Comparison of engine performance and particle emissions among used

Figure 5-7. In this figure, the vertical axis represents the percentage change of engine power, brake specific fuel consumption and specific PM/PN emissions while the horizontal axis indicates biodiesel proportion in the blends. Neat diesel was used as a reference fuel to calculate the percentage changes. There is a significant difference among all four biodiesels used. For example, C810 provides the highest reduction in particle mass and number but the penalty for that is also highest, around 25% reduction in engine brake power and an additional 25% increase of specific fuel consumption due to that reduced engine power. This fuel and power penalty is lowest for C1618 but particle mass and number reduction is also the lowest in this fuel blend. Therefore it is necessary to make a trade-off between particle emission reduction, fuel and power penalty, ensuring maximum benefit; not just for emission levels but for engine power and fuel economy as well.

Considering the aforementioned factors, C1822 seems to have advantage over the rest of the fuels, as it maintains the lowest power and fuel penalty regardless of the blending ratio to diesel and provide a reasonable reduction in engine exhaust particle emissions. The evidence suggests that biodiesels with a longer carbon chain length and higher degree of unsaturation might be a solution to reduce particle emissions to a certain extent with less fuel and engine power penalty.

Figure 5-7: Comparison of engine performance (power, BSFC) and particle emissions (PM, PN) among biodiesels and their blends where petro-diesel was used as a reference

5.4 Conclusions

In conclusion, biodiesel fuels with shorter carbon chain lengths and higher degrees of saturation have more potential to decrease engine exhaust particle emissions. With the increase of carbon chain length and degree of unsaturation, particle emissions also increase. Particle size also depends on type of fuel used. Fuel or fuel mix responsible for higher PM and PN emissions was also found to have a larger particle median size. This indicates that coagulation plays a role in overall engine exhaust particle size. Particle emissions increase linearly with fuel viscosity and surface tension but only for higher diesel-biodiesel blend percentages (B100, B50). On the other side PM emission reduces consistently with fuel oxygen content regardless of the proportion of biodiesel in the blends. High fuel injection pressure used by common rail injection systems might minimise the effects of small variation in fuel viscosity and surface tension on particle emissions. As the fuel oxygen content increases with the decrease of FAME carbon chain length it is not clear whether it is the FAME carbon chain length or the oxygen content that is the driving force which decreases particle emissions. The results support the view that chemical composition (i.e. carbon chain length, degree of unsaturation, oxygen content) of biodiesel is more important than its physical properties (i.e. viscosity, surface tension) in regards to reducing engine exhaust particle emissions.

5.5 Acknowledgement

Authors sincerely acknowledge Mr. Scott Abbett and Mr. Noel Hartnett technicians in the Biofuel Engine Research Facilities (BERF) at Queensland University of Technology (QUT) and PhD student Mohammad Aminul Islam for their valuable support during experimental work. This work was supported by the Australian Research Council under grants LP110200158,DP1097125 DP130104904 and DP120100126.

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