Type Ia supernovae (SNe Ia) are believed to be thermonuclear explosions of white dwarfs (WDs), but to date the explosion mechanism and progenitor systems remain elusive. I will present our works on a new channel that could possibly explain the majority of type Ia supernovae. In this scenario, a SN Ia is produced from a direct, "head-on" collision of two WDs in a common field tripe system. Such collisions are shown to lead to successful detonations from strong shocks. We show that the collision models reproduce several robust observational features of SNe Ia, especially establishing that the full range of Ni56 necessary for all SNe Ia can be produced by direct collisions of typical WDs. By systematically studying the SNe Ia nebular-phase spectra, we discover that bi-modal velocity distributions in Ni56 ejecta are common, which are naturally expected from the direct collision models due to the existence of two centres of explosions.