The ignition delay is defined as the period (crank angle) from the initiation of the injection to the time that heat release rate rises suddenly. In some cases, two-stage ignition could be observed. The first rise of the heat release rate is defined asθig1, which is considered to be the initiation of low temperature oxidation process. The second rise is defined as θig2, which corresponds to the occurrence of the hot flame. Here, the ignition delay Δθ[deg.] (τ[ms]) is defined as the difference between the initiation of the injection timing, θigj, and the second rise of the heat release rate, θig2.
The temperature of the gas in the cylinder was estimated from the state equation of ideal gas using the pressure that was measured. In this study, the engine was not driven with only gaseous fuel because the temperature of the gas was too low to be ignited. Therefore, the ignition delay of light oil spray under the condition of gaseous fuel mixture with air is discussed in this study.
Fig. 1 Configuration of combustion chamber
Table 1 Experimental conditions
3. EFFECT OF SURROUNDING CONDITIONS ON IGNITION DELAY
Figure 2 shows the initiation of the ignition delay, Δθ2, versus fuel injection timing, θinj. In the normal diesel operation, the ignition delay in inlet air temperature of 291 K (18℃) presented almost the same value in that of 307 K (34℃). The ignition delay presented the minimum value near 5 deg.BTDC of the initiation of the injection. As shown in Fig. 3, the ignition delay of Δθ[deg.] (or τ [ms]) is expressed as the function of inverse of gas temperature at the initiation of the injection (Arrhenius' expression). The solid line shows the ignition delay in T2=291 K. It includes the results at very early injection timings, for example, about 50 deg.BTDC. As the temperature of inlet air is high, the temperature of the gas at the injection timing increases and this decreases ignition delay. However, when the temperature of inlet air is high, less mass of air is induced in the cylinder. As this phenomenon makes the molar concentration decrease, ignition delay becomes shorter [11]. Therefore, both effects balanced with each other and the ignition delay in higher temperature of inlet air shows the same value as that in lower temperature.
Figure 4 shows the result of ignition delay using hydrogen as gaseous fuel that was induced from the intake port. Generally, the ignition delay was larger than that in the normal diesel combustion. The temperature of the gas in the cylinder was estimated from the state equation of ideal gas using pressure data. As a result, the temperatures for both cases were almost the same. The gas temperature in the compression stroke is almost the same, because the ratio of the specific heats, κ, is almost the same.
Fig. 2 Ignition delay, Δθ, versus initiation of light oil injection, θinj. Effect of inlet air temperature on ignition delay (Ta=291 K and 307 K).
Fig. 3 Arrenius' expression of ignition delay, Δθ. Effect of inlet air temperature on ignition delay (Ta=291 K and 307 K).
