Patent Publication Number: US-2012031371-A1

Title: Method for operating an internal combustion engine having spark ignition

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to German Patent Application No. 102010033394.8, filed Aug. 4, 2010, which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The technical field relates to a method for operating an internal combustion engine having a spark ignition unit. A liquid fuel is mixed with air in an injection procedure and, after ignition by an ignition spark of the spark ignition unit, combusted while discharging mechanical power. A liquefied gas is combusted as the liquid fuel. 
     BACKGROUND 
     A method is known from the publication U.S. Pat. No. 4,430,978, in which a fuel injection device and an injection system inject liquefied gas (LPG, liquefied petroleum gas) into at least one chamber, which mixes fuel and air, from a storage means, which keeps the pressurized liquefied gas in its liquid state. The fuel injection device is adapted to receive the pressurized liquefied gas from the storage means and deliver it in its liquid state into the air/fuel mixing chamber. 
     In the case of such an injection method, which only injects the liquefied gas into an intake duct or into an air/fuel mixing chamber, the resulting air/combustion gas mixture is typically supplied to a combustion chamber in homogenized form, so that an extremely fuel-lean phase or a charge stratification cannot form between the liquid gas layer and the air layer of a combustion chamber in the cylinder head. Therefore, extremely lean stages of the air-liquid gas mixture cannot be implemented using the known method 
     Therefore, at least one object is to specify a method in which not only can a homogenized and/or stoichiometric air-liquid gas mixture be supplied to the combustion chamber in the cylinder head before an ignition procedure, but rather also the particularly efficient charge stratification of air layers and liquid gas layers can be used. In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background. 
     SUMMARY 
     A method is provided for operating an internal combustion engine having a spark ignition unit is proposed. A liquid fuel is mixed with air in an injection procedure and, after ignition by the spark ignition unit, combusted while discharging mechanical power. A liquefied gas is combusted in a gaseous aggregate state as the liquid fuel. The liquefied gas is at least partially injected into the combustion cylinder using a direct injection nozzle before the ignition by the spark ignition unit. A cooling unit cools the liquefied gas at least in one line. 
     This method has the advantage that not only can a homogenized air-liquid gas mixture in the combustion cylinder be supplied in compressed form to a combustion chamber by the cylinder piston, but rather it is also possible to achieve a charge stratification through the direct injection. The engine can thus be operated having oxygen excess at an optimum thermodynamic operating point, which increases the efficiency in part-load operation in particular. Lower consumption and lower carbon dioxide emissions simultaneously result therefrom. Significant consumption savings can be achieved using this possibility of so-called lean operation. In addition, cooling of the liquefied gas is achieved, so that the liquid gas fuel flows in a noncritical liquefied phase in a supply line under high pressure and/or in a return line, whereby the safety of the method is increased. 
     For this purpose, in one performance form of the method, the following method steps are performed. Firstly, supply lines are provided for the liquefied gas to the direct injection nozzle, which protrudes into a combustion chamber of a cylinder head. Then, with the aid of the direct injection nozzle, direct injection of a liquefied gas quantity under high pressure into the combustion chamber of the cylinder head can be performed during an intake phase of a reciprocating piston or a compression phase. In the compression phase, a cylinder volume which is charged with air or with a liquid gas-air mixture is compressed in the combustion cylinder using the reciprocating piston in the direction of the combustion chamber in the cylinder head. Close to an apex of the reciprocating piston, the ignition of the liquid gas-air mixture, this is under high pressure, by means of an ignition spark in the combustion chamber follows. The at least partially injected liquefied gas quantity is then combusted while discharging mechanical power via a connecting rod to a crankshaft. 
     To perform these method steps, the liquefied gas is supplied from a pressurized liquid gas container to the direct injection nozzle using a high-pressure pump via the supply lines. The fuel mass proportion which is combusted, in the form of the so-called MFB value (mass fraction burned), during the combustion is greater than approximately 50% on a scale from approximately 0% to approximately 100% using the method of direct injection of the liquefied gas into the combustion cylinder or into the combustion chamber of the combustion cylinder, whereby more fuel mass is converted into mechanical power. 
     Moreover, through the cooling effect of the atomized and vaporized liquefied gas directly into the combustion chamber, premature ignition of the liquid gas-air mixture in the combustion chamber can be prevented, so that knocking due to the direct injection is prevented in relation to a typical injection method into an intake duct of the internal combustion engine or into a mixing chamber of the internal combustion engine, as is known from the above publication. 
     In addition, this method has the advantage that it uses the reduced flashpoint of the liquefied gas in relation to typical liquid fuels for the purpose of thermally stressing the cylinder head less. In addition, through the reduced flashpoint in relation to typical liquid fuels such as gasoline, the nitrogen oxide emission is reduced in spite of charge stratification, which is to be attributed to the reduced flashpoint of the liquefied gas, since the nitrogen oxide formation is not solely dependent on the oxygen excess, but rather also on the combustion temperature, the nitrogen oxide formation also decreasing with reduced combustion temperature. 
     In addition, it is possible through the cooling effect in the case of direct injection of the liquefied gas in the combustion cylinder during the compression phase of the second stroke of a four-stroke internal combustion engine to achieve an increased oxygen uptake in the combustion cylinder and thus allow the above-mentioned increase of the MFB value (mass fraction burned) while simultaneously reducing nitrogen oxides because of the reduced temperature of the flashpoint of liquefied gas. 
     Through the cooling effect of the liquefied gas, an increased oxygen uptake can be caused in the combustion cylinder in the case of direct injection in the compression phase and a charge stratification can be achieved. A fuel-lean combustion phase is thus made possible as the third stroke of a four-stroke internal combustion engine, particularly because homogenized liquid gas-air mixture is not present at the moment of ignition due to the charge stratification. 
     In addition, a further variation of the direct injection allows multiple direct injections of liquid gas, after ignition of an initially lean fuel-air mixture having charge stratification has already occurred in the combustion chamber. This means that in this method, firstly an ultra-lean liquid gas-air mixture is ignited with the aid of the spark ignition unit, and subsequently during the power stroke or the third stroke of a four-stroke internal combustion engine, the mixture is enriched with liquefied gas by dosing using multiple direct injections. The consumption of liquefied gas can thus be minimized and a higher power output to the crankshaft via the connecting rod can be achieved simultaneously. 
     An internal combustion engine having direct injection unit of a liquefied gas has a direct injection unit having a direct injection nozzle in a cylinder head. The direct injection nozzle is connected via an injection valve to a fuel supply line. A return line, which is connected to the injection valve for returning excess fuel, or the supply line for the fuel has a cooling unit. This internal combustion engine has the advantage due to the cooling unit in at least one of the fuel lines that the liquefied gas can be supplied in a noncritical liquid state via a high-pressure pump to the fuel supply line, whereby the safety of the internal combustion engine having direct injection unit for liquefied gas is improved. 
     A computer program is also provided which, when it runs in an engine controller, causes the engine controller to perform one of the described methods. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and: 
         FIG. 1  shows a perspective view in partial section of a cylinder head of a four-stroke internal combustion engine according to a first embodiment; and 
         FIG. 2  shows a perspective view in partial section of a cylinder head of a four-stroke internal combustion engine according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit application and uses. Furthermore, there is no intention to be bound by any theory presented in the preceding or summary or the following detailed description. 
       FIG. 1  shows a perspective view in partial section of a cylinder head  7  of a four-stroke internal combustion engine  1  according to a first embodiment. The cylinder head  7  of an internal combustion engine  1  has, in addition to two inlet valves  12  and  13 , an outlet valve  14 . In addition, a spark plug  15  of a spark ignition unit  2  is shown in  FIG. 1 . A reciprocating piston  8 , which cooperates with a connecting rod  9 , is located in a compression phase of the four-stroke internal combustion engine  1 , in which, for example, air charged by a turbocharger is compressed, while liquefied gas  16  is injected via a direct injection nozzle  5  directly into the combustion chamber  6 . This liquefied gas is supplied to a direct injection nozzle  5  from a liquid gas container  10  via a high-pressure pump  11  and a high-pressure line  4 , an electronically controlled direct injection valve  17 , which is controlled by a control and regulating unit  19  via a control line  21 , regulates an injection quantity of the liquefied gas and an injection time span as a function of sensors  20 , which cooperate with a gas pedal of the vehicle and a crankshaft of the internal combustion engine, for example, and are connected via signal lines  22  to the control and regulating unit  19 . 
     Using such direct injection, as shown in  FIG. 1 , multiple injections of the liquefied gas during the combustion procedure can be provided during the power stroke. For example, at the apex of the reciprocating piston, a lean stage can be ignited with the aid of the ignition device  2  and further liquefied gas can be injected by further injection procedures as the reciprocating piston  8  is pressed down into the combustion cylinder  3 , until finally a stoichiometric air-liquid gas mixture stage or even an enriched air-liquid gas mixture stage is achieved upon conclusion of the power stroke. In addition, a return line  18  from the direct injection nozzle  5  to the liquid gas container  10  via a check valve  23  is provided for excess supplied liquefied gas. In  FIG. 1 , a cooling unit  24  is provided in the return line  18  for excess fuel, which cools the fuel in such a way that it can be supplied in a noncritical liquid aggregate state via the check valve  23  of the high-pressure pump  11 . 
       FIG. 2  shows a perspective view in partial section of a cylinder head  7  of a four-stroke internal combustion engine  1  according to a second embodiment. Components having identical functions as in  FIG. 1  are identified by identical reference numerals and are not explained further. 
     The embodiment of  FIG. 2  differs from the embodiment according to  FIG. 1  in that, additionally or alternatively to the cooling unit  24  in the return line, a cooling unit  25  is provided in the high-pressure line  4  downstream from the high-pressure pump  11 , in order to ensure that after the heating by the compression process in the high-pressure pump  11 , the liquefied gas is supplied in a noncritical liquefied aggregate state to the direct injection valve  17 . 
     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 examples, and are not intended to limit the scope, applicability, or configuration in any way. Rather, the foregoing summary and detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and their legal equivalents.