GB2487589A

GB2487589A - Method for operating a diesel/natural-gas internal combustion engine

Info

Publication number: GB2487589A
Application number: GB1101565.8A
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: GB2487589B; GB201101565D0

Abstract

A method for operating a diÂesel/natural-gas dual fuel internal combustion engine to reduce emissions of unburned natural gas. The method comprising: estimating a value of a parameter indicative of an equivalence mass ratio of air to natural-gas inside a combustion chamber 14 of the engine 10 in an engine cycle, and performing a post inÂjection of diesel fuel into the combustion chamber 14 in the engine cycle, if the estimated value of the parameter is below a threshold value thereof. 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 cycle. The method combusts the unburned natural gas by the post injection of a small quantity of diesel to provide additional heat during the expansion stroke without increasing engine torque.

Description

METHCSD FDR OPERATING A DIESEL/NATURAL-GAS INTERNAL CCt4BUSTION ENGINE TEHNR?JL 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.

BAKGRKJ

It is known that diesel/natural-gas IC engines are currently used in some automotive applications, specially in corrirnercial vehicles such as vans or pick-ups, which are principally, even if not exclusively, 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 comprises 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 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 chambers. When the engine is properly warmed 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 charters, 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-gas, 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 cort-pletely inducted into the corresponding combustion chamber, 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 irrinediately 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 iquite 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 emis-sions.

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

DISaJOSURE 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 comprises two main steps.

The first step provides for estimating a value of a parameter indica- tive of an equivalence mass ratio of air to natural-gas inside a com-bustion chamber of the engine in an engine cycle.

The equivalence mass ratio is defined as the ratio of the gas-to-air mass ratio in an engine cycle to the stoichiometric gas-to-air mass ratio. Therefore, the parameter indicative of the equivalence mass ratio can be the equivalence mass ratio itself or another parameter proportionally correlated thereof, such as for example the gas-to-air mass ratio.

The second step provides for performing at least a post injection of diesel fuel into the combustion chamber in the engine cycle, if the estimated value of the above mentioned parameter is below a predeter-mined threshold value thereof, typically a threshold value below which the natural-gas is expected to not properly burn into the com-bustion chambers of the engine, or even to completely misfire, if the injection of diesel fuel were performed according to a conventional strategy.

A post injection is defined as an injection of a small quantity of diesel fuel which is performed during the expansion stroke of the en-gine piston, while the latter is still near to the top dead center (TDC) position, so that the post-injected fuel burns inside the com- bust ion chamber without producing a great effect on the engine tor-que.

Thanks to this solution, the heat released by the combustion of the post-injected fuel advantageously ignites also the small quantities of natural-gas which are normally expected to not bum, thereby re-ducing the emissions of unburned natural-gas in all engine operating conditions and especially after engine cut-of fs.

According to an aspect of the invention, the value of the parameter indicative of the equivalence mass ratio can be calculated as a func-tion of a value of a quantity of natural-gas entering the combustion chamber in the engine cycle and of a value of a quantity of air en-tering the combustion chamber in the same engine cycle.

Advantageously, these values allow to calculate the gas-to-air mass ratio in the engine cycle concerned, which is proportionally corre-lated to the equivalence mass ratio.

Moreover, the determination of the above mentioned values can be at-tained in quite simple ways.

For example, the value of the air quantity can be simply measured by means of a conventional mass flow sensor located in an intake mani-fold of the engine.

The natural-gas quantity value can be calculated as a function of a value of a quantity of natural-gas to be injected into the intake ma-nifold in the engine cycle, a value of a parameter indicative of a percentage of this quantity of fuel that effectively enters the com-bustion charter in the engine cycle, and a value of a quantity of natural-gas accumulated in the intake manifold due to a preceding en-gine cycle.

According to an aspect of the invention, the value of the quantity of natural-gas to be injected can be attained through a map which is provided for receiving as input a value of engine speed and a value of a parameter indicative of an engine torque demand, such as for ex-ample a parameter indicative of a position of an accelerator pedal or other accelerator device of the engine, and for returning as output the value of the natural-gas quantity to be injected.

Similarly, the value of the percentage parameter can be attained by means of another map provided for receiving as input a value of en-gine speed and a value of engine torque, and for returning as output a value of the percentage parameter.

Advantageously, these maps can be empirically determined during a ca- libration activity performed on a test bench engine and then irnple-mented in the control system of many engines of the same kind.

According to another aspect of the invention, the percentage parame- ter value returned by the map can be corrected on the basis of a val- ue of the intake manifold pressure and/or a value of the intake mani-fold temperature and/or a value of a parameter indicative of a timing of the natural-gas injection, such as for example the time (or crank angle) at which the natural-gas injection of the engine cycle con-cerned ends.

This aspect of the invention has the advantage of providing a more reliable value of the percentage parameter.

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.

BRIEF DESCRIPTI1 OF T1Th 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.

DETAILED DESCRIPTICfl The diesel/natural-gas internal combustion (IC) engine 10 comprises a cylinder 11 and a reciprocating piston 12, which is accommodated 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 corn-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 connected with an intake manifold 17, which is located at the end of an intake pipe 18, in order to supply fresh air from the environment into the combustion charter 14.

The exhaust port 16 leads to an exhaust manifold 19, which is con-nected to an exhaust pipe 20, in order to discharge exhaust gas from the combustion chamber 14 into the environment.

The cylinder head 13 is provided with a diesel fuel injector 21, which is arranged for injecting diesel fuel directly inside the com-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 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 corqrises 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 S 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.

In this way, the injected natural-gas mingles with the air in the in-take manifold 17 and it is inducted therewith into the ccmbustion chamber 14, where it is ignited by the heat released by the cortus-tion of the diesel fuel.

As shown in figure 2, the control unit 29 establishes the quantity of natural-gas to be injected for each generic ith engine cycle by means of an empirically determined map 40.

This map 40 receives as input the current value ES of the engine speed and the current value APP of the position of an accelerator pedal 30 associated to the IC engine 10, and returns as output a val-ue dQng,inj[i] of the mass quantity of natural-gas to be injected during the ith engine cycle.

The current value ES of the engine speed and the current value APP of the accelerator pedal position are determined by the control unit 29 according to conventional strategies.

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

For this reason, a calculation module 41 allows the control unit 29 to estimate the mass quantity of natural-gas which is expected to be actually inducted into the combustion chamber 14 during each individ-ual engine cycle, according to the following equation: dQng,ind[i] = Qng[i] + dQng,inj[i] *17 wherein dQng,ind[i] is the value of the mass quantity of natural-gas inducted into the combustion charter 14 during the generic 1th engine cycle; dQng, inj [ii is the value of the mass quantity of natural-gas to be injected into the intake manifold 17 during the same ith engine cycle; q is a value of an efficiency parameter indicative of the per-centage of dQng, inj [iii that effectively enters the combustion chamber 14 during the ith engine cycle; and Qng[i] is the value of the mass quantity of natural-gas which is already present in the intake rrtani-fold 17 at the beginning of the ith engine cycle due to the preceding (i_l)th engine cycle.

The value r of the efficiency parameter is determined by the control unit 29 through an empirically determined map 42, which receives as input the current value ES of the engine speed and the current value ET of engine torque, and returns as output a rough value q of the efficiency parameter.

The current value ET of the engine torque is measured or calculated by the control unit 29 according to a conventional strategy.

The rough value qk provided by the map 42 is corrected through a cor-rection module 43, in order to obtain the more reliable value rj of the efficiency parameter.

This correction is performed on the basis of the current value P of the intake manifold pressure, the current value T of the intake mani-fold temperature, and the current value t of the time at which the natural-gas injection ends.

The current values of these additional engine operating parameters are measured or calculated by the control unit 29 according to con-ventional strategies.

Furthermore, the value Qng[i) of the mass quantity of natural-gas al-ready accumulated into the intake manifold 17 at the beginning of the generic ith engine cycle is calculated according to the following equ-ation: Qng[i] = Qng[i -1}+ dQng,inj[i -1] -dQng,ind[i -1] wherein Qng[i-lJ is the value of the mass quantity of natural-gas that was accumulated into the intake manifold 17 at the beginning of the preceding (i_l)th engine cycle; dQng,inj[i-lJ is the value of the mass quantity of natural-gas injected into the intake manifold 17 during the (i_I)th engine cycle; and dQng,ind[i-l] is the value of the mass quantity of natural-gas inducted into the combustion chamber 14 during the same (i_l)th engine cycle.

It should be understood that, for the very first engine cycle in which the natural-gas is injected, namely the first engine cycle af- ter the warm-up of the IC engine 10, the values Qng[i-1], dQng,inj{i-I] and dQng,ind[i-1] are set to zero, whereupon they are determined for each engine cycle in view of the estimations performed in the preceding one.

In this way, the control unit 29 is effectively able to estimate the value dQng,ind[i] of the mass quantity of natural-gas inducted into the combustion chanter 14 during each engine cycle.

This estimated value dQng,ind[i] is used in a calculation module 44 for estimating a value EqR[iJ of the equivalence mass ratio of air to natural-gas inside the combustion chamber 14, according to the fol-lowing equation: EqR[i] = dQng, ind[i] 1--dA[i] LNG)ST wherein EqR[iJ is the value of the equivalence mass ratio of air to natural-gas for the generic ith engine cycle; dA[i] is the value of the mass quantity of air inducted into the combustion chamber 14 dur-ing the same ith engine cycle; (A/NG)s'r is the stoichiometric ratio of air to natural-gas.

The value dA[i] of the air mass quantity is measured by the control unit 29 using a conventional mass flow sensor 31 located in the in- take manifold 17 of the IC engine 10, whereas the stoichiometric ra-tio (A/NG) is a constant stored in a data carrier 32 associated to the control unit 29.

The value EqR[i] of the equivalence mass ratio is then compared with a threshold value EqR[tv] thereof, which represents a limit above which the natural-gas is expected to not properly burn into the com- bustion chamber 14, or to completely misfire, if the injection of di-esel fuel for the same engine cycle were performed according to the conventional strategy.

The threshold value EqR[tv} can be empirically determined and stored into the data carrier 32.

In view of the above, if the estimated value EqR[i] of the equiva-lence mass ratio is equal or above the related threshold value EqR[tv], the control unit 29 allows the ECU 25 to injects the diesel fuel for the concerned ith engine cycle according to the above named conventional strategy.

Otherwise, if the estimated value EqR[i] of the equivalence mass ra-tio is below the related threshold value EqR[tv], the control unit 29 induces the ECU 25 to inject, during the concerned ith engine cycle, a small quantity of diesel fuel by means of one or more post injection pulses.

These post injection pulses are performed during the expansion stroke of the engine piston 12, while the latter is still near to the top dead center (TDC) position, so that the post-injected fuel burns in-side the cylinder 11 without producing great effect on the engine torque.

Besides, the heat released by the coitustion of the post-injected fuel advantageously ignite the small quantity of natural-gas which otherwise is expected to not burn.

In this way, a reduction of the emissions of unburned natural-gas in all engine operating conditions, and especially after engine cut-offs, is attained.

The operating method described above is performed with the aid of a computer program comprising a computer-code for performing the method.

This computer program is stored in the data carrier 32, so that, when the control unit 29 executes the computer program, all the steps of the method described above 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 sumary 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.

REFERENCES

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 Mass flow sensor 32 Data carrier 40 Map 41 Calculation module 42 Map 43 Correction module 44 Calculation module 150 Intake valve Exhaust valve ES Value of engine speed APP Value of accelerator pedal position ET Value of engine torque dQng,inj [ii Value of the mass quantity of natural-gas to be injected during the ith engine cycle Qng[i] Value of the mass quantity of natural-gas which is al ready present in the intake manifold at the beginning of the th engine cycle.

Value of the efficiency parameter Rough value of the efficiency parameter P Value of the intake manifold pressure T Value of the intake manifold temperature t Value of the instant at which the natural-gas injection ends dQng,ind[i] Value of the mass quantity of natural-gas inducted into the combustion chamber during the ith engine cycle dA[i] Value of the mass quantity of air inducted into the corn bustion chamber during the ith engine cycle (A/NG)ST Stoichiometric ratio of air to natural-gas EqR[i] Value of the equivalence mass ratio of air to natural-gas for the ith engine cycle EqR[tv] Threshold value of the equivalence mass ratio of air to natural-gas ciam

Claims

1. A method for operating a diesel/natural-gas internal combustion engine, comprising the steps of: -estimating a value (EgR[i]) of a parameter indicative of an equivalence mass ratio of air to natural-gas inside a com-bustion chamber (14) of the engine (10) in an engine cycle, -performing a post injection of diesel fuel into the combus- tion chamber (14) in the engine cycle, if the estimated val-ue (EgR[i]) of the parameter is below a threshold value (EgR[tv]) thereof.

2. A method according to claim 1, wherein the value (EgR[i]) of the parameter indicative of the equivalence mass ratio is calculated as a function of a value (dQng,ind[i]) of a quantity of natural-gas entering the combustion chamber (14) in the engine cycle and of a value (dA[i]) of a quantity of air entering the combustion chamber (14) in the same engine cycle.

3. A method according to claim 2, wherein the value (dA[i]) of the air quantity is measured by means of a mass flow sensor (31) lo-cated in an intake manifold (17) of the engine (10).

4. A method according to any of the precethng claim, wherein the natural-gas quantity value (dQng, md [iJ) is calculated as a func-tion of a value (dQng,inj[i]) of a quantity of natural-gas to be injected into an intake manifold (17) of the engine (10) in the engine cycle, a value of a parameter (q) indicative of a percen- tage of this quantity of fuel that effectively enters the combus-tion chamber (14) in the engine cycle, and a value (Qng[i]) of a quantity of natural-gas accumulated in the intake manifold due to a preceding engine cycle.

5. A method according to claim 4, wherein the value (dQng,inj[i]) of the quantity of natural-gas to be injected is attained through a map (40) provided for receiving as input a value (ES) of engine speed and a value (APP) of a parameter indicative of an engine torque demand, and for returning as output the value (dQng, inj [i]) of the natural-gas quantity to be injected.

6. A method according to claim 5, wherein the parameter indicative of the engine torque demand is a position of an accelerator de-vice (30) of the engine.

7. A method according tc any claim from 4 to 6, wherein the value of the percentage parameter (r is attained through a map (42) pro-vided for receiving as input a value (ES) of engine speed and a value of engine torque (ET), and for returning as output a value (q*) of the percentage parameter.

8. A method according to claim 7, wherein the percentage parameter value (*) returned by the map (42) is corrected on the basis of a value (F) of the intake apparatus pressure and/or a value (T) of the intake apparatus temperature and/or a value (t) of a para-meter indicative of a timing of the natural-gas injection.

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

10. A computer program product on which the computer program accord-ing to claim 9 is stored.

U. 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 9 stored in the data carrier (32).

12. An electromagnetic signal modulated as a carrier for a sequence of data bits representing the computer program according to claim 9.