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Operational Features of the Main Diesels in Propulsion Electrical Installations in Unsteady Modes
Victor F. Shtykov
Electro-Engineering Department of the MSTU, Electrical Equipment Chair
Abstract. In this research the author, on the example of the test of the diesel-electrical Propulsion Plant of the B-422 project vessel, proves the fact that, from the point of view of optimization of the main diesels operation conditional both under manoeuvring conditions and when a vessel is on heave of the sea, it is reasonable to use a power regulator as the main regulator for the Propulsion Plant automatic control system instead of a speed regulator.
1. Introduction
A wide application of turbo-charged diesels as main engines for marine propulsion, especially in fishing trawlers and ice breakers, results in a significant reduction of their service life, and of the specific fuel consumption, and in a decrease of their efficiency. It is connected with frequent and radical changes of a load applied to main diesels while manoeuvring and in unsteady modes, which are characteristic of the ships mentioned above. For example, the quantity of changes of the modes of operation of a fishing ship propulsion installation can reach 22-25 thousand a year. The same refers to ice-breakers. For example, the number of only the propeller reversals in an ice breaker, cruising on the northern sea route, reaches 7 thousand per month.
As a rule, the system of charging diesels with air is calculated so that in the design mode of operation the diesel has the best power and efficiency parameters. It is obtained by coordinating the fuel and air supply per cycle at the design mode. When the turbo-charged diesel is in no-load or low-load operation, the amount of the exhaust-gas energy available is insufficient for attaining the necessary speed of rotation of a gas turbine, therefore, the turbocharger does not practically create increased pressure of supercharged air. At a fast load surge the diesel speed governor provides a fast increase of the fuel feed per cycle. Owing to the inertia of the air supply system the pressure of supercharging grows considerably slower, than the quantity of the fuel, fed into the diesel cylinders. This circumstance results in a sharp decrease of the excess-air coefficient, the latter in its turn, results in the reduction of the indicated efficiency, an increase of the specific fuel, consumption to growth of thermal stressing of the cylinder-piston assembly parts.
For example, the research of the 18PC2V-400 diesels carried out by M.K. Ovsyannikov and V.A. Petukhov on vessels of the "Byelorussia" type, allowed us to come to the conclusion that the main reason for gas escape under the cylinder covers, and the afterward occurrence of cracks in them is fast load surges due to the properties of the control system, which are accompanied by a decrease of the excess-air coefficient, that results in deterioration of the process of fuel combustion, increase of the exhaust gas temperature, it also causes smoking of the engine. In this case the conditions occur for uneven heating of the cylinder-and-piston assembly components and consequently, for their non-uniform expansion.
The characteristic example can be that of the B-422 project vessels constructed in Poland for the former Soviet Union in the 1970's. These vessels were equipped with a d.c. electrical propulsion installation fitted with a quick-response automatic control electronic system. 3 diesels constructed in Poland under licence of Sulcer firm were installed in each of those ships as main engines. The diesels had the following particulars: type of engine - 6 A25; power - 816 kW; R.P.M. - 750 r.p.m.; number of cylinders - 6; piston diameter - 250 mms; piston stroke - 300 mms; speed of piston - 7,5 meters/second; cylinder space - 147,26 litres; degree of compression - 13; effective pressure - 14,7 kgs/square centimetre; pressure of supercharging - 2,26 kgs/ square centimetre. On the first 6 vessels of the type mentioned above during the first year of their operation all the 108 cylinder covers were damaged. A large order for 22 ships appeared to be under the threat of failure.
2. Results
Experimental research, conducted by the author on one of those vessels, the diesel-electric ship "Professor Nestor Smirnov" showed that the control system of the propulsion being studied effected strict limitation of the load at the nominal level while manoeuvring. However, the time of load change is not monitored leading to extreme operating conditions of the diesel with low fuel economy and with high temperature stresses because of the fast increase of the torque and the consequent not conformity in the fuel and air supply per cycle.
Fig. 1 shows the oscillograms of the transient process of the main diesel of the propulsion installation of the B-422 project when starting up the propulsion electric motor from stationary condition up to a steady speed of rotation. In 6,5 seconds after the beginning of manoeuvring the power on the generator shaft is increased up from 22% to 80% of the nominal value. The control rack of the fuel pump in 6,2 seconds moves as far as it will go and remains in such position for 3,1 seconds. The pressure of supercharging reaches the level, appropriate for the steady-state condition developed in 10 seconds. The delay of the increase of the air supply to the diesel cylinders results in a significant reduction of the excess-air coefficient, therefore, 6,5 seconds later the temperature of the exhaust gases rises as high as 650 degrees C. It is 50 degrees higher than the maximum allowable temperature (600 degrees C) and 160 degrees higher than the temperature of the exhaust gases, corresponding to the steady-state condition at the given load. The experiments made by the author on vessels of B-422 project shows that when a vessel is navigating in the rough sea the main diesels also fall in adverse conditions of work. The widest range of load fluctuations corresponds to a mode of a vessel towing a
Fig. 2 shows the change of the main parameters of the propulsion installation of the vessel "Professor Nestor Smirnov" towing a pelagic trowl of 2312 model in sea force 5. The range of the deviation of the main parameters of the propulsion installation from the average values as shown in Fig.4 are the following:
- speed of rotation of the propulsion motor - up to 1,9%;
- main diesel-generator shaft power - up to 37%;
- fuel pump control rack position - up to 56%;
- exhaust gas temperature - up to 290 degrees C;
- pressure of supercharging - up to 7,5%.
Fluctuations of the position of the fuel pump control racks in prime movers of the propulsion installation result in occurrence of a range, in which the diesels operate at a lowered excess-air coefficient and, as a consequence, at a lowered indicated efficiency, that results in increased specific fuel consumption. The
Fig. 1. The oscillograms of the transient process of the main diesel of the propulsion installation.
fluctuations of the exhaust gas temperature can promote emergence of thermo-fatique micro-cracks in the diesel cylinder covers. An example of an elementary device, most frequently used in the propulsion gears for setting limits of dynamic loads when manoeuvring a vessel, is a rate set-point device which looks like an integrator with a restricted output signal. However, the application of such set-point device permits to eliminate the sudden (step) change of a signal of the preset speed of the propulsion engine, but it does not eliminate the overloading of the main diesels under manoeuvring. Despite the considerable increase of the time of the propulsion engine reversal, considerable overloading of the prime movers is still a feature of the transient process. It is connected with the fact that, at linear growth of the propulsion engine speed, the power on its shaft is pursuant to cubic dependence. The experiments show, that with this method of control to eliminate overloading of prime movers the integration time of linear rate set-point device for ships of the B-422 project should make 225 seconds, that worsens the manoeuvring qualities of a vessel and can not be recommended for application under the requirements of safety.
For the elimination of the problem in question the author has developed a device for generating a control signal. It is a rate set-point device, the integration time constant of which automatically varies in inverse proportion to the value of the excess-air coefficient of the main diesels. The circuit of this device is shown in Fig.3.
The device includes a control position 1, consisting of a comparator 2 and an integrator 3, with a common negative feedback applied to them, an integrator output is shunted by a photoresistor of a photon-coupled pair (PCP) 4, an imitter circuit of which is connected to the output of the amplifier 5, the input of the latter is connected with the output of the block of division 6, to the inputs of which the signals are sent from the displacement transducer 7 and the pressure gauge 8. This signal is proportional to the
displacement of the fuel pump control rack and to the change of the supercharging pressure of the prime movers. When changing over the position of the control handle 1, for example, from a "zero" to "full ahead" the signal at the control position output is quickly increased. The signal at the output of the comparator 2 jumps up to a maximally possible level, causing intensive growth of the control signal value at the output of the integrator 3. It results in an increase of the load on the main diesel shaft and, hence, in an increase of the output signal of the displacement transducer 7, indicating the position of the fuel pump control rack: the output signal of the division block 6 and the amplifier 5 is increased. The resistance of the photoresister decreases, the latter results in the reduction of the input signal of the integrator 3, the decrease of the growth rate of the control signal and the decrease of the growth rate of the diesel electric propulsion installation load. The main diesel power increases results in the growth of supercharging pressure, owing to which the output signal of the supercharging pressure gauge 8 increases, but the output signal of the block of division 6 decreases. Thus, the variation rate of the signal for a preset value of the propulsion motor r.p.m. and, hence, the rate of load variation depends on the value of the output signal of the block of division, that is on the value of the ratio of the control rack position to the pressure of supercharging the main diesels.
The comparative characteristics of the start-up modes of the propulsion motor in the diesel-electric ship "Professor Nestor Smirnov":
Fig. 3. The circuit of the rate set-point device, the integration time constant of which automatically varies in inverse proportion to the value of the excess-air coefficient of the main diesels.
Mode of start-up
Power
Time of start (seconds)
Exhaust gas temperature (degrees C)
Without correction 88 7,0 650
With correction 88 10,5 545
From the analysis of the graphs, shown in Fig. 4 one may see that the increase of the acceleration time of the electric propulsion motor by 3,5 seconds only, due to the excess-air coefficient corrective action applied to the control, permits to lower the level of prime-movers overloading considerably. It is displayed in the decrease of the maximal exhaust gas temperature by 105 degrees C and an abrupt drop of r.p.m. in the dieselgenerators.
Besides, the calculations show that such correction allows to save up to 16 % of the fuel for each
transient.
CJr'3fl 3
Fig. 4. The change of the main parameters of the propulsion plant of the vessel "Professor Nestor Smimov" at the start-up of the propulsion motor with the excess-air coefficient corrective to the control signal.
3. Summary and conclusions
The graph of the power change on the main diesels at the start-up of the propulsion motor with the excess-air coefficient corrective action applied to the control signal, may be of special interest (Fig. 4).
This graph represents a direct line that proves the fact that from the point of view of optimization of the main diesel operation under manoeuvring conditions it is reasonable to use a power regulator as the main regulator for the propulsion installation automatic control system instead of a speed regulator.
In this case the mode of operation of the propulsion installation is set not by the value of the propulsion motor (engine) speed of rotation, but by its power level. After terminating the manoeuvring such control system provides a constant preset power level of the propulsion installation.
This conclusion meets the requirements to the propulsion installation control both under manoeuvring conditions and when a vessel is on heave of the sea.
References
Shtykov V.F. Diesel electrical propulsion installations in unsteady modes: controls and simulating (Monograph). Moscow Publ. House "Kolos", 160p., 1995.
