Abstract:
A system and a method for operating an internal combustion engine at a six-stroke, eight-stroke, or greater number of strokes cycle is disclosed. The combustion of the fuel and air is accomplished in two combustion steps with at least one expansion and compression process in between the two combustions, with the second combustion occurring at a stoichiometric air-fuel ratio. The first combustion at lean air-fuel ratio provides high efficiency. The products of the first combustion are subjected to a second combustion event at stoichiometric air-fuel ratio. Consequently, high conversion efficiency of nitrogen oxides in the exhaust aftertreatment device, available at stoichiometric conditions, can be achieved.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an internal combustion engine operating on a six or more stroke cycle. The combustion is divided into two parts: a first lean combustion and a second stoichiometric combustion. 
     2. Background Art 
     It is known to those skilled in the art that it is more efficient to operate an engine at an air-fuel ratio which is lean of stoichiometric (i.e., higher air-fuel ratio) rather than at stoichiometric for a comparable speed-torque condition. However, most spark-ignition engines produced, at the present, are stoichiometric engines to facilitate the high efficiency conversion of the regulated emissions: carbon monoxide, hydrocarbons, and nitrogen oxides, in an exhaust aftertreatment device. Engines operating at lean air-fuel ratios often use a reductant to reduce nitrogen oxides within an exhaust aftertreatment device, such as a lean NOx catalyst, with the reductant management and delivery system an undesirable additional requirement on the exhaust aftertreatment system. 
     The inventors herein have recognized a method for operating an engine with a lean combustion event without incurring the emission aftertreatment difficulties associated with lean combustion. Specifically, a first combustion event occurs without exhausting the lean products of combustion. The valves in the cylinder remain closed during an expansion stroke and a compression stroke. A fuel injector in the cylinder head provides fuel to the combustion gases in the cylinder. The amount of fuel added is an amount to bring the mixture in the combustion chamber to stoichiometry. The mixture undergoes a second combustion process and is subsequently exhausted into an aftertreatment device. Because the gases arriving at the exhaust aftertreatment device are stoichiometric, the exhaust aftertreatment device can convert nitrogen oxides without supplying a reductant. 
     SUMMARY OF THE INVENTION 
     A method of operating an internal combustion engine, in which fuel injectors are disposed in the cylinders, is disclosed. The method steps include: inducting air into a cylinder of the engine, compressing the air, providing a first amount of a fuel to the air, combusting the first amount of fuel in air by a first combustion, expanding and compressing products of the first combustion, providing a second amount of fuel to the products of the first combustion, and combusting the second amount of fuel. The first amount of fuel is an amount which when mixed with the air causes the contents of the cylinder to have an air-fuel ratio which is leaner than stoichiometric. The second amount of fuel is an amount which when mixed with the products of the first combustion causes the cylinder to have an air-fuel ratio which is substantially stoichiometric. 
     An advantage of the present invention is that a first combustion event may be a lean combustion event (may be stratified or homogeneous lean) without the nitrogen oxide emission aftertreatment difficulties associated with lean combustion. This is possible because the products of combustion of the lean combustion are not exhausted after the first combustion event. In the present invention, the products of the lean combustion participate in a second combustion event, which is homogeneous and stoichiometric. A three-way catalyst aftertreatment device being fed stoichiometric exhaust gases is known, by those skilled in the art, to convert the three regulated emissions: carbon monoxide, hydrocarbons, and nitrogen oxides, at high efficiency. In this way, the high efficiency of stratified combustion can be achieved at an acceptable emission level. 
     A further advantage of the present invention is that the second combustion event occurs within a mixture with a relatively high quantity of hot combustion products. The second combustion produces very low levels of nitrogen oxides. 
     Yet another advantage of the present invention is that because the second combustion event occurs in a homogeneous, stoichiometric mixture, soot production is negligible. 
     In an engine equipped with valves which allow fully flexible timing of valve events, load is controlled primarily by valve timing. However, at the lightest loads and speeds, it is found that stoichiometrically fuelled engines require throttling to ensure acceptable combustion stability. Throttling leads to a fuel efficiency penalty. The inventors of the present invention have recognized that by spreading the torque produced over six or more strokes, the power from the engine is reduced without throttling. Thus, the inventors have devised a way to operate the engine in a manner which results in lower torque without incurring pumping losses due to throttling. 
     Another advantage of the present invention is that inducted air is combusted in two events. The inventors of the present invention have recognized that two small combustion events improve engine smoothness over one large combustion event if compared over the same number of strokes. The ability to have two combustion events without opening the valves is facilitated by providing a fuel injector in communication with the cylinder head, commonly called direct injection. With a direct injection engine, fuel may be provided independently of an air intake process. In contrast, in conventional port injected engines, fuel may be supplied to the cylinder only during an intake process, that is when air carries fuel into the cylinder. The ability of a direct injection engine to supply fuel directly to the combustion chamber provides for combustion of a cylinder&#39;s charge of fresh air occurring in two parts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of a single cylinder of an internal combustion engine which may be operated according to aspects of the present invention; 
     FIG. 2 is a schematic of a multi-cylinder internal combustion engine showing exhaust aftertreatment devices; and 
     FIG. 3 is a time line of a conventional fourstroke engine of prior art and of a six-stroke engine according to an aspect of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1, a single-cylinder of an engine  10  is shown in which the valvetrain has fully flexible events. Intake valve  14  and exhaust valve  16  are actuated electromechanically or electrohydraulically. Intake valve  14  and exhaust valve  16  are opened and closed by actuators  18  and  20 , respectively. The fully flexible valves allow the valve events to occur independently of engine angular position. A single piston  12  and cylinder  26  are shown in FIG. 1; however, engine  10  may be a multi-cylinder engine. Coupled to the combustion chamber are a fuel injector  24  and a spark plug  22  to ignite the mixture. The fuel injector  24  is a means for providing fuel to the cylinder in six-stroke operation when the valves remain closed; fuel can be supplied independently of opening the intake valves. The engine control unit  40  receives input from a variety of sensors  42 , which may be a mass airflow sensor, temperature sensors, accelerator pedal position, engine rotational speed, and others. Based on the information from the sensors  42 , the engine control unit  40  manages the firing of the spark plug  22 , the intake valve  14  actuation, the exhaust valve  16  actuation, and the fuel injector  24  actuation. 
     Referring now to FIG. 2, four cylinders  52  are disposed in engine  10 . FIG. 2 showing four cylinders is merely illustrative as the present invention could be applied to an engine of any number of cylinders. Fresh air is inducted into engine  10  through intake manifold  50 . The products of combustion are exhausted through exhaust manifolds  54  and  56  and passed through exhaust aftertreatment devices  60 ,  62 , and  64 . Exhaust aftertreatment devices  60  and  62  may be close-coupled, three-way catalysts and exhaust aftertreatment device  64  may be an underbody catalyst, which may also be a three-way catalyst. A three-way catalyst oxidizes hydrocarbons and carbon monoxide and reduces nitrogen oxides. A three-way catalyst reacts most efficiently when the exhaust mixture is very close to a stoichiometric air-fuel ratio. Typically, this is accomplished by providing a stoichiometric air-fuel ratio to the combustion chamber. However, in the present invention, two combustion events are accomplished with the six or more cycles of the process. The first combustion event is at a lean air-fuel ratio. It could be either a stratified or homogeneous air-fuel mixture. A stratified mixture is one in which fuel is added to the air immediately prior to the combustion event to prevent mixing between the fuel and the air. The time elapsing between the fuel addition and ignition is controlled, to obtain the desired fuel and air mixedness. Homogeneous combustion refers to premixed fuel and air. This is achieved by injecting the fuel well before ignition to allow time for the fuel to vaporize and mix with the air. Following the first, lean combustion process, additional fuel is added to the products of combustion of the lean combustion to cause the gases in the combustion chamber to be of stoichiometric proportion. In this way, exhaust aftertreatment devices  60 ,  62 , and  64  receive stoichiometric exhaust products. 
     In the upper half of FIG. 3, a conventional four-stroke cycle  70  is shown. During the intake stroke, designated as stroke  1 , the piston moves from top dead center (TDC) to bottom dead center (BDC). The trapezoidal intake valve profile indicates that the intake valve is open during stroke  1 . As the piston moves from TDC to BDC, air is drawn into the cylinder. Often, the intake valve opens prior to TDC and closes after BDC, as shown in FIG.  3 . Stroke  2  is a compression stroke in which the piston moves from BDC to TDC. Combustion is not shown in FIG. 3 because it does not comprise a particular stroke. Instead, it is typically initiated prior to TDC of the compression stroke and continues into the expansion stroke, designated as stroke  3  of FIG.  3 . The combustion may be spark or compression ignited and may be of a homogeneous or stratified air-fuel mixture. The exhaust stroke is designated as stroke  4 ; the dashed trapezoid indicates exhaust valve opening. As the piston moves from BDC to TDC in stroke  4 , the products of combustion are pushed out of the cylinder by the piston. Following stroke  4  is an intake stroke, which is designated as stroke  1  as it indicates a repeat of the process described. In the lower section of FIG. 3, a six-stroke cycle  80  is shown. Strokes  1 - 3  are identical to those discussed in regards to the conventional four-cycle  70  (upper half of FIG.  3 ), with the distinction that a first combustion, which occurs during the end of compressionA stroke and the beginning of expansionA stroke, is a combustion process which consumes only a portion of the air. The fuel supplied for the first combustion may be injected into the combustion chamber during the compression stroke shortly before the combustion process to provide stratified-charge combustion with the combustion initiated by spark ignition. Alternatively, the fuel may be added prior to the compression stroke to allow the fuel to mix with the air prior to combustion. The first combustion may be a homogeneous-charge compression ignition, which occurs without benefit of spark ignition. Strokes  4  and  5  are a second compression stroke and a second expansion stroke, designated as compressionB and expansionB, respectively, in FIG. 3. A second combustion occurs during the end of compressions and the beginning of expansionB. Because the fuel that was supplied to support the first combustion was consumed during the first combustion, additional fuel must be added to the unreacted air. The second fuel addition may be accomplished by fuel injector  24  mounted in the cylinder head near the end of expansionA or the beginning of compressions. The fuel is added to hot combustion products and readily vaporizes. The hot combustion products cause prereaction in the added fuel; the timing of the second fuel addition may be based on ensuring that unwanted, early combustion does not occur. The second combustion may be spark ignited. Alternatively, depending on the operating condition, the second combustion may be compression ignited, which is spontaneous ignition resulting from compression heating of the fuel and air mixture. To achieve high conversion efficiency of the exhaust aftertreatment devices, the second amount of fuel added should be an amount that which, if completely reacted, would consume the fuel and air completely and form only carbon dioxide, water, and nitrogen, thus a stroichiometric mixture. After the exhaust, stroke  6 , the cycle begins again with stroke  1 , an intake stroke. 
     In prior approaches using fully flexible valves, it has been found that throttling of the intake, which leads to pumping losses, cannot be avoided at the lowest torque and speed conditions while maintaining acceptable combustion stability. The present invention is an alternative to throttling for reducing torque produced. Instead of having a combustion event every four strokes, as in a typical four-stroke cycle as shown in the upper half of FIG. 3, the strength of a single combustion stroke is separated into two combustion events over six strokes or more, shown in the lower half of FIG.  3 . The six-stroke cycle compared to the four-stroke cycle, at equal amount of fuel and air combusted per intake stroke, produces approximately two-thirds the average torque. If an eight-stroke cycle were used, the torque produced would be roughly halved. 
     The six-stroke cycle can be lengthened to an eight-stroke or more by adding an additional compression and expansion stroke in which no combustion takes place, which may precede or follow compression and expansionsA or follow compressionB and expansionsB, as shown in the lower half of FIG.  3 . Similarly, more noncombusting compression and expansion strokes may be inserted to extend the cycle. It may be found that to even out torque pulsations that, for example, each cylinder might be operated on an eight-stroke cycle with an additional compression and expansion strokes inserted in between different strokes in various cylinders. 
     While several examples for carrying out the invention have been described, those familiar with the art to which this invention relates will recognize alternative designs and embodiments for practicing the invention. Thus, the above-described embodiments are intended to be illustrative of the invention, which may be modified within the scope of the following claims.