alcohols can reduce engine emissions as well as enhance engine performance [1]. The technical and economic feasibility of alcohols also play a part in selection of alcohol as an additive to gasoline. It is observed that the alcohols act as octane number enhancer in gasoline which improves its combustion and hence reduces the emissions [2]. Lower alcohols having carbon atoms up to three can improve engine knock resistance when added to gasoline, however higher alcohols shows degraded knock resistance in gasoline engines. Many researchers have focused their prime interest on ethanol as an additive to gasoline. A lot of research has already been carried out on ethanol-gasoline blends. It has been experienced that engine performance and emissions improve in a gasoline engine while using ethanol-gasoline blends. Generally, brake specific fuel consumption, brake power and brake thermal efficiency get improve as the ethanol content is increased to a certain level in the gasoline [3-4]. However, a different outcome was observed in terms of decrement of brake specific fuel consumption as reported by [5]. Ethanol is considered as a clean fuel and various studies prove its implementation in the gasoline engines. Ethanol has 34.73% oxygen content by mass which improves the combustion of gasoline in the fuel blends. An extra atom of oxygen ensures the reduction of emissions of hydrocarbons and oxidation of carbon monoxide as observed by [6][7][8]. Oxides of nitrogen are considered as the most undesirable emissions and its significance can be rated by the stringent emissions level as mentioned in European Union VI Emission Norms. Many researchers found that there is an increase in concentration of oxides of nitrogen with increasing ethanol percentage in the gasoline [3] [6]. Although there are several studies which shows that ethanol, when added up to a certain limit in gasoline, decreases the NO x emissions [8][11] [12]. A few studies also stated that NO x emissions depend on the operating conditions rather than the percentage of ethanol in fuel [5]. In addition to that it was reported by [8] that ethanol content allows gasoline engines to operate at higher compression as the knocking resistance of fuel improves.
Butanol is a higher alcohol and nearly bears the similar physical attributes to ethanol. Butanol is also a by-product of fermentation process in which acetone-butanol-ethanol (ABE) is produced. Butanol possesses higher heating value than ethanol that encourages researchers to adapt butanol in engines as a fuel. A number of studies showed that carbon monoxide and hydrocarbons in emissions decreased using butanol-gasoline blends [14][15][16][17]. NO x, formed at high temperatures, were found to increase on blending butanol with gasoline as reported by [16][17]. However, opposite results were obtained for NO x as studied by [14] [18]. A number of studies observed no significant changes in NO x for butanol-gasoline blends [13].

Materials
Ethanol and butanol were purchased from local chemicals supplier. Marketed gasoline was purchased from a fuel station under the regulations of the Government of India. Marketed gasoline in India contains 5% of ethanol as per the policy defined for petroleum products.

Preparation of Blends
Initially, marketed gasoline was treated with water to wash out ethanol contained in it. The marketed gasoline then separated through conical separating funnel to get pure gasoline. Seven test fuels were prepared at the time of experiments in chemistry lab. G100 represents pure gasoline that is blended with ethanol and butanol to obtain the test fuels. Test fuels prepared were G100, E5, E10, E15, B5, B10 and B15 where E and B designate ethanol and butanol blend, respectively followed by their respective percentages in the gasoline. The calorific value of all test fuels was measured on the Digital Bomb Calorimeter and density of all the test fuels was measured through the conventional method. A known volume of the test fuels was taken in a flask and mass of the considered volume was weighed in a mass balance set-up to obtain the fuel density. Fuel properties of all the test fuels are listed in the Table 1.

Experimental set up
The engine used in this study was a vertical single cylinder, four-stroke, water-cooled, multi-fuel engine with variable compression ratio. Compression ratio for gasoline mode of the engine was from 5:1 to 20:1. Engine specifications are given in Table 2. The engine shown in figure 1 was attached with an Eddy Current Dynamometer (Powermag) to achieve the specified loads. Eddy current dynamometer was fixed to a strain gauge type load cell for measuring applied load to the engine. Exhaust gas calorimeter was equipped with K type thermocouples to measure the temperature at different points. The exhaust emissions were measured by QRO 401 five gas analyser (Qrotech Make). Engine equivalence ratio was measured by analysing the exhaust contents with a precision of 0.001%. Fuel consumption rate was determined by a system consisting of a burette with two optical sensors. A differential pressure sensor was used to measure air flow rate.

Experimental procedure
For all test fuels, a single test run was made for each case. Pure gasoline (G100) was selected as reference fuel for evaluating the performance and emissions of other blended fuels. For experiments, loading at dynamometer was taken as a variable. Three different loads of 2.5 kgs, 5.0 kgs and 7.5 kgs were considered for the experiments. During the experiments engine speed was maintained at constant 1600 rpm. The compression ratio for the experiments was chosen to 8:1. Spark timing was already preset to 23° before top dead center (BTDC). The temperature of water running through engine and calorimeter was adjusted at 60° C. The engine operating parameters such as brake specific fuel consumption and brake thermal efficiency have been measured. Engine emissions such as carbon monoxide, carbon dioxide, oxides of nitrogen and unburned hydrocarbons have also been measured.  It is evident from Figure 2(a) and 2(b) that the BSFC has decreased with the increase in the load for every test fuel. BSFC is dependent on brake power and fuel consumption rate. This result suggests that the increase in brake power have dominated more the effect of increase in fuel consumption with the increased load [17]. Although heating value of ethanol is lower than gasoline, still ethanol-gasoline blends have exhibited lower BSFC in comparison to pure gasoline. One fact is that ethanol has higher heat of vaporization that makes the vapors of ethanol to absorb heat during combustion and leads to a cooling effect on the charge. This in turn makes the charge more dense. A dense charge produces more brake power [3]. But this trend changed for E15 which has higher BSFC with respect to E10. This shift in trend indicates that the lower heating value started to dominate the effect of higher heating latent heat of ethanol in the test fuel E15. Lower heating value means an increase in fuel consumption to produce same brake power [7]. From the Figure 2(b) it has been seen that adding butanol to gasoline has the same effect on BSFC as that of ethanol. This could be understand by the fact that butanol has a higher laminar flame velocity than that of pure gasoline which reduces the combustion duration to produce the same power as by pure gasoline [19]. B15 has shown the least BSFC. On comparing B15 with E15 it seems higher heating value of butanol (33 MJ/kg) as compared to ethanol (27 MJ/kg) resulted into lower BSFC of B15 [14].

Brake Thermal Efficiency
From the Figure 3(a) and 3(b) it can be seen that BTE has increased with increase in load for all the test fuels. Increase in load pushes more fuel to evaporate in the cylinder during compression stroke. This in turn decreases the charge temperature and consequently reduces the compression work. As the compression work is reduced, the net brake power increases [4,20].
Increasing ethanol content in the test fuels exhibits a better BTE than pure gasoline. This trend could be explained by the fact that the higher value of latent heat of vaporization of the ethanol increases the cooling effect on the fuel charge in the combustion chamber and it reduces the compression work [12]. The improvement in the BTE for ethanol-gasoline blends is continued till E10 test fuel. After that BTE has decreased for E15. Addition of ethanol in the gasoline increases the volatility and latent heat of the fuel blend. This results in drop of the charge temperature of the fuel blend. At the same time, further addition of ethanol in the fuel blends led to the decrement in the drop of the charge temperature as the specific heat of ethanol is higher than gasoline. Thus, adding ethanol to the fuel blend has two contradictory effects on the charge temperature. After 10% of ethanol in fuel blend, the effect of higher specific heat dominates and the drop in charge temperature decreases. Hence less work is obtained resulting in lower BTE for E15 as compared to E10 [12]. Brake thermal efficiency has increased with the increase in butanol in the test fuels as shown in the Figure 3(a). These results may be analyzed on the basis of the fact that butanol has low saturation pressure (0.33 psi) as compared to that of gasoline (4.5 psi). Lower saturation pressure of butanol allows its vapor to evaporate more. This results in decrease of charge temperature which in turn reduces the compression work. Hence, net output work increases and BTE increases [21].

Carbon Monoxide Emissions
All the fuels used have shown decrement in the concentrations of CO emissions with the load as can be seen in the Figure 4(a) and 4(b). This could be possible due to the fact that increasing engine load also increases the combustion temperature and leads to complete combustion. Another possible reason for the reduction in the concentrations of CO emissions could be the high level of excess oxygen at high loads as compared to that at low loads [4].The ethanol-gasoline blended fuels have shown lower CO emissions as compared to that of pure gasoline (G100). As the ethanol percentage blended in the gasoline increases from E5 to E15, the emissions further decrease as evident from the emissions data.E15 has shown the least CO emissions at every load than that of E5 and E10. Ethanol has less carbon than pure gasoline. Also adding ethanol to gasoline increases the oxygen in the fuel, hence oxygen-to-fuel ratio is increased for the same fuel dispersion pattern as for pure gasoline. Consequently, combustion becomes more complete for the blended fuels [3,4]. Figure 4(a). CO emission variation with load for G100, E5, E10 and E15 at CR 8 Figure 4(b). CO emission variation with load for G100, B5, B10 and B15 at CR8 Butanol-gasoline blends have shown lower CO concentrations as compared to that for pure gasoline in Figure 4(b). This happens due to the fact that the butanol-gasoline blends have higher A/F ratio and lower carbon content in comparison of pure gasoline. Thus, the fuel-air lean mixture of butanolgasoline blends ensures complete combustion of the fuel [16].B15 has shown the least CO emissions. Corresponding to the same percentages of ethanol and butanol in the gasoline, butanol-gasoline blends have shown lower CO emissions than the ethanol-gasoline blends. This is due to the fact that butanol has lower latent heat of vaporization (578.4 kJ/kg) and higher calorific value (33 MJ/kg) as compared to that of ethanol (latent heat of vaporization and calorific value of ethanol are 840 kJ/kg and 27 MJ/kg, respectively). These may result in higher combustion temperature when butanol is added to gasoline. Higher is the combustion temperature, more effective is the combustion and hence lower are the CO emissions [21].

NO x Emission
NO x emissions have increased for all the blends with increase in the load as evident from figure 5(a) and 5(b). This increase in NO x concentrations may be due to the fact that with increase in the load, incylinder temperature increases. The concentration level of NO x emission has increased with increased ethanol content in the gasoline. Ethanol causes fast flame propagation and combustion in fuel which in turn increases the combustion temperature. Hence NO x emissions increase with the increased ethanol content in the gasoline [6]. Addition of butanol in the gasoline has increased the concentration level of NO x in the emissions as compared to the NO x emission for pure gasoline as shown in the Figure 5(b). Butanol has faster flame propagation than that of pure gasoline which leads to a higher combustion gas temperature for butanol-gasoline blends. Another basis for this result could be formed on the fact that butanol has higher oxygen content and lower stoichiometric air-fuel ratio as compared to pure gasoline [22]. NO x are found to be higher for the same butanol content as compared to that for ethanol in the gasoline at medium and peak loads. Because of lower latent heat of vaporization of butanol (578.4 kJ/kg) in the fuel blends, the vapours of butanol would absorb less heat from the combustion zone and thereby the cooling effect would be lower. Hence, the cylinder temperature would be higher in case of butanolgasoline blends. In addition to that, higher heating value of butanol also leads to a higher cylinder temperature which increases the concentration of NO x in the emissions [21]. Feng et al [22] have also cited the lower latent heat of vaporisation of butanol as the primary reason for the increase in NOx emissions. Figure 5(a). NO x emission variation with load for G100, B5, B10 and B15 at CR 8 Figure 5(b). NO x emission variation with load for G100, B5, B10 and B15 at CR 8   NO x emissions have increased with increase in the load due to higher cylinder temperature at higher loads. B15 has exhibited the highest percentage rise of 234.4% in NO x emissions at the peak load with respect to the pure gasoline. In general, it has been noticed that content butanol have affected NO x emissions more thanethanol content. It can be concluded that ethanol-gasoline blends have shown better results in NO x emissions as compared to butanol-gasoline blends. Butanol's higher latent heat of vaporization must have caused these trends.