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but not surprising, to see from Figure 38c that ignition delay has an influence on engine efficiency, through the control that the delay period has on the amount of fuel burned in the premixed mode (see Figure 38a). Figure 38c shows that an increase in ignition delay from 3.5 CAD to 7 CAD tends to raise the engine efficiency by about 2, or at most by 3, %-units. Figure 38b and Figure 38c show considerable scatter in the correlations for efficiency, for individual fuel groupings. Some of this scatter could be expected to be due to random experimental error in measuring the engine efficiency; with the standard error increasing the efficiency being at least ±0.43%-units (see section 1.3). It should also be noted, that when CA50 points in CAD were plotted against engine efficiency (not shown here), no correlation was observed when all of the molecules were considered together. When considering the fuel groupings alkanes had a good negative correlation: engine efficiency decreased when CA50 point occurred later in the engine cycle, as was expected because the shape of the HRR curves were similar for all of the alkanes with the peak heat release rate increasing with later ignition. However, no good correlations were observed for the oxygenated fuel groups. This suggests that both the timing and the shape of HRR have an effect on the engine efficiency.

6.2.2. Effect of chemical fuel properties on engine thermal efficiency

The tested molecules were grouped according to functional groups (alkanes, alkenes, alcohols, esters, etc.) in their molecular structure (see Appendix II). Figure 38d shows engine efficiency to decrease somewhat with increasing carbon chain length for all the functional groups. This decrease in efficiency with increasing carbon chain length is believed be the result of decreasing ignition delay when carbon chain is increased.

In general, longer carbon chain molecules have shorter ignition delays which result in smaller premixed phases and thus in lower engine efficiency (see also Figure 38a

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and Figure 38b). There is some support for this statement in the literature, for example Campos-Fernández et al. [193] observed that addition of 20%, 25% and 30% of butanol (C4) into fossil diesel fuel increased brake thermal efficiency slightly more than the corresponding additions of pentanol (C5); Balamurugan and Nalini [188] observed that the addition of 4% butanol into fossil diesel fuel increased the brake thermal efficiency by about 7.34 %-units more than the increase caused by the addition of 4% pentanol. Closer observation of Figure 38d suggests that the effect on efficiency beyond chain length of C11 is very small, save for the case of the very viscous primary alcohol hexadecanol (C16). Molecules with carbon chain length longer than C11 were heated prior to being injected into the combustion chamber and this may have had an effect of reducing the sensitivity of the efficiency to carbon chain length.

Figure 38d also suggests that alkanes and alkenes achieve higher engine thermal efficiency than oxygenated fuel molecules with the same carbon chain length. This suggestion is supported by the results of several other studies which have shown that, for example, methyl esters [186,272,273] and fatty acids [274,275] had lower engine efficiency than diesel fuel. The observed difference between the engine efficiency of alkanes and oxygenated fuel molecules was explored further in Figure 39a, which shows the energy release rates of several C9 molecules, one each of an alkane, methyl ketone, methyl ester, acid, and alcohol. Figure 39a shows distinctly different patterns of energy release between the alkane molecule (nonane) and the four oxygenated molecules, with nonane having shorter ignition delay, a smaller premixed combustion phase and a long diffusion controlled phase. The fact that nonane has higher efficiency than the oxygenated molecule (see Figure 38d) is, therefore, surprising. In principle, one might expect the oxygenated fuel molecules to have a higher engine efficiency than the alkane molecule (nonane), because the oxygenated molecules have the majority of their energy released rapidly and close to TDC, which is closer to the theoretical air-standard Otto-cycle which has the highest thermal engine efficiency amongst the internal combustion engine-related air-standard cycles with same compression ratio [1]. The effect of molecular oxygen on thermal efficiency is explored further in Figure 39b.

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Figure 39. (a) Heat release rate of C9 molecules from five different functional groups (numbers indicate the general engine efficiency of the functional groups with 1 being the highest and 5 the lowest) and (b) the effect of fuel-bound oxygen on engine efficiency.

Figure 39b shows the thermal engine efficiency plotted against the percentage of oxygen (wt-%) in the various fuel molecules tested. Before discussing Figure 39b, it should be noted that hexadecanol, labelled C16 in Figure 39b, can be seen to have a very low efficiency of ~31% and further investigation of the energy release rate showed that hexadecanol combusted only partially and this is likely to have been due to its high viscosity (Appendix II) and likely poor combustible mixture preparation.

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Figure 39b shows quite a poor correlation (R² = 0.35) between efficiency and wt-%

of oxygen in the fuel molecule. Despite the poor correlation of Figure 39b, it is possible to see that the non-oxygenated molecules (plotted on the y-axis at zero wt-

%) are clustered around an average efficiency of ~39.5% while the oxygen-bearing molecules are clustered around an average efficiency of ~38%. This is only a tentative observation, as the experimental error in measuring the thermal efficiency was estimated at ±0.43 %-units, which tend to swamp any effect that increasing molecular oxygen might have on efficiency. Reports in the literature on the likely effect of molecular oxygen on efficiency are not unequivocal [181,183,184,276], probably because any effect is likely to be small and not readily measured. According to Nabi [181], the fuel oxygen content has no effect on engine efficiency below 30 % oxygen and has a negative correlation with efficiency with oxygen over 30

wt-%. Labeckas and Slavinskas [183] found that efficiency started to decline rapidly beyond 10 wt-% of rapeseed methyl ester added to fossil diesel fuel.

Figure 40 explores potential changes in engine efficiency caused by the following changes in the fuel molecular structure: Figure 40a, molecular branching; Figure 40b, moving either a hydroxyl or a carbonyl group along the carbon chain of a molecule;

Figure 40c, the level of unsaturation (reduction in hydrogen/carbon ratio). Taken together, the data in Figure 40 does not indicate clear influences of these molecular features on engine thermal efficiency. Any influence of molecular branching (Figure 40a) appears to be small, no greater than ±0.5%-units and not significantly greater than the experimental error in the measurement of the engine thermal efficiency.

Similarly, trends in engine efficiency versus the functional group position (Figure 40b) are not clear, with the ±0.75 %-units change in efficiency for individual molecular groups not being significantly greater than the experimental error in the measurement of the thermal engine efficiency (±0.43%). In Figure 40b, some of the homologous series (e.g. 1-, 2-, and 3-octanol; 1-, 2-, 3-nonanol; 1- and 2-undecanol) show the efficiency rising with transition of the hydroxyl group (OH) away from the primary position; this transition caused lengthening of the ignition delay period and is therefore consistent with Figure 38b and Figure 38c, which shows efficiency increasing with longer ignition delay, but this can only be considered as a tentative observation. Lastly, Figure 40c suggests that the degree of unsaturation is associated

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with lower thermal efficiency which is inconsistent with the fact that greater level of unsaturation caused longer ignition delays.

Figure 40. Effect on engine efficiency of (a) branching (arrows indicate the direction of adding two methyl branches to an alcohol), (b) moving a hydroxyl group or a carbonyl group along the carbon chain length of a fuel molecule and (c) level of unsaturation.

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6.2.3. Effect of physical fuel properties on thermal engine