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Ignition time is measured in premixed preheated hydrogen-air flows excited by a repetitive nanosecond pulse discharge in a plane-to-plane geometry. ICCD images of the plasma and the flame demonstrate that mild preheating of the fuel-air flow greatly improves plasma stability and precludes filament formation. At the initial temperatures of T0 = 100-200°C, hydrogen-air plasmas remain stable and uniform up to at least P = 150 torr, and ignition occurs in a large volume. In contrast, ignition in less uniform preheated ethylene-air plasmas occurs locally, near the electrode edges, with flame propagating toward the center of the plasma. Ignition time in hydrogen-air mixtures is measured at initial temperatures of T0 = 100-200°C, pressures of P = 40-150 torr, equivalence ratios of φ = 0.5-1.2, and pulse repetition rates of ν = 10-40 kHz. The results of ignition time measurements are compared with the predictions of the hydrogen-air plasma chemistry model, showing good agreement. Nitrogen emission spectra are used to measure time-resolved temperature in air and hydrogen-air plasmas. The results show that ignition begins at the plasma temperature of T ≈ 700 K and results in a rapid temperature rise. By turning off dominant plasma chemical radical generation processes in kinetic modeling calculations, while keeping discharge energy loading the same, it is demonstrated that ignition is driven by additional energy release in reactions of plasma-generated radicals with hydrogen. To determine if plasma-generated radicals may reduce ignition temperature, discharge pulse burst was terminated before the onset of ignition, and ignition delay time was measured versus plasma temperature at the end of the burst. Experimental ignition delay time is in reasonably good agreement with kinetic modeling calculations. The kinetic model predicts significant plasma-assisted ignition threshold temperature reduction at the- - present conditions compared to thermal ignition, up to ΔT = 180 K.