The influence of fatty acid methyl ester profiles on inter-cycle variability in a heavy duty compression ignition engine
Introduction
Inter-cycle variability (or combustion variability) resulting mainly from in-cylinder cycle-to-cycle fluctuations in the fuel–air mixture, is a key factor that affects both engine efficiency as well as emissions [1]. A thorough understanding of this phenomenon is now pressing particularly with the advent of biodiesel fuels and blends where variability in the molecular structure of the fuel, due to different feedstocks, could lead to significant inter-cycle variations. Previous studies of combustion variability focused mainly on spark ignition engines and to a lesser extent on compression ignition engines operating with commercial diesel and alternative fuels including LPG and some biodiesels [2], [3], [4], [5], [6], [7]. Kouremenos et al. [6] used a single-cylinder diesel engine to study the following key parameters: peak pressure, peak pressure location, maximum rate of pressure rise, and the crank angle at which the maximum rate of pressure rise occurs. This work showed that the fuel pump system has no influence on the cyclic pressure variation. Also using a single-cylinder compression ignition engine, Selim reported an increase in inter-cycle variability when operating with gaseous dual fuels [5] and a reduction with jojoba methyl ester [7].
Biodiesels are mixtures of fatty acid methyl (or ethyl) esters derived from vegetable oils, animal fat, or algae by transesterification with the aids of methanol (or ethanol) as a solvent to lower the viscosity and surface tension sufficiently to enable adequate atomisation of the sprays in compression ignition engines [8], [9], [10], [11], [12], [13], [14]. A key feature of biodiesels, which makes them different from conventional petro-diesel, is the oxygen-bound in the fuels. While there are always two oxygen atoms in one fatty acid methyl ester, the oxygen content in the biofuels depends on the fatty acid ester profile, specifically carbon chain length and unsaturation level.
While oxygen in the fuel can enhance the combustion process, it results in a lower heating value. At the same lambda/equivalence ratio, mixtures of oxygenated fuel and air are always leaner compared to mixtures of hydrocarbon fuels and air [15], [16]. Moser et al. [17] compared blends of ultra low sulfur diesel with 20% of soy bean methyl esters and observed that oxygen in the fuel improves lubricity, increases kinematic viscosity, lowers sulfur content, and results in inferior oxidative stability. In order to account for the oxygen content in the fuel, Muller et al. [15] proposed that the oxygen ratio (OR) and oxygen fuel ratio (OFR) are more appropriate measures of stoichiometry than the standard air to fuel ratio (AFR). The OFR and OR are defined, respectively, as the ratio of total oxygen atoms in the fuels (OFR) or fuels and oxidizers (OR) to the total oxygen atoms required for stoichiometric combustion [15]. Higher oxygen content in a fuel results in a higher oxygen fuel ratio, and also a higher difference between the oxygen ratio and lambda. For instance, the oxygen ratio for hydrocarbon fuels (where OFR = 0) is exactly the same as lambda, while in an oxygenated fuel where OFR = 0.5, the difference between the oxygen ratio (OR) and lambda is 100% [15]. The differences between the oxygen ratio and lambda for the biodiesels tested here range from 3–7%.
Results from earlier studies of the effects of oxygen content in the biodiesels on combustion efficiency and emission remain controversial [18], [19], [20], [21], [22], [23]. Lapuerta et al. [22] and Yuan et al. [23] observed that the presence of oxygen in biodiesels does not lead to increases in NOx formation. However, others [19], [20], [21] found that oxygen in the fuels does result in increased NOx, attributable to higher adiabatic flame temperature, as well as reporting increased combustion efficiency and lower soot.
This paper investigates the inter-cycle variability arising from the use of biodiesels and biodiesel blends in a common-rail diesel engine. A range of fatty acid methyl esters, with different carbon chain length and unsaturation degree, are tested as pure fuels as well as blends with commercial diesel. The oxygen ratio is applied as an indicator of combustion stoichiometry and correlations between the inter-cycle parameters and OR are drawn.
Section snippets
Experimental facilities
Experiments were conducted with a modern 6-cylinder inline, turbocharged, aftercooled, common-rail Cummins diesel engine (ISBe22031), typical of those used in buses or medium sized trucks. Fig. 1 shows a detailed schematic of the engine setup along with the pressure and crank angle data acquisition system. Each cylinder has two inlet and two exhaust valves. Cylinders two to five share their inlet ports with their adjacent cylinders while cylinders one and six each have one of their inlet valves
Fuel selection
Four biodiesels (C810, C1214, C1618, and C1875) covering a range of iodine numbers and saponification values were tested here along with commercial diesel. Selected properties of these fuels are shown in Table 2. The first two biofuels (C810 and C1214) have close iodine values (1 and 8) but different saponification numbers (330 and 233) while the other two (C1618 and C1875) have different iodine values (65 and 105) but close saponification numbers (195 and 185). The differences in iodine values
Results and discussion
As highlighted earlier, the oxygen ratio, OR (defined as the total oxygen atoms in the fuels and oxidizers to the total oxygen atoms required for stoichiometric combustion) is adopted here as a more relevant parameter for characterizing biodiesels and is hence adopted for displaying some of the data reported in the remainder of this paper. The relationship between OR and AFR for the biodiesels and commercial diesel used here is illustrated in Fig. 2. At the same combustion stoichiometry, a
Conclusion
The effects of the structure of fatty acid methyl esters on inter-cycle combustion variability is analyzed here using a modern six-cylinder inline, turbocharged, aftercooled, common-rail engine. The biodiesels used covered a range of carbon chain length and unsaturation levels. The following conclusions are drawn:
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The cyclic variability in the combustion peak pressure timing and the maximum rate of pressure rise was similar for all fuels except for C810 and only at low rpm where higher cyclic
Acknowledgment
This research is supported by the Australian Research Council. The authors would like to thank Mr. M. Rahman, Ms. S. Stevanovic, Mr. M. Babaie, Mr. A. Pourkhesalian, Mr. M. Islam, Mr. J. Islam, and Dr. H. Wang for assistance with the experiments. The support of QUT technical staffs, Mr. N. Hartnett and Mr. Scott Abbet are greatly appreciated.
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