DK179623B1

DK179623B1 - A large turbocharged two-stroke compression-ignited internal combustion engine and method of operation thereof

Info

Publication number: DK179623B1
Application number: DKPA201770489A
Authority: DK
Inventors: Jensen Kim
Original assignee: MAN Energy Solutions
Priority date: 2017-06-23
Filing date: 2017-06-23
Publication date: 2019-03-05
Also published as: DK201770489A1

Abstract

A large two-stroke compression-igniting internal combustion engine comprising a plurality of cylinders (1) with pistons (21) therein, said pistons (21) during engine operation reciprocating between a BDC and a TDC. The pistons (21) are operably connected to a crankshaft (22) via piston rods, crossheads (23) and connecting rods. The crankshaft (22) rotates with a certain rotational speed during operation of the engine. The engine also has a fuel injection system comprising one or more fuel valves (30)associated with each cylinder (1) for injecting fuel into said cylinders (1) for combustion. An electronic control unit (50) is configured to control the timing of the fuel injection relative to the crank angle of the cylinder (1) concerned by controlling opening and closing of the fuel valves (30) concerned. The electronic control unit (50) is configured to operate the engine at least in a particular rotational speed range with delayed fuel injection by the electronic control unit (50) performing at least one pre-injection after TDC, followed by a main injection.

Description

A LARGE TURBOCHARGED TWO-STROKE COMPRESSION-IGNITED INTERNAL COMBUSTION ENGINE AND METHOD OF OPERATION THEREOF

TECHNICAL FIELD

The present disclosure relates to a large turbocharged twostroke compression-igniting internal combustion engine with crossheads and to a method for operating such an engine.

BACKGROUND

Large turbocharged two-stroke compression-igniting crosshead internal combustion engine with are typically used as prime movers in large ocean going ships, such as container ships or in power plants.

In particular, when operated in ocean-going ships torsional vibrations can be challenging to control. Such torsional vibrations occur since the propeller shaft connecting the engine to the propeller is torsionally relatively flexible and this torsionally relatively flexible system is exposed to variating tangential pressure (torque) from the engine. This varying tangential pressure from the engine is caused by the cyclic process in each cylinder and repeated for each crankshaft revolution. This cyclic process in each cylinder generates large variations in crankshaft torque. During compression the torque is negative, while its positive during expansion. This is illustrated in Fig. 5, showing the cylinder pressure P and torque Q from one cylinder as uninterrupted lines and the combined torque from six cylinders as an interrupted line. By distributing a plurality of cylinders over a revolution, variations in crankshaft torque are reduced, but still significant. In the example in fig. 5, there are actually six periods for each revolution where the crankshaft torque is negative.

The problem with torsional vibrations in the loaddriveshaft-engine system is pronounced in 4, 5, 6 and 7 cylinder engines. These vibrations are critical, when considering the flexibility of the driveshaft between engine and load, for instance a propeller the inertia of engine and propeller, combined with the flexible shaft connecting them results in resonances. When running close to a resonance, the excitations from the torque variations become critical.

Torsional dampers, of the spring and or viscous type are deployed to reduce the problem of torsional vibrations.

However, torsional dampers present a significant cost increase.

Further, even with torsional dampers, these engines often have a barred speed range, i.e. a speed

e.

range in which continues operation is not allowed, because the high stresses in the shaft reduce lifetime.

Simulations and measurements have shown that delay of ignition/combustion effects the cylinder pressure in a way, which significantly reduces certain important orders of the torque variations. Thus, torsional excitation can be reduced by delaying the fuel injection. However, delaying fuel injection beyond 10° crank angle after top dead center

(TDC) is normally not possible diesel knocking.	due to the	occurrence of
WO9429585 discloses a large	two-stroke	compression-
igniting internal combustion	engine and	a method of
operating it according to the preamble of cl	aims 1 and 6,
respectively.
SUMMARY
In view of the above it is an object of	the present
invention to provide a large	two-stroke	compression-

igniting engine that, at least in a given RPM bandwidth can operate with a very late timed fuel injection delay in order to overcome or at least reduce the problems mentioned above.

The foregoing and other objects are achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures.

According to a first aspect, there is provided a large twostroke compression-igniting internal combustion engine comprising a plurality of cylinders with pistons therein, the pistons during engine operation reciprocating between a BDC and a TDC, the pistons being operably connected to a crankshaft via piston rods, crossheads and connecting rods, the crankshaft rotating with a certain rotational speed during operation of the engine, a fuel injection system comprising one or more fuel valves associated with each cylinder for injecting fuel into the cylinders for combustion, an electronic control unit being configured to control the timing of the fuel injection relative to the crank angle of the cylinder concerned by controlling opening and closing of the fuel valves concerned, the electronic control unit being configured to operate the engine at least in a particular rotational speed range with delayed fuel injection by the electronic control unit performing at least one pre-injection after TDC, followed by a main injection.

Pressure and temperature in the combustion chamber affect the occurrence of knocking. When delaying combustion, both temperature and pressure drops because of expansion of the air in the combustion chamber. By performing at least one pre-injection after TDC, i.e. after TDC = zero, the temperature in the combustion chamber is kept at a higher level, thereby increasing the maximum acceptable delay of the main injection without the risk of diesel knocking.

According to a first possible implementation of the first aspect the electronic control unit is configured to perform the main injection preferably later than 12° after TDC, more preferably later than 13° after TDC, even more preferably later than 14° after TDC, and most preferably later than 15° after TDC.

According to a second possible implementation of the first aspect the at least one pre-injection comprises an amount of fuel injection that is significantly lower than the amount of fuel injected in the main injection at full engine load.

According to a third possible implementation of the first aspect the at least one pre-injection comprises amount of fuel that is substantially the same for all engine loads.

According to a fourth possible implementation of the first aspect the electronic control unit is configured to preinject an amount of fuel sufficient for ensuring that the temperature in the cylinder concerned at the delayed main injection is substantially equal to the temperature in the cylinder concerned at TDC.

According to a fifth possible implementation of the first aspect the fuel for the main injection a gaseous fuel and fuel for the pre-injection is an ignition liquid, ignition liquid also being injected simultaneously with the main injection.

According to a second aspect there is provided a method of operating a large two-stroke compression-igniting internal combustion engine that comprises a plurality of cylinders with pistons therein, the pistons during engine operation reciprocating between a BDC and a TDC, the pistons being operably connected to a crankshaft via piston rods, crossheads and connecting rods, the crankshaft rotating with a certain rotational speed during operation of the engine, a fuel injection system comprising one or more fuel valves associated with each cylinder for injecting fuel into the cylinders for combustion, the method comprising at least in a particular rotational speed range with delayed fuel injection: performing at least one preinjection after TDC, followed by a main injection.

According to a first possible implementation of the second aspect the main injection is preferably performed later than 12° after TDC, more preferably later than 13° after TDC, even more preferably later than 14° after TDC, and most preferably later than 15° after TDC.

According to a second possible implementation of the second aspect the at least one pre-injection comprises an amount of fuel injection that is significantly lower than the amount of fuel injected in the main injection at full engine load.

According to a third possible implementation of the second aspect the method comprises pre-injecting an amount of fuel sufficient for ensuring that the temperature in the cylinder concerned at the delayed main injection is substantially equal to the temperature in the cylinder concerned at TDC.

According to a fourth possible implementation of the second aspect the method comprises pre-injecting an ignition liquid, followed by a main injection, wherein the main injection comprises injecting a gaseous fuel and a small amount of ignition liquid.

These and other aspects of the invention will be apparent from and the embodiments described below.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed portion of the present disclosure, the invention will be explained in more detail with reference to the example embodiments shown in the drawings, in which:

Fig. 1 is an elevated view showing the fore end and one lateral side of a large two-stroke compression-ignited turbocharged engine according to an example embodiment, Fig. 2 is an elevated view showing the aft end and the other lateral side of the engine of Fig. 1,

Fig. 3 is a diagrammatic representation the engine according to Fig. 1 with its intake and exhaust systems, Fig. 4 is a partially cut open side view of a marine vessel provided with the engine of Figs. 1-3,

Fig. 5 is a diagram illustrating the torque variations produced by the engine of Figs. 1-3,

Fig. 6 is a diagram illustrating the effect of the torque variations produced by the engine of Figs. 1-3, and

Fig. 7 is a diagram illustrating the combustion chamber temperature and pressure for a prior art engine and for the engine according to Figs. 1-3.

DETAILED DESCRIPTION

In the following detailed description, a large two-stroke compression-igniting engine and a method for operating a large two-stroke engine compression-igniting engine will be described by the example embodiments. Figs. 1 to 3 show a large low speed turbocharged two-stroke diesel engine with a crankshaft 22, connecting rods, crossheads 23 and piston rods. Fig. 3 shows a diagrammatic representation of a large low speed turbocharged two-stroke diesel engine with its intake and exhaust systems. In this example embodiment, the engine has six cylinders 1 in line. Large turbocharged two-stroke diesel engines have typically between five and sixteen cylinders in line, carried by an engine frame 24. The engine may e.g. be used as the main engine in an ocean-going vessel or as a stationary engine for operating a generator in a power station. The total output of the engine may, for example, range from 5,000 to 110,000 kW.

The engine is a diesel (compression-igniting) engine of the two-stroke uniflow type with scavenge ports 19 in the form a ring of piston-controlled ports at the lower region of the cylinders 1 and an exhaust valve 4 at the top of the cylinders 1. Thus, the flow in the combustion chamber is always from the bottom to the top and thus the engine is of the so called uniflow type. The scavenging air is passed from the scavenging air receiver 2 to the scavenging air ports 19 of the individual cylinders 1. A reciprocating piston 21 in the cylinder 1 compresses the scavenging air in the combustion chamber 14. Fuel is injected via two or three fuel valves 30 that are arranged in the cylinder cover 26 into the combustion chamber 14. The timing of the fuel injection is controlled by an electronic control unit 50 that is connected via signal lines (illustrated as interrupted lines in Fig. 3) to the fuel valves 30.

Combustion follows and exhaust gas is generated. When an exhaust valve 4 is opened, the exhaust gas flows through an exhaust duct 20 associated with the cylinder 1 concerned into an exhaust gas receiver 3 and onwards through a first exhaust conduit 18 to a turbine 6 of the turbocharger 5, from which the exhaust gas flows away through a second exhaust conduit 7. Through a shaft 8, the turbine 6 drives a compressor 9 supplied via an air inlet 10.

The compressor 9 delivers pressurized charging air to a charging air conduit 11 leading to the charging air receiver 2. The scavenging air in the conduit 11 passes through an intercooler 12 for cooling the charging air. The cooled charging air passes via an auxiliary blower 16 driven by an electric motor 17 that pressurizes the charging air flow in low or partial load conditions to the charging air receiver 2. At higher loads the turbocharger compressor 9 delivers sufficient compressed scavenging air and then the auxiliary blower 16 is bypassed via a nonreturn valve 15.

The cylinders 1 are formed in a cylinder liner 13. The cylinder liners 13 are carried by a cylinder frame 25 that is supported by the engine frame 24.

In a reciprocating engine, the dead center is the position of a piston in which it is farthest from, or nearest to, the crankshaft. The former is known as top dead center (TDC) while the latter is known as bottom dead center (BDC).

Fig. 4 illustrates the engine of Figs. 1 - 3 installed in a large marine vessel 40. The engine 1 is installed in an engine room that is relatively close to the stern of the marine vessel 40. A propeller shaft 42 connects the engine to a stern mounted propeller 44. A torsional damper (not shown) can be installed between the propeller shaft 42 and the engine.

Fig. 5 is a graph illustrating the torque variations created by the engine caused by the cyclic process in each cylinder during an engine cycle. The engine cycle is illustrated on the horizontal axis in degrees. During compression the torque is negative, while its positive during expansion. Fig. 5, shows the cylinder pressure P (bar) on the vertical axis and torque Q from one cylinder in uninterrupted lines and the combined torque from six cylinders as an interrupted line. The interrupted line clearly shows that the torque fluctuations are significant and actually the torque goes slightly below zero six times for each revolution of the six-cylinder engine.

Fig. 6 is a graph illustrating the magnitude of the effect of the torsional vibrations/expectations as stress in MPa in the drive shaft set out against the engine speed in RPM for a prior art engine.

The graph shows that there is a peak around 46 RPM. The large peak around 46 RPM results in a barred speed range between approximately 42 and 49 RPM, i.e. between the 2 vertically extending dashed lines. The magnitude of the stress in the drive shaft caused by the torsional vibrations, especially around the peak, can be reduced by a late main fuel injection (enabled and preceded by a small pre-injection).

The graph shows two rpm dependent stress limits in the form of the two dashed lines of the chain type. Stress levels below the lower chain line are acceptable for continuous operation. Stress levels below the higher chain line I never acceptable. Stress levels between the lower and higher chain line are acceptable for a limited period of time.

Fig. 7 illustrates the timing of the fuel injection event for a single cylinder. The interrupted lines show the events for a prior art engine, while the continuous lines show the events for an engine and method according to the present disclosure. The lines indicated with P illustrate the pressure in the combustion chamber 14 whilst the lines indicated with T illustrate the temperature in the combustion chamber. On the horizontal axis the crank angle relative to TDC is illustrated in degrees and on the vertical axis pressure in the combustion chamber is shown in bar.

In the prior art engine and method, the fuel injection is delayed to 5° after TDC. Between TDC 0 and the fuel injection at 5°, both temperature and pressure in the combustion chamber 14 fall. At 5° after TDC, fuel is injected and from this moment, the temperature in the combustion chamber rises until each reaches their respective maximum.

In the engine according to the present disclosure, a small pre—injection is performed by the electronic control unit 50 by operating the fuel valves 30. The pre-injection is performed after TDC 0. Preferably, the pre- injection is performed between 6-10° after TDC, more preferably around 7-8° and most preferably around 8° after TDC. The preinjection is a fuel injection with a relatively small amount of fuel compared to the main injection that will follow. The pre-injection injects an amount of fuel that is sufficient for ensuring that the temperature in the combustion chamber 14 does not significantly fall below the temperature at TDC 0 until approximate 10° after TDC has been reached. The main injection follows later, controlled by the electronic control unit 50. The pre-injection can be performed as a single injection or as a series of multiple small pre-injections, and the electronic control unit 50 is in an embodiment configured accordingly. The main injection is in an embodiment delayed until up to 25° after TDC. Preferably, the main injection is performed at least 12° after TDC, more preferably at least 13° after TDC and even more preferably at least 14° after TDC and most preferably at least 15° after TDC. Tests and simulations have shown that domain objection can be timed as late as 20-25° without diesel knocking or other combustion problems when a pre-injection is performed shortly after TDC.

The delayed injection is typically detrimental to fuel efficiency, and therefore the delayed injection is normally only applied in the range of engine speed with torsional vibrations and resonance problems. Thus, the electronic control unit 50 is in an embodiment configured to apply the pre-injection and late main injection only in a predetermined speed range of the engine associated with torsional operation problems. Of course, double injections (a pre-injection followed by a late timed main injection) could also be used for other purposes such as e.g. the reduction of NOx emissions.

With the engine and method according to the present disclosure the main injection event can be delayed well beyond 10° after TDC, thereby reducing torsional expectations and thus reducing problems related to torsional vibrations in engine - shaft - load systems.

The method an engine according to the present disclosure can be used for conventional fuels, such as marine diesel or heavy fuel oil as well as for alternative fuels, such as gaseous fuels.

In the case of gaseous fuel, the pre-injection will typically be performed within an ignition liquid, such as marine diesel. The main injection will be an injection of a small amount of ignition liquid together with the main amount of gaseous fuel.

According to an embodiment, each cylinder of an engine may run on a different cycle process. Thus, the pre-injection followed by a late timed main injection can be applied to one or more selected cylinders while others run on a conventional cycle with a single fuel injection per cycle.

In an embodiment, the type of fuel very between cylinders. The invention has been described in conjunction with various embodiments herein. However, other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.

The reference signs used in the claims shall not be construed as limiting the scope.

Claims

Large compression ignition large two-stroke internal combustion engine comprising:

a plurality of cylinders (1) having pistons therein, the pistons (21) moving back and forth between a lower dead center and an upper dead center during engine operation, the pistons (21) being operatively connected to a crankshaft (22) via piston rods, crossheads (23) and connecting rods, the crankshaft (22) rotating at a certain rotational speed during operation of the engine, a fuel injection system comprising one or more fuel nozzles (30) connected to each cylinder (1) for injecting fuel into the cylinders ( 1) for combustion, an electronic control unit (50) configured to control the timing of the fuel injection relative to the crankshaft angle of the cylinder (1) in question by controlling the opening and closing of the fuel nozzles (30) in question, characterized in that the electronic control unit (50) is configured to drive the engine at least in a particular rotational speed range with delayed fuel injection by means of the electronic control unit (50) by performing at least one pre-injection after upper dead center, followed by a main injection.

The engine of claim 1, wherein the electronic control unit (50) is configured to perform the main injection preferably later than 12 ° after upper dead center, more preferably later than 13 ° after upper dead center, even more preferably later than 14 ° after upper dead center, and most preferably later than 15 ° after upper dead center.

The engine of claim 1 or 2, wherein the at least one pre-injection comprises an amount of fuel injection that is substantially less than the amount of fuel injected into the main injection at full engine load.

An engine according to any one of claims 1 to 3, wherein the electronic control unit (50) is configured to pre-inject a sufficient amount of fuel to ensure that the temperature in said cylinder (1) during the delayed main injection is substantially corresponds to the temperature of the cylinder in question at the upper dead center.

An engine according to any one of claims 1 to 4, wherein the fuel for the main injection is a gaseous fuel and the fuel for the pre-injection is an ignition fluid, which ignition fluid is also injected simultaneously with the main injection.

A method of operating a large two-stroke compression-ignition internal combustion engine comprising:

a plurality of cylinders (1) with pistons (21) therein, the pistons moving back and forth between a lower dead center and an upper dead center during engine operation, the pistons (21) being operatively connected to a crankshaft (22) via cylinder rods, crossheads (23) and connecting rods, the crankshaft (22) rotating at a certain rotational speed during engine operation, a fuel injection system comprising one or more fuel nozzles (30) connected to each cylinder (1) for injecting fuel into the cylinders (1); ) for combustion, characterized in that the method comprises at least in a special rotational speed range with delayed fuel injection:

performing at least one pre-injection after upper dead center, followed by a main injection.

A method according to claim 6, wherein the main injection is preferably performed later than 12 ° after upper dead center, more preferably later than 13 ° after upper dead center, even more preferably later than 14 ° after upper dead center and most preferably later than 15 ° after upper dead center.

The method of claim 6 or 7, wherein the at least one pre-injection comprises an amount of fuel injection that is substantially less than the amount of fuel injected into the main injection at full engine load.

A method according to any one of claims 6 5 to 8, comprising pre-injecting a sufficient amount of fuel to ensure that the temperature in said cylinder at the delayed main injection substantially corresponds to the temperature in said cylinder at upper dead center .

A method according to any one of claims 6 to 9, comprising pre-injecting an ignition fluid, followed by a head injection, the main injection comprising injecting a

15 gaseous fuel and a small amount of ignition fluid.