Patent Description:
In multi-cylinder spark-ignition internal combustion engines, studies reveal that undesired combustion modes create explosion waves or over-pressures that can ultimately cause irreversible destruction of the engine parts. In modern automobile vehicles, these undesired combustions modes are usually detected by processing signals from a vibration sensor located in the lower engine block called knock sensor. The knock sensor is configured to measure the engine block vibrations during a specific crank angular window, and to detect abnormal ignition events. Nevertheless, knock sensor detection is limited to combustions that produce noticeable vibration patterns in a predefined timing range.

One particular abnormal combustion type called glow-ignition occurs when the combustion is initiated in a cylinder by a hot spot or an incandescent particle before the normal spark. The combustion after a glow-ignition event has a normal behavior in the sense that the propagation is done with a flame front like in a normal combustion, but glow-ignition generates undesired high maximum pressure in the cylinder. Occurrences of this type of abnormal combustions do not generate pressure peaks that may be sensed using conventional knock sensors.

As an alternative to knock sensor detection, it is known to process signals from sensors already installed in the engine to detect abnormal combustion modes or misfires, such as for example signals from a crank position sensor. As it is known, the crank position sensor is designed to cooperate with a target wheel that is rotationally integral with the crankshaft. The target wheel has a peripheral pattern of teeth and slots and the sensor is arranged to detect the passage of the teeth, generating a corresponding sensor signal with a pulse train. The signal generated by the crank position sensor permits determining the instantaneous position and rotational speed of the crankshaft.

<CIT>, for example, uses the crank sensor signal for identifying stochastic pre-ignition. The method described therein comprises a pre-ignition detection module that determines whether a pre-ignition event has occurred based on so-called "periods", where a period is the duration between two consecutive timestamps (corresponding to a pulse generated by a tooth of the target wheel). More specifically, the method compares differences between time periods (deltas).

Another example of a similar system is shown in <CIT>.

An object of the present invention is to provide an improved method for detecting glow-ignition events in an internal combustion engine.

According to the present invention, a method for monitoring glow ignition in an internal combustion engine comprises among others the following steps:.

In the context of the invention, the term glow-ignition is used in its conventional meaning and thus designates, for a spark-ignition engine, an ignition occurring early in the combustion cycle, typically during the compression stroke. Glow-ignition is generally generated by a hot spot or an incandescent particle in the cylinder.

The invention is made on the insight that glow ignition, while not causing vibrations, provokes a significant early rise of the cylinder pressure. The high pressure of a glow-ignition is generated during the compression stroke of the respective cylinder. The pressure change provoked by the glow-ignition goes against the movement of the piston towards the firing top dead center, and further increases the ongoing deceleration of the piston in the cylinder.

That is, early rise of pressure due to glow-ignition slows down the piston before the firing top dead center noticeably more than for a normal combustion. As a result, when glow ignition is present, the crankshaft speed drops, towards the end of the compression phase, to lower values and at a steeper rate, as compared to a normal combustion. The method of the invention proposes monitoring, for an engine cylinder, the deceleration of the crankshaft occurring in relation to the compression in the respective cylinder, and determining a glow indicator based on a deceleration gradient of the crankshaft that characterizes the steepness of the deceleration, which will be affected by the presence of glow ignition.

It will be appreciated that in the present method, the deceleration gradient is determined between the first point defined by a predetermined angular position and the second point that is the lowest speed point reached during the deceleration. The first point is defined by calibration, typically to be in a part of the deceleration phase that remains substantially stable for both normal and abnormal combustions. It has however been observed that the position of the second point depends on the occurrence of glow ignition, in which case the engine speed drops to lower values at an earlier crank position.

It may further be noted that in the present method the deceleration gradient is preferably computed over a significant portion of the deceleration, much larger than the crank angle separating two teeth of a target wheel. In other words, the deceleration gradient is determined over a crank angle range corresponding to a plurality of teeth of the target wheel, which is more reliable than a gradient that would be computed between two consecutive teeth, more likely to be affected by artefacts.

The point of lowest speed is preferably the point of minimum crankshaft speed reached in the detection window, but may alternatively lie within a predetermined deviation from the minimum crankshaft speed.

The predetermined crankshaft angle of the first point may be between -<NUM>° and -<NUM>° before the firing top dead center, preferably between -<NUM> and -<NUM>°.

The detection window is calibrated to encompass the phase of interest of the engine cycle, and should at least extend from the angular position of the first point to an angular position including the minimum speed of a normal combustion. In practice, the detection window may, e.g., range from -<NUM>° to +<NUM>° of crank angle. The point of minimum speed is thus determined in the window as the point of lowest speed.

The crankshaft position may be conventionally monitored by means of a crank position sensor arranged to sense a target wheel comprising a predetermined tooth/slot pattern and rotationally coupled with the crankshaft.

As regards the predetermined deceleration threshold, it may be defined in any appropriate manner. In particular, it may be based on earlier threshold values.

In embodiments, the deceleration threshold Thrshdecel may be calculated using the formula <MAT> wherein Avgdecel is the average of previous deceleration gradient values; and Offsetdecel is a predetermined offset characterizing the limit between a normal combustion and a glow-ignition combustion mode.

In particular, the average of the deceleration gradient Avgdecel may be calculated for one or more previous values of deceleration gradient for all engine cylinders. Furthermore, Offsetdecel may be a fraction of Avgdecel, preferably the half of the average of the deceleration gradient.

Further details and advantages of the present invention will be apparent from the following detailed description of not limiting embodiments with reference to the attached drawings, wherein:.

As it is known in the art, in a spark-ignition internal combustion engine such as a gasoline operated engine, a number of sensors are provided to monitor engine parameters, including a crank position sensor for monitoring the angular position of the crankshaft. The crank position sensor is typically arranged to cooperate with a toothed wheel, or target wheel, that is rotationally integral with the crankshaft, generating a sensor signal with a pulse train as the respective teeth pass in front of the sensor. The crankshaft rotational speed is determined base on this signal. The crankshaft speed is of course also referred to as engine speed, expressed in RPM, and both terms may be used as equivalents herein.

Referring to <FIG>, a crank position sensor <NUM> is arranged to cooperate with a target wheel <NUM> secured in rotation to a crankshaft <NUM> of an engine. The crankshaft <NUM> is connected to a piston <NUM> mounted in an engine cylinder <NUM>, defining therewith a combustion chamber <NUM>. Although <FIG> only shows one piston <NUM> and one cylinder <NUM>, the engine generally includes <NUM>, <NUM> or up to <NUM> cylinders.

Conventionally, the target wheel <NUM> may be an N-toothed wheel <NUM> comprising a plurality of teeth <NUM> spaced by gaps. Here, the target wheel <NUM> comprises <NUM> teeth <NUM> that are equally spaced by gaps, as well as a larger gap <NUM> where <NUM> equally spaced teeth are missing. Hence, two consecutive teeth are separated by a rotational distance of approximately <NUM>° (<NUM>°/<NUM>=<NUM>°), except around the gap <NUM>. The gap <NUM> serves as a synchronizing reference.

The crank position sensor <NUM> monitors passing of the teeth <NUM> of the rotating target wheel <NUM>. The sensor <NUM> may for example comprise a Hall Effect sensor or an inductive sensor. The sensor shown in <FIG> creates a crank position signal comprising pulses generated each time a tooth of the target wheel <NUM> passes in front thereof.

In other embodiments any suitable combination of target wheel and sensor unit may be used. For example, the target wheel may alternatively be a wheel with visual signs associated with an optical sensor.

The crank position sensor <NUM> is connected to a control module <NUM>, which receives the signal generated by the sensor <NUM> and determines the crankshaft position based on the pulse train. The control module <NUM> is configured to generate a timestamp for each pulse, and subsequently determines the time period between two teeth/pulses, and calculates a rotational speed of the crankshaft based on the period and the distance between the two teeth. The crankshaft speed is thus determined for each predetermined angular position defined by a target wheel tooth.

As used herein, the term normal combustion designates combustion that is initiated by a spark of the spark plug commanded by an engine control unit (ECU). Normal combustion creates normal conditions leading to combustion of the air/fuel mixture and providing torque, without occurrence of abnormal phenomena such as knock, pre-ignition or glow-ignition. The terms glow-ignition and pre-ignition refer to events initiating an abnormal combustion as opposed to combustion in normal conditions.

The present method for identifying glow-ignition in an internal combustion engine is based on the determination of a decelaration gradient of the crankshaft during the deceleration phase thereof prior to firing in a given engine cylinder. The deceleration gradient is however determined for a specific part of this deceleration phase. Indeed, it has been observed that the presence of glow ignition substantially affects the crankshaft speed.

Let us turn to <FIG>, which shows a graph of the in-cylinder pressure vs. crank angle (CA) in an engine cylinder for a normal combustion cycle represented by line <NUM>, a combustion cycle where glow ignition is present represented by line <NUM> and two combustion cycles where pre-ignition is present, indicated <NUM>. The graph in <FIG> represents the crankshaft speed vs. CA for the same combustion cycles. The pressure graph reflects what is truly happening in the combustion chamber. One can indeed observe that glow ignition and pre-ignition are characterized by significant pressure peaks, however at different times. Whereas glow ignition occurs in the late compression phase, before the firing top dead center, pre-ignition occurs later, rather after top dead center.

Looking at the crankshaft rotational speed, one can notice that it describes a sine-wave like curve. Indeed, the crankshaft speed varies in correlation with the piston strokes in the various cylinders. It can be seen that the crankshaft speed decreases before the firing TDC (due to compression stroke) and increases thereafter (due to combustion/expansion). For a <NUM> cylinder engine, the crankshaft speed will describe <NUM> sine wave periods over a complete combustion cycle (<NUM>°).

In <FIG>, it can be further noticed that when glow ignition is present, the crankshaft speed drops, towards the end of the compression phase, to lower values and at a steeper rate, as compared to a normal combustion. The minimum of the curve moves to the left, i.e. to earlier CA values. As can also be seen, pre-ignition events rather affect the engine speed after f-TDC. Hence, the analysis of the engine speed in this region of the engine speed curve permits identifying the occurrence of glow iginition.

An embodiment of the present method for identifying glow-ignition will now be explained in detail. In a first step, the crankshaft speed is monitored. This is typically done based on the signal of the crank position sensor <NUM>. As it is known, the engine ECU or a module thereof, is conventionally configured to analyze the sensor signal in order to attribute a timestamp to each crank angle position. An instantaneous engine speed is calculated for each crank position (corresponding to each tooth) based on the time period between two consecutive pulses/teeth. In a conventional engine, the engine speed is constantly monitored by the crank sensor, whereby no additional sensor is required for implementing the present method.

As it is known in the art, the raw sensor data are advantageously processed in the ECU to reduce the influence of measurement artefacts due to design imperfections, such as e.g. a backlash in the sensor wheel center position, or interferences caused by vibrations. The aim of such treatment is to obtain a rather smooth sine-line speed variation. Processing may include any suitable signal processing method known in the art to smoothen the resulting curve such as for example gap interpolation combined with dual stage Gaussian filter. This is only an example and those skilled in the art may apply any appropriate signal preparation.

The ECU maintains map with the crankshaft angular position and corresponding rotational speed over one or more engine cycle, for access by all modules and functions. This map thus stores points defined by the couple (CA; RPM).

For the purpose of glow detection, engine speed data, together with corresponding angular positions, are received at a glow detection module (also generally part of the ECU). At the level of one cylinder, the glow detection module will consider the speed and angular position data in a detection window corresponding to the crankshaft deceleration phase related to a compression stroke of this cylinder. Turning to <FIG>, the engine speed is plotted vs. crank angle. Each dot represents a point (CA; RPM), as read from the engine speed map. As is customary in the art, the crank angle value <NUM>° corresponds to the firing Top Dead Center (f-TDC) of the monitored cylinder. One will recognize the sine wave described by the engine speed, with a minimum near the f-TDC. The engine speed decrease occurs mostly on the left of f-TDC, i.e. during compression stroke (CA<<NUM>°). After passing through the minimum, the engine speed increases again due to the combustion in the respective cylinder.

The glow ignition indicator used in the present method is a deceleration gradient computed over a crank angle range spanning over several speed points, thus over several teeth of the target wheel. The deceleration gradient is thus between two points:.

Pmin corresponds to the point with minimum speed reached during the deceleration. It is easily determined by comparison from the values stored in the engine speed table.

Pinit is a predefined point that is typically determined by calibration. It is set so that there is a minimum crank angle difference between Pinit and Pmin. Pinit is preferably defined as corresponding to a predefined crank angle (e.g. tooth number N of the target wheel). Pinit is selected to be on a part of the deceleration curve that is not affected by the occurrence of abnormal combustion.

Once the values of Pinit and Pmin have been determined from the PRM-CA map within the detection window of interest, the deceleration gradient may be calculated as: <MAT>.

As discussed above, in the presence of glow ignition, Pmin will move to the left and to lower values, causing ΔNdecel to increase and Δαdecel to decrease. As a result, the deceleration gradient <MAT> increases.

It may be observed that the deceleration gradient is determined on the basis of a variable tooth of the target wheel, since in case of glow ignition the minimum speed (Pinit) will occur earlier than for a normal combustion (and thus for a different tooth). By contrast the initial point Pinit corresponds to a fixed tooth.

The so-determined glow indicator is compared to a detection threshold. When the glow indicator exceeds the detection threshold, glow ignition is considered to have occurred in the corresponding combustion cycle. Otherwise, the combustion is considered as normal.

In particular, the detection threshold Thrshdecel may be determined as: <MAT>.

Avgdecel is an average of the last computed deceleration gradient (last cycle) for all engine cylinders. To reduce the weight of abnormal combustions in the calculation of the average, the deceleration gradients are weighted by multiplying deceleration gradient of normal combustion modes by <NUM>, and by multiplying deceleration gradients of abnormal combustion modes by an adjustable weight value inferior to <NUM>.

The predetermined offset may have any suitable value that, like for example half of the average deceleration.

The graph of <FIG> shows the evolution of the deceleration gradient over time for an engine with three cylinders. The points in the lower region of the graph -oscillating about the value <NUM>, correspond to normal combustions. The continuous line oscillating about the value <NUM> represents the detection threshold Thrshdecel. Finally points representing combustions where combustion has occurred are indicated for each cylinder (cyl. #<NUM>, cyl. #<NUM> and cyl.

Claim 1:
A method for monitoring glow ignition in an internal combustion engine, said method comprising:
monitoring a crankshaft speed of said engine over a detection window corresponding to a crankshaft deceleration phase related to a compression stroke in a given engine cylinder;
determining a first point (Pinit) defined by a predetermined crankshaft angle (αinit) and corresponding speed (Ninit) during said deceleration, the angle being in the detection window;
determining a second point (Pmin) defined by a crankshaft angle and corresponding speed (Nmin) when the lowest speed (αmin) is reached within said detection window;
determining a glow indicator based on a crankshaft speed deceleration gradient between the first and the second points;
comparing said glow indicator to a predetermined threshold in order to detect the presence of glow ignition when the glow indicator exceeds the detection threshold and the absence of glow ignition when the glow indicator does not exceed the detection threshold,
wherein the deceleration gradient is calculated using the formula <MAT>
wherein Ninit and αinit are the engine speed and crank angle defining said first point; and Nmin and αmin are the engine speed and crank angle defining said second point.