A SURGE CONTROL METHOD

TECHNICAL FIELD

The present invention relates to a method and a system for controlling an engine having a turbo charger and provided with an EGR valve.

BACKGROUND

Diesel engines for use in heavy vehicles such as trucks and buses are sometimes provided with a turbo charger that may comprise a Variable Turbine Geometry (VTG) also termed Variable Geometry Turbocharger or Variable Geometry Turbine (VGT). Such an engine can typically be provided with an EGR (Exhaust Gas Recirculation) valve. Other engines for other uses may also be provided with a turbo charger in combination with EGR. One reason for employing EGR technology is that it facilitates fulfillment of emission requirements for i.a. diesel engines.

A turbo charger comprises a turbine with a variable geometry powering a compressor for feeding the air intake of the combustion engine with compressed air. If the difference in pressure upstream and downstream the compressor, i.e. the pressure difference over the compressor is high, the compressor may be unable to maintain the pressure difference and there will be a reverse gas mass flow through the compressor. This is sometimes termed surging.

One scenario that may cause surging is when there is a rapid decrease of fuel feed. In such a scenario the turbine powering the compressor may experience a rapid speed drop resulting in a rapid power drop of the compressor. The turbo pressure downstream the compressor may then be high and at the same time the compressor may be operating at low power. If this occurs there is a risk that the high turbo pressure cannot be maintained and there will be a reverse gas mass flow through the compressor, surging.

A reverse gas mass flow through the compressor is highly unwanted for a number of different reasons. Firstly, such an event will generate a bang having a fairly loud volume, which of course is disturbing for a driver of a vehicle propelled by the engine and people in the vicinity of the vehicle. Secondly, the compressor of the turbo charger will experience an abnormal operation condition that may shorten the life time of the compressor or even directly damage the compressor. Thirdly, there will be a drop in turbo pressure resulting in an instant drop in torque generated by the engine, which will be felt by the driver of the motor vehicle.

It is therefore desirable to avoid a reverse gas mass flow through the compressor of a turbo charged engine powering a motor vehicle. Hence there exists a need for a method of controlling a turbo charged combustion engine which prevents a reverse mass flow through the compressor of the turbo charger.

SUMMARY

It is an object of the present invention to provide a method and a system that is able to prevent a reverse gas mass flow through the compressor of a turbo charged combustion engine for powering a motor vehicle such as a truck or a bus.

This object 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 reverse gas mass flow through a compressor of a turbo charged combustion engine, the control unit of an engine for powering a motor vehicle such as a truck or a bus is provided with a detector for detecting a rapid decrease in fuel feed. If the negative fuel feed rate is below some predetermined threshold value, the control unit triggers a mode where a control unit is adapted to prevent surging through the turbo compressor.

In one embodiment, when the turbo charger is provided with a Variable Turbine Geometry (VTG), the control system begins to control the VTG to generate a high power when in the mode for preventing compressor surging. The high power generated by the turbine will prevent a fast power drop which could lead to surging.

In one embodiment the control system begins to control an EGR valve to a more open position to let gas flow in the reverse direction in the EGR path when in the mode for preventing compressor surging, thereby lowering the charge gas pressure so that surging through the turbo compressor can be prevented.

In one embodiment the control system stores a map comprising optimal EGR valve positions and optimal closed Variable Turbine Geometry positions for different engine operation states, and controls the Variable Turbine Geometry and / or the EGR valve position to the positions corresponding to the current engine operation conditions of the map. Hereby the risk for surging can be further reduced.

Using the control method in accordance with the invention will result in smooth reduction in the charge gas pressure, thus preventing surging through the turbo compressor, which otherwise may occur in the event of a rapid fuel feed reduction.

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. 1 is a general partial view of engine including a turbo charger with VTG and EGR.

- Fig. 2 is a flow chart illustrating steps performed in a control procedure when controlling a combustion engine preventing a reverse gas mass flow in accordance with one embodiment.

- Fig. 3 is a flow chart illustrating steps performed in a control procedure when controlling a combustion engine preventing a reverse gas mass flow in accordance with another embodiment.

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 or not having a controllable turbine. The turbo charger comprises a compressor 102 driven by a turbine 103. Furthermore the engine comprises an EGR valve 107. The EGR valve 107 controls the amount of exhaust gas that is re-circulated to the air inlet of the engine 100.

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 the EGR valve position and the position of the VTG and also other parameters used for controlling the engine. In addition sensors provide sensor signals to the ECU 106. Using the sensor signals, 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 Fig. 2 a flow chart illustrating steps performed in a control procedure performed by an ECU when controlling an engine provided with a VTG to prevent a reverse gas mass flow through a turbo compressor of the engine is shown. In a first step 201, the ECU detects a rapid decrease in fuel feed, i.e. the fuel rate change is determined to be below some pre- determined level. Upon determining that the fuel rate is strongly negative and below some pre-determined value, the ECU switches control mode and proceeds to a second step 203. In the second step 203 the position of the VTG is controlled to generate a maximum turbine speed in order to aid the compressor in maintaining the turbo pressure. Hence, by controlling the VTG position such that the turbine speed is maximized at the reduced fuel feed the risk of a rapid fuel feed reduction leading to a reverse gas mass flow through the compressor is reduced and can in most cases be prevented.

The VTG position that the control system is adapted to control to maximize the turbine power / turbine speed, can for example be given by a map storing an optimal closed VTG position providing a maximum turbine speed for each different state that the engine can operate in.

Because surging most often occurs at high engine load and within a relatively narrow engine speed range, the control system can be adapted to control the VTG to the position corresponding to maximum power at those conditions. In other words, the control system can be simplified to adapt the VTG position providing a maximal turbine power / turbine speed for one gas mass flow, i.e. the gas mass flow corresponding to the operational condition where surging is likely to occur.

Next in a third step 205 the ECU checks if a condition for exiting the mode for preventing a reverse gas mass flow through the turbo compressor is fulfilled. If so the procedure returns to step 201, via a fourth step 207 where the control system is reset to the original, regular, control mode, else the procedure stays in the mode for preventing reverse gas mass flow through the turbo compressor and the procedure returns to step 203.

In Fig. 3 a flow chart illustrating steps performed in a control procedure performed by an ECU when controlling an engine provided with an EGR system to prevent a reverse mass flow through a turbo compressor of the engine in accordance with another embodiment of the present invention is shown. First in a first step 301 the ECU detects a rapid decrease in fuel feed, i.e. the fuel rate change is determined to be below some pre-determined level. Upon determining that the fuel rate is strongly negative, i.e. below some pre-determined value, the ECU switches control mode and proceeds to a second step 303. In the second step 303 the EGR valve is opened to a more open position in order to reduce the turbo pressure by creating another gas outlet path via a reverse gas mass flow through the EGR path. Hence by controlling the EGR valve to a more open position the risk of a rapid fuel feed reduction leading to a reverse gas mass flow through the compressor is reduced because the high pressure gas of the turbo charger can exit through the EGR path instead of having to exit through the compressor of the turbo charger.

Next in a third step 305 the ECU checks if a condition for exiting the mode for preventing a reverse gas mass flow through the turbo compressor is fulfilled. If so the procedure returns to step 301 via a fourth step 307 where the control system is reset to the original, regular, control mode, else the procedure stays in the mode for preventing reverse gas mass flow through the turbo compressor and the procedure returns to step 203.

The condition used to exit the mode for preventing a reverse gas mass flow through the turbo charger can for example be a determination that the negative fuel rate change is determined to be above some pre-determined level. The level can be but does not have to be the same level where the mode for preventing a reverse gas mass flow through the turbo charger is triggered.

In accordance with a third embodiment of the present invention the control systems as described above in conjunction with Figs. 2 and 3 are combined. Thus, upon detection of a strong negative fuel decrease rate in an engine provided with both a VTG and a an EGR system, the ECU both controls the VTG to a position generating a maximum turbine speed in order to aid the compressor in maintaining the turbo pressure and controls the EGR valve to a more open position in order to reduce the turbo pressure by creating another gas outlet path via a reverse gas mass flow through the EGR path.

In such an embodiment where both the VTG and EGR are controlled, the control system can be adapted to take into account the higher exhaust gas pressure generated by a more closed VTG position. Hence, if the exhaust gas pressure is increased above the charger gas pressure, there cannot be a gas flow in the reverse direction of the EGR path. By storing a map in the control system that provides the control system with combined positions for the VTG and EGR for different states of operation of the engine, it can be ensured that there is always higher charge gas pressure than exhaust gas pressure. This is obtained by storing VTG positions that are more open when a VTG position providing maximum turbine power otherwise would result in an exhaust gas pressure exceeding the charger gas pressure.