In the past decade, the quality of Heavy Fuel Oil (HFO) has not improved or even declined in general. The progress in refinery techniques allowed to use a higher percentage of the crude oil for light fuels and other components. Therefore, the concentration of heavy components and agglomerates in the residual fuel increased. On the other hand, a better exhaust quality without compromising the superior fuel consumption of HFO-fuelled engines can only be gained by improved fuel injection. High injection pressures, rate shaping and electronic control of injection timing as well as new injection techniques like Common-Rail will be common features of engines for the next decades. Unfortunately, electronic controlled unit pumps and moreover, Common Rail systems are very sensitive to fuel quality, especially the content of water, sediments, agglomerates and other sludge. If ran on fuel processed by today’s fuel preparation system, wear would be excessive or damages will take place. On the other hand, if conventional on-board purification is intensified, the percentage of fuel deposit will rise and lower the over-all efficiency. To observe effects of fuel composition, a single cylinder research engine (250 mm bore/320 mm stroke) is fuelled by HFO in typical operation modes including different speeds and loads. Diesel fuel was also used to compare the results with another fuel of e.g. different sulphur content to separate effects on particulate matter. The high, typical sulphur content of the used HFO of about 3% increases the particulate matter; approx. 30% are sulphur products. The Soluble Organic Fraction (SOF, unburned fuel and oil) of PM is significantly higher compared to diesel fuel (50...70% to 10%). This impacts the particulate size distribution as well: although the main fraction of particulates consist of a size from 100...400nm for diesel and HFO, the number of small particulates (approx. 50 nm) is different, depending on the Sulphur content as this size consists of small condensates; therefore, the number may also be impacted by ambient air humidity. By use of different components for fuel preparation, the size, distribution and content of agglomerates can be impacted, but re-agglomeration occurs fast within minutes and must be taken into account. The differences in agglomerates does not result in a different total exhaust particulate mass. The slight effects found within the test are not significant and are in addition to that affected by ambient conditions. The fuel preparation, especially the use of homogenization equipment, needs more development efforts. For example, a mechanical homogenization apparatus can crack agglomerates and improve fuel homogeneity, but may fail to bring the size of agglomerates to a dimension that the filter is relieved.

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