As shown in Fig. 6(a), the ULSD showed the
larger reduction rate of the spray tip penetration according to the
increase of the ambient gas density, compared to DME fuel (ULSD:
about 42.5%, DME: about 18.5%). This is the reason why the DME spray
is relatively shorter than ULSD spray due to the low fuel density and
the active vaporization. In other words, it means that the DME spray
for the variation of the ambient gas density is not significant,
compared to ULSD spray. Also, the DME spray cone angle is wider
than ULSD spray, and the increased ambient gas density leads to
gradual increase of the spray cone angle of both fuels.