In the room temperature, the value of κ for hydrogen is 1.40 while the value of κ for air is 1.40. For example, under the condition of temperature of 600 K and pressure of 3 MPa, the value of κ for hydrogen is 1.396 while 1.386 for air. However, in hydrogen, the heat loss to the wall is large because the diffusion coefficient of hydrogen is larger than that of air. Eventually, the temperature of gas in hydrogen-air mixture was almost the same as that in the normal diesel combustion. Figure 5 presents Arrhenius' expression of the ignition delay. When the temperature of the initiation of injection timing, T_inj, is the same, the ignition delay increased in larger equivalence ratio than that in the normal diesel combustion, because the molar concentration of oxygen became smaller. Figure 6 shows the effect of equivalence ratio on mass of air, that is to say, molar concentration of oxygen, induced from the intake port. In hydrogen-air mixture, when the equivalence ratio increased, the oxygen in the inlet gas decreased.

Figure 7 shows the result of the ignition delay using propane or methane as gaseous fuel that was induced from the intake port. The ignition delay in air-propane mixture was the longest while the ignition delay in normal diesel condition was the shortest. The value of ratio of the specific heats, which is functions of temperature and pressure, changes in adding gaseous fuel in the inlet air, causing gas temperature decrease. For example, under the condition of temperature of 500 K and pressure of 2 MPa, the values of the specific heats for air, methane and propane are 1.397, 1.229 and 1.107, respectively [26]. It is considered that the lower temperature of compressed gas leads to longer ignition delay. Figure 8 presents the ignition delay of 23 cycles in Arrhenius' expression. The temperature of the inlet gas for air-propane was 293K, which was almost the same as in for the normal diesel combustion (291K). The data includes wide range of the injection timing between 50 deg.BTDC and TDC. When the temperature is low, the ignition delay in air-propane mixture agreed with the normal diesel operation. In retarded injection timing, the ignition delay decreased due to high temperature of gas and the fluctuation of every cycle increased.

　

Fig. 4. Ignition delay, Δθ₂, versus initiation of light oil injection, θ_inj (Air-hydrogen mixture, T_a=307 K)

　

Fig. 5 Arrhenius' expression of ignition delay, Δθ. (Air-hydrogen mixture, T_a=307 K)

　

Fig. 6 Effect of equivalence ratio on mass of air induced from intake port

　

　

Fig. 7 Ignition delay. Δθ, versus initiation of light oil injection θ_inj. (Air- propane mixture)

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