R&D on a Medium-speed, Four-cycle Diesel Engine Using Heavy fuel oil

Transcription

1 1999C R&D on a Medium-speed, Four-cycle Diesel Engine Using Heavy fuel oil 1. R&D contents 1.1 Background and R&D objectives In order to meet increasing demand for light oil and intermediate fraction, the yields of these substances were elevated through sophistication of cracking technology. As a result, it was inevitable that the quality of heavy oil would be affected, and it is expected that this trend will advance steadily in the future. Of the prime movers that burn fuel to obtain dynamic power, the diesel engine is unique in that it uses a broad range of fuels from light oil to heavy fuel oil. In the low-speed, two-cycle diesel engine for ships, for example, very heavy fuel oils are widely used. In contrast, the medium-speed, four-cycle diesel engine runs in short strokes at high speed as compared to the low-speed, two-cycle diesel engine. Consequently, although the equipment cost of this engine is relatively low and although it is widely used for electric power generation, combustion time is not long and the combustion space is flattened, so that it is difficult to obtain good combustion. Progress in application of this engine to heavy fuel oils has not yet equaled that for the low-speed, two-cycle diesel engine. Through establishment of fundamental combustion technology for reducing pollution (low NOx, low Particulate Matter (PM)) when using heavy fuel oils in medium-speed, four-cycle engines, dissemination of this kind of engine and expanded use of heavy fuel oils will be made possible, and from the standpoint of energy resource diversification and overall economy, the petroleum industry can be stabilized and vitalized. Accordingly, heavy fuel oil injection system and combustion system fundamental technologies were established for reaching the targets posted in Table 1-1. Table 1-1 Final objectives of R&D PM Thermal efficiency (%) Present status Around or above Around 40 Target 950 or below 0.1 or below 40 or above 1.2 Study contents (1) Computations In seeking to optimize engine injection system specifications and combustion chamber configuration, injection system computations were made for determining injection pressure and injection timing. For optimization of spray formation in the combustion chamber, spray flow dynamics were calculated. Since the vaporization characteristic of heavy fuel oil is substantially poorer than that of light oil or Marine Diesel fuel, a computation code was developed and used in consideration of the physical properties of heavy fuel oil. 1

2 Based on the results of the aforesaid computations, a fuel injection system and sample components of a fuel system were designed and manufactured for use with the high-speed and medium-speed four-cycle, single-cylinder engine used in elemental research. (2) Study of heavy fuel oil combustion using a high-speed, four-cycle, single-cylinder engine (undertaken in 1996 and 1997) Combustion tests were conducted using Marine Diesel fuel and various types of heavy fuel oil, and the impacts of fuel properties on combustion were determined. The possibilities for improving combustion with Marine Diesel fuel and heavy fuel oil stratified injection system were investigated, and tests were performed for improving combustion through optimization of injection system specifications. (3) Design and manufacture of medium-speed, four-cycle, single-cylinder engine A medium-speed, four-cycle, single-cylinder engine was designed, manufactured and installed. It was then used to verify the results of technology for improving combustion obtained with the high-speed, four-cycle, single-cylinder engine. (4) Elemental research Single unit tests were conducted on Marine Diesel fuel/heavy fuel oil stratified injection systems for the medium-speed, four-cycle, single-cylinder engine. Single unit tests were performed for determining stratified injection system supplies and injection characteristics. In addition, injection rate and percentages of Marine Diesel fuel and of heavy fuel oil were measured using a rotary type injection rate meter. (5) Study of heavy fuel oil combustion using a medium-speed, four-cycle, single-cylinder engine At first, injection system specifications and combustion chamber configuration were optimized, using a regular injection system. Then tests were performed for improvement of combustion by means of Marine Diesel fuel/heavy fuel oil stratified injection system. For comparisons, tests were also conducted in cases in which Marine Diesel fuel and heavy fuel oil were mixed beforehand. (6) Plan and design of Marine Diesel fuel/heavy fuel oil stratified injection system for actual engine Based on the results of research conducted thus far, Marine Diesel fuel/heavy fuel oil stratified injection system for medium-speed, four-cycle practical engine was planned and designed. 2. R&D results and analysis thereof 2.1 Computations In an effort to revise the fuel property values of a fuel spray flow computation code (KIVAII code), spray flow computations were made in this research and the impacts of fuel properties on spray formation were investigated. It was found that at the final moment of injection heavy fuel oil vaporizes only to about half the cetane. Positive measures must be taken to improve spray formation capacity. 2

3 It was learned that fuel dispersion in combustion chamber spaces can be optimized and vaporization characteristic can be improved by raising pressure through contraction of nozzle hole area and by increasing the number of nozzle holes, but within a range so that neighboring spray streams do not greatly interfere with each other. After investigating the impact of such things as injection nozzle specifications and piston configuration, trial components were manufactured. 2.2 Study of heavy fuel oil combustion using a high-speed, four-cycle, single-cylinder engine In order to improve the effectiveness of the stratified injection system, studies of combustion were undertaken in 1996 and 1997, using a high-speed, four-cycle single- cylinder engine. The properties of fuel used in those tests are presented in Table 2-1. It was confirmed that by elevating injection pressure and by optimizing combustion chamber shape, combustion performance equivalent to that of Marine Diesel fuel can be obtained with a Marine Diesel fuel/heavy fuel oil stratified injection system even when the percentage of heavy fuel oil is about 90%. Table 2-1 Properties of fuel used in combustion tests Marine Trial heavy fuel oil for Trial heavy fuel oil for Diesel fuel high-speed engine tests medium-speed engine tests Density 15 C Kinematic viscosity 50 C Residual carbon Asphalten Saturation component Aromatic component Resin component Sulfur component Flash point Lower calorific value Hu 2.3 Design and manufacture of medium-speed, four-cycle, single-cylinder engine A medium-speed, four-cycle, single-cylinder engine was designed and manufactured for use in combustion tests. A section view of the engine is shown in Figure 2-1. Cylinders were inclined on the assumption of a V-type engine often used with this class of four-cycle, power-generating engine. The completed engine is shown in Figure 2-2. Major specifications are presented in Table 2-2. Table 2-2 Major specifications of medium-speed, four-cycle, single-cylinder engine Bore Stroke Engine speed Actual engine equivalent output Actual engine equivalent mean effective pressure Mean piston speed 3

4 The electronically controlled fuel injection pump shown in Figure 2-3 was installed in a medium-speed, four-cycle, single-cylinder engine. With a regular Bosch fuel injection pump, injection timing and effective plunger stroke are implemented by changing the relationship between barrel oil supply port and lead to which the plunger has been attached. On the other hand, with the electronically controlled fuel injection pump, after the solenoid valve (normally open) attached to the top of the pump has been closed at requisite timing, it then opens after a prescribed period of time. In this way, flexible operation can be made possible. Fuel injection pump Suction room Valve system Piston Cam shaft Conduct Cam shaft Crank shaft 1st balancer Figure 2-1 2nd balancer Section view of Figure 2-2 medium-speed, four-cycle, single-cylinder engine Photo of exterior 4

5 To injection valve Heavy fuel oil Solenoid valve To fuel oil supply line Marine Diesel fuel Plunger Fuel confluence component Figure 2-3 Roller Electronically controlled fuel injection pump Figure 2-4 Fuel valve for stratified injection 2.4 Elemental research Shown in Figure 2-4 is a stratified injection valve for medium-speed, four-cycle, single-cylinder engines. In the needle chamber of the fuel valve body there is a component for confluence of Marine Diesel fuel and heavy fuel oil, with heavy fuel oil being supplied through a check valve. Figure 2-5 gives an example of heavy fuel oil injection status during engine testing. Following signals from two solenoid valves arranged in series, the supply pressure on the side of flow behind the solenoid valves rises. We can see that heavy fuel oil is supplied through a check valve because the injection pipe pressure rises. Tests to confirm operation with the injection system alone were performed prior to engine testing. The test device is shown in Figure 2-6. It is equipped with a slit that rotates in tandem with the spray, and the spray is collected at a 2 crank pitch angle. In this way the percentages of Marine Diesel fuel and of heavy fuel oil were determined. Measurement results are shown in Figure 2-7. The data thus obtained was used in analysis of the data on combustion by a medium-speed, four-cycle, single-cylinder engine. 5

6 Cylinder pressure MPa Cylinder pressure Supply pressure (flow behind solenoid valve) MPa Injection pipe pressure Needle valve lift Supply of heavy fuel oil Solenoid valve signal 2 Injection pipe pressure MPa Solenoid valve signal 1 Figure 2-5 Crank angle ADTC Heavy fuel oil supply status (a) Complete device (b) Enlarged view of rotary slit Figure 2-6 Stratified injection single-unit test device 6

7 Percentage within each slit Percentage against full injected amount Heavy fuel oil Marine diesel fuel Percentage within each slit Percentage against full injected amount Heavy fuel oil Marine diesel fuel Slit (a) Heavy fuel oil percentage 78% (b) Heavy fuel oil percentage 45% Slit Figure 2-7 Measurements of Marine Diesel fuel and heavy fuel oil percentages 2.5 Study of heavy fuel oil combustion using a medium-speed, four-cycle, single-cylinder engine (1) Improvements with regular injection system At first, tests for improvement of combustion were conducted, using a regular injection system. An attempt was made to lower PM through promotion of spray atomization by reducing nozzle hole diameter and by elevating injection pressure. Specifications were also sought so as not to exacerbate fuel consumption and NOx emissions. The results are presented in Figure 2-8. In this way, it was found that PM can be halved and the trade-off between NOx and fuel consumption can be improved somewhat. Cylinder pressure MPa Needle valve lift Nozzle hole specifications 9 hold x 0.64mm 9 hold x 0.50mm Injection pressure MPa PM mg/nm 3 Nozzle hole specifications 9 hold x 0.64mm 9 hold x 0.50mm Magnitude of change in thermal efficiency % Figure 2-8 Crank angle deg Improvement of combustion through small-diameter nozzle hole 7

8 (2) Improvements with stratified injection system Tests of improvement in combustion when using a stratified injection system were performed. Figure 2-9 shows cylinder internal pressure when the percentage of heavy fuel oil has been changed, plus the heat release rate derived from analysis of the same. With 93% heavy fuel oil, as opposed to 100% heavy fuel oil, the heat generation rate rises from the initial period to a peak, and after burning is improved. Presented in Figure 2-10 are the changes in PM and in thermal efficiency in relation to heavy fuel oil percentage. As heavy fuel oil gradually decreases from 100%, PM diminishes and NOx rises. The relationships between NOx, thermal efficiency and PM are shown in Figure Research targets respecting NOx, thermal efficiency and PM were all reached. To confirm the effectiveness of stratified injection, Marine Diesel fuel and heavy fuel oil were mixed in advance and combustion tests were performed. In Figure 2-11, the "" indicates that in the case of a mixture of Marine Diesel fuel and heavy fuel oil, combustion is improved with stratified injection of the same fuel percentage. In other words, with stratified injection, a large improvement in combustion can be obtained with a low percentage of Marine Diesel fuel. In Figure 2-12, the heat release rate of mixed fuel has been subtracted from the heat release rate of stratified injection. The figure shows that the heat release rate of stratified injection is high on the + side. The pattern of stratified injection inferred from single-unit test results is shown in the figure. The figure indicates that because the percentage of Marine Diesel fuel is high at the injection initial period and late period, heat release rate becomes high with stratified injection within a corresponding time period, and improvements can be realized in ignition and after burning. Cylinder pressure MPa Marine Diesel fuel Heavy fuel oil 93% Heavy fuel oil 100% Heat release rate J/deg Figure 2-9 Crank angle ATDC Changes in heat release rate 8

9 Symbol Injection System Injection timing Stratified injection -6.0ATDC -3.5 Mixed fuel -3.5 PM mg/m 3 Target area Stratified injection Heavy fuel oil percentage Thermal efficiency PM mg/m 3 Thermal efficiency Target area Heavy fuel oil percentage Figure 2-10 NOx and PM versus heavy fuel oil percentage Figure 2-11 Relationships among NOx, thermal efficiency and PM 9

10 Fuel percentage in case of stratified injection Heavy fuel oil Case of mixed injection Marine Diesel fuel Differences in heat release rate (stratified-mixed) kj/deg Figure 2-12 Crank angle ATDC Superiority of stratified injection system over mixed fuel (heavy fuel oil percentage 80%) 2.6 Planning and design of Marine Diesel fuel/heavy fuel oil stratified injection system for practical engine With a practical engine, metering of heavy fuel oil must be done accurately. Accordingly, a volumetric type supply system was planned and designed. The pistons were established to complete stroke in response to inflow and outflow of working oil, accomplished by opening and closing of the solenoid valves. With a format in which heavy fuel oil is forced into the system, it is possible to adjust the supply volume of heavy fuel oil by changing the piston stroke. 3. Results of empirical research (1) Computations The fuel property values of a fuel spray flow computation code were revised and the impact of fuel properties on spray formation was investigated. It was learned that fuel dispersion in combustion chamber spaces can be optimized and vaporization characteristic can be improved by raising injection pressure through contraction of nozzle hole area and by increasing the number of nozzle holes, but within a range so that neighboring fuel spray do not greatly interfere with each other. 10

11 (2) Study of heavy fuel oil combustion using a high-speed, four-cycle, single-cylinder engine It was confirmed that by elevating injection pressure and by optimizing combustion chamber shape, combustion performance equivalent to that of Marine Diesel fuel can be obtained with an Marine Diesel fuel/heavy fuel oil stratified injection system even when the percentage of heavy fuel oil is about 90%. (3) Design and manufacture of medium-speed, four-cycle, single-cylinder engine A single-cylinder engine was completed having a structure in which the cylinder was inclined on the assumption of a V-type engine, as often used with this class of four cycle, power-generating engine. Flexible operation becomes possible by introducing an electronically controlled fuel injection system. (4) Elemental research Tests were performed to confirm stable operation with the stratified injection system alone. By collecting spray at a 2 crank pitch angle using a slit that rotates in tandem with the spray, it was possible to determine the percentages of Marine Diesel fuel and of heavy fuel oil injections. (5) Study of heavy fuel oil combustion using a medium-speed, four-cycle, single-cylinder engine At first, tests for improvement of combustion were conducted, using a regular injection system. An attempt was made to lower exhaust smoke through promotion of spray atomization by reducing nozzle hole diameter and by elevating pressure. In this way, PM could be halved and the trade-off between NOx and fuel consumption could be improved somewhat. In addition, tests were performed for improving combustion by means of a Marine Diesel fuel/heavy fuel oil stratified injection system. Improvements in early period combustion and in late period combustion were higher with the stratified injection system than when Marine Diesel fuel and heavy fuel oil have been mixed in advance; and with the stratified injection system, a high improvement in combustion could be obtained with a low percentage of Marine Diesel fuel. With 950ppm of NOx, 0.07g/m 3 of PM and 41% thermal efficiency, all target values could be cleared. (6) Planning and design of A heavy oil/heavy fuel oil stratified injection system for a actual engine A volumetric type supply system was planned and designed so that metering of heavy fuel oil can be done accurately, and the system in turn was designed to be arranged on actual engine. 11

12 4. Synopsis In the present R&D, an attempt was made to improve combustion with a regular injection system so as to improve combustion by a medium-speed, four-cycle, diesel engine for which heavy fuel oil is used. Thereafter it was confirmed that combustion can be effectively improved with an A heavy oil/heavy fuel oil stratified injection system. In making these results practical, the following issues are relevant. 1) As the problems of CO 2 and exhaust gases become more critical, even higher thermal efficiency and lower pollution will become necessary from the time of planning for the third period of research. 2) The reliability of total engine systems employing practical engines with many cylinders will have to be included as a subject of investigation. Consequently, even higher targets will have to be posted, and combustion tests will have to be conducted in which all types of combustion improvement technology are incorporated, not only stratified injection, but also cycle revision, for instance, injection mode control, and water injection. Obstacles to practical application will have to be identified and improvements will have to be implemented for overcoming them. Copyright 1999 Petroleum Energy Center all rights reserved. 12