from waste olive oil run in a three cylinder, four-stroke Perkins diesel engine where the BSFC was 7% greater (Dorado et al., 2003). The reduction in fuel consumption may be a consequence of the lower energy density of biodiesel (33–42 MJ/kg) com- pared with diesel (46 MJ/kg). The variation in the reduction in BSFC may also be due to the different combustion conditions found in the different engines.
Ethanol
Ethanol has an energy content of 23.5 MJ/l and therefore contains 32.5% less energy than petrol. The most common use of ethanol in the USA is as an E85 (85% ethanol) blend where the energy content is 71.95% of petrol. This should result in 5–12%
decrease in fuel economy. A study of the 2007 EPA data estimates that the mean combined fuel economy is 73.4% (Roberts, 2007), although in flexible fuel cars there was little difference in fuel economy between E85 and petrol. The performance of a petrol blend with either 20% ethanol or 20% methanol has been compared with unleaded petrol. The results in terms of maximum brake torque (power) at different engine speeds are given in Fig. 8.5. Both ethanol and methanol improved the brake torque over the unleaded petrol which in both cases was probably due to better anti- knock characteristics and increased volumetric efficiency due to higher oxygen con- tent (Agarwal, 2007). The fuel economy and power generation has not been reported for the other biofuels.
Biodiesel
There have been a large number of studies on the exhaust emissions from engines using a variety of biodiesel types and concentrations (Graboski and McCormick, 1998; Willianson and Badr, 1998; EPA, 2002). However, it is difficult to compare results as different engines, conditions, and blends have been used. Figure 8.6 shows the mean of a number of studies on the effect of using 100% rapeseed biodiesel on the important engine emissions: hydrocarbons (HC), CO, nitrous oxides (NOx), and PM. Rapeseed biodiesel is the main biodiesel produced in the EU and the consensus shows a considerable reduction in the emission of HC and PM and a small increase in NOx. The increase in NOx was probably due to an increase in combustion tem- perature. The mean of the three studies on the effect of sunflower biodiesel on engine emissions is shown in Fig. 8.7. With sunflower biodiesel, the reduction in HC was the
Fig. 8.6. Mean values for the effects of rapeseed biodiesel on the emissions from a number of engines. (From Peterson et al., 1996; Williamson and Badr, 1998; Makareviciene and Janulis, 2003; Labeckas and Slavinskas, 2006.)
–50 –40 –30 –20 –10 0 10
Reduction(%)
HC CO Nox PM
% reduction
–60 –50 –40 –30 –20 –10 0 10
Reduction(%)
HC CO Nox PM
% reduction
Fig. 8.7. Mean values of the effects of sunflower biodiesel on the emissions from a number of engines. (From Monyem and van Gerpen, 2001; EPA, 2002.)
same as rapeseed biodiesel, and CO and PM were further reduced, but NOx emissions increased. The emissions from an engine fuelled with 100% waste olive oil biodiesel at different loads are shown in Fig. 8.8 (Dorado et al., 2003). As the load increases the reduction in CO, NOx and sulfur dioxide decreases to zero at the highest load.
A different result was observed when a 50% sunflower biodiesel blend was used in a marine diesel engine (Fig. 8.9) (Kalligeros et al., 2003). With 50% sunflower biodiesel, the emissions decrease as for waste olive oil biodiesel but do not reach zero at the highest load. The advantages of using biodiesel to reduce emissions may therefore be elim inated when the engine is used at high loads. However, the reduction in emissions may depend on the test engine used.
Fig. 8.8. The effect of waste olive oil biodiesel (100%) on the percentage of changes in emissions from a diesel engine compared with diesel at various loads (Nm). (From Dorado et al., 2003.) -60
-50 -40 -30 -20 -10 0 10
Change(%)
80 247 370 530
Load (Nm) CO CO2 Nox SO2
Fig. 8.9. The percentage of change in emissions when 50% sunflower biodiesel blend is used in a marine diesel engine at various loads (kW) compared with diesel. (From Kalligeroset al., 2003.)
-70 -60 -50 -40 -30 -20 -10 0
Change(%)
0.01 0.95 1.9 2.85 3.8
Load (kW)
Nox HC CO Particulates
The effect of increasing concentrations of biodiesel on engine emissions is shown in Figs 8.10 and 8.11. As the concentration of commercial biodiesel in blends increased, the emission of CO was reduced and NOx increased (Fig. 8.10). When soybean biodiesel was tested in contrast to commercial biodiesel, CO was not reduced significantly but HC and PM were reduced and NOx increased.
In general, emissions from diesel engines running on blends or 100% biodiesel showed a reduction in CO, HC and PM, but an increase in nitrous oxide (NOx) levels.
The reason for this change in emissions is thought to be the higher oxygen content of biodiesel, which gives a more complete combustion of the fuel and this reduces CO, HC and PM. The Environmental Protection Agency (EPA) has compiled the results of a number of studies on the effect of biodiesel content on emissions and the results were
Fig. 8.10. The effect of various concentra- tions of commercial biodiesel added to diesel in a four-stroke direct injection single cylinder diesel outboard engine. (Redrawn from Murillo et al., 2007.)
0 1 2 3 4 5 6 7 8 9
0 10 20 30 50 100
Biodiesel % NOx(g/kWh)
9 9.2 9.4 9.6 9.8 10 10.2 10.4 10.6 10.8
CO(g/kWh)
Nox CO
-80 -60 -40 -20 0 20 40
Changeinemissioncomparedwith diesel(%)
10 20 30 50 100
Soya biodiesel (%)
Nox CO HC PM
Fig. 8.11. Percentage of change in steady state emissions from a soy biodiesel fuelled Navistar HEUI diesel engine. (From Graboski and McCormick, 1998.)
similar to those observed in Figs 8.10 and 8.11. In a study using a MAN diesel bus engine, the fuel injection characteristics were different for diesel and rapeseed-derived biodiesel (Kegl, 2008). The biodiesel when injected into the engine forms a longer and narrower spray than mineral diesel, caused by a higher injection pressure, increased by low fuel vaporization and atomization due to higher surface tension and viscosity.
The reasons for the increased NOx production when using biodiesel may be the higher combustion temperature and injection characteristics. The increase in nitrous oxide (NOx) is probably due to the raised combustion temperature which is known to increase NOx formation. Advanced injection is caused by the higher bulk modulus of compressibility of biodiesel which allows the pressure wave from the pump to the nozzle to speed up, therefore advancing the timing. It has been observed that retard- ing the timing can in some way reduce NOx emissions. In order to reduce the emission of NOx with biodiesel, the injection timing was altered and the optimum setting was found to be 19° (°CA BTDC, degree of crankshaft angle before top dead centre) compared with 23° for diesel. The effect of altering the injection timing on emissions of CO and NOx is shown in Fig. 8.12. The lowest NOx emission was obtained at 21°, but the lowest CO emission was at 24°. In all the studies on emissions, no evidence has been given that the engines were optimized for biodiesel, and therefore modifica- tions such as altering the timing may reduce emissions of NOx.
Another way of reducing NOx production is to use exhaust gas recycling (EGR).
Diesel engines fuelled with Jatropha oil biodiesel produce more NOx than diesel (Pradeep and Sharma, 2007). In this case exhaust gas recirculation was tested as a system to reduce NOx. The exhaust gases consisting of carbon dioxide and nitrogen are recirculated and injected into the engine inlet, reducing the oxygen concentration and combustion temperature which reduces NOx. The level of recirculation is critical because if the oxygen is reduced too far, incomplete combustion will produce higher levels of hydrocarbon, CO and smoke. In this case with a single cylinder diesel engine, the optimum recirculation was 15% as can be seen in Fig. 8.13.
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Emissions(ppm)
Nox CO
21° 24° 27°
Fig. 8.12. Effect of injection advance on emissions, the normal setting for diesel is 23°.
(From Carraretto et al., 2004.)
Whatever biodiesel is used the scale of reduction in emissions will also be depend- ent on the engine characteristics such as combustion chamber design, injector nozzle, injection pressure, air–fuel mixture, load and other features. Therefore, the reduction in emission will vary from one diesel engine design to another.
Ethanol
Small quantities of ethanol (3–6%) have been added to petrol to increase the oxygen content to ensure complete combustion and reduce the emission of HC and PM. When high concentrations of ethanol are used such as the E85 fuel in a standard petrol engine, CO and NOx are reduced compared with petrol but there is an increase in HC (Fig. 8.14). In the flexible fuel engine where the conditions are optimized for E85 fuel, emissions of CO and NOx were reduced but HC and methane were increased.
Fig. 8.13. Comparison of combustion duration (degrees) and NOx emissions for Jatropha sp.-derived biodiesel and diesel using exhaust gas recirculation. (From Pradeep and Sharma, 2007.)
0 20 40 60 80 100 120 140
Diesel Biodiesel Biodiesel + 15%
EGR Combustion time Nox NOx(ppm ⫻10),duration(deg.)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Emissions(g/km)
Hydro- carbons
CO Nox CH4
E85 standard E85 flex
Fig. 8.14. The effect of E85 on the emis- sions from a standard engine and a flexible fuel engine. CO, carbon monoxide; NOx, nitrous oxides, CH4, methane. (From Wang et al., 1999.)
Dimethyl ether (DME)
A number of studies have been carried out on the emissions from a compression igni- tion engine (diesel) running on DME and DME blends. DME has been shown to pro- duce low noise, smoke-free combustion and reduced NOx when used in an internal combustion engine (Huang et al., 2006). DME, because of its high cetane number and low boiling point, has been used at 100% or as an oxygenated addition to diesel. When DME was used in a diesel engine, it reduced NOx and SOx emissions and was sootless (Semelsberger et al., 2006). Large motor manufacturers are developing truck and bus transport fuelled by DME. The emission levels from these development vehicles when run on DME show virtually no PM and low levels (0.5–2.0 g/kWh) of NOx.