2. A very delicate subject of the analysis of combustion characteristics is challenged. Are the test results affected by the effect of the matching of the fuel injection nozzle with the oil kind or the combustion chamber, i.e., when the injection nozzle optimized for BFO is used?
[Author's reply]
1. The Specific Fuel Consumption [g/kW・h] was obtained by measuring the consumption of GO and BFO under the condition where the engine output is constant at 77.2 kW (the load factor is 75%) by the electronic balance. The rate of heat release of BFO was corrected by multiplying the ratio of the heat generation of BFO to that of GO with the original value so as to eliminate the effect by the difference in heat generation of both fuels. Thus, the difference in the rate of heat release shown in Figs. 11 and 12 is considered to be attributable to the difference in the combustion characteristics by the fuel properties.
The CO2 emission was calculated by the quantity G' of the dried combustion gas by the formula (1) mentioned below from the ratio of content C (0.859 for GO, and 0.855 for BFO) of the carbon in the test fuel oil, and the CO2 concentration and CO concentration of the measured exhaust gas, and obtained from the formula(2).
G' = (22.4C/12)/(CO2+CO) [Nm3/kg] (1)
CO2 = G' (CO2)/(22.4/44) = (44C/12)(CO2/(CO2 + CO)) [kg/kg] (2)
On the other hand, in a case of the complete combustion of the fuel from the basic chemical reaction formula (C + O2 = CO2 4- 407 kJ/kmol) in the combustion of carbon, CO2 of approximately 3.15 kg is emitted from the fuel of I kg both for GO and BFO.
That means, the relationship between the rate of fuel consumption and the CO2 emission can be expressed by the following formula.
CO2 = (44C/12)(CO2/(CO2 + CO))(SFC) [g/kW・h]
Or
CO2 = 3.15(SFC) [g/kW・h]
In this test, the C02 emission was obtained using these two methods, but the values from both methods were almost same because the quantity of CO is extremely smaller than that of C02.
2. In the residual fuel containing the residues, a problem is presented in the point that no residues can be evaporated at the in-cylinder air temperature before the ignition, and it is necessary to promote evaporation of the residues by realizing the active combustion of the distillation parts. Though the evaporation characteristic of BFO can not be solved by the physical conditions such as the nozzle diameter and the injection pressure, impingement of the non-evaporated fuel spray with the piston can be avoided by reducing the nozzle diameter to reduce the fuel spray travel during the ignition delay.
That means, in considering the matching of the injection system with the combustion chamber in using BFO, the situation leading to adhesion to the liquid phase part as indicated in Fig. 14 can be avoided by combining the shallow-pan type combustion chamber with the relatively small nozzle diameter as employed in the present large-speed marine diesel engine.
However, such a situation is likely to occur with medium and small-sized engines when the residual fuel is used, and generation of black smoke and slow combustion in the diffused combustion period will present a problem. However, impingement of the fuel spray includes not only such an adverse effect but also the combustion promoting effect for the evaporated fuel spray5), and it is believed to be necessary to consider the combination of the shape of the combustion chamber with the injection system, taking into account both aspects.
