Patent Publication Number: US-7581531-B2

Title: Method for operating an internal combustion engine

Description:
BACKGROUND INFORMATION 
   The overall efficiency of internal combustion engines, as they are currently used in motor vehicles, is typically maximal at those operating points which are close to full load and at low to moderate engine speeds. In partial-load operation, the energy contained in the fuel is not optimally utilized. As a result, the fuel consumption is higher than would be necessary per se. However, such partial-load operation is the normal operation in motor vehicles having high-performance internal combustion engines. 
   One may attempt to keep the operating point in the range of optimum efficiency as continuously as possible via an optimum design of manual shift transmissions and shifting strategies in automatic transmissions, for example, having a continuous transmission ratio. Another possibility is the concept of so-called “half-engine operation” in which one part of the cylinders operates at a comparatively high load and thus at a comparatively good efficiency. The other cylinders are shut off by interrupting the injection of fuel into these cylinders. For example, in an eight-cylinder internal combustion engine, four cylinders are shut off in this way. 
   An object of the present invention is to provide a method which allows low-emission operation of an internal combustion engine as much as possible while simultaneously allowing low fuel consumption. 
   SUMMARY OF THE INVENTION 
   The present invention allows compensation for a disadvantageous temperature loss in a shut off cylinder by brief and possibly repeated “heating operation.” A shut off cylinder thus cools down less during the cylinder shut off period. When the cylinder shut off period ends, good mixture preparation is possible in the “preheated” cylinder which is now operating again, which in turn results in low emissions and a favorable fuel consumption in the internal combustion engine. Due to the at least essentially torque-neutral combustion of the fuel, the measure according to the present invention does not affect or at least does not detectably affect the comfort in the operation of the internal combustion engine, and without the cylinders which are not shut off having to depart from the optimal operating point for efficiency (high load). This has a favorable effect on the fuel consumption of the internal combustion engine. It is noted here that the method according to the present invention not only provides advantages during half-engine operation, but rather also during overrun fuel cutoff, for example, and the method according to the present invention may be used both in internal combustion engines having intake-manifold fuel injection and also in internal combustion engines having direct fuel injection. 
   In a preferred refinement of the method according to the present invention, fresh combustion air is introduced into the at least one shut off cylinder during the cylinder shut off period only in connection with the operating cycle(s) during which fuel is combusted. The work connected with the charge change is thus saved or at least reduced during a majority of the cylinder shut off period, and as much residual gas as possible may remain enclosed in the cylinder, which is also advantageous. The work needed for dragging along the shut off cylinder is thus reduced and the cooling of the corresponding cylinder combustion chamber is reduced via the resulting higher temperature level. 
   This may in turn be implemented in particular simply by opening at least one intake valve of the at least one shut off cylinder during the cylinder shut off period only in connection with the operating cycle(s) during which the fuel is combusted. 
   The additional fuel consumption due to the injection during the cylinder shut off period is minimal if precisely enough fuel and/or air is introduced into the cylinder, which is shut off per se, to at least approximately compensate, by the combustion of the fuel, for the pressure and/or temperature loss which occurred during preceding operating cycles since the last combustion. This may be implemented easily by opening the at least one intake valve of the at least one shut off cylinder for a significantly shorter time than a corresponding intake stroke lasts. 
   It is also suggested that at least one exhaust valve of the at least one shut off cylinder remain continuously closed during the cylinder shut off period. Therefore, a maximum residual gas quantity remains in the cylinder combustion chamber, which in turn minimizes the work needed for dragging along the shut off cylinder and the temperature loss. 
   In an advantageous refinement of the method according to the present invention, the instant of injection and/or combustion of fuel into the at least one shut off cylinder is made as a function of a temperature of the internal combustion engine and/or a number of operating cycles since the last combustion and/or a current engine speed. This ensures that the temperature and/or pressure of the internal combustion engine is held as accurately as possible at a desired level, without an unnecessarily large number of injections being necessary, which would unnecessarily worsen the fuel consumption and the emission behavior. 
   A simple possibility for the torque-neutral combustion suggested according to the present invention is to combust the introduced fuel at the end of an expansion stroke. At this instant, the piston of the corresponding cylinder is in the area of its bottom dead center, the lever arm on the crankshaft is thus comparatively poor and the cylinder pressure is comparatively low. Another possibility for a torque-neutral combustion is simply to inject such a small quantity of fuel at the end of a compression stroke that leakage losses and the cooling of the combustion chamber in drag operation are precisely compensated for by its combustion, but no or no noteworthy torque is produced. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic illustration of an internal combustion engine. 
       FIG. 2  shows a flow chart of a method for operating the internal combustion engine of  FIG. 1 . 
   

   DETAILED DESCRIPTION 
   An internal combustion engine carries reference numeral  10  as a whole in  FIG. 1 . It is used for driving a motor vehicle (not shown in  FIG. 1 ). Internal combustion engine  10  includes multiple cylinders  11  having combustion chambers  12 , of which only two are shown in  FIG. 1  for the sake of simplicity. The totality of cylinders  11  is composed of a first partial set  14  of cylinders  11  and a second partial set  16  of cylinders  11 . If a total of eight cylinders  11  are assumed, for example, first partial set  14  may include four cylinders  11  and second partial set  16  may also include four cylinders  11 . 
   Combustion air reaches combustion chambers  12  in each case via an intake valve  18  or  20  and an intake manifold  22  or  24 , respectively. A throttle valve  26  or  28  is situated in each intake manifold  22  or  24  belonging to a partial set  14  or  16 , respectively. Fuel reaches combustion chambers  12  in each case directly via injectors  30  and  32 . However, the following statements may also be applied to an internal combustion engine having intake manifold injection. 
   In the present internal combustion engine, a fuel pressure accumulator  34  or  36 , referred to as a “rail,” to which particular injectors  30  or  32  are connected, is assigned to each partial set  14  and  16  of combustion chambers  12 . A fuel-air mixture located in combustion chambers  12  is ignited by a corresponding spark plug  38  or  40 , and the hot combustion gases are discharged to an exhaust pipe  46  via exhaust valves  42  and  44 . 
   Intake valves  18  and  20  and exhaust valves  42  and  44  are equipped with a variable valve gear (not shown), which allows them to be opened and closed completely independently of the position of a crankshaft or camshaft (neither shown) of internal combustion engine  10 . The operation of internal combustion engine  10  is controlled and/or regulated by a control and regulating unit  48 . This unit receives signals from various sensors, such as an accelerator pedal of the motor vehicle, using which a user may express a torque request, and from temperature, pressure, and other sensors which detect the current operating state of internal combustion engine  10 . 
   To keep the fuel consumption of internal combustion engine  10  as low as possible during operation, if only moderate power is required of internal combustion engine  10 , first partial set  14  of combustion chambers  12  of cylinders  11  may be shut off by interrupting the injection of fuel by injectors  30 . In this case, the torque of internal combustion engine  10  is only still produced by the remaining second partial set  16  of cylinders  11  or combustion chambers  12 , whose injectors  32  still directly inject fuel. If a higher output is again needed from internal combustion engine  10 , the injection of fuel by injectors  30  into cylinders  11  or combustion chambers  12  of first partial set  14  is resumed. If fuel is injected into all combustion chambers  12  of first partial set  14  and second partial set  16 , this is referred to as “full-engine operation”; in contrast, if the fuel supply to first partial set  14  of combustion chambers  12  is interrupted, this is referred to as “half-engine operation.” 
   In half-engine operation of internal combustion engine  10  shown in  FIG. 1 , however, not only is the injection of fuel by injectors  30  interrupted, but rather intake valves  18  and exhaust valves  42  of corresponding cylinders  11  of first partial set  14  are permanently closed to save the work connected to the charge change in partial set  14  of cylinders  11 . In addition, a comparatively large quantity of residual gas is thus enclosed in combustion chambers  12  of cylinders  11  of partial set  14 . The work needed for dragging along shut off cylinders  11  of partial set  14  is thus reduced, and cylinders  11  of partial set  14  only cool off comparatively little from the higher temperature level. 
   However, because combustion chambers  12  of first partial set  14  of cylinders  11  are not closed gas-tight due to leaks at intake valves  18 , exhaust valves  42 , and scraper rings (not shown in  FIG. 1 ) of the pistons (also not shown), the mean pressure and the temperature in combustion chambers  12  of first partial set  14  of cylinders  11  gradually sink in half-engine operation, i.e., when cylinders  11  are deactivated. The work to be applied during an operating cycle for the movement of the pistons of cylinders  11  of shut off first partial set  14  of combustion chambers  12  in turn increases, and corresponding cylinders  11  cool off more strongly, which may be disadvantageous in regard to the emissions arising when first partial set  14  is reactivated. To prevent this, a method is followed which will be explained in greater detail with reference to  FIG. 2 . This method is stored in the form of a computer program in a memory of control and regulating unit  48 . 
   After a start in  50 , it is checked in  52  whether a shut off period bit B_off has the value “true.” This would mean that the shut off period of first partial set  14  of cylinders  11 , i.e., half-engine operation, has been implemented by control and regulating unit  48 . If the answer in  52  is yes, a counter n is set to zero in  54 . Subsequently, counter n is incremented by 1 in  56 . In  58 , it is checked whether counter n is greater than a limiting value G. If the answer in  58  is no, the sequence jumps back to before  56 . In contrast, if the answer in  58  is yes, on the one hand, the valve gear of intake valves  18  is briefly opened during an intake stroke of an operating cycle in  60 . The opening duration is significantly shorter than the duration of the total intake stroke. On the other hand, injectors  30  are activated, so that they inject a small quantity of fuel into combustion chambers  12  of cylinders  11  of first partial set  14 . 
   Spark plugs  38  are then activated in such a way that the air-fuel mixture now present in combustion chambers  12  of first partial set  14  of cylinders  11  is combusted at the end of the following expansion stroke. Exhaust valves  42  remain closed during the entire half-engine operation, however. Almost no torque is produced by the combustion of the fuel-air mixture at the end of an expansion stroke. Instead, the mean pressure and temperature are increased in combustion chambers  12  of first partial set  14  of cylinders  11 . 
   Alternatively, fuel may also be injected and combusted entirely normally at the end of a compression stroke in first partial set  14  of cylinders  11 . The quantity is solely to be selected as so small that leakage and cooling caused by drag operation are just compensated for, but no or no noticeable torque is produced. The advantage of such injection and combustion in the compression stroke is better emission behavior because of the higher temperatures in combustion chamber  12  in this operating phase. 
   From block  60 , the sequence jumps back to before block  52 . If it is established in  52  that bit B_off still has the value “true,” intake valves  18  remain closed and no fuel is injected by injectors  30  until a specific number G of operating cycles is again exceeded in block  58 . Thus, fresh combustion air is only introduced into combustion chambers  12  of first partial set  14  of cylinders  11  in connection with the operating cycle during which fuel is injected once by injectors  30  and subsequently combusted in a torque-neutral way. In contrast, if the answer in  52  is no, this means that the half-engine operation has ended. Fresh air is thus again also continuously supplied to combustion chambers  12  of cylinders  11  of first partial set  14 , and fuel is also injected into these combustion chambers by injectors  30  in such a way that a normal torque is produced. The method then ends in  62 .