GB2487590A

GB2487590A - Method for operating a diesel/natural-gas internal combustion engine

Info

Publication number: GB2487590A
Application number: GB1101567.4A
Authority: GB
Inventors: Alberto Vassallo; Giovanni Avolio
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2011-01-28
Filing date: 2011-01-28
Publication date: 2012-08-01
Anticipated expiration: 2031-01-28
Also published as: GB2487590B; GB201101567D0

Abstract

A method for operating a diÂesel/natural-gas dual fuel internal combustion engine 10 to reduce emissions of unburned natural gas, comprising the steps of: monitoring a value of a parameter indicative of an engine torque demand, such as the position of an accelerator pedal, monitoring a value of a change-rate of this parameter, preventing the natural-gas from being injected into an inÂtake apparatus 17 of the engine, if the value of the engine torque demand parameter decreases below a threshold value thereof at a change-rate value that exceeds a change-rate threshold value. The method addresses the issue of unburned natural gas remaining in the intake manifold 15 as a lean fuel/air mix immediately after engine demand stops, and then being inducted into the combustion chamber 14 during the next few cycles. The method stops injecting the natural gas a few cycles before the diesel injection drastically reduces so that these few diesel only cycles can burn up the remaining natural gas.

Description

NETHCZ) FOR OPERATING A DThSF1/NMURAL-(S INTERNAL CUI&JSTICV E2CINE TEQftIflL FIflD The present invention relates to a method for operating a double fuel internal combustion engine, in particular an internal combustion en- gine (IC) which burns diesel fuel and compressed natural-gas, typi-cally methane.

It is known that diesel/natural-gas IC engines are currently used in some automotive applications, specially in commercial vehicles such as vans or pick-ups, which are principally, even if not exclusively 4j1 destined to be sold in developing countries that are rich of natural-gas.

As a matter of fact, a diesel/natural-gas IC engine is a conventional direct injection diesel engine, which is further equipped with a. nat-ural-gas supplying apparatus.

This apparatus corrvrises a natural-gas injector located in the intake manifold of the engine, a tank containing natural-gas under pressure,.

a pressure regulation valve connecting the tank to the injector, and -1: a dedicated control unit which controls the injection of natural-gas inside the intake manifold.

This control unit is generally referred as slave control unit, be-cause it is operatively subordinate to the engine control unit (ECU) that controls the injection of diesel fuel.

At the start of a diesel/natural-gas engine, only diesel fuel is sup-plied into the combustion charters. When the engine is properly warned up, the ECU reduces the quantity of diesel fuel that is in-jected into the combustion chambers (with respect to the conventional operation of a Diesel engine), while the slave control unit activates the natural-gas injector to inject dosed quantity of natural-gas into the intake manifold. This quantity of natural-gas mingles with the air and it is inducted therewith into the combustion chambers, where it is ignited by the heat released by the combustion of the diesel fluid.

The quantities of diesel fuel and natural-gas injected per engine cycle are generally regulated, by the ECU and by the slave control unit respectively, with the aim of optimizing the engine efficiency and thus reducing fuel consumption and pollutant emission.

However, it has been found that after each. engine cut-off (namely a complete release of the accelerator pedal), the exhaust gas of a di-esel/natural-gas IC engine generally contains an undue quantity of unburned natural-gas1 which makes this kind of engine very critical to fulfill the severe antipollution regulations of many countries.

The cause of this drawback is mainly in that, during the normal oper-ation of the engine, the natural-gas injected per engine cycle is nOt -.----completely inducted into the corresponding combustion charter, so that a small portion thereof always remains in the intake manifold and it is inducted into the combustion chamber only during the next engine cycle.

As a consequence, when an engine cut-off occurs, even if the slave control unit irrmediately interrupts the injection of natural-gas, it happens that in the next engine cycle the combustion chambers are still charged with a lean mixture of air and a small quantity of nat-ural-gas.

Besides, in this next engine cycle the ECU strongly decreases also the injection of diesel fuel, so that the heat released by the diesel fuel combustion is generally insufficient to ignite that lean mixture of air and natural-gas.

An object of an embodiment of the present invention is to solve, or at least to positively reduce, this drawback of the known di- esel/natural-gas IC engines, in order to decrease the pollutant ernis-sions.

Another object is to attain this end with a simple, rational and ra-ther inexpensive solution.

* DISCLOSURE

These and/or other objects are attained by the embodiments of the in-vention according to the independent claims. The dependent claims concern preferred or particularly advantageous features of the em--bodiments Of the invention.

In particular, an embodiment of the invention provides a method for operating a diesel/natural-gas internal combustion engine, typically of a motor vehicle, which conprises the steps of: -monitoring a value of a parameter indicative of an engine tor-que demand, -monitoring a value of a change-rate of this parameter, -preventing the natural-gas to be injected into an intake appa-ratus of the engine, if the value of the engine torque demand parameter decreases below a threshold value thereof at a change-rate value whose modulus exceeds a modulus of a change-rate threshold value.

The threshold value of the change-rate should be sufficiently high to represent an abrupt variation of the torque demand parameter, which is typical of an engine cut-off.

The threshold value of the torque demand should be near to the mini-mum value thereof, in order to be outside of the range of values that are generally required during the normal operation of the engine, but not so small to be inside the range of values required when the en-gine is already in a cut-off state.

Indeed, even during a cut-off state, the engine control unit (ECU) generally requires the engine to produce a certain torque for pre- venting the engine to stop and eventually for operating auxiliary ap-paratuses of the vehicle, such as for example the air-conditioner of the driver and passengers cabin.

As a matter of fact, the threshold values of the engine torque demand and of its change-rate should be chosen so as to represent an engine operating stage that anticipates a cut-off.

In this way, the method according to the above mentioned embodiment of the invention advantageously allows to interrupt the injection of natural-gas few engine cycles before the injection of diesel fuel is drastically reduced in response of the engine cut-off, so that these few engines cycle are sufficient for the natural-gas already accumu- lated in the intake manifold to be inducted into the corrbustion chain-bers and burned therein.

According to an aspect of the invention, the change-rate value of the parameter is calculated as a function of a difference between two mo- nitored values of the engine torque demand and of a time elapsed be-tween their respective monitorings.

In this way, the method advantageously allows to monitor the value of the change-rate of the parameter without the need of any additional sensor.

According to another aspect of the invention, the engine torque de-mand parameter is a position of an accelerator device of the engine, typically an accelerator pedal, which can be measured by means of a position sensor associated thereto.

This aspect has the advantage that the position of the accelerator device directly and flrirnediately reflects the need for torque of the driver, and that a position sensor is generally already associated to the accelerator device of the engine, in order to allow the ECU to control the injection of diesel fuel, so that this solution does not involve additional cost.

However, it should be understood that there may be special applica-tions in which the diesel/natural-gas engine torque demand does not comes from the driver through the accelerator device, but for example from an automatic control system of the vehicle.

According to another embodiment of the invention, the method can be carried out with the help of a computer program comprising a program-code for carrying out all the steps of the method described above, and in the form of a computer program product comprising the computer program.

The computer program product can be embodied as a diesel/natural-gas internal combustion engine, typically of a motor vehicle, comprising a control unit, a data carrier associated to the control unit and the computer program stored in the data carrier, so that, when the con-trol unit executes the computer program, all the steps of the method described above are carried out.

The method can be also embodied as an electromagnetic signal, said signal being modulated to carry a sequence of data bits which repre-sent a computer program to carry out all steps of the method.

BR1 DESCRIflICt4 OF THE DRAWINGS The present invention will now be described, by way of example, with reference to the accompanying drawings.

Figure 1 is a schematic representation of a diesel/natural-gas inter-nal combustion (IC) engine of a motor vehicle.

Figure 2 is a flowchart of a method for operating the IC engine of figure 1.

AILFD DESRIPflN The diesel/natural-gas internal combustion (IC) engine 10 comprises a cylinder 11 and a reciprocating piston 12, which is acconrnodated in-side the cylinder 11 and is conventionally joined to a rotating crankshaft (not shown).

The cylinder 11 is closed by a cylinder head 13, so that the piston 12, the cylinder 11 and the cylinder head 13 globally delimit a com-bustion chamber 14.

The combustion chamber 14 is provided with an intake port 15 and an exhaust port 16, which are realized in the cylinder head 13 and are conventionally opened and closed by means of an intake valve 150 and an exhaust valve 160 respectively.

The intake port 15 is associated with an intake apparatus for supply-ing fresh air fran the environment into the combustion chamber 14, which intake apparatus comprises an intake pipe 18 leading into an intake manifold 17 connected to the intake port 15.

The exhaust port 16 is associated with an exhaust apparatus for dis- charging exhaust gas from the combustion chamber 14 into the environ- ment, which exhaust apparatus comprises an exhaust manifold 19 con-nected to the exhaust port 16 and leading to an exhaust pipe 20.

The cylinder head 13 is provided with a diesel fuel injector 21, which is arranged for injecting diesel fuel directly inside the corn-bustion chamber 14.

The diesel fuel injector 21 is connected to a diesel fuel rail 22 of a diesel fuel supplying apparatus,. which further comprises a diesel fuel tank 23, an electrically driven pump 24 for drawing the diesel S fuel from the diesel fuel tank 23 and for delivering it under pres-sure in the diesel fuel rail 22, and an engine control unit (ECU) 25 for controlling the operation of the pump 24 and of the diesel fuel injector 21.

The IC engine 10 is additionally equipped with a natural-gas supply-ing apparatus, which schematically comprises a tank 26 containing natural-gas under pressure, an injector 27 located in the intake ma-nifold 17 and connected to the tank 26 via a pressure regulation valve 28, and a control unit 29 for controlling the operation of the injector 27.

Immediately after the start of the IC engine 10, the control unit 29 prevents the injector 27 from injecting natural-gas into the intake manifold 17, so that the IC engine 10 operates only with the diesel fuel that is directly injected into the combustion chamber 14 by the diesel fuel injector 21.

When the IC engine 10 is properly warmed up, the ECU 25 reduces the quantity of diesel fuel that is injected into the combustion chamber 14, while the control unit 29 activates the injector 27 to inject dosed quantity of natural-gas into the intake manifold 17 of the IC engine 10.

Both the diesel fuel and the natural-gas are injected once per engine cycle, through one or more injection pulses, according to convention-al strategies that are performed by the ECU 25 and by the control unit 29 respectively.

However, it generally happens that the injected quantity dQng, inj [ii of natural-gas is not completely inducted into the combustion charter 14 during the corresponding i' engine cycle, so that a small fraction of natural-gas accumulates into the intake manifold 17, and it is in-ducted into the combustion charter 14 during the next (i+l)th engine cycle.

In order to prevent that this fact causes undue emission of unburned natural-gas, the control unit 29 is provided for cyclically perform-ing the procedure shown in figure 2, typically once per engine cycle after the engine warm-up.

This procedure firstly provides for measuring the current value APP of the position of an accelerator pedal 30 associated to the IC en- gine 10, which position represents the engine torque currently de-manded by the driver, and for calculating the current value CR of the change-rate over the time of this position, namely the velocity of the accelerator pedal 30.

Conventionally, the more the accelerator pedal is pressed by the driver the more its position increases or, in other words, the posi-tion of the accelerator pedal is considered to increase as the engine torque demand increases.

As a consequence, the velocity of the pedal is positive, if the acce-lerator pedal 30 is pressed by the driver towards an increased engine torque demand, while it is negative, if the accelerator pedal 30 is released by the driver so as to move towards a reduced engine torque demand.

The position of the accelerator pedal 30 is measured by means of a conventional position sensor 31 associated to the accelerator pedal itself.

The velocity of the accelerator pedal 30 is calculated by the control unit 29 on the base of at least two values of the accelerator pedal position, for example on the basis of the current measured value APP and the value measured during the preceding engine cycle, and on the basis of the time elapsed between this two measurements.

More specifically, the change rate value CR is calculated as the ra-tio of a difference between the above named values of the accelerator pedal position to the time elapsed between them.

Nevertheless, it should be understood that the calculation of the value CR of the change-rate could be inproved and/or performed in many other alternative ways.

According to the procedure of figure 2, the control unit 29 checks whether the current value APP of the accelerator pedal position is below a threshold value APPth thereof, which is near but a little greater than the minimum value that the accelerator pedal position reaches when the accelerator pedal is at the end of its release stroke.

As long as the current value APP of the accelerator pedal position is equal or above the threshold value APPth, the control unit 29 contin- ues to control the natural-gas injection according to the convention-al strategy and the procedure steps described above repeated.

When it happens that the current value APP of the accelerator pedal position is below the threshold value APPth, the control unit 29 checks whether the current value CR of the accelerator pedal velocity is below a threshold value CRth thereof.

The threshold value CRth is negative, so as to be indicative of an accelerator pedal release movement, but its modulus is sufficiently high to indicate that that release movement is performed at an high velocity, which is typical of an engine cut-off.

If the current value CR of the accelerator pedal velocity is equal or above the (negative) threshold value CRth or, in other word, if the position of the accelerator pedal 30 is increasing at any velocity or decreasing at a velocity that is equal or smaller than the modulus of the threshold value CRth, the control unit 29 continues to control the natural-gas injection according to the conventional strategy and the procedure steps described above repeated.

If conversely the current value CR of the accelerator pedal velocity is below the (negative) threshold value CRth or, in other word, if the position of the accelerator pedal 30 is decreasing at a velocity that exceeds the modulus of the threshold value CRth, the control unit 29 corrirrtands the injector 27 to abruptly and completely interrupt the injection of natural-gas.

As a matter of fact, the global effect of the conditional steps de-scribed above, respectively indicated with 40 and 50 in figure 2, is that the control unit 29 commands the injector 27 to interrupt the injection of natural-gas, if the accelerator pedal position decreases below the threshold value APPth at a velocity whose modulus exceeds the modulus of the threshold value CRth.

In this way, the procedure advantageously allows to interrupt the in-jection of natural-gas few engine cycles before the ECU 25 interrupts the injection of diesel fuel in response of the engine cut-off, so that the quantity of natural-gas already accumulated in the intake manifold 17 can be inducted into the corrbustion chaither 14 and burned therein.

The threshold values APPth and CFtth are constants, which are ernpiri-cally determined by means of a calibration activity performed on a test bench engine and stored in a data carrier 32 associated to the control unit 29.

The data carrier 32 contains also a conputer program comprising a computer-code for performing the procedure described above, so that, when the control unit 29 executes the computer program, all the steps of the procedure are carried out.

Even if figure 1 shows only one cylinder 11, it should be understood that the IC engine 10 can comprise a plurality of cylinders 11, each of which will be provided with a reciprocating piston 12 connected to the crankshaft and will be closed by the cylinder head 13, in order to delimit a respective combustion chamber 14. Each combustion cham-ber 14 will be provided with respective intake port 15 and exhaust port 16, connected to the intake manifold 17 and the exhaust manifold 19 respectively, and with a diesel fuel injector 21 connected to the diesel fuel rail 22.

While at least one exemplary embodiment has been presented in the foregoing summary and detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only exam- ples, and are not intended to limit, the scope, applicability, or con- figuration in any way. Rather, the forgoing summary and detailed de-scription will provide those skilled in the art with a convenient road map for implementing at least one exemplary embodiment, it being understood that various changes may be made in the function and ar-rangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and in their legal equivalents.

REFENES

IC engine 11 Cylinder 12 Piston 13 Cylinder head 14 Combustion chamber Intake port 16 Exhaust port 17 Intake manifold 18 Intake pipe 19 Exhaust manifold Exhaust pipe 21 Diesel fuel injector 22 Diesel fuel rail 23 Diesel fuel tank 24 Pump

ECU

26 Tank 27 Injector 28 Pressure regulation valve 29 Control unit Accelerator pedal 31 Position sensor 32 Data carrier 40 Conditional step Conditional step Intake valve Exhaust valve APP Current value of the accelerator pedal position APPth Threshold value of the accelerator pedal position CR Current value of the change-rate (velocity) of the ace lerator pedal CRth Threshold value of the change-rate (velocity) of the ac celerator pedal aarc

Claims

1. A method for operating a diesel/natural-gas internal cor±ustion engine (10), comprising the steps of: -monitoring a value (APP) of a parameter indicative of an en-gine torque demand, -monitoring a value (CR) of a change-rate of this parameter, -preventing the natural-gas to be injected into an intake ap-paratus (17) of the engine, if the value (APP) of the engine torque demand parameter decreases below a threshold value (APPth) thereof at a change-rate value (CR) whose modulus exceeds a modulus of a change-rate threshold value (CRth).

2. Method according to claim 1, wherein the change-rate value (CR) of the parameter is calculated as a function of a difference be-tween two monitored values of the engine torque demand and of a time elapsed between their monitorings.

3. Method according to claim 1 or 2, wherein the engine torque de-mand parameter is a position of an accelerator device (30) of the engine (10).

4. Method according to claim 3, wherein the value (APP) of the acce-lerator device position is measured by means of a position sensor (31) associated to the accelerator device (30).

5. A computer program comprising a computer-code for performing the method according to any of the preceding claims.

6. A computer program product on which the computer program accord-ing to claim 5 is stored.

7. A diesel/natural-gas internal combustion engine (10) comprising a control unit (29), a data carrier (32) associated to the control unit (29) and a computer program according to claim 5 stored in the data carrier (32).

8. An electromagnetic signal modulated as a carrier for a sequence of data bits representing the computer program according to claim 5.