Thus, the two ignition delays shown on the photos are over 6 deg. in case (a) and 4 deg. in case (b).
The amount of soot at the impingement point indicates the fuel-air mixing condition in the flames. At 12.5｡?TDC, more soot can be seen in case (a) than in case (b). In case (a) at 22.5｡?TDC, which is after the end of injection, unburnt stripe-shaped soot remains, while all the soot is quickly reburnt in case (b). At 32.5｡?TDC, the flame is still burning in case (a), whereas in case (b) the combustion is already completed.
According to the above-mentioned experimental results, it is concluded that there are two main problems using low-grade fuel: poor ignition characteristics and a long after-burning phase. Normally, the first injected fuel ignites, and fuel injected last causes after-burning. Thus, the control of first and last injected fuel is considered to be most important.
Therefore, a new fuel injection system is proposed. This so-called Stratified Fuel Injection System functions as follows: at the beginning and the end of the injection process, a light fuel such as marine diesel oil (MDO) is injected. Between theses two phases, low-grade BFO is injected through the same nozzle. The BFO injection is 'sandwiched' over time.
3. EXPERIMENTAL APPARATUS
3.1 Test Engine
A supercharged single-cylinder engine with 170 mm bore (H-170 engine) is used for the test runs. The specifications of this engine are given in Table 1.
3.2 Working Principle of the Stratified Fuel Injection System
The working principle of the Stratified Fuel Injection System is shown in Fig. 3. The whole system consists of an MDO injection pump, a fuel injection nozzle and a BFO supply unit. The MDO injection pump is equipped with the non-return valve X and the fuel injection nozzle with the non-return valve Y. The fuel injection nozzle has a special BFO passage, which is connected to the MDO passage. The BFO supply unit feeds the required quantity of BFO into the passage of the injection nozzle.
The working principle is as follows. Before the start of injection, the BFO is fed into the injection nozzle at a pressure which is higher than the opening pressure of the non-return valve X (located in the injection pump), but lower than the opening pressure of the needle of the injection nozzle, as shown in Fig. 3 (a). During the period of BFO supply, a certain quantity of MDO flows back to the MDO injection pump, passing the non-return valve X. Some MDO remains in the tip of the nozzle (Fig. 3 (a)). When the injection starts, the non-return valve Y blocks the passage of BFO. The MDO in the tip of the nozzle is injected first. Then, the BFO in the MDO passage is fully injected, followed by MDO from the same passage (Fig. 3 (b)).
4. EXPERIMENTAL RESULTS USING THE STRATIFIED FUEL INJECTION SYSTEM
Fig. 4 shows the results obtained with the H-170 engine and the Stratified Fuel Injection System at full load. The abscissas in Fig. 4 (a)〜(c) represent the percentage of MDO added to the BFO. 0% MDO means 100% pure BFO. Fig. 4 (a) shows that adding 10% of MDO leads to an ignition delay which is close to that of pure MDO.
Fig. 4 (b) shows the measurement of particulate matter (PM) in the exhaust gas. As expected, BFO emits much more PM than MDO. With an addition of 10% MDO using the Stratified Fuel Injection System, the PM is reduced to the same value as for pure MDO. Fig.4 (c) shows the change in specific fuel consumption (SFC) with MDO%. Each SFC data is converted to the heat value of MDO. It also shows that 10% addition of MDO improves the SFC drastically. According to these results, it is concluded that the system is highly effective. Even though the percentage of MDO is relatively low, good combustion characteristics, almost equal to those of pure MDO, can be obtained.
Fig. 4 (d) and (e) show the heat release rate and the cylinder pressure. With regard to the cost of the fuel, a small percentage of MDO is desirable. Therefore, a further reduction in the amount of MDO to 4% is examined. Comparing the addition rates of 11% and 4% MDO, the 11% addition shows almost the same combustion characteristics as pure MDO, whereas the 4% addition shows a much lower heat release rate during the main combustion duration. The maximum cylinder pressure becomes much lower. It is concluded that 4% of MDO is not sufficient.
5. EXPIANATION OF THE HIGH EFFECTIVENESS OF THE SYSTEM THROUGH FUEL-SPRAY SIMULATION
To understand the characteristics of the fuel spray in detail, numerical simulation is performed using the software package KIVA [2]. In Fig.5 the calculation conditions and the results are shown. The purpose of this simulation is to examine the distribution of the fuel particles within the fuel spray, i.e. the location of the fuel particles injected at the beginning and those injected at the end of the injection duration within the spray.
In Fig. 5 (a), the distribution of the fuel particles at the beginning of the injection duration is examined. On the left, the model injection rate is shown and on the right the resulting distribution of the particles. The black dots of the spray represent the fuel injected at the beginning (first 1/8 of the injection duration). They are marked in black in the model injection rate. The other particles are marked in gray.
The fuel particles injected at the beginning of the injection duration are first located at the tip of the spray (t=0.9 ms). These particles are slowed down by the air drag. The later injected particles of the fuel spray penetrate ethe earlier injected ones at a high velocity and push them to the side (t=1.5 ms). At t=2.4 ms, all the early injected fuel particles have been pushed to the side of the spray.
In Fig. 5 (b), the distribution of the fuel particles injected at the end of the injection duration is examined; the black dots represent the fuel particles injected at the end (1ast 1/4 of the injection duration). As shown in Fig. 5 (a), the fuel particles injected at the beginning are pushed from the center to the side of the spray where a sufficient amount of air is available for the combustion. However, the fuel injected at the end remains in the center of the spray (t=4. 2 ms to t=5.4 ms), because no spray is following to push it to the side.
The results from this calculation explain the high level of effectiveness of the Stratified Fuel Injection System. If the black particles of the spray in Fig. 5 (a) represent a light fuel like MDO, then the early igniting MDO flame surrounds the gray particles representing the low-grade fuel. Then the temperature of the low-grade fuel spray will soon rise and the evaporation of the low-grade fuel droplets is accelerated. As mentioned above, the fuel particles injected last remain in the center of the spray, where the air supply is not sufficient. This difficulty is the reason for the after burning and the long combustion duration of low-grade fuel. If the fuel injected last is a fuel like MDO, this problem would be much less severe. The effectiveness of the Stratified Fuel Injection System is explained clearly with this simulation.
