Ethanol Blended in Spark Ignition Engines
3. COMBUSTION CHARACTERISTICS OF GASOLINE-ETHANOL BLENDS IN LOW COMPOSITION
Differences in fuel properties require different setting operational parameters of machines for getting aim of combustion processes, namely large power, low emissions and fuel efficiency. Many studies applying the ethanol in spark ignition engines, either as a mixture with gasoline as well as a pure fuel. That's because some of the properties of ethanol are closer to gasoline so that the fuel is more properly used in spark ignition engines.
Table 1. Comparisons of Properties of Gasoline and Ethanol (5)
Properties Unit Gasoline/diesel Ethanol
Chemical formula - C5 – C12 C2H5OH
Molecular weight kg/kmol 114,15 46.07
C-fraction % mass 87,4 52,2
O-fraction % mass 0 34,7
H-fraction % massa 12,6 13,0
H/C Ratio atom 1,795 3
O/C Ratio atom 0 0,5
Specific gravity - 0,7 – 0,78 0,794
Density (at 15oC) kg/m3 750 – 765 785 – 809,9
Stoichiometric air-fuel ratio w/w 14,2 – 15,1 8,97
Kinematic viscousity mm2/s 0,5 – 0,6 1.2 – 1,5
Reid vapour pressure (at
37,8oC) kPa 53 – 60 17
Research Octane Number - 91 – 100 108,61 – 110
Motor Octane Number - 82 – 92 92
Cetane number - 8 5 – 20
Enthalpyy of formation
a) Liquid kJ/kmol -259,28 -224,1
b) Gas kJ/kmol -277 -234,6
Higher Heating Value (HHV) MJ/kg 47,3 29,7
Lower Heating Value (LHV) MJ/kg 44 26,9
LHV at stoichiometric mixture MJ/kg 2,77 2,7
Heat Laten Vaporization kJ/kg 380 – 400 900 – 920
Spesific Heat
a) Liquid kJ/kgK 2,4 1,7
b) Gas kJ/kgK 2,5 1,93
Freezing point oC -40 -114
Boiling point oC 27 – 225 78
Flash point oC -45 s/d -13 12 – 20
Auto ignition temp oC 257 425
Vapour Flammability Limits % vol 0,6 – 8 3,5 – 15
Laminar flame speed at 100kPa, 325K
cm/s
-33 -39
Distillation
a) Initial boiling point
% 45 78
b) 10 % 54 78
c) 50 % 96 78
d) 90 % 168 79
e) End boiling point
% 207 79
Water solubility % 0 100
Aromatics volume % 27,6 0
Toxic in large
A number of studies have shown that addition of ethanol into gasoline up to 20%
will increase engine performances, such as; power, torque, cylinders pressure and lower CO, HC and NOx compared to gasoline, without major changes in operation of machine settings. This is due to the low concentration of ethanol in gasoline is an octane booster to raise RON gasoline, thereby increasing engine power, torque and reduce detonation.
While the content of oxygen in ethanol helps complete combustion process that produces CO and HC emissions are lower.
3.1 CO and HC Emission
Combustion characteristics of ethanol are not only dependent on the percentage of ethanol in gasoline, but also influenced by the type and design of machines, injection and control systems as well as emissions handling system (6). The influence of concentration of ethanol to reduce CO and HC evidenced by several studies, including by Yang et al (4, 7). With addition of ethanol to 5% able to reduce CO emissions by 20%
and HC by 5.2%. A bigger drop will be generated if the percentage of ethanol added, where if ethanol up to 10% will reduce emissions of CO and HC respectively up to 45%
and 73% (9, 10, 11). Similarly, if ethanol is added to 20%, it will reduce emissions of CO and HC to 67% and 73% (6).
With the above results, contribution of oxygen in ethanol proved to be very effective to reduce emissions of CO and HC. This is due; the addition of ethanol 5-20%
can increase the value of lambda 0.7 to 8.4% (6). The improvements make process of can be achieved.
The condition is greatly assisting the process thus CO emissions continue to decrease with increasing concentrations of ethanol up to 20%. The HC emissions at a concentration of ethanol to 5% only able to decline to 5-7%, because HC is not only influenced by the availability of oxygen in the fuel but by several factors such as compression ratio and ignition timing. Nonetheless these emissions decreased significantly when ethanol added to 20% (6, 9).
It can be concluded that a mixture of gasoline and ethanol up to 20% do not require changes in operational settings on the gasoline engine. This is due to ethanol more as an octane booster that reduces detonation and increases the power and torque of the engine.
3.2 Nox and CO2 Emission
In contrast on NOx and CO2 emissions are increasing in addition to 20% ethanol.
This is due to the oxygen in ethanol has trigger on increase in cylinder temperature thereby forming NOx. Similarly, CO2 emissions will increase if the concentration of ethanol in gasoline increases. As mentioned earlier, oxygen in ethanol will create the conditions to reach stoichiometric mixture so product of combustion of hydrocarbon fuels particularly CO2 and H2O will increase.
Yung-Chen Yao (12) has successfully studies on combustion of gasoline and ethanol blended up to 15% without any adjustments on a machine), both in neither
ratios still in engine standard while duration of combustion too shorter with increasing concentrations of ethanol.
3.3 Engine Performances
Combustion of mixture gasoline and ethanol to 20% (E20) gives a very significant effect on the performance of spark engine without changing the engine settings. Engine performance usually characterized by a number of parameters, namely;
power, torque and fuel consumption.
Research conducted by Gholamhassan Najafi (3) using gasoline–ethanol up to 15% showed an increase in brake power of 4.78% on each additional 2.5% ethanol in gasoline. The increase was also in line with the increase in engine rotation. Similarly, torque will be increased by 2.8% when the concentration of ethanol increased from 5% to 15%. While the specific fuel consumption decreased by 1.9% with increasing concentrations of ethanol, although the SFC increased if the engine rotation is increased (Figure 1).
For specific fuel consumption values, different results obtained by Jitendra kumar (13) where the SFC increased if the concentration of ethanol is increasing (Figure 2), although its value has decreased with the increase in engine rotation. In load variations, SFC decreases with increasing speed engine, while in speed variations show the same trend when the load increases. These results are very rational considering the calorific value of ethanol is lower than gasoline.
Figure 1. Values of BSFC vs RPM(41)
Differences both of SFC by research Gholamhassan and Jitendra although engine specifications are relatively similar, due to research conducted by Jitendra not make changes to the standard setting machine. In these conditions the SFC value always increases with increases of percentage of ethanol. While by Gholamhassan perform a variety of ignition timing which is intended to optimize engine performance and minimize exhaust emissions by using response surface methodology (response surface methodology).
Combustion duration and laminar flame speed of ethanol is short so ignition timing should be advanced. It was intended that ethanol can achieve a peak pressure shortly before complete burning. Thus will be obtained in higher combustion efficiency and lower emissions. In the spark ignition engine, fuel ignition delay by advancing the ignition timing to be implicated in the increased fuel consumption (SFC).
4. COMBUSTION CHARACTERISTICS OF GASOLINE-ETHANOL BLENDS