Abstract:
A system and method for stopping an internal combustion engine. According to one aspect of the disclosure, a shutdown sequence of the engine includes retarding ignition timing relative to ignition timing prior to initiation of the shutdown sequence, and then advancing ignition timing relative to ignition timing prior to initiation of the shutdown sequence.

Description:
FIELD 
   The present disclosure is directed toward a system and method for quick-stopping an internal combustion engine. 
   BACKGROUND AND SUMMARY 
   Internal combustion engines convert chemical energy in a fuel to mechanical energy. As part of the conversion, an engine may combust fuel and air, generating several products, or emissions. For example, when hydrocarbons are used as fuel, combustion products can include CO 2  and NO x . 
   In an attempt to reduce tailpipe emissions, efforts have been made to utilize catalysts to react with the undesired emissions, thereby releasing alternative substances from the tailpipe. Other attempts have been made to decrease emissions by decreasing the operating time of an engine, for example, by using an electric motor to provide driving power under some driving conditions. 
   The inventors herein have recognized that several issues are raised by using a catalyst, both with and without an electric motor. In particular, while catalysts can be very effective at converting tailpipe emissions when an engine is operating under normal conditions, catalyst performance can be hindered if an engine pumps oxygen, or another undesired substance, to the catalyst. Such substances can be pumped to a catalyst when an engine is stopping, in particular if combustion has ceased but the pistons are still moving. This can be a particularly pertinent problem with a hybrid electric vehicle, where an engine may frequently stop and start. 
   At least some of the issues associated with saturating a catalyst with oxygen, or another undesired substance, may be addressed by quick-stopping an engine such that when the engine is stopping, burnt combustion gases are pumped to the catalyst instead of oxygen or other undesired substances. In this way, it may be possible to limit tailpipe emissions and/or improve catalyst performance. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  schematically shows a hybrid electric vehicle including an internal combustion engine. 
       FIG. 2  schematically shows the internal combustion engine of  FIG. 1 . 
       FIG. 3  shows relative spark timing before initiating a shutdown sequence and during two different phases of a shutdown sequence. 
   

   DETAILED DESCRIPTION 
   The present disclosure relates to a strategy for quick-stopping an internal combustion engine to limit catalyst saturation from undesired substances, such as oxygen. While discussed in the context of a hybrid electric vehicle, the present disclosure is equally applicable to an internal combustion engine used in virtually any other manner. 
     FIG. 1  somewhat schematically shows a parallel/series (split) configuration hybrid electric vehicle or HEV. In the illustrated HEV, an engine  24  is coupled to a planet carrier  22  of planetary gear set  20 . A one way clutch  26  allows forward rotation and prevents backward rotation of the engine and planet carrier. The planetary gear set  20  also mechanically couples a sun gear  28  to a generator motor  30  and a ring (output) gear  32 . The generator motor  30  also mechanically links to a generator brake  34  and is electrically linked to a battery  36 . A traction motor  38  is mechanically coupled to the ring gear  32  of the planetary gear set  20  via a second gear set  40  and is electrically linked to the battery  36 . The ring gear  32  of the planetary gear set  20  and the traction motor  38  are mechanically coupled to drive wheels  42  via an output shaft  44 . 
   The planetary gear set  20  splits the engine output energy into a series path from engine  24  to generator motor  30  and a parallel path from engine  24  to drive wheels  42 . Engine speed can be controlled by varying the split to the series path while maintaining the mechanical connection through the parallel path. The traction motor  38  augments engine power to the drive wheels on the parallel path through the second gear set  40 . The traction motor  38  also provides the opportunity to use energy directly from the series path, essentially running off power created by the generator motor  30 . This reduces losses associated with converting energy into and out of chemical energy in the battery and allows substantially all engine energy, minus conversion losses, to reach drive wheels  42 . 
   Thus,  FIG. 1  shows engine  24  attached directly to planet carrier  22 , for example without a clutch that can disconnect the engine from the planet carrier. One way clutch  26  allows the shaft to rotate freely in a forward direction, but grounds the shaft to the powertrain&#39;s stationary structure when a torque attempts to rotate the shaft backwards. Brake  34  does not interrupt the connection between the sun gear  28  and generator motor  30 , but can, when energized, ground the shaft between those two components to the powertrain&#39;s stationary structure. 
   A vehicle system controller (VSC)  46  controls many components in this HEV configuration by connecting to each component&#39;s controller. An engine control unit (ECU)  48  can connect to the engine  24  via a hardwire interface. In some embodiments, ECU  48  and VSC  46  can be placed in the same unit and/or serve as the same controller. In some embodiments, ECU  48  and VSC  46  may function as independent controllers, and/or be placed in separate units. The VSC  46  can communicate with the ECU  48 , as well as a battery control unit (BCU)  45  and a transaxle management unit (TMU)  49  through a communication network such as a controller area network (CAN)  33 . The BCU  45  can connect to battery  36  via a hardwire interface. The TMU  49  controls the generator motor  30  and the traction motor  38  via a hardwire interface. The control units  46 ,  48 ,  45  and  49 , and CAN  33  can include one or more microprocessors, computers, or central processing units; one or more computer readable storage devices; one or more memory management units; and one or more input/output devices for communicating with various sensors, actuators and control circuits. 
     FIG. 2  shows an example engine and exhaust system that may be used as engine  24 . Though introduced in the context of a split configuration hybrid electric vehicle above, it should be understood that the quick-stop strategy described below with reference to engine  24  can also be applied to an engine used in a differently configured hybrid electric vehicle, a non-hybrid electric vehicle, and/or non-vehicle application. 
   Internal combustion engine  24  includes a plurality of cylinders, one cylinder of which is shown in  FIG. 2 . The engine can be controlled by a controller including an electronic engine controller and/or a vehicle system controller. Engine  24  includes combustion chamber  29  and cylinder walls  31 , with piston  35  positioned therein and connected to crankshaft  39 . Combustion chamber  29  is shown communicating with intake manifold  43  and exhaust manifold  47  via respective intake valve  52  and exhaust valve  54 . While only one intake and exhaust valve is shown, more than one may be used if desired. 
   Variable valve timing may be effectuated by variable cam timing, although this is not required. In some embodiments, independent intake cam timing can be used with independent exhaust cam timing, and in some embodiments, variable intake cam timing may be used with fixed exhaust cam timing, or vice versa. Also, various types of variable valve timing may be used, such as with hydraulic vane-type actuators  53  and  55  receiving respective cam timing control signals VCTE and VCTI from controller  48 . Cam timing (exhaust and intake) position feedback can be provided via comparison of the crank signal PIP and signals from respective cam sensors  50  and  51 . 
   In some embodiments, cam actuated exhaust valves may be used with electrically actuated intake valves. In such a case, the controller can determine whether the engine is being stopped or pre-positioned to a condition with the exhaust valve at least partially open, and if so, hold the intake valve(s) closed during at least a portion of the engine stopped duration to reduce communication between the intake and exhaust manifolds. 
   Intake manifold  43  is also shown having fuel injector  65  coupled thereto for delivering fuel in proportion to the pulse width of signal FPW from controller  48 . Fuel is delivered to fuel injector  65  by a fuel system (not shown) which can include a fuel tank, fuel pump, and fuel rail (not shown). In some embodiments, the engine may be configured for direct injection (top or side), where the fuel is injected directly into the engine cylinder. In addition, intake manifold  43  is shown communicating with optional electronic throttle  125 . 
   Distributorless ignition system  88  provides ignition spark to combustion chamber  29  via spark plug  92  in response to a signal received from ECU  48 . As described in more detail below, ignition spark timing can be controlled to help limit catalyst saturation during engine shutdown. As used herein, ignition spark time, ignition timing, and/or spark timing refer to the time at which a spark plug for a particular cylinder sparks relative to the position of that cylinder&#39;s piston, generally near the end of the compression stroke (although spark can be retarded into the power stroke). 
   Universal Exhaust Gas Oxygen (UEGO) sensor  76  is shown coupled to exhaust manifold  47  upstream of catalytic converter  70 . Alternatively, a two-state exhaust gas oxygen sensor may be substituted for UEGO sensor  76 . Two-state exhaust gas oxygen sensor  98  is shown coupled to the exhaust system downstream of catalytic converter  70 . Alternatively, sensor  98  can also be a UEGO sensor. Catalytic converter temperature is measured by temperature sensor  77 , and/or estimated based on operating conditions such as engine speed, load, air temperature, engine temperature, and/or airflow, or combinations thereof. 
   Converter  70  can include multiple catalyst bricks, in some embodiments. In some embodiments, multiple emission control devices, each which can have multiple bricks, can be used. Converter  70  can be a three-way type catalyst in some embodiments. 
   A controller including ECU  48  is shown in  FIG. 2 . ECU  48  includes a microprocessor unit  102 , input/output ports  104 , read-only memory  106 , random access memory  108 , keep alive memory  110 , and a data bus. ECU  48  is shown receiving various signals from sensors coupled to engine  24 , in addition to those signals previously discussed, including: engine coolant temperature (ECT) from temperature sensor  112  coupled to cooling sleeve  114 ; a position sensor  119  coupled to an accelerator pedal; a measurement of engine manifold pressure (MAP) from pressure sensor  122  coupled to intake manifold  43 ; a measurement (ACT) of engine air charge temperature or manifold temperature from temperature sensor  117 ; and an engine position sensor from a Hall effect sensor  118  sensing crankshaft  39  position. In one aspect of the present description, engine position sensor  118  produces a predetermined number of equally spaced pulses every revolution of the crankshaft from which engine speed (RPM) can be determined. 
   ECU  48  can control ignition timing of engine  24  while the engine is running. It should be understood that ECU  48  can also control many other aspects of engine operation, although such control need not be extensively described in order to teach the spark timing used to quick-stop the engine. In general, ECU  48  can control ignition timing (and other aspects of engine operation) so that engine  24  produces a desired torque while running. In many operating conditions, the ECU can set spark timing to occur at various times during the power stroke of a four stroke engine. Nonlimiting examples of such timing can include, but are not limited to, at top dead center (TDC), 5 degrees after TDC, 10 degrees after TDC, 15 degrees after TDC, etc. While the engine is running, adjustments can be continuously made to the spark timing. 
   Various conditions may arise in which it is desirable to shut down engine  24 . For example, the vehicle may be completely turned off and parked. As another example, an electric motor of an HEV may be capable of delivering all the power necessary to drive the HEV, thus enabling the internal combustion engine to be shut down. While an engine can be shut down simply by ceasing fuel injection and/or sparking all together, such a technique will not cause the engine to immediately stop moving. The pistons will continue to pump as the engine coasts down. If there are no combustion events, and the pistons are still pumping, air will be forced through the exhaust system, including the catalyst. The catalyst can become saturated with oxygen, or another undesired substance, under such circumstances. As a result, the catalyst will be less efficient at reducing NO x . Also, the excess air can tend to cool the catalyst, and the intake manifold fuel puddles can be depleted. 
   In some embodiments, this problem can be addressed by limiting (or even eliminating) the number of engine cycles that occur without a combustion event. Instead of simply stopping fuel injection and/or spark so that no combustion event occurs, a shutdown sequence can be initiated in which the engine is stopped. 
   A shutdown sequence can include a first phase in which the engine speed is slowed. In order to slow the engine speed, the spark can be retarded to occur relatively late during the power stroke, so as to generate less than optimal power output from the combustion event. In some embodiments, the spark can be severely retarded to occur very late in the power stroke. Such a delayed spark can help slow the engine while combustion gases, as opposed to fresh air, are delivered to the catalyst. 
   The spark can be delayed for any number of engine cycles. In some embodiments, the spark can be delayed in two or more cylinders. Furthermore, the spark can be delayed by the same amount for two or more different combustion events, or the relative amount of delay can change from one combustion event to the other. In some embodiments, the spark is progressively delayed in one or more cylinders for one or more engine cycles. 
   In some embodiments, additional and/or alternative mechanisms can be used to slow the engine. For example, an electric motor and generator assembly can be used to slow an engine in a HEV, while at the same time storing energy that can be used by the vehicle. 
   After the spark is first retarded to allow the engine to slow, the shutdown sequence can enter a second phase in which the spark is advanced into the compression stroke. In some embodiments, the spark can be advanced to occur early in the compression stroke. A premature ignition can cause the engine to slow as pressure builds on the compression stroke. By causing a combustion event to occur during the compression stroke, the engine can be rapidly stopped. In some embodiments, spark is not advanced into the compression stroke until the engine speed has slowed, during the spark retarding phase of the shutdown sequence, sufficiently so that the engine can be stopped on the first early ignition event. Even if the engine fails to stop on the first early ignition event, the net torque will be nearly zero, and the engine will continue to decelerate and stop on a subsequent compression stroke. In some embodiments, an advanced spark can be used in two or more cylinders and/or for two or more engine cycles. 
   Using the above described shutdown sequence, the engine can be stopped while the catalyst and the rest of the exhaust system contain substantially only burned exhaust gases and the cylinders contain either exhaust gases or a combustible unburned mixture. On the following restart, the ignition can be triggered for each cylinder at the normal times as the engine starts to rotate. This quick stop and restart procedure can help avoid the over saturation of the catalyst with oxygen and the subsequent loss of NO x  treatment efficiency. 
     FIG. 3  shows a flow chart  200  that demonstrates how the spark timing can be changed during an engine shutdown sequence. At  202 , a reference spark timing  210  that corresponds to a running engine is shown. It should be understood that the precise spark timing can change substantially throughout engine operation, and the illustrated spark timing is meant only to provide a reference to which spark timing during engine shutdown can be compared. The actual spark timing while an engine is running can be more advanced or retarded than shown. 
   As shown at  204 , spark timing  212  can be retarded (made to occur later in the power stroke) when an engine shutdown sequence is initiated. The amount of ignition delay can be set to provide a desired magnitude of engine deceleration. In some embodiments, increased ignition delay can correspond to increased engine deceleration. The illustrated spark timing is not limiting, but rather illustrates one possible spark timing that is retarded relative to spark timing  210 . 
   As shown at  206 , spark timing  214  can be advanced (made to occur sooner in the power stroke or in the compression stroke) to further advance the engine shutdown sequence. Again, the illustrated spark timing is not limiting, but rather illustrates one possible spark timing that is advanced relative to spark timings  210  and  212 . The spark advance can be set to begin when engine speed is sufficiently decelerated so that a limited number of advanced spark combustion events can bring the engine to a complete stop.