Supplying of Marine Diesel Engine Ecological Parameters

The by-pass system of exhaust gas for the engine 6L20 Wartsila has been observed. The requirements of Annex VI MARPOL towards nitrogen oxide concentration in ship engine exhaust gases have been provided. The purpose of research was the determination of diesel 6L20 Wartsila by-pass exhaust gases optimum volume – at this the nitrogen oxide minimal concentration in exhaust gases is assured, the minimal increase (comparing with operation mode without by-pass) – specifi c eff ective fuel consumption, supporting of necessary thermal factor diapason of engine cylinders. The research was performed for the exhaust gas by-pass diapason 0...10 % with engine load diapason 0.55...0.85 % from nominal power. Upon experimental results it has been stated that the exhaust gas by-pass usage favors the ecological parameters of ships engine operation modes – by this at the range of exploitation load 0.55...0.85 % from nominal power the nitrogen oxide concentration in exhaust gas is decreased to 1.32....12.97 %. The exhaust gas by-pass impairs the combustion process and favors the increasing of specifi c eff ective fuel consumption and increasing the temperature of exhaust gases. The exhaust gas by-pass system eff ectiveness assessment should be performed by complex estimation of the following engine operation parameters: the nitrogen oxide concentration in exhaust gas, increasing of specifi c eff ective fuel consumption(SFOC), the exhaust gas temperature. As optimal degree of exhaust gas by-pass value when the maximum decrease of nitrogen oxide emission at minimal increase of fuel consumption and simultaneous engine thermal factor handling has to be considered.


INTRODUCTION / Uvod
Sea transport is an essential part of developed countries all over the world, an exit to the global ocean aquatic area. According to the "United Nations 2019 Maritime Report", the volume of the sea transportation in 2019 has reached 116 billion tons, thus even the after world crisis in 2008-2010 the sustainable growth of the world maritime trade has been confi rmed [1].
The active development of the maritime traffi c sustains the growth of the ship and engine building [2,3]. The engine that assures the movement and functionality of the sea and river transport (regardless of their purpose, gross tonnage and region of navigation zone), is the internal combustion engine (diesel) [4]. The engine, fi xed at the sea and river vessels, generates Figure 2 Requirements of Annex VI MARPOL to the quantity of NOx into exhaust gas of ship engines Slika 2. Zahtjevi iz Priloga VI MARPOL-a za količinu NOx u ispušnom plinu brodskih motora mechanical energy by the fuel air ratio, assuring the permanent heat and mass exchange with the atmosphere during this operation [5]. It takes the air and consumes the fuel, then it is exhaust gas, containing partially the air and the product of fuel oxidation. In this matter of fact the air, coming to the engine cylinder, performs certain thermodynamic cycle, and as a result is transforming into exhaust gas -a complex gas mixture with numerous components. During the usage of hydrocarbon fuel by diesel oil-derived and atmospheric air as an oxidizing agents the exhaust gases of the ships' power plants consist of 99…99.8 % non-toxic components, namely -the incomplete-combustion products (carbon dioxide CO 2 and water vapor H 2 O) and air with the decreased content of oxygen O 2 and nitrogen N 2 . The rest (0.1…1.0 % of the volume) are the mechanical impurities, toxic for the environment and the humans [6].
When combustible fuel elements are oxidized by oxygen in the air (carbon C, hydrogen H and sulphur S) as well nitrogen N and further burning of fuel air mixture, the following toxic components appear: carbon dioxide gas CO, carbon oxide and hydrocarbon C n H m , soot C, nitrogen oxide NO X , sulphur oxide SO X , as well the high-density metals combination, contained in fuel (Fig.1). Figure 1 The mechanism of toxic components exhaust gases formation during fuel oxidation and combustion Slika 1. Mehanizam stvaranja otrovnih komponenti ispušnih plinova tijekom oksidacije i izgaranja goriva The nitrogen oxide NO X is one of the most toxic components of exhaust gas [7,8]. During normal atmospheric conditions the nitrogen is represented in the inert gas. In high pressure and temperature in particular the nitrogen very actively reacts with the oxygen. In the engine exhaust gas more than 90 % of all NO X quantity is nitrogen oxide NO that in the exhaust system and into atmosphere easily oxidizes into dioxide NO 2 and becomes an azotic acid HNO 3 . After that, the azotic acid condenses into the air, returns to the surface of the world ocean or to the island and inland part of the Earth as an acid rain and infects the environment and the humans [9,10].
The diesel engines ecological characteristics are specifi ed essentially by the content of nitrogen oxide NO X in combustion products, which signifi cantly dominates over the other harmful components of exhaust gases as per toxic index. In this respect a range of international organizations (namely International Maritime Organization -IMO) incorporated strict requirements whose implementation assures ecological parameters of ship engine operation [11].
The nitrogen oxide concentration in the ships' power plant exhaust gases are determined by the requirements of the Annex VI MARPOL, depending on the year of ship building and engine revolution per minute. According to the standards Tier-I, Tier-II, Tier-III, (related to the diesel vessels built after 2000, 2011 and 2016) the maximum quantity of NO X into exhaust gas should not be over the limits determined by the specifi c formula (Fig. 2) [12].

LITERATURE REVIEW / Pregled literature
At present, beyond the controlled sea and river transport engine parameters, the high profi le is given to such ecological parameters as nitrogen oxide concentration in exhaust gases [13,14]. This parameter is controlled by the international requirements and is mandatory for ship power plants operation, both in the world ocean aquatic areas and in the territorial waters of seafaring countries [15,16].
Nitrogen oxide formation during fuel combustion occurres when temperature into engine cylinder increase 1500 K and high oxygen concentration condition during atmospheric nitrogen oxidating is controlled in the burning process. In this respect, all methods that assure decreased NO X emission are focused on changes of stoichiometric proportion fuel-air, that leads to deterioration of mixing process, of oxidation and burning [17].
The decreased NO X concentration into exhaust gas is achieved by means of: -by infl uence to the operation process into engine cylinder [18][19][20]; -construction and operation parameters changes of high pressure fuel equipment [21][22][23]; -infusion of reagents into exhaust gas during their passing through special reactors [24][25][26]; -the usage of exhaust gas system operation assured either their recirculation or (Exhaust gas recirculation -EGR) [27,28], or their by-pass (Exhaust gas wastegate -EWG) [29]. Exhaust gas recirculation systems (EGR) are used, as a rule, for low speed engine [30,31]. For the medium speed engine the operation of exhaust gas fl ow could be performed by their by-pass (EWG) [32,33]. In this case a part of engine exhaust gas immediately falls into exhaust gas manifold beside exhaust gas turbocharger (TC). At this the rotation rate is decreased as well as pressure and quantity of pressurized air into cylinder. The exhaust gas by-pass is assured by special valve permitting to direct a part of exhaust gas not into TC but specifi cally into to the exhaust pipe [34]. At present, the EWG system is installed into medium speed engine and assures the functions of main and auxiliary engines [35].
The operation and exploitation characteristics of sea and river ships' engine TC are assured by complex coordination of two components -TC compressor (charges air into engine cylinder) and exhaust gas side of turbine (used exhaust gas energy) [36]. The steady running of engine TC is characterized by the equality of turbo charger power N T and TC compressor side N C : , and the relevant diff erence of their power  ТС not exceed 0.5 % The operation process, taking part into the sea and river ships' engines, is in tight connection with the process of charge air and exhaust gas. To adjust the operation process in the combustion chamber and the quantity of air, supplied by the TC compressor side, the following measures of automatic regulation are applied: -by-pass a part of exhaust gas besides turbine [37]; -the turn of blades of turbine [38]; -the changes of open fl ow area at turbine entrance [39].
The most practical usage has been provided in the system with automatic regulation with by-pass of exhaust gas beside turbine. It helps to assure the engine with necessary quantity of air at all operation modes and to reduce the maximum turbocharger shaft revolution.
The usage of exhaust gas by-pass beside turbine is possible for the one and two step pressurized system (Fig. 3). In the one step pressurized system (Fig. 3, a) after exhaust manifold the exhaust gas fl ow is going directly to the turbine, and a part of fl ow (which in the engines of diff erent producers is located between 0…15 %) is redirected to the exhaust pipe. For two step pressurized system (Fig. 3, b) the by-pass of a part of exhaust gas (moved from engine cylinders to the exhaust manifold then to the turbine ) can be assured either to the TC of low pressure or (as in one stage charger) into the exhaust gas funnel. During the by-pass of exhaust gas fl ow, it has been throttling to the pressure less than the exhaust gas pressure beside TC [40]. This prevents gas injection and stagnation of gas fl ow, coming from turbine. Notwithstanding a big cycle of performed research on the engine with two stage TC, there arestill a lot of uncertain moments concerning the thermo-gas dynamic fl ow [41,42].
The by-pass exhaust gas "waste-gate" systems are used, as a rule, to decrease the power of turbocharger in case of long work engine used on the maximum load mode or at instant high increase of load. At these terms of operation the turbocharger revolution frequencies could surpass the maximum allowed value, that is why with a help of by-pass the quantity of exhaust gas coming to the turbocharger is decreased. The usage of EWGsystem for decreasing NO X emission for the current moment has no accomplished research with the confi rmed results. In some of the previous works [9,31,32] the infl uence of EGR-system on ecological compatibility has been studied.
At the same time, the operation of the exhaust gases recirculation, as a method of assured ship engine ecological parameters, appears almost not to be studied, whose positive results of appliance will diff er by scientifi c novelty, by actuality, and will fi nd a practical use in the maritime transportat.
The research has been conducted on three similar types of medium speed engine 6L20 of Wartsila with electronical operating system of fuel supplying phases, air and gas assignment, being part of ship's power plant as diesel generators. The nominal power of Auxiliary engine was N enom =1200 kW at with speed 1000 min -1 . The engines have had almost the same running hours and have been used on the equal loads. As operation system of exhaust gas on these engines the system of EWG has been installed. The appliance of this system is recommended by Wartsila fi rstly to limit the pressure of charge air and to prevent the surging eff ects of TC at high load and as additional option -to reduce NO X . According to the project documentation the EWG system assures exhaust gas by-pass system in the range 0…10 %. The principal scheme of EWG system of the engine 6L20 Wartsila is shown in Fig. 4. Figure 4 The principal scheme of medium speed engine 6L20 Wartsila with exhaust gas operating system EWG: 1 -controller of by-pass valve position; 2 -pneumatic drive of by-pass valve; 3 -by-pass valve (wastegate); 4,5 -main and by-pass gas fl ow pipeline; 6 -air cooler; 7scavenging air receiver; 8 -engine cylinders; C1, C2 -control points of gas fl ow; T,C -turbine and compressor of turbocharger The air pressurized by the compressor of TC moves into the air cooler 6 after that going in the engine cylinder 8 through scavenging air receiver 7. In engine (traditionally for medium speed diesel engine Wartsila) the impulse system of TC is realized, at which the exhaust gas from the engine cylinder 8 per separate exhaust gas line going into the turbine of turbocharger. Depending on the by-pass valve position 3 (the transposition is assured by pneumatic actuator 2 and regulated by controller 1) the exhaust gas is going either to the turbine 4, or to the by-pass 5.
The consumption of exhaust gas into lines 4 and 5 is determined in points C1 and C2 by using fl owmeter MT100s of "Siemens AG" (Germany). The sensibility of fl owmeters MT100 is determined as 0.07…0.2 nm 3 /s, the operation temperature is up to 454°С, assuring their functionality at all range of engine operation loads [43]. The fl ow meters MT100 comply with the requirements of the Environmental Protection Agency (EPA) Continuous Emission Monitoring System.
During the experiment in point C1 by means of gas detector Testo350XL the concentration of NO X in exhaust gas has been determined [44]. The NO X concentration control is performed in the exhaust funnel at the distance of 10 m from turbocharger output that corresponds to the NO X Technical fi le requirements.
The measuring of exhaust gas fl owmeter and NO X concentration into exhaust gas were held within 10 minutes at 15 second intervals. The obtained results have been averaged.
The operating economy parameter of any engine is fuel consumption. For the ship's engines its value has to be determined not only per unit time but also relevant to the eff ective power. This allows us to compare the economic eff ectiveness of diesel engines with diff erent power (the engine value ranges from 100 to 100000 kW). Taking into consideration that in previous research the meaning of all parameters was fi xed only at installed modes, the value of fuel consumption has been determined as an average result for all periods of each stage of experiment.
SFOC b e has been determined by means of ships' measuring tools -the fl owmeters, installed on the fuel line of fuel inlet to the high pressure of fuel pumps, and timer and has been analyzed as per formula where G h -timing consumption of fuel, kg/h, which analyzed as per formula where V f -fuel volume, going through the fl ow meter, m 3 ; r f -fuel density at corresponding temperature, kg/m 3 ; t -time, during that the experiment has been performed on necessary speed mode, hour; -engine power on diff erent speed modes, kW, [45,46]. The specifi city of ships' engine power identifi cation at sea and river transport assuring the auxiliary function (the diesel generator engine) is a possibility to identify it as per electric user capacity. There is no necessity to take performance of the engine (that simplifi es experiment technology) and a possibility to measure the direct power, for example with watt-meter (that increase the accuracy of experiment). The engine power is analyzed as per formula where -eff ective power of generator engine, kW (is determined as per wattmeter in central controlling point); gen  -electrical generator coeffi cient of effi ciency on relevant exploitation mode (taking into consideration the compliance with the characteristics and instruction manual of electrical generator).
The degree of exhaust gases by pass is analyzed as per formula: (1) where G wg -the volume of exhaust gases , passing through by pass valve, kg/s (has been measuring in point 9 by means fl ow meter MT100S); G Ʃ -a summary quantity of exhaust gases coming into blowoff pipeline from TC at dully closed by pas valve, kg/s (measured at point 1 by means fl ow meter MT100S).
The inaccuracy during measurement of gases consumption, determined by fl ow meter MT100S, did not exceed ±0.5 %, the inaccuracy during measuring of exhaust gases NO X emission by gas analyzer Testo350XL has been fi xed as ±3.5 %, the inaccuracy in checking of specifi c eff ective fuel consumption did not exceed ±2.5 %.

RESULTS / Rezultati
The engine, where all experimental research has been performed, has assured the power for the constant consumer groups. At this (depending on researched modes) its power was 660, 780, 900, 1020 kW, that was in compliance with 55, 65, 75 and 85 % from nominal load -0,55N enom , 0,65N enom , 0,75N enom , 0,85N enom . The inaccuracy in power changing did not exceed ±1.5 %.
The ship's power plant contained three single type engines, in this respect in case when the quantity of energy consumer and its power was changing, the required load was redirected to the engine which was not engaged in the experiment, thus the engine engaged in the experiment was used under permanent load. Besides this, during experiment on engine the permanent temperature mode was held in the lubricant and cooling systems. During the experiment, the engine has been under permanent load within 2.5…3 hours with the stable position of by-pass valve on each of the experiment mode. Considering the long period of experiment performance, the exhaust gas consumption checking persistence has been fully neutralized and has no impact on the results.
To identify the degree of wastegate opening, initially, in point C1 the general consumption of gas has been identifi ed G Ʃ , outgoing from engine cylinder and going through the exhaust gas manifold 4 (at dully closed valve 3). After that, at changed position of wastegate 3 in point C2 the exhaust gas consumption G wg has been identifi ed through by-pass pipeline 5 and the degree of exhaust gas by-passing . has been rated per formula (1). The following measurements have been performed as per two schemes (Fig. 5): 1) at constant position of by-pass valve the load to engine has been changed and then the NO X ratio concentration in exhaust gas and SFOC b e , have been determined, for example at constant ratio =10.0 % and diff erent exploitation meaning , corresponding to 55, 65, 75, 85 % from nominal power; further the position of by-pass valve has been changed ( = 8.0, 6.0, 4.0 %) and for every ratio in mentioned diapason the loading to engine has been changed again and the checking of NO X and b e has repeated; 2) at constant engine load the by-pass valve position has been changed and then the NO X emission has been determined and the economic parameter of engine -b e , for example, at constant ratio = 0,85 N enom and diff erent meanings ( =10.0, 8.0, 6.0, 4.0 %)); then the engine load meaning has been changed (0,55N enom , 0,65N enom , 0,75N enom ) and for every meaning in the mentioned diapason the position of by-pass valve has been changed again and the measurement of NO X and b e was repeated. This helped to get more experimental meanings and to increase their informative content. Thus received experimental meanings have shown good convergence that confi rmed corrective way of the performed measurements. As criteria of engine thermal factor the temperature average value of exhaust gas on engine cylinders have been taken t g , measuring of that value has been assured by the ship's diagnostic system Doctor. The temperature value of exhaust gas is recommended by the manufacturer of the engine as well by Scientifi cs used as the evaluation criteria in te working process and condition of high pressure fuel equipment [48]. The indicator of value t g has been performed during all time of experiment. The research results are generalized in Table 1 and provided in Fig. 6.  -specifi c eff ective fuel oil consumption without using EWG system and by using EWG system with diff erent degree of by-pass , g/(kW×h); -NO X emission without using of EWG system and by using EWG system with diff erent degree of by-pass, g/(kW×h).
The meanings EWG 0 , e e b b and are taken from table 1 for relevant load meanings and bypass degree δ EWG . The changes Δb e and ΔNO X for diff erent engine loads and diff erent degree of exhaust gas by-pass are shown on the Fig. 7.

DISCUSSION / Rasprava
The by-pass exhaust gases system (Exhaust gas wastegate -EWG) are recommended and used by certain engine manufactured (for example Wartsila) to reduce the pressure of charged air to reduce the overload engine. The EWG system assures the by-pass of exhaust gas in cylinder in diapason 0…10 % from their general volume directly to the exhaust funnel without using their energy into TC. The EWG system can be used for assuring the ecological parameters of engine operation (namely to decrease the NO X emission with exhaust gas) in all range of engine operation modes.
During experimental research to determine the EWG system infl uence on sea and river ships' engines exploitation parameters simultaneous with operation modes' process ( such as cylinder pressure, power, fuel consumption) the exhaust gas volume fl ow rate and its temperature on separate engine cylinder have been controlled. The fi rst (the volume fl ow rate) has assured the control and regulation of EWG system by-pass degree, the second (temperature) has prevented the engine from thermo overheat.
The usage of EWG system assured the degree of NO X concentration into exhaust gas. Thus, (by poor combustion process) the engine power is decreasing and specifi c eff ective fuel consumption is increasing. Besides this, the excessive increasing of exhaust gas quantity leads to the intensive engine thermal factor.
The application of the EWG system has a complex impact on the economic and ecological engine operation parameters. According to the experimental data on every engine loads with increased exhaust gas by-pass degree δ EWG, the meanings Δb e are also increasing (relevant to specifi c eff ective fuel consumption increasing) as well the ΔNO X (relevant to nitrogen oxides emission decrease). The Δb e increasing with growth of by-pass degree δ EWG is connected to the decreasing of turbocharger power, with stoichiometric air-fuel ratio changing, with combustion process degradation and leads to degradation of diesel effi ciency operation (decreasing of NO X quantity in exhaust gas).
The usage of exhaust gas by-pass system favoring the refi ning of medium-speed engine ecological parameters, namely at this in diapason of operation loads (0.55…0.85) N enom the exhaust gas of NO X emission degree is decreasing on 1.32…12.97 %. The utmost level of NO X emission corresponds to 75…85 % load -the most widespread operation modes of medium-speed engine during their appliance as dieselgenerators incorporated in the ship's power plant.
The degradation of the charge air supply process during EWG usage favors fuel burning process shifting to the expansion line and provokes the growth of engine thermal factor (that is possible to evaluate as per temperature value of exhaust gas t g ). According to the manual engine operation rules 6L20 Wartsila, to assure the temperature stress the value of temperature exhaust gas t g should not exceed 300°С. For operation modes, corresponding to (0.55…0.65)N enom , the usage of EWG system can be possible only in the diapason δ EWG = 0…6 %, because for the majority of values δ EWG the level of exhaust gas temperature t g increases the recommended limits. For the modes, relevant to the loads diapason (0.75…0.85)N enom , in all range of gases by-pass changes δ EWG = 0…10 % occur their temperature growth, but even at level δ EWG = 10 % the value t g do not exceed maximum accepted limits, that assure the acceptable level of engine thermal factor. Obtained results are in good agreement with data provided in a number of papers devoted to similar research [49][50][51].

CONCLUSION / Zaključak
For ships' medium-speed engine with electronical operation, as a method assured the compliance of Annex VI MARPOL requirements, the exhaust gases by-pass system can be usednamely EWG system, at which a part of combustion products are going to exhaust funnel passing through turbocharger. The electronic engine operations allow to assure this process in smooth mode at range 0…10 % from total exhaust gas volume, going out from the engine cylinder.
The analysis of research results, performed for ships' medium-speed engine 6L20 Wartsila (used at sea and river ships as auxiliary generator engine), allows to conclude the following: 1) the increase of by-pass exhaust gas level at diapason 4….10 % and favor the decreasing of nitrogen oxides emission from 8.48 g/(kW×h) to 7.28 g/(kW×h) and depend on engine load; at this the relevant decreasing of ΔNO Х emission is within the limits 1.32…12.97 %; this proportionally improves the diesel ecological safety capacity works, which is especially important for the auxiliary engines constantly working during ships' location in water area and on the territory of sea ports; 2) the utmost level of nitrogen oxide concentration decreasing in exhaust gas correspond to the maximum level of bypass exhaust gas and maximum load of engine mode (in performed experiments 10 % and 0.85N enom consequently); expressly such modes of auxiliary engines are the most widespread and long termed; 3) the usage of EWG system decreases the exhaust gas emission, going from turbine , that leads to turbocharger capacity, to the decreasing of charge air quantity, going to the engine cylinder, and growth the specifi c eff ective fuel consumption; the changing of this parameter is a negative aspect in case if we use the exhaust gas recirculation system; 4) for the operative engine modes close to nominal ones (in performed research (0.75…0.85)N enom ) by using by-pass exhaust gas system the relevant increase of specifi c eff ective fuel consumption is determined as 0.41…2.14 %; at this taking into consideration the maximum (up to 4.74…12.97 %) NO X emission decrease at current exploitation modes, the improvement of engine ecological operation parameters is a prevalent factor for current charges diapason, that's why the usage of EWG system is eff ectual and can be recommended as a method to assure the ecological compliance of ships' engine; 5) at load (0.55…0.65)N enom the increase of fuel consumption at use of EWG system can reach 1.06…2.89 %; taking into consideration that in current load option the EWG usage assure the NO X emission on 1.32…6.27 %, the appliance of by-pass exhaust gas for current diapason is not reasonable; 6) the estimation of EWG system eff ectivity as one of measure the comply to the Annex VI MARPOL requirements on NO X emission reducing, should be performed by complex estimation of the following parameters of engine operation: NO X quantity in exhaust gas, the increasing of specifi c eff ective fuel consumption Δb e , the exhaust gas temperature t g . As optimal by-pass exhaust gas the values corresponding to maximum NO X emission decreasing at minimum increase of fuel consumption and simultaneous maintaining of t g within the limits, not exceeding the thermal factor acceptable level; the additional temperature control of exhaust gas need to be performed during all time of waste gate system usage; 7) for the considered ship engine 6L20 Wartsila (where the research has been performed) the usage of EWG system is reasonable for load increasing the value 0.75N enom . At this decreasing of nitrogen oxide emission on 8.40…12.47 %, assured the NOx concentration in exhaust gas at level 7.34…8.19 g/(kW×h). The increase of specifi c eff ective fuel oil consumption at current modes is within the limits 0.41…2.14 %. For the load (0.55…0.65)N enom the NO X emission decrease also is relevant (on 1.32…6.27 %), but the specifi c eff ective fuel oil consumption is increasing on 2.89…4.13 %, besides this at 8…10 % by-pass of exhaust gas the level of engine thermal factor increase the acceptable limits; in this manner the usage of wastegate system in diapason of diesel loads (0.55…0.65)N enom is non eff ective from the economic and as well from the exploitation point of view. The usage of EWG change the stochiometric proportion of fuel-air ration that degradant the combustion process and favor the increasing of specifi c eff ective fuel consumption. Notwithstanding the specifi c increase of fuel consumption, the usage of EWG system could be recommended in specifi c region of world ocean, when the dominant parameter during ship power plant exploitation became their ecological parameters.
The provided results confi rm the usage of EWG system to decrease the NO X emission level. But the most reasonable usage could be its appliance as additional measure in complex with exhaust gas recirculation system EGR that required additional research.

ACKNOWLEDGMENTS / Zahvale
Presented studies were conducted in accordance with the Research plan of National University "Odessa Maritime Academy", Project "Improvement of effi ciency and environmental indicators of ship power plants on the basis of modern engineering and IT processes".