A TURBINE SPEED CONTROL METHOD

5 TECHNICAL FIELD

The present invention relates to a method and a system for controlling the turbine speed of an engine provided with a turbo charger having a turbine with controllable speed, such as a Variable Turbine Geometry (VTG) turbo charger. 0 BACKGROUND

Diesel engines for use in heavy vehicles such as trucks and buses are sometimes provided with a Variable Turbine Geometry (VTG) also termed Variable Geometry Turbocharger (VGT). 5 A VTG turbo charger comprises a turbine with a variable geometry powering a compressor for feeding the air inlet of the combustion engine with compressed air. The position of a VTG can be changed when the engine is running by a control system adapted to control the VTG. The position to which the control system controls the VTG is determined by engine calibration. The engine calibration is set to fulfill emission restrictions and to meet0 performance requirements set by the manufacturer, hi some modes of operation of the engine there is a risk that the speed of the turbine exceeds a maximal tolerable speed. A too high turbine speed can lead to a break down of the turbine and is hence important to prevent. In addition the control method used to prevent a turbine speed above the maximum allowable turbine speed should also result in a smooth operation of the engine. 5

Existing control methods for preventing a too high turbine speed simply turn off the fuel feed to the engine when there is a risk of a too high turbine speed. Such a control method will result in an abrupt disruption of the power supplied by the engine to the motor vehicle propelled by the engine. ,0

Hence, there exists a need for a method and a system for controlling the speed of a turbine of a VTG engine, which ensures that the maximum turbine speed is not exceeded and which

at the same time improves engine performance by minimizing the risk of having to turn off the fuel supply to the engine.

SUMMARY It is an object of the present invention to provide a method and a system that is able to prevent a too high turbine speed of a turbocharged engine powering a motor vehicle such as a truck or a bus.

It is another object of the present invention to provide a method and a system for controlling the speed of a turbine of a VTG engine, which ensures that the maximum turbine speed is not exceeded and which at the same time improves engine performance by minimizing the risk of having to turn off the fuel supply to the engine.

These objects and others are obtained by the method, system and computer program product as set out in the appended claims. Thus, in order to prevent a controllable turbine of a turbo charger of a combustion engine adapted to power a motor vehicle from overspeeding, the system can be adapted to start controlling the controllable turbine to a state acting to reduce the turbine speed when the turbine speed exceeds a first turbine speed level. The engine control system is further adapted to control the fuel feed to the engine to a reduced fuel feed when the turbine speed exceeds a second, turbine speed level. Hereby, a turbine overspeed protection is achieved without having to completely turn off the fuel feed.

In accordance with one embodiment the control system is adapted to combine control of the fuel feed and control of a controllable turbine, such as a VTG. Hereby an efficient and powerful control system is obtained which heavily reduces or even eliminates the risk of having to completely turn off the fuel feed to avoid turbine overspeeding.

In accordance with one embodiment the fuel feed is reduced more the higher the turbine speed is. This can for example be obtained by reducing the fuel feed in proportion to the difference between a current turbine speed and some turbine speed level, when the turbine speed exceeds the turbine speed level. Hereby a control system taking into account the

actual turbine speed is obtained which further reduces the risk of reaching a turbine speed requiring a complete fuel feed turn off.

The control method used by the control system can for example be adapted to control a Variable Turbine Geometry (VTG), or another turbine having a controllable speed from overspeeding. Overspeeding is prevented by controlling both the control signal to the turbine and the fuel feed in response to the current turbine speed.

In accordance with one embodiment of the present invention, the control system used to prevent turbine overspeeding operates in three different modes in response to the current turbine speed. In a first mode when the turbine exceeds a first, pre-determined, speed, the control system is adapted to reduce the turbine speed by changing the VTG position, in the case of a VTG turbocharger. In a second mode if the turbine speed exceeds a second, predetermined, speed higher than the first turbine speed, the control system is adapted to reduce the fuel feed to the engine to reduce the turbine speed. In a third mode if the turbine speed reaches a third, pre-determined, speed higher than the second turbine speed, the control system is adapted to cut off the fuel feed to the engine to reduce the turbine speed. In particular the third, pre-determined, speed may be equal to the maximum allowed turbine speed.

Using the method and system in accordance with the invention will provide a smooth and robust turbine overspeed protection, which provides a better driving experience.

In addition, by reducing the risk of having to completely turn off the fuel supply, the risk of repeated fuel turn off, so-called cycling is reduced or even eliminated. Repeated fuel turn off occurs if the fuel is turn off and the turbine speed is reduced where after when the fuel is turned on turbine speed again increases and reaches a speed where the fuel needs to be turned off. Such a repeated fuel turn off will negatively impact the lifetime of the turbocharger.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail by way of non-limiting examples and with reference to the accompanying drawings, in which:

- Fig. l is a general partial view of an engine including a turbo charger with VTG,

- Fig. 2 is a view illustrating different modes when controlling turbine speed,

- Fig. 3 is a view of a turbine speed controller, and

- Fig. 4 is a flow chart illustrating steps performed in a control procedure when controlling a combustion engine for preventing turbine overspeeding.

DETAILED DESCRIPTION

In Fig.l selected parts of an engine 100 of a motor vehicle 10 is schematically depicted. The engine depicted in Fig. 1 can for example be designed to be part of a truck or any other heavy vehicle such as a bus or the like. The exemplary engine 100 in Fig. 1 is a diesel engine provided with a turbocharger and having five cylinders 105. The turbo charger can be of any type for example a turbo charger having a Variable Turbine Geometry (VTG) or another turbo charger having a controllable turbine, such as a turbo charger with waste gate. The turbo charger comprises a compressor 102 driven by a turbine 103. The engine is supplied with fuel from a fuel tank 104.

The engine is controlled by an electronic control unit (ECU) 106. The ECU 106 is connected to the engine to control the engine. For example, the ECU can be adapted to control fuel feed and the position of the VTG and other parameters used for controlling the engine. In addition sensors provide sensor signals to the ECU 106. Using the sensor signals from the motor vehicle 10, the ECU 106 exercises control of the engine using some programmed computer instructions or similar means. Typically, the programmed computer instructions are provided in the form of a computer program product 110 stored on a readable digital storage medium 108, such as memory card, a Read Only Memory (ROM) a Random Access Memory (RAM), an EPROM, an EEPROM or a flash memory.

In order to prevent the turbine of the VTG from overspeeding the control unit exercising control over the engine is adapted to detect a high turbine speed and take measures before

the turbine speed reaches a state where it is necessary to cut off the fuel supply in order to prevent the turbine from overspeeding.

Hence, as depicted in Fig. 2, if the turbine speed reaches a first level the control system is adapted to detect this and in response to such a detected high turbine speed starts to adjust the VTG position to a more open position to reduce the turbine speed. Typically the VTG can be controlled to a more open or maximally open position to reduce the turbine speed. If, despite the action taken when the turbine speed reaches the first level, the turbine speed increases and reaches a second level, the control system is adapted to detect this and in response to such a detected high turbine speed, above the second speed level, starts to reduce the fuel feed to the engine without completely cutting off the fuel feed. In particular the fuel feed can be reduced in a smooth way to not severely impact the driving experience. For example, when the turbine speed exceeds the second speed level, the fuel feed is reduced in proportion to the difference between the current turbine speed and the second speed level.

Only as a last emergency measure the fuel feed is turned off. This is preferably only performed if the turbine speed reaches a third level, which can be equal to the maximum turbine speed, despite the measures taken when the turbine speed exceeds level one and level two. In addition the VTG or waste gate or an EGR valve can be fully opened.

In Fig. 3, a view of a controller unit adapted to control a VTG and engine fuel supply in accordance with the principles set out above in conjunction with Fig. 2 is depicted. Thus, a controller unit 106, in particular an electronic control unit (ECU) can be adapted to receive the current turbine speed as input signal 301 and also be fed with three different turbine speed levels, level 1, level 2 and level 3. For example the third level, level 3, may correspond to the maximum allowed turbine speed and the level 1 and level 2 may correspond to a percentage of the level 3 speed, respectively. For example level 1 may correspond to 80% of the level 3 speed and level 2 may correspond to 90 % of the level 3 speed. Using the current turbine speed as input signal the controller unit 106 controls the VTG and fuel feed as described below in conjunction with Fig. 4 when the turbine speed reaches a high speed.

In Fig. 4, a flow chart illustrating steps performed in a control procedure performed by a control unit when controlling a combustion engine for preventing turbine overspeeding is shown. In a first step 401 three different turbine speed levels are set or loaded in a control system for preventing turbine overspeed and the turbine speed is controlled in a regular manner. Next, in a second step 403 the control unit begins to check if the current turbine speed is above a first turbine speed, if the turbine speed exceeds the first turbine speed level the procedure proceeds to a third step 405, else the procedure returns to step 401.

In the third step 405, the control unit begins to reduce the turbine speed in a first control mode by adjusting the VTG position. Next, in a fourth step 407 the control unit again checks the turbine speed. If in step 407 the turbine speed is below the first speed level, the procedure returns to step 401. If the turbine speed still exceeds the first level but not the second level the procedure remains in the first turbine speed control mode and the procedure returns to step 405. If, in step 407 it is determined that the turbine speed exceeds a second turbine speed level the procedure proceeds to a fifth step 409.

In step 409, the control unit begins to reduce the turbine speed in a second control mode by reducing the fuel feed to the engine. The fuel reduction can for example be performed in a smooth manner, as described above, to not cause an abrupt behavior change of the engine. In the second mode when the fuel feed is reduced it is preferred, but not necessary to continue to control the VTG to an opened position. Next, in a sixth step 411 the control unit again checks the turbine speed. If in step 411 the turbine speed is below the second speed level, the procedure returns to step 405. If the turbine speed still exceeds the second level but not the third level, the procedure remains in the second turbine speed control mode and the procedure returns to step 409. If, in step 411 it is determined that the turbine speed exceeds a third turbine speed level the procedure proceeds to a seventh step 413.

In step 413, the control unit begins to reduce the turbine speed in a third control mode by terminating the fuel feed to the engine. Next, in an eighth step 415 the control unit again checks the turbine speed. If in step 415 the turbine speed is below the third speed level, the procedure returns to step 409. If the turbine speed still exceeds the third level, the procedure remains in the third turbine speed control mode and the procedure returns to step 413.

Using the method and system as described herein will provide a smooth and robust turbine overspeed protection, which provides a better driving experience. The use of the method and system as described herein will also increase the lifetime of the turbocharger by reducing or eliminating repeated fuel turn off events caused by turbo charger over speeding.