  Two major processes have been proposed to convert the coronal magnetic energy
into the kinetic energy of a coronal mass ejection (CME): resistive magnetic
reconnection and ideal macroscopic magnetohydrodynamic instability of magnetic
flux rope. However, it remains elusive whether both processes play a comparable
role or one of them prevails during a particular eruption. To shed light on
this issue, we carefully studied energetic but flareless CMEs, \textit{i.e.},
fast CMEs not accompanied by any flares. Through searching the Coordinated Data
Analysis Workshops (CDAW) database of CMEs observed in Solar Cycle 23, we found
13 such events with speeds larger than 1000 km s$^{-1}$. Other common
observational features of these events are: (1) none of them originated in
active regions; they were associated with eruptions of well-developed long
filaments in quiet-Sun regions, (2) no apparent enhancement of flare emissions
was present in soft X-ray, EUV and microwave data. Further studies of two
events reveal that (1) the reconnection electric fields, as inferred from the
product of the separation speed of post-eruption ribbons and the photospheric
magnetic field measurement, were in general weak; (2) the period with a
measurable reconnection electric field is considerably shorter than the total
filament-CME acceleration time. These observations indicate that, for these
fast CMEs, the magnetic energy was released mainly via the ideal flux rope
instability through the work done by the large scale Lorentz force acting on
the rope currents rather than via magnetic reconnections. We also suggest that
reconnections play a less important role in accelerating CMEs in quiet Sun
regions of weak magnetic field than those in active regions of strong magnetic
field.
