This paper addressed the ignition and detonation initiation investigation of two-phase valveless pulse detonation engines (PDEs) in different operational cases. To quantify the ignition and detonation initiation performance, the parameters detonation initiation time and deflagration-to-detonation transition (DDT) distance were examined. Detonation initiation time was defined as the time between the times when the spark plug received the ignition signal and when detonation initiated by DDT, which was the sum of the ignition time and DDT time. In order to observe the effects of ignition energy, operating frequency of the PDE, liquid fuel type, and inner diameter of the PDE on detonation initiation characteristics, proof-of-principle experiments of PDEs with inner diameters of 50 mm and 120 mm were carried out. Gasoline and kerosene were used as the liquid fuels of PDEs. A conventional Schelkin spiral is used to obtain DDT in liquid fuellair mixtures. The results indicated that the ignition energy, operating frequency, fuel type, and PDE diameter had important effects on detonation initiation time. As the ignition energy increased, the detonation initiation time decreased, and the average thrust of PDE increased while the DDT distance didn't change notably. As the operating frequency increased, the detonation initiation time and the optimum equivalence ratio required to stabilize multi-cycle detonation decreased while the ignition energy effect on detonation initiation time degenerated. The detonation initiation time and DDT distance of kerosenelair was longer than that of gasolinelair. As the operating frequency increased, the difference between the detonation initiation time of kerosenelair and that of gasolinelair decreased. The experimental data suggested that the detonation initiation time and DDT distance increased at the increased PDE diameter.