GB2310286A - Misfire detection for an engine - Google Patents

Description

MISFIRE DETECTOR FOR AN ENGINE.

The present invention relates to a misfire detector for an engine, for example, a four cylinder diesel engine.

US 4, 212,278 relates to a safety apparatus to monitor the ratio of engine output torque to supplied fuel in order to detect engine misfire or seizure. However, it is not possible to detect a misfire of a specific cylinder; it is only possible to detect that a misfire has occurred.

According to a first aspect of the invention, there is provided a misfire detector for an internal combustion engine, comprising means for measuring speed changes produced by a cylinder of the engine in a plurality of cylinder firing cycles, means for deriving a relationship between the speed changes and amounts of fuel supplied to the cylinder during the plurality of cylinder firing cycles, and means for deriving from the relationship an indication of whether the cylinder is misfiring.

According to a second aspect of the invention, there is provided a method of detecting misfire in an internal combustion engine, comprising measuring speed changes produced by a cylinder of the engine in a plurality of cylinder firing cycles, deriving a relationship between the speed changes and amounts of fuel supplied to the cylinder during the plurality of cylinder firing cycles, and deriving from the relationship an indication of whether the cylinder is misfiring.

Preferably the speed changes produced by each cylinder of the engine are measured and the relationship is derived for each of the cylinders to allow a misfiring cylinder to be identified.

The relationship may be derived by fitting a linear function to the set of corresponding speed changes and fuel amounts, for instance by least squares fitting. The indication may be derived by deriving the gradient of the linear function and indicating misfire if the gradient is less than a predetermined gradient. As an alternative, the relationship may be derived by forming the ratio of the scatter of the speed changes to the scatter of the fuel amounts. The indication may be derived by indicating misfire if the ratio is less than a predetermined ratio.

Preferably, the amount of fuel supplied to each cylinder is determined by a position sensor measuring axial position of a pump rotor.

It is thus possible to detect the misfire of a cylinder in the engine.

The invention will now be described in more detail, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of an engine misfire detector arrangement according to an embodiment of the present invention; Figure 2 is a flow diagram illustrating the functional steps of the arrangement of Figure 1; Figures 3 to 6 are scatter diagrams for the embodiment of Figure 1; and Figures 7 and 8 are schematic diagrams illustrating a method of measuring changes in engine speed for use with the embodiment of Figure 1.

Figure 1 shows a diesel engine 1 having four cylinders la, ib, ic, id. A flywheel 2 is connected to a crankshaft of the engine 1 and has four teeth 3a, 3b, 3c, 3d equally spaced about the circumference of the flywheel 2. A sensor 4, such as a variable reluctance transducer, is disposed adjacent the flywheel 2 to detect the passing of the teeth 3a, 3b, 3c, 3d. An electronically programmable injector control (EPIC) fuel pump 5 having an axially movable rotor and a position sensor to monitor the axial position of the rotor is connected to each cylinder la, ib, ic, id in order to supply measured amounts of fuel thereto. The amount of fuel delivered by the fuel pump 5 is measured in rotor position units. A processor 6 is connected to the sensor 4 and the fuel sensor. In addition to the sensors mentioned above, other sensors such as load demand and temperature sensors may be required for the engine to function.

However, such sensors will not be described in any further detail, since they are not considered essential in order to understand the present invention.

The operation of the arrangement of Figure 1 will now be described with reference to Figures 2 to 6.

The firing cycle of the engine 1 is in the following order: the first cylinder la, the second cylinder ib, the third cylinder ic and the fourth cylinder id.

Referring to Figure 2, memory locations for receiving information relating to the position of the rotor, associated changes in engine speed corresponding to the firing of the four cylinders la, 1b, ic, id, and a firing cycle counter for counting the number of firing cycles are cleared (step S1).

The processor 6 then determines whether the engine conditions are suitable for accurate detection of engine misfire to take place (step S2).

If the engine conditions are not suitable, the processor 6 waits until the conditions are suitable for accurate detection of engine misfire. Such conditions do not include, for example, transient conditions and conditions of transmission vibration.

Upon detection of suitable conditions, the processor 6 proceeds to step S3 and awaits an engine cycle interrupt before reading the last four rotor positions of the rotor (indicating the amount of fuel delivered to each of the four cylinders 7 a, ib, ic, 1d) during the current firing cycle. The changes in engine speed corresponding to the firing of the four cylinders la, ib, ic, 1d are also read.

The processor 6 then ascertains whether the engine conditions have changed since the start of the engine firing cycle (step S4). If the engine conditions have changed, the processor returns to step S2 and awaits suitable conditions for accurate detection of engine misfire.

If the engine conditions have not changed since the start of the firing cycle, the processor 6 stores the four positions of the rotor and the associated changes in engine speed (step S5). The engine firing cycle counter is then incremented by 1 (step 56) and the processor 6 monitors the firing cycle counter to ascertain whether the firing cycle counter is greater than or equal to 100 (step 57) and if not, the processor 6 returns to step S2 and continues as described above. Once the firing cycle counter reaches 100, the processor 6 determines a linear relationship, for each of the four cylinders la, ib, ic, id, between the amount of fuel supplied to each cylinder and the corresponding changes in engine speed and determines the gradient of the linear relationship (step S8). A fault code signifying a misfiring cylinder is recorded in respect of any of the cylinders whose corresponding linear relationship has a gradient of less than 0.1 rpm/rotor unit (step S9). The processor 6 then begins to monitor the engine for misfires once more by returning to step S1 described above.

The linear relationships can be determined by any known method, for example, a least square fit method.

Referring to Figure 3, a first straight line is fitted to the scatter of the rotor position and corresponding changes in engine speed for the first cylinder la. The gradient of the first straight line is 0.64 rpm/rotor unit.

Similarly, a second straight line is fitted to the scatter of rotor positions and corresponding changes in engine speed for the second cylinder 1 b in Figure 4. The gradient of the second straight line is 0.81 rpm/rotor unit. Since the gradients of the first and second straight lines are greater than 0.1 rpm/rotor unit, this indicates that the first and second cylinders la, ib are not misfiring.

A third straight line is fitted to the scatter for the third cylinder ic in Figure 5. The gradient of the third straight line is -0.04 rpm/rotor unit.

This value signifies that the third cylinder ic is misfiring.

Figure 6 shows a fourth straight line corresponding to the performance of the fourth cylinder id. The gradient of the fourth straight line is 1.27 rpm/rotor units. Thus, this value signifies that the fourth cylinder id is not misfiring.

Another method of detecting a misfiring cylinder involves measuring the scatters of the engine speed changes and the fuelling values by, for example, calculating the standard deviations. The ratio of the scatter in engine speed changes with respect to the scatter in fuelling values may be used as a criterion for misfire detection. A small scatter in the engine speed changes corresponding to a large scatter in the fuelling values would signify a misfire.

In order to measure the respective changes in engine speed corresponding to the firing of each cylinder of the engine 1, the four teeth 3a, 3b, 3c, 3d are equally spaced at 900 from each other about the circumference of the flywheel 2. The first and third teeth 3a, 3c correspond to the firing of the first and third cylinders la, ic and the second and fourth cylinders ib, id, respectively, and are located on the flywheel 2 at 5 before top dead centre (TDC) firing of the corresponding cylinders la, ib, Ic, id as shown in Figure 7. When one of the four cylinders la, ib, ic, id fires, the average engine speed is calculated over 90" from the tooth 5 before TDC to the tooth 85" after TDC firing as a function of the time taken tiaic, tlb,ld for the teeth 3a, 3b, 3c, 3d to pass in front of the sensor 4. The change in engine speed is calculated by subtracting a previous engine speed from the current engine speed.

Figures 7 and 8 show a firing order of la, Ib, ic, id.

Referring to Figure 8, each pulse represents the passing of a tooth in front of the sensor 4. In a given firing cycle, tooth 3a passes in front of the sensor 4. As the flywheel 2 rotates, the position of TDC firing of the first cylinder la is shown by a first arrow TDC1 (50 after tooth 3a passes in front of the sensor). The flywheel 2 rotates further and the second tooth 3b passes in front of the sensor 4. The processor 6 measures the time taken between the passage of the second and third teeth 3b, 3c in front of the sensor 4. The passage of the fourth and first teeth 3d, 3a in front of the sensor (corresponding to TDC firing of the second cylinder 1b) is similarly timed by the processor 6. The times taken for the passage between the second and third teeth 3b, 3c and between the fourth and first teeth 3d, 3a are used to calculate the change in engine speed between TDC firing of the first cylinder and TDC firing of the second cylinder. The same process is used to calculate changes in engine speed between TDC firing of the second and third cylinders ib, Ic, the third and fourth cylinders ic, id and the fourth and first cylinders id, la.

Although in the present example misfire detection has been described in a single speed, load, engine temperature, gear combination context (typically, a fully warm engine operating at a target idle speed in neutral), it is not intended that the present invention be limited to this example.

Instead, misfire detection is also intended to be active at all speeds and load conditions, provided that the engine is in a steady state for the duration of the test (i.e. there are no significant changes in load or speed greater than a predetermined level and, if the level is exceeded, the misfire detection can be disabled).

Claims (10)

1. A method of detecting misfire in an internal combustion engine, comprising measuring speed changes produced by a cylinder of the engine in a plurality of cylinder firing cycles, deriving a relationship between the speed changes and amounts of fuel supplied to the cylinder during the plurality of cylinder firing cycles, and deriving from the relationship an indication of whether the cylinder is misfiring.

2. A method as claimed in Claim 1, wherein the speed changes produced by each cylinder of the engine are measured and the relationship is derived for each of the cylinders to allow a misfiring cylinder to be identified.

3. A method as claimed in Claim 1 or Claim 2, wherein the relationship is derived by fitting a linear function to the set of corresponding speed changes and fuel amounts.

4. A method as claimed in Claim 3, wherein the relationship is derived by using a least square fitting technique.

5. A method as claimed in Claim 3, wherein the relationship is derived by forming a ratio of the scatter of speed changes to the scatter of the fuel amounts.

6. A method as claimed in any one of the preceding claims, wherein the amount of fuel supplied is determined by monitoring the axial positidn of a fuel pump rotor.

7. A misfire detector for an internal combustion engine, comprising means for measuring speed changes produced by a cylinder of the engine in a plurality of cylinder firing cycles, means for deriving a relationship between the speed changes and amounts of fuel supplied to the cylinder during the plurality of cylinder firing cycles, and means for deriving from the relationship an indication of whether the cylinder is misfiring.

8. A misfire detector as claimed in Claim 7, wherein the amounts of fuel supplied are determined by monitoring the axial position of a fuel pump rotor.

9. A method of detecting misfire substantially as hereinbefore described with reference to the accompanying drawings.

10. A misfire detector substantially as hereinbefore described with reference to the accompanying drawings.