Module8:Engine Fuels and Their Effects on Emissions
  Lecture 40:Alternative Fuels (contd.)
 
                                       Table 8.18
Properties of Biodiesel Derived from Some Vegetable Oils

Properties

Rapeseed methyl ester

Jatropha methyl ester

Molecular weight
Hydrogen/carbon ratio, m/m
Oxygen content, % m/m
Relative density @ 15° C
Kinematic viscosity @ 40° C, mm2/s
Cetane number
Lower heat of combustion, MJ/kg
Sulphur content, %m/m

≈300
0.15
9-11
0.882
4.57
51.6
37.7
<0.002

≈293
0.157
10.9
0.88
4.4
57.1
38.45
< 0.020

The vegetable oil esters are practically free of sulphur and have a high cetane number ranging from 46 to 60 depending upon the feedstock. Due to presence of oxygen, biodiesels have a lower calorific value than the diesel fuels. European specifications for biodiesel or fatty acid methyl esters (FAME), EN 14214 have been issued in 2003.

Emissions

The influence of biodiesel on emissions varies depending on the type of biodiesel (soybean, rapeseed, or animal fats) and on the type of conventional diesel to which the biodiesel is added due to differences in their chemical composition and properties. The average effects of blending of biodiesel in diesel fuel on CO, HC, NOx and PM emissions compared to diesel as base fuel are shown in Fig.8.7.The Table 8.19 gives change in emissions with 20 % blend of biodiesel in diesel and 100% biodiesel compared to diesel alone. These show the average of the trends observed in a number of investigations.
Use of biodiesel results in reduction of CO, HC and PM, but slight increase in NOx emissions is obtained.

  • Reduction in CO emissions is attributed to presence of oxygen in the fuel molecule.
  • A slight increase in NOx emissions results perhaps due to advancement of dynamic injection timing with biodiesel. The methyl esters have a lower compressibility, which results in advancement of dynamic injection timing with biodiesel compared to diesel.
  • Lower SOF with biodiesel and advanced injection timing also results in lower PM emissions.
  • Volumetric fuel consumption with biodiesel is higher than diesel due to its lower heating value. An increase of 10-11 % in fuel consumption compared to diesel may be expected when comparing their heating values. An increase in volumetric fuel consumption by 0.9-2.1% with 20% blends has been obtained.

Figure 8.7: Average effect  on  diesel engine emissions resulting from  addition of biodiesel in diesel fuels

                                                                           Table 8.19

Average reduction in emissions with use of biodiesel and 20% biodiesel blends compared to diesel alone

Emission

B100

B20

HC
CO
PM
NOx
PAH
Sulphates

- 93
- 50
- 30
+13
- 80
- 100

-30
-20
-22
+2
-13
-20