Fig. 8 Arrenius' expression of ignition delay, Δθ. (Air-propane mixture)
Fig. 9 Coefficient variance of ignition delay, CV(τ2), versus initiation of injection timing
Fig. 10 Heat release rate versus crankangle
Therefore, cycle-to-cycle fluctuations of ignition delay were investigated. Here, coefficient of variance, CV(τ), was defined as the value of the standard deviation of fluctuations divided by the averaged value of ignition delay. The values of CV(τ) were plotted versus the initiation of injection timing, θinj as shown in Fig.9. When the injection timing was advanced, the value of CV(τ) decreased. Under all the conditions, the value of CV(τ) with gaseous fuel and air mixture was larger than that of the normal diesel combustion. On the contrary, when the injection timing was retarded to the top dead center of the compression, the value of CV(τ) with gaseous fuel and air mixture increased suddenly compared to the normal diesel operation.
The heat release rates of 23 cycles at the initiation of the injection timing of the TDC for the normal diesel combustion (φt=0.33) and for air-hydrogen mixture (φt= 0.37) are shown in Figs.10(a) and (b), respectively. The maximum value of heat release rate in air-hydrogen mixture was smaller than that in the normal diesel combustion. In the normal diesel combustion, the ignition delay and the following combustion of every cycle presented almost the same value and stable engine operation could be achieved. However, in air-propane mixture, the ignition delay of every cycle showed somewhat different, forcing the following combustion to fluctuate. In other gaseous fuel of methane or hydrogen, the fluctuation also occurred. It is considered that the fluctuation in air-propane mixture became especially large because the combustion became slower due to the lower temperature of the gas in the cylinder than that in other gaseous fuel.
4. PREDICTION OF IGNITION DELAY
Pressure and temperature of gas in the cylinder change from the initiation of the injection till ignition. Here, the equation proposed by Livengood-Wu was used to predict the ignition delay of light oil spray into gaseous fuel and air [27].
