TS-76
Stratified Injection System for Heavy Fuel
Shin-nosuke OSAFUNE*, Kazuhiko IWAMOTO**, Tatsuo TAKAISHI ***, Hiroyuki ISHIDA***, Koji TAKASAKI**** and Masayoshi NAKASHIMA****
ABSTRACT
Medium and large-sized diesel engines have two economic advantages: high efficiency and use of cheap fuel, the latter being made possible by the use of heavy fuel. But recently, the quality of heavy fuel is becoming worse and utilization is becoming more difficult. Main problems of such heavy fuel are long ignition delay and long after-burning. It can easily be supposed that the former is depending on the first injected fuel and the latter on the fuel injected last. In this paper a new fuel system called Stratified Fuel Injection System is proposed. This system makes it possible to inject a small amount of light fuel at the beginning and at the end of injection duration. Heavy fuel is injected in between and sandwiched by the light fuel. A prototype of that injection system is assembled and applied to a high-speed engine. As a result, thanks to only 10% addition of light fuel, the heavy fuel can be burned well just the same as pure light fuel.
Key words: Heavy Fuel, Ignition Delay, After-burning, Stratified Fuel Injection System
1. INTRODUCTION
In the near future, it is very likely that the quality of bunker fuel oil (BFO) becomes much lower than now. However, as the only advantage of the BFO is its low cost, it is unrealistic to process the BFO before the use. So it is necessary to develop a new technique to control the combustion of such low-grade fuels.
In this study some experimental results about the combustion characteristics of such low grade BFO are introduced. According to it, there are two main problems using low grade BFO: poor ignition characteristics and a long after-burning phase. 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 like marine diesel oil (MDO) with short ignition delay and good combustion characteristics is injected. Between these two phases low -grade BFO is injected through the same nozzle. The BFO injection is 'sandwiched' over time.
2. PROBLEMS OF LOW-GRADE BUNKER FUEL OIL
An experimental result about the combustion characteristics of such low grade BFO is introduced in Fig.1 [1]. In this figure, the heat release rates of two bunker fuel oils named BFO-S and BFO-A in the high-speed test engine are shown. Though the both fuels were common on the market, many 10w-speed marine engines driven with BFO -A have suffered abnormally heavy wear of the cylinder liners in a short time, whereas the same engines have not experienced any problems with BFO-S at all.
Fig. 1 shows a higher peak of the pre-mixed combustion immediately after the ignition for BFO-A. This is due to the longer ignition delay. During the diffusive combustion stage, which lasts up to a crank angle of 15゜, BFO-A has a lower heat release rate than BFO-S. The diffusive combustion is followed by after-burning, which continues up to a crank angle of 30゜ for BFO-A. The after-burning of BFO-A is longer than that of BFO-S. It is concluded that BFO-A has poorer ignition and combustion characteristics and has a longer combustion duration.
The distributions of distillation temperatures of the two fuels are also shown in Fig. 1. Comparing the two fuels, BFO-A contains more low temperature distillates (200-300 ℃) and less higher temperature distillates (300-500 ℃) than BFO-S. The shape of the distribution graph is near to that of so-called Dumbbell fuel. The black bar in this figure means the residual portion that is the vacuum residue from the refinery. It is more than 50% for both the fuels.
The poor combustion characteristics of BFO-A could be related to the fact that the distribution of distillation temperatures is near to the image of Dumbbell fuel. So, the following investigation is carried out. To compensate for the shortage of high temperature distillates (350-500 ℃) in BFO-A, a sample fuel named SF-L that contains only higher temperature distillates and little aromatic hydrocarbons is added to the BFO-A. Combustion characteristics of BFO-A are observed first. Secondly. SF-L is added to BFO-A and the combustion characteristics are observed under the same conditions in the visual engine.
Fig. 2 shows a result of high-speed visualization of the combustion process using the visual test engine [1]. It is actually a supercharged, 2-stroke single-cylinder engine with a bore of 190 mm. Pmi and engine speed at the visualization test are 15 bar and 400 rpm respectively. The injection starts at 4｡?TDC and ends at 19｡?TDC, and the maximum injection pressure is 100 MPa.
Fig. 2 (a) shows the combustion process of 100% BFO-A. Fig. 2 (b) shows the process for a mixture of 50% BFO-A and 50% SF-L. The ignition is observed first. In case (a), ignition occurs at 2.5゜ATDC, but all 8 flames do not ignite at the same time. In case (b), ignition already occurs at TDC. All 8 spray cones ignite at the same point in time. As mentioned above, the injection starts at 4゜BTDC.
