A higher bsfc of ERO5 and more oxygenated blends can be attributed, primarily, to a lower by 27.3% net heating value of ethanol relative to that of RO, 36.87 MJ/kg; nevertheless, it is probably not the main reason that leads to having higher biofuel consumption in mass per unit energy developed because the maximum values of brake thermal efficiency in the case of fuelling the loaded engine with considered blends (Fig.
The problems of incomplete burning may arise more likely when an engine operates on fuel-rich mixtures, and therefore, in spite of a higher (by 16.3%) net heating value of petrol, bsfc under heavy loads, [lambda] = 1.6, increases when more than 2.5vol% of petrol is premixed in RO.
In spite of better atomization of heavy oil droplets and an extra amount of ethanol oxygen available for the complete combustion of fuel-rich portions, a lower cetane number and net heating value of ethanol remain the main cause as to why a loaded engine running on oxygenated RO blends does not operate efficiently at speeds of 2000-2200 [min.sup.-1].
Comparing with blends ERO, the addition to petrol utilises advantages related to a three-fold higher cetane number and excellent miscibility with RO that along with its wider evaporation range, lower auto-ignition temperature and better net heating value results into the bsfc of the engine running on blend PRO2.5 lower by 3.4% and 5.5% at speeds of 2000 and 2200 [min.sup.-1].
When analysing the test results, one should remember that RO composition distinguishes from that of diesel fuel as having 10.8vol% of oxygen and oil mixing with ethanol diminish the net heating value of the blend that at higher ethanol additions may affect the emission of N[O.sub.x] due to lower cylinder gas pressure and flame temperature.
Despite the fact that adding petrol diminishes the content of biofuel oxygen, the improved net heating value, better atomisation of fuel sprays and the final combustion occurring earlier in the expansion stroke may cause higher cylinder peak temperature and create more favourable conditions for N[O.sub.x] production (Schumacher et al.
Lower CO emissions and smoke probably may occur because of inherently big oxygen content (10.8%) in RO composition which due to the lower viscosity of blends PRO7.5, better atomisation of heavy RO droplets and a higher net heating value of petrol stimulates combustion.
The density of ethanol and petrol is 13.9% and 17.6% relatively lower than that of RO (0.916 g/[cm.sup.3]) and their net heating values also differ from 26.82 MJ/kg to 42.88 MJ/kg.