GB2460163A

GB2460163A - Reducing the transient specific fuel consumption of a turbocharged i.c. engine having an electronically controlled turbine inlet flow control device

Info

Publication number: GB2460163A
Application number: GB0908552A
Authority: GB
Inventors: Khizer Tufail
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2008-05-19
Filing date: 2009-05-19
Publication date: 2009-11-25
Anticipated expiration: 2029-05-19
Also published as: CN101586501A; GB2460163B; GB0809049D0; GB0908552D0; DE102009010633A1; GB2460224A; CN101586501B

Abstract

A method for reducing transient specific fuel consumption of an engine is disclosed in which a turbocharger inlet flow control device 44 is controlled so as to reduce pressure upstream from a turbocharger 14 and improve specific fuel consumption. In particular, when the pumping mean effective pressure (PMEP) exceeds a calibratable limit, a minimum closing limit is applied to the inlet flow control device 44 so as to prevent closure of the inlet flow control device 44 beyond this limit thereby minimising increases in upstream exhaust gas pressure and reducing pumping losses. The current value of PMEP may be determined using a virtual sensor produced by (a) accelerating the engine with the electronically controlled inlet flow device set to a predetermined position; (b) recording PMEP and pressure ratio for acceleration between idle speed and normal engine speed; (c) repeating (a) and (b) over a range of opening positions; (d) repeating steps (a) to (c) for a number of acceleration rates and (e) recording the generated data in a look-up table referencing PMEP values, pressure ratio values, engine acceleration and engine speed.

Description

A Method and System for Reducing the Transient Specific Fuel Consumption of an Engine This invention relates to turbocharged internal combustion engines and more particularly to a turbocharged internal combustion engine having an exhaust gas recirculation (EGR) system.

As is known in the art, high performance, high speed engines are often equipped with turbochargers to increase power density over a wider engine operating range, and EGR systems to reduce the production of NOx emissions.

More particularly, turbochargers use a portion of the exhaust gas energy to increase the mass of the air charge (i.e., boost) delivered to the engine combustion chambers.

The larger mass of air can be burned with a larger quantity of fuel, thereby resulting in increased power, torque and fuel efficiency as compared to naturally aspirated engines.

A typical turbocharger includes a compressor and turbine coupled by a common shaft. The exhaust gas drives the turbine, which drives the compressor, which in turn, compresses ambient air and directs it into the intake manifold. A continuously variable geometry turbocharger (VGT) allows the intake airflow to be optimized continuously over a range of engine speeds. In diesel engines, this is accomplished by changing the angle of the inlet guide vanes on the turbine stator and an optimal position for the inlet guide vanes is determined from a combination of desired torque response, fuel economy and emissions requirements.

It is an object of this invention to provide an improved method for reducing the specific fuel consumption of an engine.

According to a first aspect of the invention there is provided a method for reducing the transient specific fuel consumption of an engine having a turbocharger, the turbocharger having a turbine arranged so as to be driven by the exhaust gases from the engine and an electronically controlled inlet flow control device to regulate the exhaust gas entering the turbine wherein the method comprises applying a minimum closure limit to the electronically controlled inlet flow control device so as to reduce exhaust gas pressure build-up upstream from the turbocharger turbine if a current value of pumping mean effective pressure exceeds a first limiting value of pumping mean effective pressure.

The engine may have an exhaust manifold and an inlet manifold and the turbocharger may have a compressor for selectively increasing the pressure in the inlet manifold of the engine.

Applying a minimum closure limit may comprise adding a correction factor to a normally demanded position so as to define a minimum closure position below which the electronically controlled inlet flow control device cannot be closed.

The minimum closure limit may only be applied if an operator demand is below a predetermined limit.

The operator demand may be the position of an accelerator pedal.

The method may further comprise determining the first limiting value of pumping mean effective pressure, determining the current value of pumping mean effective pressure, determining whether the current value of pumping mean effective pressure is greater than the first limiting value of pumping mean effective pressure and, if the current value of pumping mean effective pressure is greater than the first limiting value of pumping mean effective pressure, applying the minimum closure limit to the electronically controlled inlet flow control device.

A current value of pumping mean effective pressure may be determined using a virtual sensor.

The virtual sensor may be a look-up table referencing dynamically obtained values of pumping mean effective pressure, dynamically obtained values of pressure ratio and engine speed.

The virtual sensor may be produced by:- (a) accelerating the engine at a predetermined rate with the electronically controlled inlet flow control device set to a predetermined opening position; (b) recording pumping mean effective pressure and pressure ratio for the acceleration between idle speed and a maximum normal operating speed of the engine; (c) repeating steps (a) and (b) for a number of further opening positions of the electronically controlled inlet flow control device until a number of opening positions spanning the normal working range of opening positions has been recorded; (d) repeating steps (a) to (c) for a further number of predetermined engine acceleration rates spanning a normal working range of the engine; and (e) recording the generated data in a look-up table referencing the dynamically obtained values of pumping mean effective pressure, dynamically obtained values of pressure ratio, engine acceleration and engine speed.

A first limiting value of pumping mean effective pressure may be determined using a steady state look-up table referencing speed against engine pressure ratio.

The electronically controlled inlet flow control device may comprise a number of vanes moveable by an actuator and applying a minimum closure limit to the electronically controlled inlet flow control device so as to reduce exhaust gas pressure build-up upstream from the turbocharger turbine comprises restricting operation of the actuator so as to prevent the vanes from being closed beyond the minimum closure limit.

The engine may include a throttle valve to control the flow of air to the engine and the method may further comprise preventing the throttle valve from closing beyond a calibratable limit if the current value is greater than the first limiting value of pumping mean effective pressure.

The method may further comprise taking no action if the current value of pumping mean effective pressure is less than the first limiting value of current pumping mean effective pressure.

The method may further comprise providing a warning to a user of the engine if the current value of pumping mean effective pressure is greater than an upper limiting value of pumping mean effective pressure that is higher than the first limiting value of pumping mean effective pressure.

The method may further comprise reducing fuelling to the engine if the current value of pumping mean effective pressure is greater than a failure limiting value of pumping mean effective pressure that is considerably higher than the first limiting value of pumping mean effective pressure.

The closure limit may be varied based upon the difference between the current value of pumping mean effective pressure and the first limiting value of pumping mean effective pressure.

The closure limit may be increased as the difference increases.

According to a second aspect of the invention there is provided a control system for controlling an engine having a turbocharger, the turbocharger having a turbine arranged so as to be driven by the exhaust gases from the engine, the system comprising an electronic controller and an inlet flow control device controlled by the electronic controller to regulate the exhaust gas entering the turbine of the turbocharger wherein the electronic controller is operable to control the electronically controlled inlet flow control device so that it does not close below a minimum closure limit so as to reduce exhaust gas pressure build-up upstream from the turbocharger turbine if a current value of pumpinq mean effective pressure exceeds a first limiting value of pumping mean effective pressure.

The controller may be operable to apply a correction factor to a normally demanded position so as to prevent the electronically controlled inlet flow control device from being closed beyond a position corresponding to the minimum closure limit.

The operator demand may be the position of an accelerator pedal.

The minimum closure limit may be calibrated based upon the difference between the current value of pumping mean effective pressure and the first limiting value of pumping mean effective pressure.

The minimum closure limit may be increased as the difference increases.

The electronic controller may take no additional action if the current value of pumping mean effective pressure is less than the first limiting value of current pumping mean effective pressure.

The electronically controlled inlet flow control device may comprise a number of vanes moveable by an actuator and the electronic controller may be operable to control the electronically controlled inlet flow control device so as to reduce exhaust gas pressure build-up upstream from the turbocharger turbine by restricting operation of the actuator so as to prevent the vanes from being closed beyond the minimum closure limit.

The engine may include a throttle valve to control the flow of air to the engine and the electronic controller is further operable to prevent the throttle valve from closinq beyond a calibratable limit if the current value of pumpinq mean effective pressure is greater than the first limiting value of pumping mean effective pressure.

The electronic controller may be further operable to provide a warning to a user of the engine if the current value of pumping mean effective pressure is greater than an upper limiting value of pumping mean effective pressure that is higher than the first limiting value of pumping mean effective pressure.

The electronic controller may be further operable to reduce fuelling to the engine if the current value of pumping mean effective pressure is greater than a failure limiting value of pumping mean effective pressure that is considerably higher than the first limiting value of pumping mean effective pressure.

The electronic controller may be further operable to determine the first limiting value of pumping mean effective pressure, determine the current value of pumping mean effective pressure, determine whether the current value of pumping mean effective pressure is greater than the first limiting value of pumping mean effective pressure and, if the current value of pumping mean effective pressure is greater than the first limiting value of pumping mean effective pressure, control the electronically controlled inlet flow control device so that it does not close below the minimum closure limit.

A current value of pumping mean effective pressure may be determined by the controller using a virtual sensor.

Using a virtual sensor may comprise using a look-up table referencing dynamically obtained values of pumping mean effective pressure, dynamically obtained values of pressure ratio and engine speed.

Using a virtual sensor to provide a value of current pumping mean effective pressure may comprise using a virtual sensor produced by:- (a) accelerating the engine at a predetermined rate with the electronically controlled inlet flow control device set to a predetermined opening position; (b) recording pumping mean effective pressure and pressure ratio for the acceleration between idle speed and a maximum normal operating speed of the engine, (c)repeating steps (a) and (b) for a number of further opening positions of the electronically controlled inlet flow control device until a number of opening positions spanning the normal working range of opening positions has been recorded; (d) repeating steps (a) to (C) for a further number of predetermined engine acceleration rates spanning a normal (e) recording the generated data in a look-up table referencing the dynamically obtained values of pumping mean effective pressure, dynamically obtained values of pressure ratio, engine acceleration and engine speed and using the dynamic look up table so produced to provide a value of current pumping mean effective pressure.

According to a third aspect of the invention there is provided a method for producing a pumping mean effective pressure virtual sensor for use in a control system according to said second aspect of the invention having a virtual sensor to provide a value of current pumping mean effective pressure, the method comprising the steps of:- (a) accelerating the engine at a predetermined rate with the electronically controlled inlet flow control device set to a predetermined opening position; (b) recording pumping mean effective pressure and pressure ratio for the acceleration between idle speed and a maximum normal operating speed of the engine; (c) repeating steps (a) and (b) for a number of further opening positions of the electronically controlled inlet flow control device until a number of opening positions spanning the normal working range of opening positions has been recorded; (d) repeating steps (a) to (c) for a further number of predetermined engine acceleration rates spanning a normal working range of the engine; and (e) recording the generated data in a look-up table referencing the dynamically obtained values of pumping mean -10 -effective pressure, dynamically obtained values of pressure ratio, engine acceleration and engine speed.

The invention will now be described by way of example with reference to the accompanying drawing of which: FIG.1 is a schematic view of an engine system having an EGR system and a variable geometry turbocharger (VGT) according to the invention; FIG.2 is a schematic graph showing the relationship between turbocharger demand, turbocharger response and pumping mean effective pressure for a typical engine; Fig.3 is a diagram showing plots of ideal pumping mean effective pressure and a upper limit of pumping mean effective pressure; Fig.4 is a graph showing four different engine accelerations named A' to D'; Fig.5A is a chart showing the relationship between turbocharger vane restriction for various engine speed and Delta PMEP combinations; Fig.5B is a chart showing the relationship between throttle valve restriction for various engine speed and Delta PMEP combinations; and Fig.6 is a flow chart showing a method for controlling the engine and VGT shown in Fig.1 according to this invention; -11 -Referring now to FIG.1, an engine 10 is shown. The engine 10 includes an exhaust gas recirculation (EGR) system 12 and a variable geometry turbocharger 14.

The turbocharger 14 has a compressor 36 and a turbine 38 coupled by a common shaft 40 and an inlet flow control device which is in this case is a set of moveable turbine vanes 44 moved by an actuator. Closing the vanes 44 will increase the inlet flow speed, thereby increasing boost and will also increase the pressure upstream from the turbocharger. Conversely, opening the vanes 44 will decrease the inlet flow speed, thereby decreasing boost and will also decrease the pressure upstream from the turbocharger. It is known that the specific fuel consumption of an engine is related to the pumping mean effective pressure (PMEP) of the engine and in general increasing the PMEP will increase the specific fuel consumption of the engine due to the increased pumping losses. It is therefore desirable to keep the PMEP low if fuel economy is to be maximised.

It will be appreciated by those skilled in the art that: -BMEP = IMEP -FMEP -PMEP Where: IMEP is the Indicated Mean Effective Pressure over the entire four-stroke cycle; BMEP is the Brake Mean Effective Pressure over the compression and expansion strokes only; PMEP is the Pumping Mean Effective Pressure over the intake and exhaust stroke only; and -12 -FMEP is Friction Mean Effective Pressure.

That is to say, BMEP = IMEP -Losses where PMEP is one of the losses which needs to be minimised in order to improve fuel economy.

The turbocharger 14 uses exhaust gas energy to increase the mass of the air charge (i.e., boost) delivered to the engine combustion chambers 18 resulting in more torque and power as compared to a naturally aspirated, non-turbocharged engine. The exhaust gas 30 drives the turbine 38 which drives the compressor 36, which in turn, compresses ambient air 42 and directs it in the direction of arrow 43 into the intake manifold 26.

The VGT 14 is adjusted as a function of engine operating conditions such as, for example, engine speed during engine operation by varying the turbine flow area between either a relatively open position and a relatively closed position. This is accomplished by changing the angle of the inlet guide vanes 44 on the turbine 38.

The relatively open or closed positions of the guide vanes 44 is determined from the desired engine operating characteristics at various engine speeds. It is to be noted that at a given operating condition, when in the relatively open position, the boost indicated by arrow 43 is relatively low whereas when in the relatively closed position the boost is relatively high. Further, when in the closed position, the pressure in the exhaust manifold, and hence at the input to the EGR valve 34 is relatively high while in the open position the pressure is relatively low.

-13 -An engine block 16 is shown having four combustion chambers 18, each of which includes a direct-injection fuel injector 20. The duty cycle of the fuel injectors 20 is determined by an electronic controller which in this case is in the form of an engine control unit (ECU) 24 and transmitted along signal line 22.

Air enters the combustion chambers 18 through an intake manifold 26 and combustion gases are exhausted through an exhaust manifold 28 in the direction of arrow 30.

To reduce the level of NOx emissions, the engine is equipped with the EGR system 12 which comprises a conduit 32 connecting the exhaust manifold 28 to the intake manifold 26. This allows a selective portion of the exhaust gases to be circulated from the exhaust manifold 28 to the intake manifold 26 in the direction of arrow 31. An EGR valve 34 regulates the amount of exhaust gas recirculated from the exhaust manifold 28 and in the combustion chambers, the recirculated burned exhaust gas acts as an inert gas, thus lowering the flame and in cylinder gas temperature and decreasing the formation of NOx. It will be appreciated by those skilled in the art that the flow of exhaust gas through the EGR valve 34 is a function of the pressure across the valve 34 and the valve position demanded by the electrical signal provided to the EGR valve 34 on line 46 from the ECU 24. That is to say there is not a linear relationship between EGR flow rate and EGR valve 34 position.

The electrical signal on line 46 is produced by the ECU 24 from relationships stored in the ECU 24 in accordance with a computer program stored in the ECU 24.

-14 -All of the engine systems, including the EGR valve 34, VGI 14 and the fuel injectors 20 are controlled by the ECU 24. For example, the signal 46 from the ECU 24 regulates the EGR valve position, a signal on line 48 regulates the position of the VGI vanes 44 and a signal on the line 47 controls a throttle valve 49.

In the ECU 24, the command signals 46, 48 to the EGR 34 and VGT 14 actuators are calculated from measured or estimated variables and engine operating parameters by means of control algorithms. Sensors and calibratable lookup tables residing in ECU memory provide the ECU 24 with engine operating information.

An intake manifold pressure (MAP) sensor 50 provides a signal 52 to the ECU indicative of the pressure in the intake manifold 26, an air charge temperature sensor 58 provides a signal via line 60 to the ECU 24 indicative of the temperature of the intake air charge and a mass air flow sensor (MAF) provides a signal via line 66 of air flow entering the compressor portion 36. Additional sensor inputs are also received by the ECU 24 along signal line 62 such as engine coolant temperature, fuel rail pressure, fuel injector timing, engine speed, exhaust manifold pressure and throttle position. Operator inputs 68 are received along signal line 70 such as the accelerator pedal position.

Based on the sensor inputs, data stored in memory such as, for example, engine mapping data and various algorithms, the ECU 24 controls the EGR 34 to regulate the EGR flow fraction and the position of the vanes 44 in order to provide emission reduction by the EGR and high fuel economy -15 -provided by the VGI boost. In addition the ECU 24 is operable to calculate the pressure ratio of the engine 10 by using the output from the MAP sensor 50 and an algorithm or look-up table referencing engine speed against inlet manifold pressure to determine a value for exhaust gas pressure and then by performing the equation:-Pressure Ratio (PR) = Exhaust pressure/ Inlet Pressure With reference to Figs. 2 and 3 the problem to be solved will be described in greater detail along with its solution.

Referring firstly to Fig.2 there is shown the relationship between turbocharger boost pressure and time for a typical increased demand situation. It will be appreciated that the graphs are merely representative and are not intended to represent an actual case but merely demonstrate the problem and its solution.

It can be seen that the turbocharger response, that is to say the boost pressure produced by the turbocharger 14 lags behind the demanded boost during the initial increase in demand, then later in the demand cycle the actual boost pressure overshoots the demand and will thereafter undershoot and overshoot the demand in a decaying manner until eventually after two or three oscillations the actual boost corresponds with the demanded boost. It will be appreciated that filtering and control system damping will be used in practice to minimise this under-damped response so as to minimise the magnitude and number of overshoots.

-16 -This typical turbocharger response will produce a significant increase in specific fuel consumption (g/Kwhr) due to the fact that during the initial part of the demand cycle there is a large difference between demanded boost and actual or currently produced boost (see the double headed arrow marked Lag) . The effect of this is to cause the ECU 24 to command the actuator used to control the vanes 44 to close the vanes 44 more than they need to be closed to produce the final demand boost in an effort to rapidly increase the boost being produced by the turbocharger 14.

It will be appreciated that closing the vanes 44 will increase the velocity of the exhaust gas entering the turbocharger 14 thereby accelerating it to produce more boost pressure. A disadvantage of this control strategy is that the PMEP of the engine 10 is greatly increased by the action of the ECU 24 closing the vanes 44 because the pressure upstream from the turbocharger turbine 38 due to the flow restriction imposed by the closed vanes 44 will increase. As previously mentioned, the specific fuel consumption of an engine is related to PMEP and if PMEP increases specific fuel consumption also increases. Fig.2 shows the corresponding PMEP for the increased turbocharger demand from which it can be seen that there is a rapid and significant increase in PMEP due to the effect of the ECU 24 trying to reduce the Lag.

Referring now to Fig.3 there is shown a plot of ideal PMEP for various pressure ratios and speeds. It is desirable to operate the engine 10 as close to this ideal relationship as this will produce low specific fuel consumption from the engine 10. However, in practice it is not possible to operate the engine during transient events in accordance to this relationship and so a first limit -17 - (shown as limit on Fig.3) is generated based upon the idealised relationship for use in determining when action needs to be taken to inhibit normal operation of the ECU 24 so far as control of the vanes 44 is concerned.

The application of this first limit can be better understood with reference to Fig.2 where this limit is shown as a lower limit line on the PMEP graph.

It will be noted that the PMEP exceeds the limit during much of the initial stage of the demand cycle and it is when the PMEP exceeds this first or lower limit that the ECU 24 is programmed in accordance with this invention to modify its control of the vanes 44 so as to minimise the increase in PMEP.

Also shown on Fig.2 are two further limits referred to as upper limit and failure limit, these limits are derived from the lower limit and represent respectively a level above which a warning to a user of the engine 10 needs to be given to investigate system integrity (OBD warning) and a level above which there has been a system failure which will result in uncontrolled excessive specific fuel consumption if normal engine control techniques are applied and so emergency measures in the form of an automatic reduction in fuelling to the engine 10 is used.

Therefore the inventors have realised that by limitinq the back pressure generated in the exhaust manifold 28 upstream from the turbocharger turbine 38 during transient events the specific fuel consumption of the engine will be significantly improved. The effect of this improvement on overall fuel consumption (L/Km) will however depend upon the -18 -operating cycle of the engine 10. For example, if the engine 10 is being operated for the vast majority of its time at constant speed (e.g. if the engine is fitted to a vehicle travelling along a motorway) the improvement in overall fuel consumption will be minimal but if the engine is subject to many transient events (e.g. if the engine is fitted to a vehicle travelling in a town, city or along a country road) there will be a significant improvement in overall fuel consumption.

Referring now to Fig.6 there is shown one example of a method for operating the engine 10 in accordance with this invention.

The method commences at a key-on event of the motor vehicle to which the engine 10 is in this case fitted.

Then at block 210 a first or lower limit is determined for PMEP as discussed above and at block 220 a dynamic or current value for PMEP is determined as will be described hereinafter.

Then at block 230 the difference between the current value of PMEP and the first limit is determined according to the equation:-Delta PMEP = Current PMEP -PMEP limit (1) Note that, although PMEP in its true sense is negative (i.e. the bigger the PMEP loss the bigger the negative number), for the purposes of this invention values of PMEP are treated as absolute values and therefore a higher PMEP value means a higher loss.

-19 -Then at block 240 it is determined whether Delta PMEP is greater than the upper limit by using the test:-Is Delta PMEP > Upper Limit and if the answer is Yes the method advances to block 244 where a warning is sent to a user of the engine 10 (e.q.

a warning light is illuminated, a buzzer sounds or an alphanumeric display displays a message) The method then advances to block 246 where it is determined whether Delta PMEP is greater than the failure limit by using the test:-Is Delta PMEP > Failure Limit if the answer is No the method ends at block 900 and normal control of the vanes 44 and throttle valve 49 by the ECU 24 is used.

Alternatively, if the answer to block 246 is Yes the method advances to block 248 where fuelling to the engine 10 is reduced or limited to a predetermined maximum level and the method then advances to block 900 where the method ends and normal control of the vanes 44 and the throttle valve 49 by the ECU 24 is used albeit with the restriction on fuelling.

Referring back now to block 240 if the answer to the test is No then the method advances to block 242 where it is determined whether driver demand is above a limit. The limit is a level of accelerator pedal position above which -20 -vane control is not desirable as maximum power will be required. So, for example and without limitation, if the accelerator pedal is more than 75% towards its fully depressed position the test at block 242 would be passed and the method advances to blocks 400 and 410 where normal control of the vanes 44 and throttle valve 49 by the ECU 24 is used.

However if the driver demand is below the limit which in this case is 75% depressed then the method advances to block 250 where it is determined whether the current PMEP is above the lower limit using the test:-Is Delta PMEP > First(lower) Limit if the answer is No the method advances to block 400 where normal vane control is used by the ECU 24 to position the vanes 44 and then to block 410 where normal throttle valve control is used by the ECU 24 to control the position of the throttle valve 49. If the engine has no throttle valve then block 410 would be omitted. The method then advances to block 500 to determine whether the engine 10 is still operating, if it is the method returns to block 210 to repeat the process but if the engine 10 is no longer operating the method ends at block 900. It will be appreciated that if the result of equation (1) is positive then this indicates that the current level of PMEP is too high and action needs to be taken but if the result is zero or negative then the current PMEP is below the first limit and so no action is taken and the ECU 24 controls the vanes 44 normally to meet demand as indicated by block 400.

-21 -If the answer to block 250 is Yes the method advances to block 300 where the position of the vanes 44 is inhibited or more accurately the operation of the ECU 24 is modified so that the actuator used to move the vanes 44 cannot close the vanes 44 more that a minimum closure limit.

Fig.5A shows a chart indicating how control of the ECU 24 is modified. If the speed of the engine 10 is N and the value of Delta PMEP is P then the value of correction required to vane position is given at the intersection of these two values namely at the point For example if the engine speed is 1000 RPM and the value of Delta PMEP is +5 then this may produce a correction or minimum closure limit of X%. The ECU 24 then uses this value to prevent the vanes from being closed more than X% above the normal steady state value. For example, if the normal demanded position of the vanes for the current turbocharger demand is 10% open and X=10% then the vanes 44 will not be closed more than the 20% open position irrespective of any difference between demanded boost and current boost. The effect of this is to reduce the pressure in the exhaust manifold 28 upstream from the turbocharger turbine 38 thereby preventing an excessive increase in specific fuel consumption from occurring. This management of vane minimum closure position will have the effect of slowing the response of the turbocharger 14 to increases in demand but by careful calibration of the minimum closure limit this loss of response can be kept to an acceptable level. For example, the value of correction factor X% for the speed N5 and for the Delta PMEP value P5 will be different to those for the speed and Delta PMEP combination The values used to populate the chart shown in Fig.5A will be determined by experimental work and are stored in the memory of the ECU 24 as a look-up table.

-22 -In general terms the correction factor will be higher for a large Delta PMEP than it will be for a small Delta PMEP because a large PMEP will normally produce a very closed vane position whereas a small Delta PMEP will produce a less closed state. E.g. if the Delta PMEP is +5 the value of X may be 25% and the normal position of the vanes 44 would be 5% open, therefore after applying the minimum closure limit of 25% the vanes could not close beyond 30% open. But if the Delta PMEP is +1 then the vanes may normally be 25% open and the value of X may be 15% therefore after applying the minimum closure limit of 15% the vanes could not close beyond 40% open. That is to say, the closure limit is increased as the PMEP difference increases.

Referring back now to block 300, if there is no throttle valve on the engine 10 the method would then advance to block 500 to determine whether the engine 10 is still operating, if it is the method returns to block 210 to repeat the process but if the engine 10 is no longer operating the method ends at block 900.

However, in this case where a throttle valve 49 is fitted to the engine 10, the method advances to block 310 where the control of the throttle valve 49 by the ECU 24 is modified in a similar manner to that described above with respect to the vanes 44.

That is to say, the position of the throttle valve 49 is inhibited or more accurately the operation of the ECU 24 is modified so that an actuator used to move the throttle valve 49 cannot close the throttle valve more that a minimum closure limit.

-23 -After completing the operation of block 310 the method then advances to block 500 to determine whether the engine is still operating, if it is the method returns to block 210 to repeat the process but if the engine 10 is no longer operating the method ends at block 900.

Fig.5B shows a chart indicating how control of the ECU 24 is modified. If the speed of the engine 10 is N and the value of Delta PMEP is P then the value of correction required to throttle valve position is given at the intersection of these two values namely at the point For example if the engine speed is 1000 RPM and the value of Delta PMEP is +5 then this may produce a correction or minimum closure limit of Y%. The ECU 24 then uses this value to prevent the throttle valve from being closed more than Y% above the normal steady state value. For example, if the normal demanded position of the throttle valve 49 is 40% open and Y=5% then the throttle valve will not be closed more than the 45% open position irrespective of any difference between demanded boost and current boost. The effect of this is to reduce pumping losses in the engine 10 and hence reduce specific fuel consumption.

This management of throttle valve minimum closure position is determined by careful calibration and so the value of Y% will vary for different speed and Delta PMEP combinations. For example, the value of correction factor Y% for the speed N5 and for the Delta PMEP value P5 will be different to those for the speed and Delta PMEP combination The values used to populate the chart shown in Fig.5B will be determined by experimental work and are stored in the memory of the ECU 24 as a look-up table.

-24 -Referring back now to block 220 a method for determining current or dynamic PMEP will now be described with particular reference to Fig.4.

The first step in the method is to accelerate the engine 10 at a predetermined rate with the electronically controlled inlet flow control device set to a predetermined opening position. Fig.4 shows four accelerations A to D and so the first step would be to accelerate the engine 10 at the rate A' with the vanes 44 set at say 5% open and record values of pumping mean effective pressure and pressure ratio for the acceleration between idle speed (850 RPM) and a maximum normal operating speed of the engine (5000RPM) . As mentioned above the pressure ratio is determined using the output from the MAP sensor 50 and one or more algorithms stored in the memory of the ECU 24.

The next step is to repeat the acceleration A' using a second vane open position of say 10% and record the values of pumping mean effective pressure and pressure ratio for the acceleration between the idle speed and the maximum normal operating speed of the engine 10. This process is then repeated until a number of opening positions spanning the normal working range of opening positions of the vanes 44 has been recorded.

Then the whole process is repeated for the accelerations B', C' and D' which span a normal working range of the engine from low to high acceleration. It will be appreciated that acceleration is controlled by the load applied to the engine and so acceleration A' will have -25 -maximum load applied and acceleration D' will have minimum load applied to the engine 10 via the dynamometer.

This process will produce a large volume of data indicating PMEP and Pressure Ratio PR for various engine speeds and various rates of acceleration. It is to be noted that this data is produced during transient operation of the engine and so represents dynamic values rather than steady state values.

The accumulated data can then be recorded in a look-up table such as that shown in Fig.5 referencing the dynamically obtained values of pumping mean effective pressure, dynamically obtained values of pressure ratio for various engine accelerations and engine speed.

Therefore for any given engine speed, acceleration and pressure ratio PR a dynamic value of PMEP can be obtained and in this way a virtual PMEP sensor is produced.

It will be appreciated that the method described above and shown in Fig.6 would be performed by the ECU 24 and that various modifications could be made to the method such as the sequence of performing the blocks.

It will be appreciated that engine acceleration can be obtained from engine speed by various methods.

It will further be appreciated that although the invention is described with respect to an engine having a throttle valve it is equally applicable to a diesel engine having no throttle valve.

-26 -It will be appreciated by those skilled in the art that although the invention has been described by way of example with reference to one or more embodiments it is not limited to the disclosed embodiments and that one or more modifications to the disclosed embodiments or alternative embodiments could be constructed without departing from the scope of the invention as set out in the appended claims.

Claims

-27 -Claims 1. A method for reducing the transient specific fuel consumption of an engine having a turbocharger, the turbocharger having a turbine arranged so as to be driven by the exhaust gases from the engine and an electronically controlled inlet flow control device to regulate the exhaust gas entering the turbine wherein the method comprises applying a minimum closure limit to the electronically controlled inlet flow control device so as to reduce exhaust gas pressure build-up upstream from the turbocharger turbine if a current value of pumping mean effective pressure exceeds a first limiting value of pumping mean effective pressure.
2. A method as claimed in claim 1 wherein the method further comprises determining the first limiting value of pumping mean effective pressure, determining the current value of pumping mean effective pressure, determining whether the current value of pumping mean effective pressure is greater than the first limiting value of pumping mean effective pressure and, if the current value of pumping mean effective pressure is greater than the first limiting value of pumping mean effective pressure, applying the minimum closure limit to the electronically controlled inlet flow control device.
3. A method as claimed in claim 1 or in claim 2 wherein a current value of pumping mean effective pressure is determined using a virtual sensor.
4. A method as claimed in claim 3 wherein the virtual sensor is a look-up table referencing dynamically obtained -28 -values of pumping mean effective pressure, dynamically obtained values of pressure ratio and engine speed.
5. A method as claimed in any of claims 1 to 4 wherein a first limiting value of pumping mean effective pressure is determined using a steady state look-up table referencing speed against engine pressure ratio.
6. A method as claimed in any of claims 1 to 5 wherein the closure limit is varied based upon the difference between the current value of pumping mean effective pressure and the first limiting value of pumping mean effective pressure.
7. A control system for controlling an engine havinq a turbocharger, the turbocharger having a turbine arranged so as to be driven by the exhaust gases from the engine, the system comprising an electronic controller and an inlet flow control device controlled by the electronic controller to regulate the exhaust gas entering the turbine of the turbocharger wherein the electronic controller is operable to control the electronically controlled inlet flow control device so that it does not close below a minimum closure limit so as to reduce exhaust gas pressure build-up upstream from the turbocharger turbine if a current value of pumpinq mean effective pressure exceeds a first limiting value of pumping mean effective pressure.
8. A control system as claimed in claim 7 wherein the minimum closure limit is calibrated based upon the difference between the current value of pumping mean effective pressure and the first limiting value of pumping mean effective pressure.

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9. A control system as claimed in claim 7 or in claim 8 wherein the electronically controlled inlet flow control device comprises a number of vanes moveable by an actuator and the electronic controller is operable to control the electronically controlled inlet flow control device so as to reduce exhaust gas pressure build-up upstream from the turbocharger turbine by restricting operation of the actuator so as to prevent the vanes from being closed beyond the minimum closure limit.
10. A control system as claimed in any of claims 7 to 9 wherein the engine includes a throttle valve to control the flow of air to the engine and the electronic controller is further operable to prevent the throttle valve from closing beyond a calibratable limit if the current value of pumping mean effective pressure is greater than the first limiting value of pumping mean effective pressure.
11. A control system as claimed in any of claims 7 to wherein the electronic controller is further operable to reduce fuelling to the engine if the current value of pumping mean effective pressure is greater than a failure limiting value of pumping mean effective pressure that is considerably higher than the first limiting value of pumping mean effective pressure.
12. A control system as claimed in any of claims 7 to 11 wherein the electronic controller is further operable to determine the first limiting value of pumping mean effective pressure, determine the current value of pumping mean effective pressure, determine whether the current value of pumping mean effective pressure is greater than the first -30 -limiting value of pumping mean effective pressure and, if the current value of pumping mean effective pressure is greater than the first limiting value of pumping mean effective pressure, control the electronically controlled inlet flow control device so that it does not close below the minimum closure limit.
13. A control system as claimed in any of claims 7 to 12 wherein a current value of pumping mean effective pressure is determined by the controller using a virtual sensor.
14. A control system as claimed in claim 13 wherein using a virtual sensor comprises using a look-up table referencing dynamically obtained values of pumping mean effective pressure, dynamically obtained values of pressure ratio and engine speed.
15. A method for producing a pumping mean effective pressure virtual sensor for use in a control system as claimed in claim 13 or in claim 14 wherein the method comprises the steps of:- (a) accelerating the engine at a predetermined rate with the electronically controlled inlet flow control device set to a predetermined opening position; (b) recording pumping mean effective pressure and pressure ratio for the acceleration between idle speed and a maximum normal operating speed of the engine; (c) repeating steps (a) and (b) for a number of further opening positions of the electronically controlled inlet flow control device until a number of opening positions spanning the normal working range of opening positions has been recorded; -31 - (d) repeating steps (a) to (C) for a further number of predetermined engine acceleration rates spanning a normal (e) recording the generated data in a look-up table referencing the dynamically obtained values of pumping mean effective pressure, dynamically obtained values of pressure ratio, engine acceleration and engine speed.
16. A method for reducing the transient specific fuel consumption of an engine substantially as described herein with reference to the accompanying drawing.
17. A control system for controlling an engine substantially as described herein with reference to the accompanying drawing.
18. A method for producing a pumping mean effective pressure virtual sensor for use in a control system substantially as described herein with reference to Figs. 4 and 5 of the accompanying drawing.