**4.7. Emissions analysis**

cy of birrea biodiesel when compared to fossil diesel as shown in table 7. The improved thermal

The fuel consumption profiles shown in figure 6(a) indicate that birrea biodiesel performs bet‐ ter than petroleum diesel across all engine loads under review. The maximum variation be‐ tween the two fuels is 48% at engine load of 90%, and the minimum variation is 34% at engine load of 30%.The variation of specific fuel consumption also depicts birrea biodiesel to be a more economic fuel for the diesel engine than petroleum diesel. The changes in specific fuel con‐ sumption and power depend on engine design, speed and loading conditions. Engines with higher compression ratios would result in higher temperatures and pressures during combus‐ tion in the cylinder, promoting more complete combustion. Engine speed also affects the airfuel mixing process, with higher engine speed normally giving a better mixture and higher cylinder temperature and pressure. On the contrary lowering the engine speed would lower

The economic value of birrea biodiesel as a fuel in CI engine is further validated by its re‐ markably high engine torque shown in figure 6(b). For both petroleum diesel and birrea bio‐ diesel fuels, torque increases steadily to maximum values of 20.1 Nm and 27 Nm respectively and then gradually decreases to minimum values of 10 Nm and 23.1 Nm re‐ spectively at engine load of 90%. The disparity in the generated torque can be attributed to the improved combustion processes caused by increased atomisation and spray characteris‐

The brake power profiles shown in figure 6(c) indicates a gradual decrease with increase in engine load for both diesel fuels, with birrea biodiesel recording relatively high values when compared to petroleum diesel across the entire engine loads under review. This is consistent

Overall, the results in figure 6 indicate that birrea biodiesel is a suitable fuel for the compres‐ sion ignition engine. A summary of engine performance using birrea biodiesel in compari‐ son with petroleum diesel and jatropha curcas biodiesel fuels at a speed of 2500rpm and

Brake power (W) 6.84 5.00 8.95 Specific fuel consumption (g/kwhr) 0.32 0.43 0.63 Torque (N) 24.9 17.9 - Fuel flow (kg/hr) 2.16 2.15 0.62 Brake thermal efficiency (%) 67.8 65.5 24.09 Mass of air (kg/hr) 37.25 36.52 5.52 Air fuel ratio 17.25 17.01 8.9

**Table 7.** Engine performance using birrea biodiesel, petroleum diesel and jatropha biodiesel fuels.

**Performance Sclerocarya Birrea B100 Petroleum diesel Jatropha curcas**

**B100 [21]**

efficiency of biodiesel is attributed to the oxygen content and higher cetane number.

the cylinder temperature and this can lead to poor vaporization and atomization.

tics for biodiesel fuel.

Legend: B100 = 100% biodiesel

with the high torque shown in figure 6(b).

202 Advances in Internal Combustion Engines and Fuel Technologies

engine load of 30% is presented in table 7.

This section compares emission levels of unburned hydrocarbon (HC), carbon monoxide (CO), and carbon dioxide (CO2) when the engine under review runs on petroleum diesel and on birrea biodiesel fuel (B100). The experimental data recorded for the three pollutants are presented in figure 7(a), (b), and (c). Typical engine combustion reaction is summarised by equation 5.

$$\text{Fuel} \, + \,\text{Air} \, \{\text{N}\_2 + \text{O}\_2\} \text{=} \text{CO}\_2 \, + \text{CO} + \text{H}\_2\text{O} + \text{N}\_2 + \text{O}\_2 + \text{HC} \, \text{ } + \text{O}\_3 + \text{NO}\_2 \tag{5}$$

This section however focuses on HC, CO and CO2 only. Figure 7(a) shows the data on emis‐ sion levels of HC recorded when the engine was using petroleum diesel and birrea biodie‐ sel. One of the most discernible trends connected to the data in figure 7(a) is that combustion of birrea biodiesel provides a significant reduction in unburned HC.

The difference in magnitude of HC emissions between the two diesel fuels increases with increase in load in a near exponential relationship. Based on average values, combustion of birrea biodiesel provides a reduction of unburned HC of approximately 59.4% at the com‐ pression ratio of 16:1. The lower HC emissions may be attributed to the availability of oxy‐ gen and high cetane number in biodiesel, which facilitates better combustion. It is the view of the authors that the relatively low level of HC emissions recorded when the engine was run using birrea biodiesel is linked to the quality of the biodiesel in terms of kinematic vis‐ cosity profile which complies favourably well with more stringent international standards such as the European standard (EN 14214), as demonstrated by figure 5.

Figures 7(b) and (c) show variation of CO and CO2 emission levels respectively with in‐ crease in engine load.

The data in figures 7 (b) and (c) should be viewed and discussed in parallel to enable the correlation between CO and CO2 emission levels to be identified and explained for the op‐ erational conditions under review.

Considering the results in figures 7(b) and (c), it can be seen that CO and CO2 emissions of petroleum diesel tend to increase with increase in engine load, while the same emissions for birrea biodiesel tend to increase gradually with increase in load for low load ratings. How‐ ever, the data in figure 7(b) show that both diesel fuels recorded the same average value of 1.5% by volume of CO emissions, while figure 7(c) depicts a slightly higher average value of CO2 for birrea biodiesel. CO is one of the consequences of incomplete fuel combustion. Less CO is generated with biodiesels than diesel for engine load below 60%. Concentration of oxygen during combustion would enhance the oxidation rate of CO and lead to less CO for‐ mation. This is a major advantage of oxygenated fuels like biodiesel. However, at higher en‐ gine loads, the lower temperatures could hinder the conversion rate of CO to CO2, leading to higher CO emissions. These effects are mainly attributed to the complex interactions be‐ tween combustion dynamics and physicochemical properties of the fuel. The combustion ef‐ ficiency depends on the engine design, injection system, air-fuel mixture, and the loading and speed conditions. The physicochemical properties of birrea biodiesel may be affected by feedstock variations, conversion process and separation efficiency. Factors such as chemical compositions, carbon-chain lengths, degree of saturation and impurities also influence the performance of the biodiesel as a fuel in the diesel engine.

Overall results indicate that using birrea biodiesel in a compression ignition engine provides significant reduction in HC emission levels than petroleum diesel, while levels of CO and

Sclerocarya Birrea Biodiesel as an Alternative Fuel for Compression Ignition Engines

http://dx.doi.org/10.5772/54215

205

**5. Economic feasibility of using Sclerocarya birrea to produce biodiesel**

The future outlook of the Sclerocarya birrea biodiesel fuel is bright on the basis of abun‐ dance and minimal conflict with food security. As discussed in section 2, Sclerocarya birrea plant is abundant in Botswana and an almost negligible fraction is utilised for purposes that use the fruit pulp and juice, discarding the seed as a byproduct. Thousands of tons of the fruits are left to rot annually as an untapped resource. The major economic factor to consider with respect to the input costs of biodiesel production is the feedstock, which is about 80% of the total operating cost [37]. Other important costs relate to the geographical area of the feedstock, variability in crop production from season to season, labor and production inputs including methanol and catalyst. These costs depend on the prices of the biomass used and the size and type of the production plant. Other important factors that would determine the production cost of birrea biodiesel are the yield and value of the byproducts of the biodiesel production process, such as oilseed cake (a protein-rich animal feed) and glycerine (used in the production of soap and as a pharmaceutical medium). Since birrea nut oil has not been studied as a potential feedstock for production of biodiesel, precise production costs are yet

This study aims to establish the technical properties of the biodiesel as a suitable fuel for the compression ignition engine, thereby providing a basis upon which socio economic feasibili‐ ty analysis can be done to further the research. Factors including actual yield of birrea fruits

Production of biodiesel from sclerocarya birrea will however have other obvious social im‐ pacts. It will provide a source of income and employment for many families. Thousands of rural people who are largely unemployed would earn income from the gathering and proc‐ essing of the raw material into biodiesel. Thus the logistics of harvesting the raw material is deemed simple and cost-effective. Furthermore, the fact that sclerocarya birrea tree thrives and produces abundantly under natural (unoptimised) conditions implies a substantial re‐ duction in overall costs of producing biodiesel from this plant species. Optimising growing

Although birrea fruit production is seasonal under natural conditions (follows the rain sea‐ son), nut availability for oil extraction can be perennial. When the hard woody seed has been extracted from the fruit skin and pulp and allowed to dry, it can stay for more than a year with no damage to the nut in its cavity. Thus there are two options for keeping this raw material inventory. One option is to keep hard seeds and only crack them when oil extrac‐ tion is about to commence. This eliminates (or largely minimises) inventory holding costs on the delicate seed bearing the oil. The other option is to crack all the seeds and keep in stock the oil bearing nuts, but this requires special preservation facilities which come at a cost.

per hectare and associated production and logistics costs will need investigation.

CO2 emissions are quite comparable.

to be established.

conditions, if desired, may increase the yield.

**Figure 7.** a): Unburned hydrocarbon (HC) Emissions of the two diesel fuels (b): CO Emissions of petroleum diesel and birrea biodiesel (B100) (c): CO2 Emissions of petroleum diesel and birrea biodiesel (B100)

Overall results indicate that using birrea biodiesel in a compression ignition engine provides significant reduction in HC emission levels than petroleum diesel, while levels of CO and CO2 emissions are quite comparable.

ficiency depends on the engine design, injection system, air-fuel mixture, and the loading and speed conditions. The physicochemical properties of birrea biodiesel may be affected by feedstock variations, conversion process and separation efficiency. Factors such as chemical compositions, carbon-chain lengths, degree of saturation and impurities also influence the

(a)

20 30 40 50 60 70 80 90 100

Load (%) Petrolem Diesel Birrea B100

(b)

20 30 40 50 60 70 80 90 100

Load (%) Petroleum Diesel Birrea B100

(c)

**Figure 7.** a): Unburned hydrocarbon (HC) Emissions of the two diesel fuels (b): CO Emissions of petroleum diesel and

birrea biodiesel (B100) (c): CO2 Emissions of petroleum diesel and birrea biodiesel (B100)

20 30 40 50 60 70 80 90 100

Load (%) Petroleum Diesel Birrea B100

performance of the biodiesel as a fuel in the diesel engine.

0.5

2

4

6

CO2 Emissions (%)

8

10

1

1.5

CO Emissions (%)

2

2.5

HC Emissions (ppm)

204 Advances in Internal Combustion Engines and Fuel Technologies
