Abstract:
The present invention relates to a method of controlling the combustion phase of a fuel mixture of a spark-ignition supercharged internal-combustion engine comprising at least one cylinder ( 12 ) with a combustion chamber ( 14 ), at least one fuel supply means ( 48, 52 ) and spark ignition means ( 38 ). 
     According to the invention, the method consists, for high loads and low speeds of said engine, in determining, during the combustion of the fuel mixture, the value of crank angle (θ′) where the maximum cylinder pressure (P max ) occurs in the combustion chamber ; in comparing the value thus determined with a maximum angle threshold value (θ max ) representative of an abnormal combustion in said chamber ; in detecting the start of an abnormal combustion when the determined value reaches said threshold value and when the ignition means are not actuated ; and in feeding an amount of another fuel into the fuel mixture in order to modify the energy index of this mixture so as to reduce the crank angle where the maximum cylinder pressure occurs.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method of controlling the combustion phase of a fuel mixture of a spark-ignition supercharged internal-combustion engine, notably of gasoline type. 
         [0002]    It more particularly relates to such an engine with indirect fuel injection, but it does not rule out in any way an engine with direct fuel injection. 
         [0003]    It essentially concerns engines that have undergone downsizing. This operation is intended to reduce the size and/or the capacity of the engine while keeping the same power and/or the same torque as conventional engines. 
       BACKGROUND OF THE INVENTION 
       [0004]    Generally, this type of engine comprises a combustion chamber within which a fuel mixture undergoes a compression phase, followed by a combustion phase under the effect of spark ignition means, such as a plug. 
         [0005]    It has been observed that this fuel mixture can follow an abnormal combustion that generates mechanical and/or thermal stresses some of which can seriously damage the engine. 
         [0006]    This abnormal combustion is essentially due to a pre-ignition (or self-ignition) of the fuel mixture before the plug initiates ignition of the mixture present in the combustion chamber. 
         [0007]    In fact, considering the high pressures and the high temperatures reached in this combustion chamber as a result of supercharging, combustion start can occur sporadically well before the time when ignition of the fuel mixture by the plug occurs. 
         [0008]    In cases where this abnormal combustion due to pre-ignition or self-ignition occurs suddenly, in a random and sporadic manner, it is referred to as rumble. 
         [0009]    The latter abnormal combustion leads to very high pressure levels (of the order of 120 to 250 bars) and to a thermal transfer increase that may cause partial or total destruction of the moving elements of the engine, such as the piston or the piston rod. 
         [0010]    Furthermore, it has been observed that this abnormal combustion takes place at high loads and generally at low engine speeds. 
         [0011]    More precisely, this abnormal combustion appears when an ignition sub-advance is achieved as a result of circumstances linked with incipient engine knock, a phenomenon that then requires to decrease this advance and thus to increase rumble risks at low engine speed. 
         [0012]    One solution for preventing this risk consists in limiting the maximum cylinder pressure angle so as to avoid generating temperature and pressure conditions favouring such rumble. 
         [0013]    This solution involves the drawback of not allowing to exploit all of the performance potential of this spark-ignition engine. In fact, the load increase via the supercharging pressure generates engine knock, which requires application of a conventional ignition sub-advance. This sub-advance moves forward the combustion and therefore the maximum cylinder pressure angle that must be limited to a usual value of 35° crank angle after the TDC (combustion Top Dead Centre). The maximum performances of the engine at low speed are thus limited to the load for which the ignition advance adjustment corresponds to these two combined criteria. 
         [0014]    The present invention thus aims to overcome the aforementioned drawbacks by means of a combustion method allowing rumble appearance risks to be limited. 
       SUMMARY OF THE INVENTION 
       [0015]    The present invention thus relates to a method of controlling the combustion phase of a fuel mixture of a spark-ignition supercharged internal-combustion engine, notably of gasoline type, wherein the engine comprises at least one cylinder with a combustion chamber, at least one fuel supply means, at least one intake means, at least one exhaust means and spark ignition means, characterized in that it consists, for high loads and low speeds of said engine: 
         [0016]    during the combustion of the fuel mixture, in determining the value of the crank angle where the maximum cylinder pressure occurs in the combustion chamber, 
         [0017]    in comparing the value thus determined with a maximum angle threshold value representative of an abnormal combustion in said chamber, 
         [0018]    in detecting the start of an abnormal combustion when the determined value reaches said threshold value and when the ignition means are not actuated, 
         [0019]    in feeding another fuel into the fuel mixture in order to vary the global octane number of this fuel mixture so as to reduce the crank angle where the maximum cylinder pressure occurs. 
         [0020]    The method can consist in increasing the global octane number of the fuel mixture. 
         [0021]    The method can consist in feeding another fuel of gas type into the fuel mixture. 
         [0022]    Advantageously, the method can consist in feeding a gaseous fuel of VNG type. 
         [0023]    The method can consist in preparing the fuel mixture from a liquid fuel. 
         [0024]    The method can consist in preparing the fuel mixture from a gasoline fuel. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0025]    Other features and advantages of the invention will be clear from reading the description hereafter, with reference to the accompanying figures wherein: 
           [0026]      FIG. 1  shows an engine using the combustion phase control method according to the invention, 
           [0027]      FIG. 2  shows pressure curves (P in bars) as a function of the crank angle (°V) for an engine of the prior art and for an engine using the method according to the invention, and 
           [0028]      FIG. 3  shows curves representative of the torque (C in N.m) as a function of the engine speed (R in rpm) for a conventional engine and for an engine using the method according to the invention (Inv). 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    The engine illustrated in  FIG. 1  is a spark-ignition supercharged internal-combustion engine  10 . 
         [0030]    This engine has the specific feature of running with a first combustion mode referred to as single carburetion, according to which a single fuel is used, or with another combustion mode, referred to as multi-carburetion, allowing several fuels of different nature to be associated. 
         [0031]    By way of non limitative example, the fuel used for the single carburetion mode is a first liquid fuel (gasoline, ethanol, biofuel, etc.) with an anti-knock index (RON) of the order of 90 to 95 for example, which is associated, for multi-carburetion running mode, with at least another liquid or gaseous fuel (VNG (Vehicular Natural Gas), LPG (Liquefied Petroleum Gas), methane) having a higher RON than the first fuel. 
         [0032]    For reasons of simplification of the following description, the single carburetion mentioned uses a liquid fuel of gasoline type and for the multi-carburetion, which is here a bicarburetion, the gasoline type liquid fuel is associated with a VNG type gaseous fuel. 
         [0033]    The engine illustrated in  FIG. 1  comprises at least one cylinder  12 , four cylinders here, housing a piston with a reciprocating rectilinear displacement (not shown) delimiting a combustion chamber  14  within which combustion of a fuel mixture takes place. 
         [0034]    The cylinder comprises air intake means  16  with at least one valve  18  associated with an intake pipe  20 . This cylinder also comprises burnt gas exhaust means  22  with at least one exhaust valve  24  controlling an exhaust pipe  26 . 
         [0035]    Exhaust pipes  26  are connected to an exhaust manifold  28  which is in turn connected to an exhaust line  30 . 
         [0036]    A supercharging device  32 , a turbocompressor for example, is arranged on this exhaust line. This turbocompressor comprises a turbine  34  scavenged by the exhaust gas circulating in the exhaust line, and a compressor  36  rotatingly connected to the turbine so as to enable compression of the outside air and to allow this intake air under pressure (or supercharged air) into intake pipes  20 . 
         [0037]    This cylinder also comprises spark ignition means  38 , a spark plug for example, enabling to generate one or more sparks allowing to ignite the fuel mixture present in the combustion chamber of the cylinder. 
         [0038]    Means  40  allowing to know the state of progress of the combustion in combustion chamber  14  are also provided. By way of example illustrated in  FIG. 1 , these means are a pressure detector  42  allowing to measure the evolution of the pressure in the combustion chamber, referred to as cylinder pressure. This pressure is one of the parameters representative of the state of progress of the fuel mixture combustion. 
         [0039]    Of course, any other means allowing to know the state of progress of the combustion can be used, such as knock detection means usually arranged on the cylinder housing of the engine, such as an accelerometer that allows to generate a signal representative of vibrational waves. 
         [0040]    In order to be able to determine the position of the piston within the cylinder, the engine also carries an angular sensor  44  arranged opposite a target  46  carried by the crankshaft (not shown) to which the piston is connected. 
         [0041]    This cylinder also comprises first fuel supply means  48 , here in form of an injector  50  for a fuel in liquid form, of gasoline type, allowing the fuel to be fed into intake pipe  20 . 
         [0042]    This cylinder also comprises second fuel supply means  52  with a gaseous fuel (VNG) injector  54  allowing the gaseous fuel to be fed into intake pipe  20 . 
         [0043]    The engine comprises a computing and control unit  56  referred to as engine calculator, allowing the operation of this engine to be controlled. 
         [0044]    More particularly, this calculator is directly or indirectly connected by conductors to the various detectors, sondes and/or detection means (water temperature, oil temperature, pressure in the combustion chamber) the engine is provided with. The signals received are processed and the calculator then controls through control lines the components of this engine so as to ensure smooth running thereof. 
         [0045]    As illustrated in  FIG. 1  by way of example, pressure detectors  42  are connected to this calculator by a conductor  58  so as to send a signal representative of the pressure prevailing in the cylinders, angular sensor  44  is connected to this calculator by a conductor  60  so as to determine the positions of the piston during the displacement thereof, spark plugs  38  are connected by control lines  62  to the calculator so as to control the ignition time of the fuel mixture and the controls of injectors  50 ,  54  are connected by control lines  64 ,  66  to calculator  56  so as to control the fuel injection parameters, such as the amount of fuel injected or the injection time. 
         [0046]    This calculator furthermore comprises maps or data charts allowing to evaluate the parameters required for its operation, according to the various engine running conditions, such as the engine speed or the power requested by the driver. 
         [0047]    When this engine runs at high load and low speed (between 1000 and 2500 rpm), engine knock requires an ignition sub-advance so as to get out of this abnormal combustion mode. 
         [0048]    This sub-advance moves the maximum cylinder pressure angle θ max  away from the top dead centre (PMH in the figure) and this angle is generally limited to a usual value of about 35 crank angle degrees after the TDC, as illustrated in  FIG. 2 , 
         [0049]    In cases where this crank angle value θ max  is approached or exceeded, the risks as regards a rumble type abnormal combustion are increased. 
         [0050]    In order to prevent this, the progress of the fuel mixture combustion in combustion chamber  14  is controlled. 
         [0051]    With reference to  FIG. 2 , curve AA shows the conventional evolution of the cylinder pressure in combustion chamber  14  of an engine running in single carburetion mode, as a function of the crank angle (°V), after intake of the fuel mixture into this chamber. 
         [0052]    The piston compresses the fuel mixture present in the combustion chamber to the neighbourhood of the TDC where this mixture is ignited by spark plug  38 . As a result of this ignition, the cylinder pressure increases up to a value P max  at a threshold crank angle θ max  of about 35° after the TDC. This cylinder pressure then decreases until the BDC is reached, where the pressure in the combustion chamber is close to atmospheric pressure. 
         [0053]    Above this usual value of 35°, fuel mixture self-ignition risks may appear. 
         [0054]    In fact, if the timing advance is set too late (which generates a maximum cylinder pressure angle above 35°), the pressure and temperature conditions in combustion chamber  14 , just before the spark plug fires, are such that a sudden self-ignition (rumble) can occur. 
         [0055]    This value of 35 crank angle degrees advantageously corresponds to a mean obtained from experiments on the engine test bench. 
         [0056]    In order to limit risks of rumble type abnormal combustion appearance, it is necessary that the value of the measured crank angle of the measured maximum cylinder pressure does not exceed the set crank angle threshold value θ max  of this pressure. 
         [0057]    Pressure detector  42  therefore measures the pressure in chamber  14  during the combustion phase between the TDC and the BDC, and it sends the corresponding signal through conductor  58  to the calculator. 
         [0058]    Advantageously, this maximum angle value can first be validated on the engine test bench, which thus allows to do without the pressure detectors and the link with calculator  56 . 
         [0059]    These maps of the cylinder pressure angle as a function of the load, ignition advance, engine speed, supercharging pressure, are integrated in the calculator, thus sparing continuous use of these detectors in the vehicle. 
         [0060]    Simultaneously, angular sensor  44  communicates the crankshaft angular position signal to this calculator that determines the position of the piston therefrom. 
         [0061]    From these two signals (pressure and angular position), the calculator determines the value of crank angle θ′ after the TDC where the maximum cylinder pressure is reached. 
         [0062]    This determined value of angle θ′ is then compared with the maximum angle threshold value θ max  that is representative of the maximum limit from which rumble appearance risks are high. 
         [0063]    This threshold value is advantageously contained in the calculator data charts and, in the example described, it is around 35° after the TDC. 
         [0064]    This allows the calculator to detect the risk of rumble type abnormal combustion start when the determined angle value approaches or reaches maximum angle threshold value θ max , and when this value is determined early in the cycle, i.e. before the spark plug fires. 
         [0065]    Thus, when this threshold value is approached or even reached, the calculator controls the fuel injectors so as to switch from an engine in single carburetion mode (gasoline) to an engine in bicarburetion mode (gasoline and gas). 
         [0066]    Concomitant injection of a second gaseous fuel, of VNG type for example, then allows to modify the global octane number of this mixture and thus to increase the knock resistance. This in turn allows to increase the load and/or the ignition advance while maintaining the maximum cylinder pressure angle below the maximum value allowed. 
         [0067]    This second fuel affords the advantage of having self-ignition resistance characteristics (RON) that allow the combustion to be retimed by modifying the ignition advance. 
         [0068]    Via this concomitant injection, it is thus possible to increase the engine performances notably, while remaining far from the rumble risk zone. 
         [0069]    More precisely, during the engine intake phase, calculator  56  controls injectors  50  and  54  through control lines  64 ,  66  in such a way that part of the mass of the gasoline initially injected through injector  50  into pipe  20  is replaced by an injection of VNG through injector  54 . 
         [0070]    This allows to modify the octane number of the initial fuel mixture and to increase this global octane number of the fuel mixture obtained as a result of the presence of the VNG fuel with a higher RON than the gasoline. 
         [0071]    The optimum amount of gas to be injected corresponds to the global octane number required in order to obtain mixing of the two fuels so that the operating point of the engine is at the desired torque, with an ignition advance to the limit of engine knock and generating a maximum pressure angle less than or equal to 35° after the TDC. 
         [0072]    This concomitant injection of a second gaseous fuel allows to maintain maximum cylinder pressure angle θ′ (see  FIG. 2 ) below threshold value θ max  with the additional advantage of remaining at a fuel/air ratio of 1 for the fuel mixture. 
         [0073]    Of course, the person skilled in the art is able to determine the mass proportion of VNG in relation to gasoline required to obtain the desired RON so as to move away from the knock risk zone while remaining within the allowed maximum cylinder pressure angle criteria, thus limiting rumble risks. 
         [0074]    This also allows to increase the compression ratio of engines running on gasoline, with all the advantages involved. 
         [0075]    By means of the invention, as can be seen in  FIG. 3 , the torque obtained with a concomitant VNG injection allows to obtain a higher torque at a lower engine speed (curve Inv) than the torque obtained with an engine of the prior art (curve AA). 
         [0076]    The present invention is not limited to the embodiment example described and it encompasses any equivalent or variant covered by the present invention.