It must be noted that the appearance of the exhaust gas changed
from highly transparent to slightly opaque at fuel temperatures
lower than approximately 70 C. At such low temperatures
blue smoke was observed, most likely because the time required
for full evaporation of the poorly atomized oil was insufficient.
The vegetable oil then starts to break down into glycerol and free
fatty acids, which is known to give bluish smoke [42]. The observation
of smoke in the exhaust prescribed the lower temperature
limit in this parameter study