DK201500282A1

DK201500282A1 - A large turbocharged two-stroke self-igniting internal combustion engine with an egr control system

Info

Publication number: DK201500282A1
Application number: DKPA201500282A
Authority: DK
Inventors: Casper Hededal Svendsen; Kræn Vodder Nielsen
Original assignee: Man Diesel & Turbo Filial Af Man Diesel & Turbo Se Tyskland
Priority date: 2015-05-12
Filing date: 2015-05-12
Publication date: 2016-11-28
Also published as: JP6203325B2; KR20160133363A; CN106150775A; DK178748B1; CN106150775B; JP2016211549A; KR101800637B1

Abstract

A large turbocharged two-stroke self-igniting internal combustion engine uniflow type. The engine has a plurality 5 of cylinders (1) with scavenge ports (17) and an exhaust valve (4), an intake system through which scavenging gas is introduced into the cylinders (1), the intake system comprising a scavenge gas receiver (2) connected to the cylinders (1) via the scavenge ports 10 (17), an exhaust system through which exhaust gas produced in the cylinders is exhausted, the exhaust system comprising an exhaust gas receiver (3) connected to the cylinders via the exhaust valves (4), a turbocharger (5) having a turbine (8) that drives a 15 compressor (9) with the turbine (8) in the exhaust system and the compressor (9) in the intake system, the compressor delivering a flow of scavenge air to the scavenge gas receiver, a fuel system for delivering a flow of fuel to the cylinders (1), an EGR system for 20 conveying a flow exhaust gas from the exhaust system to the intake system and comprising at least one variable or fixed speed blower (22), a first sensor (27) providing a signal representative of the oxygen concentration Os in the scavenge gas receiver (2), and a controller (50) 25 configured to control the flow of exhaust gas through the EGR system. The controller (50) is configured to use the signal from the first sensor for feedback control, and the controller (50) is configured to use an estimate of a required recirculated exhaust gas flow for feed forward 30 control.

Description

A LARGE TURBOCHARGED TWO-STROKE SELF-IGNITING INTERNAL COMBUSTION ENGINE WITH AN EGR CONTROL SYSTEM

TECHNICAL FIELD

The present disclosure relates to exhaust gas recirculation (EGR) systems for large turbocharged two-stroke self-igniting internal combustion engines, and more particularly to control of the operation of the EGR system.

BACKGROUND

Large turbocharged two-stroke self-igniting internal combustion engines are typically used in propulsion systems of large ships or as prime mover in power plants. The height of these engines is typically not crucial, and therefore they are constructed with crossheads in order to avoid lateral loads on the pistons. Typically, these engines are operated with heavy fuel oil or with fuel oil.

Emissions from marine diesel engines are subject to restriction due to awareness of environmental effects of the emissions. The Tier III restrictions, limiting the emission of N0X from marine diesels in selected areas, as was presented by International Maritime Organization (2013) will be introduced in 2016. This motivates the ship industry to develop technologies that reduce the emissions of NOx. One of such technologies is Exhaust Gas Recirculation (EGR) which has been applied to four-stroke engines in the automotive industry for several decades.

The principle of EGR is to recirculate part of the exhaust gas back to the scavenge manifold of the engine. This decreases the scavenge oxygen level in the scavenging gas and in turn decreases the formation of NOx gas during combustion. Unfortunately, lowering the oxygen content of the scavenging gas also affects combustion efficiency. At excessively low scavenge air oxygen levels the engine will produce undesirable visible smoke.

Until recently, these large turbocharged two-stroke self-igniting internal combustion engines were not operated with EGR. This is changing due to increasingly strict emission requirements, in particular due to requirements for reducing the NOx content in the exhaust gas. EGR is a measure well known from the field of the significantly smaller four-stroke self-igniting internal combustion engines. However, the EGR technology of the smaller four-stroke engines cannot be simply applied to the much larger two-stroke engines for various reasons listed below.

One of these reasons is the difference in the effort available for commissioning an EGR controller for an EGR control system developed of a large turbocharged two-stroke diesel engine and the EGR system in the automotive industry, respectively. Each automotive engine design is thoroughly tested on a test bench before releasing for large scale production. In opposition to this, the specific large two-stroke engine designs are produced in very low numbers, they are sometimes not tested until the first engine is produced and even then very limited test time is available due to very high test running cost. It is furthermore possible that a large two-stroke engine will be reconfigured during its time of operation. The consequences of these practical issues are that manual tuning for the individual design is not applicable and that observer design based on a priori data is impractical. This means that the control design must be robust not only towards changes in system behaviour but also towards imprecise design data.

Another reason is the fact that there is a positive pressure difference between the exhaust gas side and the intake side of a four-stroke engine, i.e. the positive pressure difference will cause the exhaust gas to be recirculated to flow to the intake side without the need for blowers or the like. However, in a large turbo charged two-stroke engines there is a negative pressure difference between the exhaust gas side and the intake side, and the charging air would flow towards the exhaust side if there was established a simple conduit between the intake side and the exhaust side, as is done in the smaller four-stroke engines. Thus, an EGR system of a two-stroke engine requires a blower or pump to force a portion of the exhaust gas into the charging air i.e. in a large turbo charged two-stroke diesel engine, the exhaust gas is recirculated by use of a combination of blowers and valves to overcome the pressure difference between the exhaust system and intake system.

Further, the use of heavy fuel oil causes the exhaust gas of a large two-stroke engine to be much more aggressive than in a four-stroke engine due to the high sulfur content of the heavy fuel oil which results in sulfuric acid being present in relatively high concentrations in the exhaust gas, which poses a challenge to the components of the exhaust gas system and in the case of EGR, to the components of the EGR system and the intake system.

In order to meet the emission requirements for both NOx and sooth, it is necessary to control the oxygen concentration in the scavenge gas receiver accurately because soot formation will exceed the acceptable limit if the oxygen concentration is too low and NOx emissions will exceed the acceptable limit if the oxygen concentration is too high. A load dependent scavenge oxygen concentration (0S) setpoint is predefined. The actual oxygen concentration 0S is measured and the setpoint is reached by feedback control of this measurement, by using the EGR blower speeds and EGR valve openings as actuators.

Thus, in order to control the EGR flow accurately, it is necessary to know the oxygen (02) content in the scavenge gas accurately and promptly. Based on the measured oxygen content a closed control loop can be used to adjust the amount of recirculated exhaust gas and thereby the oxygen content in the scavenge gas.

However, the measurement of the oxygen concentration in the scavenge gas with presently available sensor technology is slow due to the harsh conditions in the scavenge gas receiver. This is not a problem in steady-state operation, but gives rise to significant challenges in transient operation, e.g. when the ship as to accelerate or has to slow down. The slow measurement can lead to undesirable oscillations in the feedback control loop when the feedback gain is high. However, a low feedback gain leaves the system vulnerable toward disturbances such as changes in the fuel flow (load changes). In prior art systems a compromise between oscillation and disturbance rejection is necessary.

SUMMARY

It is an object of the invention to provide a large turbocharged two-stroke self-igniting internal combustion engine uniflow type with an EGR system that overcomes or at least reduces the problems indicated 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 turbocharged two-stroke self-igniting internal combustion engine uniflow type, said engine comprising a plurality of cylinders with scavenge ports at their lower end and an exhaust valve at their upper end, an intake system through which scavenging gas is introduced into the cylinders, said intake system comprising a scavenge gas receiver connected to said cylinders via said scavenge ports, an exhaust system through which exhaust gas produced in the cylinders is exhausted, said exhaust system comprising an exhaust gas receiver connected to said cylinders via said exhaust valves, a turbocharger having a turbine that drives a compressor with said turbine in the exhaust system and said compressor in the intake system, said compressor delivering a flow of scavenge air to the scavenge gas receiver, a fuel system for delivering a flow of fuel to said cylinders, an EGR system for conveying a flow exhaust gas from the exhaust system to the intake system and comprising at least one blower, a first sensor providing a signal representative of the oxygen concentration 0S in the scavenge gas receiver, and a controller configured to control the flow of exhaust gas through said EGR system, said controller being configured to use the signal from said first sensor for feedback control, and said controller being configured to use an estimate of a required recirculated exhaust gas flow for feed forward control.

By providing feed forward control based on an estimate in combination with a feedback control that uses a relatively slow sensor, transient performance can be significantly improved while maintaining high accuracy in stationary state.

In a first possible implementation form of the first aspect said controller is configured to control the flow of exhaust gas through said EGR system to keep the oxygen level in said scavenge gas receiver close to an oxygen concentration setpoint.

In a further possible implementation form of the first aspect said controller is configured to use the signal from said first sensor in feedback control to keep the oxygen content in the scavenge gas receiver close to a setpoint, and said controller is configured to use flow measurements and/or estimates of the fuel flow, the EGR flow and/or the compressor flow in feed forward control to keep the oxygen content in the scavenge gas receiver close to said setpoint.

In a further possible implementation form of the first aspect said feedback control is dominant in steady state operation of said engine and wherein said feed forward control is dominant in transient operation of said engine .

In a further possible implementation form of the first aspect the signal of said first sensor has latency relative to actual changes in the oxygen concentration Os in said scavenge gas received, and wherein said flow measurements and/or estimates of the fuel flow, the EGR flow and/or the compressor flow can be measured or determined instantaneously.

In a further possible implementation form of the first aspect said feed forward control improves transient performance and wherein said feedback control minimizes the control error in stationary state.

In a further possible implementation form of the first aspect said controller is uses a control law derived from a model of the EGR system.

In a further possible implementation form of the first aspect said controller is configured to estimate the EGR flow based on speed of the variable speed blower, upstream and downstream pressures of the variable speed blower and a map of the variable speed blower that is preferably in non-dimensional parameters.

In a further possible implementation form of the first aspect said controller is configured to estimate said downstream pressure from a pressure sensor and form a valve pressure drop signal over an EGR valve in said EGR system upstream or downstream of said variable speed blower .

In a further possible implementation form of the first aspect said controller is configured to estimate said upstream pressure from said downstream pressure and a variable speed blower pressure rise measurement.

In a further possible implementation form of the first aspect said fuel flow estimate is based on either a load signal uioad or a fuel index Yf and the engine speed Qeng signals, preferably along with respective proportionality constants .

In a further possible implementation form of the first aspect said controller uses the signal from said first sensor, the load uioad or fuel index Yf and engine speed ω0, the compressor speed mt, the speed of the variable speed blower) , the scavenge pressure pscav, the valve pressure drop Δρν and variable speed blower pressure rise Apb for controlling the flow exhaust gas from the exhaust system to the intake system.

In a further possible implementation form of the first aspect said controller is configured to adjust the speed of said variable speed blower in order to control the flow of exhaust gas through said EGR system.

In a further possible implementation form of the first aspect said EGR valve is an adjustable valve and wherein said controller is configured to adjust the opening of said EGR valve in order to control the flow of exhaust gas through said EGR system.

In a further possible implementation form of the first aspect the blower is a variable speed blower.

According to a second aspect, there is provided a method for controlling the flow exhaust gas from the exhaust system to the intake system of a large turbocharged two-stroke self-igniting internal combustion engine uniflow type, said method comprising recirculating a portion of the exhaust gas that is produced by cylinders of the engine, controlling the flow of recirculated exhaust gas to keep an oxygen level in scavenge gas receive close to an oxygen concentration setpoint, using a measured oxygen content in said scavenge gas receiver for feedback control, using an estimate of a required recirculated exhaust gas flow for feed forward control.

In a first possible implementation form of the second aspect the method further comprises using said feedback control for minimizing the control error in stationary state, and using said feed forward control to improve transient performance.

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 a front view of a large two-stroke diesel engine according to an exemplary embodiment,

Fig. 2 is a side view of the large two-stroke engine of Fig. 1,

Fig. 3 is a cross-sectional diagrammatic representation the large two-stroke engine according to Fig. 1,

Fig. 4 is a diagrammatic representation of the engine of Fig. 1 illustrating the intake system, exhaust system and the EGR system in greater detail,

Fig. 5 is a diagrammatic representation of another embodiment the engine of Fig. 1 illustrating the intake system, exhaust system and the EGR system in greater detail, and

Fig, 6 is a diagrammatic representation of an example embodiment of a controller used in the engine of Fig. 1.

DETAILED DESCRIPTION

In the following detailed description, the large low speed two-stroke turbocharged self-igniting internal combustion 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 42 and crossheads 43. Figure 3 shows a diagrammatic representation of the large low speed turbocharged two-stroke diesel engine with its intake and exhaust systems in sectional view. In this example embodiment the engine has six cylinders 1 in line, e.g. the engine is a single line of cylinders. For illustration purposes only, Fig. 1 shows the engine having a quantity of six cylinders 1. It should be apparent that virtually any other quantity of cylinders 1 may be employed without departing from aspects of the present invention. Large turbocharged two-stroke diesel engines typically have between four and sixteen cylinders in line, carried by an engine frame 45. 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 has an intake system and an exhaust system. Turbocharging is provided by a turbocharger 5 having a turbine 8 in exhaust system that drives a compressor 9 in the intake system. The engine has a fuel system that delivers fuel to the cylinders.

The engine is a diesel (self-igniting) engine of the two-stroke uniflow type with scavenge ports 17 at the lower region of the cylinders 1 and an exhaust valve 4 at the top of the cylinders 1. The engine can be operated on various types of fuel, such as e.g. marine diesel, heavy fuel, or gas (LPG, LNG, Methanol, Ethanol).

The scavenge gas is passed from the scavenge gas receiver 2 to the scavenge ports 17 of the individual cylinders 1. A piston 41 in the cylinder 1 compresses the scavenge gas, fuel is injected via fuel valves (not shown) in the top of the cylinders 1 and combustion follows and exhaust gas is generated. When an exhaust valve 4 is opened, the exhaust gas flows through an exhaust duct 6 associated with the cylinder 1 concerned into the exhaust gas receiver 3 and onwards through an exhaust conduit 33 to a turbine 8 of a turbocharger 5, from which the exhaust gas flows away through an exhaust conduit 7. Through a shaft 12, the turbine 8 of the turbocharger 5 drives a compressor 9 supplied via an air inlet 10. The compressor 9 delivers pressurized scavenge air to a scavenge air conduit 11 leading to the scavenge gas receiver 2. The engine can have more than one turbocharger 5, as is well known in the art.

The scavenge gas receiver 2 has an elongated hollow cylindrical body constructed from e.g. plate metal and an essentially circular cross-sectional outline to form a hollow cylinder. The scavenge gas receiver 2 extends along the full length of the engine and supplies all of the cylinders 1 with scavenge air. The scavenge gas receiver 2 has a substantial cross-sectional diameter and a large overall volume, which is required in order to prevent any pressure fluctuations caused by the scavenge ports 17 of the individual cylinders 1 opening and taking in scavenge gas, i.e. to ensure substantially constant pressure in the scavenge gas receiver 2 despite the irregular consumption of scavenge air by the individual cylinders 1. Typically, the diameter of the scavenge gas receiver 2 is larger than the diameter of the pistons 1.

In an embodiment, e.g. for very large engines with a high number of cylinders 1 and a great overall engine length, the engine may be provided with two scavenge gas receivers 2, each having its own housing, with one of the scavenge gas receivers 2 covering approximately half of the cylinders 1 at one end of the line of cylinders 1 and the other scavenge gas receiver 2 covering the other approximately half of the cylinders 1 at the opposite end of the line of cylinders 1. In this embodiment, the number of EGR systems/strings is preferably increased accordingly, so an engine according to this embodiment could be provided with four EGR blowers i.e. two blowers in each EGR string.

The exhaust gas receiver 3 has an elongated hollow cylindrical body constructed from e.g. plate metal and an essentially circular cross-sectional outline. The plate metal is covered by a layer of insulation material to avoid heat loss. The exhaust gas receiver 3 extends along the full length of the engine and receives exhaust gas from all of the cylinders 1 via the individual exhaust ducts 6 that extend into the exhaust gas receiver 3. The exhaust gas receiver 3 has a considerable cross-sectional diameter and a large volume, which is necessary in order to minimize pressure fluctuations caused by the exhaust valves 4 of the individual cylinders 1 opening sending exhaust gas jets at high speed into the exhaust gas receiver 3, i.e. to ensure a substantially constant pressure in the exhaust gas receiver 3 despite the intermittent delivery of exhaust gas by the individual cylinders 1. Typically, the diameter of the exhaust gas receiver 3 is larger than the diameter of the pistons 1.

In an embodiment, e.g. for very large engines with a high number of cylinders 1 and a great overall engine length, the engine may be provided with two exhaust gas receivers 3, with one of the exhaust gas receivers 3 covering approximately half of the cylinders 1 at one end of the line of cylinders 1 and the other exhaust gas receiver 3 covering the other approximately half of the cylinders 1 at the opposite end of the line of cylinders.

Referring now to Fig. 4 the intake system, the exhaust system and the EGR system of the engine is shown in greater detail.

Scavenge air is routed to the compressor 9 of the turbocharger 5 via an inlet conduit 10. The compressor 9 compresses the scavenge air and a scavenge air conduit 11 routes the compressed scavenge air to the scavenge gas receiver 2. The scavenge air in the conduit 11 passes through an intercooler (not shown) for cooling the compressed scavenge air - that leaves the compressor 9 at approximately up to 200 "C - to a temperature between 5 and 80 °C. The cooled scavenge air passes via an auxiliary blower 16 driven by a drive motor that pressurizes the scavenge air flow at low or partial load conditions to the charging gas receiver 2. At higher loads the compressor 9 delivers sufficient compressed scavenge air and then the auxiliary blower is bypassed via a non-return valve (not shown).

The scavenge air conduit 11 passes a summation point 28 where recirculated exhaust gas from the EGR system is added to the scavenge air and leads the scavenge air mixed with recirculated exhaust gas to the inlet of the scavenge gas receiver 2. From the scavenge gas receiver 2 the mixture of scavenge air and recirculated exhaust gas takes part in the combustion process in the cylinders 1. The exhaust gas thus produced in the cylinders 1 is received in the exhaust gas receiver 3. Thus, the combustion process is performed with scavenge air mixed with circulated exhaust gas, thereby, allowing low NOx emission levels.

The EGR system extends between the exhaust system and the intake system. The EGR system has an EGR conduit 20 that routes a portion of the exhaust gas to the intake system. In the present embodiment the EGR conduit 20 connects to the exhaust gas receiver 3, but it is understood that the exhaust gas could be taken directly from cylinders 1 or from any other part of the exhaust system upstream of the turbine 8 of the turbocharger 5, e.g. by branching off from exhaust conduit 33.

In a large turbocharged two-stroke internal self-igniting combustion engine the exhaust gases typically contain a relatively high amount of aggressive substances such as e.g. sulfuric acid due to the high sulfur content of the fuel.

Therefore, the EGR system includes a scrubber 21, which may be a wet scrubber in the EGR conduit 20 for cleaning the recirculated exhaust gases to avoid that contaminated unclean recirculated exhaust gas is reintroduced into the cylinders 1, and to avoid contamination of the EGR system downstream of the scrubber 21, as well as the scavenge gas receiver 2 and the auxiliary blower 16. The scrubber is placed in the upstream part of the EGR system to have most effect.

Generally, in a large turbocharged two-stroke self-igniting internal combustion engine the pressure in the scavenge gas on the inlet side of a cylinder 1 will be higher than the pressure in the exhaust gas on the outlet side of the cylinder 1 concerned, otherwise, scavenging could not take place because the pressure dictated flow direction would be in the wrong direction towards the intake side. This aspect of large turbocharged two-stroke internal combustion engines renders it impossible to simply allow exhaust gas to flow from the exhaust system through an EGR conduit to the intake system for exhaust gas recirculation without the assistance of blowers or the like. Therefore, the EGR system includes at least one blower 22,23 to force exhaust gas from the exhaust system through the EGR system to the intake system.

In the present embodiment the EGR conduit 20 is split into two strings downstream of the scrubber 21, with each string including a variable- or fixed speed EGR blower 22, 23 and each string including an adjustable EGR valve 24, 25.

The controller 50 is provided to control the flow through the EGR system such that the actual oxygen concentration in the scavenge gas receiver 2 is kept as close as possible to a predetermined oxygen concentration set point.

The speed of each of the EGR blowers 22, 23 is controlled independently by a controller 50 (Fig. 6) and/or the setting of each of the EGR valves 24, 25 is controlled by the controller 50. The pressure upstream and/or downstream of the EGR valve 24, 25 is measured by a sensor and communicated to the controller 50. Preferably, the speed of the EGR blowers 22, 23 is measured and communicated to the controller 50 so that the controller 50 can control the speed of the EGR blowers 22, 23, e.g. in a feedback control loop. The pressure rise Apbover the respective blower 22, 23 is measured and communicated to the controller 50.

The oxygen content Os in the scavenge gas receiver 2 is measured with a first sensor 27. A signal of the first sensor 27 is communicated to the controller 50. The first sensor 27 is a robust but very slow (high latency) sensor that can handle the harsh conditions in the scavenge gas receiver 2.

The speed of the turbocharger shaft 12 is measured for determining the speed o)t of the compressor 8.

Feedback control is vulnerable to disturbances such as changes in fuel flow. However, the fuel flow is estimated from signals that are available in the control system (e.g. the load signal).

The overall principle of the controller 50 is to control the EGR system by using both the Os measurement and estimates or measurements of the following flows: • Fuel flow, rhf. • EGR flow, megr. • Compressor flow, mc.

The flow measurements/estimates are used as feed forward in a control law used by the controller 50 to improve transient performance, while integral feedback of the Os measurement keeps the control error at zero in the stationary state.

This controller 50 uses a specific control law based on the overall principle explained above. The control design uses a (simplified) control oriented model of the EGR system.

Stability analysis has shown exponential convergence of the control error, which is a good indicator of robustness toward unmodelled dynamics (the difference between the simple model and reality). The stability analysis is not presented in further detail here.

The fuel flow estimate is based on either the load signal (uioad) or the fuel index (Yf) and the engine speed (ω6ί15) signals along with respective proportionality constants. Only one of these estimates are used.

The EGR flow estimate is based on the speed of the variable speed blower 22,23, upstream and downstream pressures of the variable speed blower 22,23 and an EGR blower map in non-dimensional parameters (flow and head coefficients).

The downstream pressure is estimated from the scavenge pressure and valve pressure drop measurement signals

The upstream pressure is calculated from the downstream pressure and the blower pressure rise measurement

The pressure ratio is then:

Head coefficient (rpb) is calculated from pressure ratio, blower speed (a>fc), and approximated constants; specific heat (cp) , upstream temperature (Tus) t ratio of specific heats (γ) and blower radius (Rb

) .

The EGR blower map (CqjC-l^) converts from head coefficient to flow coefficient (0ft)

The EGR flow is calculated as (Rs is the gas constant)

The EGR flow is calculated as the sum of the blower flows

Estimation from a compressor map is not feasible as maps that cover all operating points are not practically available for each engine. Instead, only the compressor speed (æt) is used in a rather imprecise model for approximation

The parameter a is predefined whereas Θ is continuously estimated (more about the adaptive part later).

The model used in the control law is based on the following model of 0S during stationary conditions

An inverted expression of the model is defined as the function h(0, d, 0S)

The dynamics of the gas mixing and 0S sensor 27 are lumped together as a first order system with known time constant τ and a time delay τdelay The dynamics can be expressed as

The redundancy of having both a model and a measurement of 0S is used for continuously estimating the parameter Θ that is expected to vary slightly. The following nonlinear parameter estimator is used (at each controller update). Note the tuning parameter k.

The control law is based on the inversion of the static part of the model, h(6,d,0s), with use of the latest estimate Θ, the known vector signal d and the scavenge oxygen setpoint:

As expressed in the control law the direct inversion h might return values outside the actuator limits. In special cases even undefined values. Whenever the value is not within the actuator limits, the controller 50 chooses the maximum EGR flow. For practical purposes the undefined values can be handled by checking the denominator during the calculation of h.

The parameter estimator represents the integral part of the controller. It makes the Os error converge to zero during stationary conditions, thus allowing for a control law without explicit transient detection.

This is a non-exhaustive list of the signals that can be used by the controller 50: • Scavenge oxygen measurement (Os) . • Load (uioad) or fuel index (Yf) and engine speed (o)c) . • Compressor speed (o)t) . • EGR blower speed (cob) . • Scavenge pressure (pSCav) · • EGR valve pressure drop (Δρν) . • EGR blower pressure rise (Apb) .

The controller 50 can use all or several of the above signals. Since the scavenge oxygen measurement is slow, at least one of the other signals from which an estimate of the oxygen concentration in the scavenge gas receiver can be obtained promptly is used for the server-based (feed forward) control.

This is a non-exhaustive list of tuning parameters that can be used to tune the controller 50: • Parameter estimator gain (k). • Oxygen time constant (τ) . • Oxygen delay (τdelay) . • Compressor flow approximation exponent (a).

The controller 50 can use all or several of the above tuning parameters that are specific for the engine concerned.

This is a non-exhaustive list of basic parameters that can be used by the controller 50: • Fuel proportionality (kioad or kY) . • EGR blower upstream temperature (Tus) . • EGR blower specific heat (cp) . • EGR blower ratio of specific heats (y). • EGR blower gas constant (Rs) . • EGR blower radius (Rb). • EGR blower map (c0, cv c2) . • Ambient oxygen fraction (Oa) . • Stoichiometric oxygen-fuel ratio (Ay) . • Maximum EGR flow {ihegrmax) .

In the present embodiment the outer control loop specifies outer EGR flow rather than actual actuator values (EGR blower speeds and EGR valve openings). Thus, the controller (50) also includes an inner loop that controls the EGR flow, e.g. by adjusting the speed of the variable speed blower(s) 22,23 and/or the opening of the EGR valves 24,25. In an embodiment the inner loop could use a basic feedback method.

Fig. 5 shows an embodiment that is essentially identical to the embodiment of Fig. 4, except that the EGR conduit 20 is not split into two strings, so that the e.g. are system can suffice with a single blower 20 and a single EGR valve 24.

Fig. 6 shows a diagrammatic representation of the controller 50 including a feedback - and feedforward based Os-controller, a flow controller and a flow estimator .

Fig. 6 shows the input of the desired oxygen concentration (Os setpoint) to the feedback - and feedforward based Os-controller. The feedback - and feedforward based Os-controller is also in receipt of signals representative of the engine load, the compressor speed and the measured Os.

The feedback - and feedforward based Os-controller is also in receipt of a signal indicating the flow estimate from the flow estimator.

The Feedback - and feedforward based Os-controller determines a flow setpoint on the basis of the received signals. The flow controller is in receipt of the flow setpoint determined by the feedback - and feedforward based Os-controller and the flow controller issues an EGR blower speed setpoint and/or an EGR valve opening setpoint.

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. A single processor or other unit may fulfill the functions of several items recited in the claims. 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

1. A large turbocharged two-stroke self-igniting internal combustion engine uniflow type, said engine comprising: a plurality of cylinders (1) with scavenge ports (17) at their lower end and an exhaust valve (4) at their upper end, an intake system through which scavenging gas is introduced into the cylinders (1), said intake system comprising a scavenge gas receiver (2) connected to said cylinders (1) via said scavenge ports (17), an exhaust system through which exhaust gas produced in the cylinders is exhausted, said exhaust system comprising an exhaust gas receiver (3) connected to said cylinders via said exhaust valves (4), a turbocharger (5) having a turbine (8) that drives a compressor (9) with said turbine (8) in the exhaust system and said compressor (9) in the intake system, said compressor delivering a flow of scavenge air to the scavenge gas receiver, a fuel system for delivering a flow of fuel to said cylinders (1), an EGR system for conveying a flow of exhaust gas from the exhaust system to the intake system and comprising at least one blower (22), a first sensor (27) providing a signal representative of the oxygen concentration (0S> in the scavenge gas receiver (2), and a controller (50) configured to control the flow of exhaust gas through said EGR system, said controller (50) being configured to use the signal from said first sensor for feedback control, and said controller (50) being configured to use an estimate of a required recirculated exhaust gas flow for feed forward control.

2. An engine according to claim 1, wherein said controller (50) is configured to control the flow of exhaust gas through said EGR system to keep the oxygen level in said scavenge gas receiver (2) close to an oxygen concentration setpoint.

3. An engine according to claim 1 or 2, wherein said controller (50) is configured to use the signal from said first sensor (27) in feedback control to keep the oxygen content in the scavenge gas receiver (2) close to a setpoint, and wherein said controller (50) is configured to use flow measurements and/or estimates of the fuel flow, the EGR flow and/or the compressor flow in feed forward control to keep the oxygen content in the scavenge gas receiver (2) close to said setpoint.

4. An engine according to claim 1, wherein said feedback control is dominant in steady state operation of said engine and wherein said feed forward control is dominant in transient operation of said engine.

5. An engine according to any one of claims 1 to 4, wherein the signal of said first sensor (27) has latency relative to actual changes in the oxygen concentration (0S) in said scavenge gas received (2), and wherein said flow measurements and/or estimates of the fuel flow, the EGR flow and/or the compressor flow can be measured or determined instantaneously.

6. An engine according to any one of claims 1 to 5, wherein said feed forward control improves transient performance and wherein said feedback control minimizes the control error in stationary state.

7. An engine according to any one of claims 1 to 6, wherein said controller (50) is uses a control law derived from a model of the EGR system.

8. An engine according to any one of claims 1 to 7, wherein said controller (50) is configured to estimate the EGR flow based on speed of the variable speed blower, upstream and downstream pressures of the variable speed blower (22,23) and a map of the variable speed blower (22,23) that is preferably in non-dimensional parameters.

9. An engine according to claim 8, wherein said controller (50) is configured to estimate said downstream pressure from a pressure sensor and form a valve pressure drop signal over an EGR valve (24,25) in said EGR system upstream or downstream of said variable speed blower (22,23) .

10. An engine according to claim 8 or 9, wherein said controller (50) is configured to estimate said upstream pressure from said downstream pressure and a variable speed blower (22,23) pressure rise measurement.

11. An engine according to claim 8,9 or 10, wherein said fuel flow estimate is based on either a load signal (uioad) or a fuel index (Yf) and the engine speed (oeng) signals, preferably along with respective proportionality constants .

12. An engine according to claim 11, wherein said controller (50) uses the signal from said first sensor (27), the load (uioad) or fuel index (Yf) and engine speed (ω0) , the compressor speed (mt) , the speed (mb) of the variable speed blower (22,23), the scavenge pressure (Pscav) r the valve pressure drop (Apv) and variable speed blower (22,23) pressure rise (Apb) for controlling the flow exhaust gas from the exhaust system to the intake system.

13. An engine according to any one of claims 1 to 12, wherein said controller (50) is configured to adjust the speed of said variable speed blower (22, 23) in order to control the flow of exhaust gas through said EGR system.

14. An engine according to any one of claims 1 to 13, wherein said EGR valve (24,25) is an adjustable valve and wherein said controller (50) is configured to adjust the opening of said EGR valve (24, 25) in order to control the flow of exhaust gas through said EGR system.

15. A method for controlling the flow exhaust gas from the exhaust system to the intake system of a large turbocharged two-stroke self-igniting internal combustion engine uniflow type, said method comprising: recirculating a portion of the exhaust gas that is produced by cylinders of the engine, controlling the flow of recirculated exhaust gas to keep an oxygen level in scavenge gas receive close to an oxygen concentration setpoint, using a measured oxygen content in said scavenge gas receiver for feedback control, using an estimate of a required recirculated exhaust gas flow for feed forward control.

16. A method according to claim 15, further comprising using said feedback control for minimizing the control error in stationary state, and using said feed forward control to improve transient performance.