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Проектирование и конструкция судов / Ship Design and Construction
Oleksiy Bondarenko, Tetsugo Fukuda
OLEKSIY BONDARENKO, Dr. of Engineering (Ph.D.), Researcher, Department of Energy and Power System, National Maritime Research Institute of Japan, Tokyo 181004, Japan, e-mail: bond@nmri.go.jp
TETSUGO FUKUDA, Chief Researcher, Department of Energy and Power System, National Maritime Research Institute of Japan, Tokyo 181004, Japan, e-mail: tfukuda@nmri.go.jp
Consideration of propulsion engine operation in combination with ship hull air lubrication
This paper discusses the result of full-scale experiments on a four-stroke and two-stroke engines equipped with a scavenging air bypass system for use with an air lubrication system. The experimental results revealed the effect of scavenging air bleeding on engine performance in both the steady-state and transient conditions and helped to come up with ideas on extending operation limit of main propulsion engine with partial scavenging air bleeding. Furthermore, simple and reliable simulation model of a Diesel engine and scavenging air bypass system was developed and validated against available experimental data.
Key words: air bypass, air lubrication, propulsion plant, engine simulation.
Introduction
Nowadays the surging prices of fuel and tightening up regulations on exhaust gas emission together with the mandatory requirement on improvement of new build ship energy efficiency (Energy Efficiency Design Index approved on IMO MEPC 62 meeting) provoke the development and implementation of energy-saving technologies which has been greatly anticipated by the shipping industry. In this respect, air lubrication is a well-known technique and is promising one for energy saving during ship operation.
Air lubrication is an effect achieved by small air bubbles which, being injected into the turbulent boundary layer developed along a ship bottom moving in water, significantly reduce skin friction. Series of theoretical and experimental studies performed by Kato and Kodama [8] and by Kodama and Takahashi [9] showed that reduction can reach as much as 20%, and keeping in mind that frictional resistance constitutes more than 70% of total resistance of low speed ship, the effect on a real ship can be significant. The later was verified during full scale experiments on real ships. Thus in years 2005 and 2008 full scale experiments on a cement carrier ship called "Pacific Seagull" of length Lpp = 120 m, equipped with the air bubbles injection equipment, confirmed an average 5% net power-saving [7].
As shown by Hinatsu [6], frictional resistance reduction rate directly proportional to the equivalent air layer thickness, expressed by ta = Qa/(BaVs), and the pumping power for air injection is determined
by the air flow rate Qa. Thus for a ship with large breadth Ba advancing with speed Vs, the net power-saving becomes a trade-off between the power required for air injection under the hull bottom and frictional drag reduction achieved by air lubrication [9]. However, the large ocean going ships like bulk carriers or tankers, as a prime mover use the high powered marine Diesel engine directly coupled to a propeller. Such an engine is equipped with the turbocharger (TCH) which, utilizing the energy of exhaust
© Oleksiy Bondarenko, Tetsugo Fukuda, 2015
gas, supplies pressurized air required for fuel combustion. Owing to the high efficiency of today's modern TCH the amount of supplied air exceeds engine demand and as a result, the part of air can be bypassed and used for air lubrication system (ALS), thus increasing overall efficiency of the ship operation. Such a system is called scavenging air by-pass as shown in Fig. 1.
Nonetheless scavenging air bleeding, harmlessly for engine, is possible only in narrow operational range, in respect that the primary prerequisite for the good combustion process is a proper ratio of injected fuel mass and air mass charge. Depletion of the air charge due to bleeding of scavenging air deteriorates the combustion process with implication on the specific fuel oil consumption (SFOC) and exhaust gas emission which ultimately stultify the net power-savings. Moreover contradictory operational measure (slow steaming) to curtail fuel cost inevitably lowers engine load and scavenging air pressure, and eventually makes it impossible to use the scavenging air for ALS. Thus, in order to extend operational range of scavenging air bypass system it is necessary to reveal effect of scavenging air bleeding on engine performance in wide range of operating modes and find countermeasures to diminish negative consequences on engine performance. In view of complexity of the assigned tasks for full-scale experiments it is reasonably to use the simulation technique for preliminary investigation propulsion plant performance equipped with air by-pass system.
Experiment arrangement and conditions
In order to investigate the effect of scavenging air by-pass system on engine performance in both the steady and transient conditions the series of full-scale experiments on four-stroke and two-stroke engines had been performed. The collected data also were used to set up and validate the simulation model of engine.
The test engines were equipped with a scavenging air by-pass system as shown in Fig. 2. The bypass system are comprised of control valves, flow, pressure and temperature sensors as well as control system and directly connected to the engine's air manifold after charging air cooler. The test engines specification are listed in Table 1 for four-stroke and two-stroke engines respectively. The performance data were measured at 3 operating points, which represent typical loads of propulsion engine.
Table 1
4-stroke and 2-stroke engines particulars
Niigata 6L19HX Mitsui-MAN 4S50ME-T9.2
No of Cylinders -- 6 4
Bore/Stroke mm 190/260 500/2214
Power kW 750 7120
Speed rpm 1000 117
Scavenging air pressure barA 2.9 4.4
Scavenging air flow Nm3/h 4180 40932
Table 2
Experiment conditions
4st engine 2st engine
Operating points:
Power/Speed at P1, % 93 / 97 85 / 95
Power/Speed at P2, % 75 / 91 63 / 79
Power/Speed at P3, % 79 / 96 52 / 87
Air bleeding rate, % 7,14,18,25,30 5,10,15
By-pass fluctuation period, sec 7,10,15 10,12,15
By-pass fluctuation amplitude, % 25,30,40,50 20,30
Fig. 2. Scavenging air by-pass system test bench
At each measurement point the engine speed and torque were fixed and engine performance parameters as well as exhaust gas components emission were measured at different air bleeding rates ; such measurements are referred as steady-state. Besides at these points the performance of engines were measured with fluctuating air bleeding rate and are referred as transient state. The measurement points, air bleeding rates and fluctuating conditions are summarized in Table 2.
Here it should be noted that periodical change of air bleeding rate results in periodical change of air to fuel ratio that is regular occurrence in actual sea as shown by Bondarenko and Kashiwagi [3], thus our experiments also reflects engine performance change in real sea conditions.
Experiment results and analysis
During the experiments a huge amount of data were collected and for sake of compactness the most important and relevant results are reported. Here it should be noted that the experiments were done without any measures to improve engine performance against reduced scavenging air pressure such as adjustment of turbine nozzle area or auxiliary air blowers.
The primary effect of air bleeding becomes apparent in air manifold pressure drop as shown in Fig. 3 for four-stroke and two-stroke engines respectively. Consequently air pressure drop leads to depletion of cylinder air charge which is indirectly indicated in Fig. 3 by drop of oxygen concentration in the exhaust gas emission. The effects of air depletion due to by-pass operation on engines performance are reported in Fig. 4 for four-stroke and
two-stroke engines respectively. As can be seen from these figures, similar for both engines, gradual increase of air bleeding rate causes increase of SFOC and it follows from the fact that air bleeding deteriorates fuel combustion. The former is confirmed by decreasing trend of NOx emission (since NOx formation greatly depends on oxygen concentration in cylinder). These results are typical for steady-state engine operation with reduced air to fuel ratio as can be found in Benson and Ledger [1]; Heywood [5].
The most prominent results though, came from transient tests. During transient tests the load torque and engine speed were kept constant whereas air bleeding rate was changing periodically with amplitude and period shown in Table 2. The results are also depicted in Fig. 4 in terms of average values of variables. As can be seen, there is no remarkable effect on four-stroke engine performance - the transient performance coincides with that at steady-state. However, there is a tiny effect of transient operation on performance of two-stroke engine, and this difference can be explained by the different scavenging process of four-stroke and two-stroke engines [4]. Four-stroke engines within one working cycle consist of suction stroke and thus less sensitive to variation in scavenging air pressure, whereas two-stroke engines fully rely on scavenging air supplied by TCH.
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Scavenging Air Pressure □ O2 concentration
OO
--1-1-1-1-
20 25 30
Air Bleeding Rate, %
10 12 14 16
Air Bleeding Rate, %
Fig. 3 Effect of scavenging air by-pass on 4-stroke and 2-stroke engines performance
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0
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Fig. 4. Experiment results for 4-stroke and 2-stroke engines
Nevertheless, one should accept that transient operation of both the four-stroke and two-stroke engines can be considered as quasi-steady or in other words from the engine point of view the transient trajectory is assumed to be made up of series of steady-state points as engine dynamic and, in particular duration of scavenging process is much faster than the fluctuation of air bleeding rate. This implies engine and air by-pass system models can be constructed assuming quasi-steady process as explained in forthcoming section.
Engine simulation model
Important characteristic of any simulation model is a model adequacy in the framework of set research objectives. The model should appropriately describe entities important for current research at the same time simplification can be accepted for inessential and minor parameters. The model of a Diesel engine in quasi-steady approximation simulates the components constituent of the engine as if they pass successively from one steady-state operating point to another. The intermittent nature of the engine cycle is simplified through time-averaged values [11]. The main principle used in the cycle mean model of a two-stroke marine Diesel engine is the flow of air and exhaust gas through the two throttles: the first one is a cylinder unit and the second is a turbocharger turbine as illustrated in Fig. 5.
The equations of flow through the throttles together with the equation of fuel mass flow, appended with mass balance form a core part of
the model. These equations, respectively for air, exhaust gas, fuel flows and mass balance, are expressed as:
Fig. 5. Concept of cycle mean model of a Diesel engine
a JRJs I \Ya-1 \Ps> \Ps-
2
Ya+1 A Ya
yPsJ
(1)
Ge — at y.AT
Pe
V ReTe
2
(Ye \ (Pa\Y^_iPa \Ye - 1 (Pe) (Pe)
Ye + 1
(2)
Gf — zc if hp ne
Ge = Ga + Gf
(3)
(4)
where ^Av, ^AT are equivalent throttle areas of cylinder unit and turbine respectively; Ra, Re are the gas constants for air and exhaust gas respectively; ya, ye are the specific heat ratios of air and exhaust gas respectively; Ts, ps are the temperature and pressure of air at the cylinder input; Te, pe are the temperature and pressure of exhaust gas at the turbine input; m0 is the amount of fuel injected to one cylinder per
2
cycle at rated power; zc is the number of cylinders; hp is the fuel pump index (provided by the speed governor); ne is the engine rotational speed.
The engine simulation loop is closed through the simulation of a turbocharger unit, in respect that the turbine transforms energy of exhaust gas to mechanical energy and the compressor receives this mechanical energy and transforms it to the energy of compressed air, which eventually is necessary for combustion.
The torque delivered by the turbine:
^ = mrCPeTe{Gair + Gf) / _ (S)
nTC y We' J
The torque absorbed by the compressor:
ViC nrc\ \Pa' )
The engine rotational speed ne and turbocharger shaft speed n T C are calculated from equations of shafts dynamics: dne
2n Ie~^=Qe(t)_Ql(t) (7)
dnTC
2n= Qr(t) _ Qc(t) (8)
where Ql(t) is the load torque applied on the engine, in particular it can be the torque absorbed by a ship propeller.
The engine torque Qe(t) is calculated using the following equation:
Qe 2jT(Pmep _ fyr) (9)
The mean effective pressure Pmep developed in the cylinder as a result of fuel combustion is defined as:
Pmep VcPmephp (1°)
where Pm,ep is the maximum mean effective pressure that the engine can achieve and is specified by the
1.2
manufacturer; vc = 1 _ 1.1 e a2 is the combustion efficiency [1], which is a function of air excess ratio a = Ga/Gf; P^r = f(ne) is the friction mean effective pressure.
Last but not least important model elements are the scavenging air and exhaust gas flow receivers, which interconnected between flow elements: compressor, cylinder unit, turbine and air by-pass system. Applying mass and energy balances on flow receiver elements and assuming ideal gas law, the following model of exhaust gas receiver is obtained:
dm
dt
r = Ga + Gf-Ge (11)
dTP Ye , ч Te i dmr\
dne=mzez&aM+w^—fa+—) (12)
mrReTe
Pe = —Tr--(13)
ve.r
With respect to scavenging air receiver, it is more convenient to write equation with respect to pressure, assuming that the temperature is constant as it is governed by an intercooler:
dps YaRaTs
dt
Va.r
(Gc — Ga — Gbp)
(14)
The intercooler is installed between the compressor and scavenging air receiver, and is described as follows:
T TrW + knr ( Tn
1
Ya-1
<PsYY~
ViC Wa
— ) ,a -1}+1
— T
1 r1
1
( Gc0)
(15)
where Tcw denotes temperature of cooling water; kac is the cooler constant; Va r, Ve r are the receivers volume; £,a is the fuel chemical energy contained in exhaust gas; Hu is the lower calorific value of fuel; Cpe, CPa are the specific heat capacity of exhaust gas and air.
The compressor is represented by its steady-state performance map. Given the speed nTC and pressure ratio nC = ps/pa, the flow rate Gc and efficiency ijiC are calculated using interpolation. Finally, the air by-pass system is simulated as a quasi-steady flow through orifice:
Gbp — ßf
V RaTs
7
Ya+1
/Pb\Ya /РА Ya \Ps' s'
(16)
where denotes a flow area ofby-pass valve.
The empirical parameters constituent eqs. (2), (5), (9), (12) and (15) can be found from engine operational data as explained by Bondarenko and Fukuda [2].
The quasi-steady thermodynamic model of Diesel engine allows simulation of complete engine as a part of ship propulsion plant equipped with air lubrication system. The main inputs to the model are rate of fuel flow hp, load torque Qi and by-pass valve opening Any of the presented temporary variables can be calculated as output.
Fig. 6. Steady-state engine performance measured and calculated
The above described simulation approach were applied for examining the transient behavior of two-stroke engine used in by-pass experiments, where significant amount of data for steady and transient operation were collected. The most important steady-state engine variables are compared in Fig. 6.
To confirm air by-pass system operation, the measured by-pass valve position were used as input to the model, and engine responses in terms of scavenging air pressure and by-pass flow were compared with that measured during experiments. The results are reported in Fig. 6. The both figures confirm the ability of quasi-steady approach to represent real engine operation in both the steady-state and transient conditions.
Conclusion
The experiments on four-stroke and two stroke engines made it possible to reveal the effect of scavenging air by-pass system on engine performance in both the steady-state and transient condition. As was found large air bleeding rate causes drop of the scavenging air pressure in receiver and lead to dramatic engine performance deterioration, this is because air bleeding act as a power takeoff from the compressor at the same time power input from the turbine stays constant. In this respect in order to restore power balance of turbocharger and thus avoid engine performance deterioration, the additional energy should be supplied to the turbine in response to air bleeding rate increase. However the important question is: how does the required compressor assist power relate to air bleeding rate? At the preliminary stage the answer can be found through the analysis of compressor power Pc = Qc nTc by the method of small deviation [10]. The method allows of interpreting original nonlinear equation as an explicit linear combination of the small deviation of constituent arguments. Following the method, equation for compressor power is transformed as:
log(Pc) = log(CpJ + log(Ts) + log(Gc) — log^^ + log(l — nja )
U
d[log(.PC)]^n d[log(Gc)] d[log(riic)]^
dPc dPc= dCc dGc—^rccrdVic+
U
Va-l „ Va
duc
duc
5PC = 8Gc-5ï]ic + Kc8n{
(17)
c
where SPC = is the relative change of compressor power, 8GC = ^ is the relative change of air flow through compressor, 5ijiC = ^^ is the relative change of compressor efficiency, SnC = ^ is the relative
Va-1 Ya-1n Va
change of compressor pressure ratio and KC = Va is the influence coefficient.
i V c
l-uja
From the eq. (17) it is readily seen that the increment of air flow by 10% will require the same increment in power if the other parameters stay constant. However from the theory of centrifugal compressors [12] it is known that the efficiency iic drops as air flow increases thus contributing to power increment Siic = —f(5Gc). At the same time the experiment showed that the decrease of air pressure by 10 % causes only minor deterioration of engine performance as can be checked in figs.3 and 4, thus some minor change in pressure Snc = —f(SGc) may significantly reduce required power increment due to the fact that Kc > l. From the engine side the required power increment can be ensured by applying hybrid turbo compressor [13] moreover additional turbine power can be obtained by adjusting the turbine flow area utilizing a well-known technology - variable turbine area (VTA) system. Using the developed model the effect combine operation of these devices with the air by-pass system will be further investigated.
ACKNOWLEDGEMENTS
This work was supported by Grant-in-Aid for Scientific Research (KAKENHI), No. 23246153.
The authors also wish to thank Mitsui Engineering and Shipbuilding Corporation for granting an opportunity to experiment with Mitsui-MAN two stroke marine Diesel test engine.
REFERENCES
1. Benson R.S., Ledger, J.D. et al. Comparison of experimental and simulated transient responses of a turbocharged Diesel engine. SAE paper 730666, 1973.
2. Bondarenko O., Fukuda T., et al. Development of Diesel Engine Simulator for Use with Self-Propulsion Model. J. of the JIME. 2013(48);5:98-105.
3. Bondarenko O., Kashiwagi M. Dynamic behavior of ship propulsion plant in actual sea. J. of the JIME. 2010(45):76-80.
4. Dyachenko V.G. Theory of Internal Combustion Engines. Kharkiv, Evromedia, 2009, 500 p.
5. Heywood J.B. Internal Combustion Engines Fundamentals. McGraw-Hill, 1988, 917 p.
6. Hinatsu M. Recent Activity on the Energy Saving for Ships Using Air Lubrication Method. Japanese J. Multiphase Flow. 2013(27);1. (in Japanese).
7. Hoang C.L., Toda Y. et al. Full Scale Experiment for Frictional Resistance Reduction using Air Lubrication Method. Proc.: The 9th International Offshore and Polar Engineering Conf., Osaka, 2009.
8. Kato H., Kodama Y. Microbubbles as a Skin Friction Reduction Device - A Midterm Review of the Research. Proc.: The 4th Symposium on Smart Control of Turbulence, 2003.
9. Kodama Y., Takahashi T. et al. Practical application of microbubbles to ships - Large scale model experiments and a new full scale experiment. Proc.: The 5th Osaka colloquium, 2005.
10.Mezherickii A.D. Turbochargers of diesel engines. Leningrad, Shipbuilding, 1971, 192 p.
11.Rakopoulos C.D., Giakoumis E.G. Diesel Engine Transient Operation. Principles of Operation and Simulation Analysis. London, Springer, 2009, 408 p.
12.Slobodyanyuk L.I., Polyakov V.I. Ship's steam and gas turbines operation. Leningrad, Shipbuilding, 1983,360 p.
13.Yoshihisa Ono. Solutions for better engine performance at low load by Mitsubishi turbochargers. Proc.: The 23rd CIMAC Congress, 2012.
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Проектирование и конструкция судов
А.А. Бондаренко, Тётсуго Фукуда
БОНДАРЕНКО АЛЕКСЕЙ АЛЕКСАНДРОВИЧ - кандидат технических наук, научный сотрудник лаборатории энергий и энергосистем (Национальный морской исследовательский институт, Япония). Tokyo 181004, Japan. E-mail: bond@nmri.go.jp. ФУКУДА ТЁТСУГО - старший научный сотрудник лаборатории энергий и энергосистем (Национальный морской исследовательский институт, Япония). Tokyo 181004, Japan. E-mail: tfukuda@nmri.go.jp
Анализ работы главного судового двигателя совместно с системой воздушной смазки корпуса судна
Изложены результаты полномасштабных экспериментов, проведенных на четырехтактном и двухтактном судовых двигателях, оснащенных отбором наддувочного воздуха, для использования в системе воздушной смазки корпуса судна. Эксперименты позволили оценить эффект влияния отбора наддувочного воздуха на характеристики двигателя в стационарных и переходных режимах, что дало возможность определить меры по снижению влияния отбора воздуха на характеристики главного судового двигателя, оборудованного системой отбора воздуха. Кроме того, для дальнейших исследований была разработана простая, но в то же время эффективная имитационная модель дизельного двигателя, оборудованного такой системой. Эффективность математической модели подтверждена имеющимися экспериментальными данными.
Ключевые слова: отбор воздуха, воздушная смазка, судовая силовая установка, моделирование двигателя.
