FIELD OF THE INVENTION:

This invention relates to a method and a device to run a vehicle in limp home mode when crank shaft position sensor fails.

BACKGROUND OF THE INVENTION:

In a vehicle a crank shaft position sensor provides information about the speed of the engine and a cam shaft position sensor provides information about the phase of the engine i.e. providing information about which cylinder is in compression stroke. Based on the crank shaft position sensor and the cam shaft position sensor, an Engine Control Unit (ECU) in the vehicle computes the timings for injection of the fuel into the cylinder and ignition of the fuel.

The EP 569227 discloses a fuel injection control system for an internal combustion engine. The disclosure mentions about a limp home circuit that conducts an open loop control on fuel injection in place of the feedback control system when an abnormality is detected. The limp home circuit secures the permissible minimum driving function which is required to drive the internal combustion engine of a vehicle. The limp home circuit has a fuel cut unit for intercepting fuel supply to the internal combustion engine when the rotational speed of the engine exceeds a predetermined value.

ADVANTAGES OF THE INVENTION:

The proposed method and device enables the user of the vehicle to drive the vehicle to a nearest service station even though the crank shaft position sensor is not working.

The timing for fuel injection in limp home made is computed using the fuel pressure signal, without needing any additional hardware.

OBJECT OF THE INVENTION:

The invention proposes a method and a device to drive a vehicle in a limp home mode when the crank shaft position sensor fails and a cam shaft position sensor is not present in the vehicle as part of cost savings. This is achieved by using the fuel pressure signal. This enables the user to drive the vehicle with limited functionality, to a service station.

BRIEF DESCRIPTION OF THE DRAWINGS:

Figure 1 Shows the schematic of the invention

Figure 2 Shows the graphs of the crank sensor and the fuel pressure sensor

DESCRIPTION OF THE INVENTION:

Figure 1 shows an Engine Control Unit (ECU) 10 controlling a set of cylinders 12,14,16 and 18 of an internal combustion engine 20 through a set of control signals 22,24,26 and 28 respectively, for fuel injection and ignition. The internal combustion engine with four cylinders is shown only as an example. The ECU comprises a means 29 to receive a crank signal 30 from a crank shaft position sensor and a fuel pressure signal 32 generated by a fuel pressure sensor disposed in a fuel accumulator which is not shown in fig. In the rest of the document the crank shaft position sensor is referred as crank sensor and the signal generated by crank sensor is referred as crank signal. Under normal mode when the crank sensor is functioning properly, the ECU controls the fuel injection and the fuel ignition into the cylinders based on the crank signal 30, phase of the engine and the engine parameters which are not shown in the figure. The typical engine parameters are: engine speed, engine load, ambient air pressure, coolant temperature, torque demand. Etc.

This list is only for reference and is not a complete list. To control the fuel injection, phase of the engine needs to be detected; the phase of the engine refers to time or the crank shaft position when a reference cylinder approaches the compression TDC. In prior arts, typically a cam shaft position sensor provides the information on the phase of the engine. But the proposed invention assumes that there is no cam sensor available as part of the engine control system and the phase of the engine is detected by different methods when the engine is started. These methods may include monitoring of pressure in the intake, monitoring of the pressure in the fuel accumulator etc. As these methods are already known, these are not explained in this document.

The ECU also comprises a means 34 to detect peaks in the fuel pressure signal. The fuel pressure signal is typically in the form of a sine wave and detection of the peaks in the fuel pressure signal is achieved by known techniques like checking for the gradient of the fuel pressure signal for change in direction.

The ECU also comprises a computing means 36 to compute the timing for fuel injection/ignition as explained in the later part of the document.

Shown in fig. 2 are the crank signal 30 and the fuel pressure signal 32. The graph 2A represents the crank signal, the X axis representing time and the Y axis representing the amplitude of the crank signal in terms of voltage. The graph 2B represents the fuel pressure signal, the X axis representing the time and the Y axis representing the pressure.

The crank signal is in the form of a stream of pulses. There will be a reference pulse which may be wider compared to other pulses or there may be a wider gap in the pulse stream. These reference pulses are shown at to, t1, t2 and t3. These reference pulses are generated for each rotation of the crank shaft, i.e. once in 360 degrees rotation of the crank shaft. The reference pulse indicates that a reference cylinder is approaching a Top Dead Sensor (TDC). Based on the reference pulse in the crank signal and the phase signal which is not shown, the ECU 10 determines which cylinder is approaching the compression TDC and the fuel is injected into this cylinder.

The TDCs of all other cylinders will have a predefined offset with respect to the reference cylinder. Using the reference pulses of the crank shaft and phase signal, the ECU unambiguously determines the timings for fuel injection/ignition for all the cylinders.

Graph 2B shows the pressure signal which is in the form of a sine wave, the positive peaks representing TDC of one of the cylinders. By measuring the time between two consecutive peaks, the engine speed is calculated. Also to determine the angle of the crank shaft for the purpose of injection or ignition of the fuel, the measurement of time between the two consecutive peaks is used.

When the crank sensor fails, the ECU needs to work in an open loop condition to control the engine with limited functionality. This is referred as limp home mode. In limp home mode the target speed of the engine and the quantity of the fuel to be injected are predefined. This is to enable the user to drive the vehicle to the service station with limited functionality.

As the crank shaft position sensor fails, it is not possible for the ECU to determine the timing for fuel injection/ignition accurately. But an approximate calculation can be made to drive the vehicle in limp home mode, as explained below.

When the crank signal was functioning properly the ECU keeps injecting fuel into appropriate cylinder and also stores into memory the last cylinder number in which the fuel was injected.

When the crank sensor fails, the ECU monitors for the pressure signal, detects the peak, retrieves the cylinder number in which the fuel was injected last time, determines the next cylinder in which the fuel needs to be injected, computes the time at which the fuel needs to be injected and injects the fuel into the appropriate cylinder accordingly. The timing information for injecting fuel into the cylinder may also be used to ignite the fuel in the cylinder.

Based on the engine operating conditions, the timing of the fuel injection/ignition may be varied around the peak of the pressure signal considering that the two peaks are ate 360 degrees apart of the rotation of the crank shaft. Let T be the time between two peaks, then T/360 gives the time taken for crank shaft to rotate by 1 degree. Let this time be t. So to
advance or delay the injection for the next cycle by n degrees, the ECU calculates the timing for injection Ti as :

Ti = T-t*n for advancing and Ti=T+t*n for delaying by n degrees. Ti indicates the time at which the fuel is to be injected/ignited with reference to the timing of the last peak detected.

The time T between the two peaks is also used to compute the engine speed in terms of engine RPM.

Once the engine reaches the target engine speed, the ECU maintains the engine speed as close to the predefined target engine speed as possible. The injection quantity is adjusted to maintain the required engine speed, with injection/ignition released at calculated firing timing.

The description disclosed is with reference to a diesel engine where the injection of the fuel into the cylinder results in ignition of the air-fuel mixture. In the gasoline engines, a separate ignition signal is needed to ignite the fuel. The timing of injection explained in the document refers to the diesel engines. The same can be adapted to gasoline engines by using the above determined timing for generating the ignition signals.

CLAIM:

1. A method to operate an internal combustion engine in a limp home mode when the crank shaft sensor fails, the said method comprising the steps detecting peaks in a fuel pressure signal determining last fired cylinder of the said internal combustion engine determining the next cylinder to be fired based on the last cylinder fired computing the timing for the fuel injection/ignition for the next cylinder in dependence of the peaks in the pressure signal injecting/igniting the fuel in the said next cylinder

2. A method to operate an internal combustion engine according to claim 1 wherein the detection of a peak is done by checking for the change in direction of the gradient of the fuel pressure signal

3. A method to operate an internal combustion engine according to claim 1 wherein the engine speed is computed by computing the time elapsed between the two consecutive peaks of the fuel pressure signal

4. A method to operate an internal combustion engine according to claim 1 wherein the timing for the fuel injection/ignition for the next cylinder is fine-tuned in dependence of the said engine speed.

5. An Engine Control Unit (ECU) (10) to control an internal combustion engine in a limp home mode when a crank shift sensor fails, the said ECU (10) comprising:

A means (29) to receive fuel pressure signal (32)

A means (34) to detect peaks in the fuel pressure signal (32)

A computing means (36) to compute the timing for fuel injection/ignition for next cylinder in dependence of the said peaks in the said fuel pressure signal (32) and the last fired cylinder Injecting the fuel in the said next cylinder in dependence of the said timing.