Contemporary Marine Engine Designs - Report Example

Contemporary Marine Engine Designs There have been recent concerns with environmental impact of various substances used in day to day activities. The focus now is on going green, so to speak and efforts both at individual and corporate level are being made in order to curb pollution where possible. Since the late 70s, a lot of focus has been given to exhaust emissions of marine diesel engines. According to Kee-Rong, various environmental legislations have been put in place. Kee-Rong, 2000). Since January 2000, exhaust emissions controls have been enforced in marine diesel engines by the International Maritime Organization (IMO). Basically in marine diesel engines combustion comprises of a series of batch processes whereby elevated temperature and pressure is used. Every individual piston stroke constitutes a batch process such that the slower it is while maintaining the inherent thermodynamics the more efficient it will be. Exhaust gases emitted from marine diesel engines comprise oxygen, carbon dioxide, nitrogen, water vapor, oxides of sulfur and nitrogen as demonstrated in the fig 1 below. This composition is as a result of the combustion process in the engine, the fuel used and the different methods used to control these emissions. Fig 2. Two stroke crosshead diesels operate with air excess ratio of about 3.5 such that 50% of the air is available for consumption while the balance is scavenged through the cylinder. Consequently the exhaust contains about 14.5% oxygen. Most environmental legislation stipulates a limit of 15% oxygen so in case this is exceeded, adjustments have to be made accordingly. Nitrogen plays a small but fundamental role in the chemical processes experienced in the engine. (Corbett, 1997). The combustion of various hydrocarbons consequently results in the production of carbon dioxide and water .Carbon dioxide has also been the focus of various environmental legislations because of the ‘greenhouse effect’. To control such the only viable means is to employ a marine diesel engine with a high thermal efficiency and low carbon fuels .Carbon monoxide on the other hand is a toxic gas hence its elimination in the engine process is of paramount importance. Smoke intensity is a visible means of quantifying combustion quality. Smoke and opacity limits are enforced in various countries. It should be noted that the larger the marine diesel engine the more visible the exhaust smoke will be because for a given Bosch Smoke Number (BSN) the greater the diameter of the plume the greater the amount of light it will absorb. Most modern diesel engine has low smoke values resulting in an invisible plume. Nitrogen dioxide may give the plume a yellowish appearance while if water vapor condenses it will give the same a grey color. Particulate Matter (PM) comes from ash content of the diesel and oil, partly burnt lube oil, sulfates and water and very small particles of partly burned fuel. Lube oil contributes a large number of calcium salts which help neutralize sulfuric acid. There are a number of methods employed in measuring the components of exhaust gas. NOx is measured by a chemiluminescence analyzer (CLA) and an electrochemical sensor (ECS). The CLA can be heated to avoid condensation and is a common method used in measuring exhaust gas from marine diesel engines. Carbon monoxide and carbon dioxide are measured using infrared (IR) technology. The new ISO standard method for measuring particulate emissions is the dilution tunnel method whereby a sample is taken from the exhaust gases and diluted with air and the filtered at a maximum of 52 degrees centigrade. The collected material is gravimetrically determined. For smoke there are a wide variety of methods of smoke spot methods. Exhaust is passed through a soot paper and the reduction in reflectance of the soot paper is assessed. An example of scales used include the Bosch Smoke Scale and the SAE Smoke Number(Scale 1-10).Opacity methods take into consideration the percentage of light that disappears when passing through a certain path length of exhaust gas. This method is subjective as it may be influenced by the prevailing atmospheric conditions. Examples of opacity methods include Ringelmann Number (Scale 0-5) and the Hartridge smoke value. Fankhausen voices that the advanced common rail injection technology is now making it possible to provide smokeless engines. (Fankhausen, 2001). With current systems fuel injection pressure is a function of engine speed and load such that at reduced load injection pressure drops resulting in large fuel droplets on the combustion surfaces. The common rail is a manifold spanning the length of the engine as demonstrated in the fig 2.1 and 2.2 below in the Sulzer RT-flex engine. Fig 2.1 Fig 2.2 The common rail and associated pipe work are below the engine platform and easily accessible from above. The common rail is fed with heated fuel oil at a nominally high pressure of 1000 bars ready for injection. There are a number of high pressure pumps running on multi-lobe cams on the supply unit. Fuel is delivered from this common rail through a separate injection control unit for each engine cylinder to the standard fuel injection valves hydraulically operated by high pressure fuel oil. The control units, using Sulzer rail valves control fuel injection volume and injection pattern shape. In each cylinder cover the injection valves operate in concert or in uniform depending on how the electronic program has been set. In the RT-flex system the heated heavy oil is isolated from the quick acting rail valves. The Sulzer common rail system therefore has well proven fuel injection valves and provides the option of controlling individual fuel injection valves. The volumetric control of fuel injection is precise and injection pressure and rate shaping is variable. The power plant thus runs steadily at low engine speeds with reduced vibrations as the speed is regulated precisely. This results in smokeless operation at all speeds. The RT flex system also has starting air control and actuation of the exhaust valve. The exhaust valves are operated by a hydraulic pushrod and the energy to actuate it comes from a servo oil rail that operates at a nominal pressure of 200 bar pressure. Hydraulic pumps at high pressure supply the oil to the servo oil rail. (Aeberli, 2002). The actuating units for each cylinder are flexible in a way that they electronically regulate the opening and shutting patterns of the valves as shown in fig 3 below. Fig 3. For newly upgraded and designed engines either the common rail technology (CR) or the optimized injection system will be used in order to reduce emissions from modern marine diesel engines. Figure 4 below shows the results of some tests performed on a test engine applying the common rail technology with a 50% load and a constant speed of 1200rpm. Fig 4. There seems to be an increase in fuel consumption at high rail pressures but on the other hand the prevailing rail pressure seems to be influencing the emission of smoke in a positive and desirable manner. At the heart of the RT flex system is the Wartsila WECS-9500 electronic control system which has separate electronic microprocessor controlled units for each cylinder. The system is designed to be user friendly hence the reduced costs in terms of additional training costs can be enjoyed. The Sulzer RT flex engine therefore gives reduced running costs since at partial load the engine consumes less full and this lengthens the duration of time between major services. With this technology the adjustment of mechanical pumps is no longer necessary since this function has been taken over by the electronic control module. This reduces maintenance costs. The volumetric control system as exemplified by the common rail system results in a more balanced engine operation .This results in a higher predictability of the maintenance costs. There is better fuel economy and compliance with the NOx emission limit stipulated in the MARPOL 73/78 convention and consequently smokeless operation at all operating speeds. The selective shut off of injectors rather than cylinders gives a more balanced engine operation. The unit can also develop 100% power even with one servo oil pump out of operation as it incorporates a built in redundancy system. Another technological advancement is seen in the MAN B & W common rail fuel system as seen in the figure below. Clean lube oil under pressure is used as the medium that drives the fuel injection pump in the common rail system. A servo oil accumulator is incorporated in every cylinder unit in order to facilitate efficient delivery of servo oil in response to the demands placed by the injection system and in order to dampen any heavy pressure oscillations in the oil pipe system. (MAN B & W, 1997). Plunger movement is controlled by a rapid response proportional control valve (NC valve) that is controlled by a linear electric motor that gets its control signal from the electronic control module in the cylinder unit. The functionality of this system further strengthens the fact that the electronic fuel injection system plays a key role in fuel economy, reduced emissions and optimized engine performance. As seen in fig 5 below, successive pump designs are much simpler than the initial designs as they are easier to manufacture and they are actually smaller than their predecessors. Fig 5. The third generation pump is actually able to use heavy fuel oil and consequently the pump plunger is modified in such a way that the heavy fuel oil cannot enter the lube oil system. The fuel pressure acting on the plunger and the hydraulic oil pressure acting on the driving piston keeps the driving piston and the injection plunger in contact. Both ends of the plunger stroke are controlled by the fast acting hydraulic valve (NC valve) that is in turn microprocessor controlled. The ideal fuel injection cam shape in a conventional marine diesel engine generates an optimized fuel injection pattern that is required for optimal combustion and consequently optimized thermal efficiency. The optimum duration for injection in modern marine diesel engines is about 19 degrees crank on average at full load and in the second half of that cycle maximum firing pressure is achieved. For optimal thermal efficiency the fuel to be injected ,after reaching maximum firing pressure should be injected and burnt as fast as possible in order to achieve the highest expansion ratio for the heat energy released. The optimal “rate shaping” of the fuel injection system therefore is one showing increasing rate of injection towards the end of the injection cycle thus the balance of the fuel is supplied as quickly as possible. The camshaft consequently has been redesigned to accommodate these recent advances. As opposed to the camshaft based injection system the Intelligent Engine(IE) can do the same but it goes further whereby the IE can be optimized to operate efficiently over a wide range of load conditions. (MAN B & W, 2001). In many modern marine diesel engines on/off control valves in the Common Rail Injection systems are becoming common fare since they are simple to use and generally more flexible than camshaft based systems. At the start of injection in common rail systems the pressure in the rail is at set-pressure and decreases during injection since the outflow from the rail is more rapid than the inflow from the high pressure pumps. A variation of the common rail system, the Staged Common Rail system, provides a fuel injection rate close to optimal though combustion is not optimized due to uneven distribution of fuel in the combustion chamber. In this system the valve opening first injects the largest amount of fuel. This penetrates too much and reaches the next fuel valve where as experience with older marine diesel engines has shown there is the possibility of hot corrosion of the nozzle tip. There is uneven fuel injection whereby the first nozzle gets too much, the second just enough and the third nozzle gets too little fuel. The average input may be okay but the results cannot sustain efficient thermal efficiency. In this regard the IE injection system is superior in that it can perform not only a single injection pattern but also a pre injection system as it monitors all the prevailing operating parameters. The control system can select from a number of injection patterns stored in the computer database so as to achieve optimal injection. Several tests have conclusively proved that the “progressive injection” type is superior in terms of fuel economy. The “double injection” gives between 18-22% lower NOx emissions but with a higher rate of fuel consumption. The Delphi Common Rail System is another technological advancement which can be adapted for different marine diesel engines as shown in fig 6, below. It consists of injectors, common pressure accumulator and a high pressure supply pump .A high pressure regulator is optional. The module also has an integral Electronic control unit and a filter unit. The pressure level in the common pressure accumulator is regulated by the metering on the supply pump and the high pressure regulator. Injection pressures can be produced at low speeds if need be since the pressure accumulator operates independently of engine speed and load. On the rail are a series of injectors which are opened and shut by solenoid that are controlled by the Electronic Control Unit. Fig 6. DENSO is currently increasing the number of injections per combustion stroke to five up from two and raising the maximum pressure to about 180 MPa in order to comply with EURO-4 emission regulations. Other technologies in the pipeline include the adaptation of piezoelectric actuators for the injectors. The reduction of NO to nitrogen gas is one of the pertinent points in marine diesel engines in order to comply with current legislation on pollution. Presently there is the use of catalytic converters to combat this. According to Annex VI of the MARPOL 73/78 Convention limiting NOx levels are defined for marine diesel engines with an output of more than 130kW.The sulfur content in fuels is also capped at 4.5% according to Regulation 14 of Annex VI. Countries like Norway and Denmark apply an ecological tax on shipping lines and also insist that fuel with less than 1.5% sulfur has to be burned. Treatment of the exhaust gases is also a viable option in order to reduce emissions of sulfur oxides emissions to levels below 6g/kWh. (Schlemmer-Kelling, 2001). Annex VI will apply to ships constructed after January 2000 and engines in existing ships undergoing major overhauls after the same date. With a 1.6% annual fleet conversion rate these measures will reduce global ship NOx emissions by less than 0.9% per year. Carbon dioxide emissions are also coming in focus especially with the recent threat of global warming through the greenhouse effect. Efficient marine diesel engines have low carbon dioxide emissions .A Clean Development Mechanism (CDM) whereby carbon offsets between countries is possible in future has also been ratified by the Kyoto Protocol. The main gas component to be reduced in marine diesel engine exhaust emission is NOx. At the combustion level there are three main NO sources. At about 1500 K the dissociation of nitrogen and oxygen molecules occurs such that the recombination of the individual elements results in formation of NO in the thermal NO process. Marginal amounts of nitrogen aside from carbon and hydrogen are also found in fuel. Due to rapid reactions most of the nitrogen is oxidized to NO. In prompt NO production, air around the combustion front containing nitrogen is combined to NO. Marine diesel engines contain about 0.1% nitrogen and most of this is oxidized to NO in the combustion area. According to Marine Engineering Review, marine diesel engines supply of air is about 2.5 times in excess of the minimum requirement. (Marine Engineering Review, 1997). Nitrogen is usually an inert gas but at the elevated temperatures in the combustion chamber nitrogen is oxidized and initially mostly nitric oxide (NO) is formed. During the exhaust cycle some of this NO will convert to nitrogen dioxide and nitrous oxide. This mix of the oxides of nitrogen is NOx. There usually is enough oxygen from the combustive process for NO reactions to proceed depending on the fuel and air mixture in the combustion chamber. In marine diesel engines combustion occurs over different mixtures. With slightly lean fuel mixtures highest NOx concentrations will occur since there is a higher concentration of burnt gas. It should be noted that NOx rates of reaction are only significant at elevated temperatures but they slow down as soon as the temperature of the burnt gas drops. The air fuel mixture is the primary determinant in the combustion rate. A high degree of swirl is employed at a low engine speeds in order to encourage the mixing of fuel and air and this has implications on the shape of the combustion chamber. Consequently slow speed engines have a higher stroke to bore ratio than medium speed engines. Primary and secondary measures have been employed to reduce exhaust emissions in marine diesel engines. The primary measures zero in on reducing emissions at the combustion level while secondary measures combat emissions at the exhaust level. The main drawback in the use of catalytic converters is the space occupied for the catalytic converter and the necessity of a reducing agent in the converter. These hindrances make them undesirable to marine diesel engine users. As a result of this primary reduction techniques are widely employed whereby concentrations and peak temperatures are lowered in order to lower NOx concentration. Complete combustion of injected fuel also serves to reduce exhaust gas components like carbon monoxide and hydrocarbons and also reduce the amount of particulates in the exhaust emission. By tuning the combustion process most marine diesel engines comply with the Marpol Annex limits. This tuning can be achieved by tweaking the injection timing, intensity and rate profile. The spray pattern can also be modified along with the compression ratio. Delayed injection in conjunction with decreased injection duration and increased compression pressure tend to have positive implications on fuel consumption. High compression pressure, delayed fuel injection and short combustion duration has the effect of achieving a combustion approaching constant thermodynamics. This consequently results in engine design with a low NOx without significant loss in fuel consumption. Advancements in the computer field have resulted in advanced three dimensional models of the dynamics of combustion of the air and fuel mixture at the level of the combustion chamber. These techniques are known as Computational Reactive Fluid Dynamics or Computational Fluid Dynamics and as such they have been employed by marine diesel engine manufacturers to develop low NOx fuel nozzles. According to Wartsila NSD the location of flame zones in relation to metal surfaces is of paramount importance in controlling NOx emission in medium speed marine diesel engines and that the combustion space ideal for low smoke was also sufficient if not adequate for NOx. (Wartsila, 2002). Rapid mixing of bulk gas with burnt gas allows the latter to be cooled by surfaces of the combustion chamber. MAN B & W, as standard on slow speed engines introduced a slide type fuel valve which has a zero sac volume that avoids the entry of diesel fuel into the combustion chamber after the cessation of injection thus reducing HC, CO and NOx emissions. The main source of soot deposits and consequently smoke is the fuel remaining in the injector sac hole. This has been smoothed out by the use of mini sac whole fuel nozzles though they are still at the test level. As demonstrated by a Kawasaki report hot gas cooling by metal surfaces seems to be a factor in NOx reduction with the slide type valves.MTU improved injection by optimizing the number of nozzle holes, hole shape and also rate of opening and shutting of injectors. Through the use of a deep bowl combustion chamber, Yanmar Diesel used smaller nozzle holes and increased number of injection nozzle holes. High peak temperatures as stated before are achieved when the fuel and air mixture has burned and once these gases are further compressed even higher thermodynamic forces are experienced leading to increased NOx emissions. This has been combated by later injection of fuel and this is actually the best method so far of reducing NOx emissions. This delayed injection leads to reduced pressure and therefore less compression post combustion .When the rate of pressure fall due to piston motion exceeds increase in pressure due to combustion, peak pressure occurs. Later burning results in the delayed injection which increases fuel consumption since less of the combustion energy release is subjected to the expansion cycle in full. Later into the expansion process gas temperatures remain high and these results in more heat losses through the walls of the combustion chamber. Due to reduced temperatures in the later part of the combustion process there is an increase in smoke. The effect of delayed injection on fuel consumption and exhaust emissions is illustrated in fig 7 below. Fig 7. As seen previously by Aeberli, it is obvious that further research and development has improved fuel consumption. (Aeberli, 2001). By reducing the delay period in medium sized marine diesel engines pre injection can reduce thermodynamic effects in initial stages of combustion thus reducing Nox emissions and other particulates hence introducing an element of flexibility. A small fuel charge, in pre injection, is injected prior to the main charge injection in the Sulzer RT Flex common rail engine. In sequential injection as opposed to triple injection the three nozzles in the cylinder are electronically actuated with different timing while with the latter the charge is injected in separate but successive sprays. Miller supercharging and scavenge air cooling tend to lower the pre compression temperature and thus reduce the temperatures experienced in the cylinder. As a result, by improving the efficiency of the air cooler the effective scavenge air temperature can be reduced thus improving overall thermal efficiency .To achieve a lower scavenge temperature on 4 stroke engines the Miller supercharging concept is applied whereby with a high pressure turbocharger, before the piston reaches the end of the intake stroke the inlet valve is closed and there is expansion of the charge air inside the cylinder resulting in reduced temperature. This results in more fuel being burned without impacting on the peak cylinder pressure and reducing NOx emissions by about 20%.According to Caterpillar they found that more smoke at low load limited the application of Miller supercharging. When water is introduced into the combustion chamber it reduces the formation of NOx. The water reduces combustion temperature by reducing overall oxygen concentration, the energy of absorption for evaporation and increasing the specific heat capacity of gases in the cylinder. It also reduces the availability of oxygen for the NOx forming reactions. This can be achieved through water fuel emulsification which reduces smoke or through humidification whereby the water is introduced through direct injection into the cylinder. The humidification process results in the production of smoke. As a general rule 1% of water reduces NOx by about the same percentage. Using emulsified fuel improves the combustive process and reduces fuel consumption. At the cylinder level surface tension forces of the fuel are overcome by the steam pressure of water and thus the fuel experiences improved atomization. According to Wartsila this results in a more homogenous mixture of air and fuel. MAN B & W prefers the fuel water emulsion system illustrated below whereby it allows 20% emulsion of water with fuel at a maximum load. As shown in the fig 8 below a 1.5 % increase in for 26% water in fuel and about a ^% increase in fuel consumption for 48% water in fuel. Fig 8. Wartsila however claim that the system has several drawbacks because for one after every engine stop the system must be flushed to avoid the corrosion of engine parts. They also claim that emulsification is only stable for a few seconds. They however advocate for direct water injection when large NOx reductions are required. They have consequently developed separate water and fuel injection systems whereby water injection can be turned off without interfering with fuel injection. Fig 9 below shows the combined water fuel nozzle developed by Wartsila. Fig 9. According to Naval Architect (2000), the early injection of water before fuel avoids interference with the ignition and combustive process and also cools the combustion space. The timing and duration of the water injection is electronically controlled depending on the output of the marine diesel engine. Optimization of the same parameters can be achieved for different applications. Charge air water content can be increased by use of a humidifier such as the Munter HAM (Humid Air Motor) which has demonstrated a 70% NOx reduction at full load. It takes water vapor directly from seawater into a humidification tower. The water is heated by engine cooling water such that the relative humidity of the inlet air can reach about 99%.To prevent condensation the pipes between the engine and the humidifier are insulated. The operation of the Munters HAM system is illustrated fig 10 below. The exhaust heat in fast ferries is available for evaporating the seawater used in this system. The HAM provides a high NOx reduction potential with no effect on reliability, fuel oil consumption and no operating costs by the use of seawater. The HAM system requires much less auxiliary equipment as compared to other NOx reducing measures. Fig 10. Wartsila is developing a Combustion Air Humidification that has an anti polishing ring which allows higher inlet air humidity without compromising lubrication. Water in form of droplets are injected after the turbocharger. With this system the charge air is increased to about 60 g of water per kg of air. Exhaust Gas Recirculation lowers NOx by lowering the combustion temperature by increasing the specific heat capacity of the gases in the cylinder and reducing the overall consumption of oxygen. Reduction of the oxygen concentration reduces the oxygen available for NOx reactions.EGR is either achieved by modification of engine scavenging such that combustion products remain in the cylinder or by mixing the exhaust gases with the charge air outside the cylinder. External EGR in cases whereby poor quality oil is used can result in fouling and corrosion problems which are difficult to dispose of. (Egeberg, 2001). According to MAN B & W when NOx levels around 2g/kWh are required, the solution is the use of Selective Catalytic Converters. Here the exhaust gases are mixed with ammonia over a catalyst such that over 90% of the NOx is removed. Wartsila is in the process of developing a Steam Injected Diesel whereby during the compression stroke before the cylinder pressure becomes too high, steam is injected. Latent energy from the steam is then employed in the expansion stroke and the balance humidifies combustion air consequently reducing NOx emissions. References Aeberli, Kaspar. Common Rail At Sea: The Sulzer RT-flex Engine, Wärtsilä Switzerland Ltd, Winterthur, 2002. Aeberli, Kasper. The Sulzer RTA Low Speed Engine Range: Today and in the Future, 23rd CIMAC Congress, 2001. Corbett, J. J. and Fischbeck, P., “Emissions from Ships”, Science, Vol 298, 1997. Egeberg, C., Ostergaard, A. “The MC Engine and its Future Development”, 23rd CIMAC Congress, 2001. Fankhauser, S., Heim, K., “The Sulzer RT-flex Launching the Era of Common Rail on Low Speed Engines”, 23rd CIMAC Congress, 2001. Holtbecker, R., Geist, M., Emissions Technology, “Sulzer RTA Series, Exhaust Emissions Reduction Technology for Sulzer Marine Diesel Engines”, Wärtsilä NSD, July 1998. MAN B&W, "Emission Control of Two-Stroke Low-Speed Diesel Engines", MAN B&W Technical Paper, 1997. MAN B&W. “The Intelligent Engines: Development Status and Prospects”, Denmark, 2001. Marine Engineering Review. “The Green Diesel”, 1997. Naval Architect. “Water Injection and SCR Systems Cope With Emission Controls”, 2000. Schlemmer-Kelling, U. “The New Low Emissions Heavy Fuel Engines of Caterpillar Motoren (MaK)”, 23rd CIMAC Congress, 2001. Wärtsilä NSD. “The Common Rail System Gives You The Smokeless Engine”, Finland, 2002. Read More