Methanol
113
peak cylinder pressure because more fuel burned after TDC. Lower peak
cylinder pressures resulted in lower peak temperatures. As a consequence,
the NO x concentration diminished (Sayin and Uslu 2008).
4.7.2.4 Smoke Number (SN)
The emitted PM is essentially composed of soot, though some hydrocarbons,
generally referred to as a soluble organic fraction (SOF) of the particulate
emissions, are also adsorbed on the particle surface or simply emitted as liquid droplets. Smoke opacity formation occurs at the extreme air deficiency.
This air or oxygen deficiency is present locally in the very rich core of the
fuel sprays in the combustion chamber. It increases as the air–fuel ratio
decreases. Soot is produced by oxygen deficient thermal cracking of longchain molecules (Challen and Baranescu 1999).
Regarding the effect of different methanol contents on SNs, it was
observed that increasing methanol ratio in the blend reduced SNs. The
change in SNs compared to M0 implied that they diminished by 15%, 19%,
and 26% for M5, M10, and M15, respectively, at 20 Nm load and advance
injection timing as illustrated in Figure 4.15a. The presence of atomic-bound
oxygen in methanol satisfies positive chemical control over soot formation.
The tendency to generate soot by the fuel dense region inside a diesel diffusion flame sheath is reduced, so that soot-free spray combustion could be
achieved (Can, Celikten, and Usta 2004). The formation of smoke is most
strongly dependent upon the engine load. As the load increases, more fuel
is injected, and this increases the formation of smoke. The results obtained
in the study of Canakci, Sayin, and Gumus (2008) supported this statement.
For instance, in comparison to Figures 4.14c and 4.15c, it was seen that the
change of SNs increased by 6% for M10 as the engine load increased from
5 to 15 Nm at retarded injection timing. Advancing the injection timing
reduced the smoke emissions. The earlier injection led to higher temperatures during the expansion stroke and more time in which oxidation of the
soot particles occurred (Challen and Baranescu 1999). Figure 4.13a through
c presents the percentage change in SNs at different injection timings for
10 Nm load. As seen from these figures for M15, advanced injection timing
lowered in SNs by 3% and retarded injection timing raised in SNs by 3%
compared to ORG timing for M15, respectively.
Ozaktas et al. (2000) studied the effect of the compression ratio on the soot
formation for methanol–diesel fuel blends on an ASTM-CFR engine and they
compared the results with baseline diesel fuel. In that study, the selected
compression ratios were 14.8:1, 16:1, 17.4:1, 19:1, and 21:1. They expressed that
soot emission decreased with decreasing compression ratio. Soot emissions
of the blend fuels were generally lower than those of diesel fuel. The difference was more outstanding for higher compression ratios. Soot emissions
of diesel fuel were very high for these compression ratios and the methanol component is relatively very effective on the decrease of soot emission.
113
peak cylinder pressure because more fuel burned after TDC. Lower peak
cylinder pressures resulted in lower peak temperatures. As a consequence,
the NO x concentration diminished (Sayin and Uslu 2008).
4.7.2.4 Smoke Number (SN)
The emitted PM is essentially composed of soot, though some hydrocarbons,
generally referred to as a soluble organic fraction (SOF) of the particulate
emissions, are also adsorbed on the particle surface or simply emitted as liquid droplets. Smoke opacity formation occurs at the extreme air deficiency.
This air or oxygen deficiency is present locally in the very rich core of the
fuel sprays in the combustion chamber. It increases as the air–fuel ratio
decreases. Soot is produced by oxygen deficient thermal cracking of longchain molecules (Challen and Baranescu 1999).
Regarding the effect of different methanol contents on SNs, it was
observed that increasing methanol ratio in the blend reduced SNs. The
change in SNs compared to M0 implied that they diminished by 15%, 19%,
and 26% for M5, M10, and M15, respectively, at 20 Nm load and advance
injection timing as illustrated in Figure 4.15a. The presence of atomic-bound
oxygen in methanol satisfies positive chemical control over soot formation.
The tendency to generate soot by the fuel dense region inside a diesel diffusion flame sheath is reduced, so that soot-free spray combustion could be
achieved (Can, Celikten, and Usta 2004). The formation of smoke is most
strongly dependent upon the engine load. As the load increases, more fuel
is injected, and this increases the formation of smoke. The results obtained
in the study of Canakci, Sayin, and Gumus (2008) supported this statement.
For instance, in comparison to Figures 4.14c and 4.15c, it was seen that the
change of SNs increased by 6% for M10 as the engine load increased from
5 to 15 Nm at retarded injection timing. Advancing the injection timing
reduced the smoke emissions. The earlier injection led to higher temperatures during the expansion stroke and more time in which oxidation of the
soot particles occurred (Challen and Baranescu 1999). Figure 4.13a through
c presents the percentage change in SNs at different injection timings for
10 Nm load. As seen from these figures for M15, advanced injection timing
lowered in SNs by 3% and retarded injection timing raised in SNs by 3%
compared to ORG timing for M15, respectively.
Ozaktas et al. (2000) studied the effect of the compression ratio on the soot
formation for methanol–diesel fuel blends on an ASTM-CFR engine and they
compared the results with baseline diesel fuel. In that study, the selected
compression ratios were 14.8:1, 16:1, 17.4:1, 19:1, and 21:1. They expressed that
soot emission decreased with decreasing compression ratio. Soot emissions
of the blend fuels were generally lower than those of diesel fuel. The difference was more outstanding for higher compression ratios. Soot emissions
of diesel fuel were very high for these compression ratios and the methanol component is relatively very effective on the decrease of soot emission.