TS-79
Proceedings of the 6th International Symposium on Marine Engineering Tokyo, Japan, October 23rd to 27th, 2000
Ignition Delay of Light Oil Spray into Gaseous Fuel and Air Mixture in a Dual Fuel Engine
Eiji TOMITA*, Yoshisuke HAMAMOTO**, Arifin SIAGIAN*, Zhenyu PIAO* and Shogo FUJITA*
ABSTRACT
In diesel engines, ignition delay is determined as the period between the initiation of fuel injection and ignition. The ignition delay affects the initial combustion and the following combustion process and exhaust emissions. In this study, the gaseous fuel of propane, methane or hydrogen mixed with air was induced into the cylinder and light oil was injected in the cylinder as ignition source. In this study; the ignition delay in this kind of engine was focused as a fundamental study. When methane or propane is mixed with intake air, the ignition delay becomes long because the temperature of compressed gas decreases due to the decrease in the ratio of specific heats. When hydrogen is mixed with air, ignition delay becomes long because molar concentration of oxygen decreases. The concept of Livengood-Wu can be applied to the combustion in dual fuel engines by changing the value of a constant, A. In these kinds of dual fuel engine, the fluctuation of the ignition timing occurs due to the oxidation of the mixture near the TDC at retarded injection timing.
Key Words : Diesel Engine, Combustion, Dual Fuel Engine, Ignition, Gaseous Fuel
1. INTRODUCTION
In diesel engines, mixture of fuel and air is formed after injection of the fuel and ignition occurs. There exists a delay period from the initiation of the injection to the ignition. In high speed diesel engines, the ignition delay plays an important role to the initial combustion and the combustion process, yielding performance of the engine and formation of exhaust emissions, etc. Therefore, there have been many studies and equations for ignition delay are proposed [1-11].
Dual fuel engine, which operates on gaseous fuels with liquid fuel injection, is considered to be one of the key technology for using alternative fuels in order to minimize the use of diesel fuel while maintaining satisfactory engine performance [12]. Exhaust emissions and performance of Natural gas, LPG (Liquefied Petroleum Gas) and hydrogen are investigated as gaseous fuel [13-24], and VOC (Volatile Organic Compounds) is also applicable. In diesel operation, it is difficult to reduce smoke from the cylinder owing to the existence of rich region of fuel at heavy load. In dual fuel engine, smoke emissions decrease owing to lean burn in the cylinder. However, there are some problems associated with the conversion of a conventional diesel engine to dual fuel operation. The dual fuel engine exhausts much unburned fuel and carbon monoxide emissions at light load relative to the corresponding diesel performance. And cycle-to-cycle fluctuations of torque also occur at light load. A fundamental study of the fuel spray of combustion containing gaseous hydrocarbons was performed using a rapid compression machine [25].
In this study, gaseous fuel of methane, propane or hydrogen was mixed with inlet air and light oil was injected into the engine cylinder. Ignition delay was investigated by analyzing pressure data in the cylinder and compared that in normal diesel engine. And the equation proposed by Livengood-Wu was discussed.
2. EXPERIMENTAL METHOD
A direct injection diesel engine was used for this study. This engine is four-stroke cycle with single cylinder. The bore and stroke are 92 and 96 mm (volume displacement: 638cm3) and the compression ratio is 17.7. The combustion chamber is a deep dish type as shown in Fig. 1. The nozzle has four holes of which diameter is 0.26 mm and the injection angle is 150 degrees. The injection pressure is 19.6 MPa as presented in Table 1.
Gaseous fuel of methane (purity: 99%), propane (purity: 99%) or hydrogen was induced with air from an intake port and gas oil was injected in the cylinder. The cetane index of the light oil was 61. The combustion of the mixture occurs with the autoignition of the light oil in high temperature and pressure. The atmospheric temperature was 291 (or 293 K) and 307 K The main engine speed, n, was 1000 rpm and changed from 800 to 2000rpm. The amount of the light oil was 4mg/cycle, which corresponded to 0.08 in equivalence ratio. In normal diesel combustion, the amount of the light oil was changed from 11.2 to 18.7 mg/cycle in n=1000rpm. The equivalence ratio in total, φt, was from 0.25 to 0.53. The oil and water temperature were kept at 60℃. The injection timing was changed from 20 deg.BTDC to 5 deg.ATDC. In addition, the data at very early injection timing were also used.
