GB2512102A

GB2512102A - Method and apparatus for identifying unstable combustion in an internal combustion engine

Info

Publication number: GB2512102A
Application number: GB1305134.7A
Authority: GB
Inventors: Tom W Carlill
Original assignee: Perkins Engines Co Ltd
Current assignee: Perkins Engines Co Ltd
Priority date: 2013-03-20
Filing date: 2013-03-20
Publication date: 2014-09-24
Anticipated expiration: 2033-03-20
Also published as: GB2512102B; GB201305134D0

Abstract

A method of, and controller for, detecting unstable combustion (mis-fire) in an internal combustion engine (e.g. in a vehicle). The method comprises the steps of measuring an exhaust gas parameter (e.g. amount of nitrous oxide or oxygen in the exhaust gas), and detecting unstable combustion by comparing the measured exhaust gas parameter with an expected exhaust gas parameter value. Unstable combustion may be detected if the difference between the measured parameter and the expected value is greater than a threshold amount. The expected value may be determined using a map, with an input to the map comprising a start of ignition (SOI) retardation degree angle of the engine, and optionally at least one of a measured air temperature and a measured air pressure.

Description

METHOD AND APPARATUS FOR IDENTIFYING UNSTABLE COMBUSTION IN

AN INTERNAL COMBUSTION ENGINE

Technical field

The present disclosure relates to a method and apparatus for identifying unstable combustion in an internal combustion engine -

Background

In order to meet emissions requirements, internal combustion engines, for example diesel engines, often have exhaust aftertreatment devices fitted. These devices are intended to reduce or remove certain particles from the exhaust gases before they are released into the atmosphere. One example of an exhaust aftertreatment device is a selective catalytic reduction (SCR) device that is intended to remove nitrogen oxides (NOx) from the exhaust gas. A further example is a diesel particulate filter (DPF) that is intended to remove diesel particulate matter or soot from the exhaust gases.

Exhaust gas aftertreatment devices may have a minimum temperature at which they begin to operate. Therefore, it may be desirable to ensure that the device temperature does not drop below its minimum operating temperature otherwise the device may not operate correctly and emissions requirements may not be met.

There are a number of techniques that are used to increase the temperature of exhaust gas aftertreatment devices to ensure that they do not drop below their minimum operating temperature. One technique that may be used in diesel engines is to retard the start of ignition (501) of the diesel within the engine cylinders. In this technique, diesel may be injected into the cylinder at such a time during the piston cycle that ignition of the diesel begins after the piston has passed top-dead-centre (TDC) . For example, 501 may take place when the rotating shaft driving the piston has rotated 5 degrees past its TDC position, i.e. SOT is retarded by 5 degrees. The result of this is that combustion may occur later in the cycle causing the exhaust gas to be hotter at the time that the exhaust valve opens and the exhaust gases are expelled from the cylinder. As the hotter exhaust gases pass through the exhaust gas aftertreatment device, the device temperature may be increased.

A further advantage of retarding the SOl may be that the temperature of the engine cylinder after the previous combustion event remains high for longer, which allows further opportunity for soot in the cylinder to burn off, thus reducing soot emissions.

A further advantage of retarding the SOl may be that the peak combustion cylinder pressure of the engine is reduced.

By reducing the peak combustion cylinder pressure, more fuel may be injected without causing mechanical failure. By injecting more fuel, more power may be generated by the engine.

However, if the SOl is retarded too far, the pressure within the cylinder may be insufficient to induce stable combustion. This may cause unstable combustion, for example a misfire, which reduces the engine power output and increases the generation of soot from inefficient combustion. Therefore, it may be desirable to retard 501 as far as possible when the exhaust gas aftertreatment device temperature needs increasing, to burn of f more soot in the cylinders or to increase engine output power, whilst still avoiding unstable combustion within the cylinders.

Us 2012/0102923 Al describes a method of after-treatment device heating with engine combustion feedback. The method uses at least one pressure sensor disposed within at least one cylinder of the engine in order to identify an unstable combustion event, i.e. an engine misfire. If an unstable combustion event is detected by the pressure sensor, one or both of an air mass inflow rate and/or a near post fuel injection rate may be adjusted to eliminate the unstable combustion event and prevent any further unstable combustion events.

However, pressure sensors within cylinders are non-standard in a lot of engines, and the cost of the sensor and fitting the sensor may be very high. Furthermore, such pressure sensors may malfunction during the life of the engine, requiring them to be changed on a regular basis. Therefore, in a number of engines it may either not be possible, or be very expensive, to use a pressure sensor within a cylinder in order to detect misfiring.

Summary

The disclosure provides a method for detecting unstable combustion in an internal combustion engine, the method comprising the steps of: measuring an exhaust gas parameter; and detecting unstable combustion by comparing the measured exhaust gas parameter with an expected exhaust gas parameter value.

The disclosure also provides a controller for detecting unstable combustion in an internal combustion engine, the controller being configured to: measure an exhaust gas parameter; and detect unstable combustion by comparing the measured exhaust gas parameter with an expected exhaust gas parameter value.

Brief description of the drawings

A method and apparatus for detecting an unstable combustion event in an internal combustion engine is described by way of example only with reference to the accompanying drawings, in which: Figure 1 shows a schematic representation of an engine arrangement comprising an internal combustion engine, an exhaust gas aftertreatment device, an exhaust gas sensor and a controller; Figure 2 shows an example of how the measurement by the exhaust gas sensor of nitrous oxides and oxygen may vary as the start of ignition is retarded; Figure 3 shows an example vehicle in which the engine arrangement of Figure 1 may be utilised.

Detailed description

Figure 1 represents an internal combustion engine 110, for example a diesel engine, with an exhaust gas stream 115 that enters an exhaust gas aftertreatment device 120, for example a selective reduction catalyst (SCR) -The exhaust gas aftertreatment device 120 is intended to reduce or remove certain particles from the exhaust gases before they are released into the atmosphere so that emissions targets may be met. For example, an SCR device is intended to remove nitrogen oxides (NOx) from the exhaust gas before an output exhaust gas 125 is released into the atmosphere.

In the exhaust gas stream 115 upstream of the exhaust gas aftertreatment device 120 there may be located an exhaust gas sensor 140. The exhaust gas sensor 140 may send a signal 145 to a controller 130 indicating the amount of oxygen (02) and/or nitrous oxides (NOx) in the exhaust gas stream 115. The controller 130 may control aspects of the operation of the engine 110 using a control signal line 135.

Using the control signal line 135, the controller 130 may alter a start of ignition (SQl) retardation degree in the cylinders of the engine 110. An 501 retardation delay of 00 represents ignition of a fuel injected into a cylinder of the engine 110 taking place when the piston within the cylinder is at top-dead-centre (TDC) . An 501 retardation delay of, for example, 5° represents ignition of the fuel taking place when the piston within the cylinder has moved past TDC by an amount corresponding to a 5° rotation of an engine crankshaft. The 501 retardation degree may be controlled by the controller 130 by altering the fuel injection timings of the engine 110, such that in order to increase the 501 retardation degree (i.e. increase the number of degrees beyond TDC at which 501 takes place) fuel is injected into the cylinder later.

Figure 2 shows a graph of how 02 and NOx measured by the exhaust gas sensor 140 may vary with changes in the 901 retardation degree whilst the engine 110 is operating in steady state conditions, for example idling. Between a 0° 901 retardation degree (i.e. 901 taking place at TDC) and an a 901 retardation degree, for example 5° or 8°, combustion may take place normally. As can be seen from Figure 2, measurements of NOx and 02 between 0° and a may have a predictable characteristic. Whilst Figure 2 shows NOx and 02 linearly decreasing as the 901 retardation degree increases, they may instead behave in some other, predictable way, for example they may remain constant or they may increase or decrease in a linear or non-linear, but predictable, fashion.

At 501 retardation degrees greater than a, unstable combustion, for example misfiring, may take place within the cylinders as a consequence of the cylinder pressure not being sufficiently high to cause stable combustion. When misfiring is taking place, the measured NOx and 02 characteristics may change from the predictable characteristics that exist between 0° and a, as shown in Figure 2. The NOx measurement may reduce due to incomplete combustion of the fuel and the 02 measurement may rise because of residual °2 after incomplete combustion within the cylinder. However, for different engines and different operating conditions, the NOx and 02 measurements during unstable combustion may differ from the predicable characteristics between 0° and a in some other way. For example, 02 measurements may instead rapidly decrease and/or NOx measurements may rapidly increase. Regardless of how the 02 and NOx measurements behave when the 801 retardation degree is greater than a, the behaviour will be different to their behaviour at retardation degrees between 00 to a.

whilst the engine 110 is operating in steady state conditions, the controller 130 may perform an unstable combustion identification routine. In this routine, the controller may set about identifying a. This may be performed whilst the engine 110 is operating in steady state conditions because 801 retardation degree may then be the most significant factor in NOx and/or 02 measurement1 without any changes in NOx and/or 02 being caused by changes in engine speed and power output.

In a first unstable combustion identification routine, the controller 130 may adjust the fuel ignition timings so that 801 occurs at TDC, or at a small amount of 801 retardation degree, for example 2°. At this degree of retardation, combustion may take place normally. The controller 130 may then gradually increase the 801 retardation degree and record the NOx and/or 02 levels measured by the exhaust gas sensor 140. In this way, the controller 130 may move from the left towards to the right of the graph shown in Figure 2 and record the N0x and/or 02 levels at different 801 retardation degrees.

After a number of measurements have been made at increasing 801 retardation degrees, for example four measurements, the controller 130 may identify a predictable characteristic of the NOx and/or 02 levels, i.e. a trend line. Using the trend line, the controller 130 may predict an expected NOx and/or 02 measurement level for a given 801 retardation degree, for example the next increment in 801 retardation degree, which may then be compared with the actual NOx and/or 02 measurement at that given 801 retardation degree.

If the difference between the measured NOx and/or 02 and the expected NOx and/or 02 is less than a threshold amount, the controller 130 may consider combustion still to be stable, after which the process may be repeated at a greater 801 retardation degree. If, however, the difference is greater than the threshold amount, the controller 130 may consider combustion to be unstable and the 801 retardation degree at which combustion is considered to be unstable is a.

The threshold may set at a level at which small variations in NOx and/or 02 measurement may not result in the controller 130 erroneously considering combustion to be unstable, when in fact it is still stable. These small variations may be caused by variations in atmospheric conditions between measurements, exhaust gas sensor 140 inaccuracies and approximations in the trend line. The exact level of the threshold will be set in accordance with a number of factors, for example expected engine operating conditions and engine type and size.

In a second, alternative unstable combustion identification routine, the controller 130 may measure NOx and/or °2 at an 801 retardation degree using the exhaust gas sensor 140 and compare the measurement with an expected NOx and/or 02 value for that 801 retardation degree.

If the difference between the measured NOx and/or 02 and the expected NOx and/or 02 value is less than a threshold amount, the controller 130 may consider combustion still to be stable, after which the process may be repeated at a greater 501 retardation degree until unstable combustion is detected. If, however, the difference is greater than the threshold amount, the controller 130 may consider combustion to be unstable, after which the process may be repeated at a lesser 501 retardation degree until stable combustion is detected. When detected combustion goes from stable to unstable, or vice-versa, the controller 130 may know the SOT retardation degree at which combustion is considered to be

unstable, a.

Expected NOx and/or 02 values may be obtained during engine development, wherein an engine of approximately average wear may be used for measuring NOx and/or 02 at different 501 retardation degrees. From these engine development measurements, an expected NOx and/or 02 value map may be prepared, which may be used by the controller 130 to return an expected NOx and/or 02 value for a given SOT retardation degree.

During engine development, NOx and/or 02 measurements at different S0I retardation degrees may also be taken at different air temperatures and air pressures, so that the expected NOx and/or 02 value map may be prepared which also considers measured air temperature and pressure. This is because the air temperature and pressure in which an engine is operating may affect the SOT retardation degree at which unstable combustion begins. Where development testing has taken place at different air temperatures and pressures, the controller 130 may use the map to return an expected NOx and/or 02 value for a given SOT retardation degree, measured -10 -air temperature and measured air pressure. Measurement of air temperature and air pressure may be performed using any techniques well known to the skilled person, for example using a temperature sensor and a barometer.

The threshold may be set at a level at which small variations in NOx and/or 02 measurement may not result in the controller 130 erroneously considering combustion to be unstable, when in fact it is still stable. These small variations may be caused by a number of factors, for example different levels of wear between the engine 110 and the engine used during engine development for the creation of the expected N0x and/or 02 map, as well as inaccuracies in the exhaust gas sensor 140 and the temperature and pressure sensors. The exact level of the threshold will be set in accordance with a number of factors, for example expected engine operating conditions and engine type and size.

Having identified the 901 retardation degree a, the controller 130 may adjust the way in which the engine 110 is controlled to optimise engine performance, or in order to perform engine diagnostics.

If the unstable combustion identification routine described above is performed for all of the engine cylinders at once, unexpected fuel cetane levels may be identified. This may be achieved by comparing a with a predicted 901 retardation degree, 13, at which unstable combustion is expected to occur.

13 may be identified during engine development, in a similar way to the preparation of the map described above in respect -11 -of the second combustion identification routine. During engine development, a fuel with an average cetane level may be used and the 901 retardation degree at which unstable combustion occurs may be recorded as. Therefore, when the controller 130 is performing the unstable combustion identification routine for all cylinders in the engine 110 at once in order to check the fuel cetane levels, it may retrieve 3 and then compare a with 3. 3 may be dependent upon air temperature and pressure, and so the controller 130 may retrieve r for a particular measured air temperature and air pressure.

If a is greater than 1 (i.e. if unstable combustion is occurring at a higher 901 retardation degree than expected) the controller may infer that fuel cetane levels are higher than the average fuel cetane levels used during the engine development tests. This is because a fuel with higher cetane levels may more readily combust and may therefore continue to combust stably at lower cylinder pressures than a fuel with lower cetane levels. If high cetane levels are inferred, the controller 130 may adjust its engine control routines to allow for a higher level of 901 retardation.

This means that additional 901 retardation degrees may be utilised by the controller 130 in the future in order to achieve even higher exhaust gas aftertreatment device 120 temperatures, or to achieve improved cylinder soot burning, or to reduce peak combustion cylinder pressure thereby increasing power output, without causing unstable combustion -If a is less than (i.e. if unstable combustion is occurring at a lower 901 retardation degree than expected), -12 -the controller 130 may infer that fuel cetane levels are lower than the average fuel cetane levels used during the engine development tests. Therefore, the controller 130 may adjust its engine control routines to restrict the allowable SOl retardation degree, in order to prevent unstable combustion from occurring in the future. Furthermore, because low cetane levels may result in poor exhaust emission readings from the engine 110, the controller 130 may also set a low cetane flag so that if engine emissions are investigated, failure to meet emissions requirements may be explained by poor fuel cetane levels.

Therefore, by performing this testing, the controller 130 may trim its engine control routines in order to improve engine 110 performance.

Alternatively, if one of the unstable combustion identification routines described earlier is performed for one engine cylinder at a time, engine and fuel system issues may be identified.

If the identified a for one cylinder is considerably lower than the identified a for at least some of the other cylinders (i.e. unstable combustion occurs at a lower 501 retardation degree for one cylinder than it does for other cylinders) then there may be a problem with that cylinder.

The problem might be, for example, with the cylinder fuel injector. Over time, fuel injector timing may drift and the time at which fuel is being injected into a cylinder may be later in the piston cycle than the controller 130 expects.

This may result in SOl taking place in that cylinder at a -13 -higher retardation degree than intended by the controller 130, thereby causing unstable combustion in that cylinder.

The controller 130 may rectify this situation by adjusting its control routines for that particular cylinder so as to move instructed fuel injection earlier in the piston cycle, thus also moving the actual fuel injection timing to the same time as the other cylinders. An error flag may also be set for the cylinder to indicate that the cylinder is defective so that the operator may be informed that there is something wrong with the cylinder or its fuel system and have the engine 110 inspected. This may also be useful during routine inspection of the engine 110 so that a technician may more quickly identify where there are problems with the engine 110.

The problem with the cylinder might also or alternatively be caused by a lower compression ratio in that cylinder, which may be the result of, for example, mechanical failure of a gasket or a piston ring. Again, the controller 130 may compensate for this in the same way as above by adjusting its control routines to move the instructed fuel injection timings earlier in the piston cycle for that cylinder so as to restrict the allowable SQl retardation degree. By doing so, the fuel may be injected into the cylinder whilst the cylinder pressure is higher, thereby preventing unstable combustion in the cylinder. An error flag may again be set for the cylinder as described above to indicate that the cylinder is defective.

Whilst this cylinder diagnostic routine may not be able to identify exactly what the problem with a cylinder is, i.e. -14 -if it is a fuel injector issue, or a piston ring failure etc, it may still identify that there is some problem with a particular cylinder, take steps to prevent unstable combustion in that cylinder and flag that there is a problem with that cylinder so that it may be repaired.

Figure 1 also shows a controller 130 that may be configured to perform the method steps described above. The controller may receive a measurement signal 145 of NOx and/or 02 in the exhaust gas stream from the exhaust gas sensor 140, using which it may perform the unstable combustion identification routines and engine performance optimisation and diagnostics routines described above. The controller may have any number of further inputs, for example it may also have at least one of an air temperature measurement from a temperature sensor and an air pressure measurement from a barometer. The controller 130 may also have a control signal line 135 for controlling the operation of the engine 110, for example by adjusting fuel injection timings in order to alter the SOl retardation degree.

The controller 130 may be implemented as a stand-alone control unit, or may be incorporated into an engine control unit, for example a Caterpillar AG:E2v2 ECU.

The skilled person will readily appreciate that a number of alternatives to the implementations described above may be used.

For example, the exhaust gas sensor 140 may be located anywhere in the exhaust gas stream of the engine 110, for example it may be upstream of the aftertreatment device 120, -15 -within the aftertreatment device 120 or downstream of the aftertreatment device 120. Where the exhaust gas sensor 140 is located will affect the relationship of NOx and 02 with SQl retardation degree, but the relationship may still have a predictable characteristic between 00 and a and a different characteristic above a. Therefore, the 501 retardation degree at which unstable combustion occurs may still be identified using the unstable combustion identification routines described above.

Furthermore, two or more exhaust gas sensors may be used, for example a first exhaust gas sensor upstream of the aftertreatment device 120 and a second exhaust gas sensor downstream of the aftertreatment device 120. The readings from both exhaust gas sensors may be used to determine the 501 retardation degree, a, at which unstable combustion occurs.

Furthermore, the exhaust gas sensor 140 may measure only NOx or only 02, in which case only NOx or 2 measurements may be used for determining a. Alternatively, the exhaust gas sensor 140 may measure both NOx and °2, in which case both measurements may be used for determining a. Alternatively, two separate exhaust gas sensors, one measuring NOx and the other measuring 02 may be used, in which case both measurements may be used for determining a.

Furthermore, whilst the above disclosures describe the measurement of NOx and/or 02, other exhaust gas parameters may be used. For example, the level of CO2 in the exhaust gas may be measured using a CO2 sensor, or the level of hydrocarbons in the exhaust gas may be measured using a -16 -hydrocarbon (HC) sensor. In both of these examples, the sensor measurements should have a predictable characteristic during normal, stable combustion (i.e. at an SOl retardation degree of 0° to a) and a different characteristic during abnormal combustion (i.e. at an 501 retardation degree greater than a) . Therefore, unstable combustion should still be identifiable in the same way as described above in respect of NOx and/or °2 measurement.

Industrial applicability

The present disclosure provides a method and apparatus for identifying unstable combustion using an exhaust gas parameter reading. The exhaust gas parameter reading may be obtained from a NOx and/or 02 sensor in the exhaust gas stream, which is included as a standard component in engine arrangements with exhaust gas aftertreatment devices.

Therefore, unstable combustion may be identified without the need for additional, expensive, specialist sensors.

Furthermore, the method and apparatus may be utilised for trimming engine control processes by identifying fuel quality and/or cylinder and fuel line wear and optimising the control processes to improve engine performance without inducing unstable combustion.

Claims

-17 -Claims 1. A method for detecting unstable combustion in an internal combustion engine, the method comprising the steps of: measuring an exhaust gas parameter; and detecting unstable combustion by comparing the measured exhaust gas parameter with an expected exhaust gas parameter value.
2. The method of claim 1, wherein unstable combustion is detected if a difference between the measured exhaust gas parameter and the expected exhaust gas parameter value is greater than a threshold amount.
3. The method of either claim 1 or claim 2, wherein the expected exhaust gas parameter value is determined using an expected exhaust gas parameter value map, an input to the expected exhaust gas parameter value map comprising: a start of ignition retardation degree of the engine.
4. The method of claim 3, wherein the input to the expected exhaust gas parameter value map further comprises at least one of: a measured air temperature; and a measured air pressure.
5. The method of either claim 1 or claim 2, wherein the expected exhaust gas parameter value is determined using a trend line that returns an expected exhaust gas parameter value for a given start of ignition retardation degree of the engine, wherein the trend line is determined by: -18 -recording a plurality of measured exhaust gas parameter values during stable engine combustion, each measurement being taken at a different start of ignition retardation degree; and identifying a trend line from the plurality of measured exhaust gas parameter values.
6. The method of any preceding claim, wherein: a start of ignition retardation degree for all cylinders in the internal combustion engine is adjusted until unstable combustion takes place.
7. The method of claim 6, wherein the start of ignition retardation degree at which unstable combustion is detected is compared with an expected start of ignition retardation degree at which unstable combustion is expected, and if unstable combustion is detected at a start of ignition retardation degree that is less than the expected start of ignition retardation degree at which unstable combustion is expected, an engine control routine is adjusted to restrict an allowable start of ignition retardation degree.
8. The method of either claim 6 or claim 7, wherein the start of ignition retardation degree at which unstable combustion is detected is compared with an expected start of ignition retardation degree at which unstable combustion is expected, and if unstable combustion is detected at a start of ignition retardation degree that is greater than the expected start of ignition retardation degree at which unstable combustion is expected, an engine control routine -19 -is adjusted to increase an allowable start of ignition retardation degree.
9. The method of either of claims 7 or 8, wherein the expected start of ignition retardation degree at which unstable combustion is expected is determined with consideration to at least one of a measured air temperature and a measured air pressure.
10. The method of any preceding claim, further comprising the steps of: determining a start of ignition retardation degree at which unstable combustion in a cylinder of the internal combustion engine is detected; and comparing the determined start of ignition retardation degree for the cylinder with a start of ignition retardation degree at which unstable combustion in a plurality of other cylinders in the internal combustion engine is detected, wherein a cylinder in which unstable combustion occurs at a start of ignition retardation degree that is less than the start of ignition retardation degree at which unstable combustion occurs for the plurality of other cylinders in the engine is a defective cylinder.
11. The method of claim 10, wherein an engine control routine is adjusted to restrict an allowable start of ignition retardation degree for the defective cylinder.
12. The method of any preceding claim, wherein the exhaust gas parameter comprises at least one of an amount of nitrous oxides in the exhaust gas and an amount of oxygen in the exhaust gas.

-20 -
13. A controller for detecting unstable combustion in an internal combustion engine, the controller being configured to: measure an exhaust gas parameter; and detect unstable combustion by comparing the measured exhaust gas parameter with an expected exhaust gas parameter value.
14. An engine arrangement comprising: an internal combustion engine; an exhaust gas aftertreatment device arranged in an exhaust gas stream of the internal combustion engine; an exhaust gas sensor located within the exhaust gas stream; and the controller of claim 13 configured to receive a reading from the exhaust gas sensor.
15. A vehicle comprising the engine of claim 14.