过量空气系数与柴油机的性能效应.doc

過量空氣係數與柴油機的性能效應田文國1 葉榮華2摘要由於現行新造船舶為二行程柴油機具有超長行程與低轉速的趨勢，各航運公司為了經濟上的考量，紛紛採用劣質重油，這些對輪機技術要求相應提高否則對柴油引擎的性能，尤其是採用廢氣渦輪增壓機的船舶主機產生重大的影響。由於機器維修保養技術的不當，造成過量空氣(掃氣)係數的大幅降低，使排氣溫度明顯上升，使主機的熱負荷增大與效率顯著的降低。為探討此問題，本文主要提出過量空氣係數與柴油引擎性能的關係，透過新造船舶於海上公試的一系列測試數據，了解影響柴油機性能的一些重要參數。並探討如何提升過量空氣係數技術，以降低重柴油機部件熱負荷與提升機器最大設計性能。此外，於文中並舉實例，透過針對過量空氣係數採取一系列維修保養技術來降低引擎的熱負荷，以提升柴油機的性能與操作效率。Effects of excess air on the performance of marine diesel engineWen-Kwo Tien1, Rong-Hua Yeh2ABSTRACTThe use of long stroke and the enhancement of the thermal efficiency of the diesel engine have resulted in lower exhaust gas temperatures. This has decreased the efficiency of the propulsion system to some extents particularly with waste heat turbocharger. In consequence, there is an apparent decrease in the flow rate of intake and a temperature rise in exhaust gas, which may cause a larger thermal load and a lower efficiency of the main engine. The purpose of this study is to investigate the effect of the coefficient of excess air to the performance of engine. Through a series of the sea trials for newly built ships, the factors that have a great influence of engines performance are determined. In addition, methods of raising the coefficient of excess air are proposed to reduce the thermal load and accordingly enhance the performance of diesel engine. Finally, to achieve an efficient operation of the main engine, an example of trouble shooting and maintenance of the engine is given for practical applications.61.Introduction1. 台灣海洋大學商船系副教授2. 台灣海洋大學輪機系教授Diesel engines used in marine applications span a wide range of technologies and applications from small auxiliary engines to very large ocean-going propulsion engines. To combust the fuel more efficiently, a turbocharger is always mounted in the exhaust flow of the engine. A turbocharger has a turbine that powers the compressor using wasted energy from the exhaust gases. A key advantage of turbochargers is that they offer a considerable increase in engine power with only a slight increase in weight. An aftercooler is often used between the turbocharger and the engine to cool the charge air. This cooling makes the air denser and allows more air to enter the cylinder. By lowering the charge air temperature, the peak combustion temperature is also reduced, thereby reducing NOx emissions. The increased charge air density also increases power density, allowing a smaller displacement engine to do the work that would normally require a larger engine. Another benefit of aftercooling is the potential to improve brake-specific fuel consumption. Nowadays, turbocharging has been very common on diesel engines for marine and heavy machinery applications (Pulkrabek 2003). A turbocharger combined with a smart turbocharging system is able to guarantee an optimum air-fuel ratio under all conditions, contributing even more to the control of soot emissions (Woodyard 2004).Stable combustion requires fuel, oxygen, and a source of ignition. There is roughly 21% oxygen by volume in the air. A 50% excess air condition implies approximately 10.5% oxygen remains in the boiler or engine exhaust stack. Choosing an excess air level is based on the type of fuel and the degree of combustion control that can be achieved for a specific system. Excess air levels range from about 5 to 10 percents for gas or oil fired furnaces to about 15 to 50 percents for coal and wood fired units (Richard 2003). For a traditional marine diesel engine, the excess air coefficient is in the range of 180% to 220% (Hsueh 1997, Technical Report 1996). It is reported that the newly designed MAN B&W SMC-C series engines which burn IFO 380 oil have excess air coefficients over 320% (MAN B&W 1996). According to the work of Robert and Philip (2002), the beneficial effect on emissions of high excess air is clear for all regulated emissions. A 40% increase in excess air at high load reduces fuel consumption by 16%, and a 100% increase in excess air by 19%. The exhaust gas recirculation(EGR) system is a very effective technique for reducing NOx emission from a diesel engine, particularly at the high load of engine operation condition. In a turbocharged engine, however it is difficult to introduce EGR at the high load because of the high boost pressure in the air intake. Introducing and evaluating the exhaust gas recirculation system to the turbocharged engines, Yokomura et al. (2003) showed that the system with venturi is rather effective for expanding the EGR range up to high engine load conditions. Conducting an experiment, Ogawa et al. (2003) pointed out that a combination of the divided cavity, EGR and intake air throttling was effective in simultaneously eliminating knocking and reducing total hydrocarbon and NOx. At lower overall excess air ratio conditions, NOx reduction was shown with a biform mixture composition without slightly lean or extremely rich regions.The purpose of this study is to investigate the effects of excess air on the performance of long stroke, slow speed marine diesel engines. Initially, a simple model is presented and the calculated results are compared with empirical formulation proposed by Chang and Lu (1992) and available data from the engine sea trials. The dominant parameters that affect the amount of excess air of the propulsive engine are introduced and discussed. In applications, considering these parameters, several effective steps are suggested to take for the improvement of the main engines serviced for years.2. AnalysisExcess Air is defined as the amount of air in excess of the stoichiometric amount. The proper condition of stoichiometric combustion, where only enough air is supplied for complete oxidation of each hydrogen and carbon atom from the fuel, is extremely difficult to maintain. Therefore, it is common to use more air than the stoichiometric (theoretical) amount in the combustion chamber. It may increase the chances of complete combustion. It is referred as lean condition when more than stoichiometric air is present or rich condition when less than stoichiometric air is present for a given amount of fuel. For actual combustion in an engine, the coefficient of excess air, , is defined as follows(1)where L stands for the actual mass of air needed and L0 represents the theoretical mass of air needed in the cylinder for one kg fuel that completely burns. It is worthwhile to note that L0 is approximately equal to 14.5 kg in most case (Pulkrabek 2003). Actually, L0 ranges from 14.8 to 15.12, depending on fuel quality (Obert 1973, Shen 1980, and Lee 1994).Compression ratio, , is a ratio between the volume of a combustion chamber when the piston stops at the point closest to the crankshaft and the volume when the piston stops at the furthest point away from the crankshaft. The higher the compression ratio, the more mechanical energy can be obtained for an engine. It is defined as(2)where Vd, displacement volume, is the volume swept by the piston as it travels from bottom-dead-center to top-dead-center. Note that Vc is referred as clearance volume and is the minimum cylinder volume which occurs when the piston is at top-dead- center. Because of the complex shape of this space, it is usually measured directly rather than calculated.In this work, intake air is considered as an ideal gas such that the ideal gas relationship can be used, i.e.p=RTscav(3)where p is scavenging pressure, is air density, R is gas constant (=0.287 kJ/kgK), and Tscav is scavenging temperature. It is assumed that the main engines are two-stroke engines with constant pressure turbochargingand uniflow scavenging via a single hydraulically operated exhaust valve in the cylinder head. Substituting equations (2) and (3) into equation yields(4)Furthermore, a famous and straightforward empirical relationship for excess air coefficient is expressed as (Chang and Lu 1992) (5)where qw is the ratio of heat removed by coolant, i is indicated thermal efficiency, Hu is lower heating value of fuel, Cpg and Cpa are specific heats of gas and air for constant pressure, Texh is exhaust temperature of the gas, and is the coefficient of scavaging.3. Data of Sea TrialUsually, after the offer has been accepted or a buying agreement in written form of a ship has been signed, a sea trial is necessary. The sea trial data is valuable particularly when these data are analyzed. Sea trial analysis can help fix problems when they appear, and better yet, it can be used in the design process of future vessels to help avoid these problems from the outset. Abnormal exhaust is an indicator that an engine is running abnormally. Items checked underway will give a more accurate assessment. Sea trial data is collected to determine if the engines are running within the specified parameters. Engine information such as engine rpm, engine power, gear box ratio, and fuel consumption are obtained.The conditions for the tests were calm wind and flat seas in brackish water of 20 meters depth. Speed was calculated with measured time across a run of a known distance. Vessel trim was estimated with a bubble inclinometer, and fuel rate was available through the engines digital readout. There are estimates and predictions used in the analysis, as well as potential sources of measurement error. Given good measurement data and an accurate definition of the propeller (particularly pitch and cup), the accuracy should be well within 10%, in most cases within 5%. Fortunately, the purpose of these analyses is to find indicators and trends, so this small inaccuracy is acceptable.For a two-stroke diesel engine, air is first ingested and then compressed in the engines cylinders. After the inlet air is compressed, the pressure in the cylinder is very high. Assume this process to be followed by an isentropic compression stroke, the pressure in the cylinder after compression can be easily predicted bypcomp=pscav1.4(6)Table 1 presents the comparison of predicted and measured pressures in cylinders of engine after compression. It shows that the measured pressures are a little less than the predicted ones. In addition, the largest relative error between the two data is less than 5%.A group of newly built MAN B&W vessels, burning heavy diesel oil IF 180, were arranged to be tested. Various load conditions such as 50%, 75%, 90% and 100% were put into operations and the corresponding engine speeds, brake powers, and specific fuel consumptions were listed in Tables 2-5. Normally, the exhaust temperature increases with engines load. However, from Tables 2 and 4, it was shown that the exhaust temperature did not necessarily increase with engines load due to the variation of excess air supply. Such a phenomenon can be clearly explained from the observation of Figure 1. In this figure, the effect of engines load on excess air coefficient is displayed. Apparently, larger amount of fresh air supplied for combustion causes that the exhaust temperature for 90% load is lower than that for 75% load. For partial load, the efficiency of turbocharger is reduced due to the decrease of the mass flow rate of exhaust gas. The scavenging pressure is thus lowered which in turn reduces the excess air. When the engine is operated under normal rated power, more excess air is obtained because of a higher efficiency of waste heat turbocharger. This results in a lower average exhaust temperature of the main engine. It should be pointed out that fuel input is increased while accelerating the engine. Ideally, the air input should be increased to match the increase of fuel for complete combustion. However, due to the lag and slow response of the turbocharger, the thermal load is higher and the excess coefficient is lower under this circumstance. In addition, the excess air coefficients are all well above three for these types of engines irrespective of load conditions.4. ExampleA container vessel was built in 1994. The main engine of MAN B&W 7S60MCC (15820kW X 105r/min) was used. Due to long-term negligence in maintenance, the engines performance decreased sharply in 2003. It was reported that the engine exhaust temperature increased to 420, which caused some cylinder exhaust valve and liner damaged. The major parameter records were shown in Table 6. The ship was then sent to dock for major repairs. Several steps, listed below, are taken with respect to the above mentioned problem.4.1 An improvement in the performance of the turbocharger.(1) Remove the cover on the manifold and clean the sediments on the grid which is installed in the expansion joint of compressor inlet.(2) Disassemble the turbocharger and clean the both sides of turbine and compressor. Check and measure the corrosion of the moving blade. Replace the excessively deformed nozzle ring to increase the efficiencies of turbine and compressor.(3) Clean the waste heat boiler and the stack to decrease the back pressure of turbine exit.4.2 Reduction of the airflow resistance.(1)Circulate the chemicals to clean the airside of the air cooler.(2)Clean the scavenging manifold, scavenging chamber and port of each cylinder.4.3 A decrease of the scavenging temperature.Unobstruct and clean water sides tube nest of the air cooler to decrease the scavenging temperature and increase the amount of intake air.4.4 A Check of the wear conditions of the cylinder and piston.Remove the pistons and replace the unduly worn cylinders and piston rings.4.5 Improvements in fuel injection.(1) Reinforce the pretreatment of heavy fuel oil, including effective and sufficient cleaning of the fuel through a centrifugal separator and appropriate filtering and heating the oil.(2) Check the function of fuel injection system, including the proper pressure of injection pump and atomization of the nozzle (injector).An immediate improvement in engines performance was observed after the above- stated steps were carried out. The scavenging pressure increased from 0.162 MPa to 0.207 MPa and the average exhaust temperature dropped from 423 to 336 under 90% load condition. In addition, the pressures of compression and explosion rose accordingly. Other output conditions of the main engine are shown in Table 6. Figure 2 plots the effects of engines load of excess air coefficient before/after overhaul. It is shown that is always less than 2.5 and decreases with increasing load before the engine of the vessel is overhauled. However, after the engine was overhauled and maintained, an apparent raise in excess air coefficient is obtained. Note that increases with engines load due to sufficient air supply from the scavenging manifold and ports of cylinders. Figure 3 depicts the comparison of the data measured aboard and empirical formula of previous study. In this figure, is given as a function of Texh on the conditions that i=45%, qw=20% and Hu=41000 kJ/kg. Most of the measured data compare favorably with the empirical relationship. In addition, it is worthwhile to note that the excess air coefficients of the new vessels are all above 3, however, becomes lower than 2.5 after a certain period of operation. A good maintenance of the main engine may raise the excess air coefficient but the effect is still limited.5. ConclusionIn this study, the effects of excess air coefficients for diesel engines are investigated. It is shown that plays an important role in diesel engine during combustion. Furthermore, a larger may change the condition of engines combustion for diesel engines, which will burn fuel oil more efficiently and emit the complete heating value, and decreases fuel consumption as well as the thermal load of engine parts. Consequently, it will keep the best engine performance.To increase during operation, it will apparently improve the performance of the main diesel engine. Although the coefficients of excess air cant be measured on-site while the engine is running, an immediate assessment can be made from the scavenging air pressure, compression pressure, average exhaust temperature and thermal load through this work. In this regard, it is possible to achieve good performance of the engine at all times.To efficiently operate the marine diesel engine, there has been a recent tendency to design engine with long stroke to achieve the required power at a relatively low engine speed. Furthermore, to consider economical reasons for shipping company, the use of low quality fuel is pretty common which has led to deteriorating the performance of main diesel engine. Nowadays, full atomization of the fuel for the injector is designed and larger amount of excess air is supplied to improve the combustibility of the low quality fuel,. 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