Patent Publication Number: US-8978601-B2

Title: Six-stroke engine system with blowdown exhaust system

Description:
TECHNICAL FIELD 
     This patent disclosure relates generally to internal combustion engines and, more particularly, to internal combustion engines that are configured to operate on a six-stroke internal combustion cycle. 
     BACKGROUND 
     Internal combustion engines operating on a six-stroke cycle are generally known in the art. In a six-stroke cycle, a piston reciprocally disposed in a cylinder moves through an intake stroke from a top dead center (TDC) position to a bottom dead center (BDC) position to admit air or an air mixture that includes fuel and/or recirculated exhaust gas into the cylinder. During a compression stroke, the piston moves towards the TDC position to compress the air mixture. During this process, an initial or additional fuel charge may be introduced to the cylinder by an injector. Ignition of the compressed mixture increases the pressure in the cylinder and forces the piston towards the BDC position during a first power stroke. In accordance with the six-stroke cycle, the piston performs a second compression stroke in which it recompresses the combustion products remaining in the cylinder after the first combustion or power stroke. During this recompression, any exhaust valves associated with the cylinder remain generally closed to assist cylinder recompression. Optionally, a second fuel charge and/or additional air may be introduced into the cylinder during recompression to assist igniting the residual combustion products and produce a second power stroke. Following the second power stroke, the cylinder undergoes an exhaust stroke when the exhaust valve or valves open to permit the substantial evacuation of combustion products from the cylinder. One example of an internal combustion engine configured to operate on a six-stroke engine can be found in U.S. Pat. No. 7,418,928. This disclosure relates to a method of operating an engine that includes compressing part of the combustion gas after a first combustion stroke of the piston as well as an additional combustion stroke during a six-stroke cycle of the engine. 
     Some possible advantages of the six-stroke cycle over the more common four-stroke cycle can include reduced emissions and improved fuel efficiency. For example, the second combustion event and second power stroke can provide for a more complete combustion of soot and/or fuel that may remain in the cylinder after the first combustion event. Although the six-stroke method provides some advantages, its implementation with other technologies and its compatibility with other technologies has not yet been entirely understood. 
     SUMMARY 
     In one aspect, the disclosure describes an internal combustion engine system operating on a six-stroke cycle including an engine. The engine includes a combustion chamber having a piston reciprocally disposed in a cylinder to move between a top dead center position and a bottom dead center position. The combustion chamber further includes an exhaust valve adapted to open and close to selectively expel exhaust gasses from the combustion chamber during an exhaust stroke, and a blowdown exhaust valve adapted to open and close to selectively expel blowdown exhaust gasses from the combustion chamber during a recompression stroke. The engine system also includes an exhaust line communicating with the engine to direct exhaust gasses out of the combustion chamber, and a blowdown exhaust line communicating with the engine to direct blowdown exhaust gasses out of the combustion chamber and into the exhaust line. The blowdown exhaust gasses are expelled through the blowdown exhaust valve during the recompression stroke, and the exhaust gasses are expelled through the exhaust valve during the exhaust stroke. 
     In another aspect, the disclosure describes a method of reducing emissions from an internal combustion engine operating a six-stroke cycle. The method includes introducing air into a combustion chamber of the internal combustion engine during an intake stroke. The method also includes compressing the air in the combustion chamber during a first compression stroke. The method includes introducing a first fuel charge into the combustion chamber during the first compression stroke to form a compressed fuel and air mixture, and combusting the compressed fuel and air mixture in the combustion chamber at the completion of the first compression stroke, thereby expanding the fuel and air mixture during a first power stroke and resulting in intermediate combustion products within the combustion chamber. The method includes compressing at least part of the intermediate combustion products within the combustion chamber during a second compression stroke, and opening a blowdown exhaust valve to expel at least a portion of the intermediate combustion products from the combustion chamber into a blowdown exhaust line as blowdown exhaust gasses between commencement of the first power stroke and completion of the second compression stroke. The method includes closing the blowdown exhaust valve to halt expulsion of blowdown exhaust gasses from the combustion chamber between commencement of the first power stroke and completion of the second compression stroke. The method includes combusting the compressed fuel and air mixture in the combustion chamber at the completion of the second compression stroke, thereby expanding the fuel and air mixture during a second power stroke and resulting in second combustion products within the combustion chamber. The method also includes opening an exhaust valve to expel at least a portion of the second combustion products from the combustion chamber into an exhaust line as exhaust gasses between commencement of the second power stroke and the completion of an exhaust stroke, and directing the blowdown exhaust gasses from the blowdown exhaust line into the exhaust line. 
     In yet another embodiment, the disclosure describes a machine that includes an engine. The engine includes a combustion chamber including a piston reciprocally disposed in a cylinder to move between a top dead center position and a bottom dead center position. The combustion chamber further includes an exhaust valve adapted to open and close to selectively expel exhaust gasses from the combustion chamber during an exhaust stroke, and a blowdown exhaust valve adapted to open and close to selectively expel blowdown exhaust gasses from the combustion chamber during a recompression stroke. The engine also includes an exhaust line communicating with the engine to direct exhaust gasses out of the combustion chamber, and a blowdown exhaust line communicating with the engine to direct blowdown exhaust gasses out of the combustion chamber and into the exhaust line. The blowdown exhaust gasses are expelled through the blowdown exhaust valve during the recompression stroke, and exhaust gasses are expelled through the exhaust valve during an exhaust stroke. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an engine system having an internal combustion engine adapted for operation in accordance with a six-stroke combustion cycle and associated systems and components for performing the combustion process in accordance with the disclosure. 
         FIGS. 2-8  are cross-sectional views representing an engine cylinder and a piston movably disposed therein at various points during a six-stroke combustion cycle in accordance with the disclosure. 
         FIG. 9  is a chart representing the lift of an intake valve and an exhaust valve for an engine cylinder as measured against crankshaft angle for a six-stroke combustion cycle in accordance with the disclosure. 
         FIG. 10  is a chart illustrating a trace of the internal cylinder pressure as measured against crankshaft angle for a six-stroke combustion cycle in accordance with the disclosure. 
         FIG. 11  is a block diagram of another embodiment of an engine system having an internal combustion engine in accordance with the disclosure. 
         FIG. 12  is a cross-sectional view representing an engine cylinder and a piston movably disposed therein at a point during a six-stroke combustion cycle in accordance with the disclosure. 
         FIG. 13  is a flowchart depicting a method of operating an engine system having an internal combustion engine in accordance with the disclosure. 
         FIG. 14  is a flowchart depicting another method of operating an engine system having an internal combustion engine in accordance with the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure generally relates to an internal combustion engine and, more particularly, to one adapted to perform a six-stroke cycle for reduced emissions and improved efficiencies. Internal combustion engines burn a hydrocarbon-based fuel or another combustible fuel to convert the potential or chemical energy therein to mechanical power. In one embodiment, the disclosed engine may be a compression ignition engine, such as a diesel engine, in which a mixture of air and fuel is compressed in a cylinder to raise the pressure and temperature of the mixture to a point of at which auto-ignition or spontaneous ignition occurs. Compression ignition engines typically lack sparkplugs, which are typically associated with cylinders of gasoline burning engines. In the present disclosure, the utilization of different fuels such as gasoline and different ignition methods, for example, use of diesel as a pilot fuel to ignite gasoline or natural gas, are contemplated and fall within the scope of the disclosure. 
     Now referring to  FIG. 1 , wherein like reference numbers refer to like elements, there is illustrated a block diagram representing an internal combustion engine system  100 . The engine system  100  includes an internal combustion engine  102  and, in particular, a diesel engine that combusts a mixture of air and diesel fuel. In the present description, it is contemplated that the air provided to the cylinder may be in the form of a mixture of air and exhaust gas. The illustrated internal combustion engine  102  includes an engine block  104  in which a plurality of combustion chambers  106  are disposed. Although six combustion chambers  106  are shown in an inline configuration, in other embodiments fewer or more combustion chambers may be included or another configuration such as a V-configuration may be employed. The engine system  100  can be utilized in any suitable application including mobile applications such as motor vehicles, work machines, locomotives or marine engines, and stationary applications such as electrical power generators. 
     To supply the fuel that the engine  102  burns during the combustion process, a fuel system  110  is operatively associated with the engine system  100 . The fuel system  110  includes a fuel reservoir  112  that can accommodate a hydrocarbon-based fuel such as liquid diesel fuel. Although only one fuel reservoir is depicted in the illustrated embodiment, it will be appreciated that in other embodiments additional reservoirs may be included that accommodate the same or different types of fuels that may also be burned during the combustion process. Because the fuel reservoir  112  is often situated in a remote location with respect to the engine  102 , a fuel line  114  can be disposed through the engine system  100  to direct the fuel from the fuel reservoir to the engine. To pressurize the fuel and force it through the fuel line  114 , a fuel pump  116  can be disposed in the fuel line. An optional fuel conditioner  118  may also be disposed in the fuel line  114  to filter the fuel or otherwise condition the fuel by, for example, introducing additives to the fuel, heating the fuel, removing water and the like. 
     To introduce the fuel to the combustion chambers  106 , the fuel line  114  may be in fluid communication with one or more fuel injectors  120  that are associated with the combustion chambers. In the illustrated embodiment, one fuel injector  120  is associated with each combustion chamber but in other embodiments different numbers of injectors might be included. Additionally, while the illustrated embodiment depicts the fuel line  114  terminating at the fuel injectors, the fuel line may establish a fuel loop that continuously circulates fuel through the plurality of injectors and, optionally, delivers unused fuel back to the fuel reservoir  112 . The fuel injectors  120  can be electrically actuated devices that selectively introduce a measured or predetermined quantity of fuel to each combustion chamber  106 . In other embodiments, introduction methods other than fuel injectors, such as a carburetor or the like, can be utilized. 
     To supply the air that is combusted with the fuel in the combustion chambers  106 , a hollow runner or intake manifold  130  can be formed in or attached to the engine block  104  such that it extends over or proximate to each of the combustion chambers. The intake manifold  130  can communicate with an intake line  132  that directs air to the internal combustion engine  102 . Fluid communication between the intake manifold  130  and the combustion chambers  106  can be established by a plurality of intake runners  134  extending from the intake manifold. One or more intake valves  136  can be associated with each combustion chamber  106  and can open and close to selectively introduce the intake air from the intake manifold  130  to the combustion chamber. While the illustrated embodiment depicts the intake valves at the top of the combustion chamber  106 , in other embodiments the intake valves may be placed at other locations such as through a sidewall of the combustion chamber. To direct the exhaust gasses produced by combustion of the air/fuel mixture out of the combustion chambers  106 , an exhaust manifold  140  communicating with an exhaust line  142  can also be disposed in or proximate to the engine block  104 . The exhaust manifold  140  can communicate with the combustion chambers  106  by exhaust runners  144  extending from the exhaust manifold  140 . The exhaust manifold  140  can receive exhaust gasses by selective opening and closing of one or more exhaust valves  146  associated with each chamber. 
     To actuate the intake valves  136  and the exhaust valves  146 , the illustrated embodiment depicts an overhead camshaft  148  that is disposed over the engine block  104  and operatively engages the valves, but other valve activation arrangements and structures can be used. As will be familiar to those of skill in the art, the camshaft  148  can include a plurality of eccentric lobes disposed along its length that, as the camshaft rotates, cause the intake and exhaust valves  136 ,  146  to displace or move up and down in an alternating manner with respect to the combustion chambers  106 . The placement or configuration of the lobes along the camshaft  148  controls or determines the gas flow through the internal combustion engine  102 . In an embodiment, the camshaft  148  can be configured to selectively control the relative timing and the duration of the valve opening and closing events through a process referred to as variable valve timing. Various arrangements for achieving variable valve timing are known. In one embodiment, contoured lobes formed on the camshaft  148  are manipulated to alter the timing and duration of valve events by moving the camshaft along its axis to expose the valve activators to changing lobe contours. To implement these adjustments in the illustrated embodiment, the camshaft  148  can be associated with a camshaft actuator  149 . As is known in the art, other methods exist for implementing variable valve timing such as additional actuators acting on the individual valve stems and the like. 
     To assist in directing the intake air to and exhaust gasses from the internal combustion engine  102 , the engine system  100  can include a turbocharger  150 . The turbocharger  150  includes a compressor  152  disposed in the intake line  132  that compresses intake air drawn from the atmosphere and directs the compressed air to the intake manifold  130 . Although a single turbocharger  150  is shown, more than one such device connected in series and/or in parallel with another can be used. To power the compressor  152 , a turbine  156  can be disposed in the exhaust line  142  and can receive pressurized exhaust gasses from the exhaust manifold  140 . The pressurized exhaust gasses directed through the turbine  156  can rotate a turbine wheel having a series of blades thereon, which powers a shaft that causes a compressor wheel to rotate within the compressor housing. 
     To filter debris from intake air drawn from the atmosphere, an air filter  160  can be disposed upstream of the compressor  152 . In some embodiments, the engine system  100  may be open-throttled wherein the compressor  152  draws air directly from the atmosphere with no intervening controls or adjustability. In such systems, engine speed is primarily controlled by the amount of and timing at which fuel is introduced to the combustion chambers. However, in other embodiments, to assist in controlling or governing the amount of air drawn into the engine system  100 , an adjustable governor or intake throttle  162  can be disposed in the intake line  132  between the air filter  160  and the compressor  152  to provide a means of controlling the air intake of the engine, but other means, such as by use of variable valve timing, can be used for this purpose. Because the intake air may become heated during compression, an intercooler  166  such as an air-to-air heat exchanger can be disposed in the intake line  132  between the compressor  152  and the intake manifold  130  to cool the compressed air. 
     To reduce emissions and assist adjusted control over the combustion process, the engine system  100  can mix the intake air with a portion of the exhaust gasses drawn from the exhaust system of the engine through a system or process called exhaust gas recirculation (EGR). The EGR system forms an intake air/exhaust gas mixture that is introduced to the combustion chambers. In one aspect, addition of exhaust gasses to the intake air displaces the relative amount of oxygen in the combustion chamber during combustion that results in a lower combustion temperature and reduces the generation of nitrogen oxides. Two exemplary EGR systems are shown associated with the engine system  100  in  FIG. 1 , but it should be appreciated that these illustrations are exemplary and that either one, both, or neither can be used on the engine. It is contemplated that selection of an EGR system of a particular type may depend on the particular requirements of each engine application. 
     In the first embodiment, a high-pressure EGR system  170  operates to direct high-pressure exhaust gasses to the intake manifold  130 . The high-pressure EGR system  170  includes a high-pressure EGR line  172  that communicates with the exhaust line  142  downstream of the exhaust manifold  140  and upstream of the turbine  156  to receive the high-pressure exhaust gasses being expelled from the combustion chambers  106 . The system is thus referred to as a high-pressure EGR system  170  because the exhaust gasses received have yet to depressurize through the turbine  156 . The high-pressure EGR line  172  is also in fluid communication with the intake manifold  130 . To control the amount or quantity of the exhaust gasses combined with the intake air, the high-pressure EGR system  170  can include an adjustable EGR valve  174  disposed along the high-pressure EGR line  172 . Hence, the ratio of exhaust gasses mixed with intake air can be varied during operation by adjustment of the adjustable EGR valve  174 . Because the exhaust gasses may be at a sufficiently high temperature that may affect the combustion process, the high-pressure EGR system can also include an EGR cooler  176  disposed along the high-pressure EGR line  172  to cool the exhaust gasses. 
     In the second embodiment, a low-pressure EGR system  180  directs low-pressure exhaust gasses to the intake line  132  before it reaches the intake manifold  130 . The low-pressure EGR system  180  includes a low-pressure EGR line  182  that communicates with the exhaust line  142  downstream of the turbine  156  so that it receives low-pressure exhaust gasses that have depressurized through the turbine. The low-pressure exhaust gasses are delivered to the engine intake system upstream of the compressor  152  so they can mix and be compressed with the incoming air. The system is thus referred to as a low-pressure EGR system because it operates using depressurized exhaust gasses. To control the quantity of exhaust gasses re-circulated, the low-pressure EGR line  182  can also include an adjustable EGR valve  184 . 
     To further reduce emissions generated by the combustion process, the engine system  100  can include one or more after-treatment devices disposed along the exhaust line  142  that treat the exhaust gasses before they are discharged to the atmosphere. One example of an after-treatment device is a diesel particulate filter (DPF)  190  that can trap or capture particulate matter in the exhaust gasses. As the DPF becomes filled with particulate matter, it undergoes a process known as regeneration in which the particulate matter is oxidized. Regeneration may be done either passively or actively. Passive regeneration utilizes heat inherently produced by the engine to burn or incinerate the captured particulate matter. Active regeneration generally requires higher temperature and employs an added heat source such as a burner to heat the DPF. Another after-treatment device that may be included with the engine system is a selective catalytic reduction (SCR) system  192 . In an SCR system  192 , the exhaust gasses are combined with a reductant agent such as ammonia or urea and are directed through a catalyst that chemically converts or reduces the nitrogen oxides in the exhaust gasses to nitrogen and water. To provide the reductant agent, a separate storage tank  194 , which is placed in fluid transfer with the SCR catalyst, may be associated with the SCR system. A diesel oxidation catalyst  196  is a similar after-treatment device that includes metals such as palladium and platinum that can act as catalysts to convert hydrocarbons and carbon monoxide in the exhaust gasses to carbon dioxide. Other types of catalytic converters, three way converters, mufflers and the like can also be included as possible after-treatment devices. 
     Reduction of emissions generated by the combustion process and a means to control the peak cylinder pressure, and thus the power generated by the second combustion stroke, can also be achieved by utilizing a blowdown exhaust system  301  that is associated with the engine.  FIG. 11  illustrates an engine system  300  that includes a blowdown exhaust system  301  to reduce emissions generated by an internal combustion engine  302  and to control the peak cylinder pressures in the engine cylinders during the second combustion stroke. Along these lines, the blowdown exhaust system  301  is configured to bleed off a predetermined amount of exhaust gas (and other combustion byproducts) from each engine cylinder while the cylinder is undergoing a recompression stroke. The selective withdrawal of exhaust gas from the cylinders can be accomplished by variably controlling blowdown exhaust valves  310 . In this way, the materials present in the cylinder at the initiation of the second combustion stroke can be better controlled and, thus, the power output, peak cylinder pressure and emissions generated by the second combustion stroke can be controlled as well. 
     In  FIG. 11 , various components and systems shown in  FIG. 1  have been omitted for clarity but is should be appreciated that such components and systems can be part of the engine system  300 , as applicable. In reference to the embodiment illustrated in  FIG. 11 , the illustrated blowdown exhaust system  301  includes a blowdown exhaust line  305  separate from the exhaust line  142 . In embodiments that include a blowdown exhaust system  301 , fluid communication between the combustion chamber  306  and the blowdown exhaust line  305  can be established by blowdown exhaust runners  307  extending from the blowdown exhaust line. As shown, the blowdown exhaust runners  307  are formed separate from the exhaust runners  144 , which interconnect the combustion chamber  306  with the exhaust manifold  140 . 
     One or more blowdown exhaust valves  310  can be associated with each combustion chamber  306  and can open and close to selectively expel blowdown exhaust gasses from the combustion chamber to the blowdown exhaust line  305 . Thus, two separate paths for exhaust gas from the cylinders are created—the main path for exhaust gas passing through the exhaust valves  146 , and a parallel path for blowdown exhaust gas passing through the blowdown exhaust valves  310 . The blowdown exhaust line  305  directs the blowdown exhaust gasses into the exhaust line  142 .  FIG. 11  shows the blowdown exhaust line  305  directing the blowdown exhaust gasses into the exhaust line  142  downstream of the turbine  156 . The blowdown gas line  305  can alternatively introduce the blowdown exhaust gasses into the exhaust line upstream of the turbine  156 . 
     Returning now to  FIG. 1 , to coordinate and control the various systems and components associated with the engine system  100 , the system can include an electronic or computerized control unit, module or controller  200 . The controller  200  is adapted to monitor various operating parameters and to responsively regulate various variables and functions affecting engine operation. The controller  200  can include a microprocessor, an application specific integrated circuit (“ASIC”), or other appropriate circuitry and can have memory or other data storage capabilities. The controller can include functions, steps, routines, data tables, data maps, charts and the like saved in and executable from electronic memory means that are readable and writable to control the engine system. Although in  FIG. 1 , the controller  200  is illustrated as a single, discrete unit, in other embodiments, the controller and its functions may be distributed among a plurality of distinct and separate components. To receive operating parameters and send control commands or instructions, the controller can be operatively associated with and can communicate with various sensors and controls on the engine system  100 . Communication between the controller and the sensors can be established by sending and receiving digital or analog signals across electronic communication lines or communication busses. In  FIG. 1 , the various communication and command channels are indicated in dashed lines for illustration purposes. 
     For example, to monitor the pressure and/or temperature in the combustion chambers  106 , the controller  200  may communicate with chamber sensors  210  such as a transducer or the like, one of which may be associated with each combustion chamber  106  in the engine block  104 . The chamber sensors  210  can monitor the combustion chamber conditions directly or indirectly, for example, by measuring the backpressure exerted against the intake or exhaust valves, or other components that directly or indirectly communicate with the combustion cylinder such as glow plugs. During combustion, the chamber sensors  210  and the controller  200  can indirectly measure the pressure in the combustion chamber  106 . The controller can also communicate with an intake manifold sensor  212  disposed in the intake manifold  130  and that can sense or measure the conditions therein. To monitor the conditions such as pressure and/or temperature in the exhaust manifold  140 , the controller  200  can similarly communicate with an exhaust manifold sensor  214  disposed in the exhaust manifold  140 . From the temperature of the exhaust gasses in the exhaust manifold  140 , the controller  200  may be able to infer the temperature at which combustion in the combustion chambers  106  is occurring. 
     To measure the flow rate, pressure and/or temperature of the air entering the engine, the controller  200  can communicate with an intake air sensor  220 . The intake air sensor  220  may be associated with, as shown, the intake air filter  160  or another intake system component such as the intake manifold. The intake air sensor  220  may also determined or sense the barometric pressure or other environmental conditions in which the engine system is operating. 
     For controlling the combustion process, the controller  200  can communicate with injector controls  230  that can control the fuel injectors  120  operatively associated with the combustion chambers  106 . The injector controls  230  can selectively activate or deactivate the fuel injectors  120  to determine the timing of introduction and the quantity of fuel introduced by each fuel injector, for example, by further monitoring and control of the injection pressure of fuel provided to the fuel injectors  120 . Regarding control of valve timing, the controller  200  can also communicate with a camshaft control  232  that is operatively associated with the camshaft  148  and/or camshaft actuator  149  to control the variable valve timing, when such a capability is used. 
     In embodiments having an intake throttle  162 , the controller  200  can communicate with a throttle control  240  associated with the throttle and that can control the amount of air drawn into the engine system  100 . Alternatively, the amount of air used by the engine may be controlled by variably controlling the intake valves in accordance with a Miller cycle, which includes maintaining intake valves open for a period during the compression stroke and/or closing intake valves early during an intake stroke to thus reduce the amount of air compressed in the cylinder during operation. The controller  200  can also be operatively associated with either or both of the high-pressure EGR system  170  and/or the low-pressure EGR system  180 . For example, the controller  200  is communicatively linked to a high-pressure EGR control  242  associated with the adjustable EGR valve  174  disposed in the high-pressure EGR line  182 . Similarly, the controller  220  can also be communicatively linked to a low-pressure EGR control  244  associated with the adjustable EGR valve  184  in the low-pressure EGR line  182 . The controller  220  can thereby adjust the amount of exhaust gasses and the ratio of intake air/exhaust gasses introduced to the combustion process. 
     The engine system  100  can operate in accordance with a six-stroke combustion cycle in which the reciprocal piston disposed in the combustion chamber makes six or more strokes between the top dead center (TDC) position and bottom dead center (BDC) position during each cycle. A representative series of six strokes and the accompanying operations of the engine components associated with the combustion chamber  106  are illustrated in  FIGS. 2-8  and the valve lift and related cylinder pressure are charted with respect to crank angle in  FIGS. 9 and 10 . Additional strokes, for example, 8-stroke or 10-stroke operation and the like, which would include one or more successive recompressions, are not discussed in detail herein as they would be similar to the recompression and recombustion that is discussed, but are contemplated to be within the scope of the disclosure. 
     The strokes are performed by a reciprocal piston  250  that is slidably disposed in a cylinder  252  bored into the engine block. One end of the cylinder  252  is closed off by a flame deck surface  254  so that the combustion chamber  106  defines an enclosed space between the piston  250 , the flame deck surface and the inner wall of the cylinder. The reciprocal piston  250  moves between the TDC position where the piston is closest to the flame deck surface  254  and the BDC position where the piston is furthest from the flame deck surface. The motion of the piston  250  with respect to the flame deck surface  254  thereby defines a variable volume  258  that expands and contracts. 
     Referring to  FIG. 2 , the six-stroke cycle starts with an intake stroke during which the piston  250  moves from the TDC position to the BDC position causing the variable volume  258  to expand. During this stroke, the intake valve  136  is opened so that air or an air/fuel mixture may be directed into the combustion chamber  106 , as represented by the exemplary positive bell-shaped intake curve  270  indicating intake valve lift in  FIG. 9 . The duration of the intake valve opening and the shape of the intake curve  270  may optionally be adjusted to control the amount of air provided to the cylinder. Referring to  FIG. 3 , once the piston  250  reaches the BDC position, the intake valve  136  closes and the piston can perform a first compression stroke moving back toward the TCD position and compressing the variable volume  258  that has been filled with air during the intake stroke. As indicated by the upward slope of the first compression curve  280  in  FIG. 10 , this motion increases pressure and relatedly temperature in the combustion chamber. In diesel engines, the compression ratio can be on the order of 15:1 although other compression ratios are common. 
     As illustrated in  FIG. 4 , in those embodiments in which air or a mixture of air with exhaust gas is initially drawn into the combustion chamber  106 , the fuel injector  120  can introduce a first fuel charge  260  into the variable volume  258  to create an air/fuel mixture as the piston  250  approaches the TDC position. The quantity of the first fuel charge  260  can be such that the resulting air/fuel mixture is lean, meaning there is an excess amount of oxygen to the quantity of fuel intended to be combusted. At an instance when the piston  250  is at or close to the TDC position and the pressure and temperature are at or near a first maximum pressure, as indicated by point  282  in  FIG. 10 , the air/fuel mixture may ignite. In embodiments where the fuel is less reactive, such as in gasoline burning engines, ignition may be induced by a sparkplug, by ignition of a pilot fuel or the like. During a first power stroke, the combusting air/fuel mixture expands forcing the piston  250  back to the BDC position as indicated in  FIGS. 4 to 5 . The piston  250  can be linked or connected to a crankshaft  256  so that its linear motion is converted to rotational motion that can be used to power an application or machine. The expansion of the variable volume  258  during the first power stroke also reduces the pressure in the combustion chamber  106  as indicated by the downward sloping first expansion curve  284  in  FIG. 10 . At this stage, the variable volume contains the resulting combustion products  262  that may include unburned fuel, soot, ash and excess oxygen from the intake air. 
     Referring to  FIG. 6 , in the six-stroke cycle, the piston  250  can perform another compression stroke in which it compresses the combustion products  262  in the variable volume  258  by moving back to the TDC position. During the second compression stroke, both the intake valve  136  and exhaust valve,  146  are typically closed so that pressure increases in the variable volume as indicate by the second compression curve  286  in  FIG. 10 . However, in some embodiments, to prevent too large a pressure spike, the exhaust valve  146  may be briefly opened to discharge some of the contents as blowdown exhaust gasses in a process referred to as blowdown, as indicated by the small blowdown curve  272  in  FIG. 9 . 
     In reference to the embodiment illustrated in  FIG. 11 , which includes a dedicated blowdown exhaust valve  310  associated with each cylinder,  FIG. 12  illustrates an embodiment of a combustion chamber  306  of an engine  302  during the second compression stroke in an engine system  300  featuring a blowdown exhaust system  301 . As shown in  FIG. 12 , the blowdown exhaust valve  310 , rather than the main exhaust valves  146 , may briefly open during the second compression stroke to discharge some of the combustion products  362  out of the variable volume  358  as blowdown exhaust gasses. The blowdown exhaust gasses can be directed into the blowdown exhaust line  305  through the blowdown exhaust runners  307 . The blowdown exhaust line  305  directs the blowdown exhaust gasses to a point in the engine system  300  to be expelled from the engine system, such as into exhaust line  142  upstream or downstream from turbine  156 . The specific timing for selectively opening and closing the blowdown exhaust valve  310  can be achieved with variable valve timing or extended valve actuation, as both techniques are known in the art. Such selective valve activation may be adjusted based on engine operating parameters that are indicative of or serve as a basis for calculating the amount of exhaust gas that will thus be expelled from the cylinders. Exemplary engine parameters that are suitable for such determination can include, but not be limited to, cylinder pressure, exhaust temperature, exhaust gas pressure in the exhaust manifold, blowdown valve timing and duration, and others. 
     When the piston  250  reaches the TDC position shown in  FIG. 6 , the fuel injector  120  can introduce a second fuel charge  264  into the combustion chamber  106  that can intermix with the combustion products  262  from the previous combustion event. Referring to  FIG. 10 , at this instance, the pressure in the compressed variable volume  258  will be at a second maximum pressure  288 . The second maximum pressure  288  may be greater than the first maximum pressure  282  or may be otherwise controlled to be about the same or lower than the first pressure. 
     The quantity of the second fuel charge  264  introduced to the cylinder, in conjunction with oxygen that may remain within the cylinder, can be selected such that stoichiometric or near stoichiometric conditions for combustion are provided within the combustion chamber  106 . At stoichiometric conditions, the ratio of fuel to air is such that substantially the entire second fuel charge will react with all the remaining oxygen in the combustion products  262 . When the piston  250  is at or near the TDC position and combustion chamber  106  reaches the second maximum pressure  288 , the second fuel charge  264  and the previous combustion products  262  may spontaneously ignite. Referring to  FIGS. 6 to 7 , the second ignition and resulting second combustion expands the contents of the variable volume  258  forcing the piston toward the BDC position resulting in a second power stroke driving the crankshaft  256 . The second power stroke also reduces the pressure in the cylinder  252  as indicated by the downward slopping second expansion curve  290  in  FIG. 10 . 
     The second combustion event can further incinerate the unburned combustion products from the initial combustion event such as unburned fuel and soot. The quantity or amount of hydrocarbons in the resulting second combustion products  266  remaining in the cylinder  252  may also be reduced. Referring to  FIG. 8 , an exhaust stroke can be performed during which the momentum of the crankshaft  256  moves the piston  250  back to the TDC position with the exhaust valve  146  opened to discharge the second combustion products to the exhaust system. Alternatively, additional recompression and re-combustion strokes can be performed. With the exhaust valve opened as indicated by the bell-shaped exhaust curve  274  in  FIG. 9 , the pressure in the cylinder can return to its initial pressure as indicated by the low, flat exhaust curve  292  in  FIG. 10 . 
     INDUSTRIAL APPLICABILITY 
     The industrial application for the apparatus and methods of a six-stroke engine system with blowdown exhaust system as described herein should be readily appreciated from the foregoing discussion. The present disclosure is applicable to any type of machine utilizing an internal combustion engine performing a six-stroke combustion cycle. It may be particularly useful in increasing efficiency of machines with six-stroke internal combustion engines. 
     Utilizing the apparatus taught in this disclosure can increase the efficiency of the engine  302  by reducing the pressure in the engine&#39;s combustion chambers during the second compression stroke of the piston. Referring to  FIGS. 12 and 13 , expelling a portion of the combustion products  362  from the variable volume  358  through the blowdown exhaust valves  310  after the first power stroke can reduce the volume or amount of material remaining within the variable volume for the piston  350  to compress during the second compression stroke. Reducing the combustion products remaining in the variable volume  358  results in less force required to compress that material. The engine  302 , thus, may work more efficiently, i.e., a larger percentage of engine power generated can be used to perform work rather than being consumed to operate the engine, when a portion of the combustion products  362  are expelled from the variable volume as blowdown exhaust gasses after the first power stroke. This is because the engine can use less energy to compress the combustion products remaining in the variable volume  358 . The relationship between efficiency and the amount of blowdown gasses expelled is generally inversely related such that expelling large amounts of combustion products  362  from the variable volume  358  results in relatively greater efficiency, while expelling small amounts of or no combustion products results in relatively lower increased efficiency. Another benefit of reducing the amount of material to compress within the variable volume  358  is reduction of the peak cylinder pressure experienced in the combustion chamber  306  during the second compression stroke and the resulting forces applied to the engine  302  components such as the piston  350 , the cylinder  352 , and other components. 
     A tradeoff exists, however, between the increased efficiency of the engine  302  and the amount of emissions produced by the engine. When large amounts of combustion products  362  are expelled as blowdown exhaust gasses, a greater portion of expelled combustion products, which are not re-combusted during the second power stroke, increase the feed-gas emissions of the engine  302 . Therefore, to maximize efficiency, the engine  302  can expel large amounts of combustion products  362  between the first power stroke and the second power stroke. However, to minimize emissions, the engine  302  can expel low amounts of or no combustion products between the first and second power strokes. 
       FIG. 13  illustrates a representative flowchart of one method  400  of operating engine system  300  featuring a blowdown exhaust system  301 . After starting at  401 , the method includes opening the intake valves  136  during an intake stroke to introduce air into the combustion chamber  306  at  402 . Once the piston  350  reaches the BDC position, the intake valves  136  close and the first compression stroke compresses the air in the combustion chamber  306  at  404 . At some point during the first compression stroke, fuel can be introduced into the combustion chamber  306  to create an air/fuel mixture at  406 . At a time near the time when the piston  350  reaches the TDC position, the air/fuel mixture may combust at  408 , expanding against the piston during a first power stroke and forcing the piston back to the BDC position. In a second compression stroke, the piston  350  can compress the combustion products  362  in the combustion chamber  306  at  410 . During the second compression stroke, the blowdown exhaust valve  310  can open to expel a portion of the combustion products  362  as blowdown exhaust gasses at  412 . The blowdown exhaust line  305  directs the blowdown exhaust gasses into the exhaust line  142  either upstream or downstream of the turbine  156 , or anywhere else be expelled from the engine system  300 . Once the piston  350  reaches the TDC position, additional fuel can be introduced into the combustion chamber  306  to mix with the remaining combustion products  362 . The compressed air/fuel/combustion product mixture combusts at  414 , forcing the piston  350  towards the BDC position during a second power stroke. During the exhaust stroke, the exhaust valves  146  open expelling the combustion products  362  from the combustion chamber  306  as exhaust gasses at  416 . 
       FIG. 14  illustrates another representative flowchart of a method  500  of operating the engine system  300  featuring a blowdown exhaust system  301 . The illustrated method includes configuring a controller, such as controller  200 , to monitor engine system parameters and to actuate the blowdown exhaust valve  310 . In the illustrated method, after starting at  501 , the controller  200  measures or otherwise determines a first engine parameter at  502 , such as engine load, engine speed, or any other suitable parameter. Based on the first engine parameter, the controller  200  determines a second engine parameter setpoint at  504 . The second engine parameter setpoint can be a target value for exhaust temperature, blowdown exhaust temperature, peak cylinder pressure, air temperature, or any other parameter indicative of engine behavior and that corresponds with the first engine parameter. The illustrated method also includes sensing or otherwise measuring a second engine parameter  506 . The controller  200  can then compare the second engine parameter setpoint to the measured second engine parameter  508 . Based on the difference between the second engine parameter setpoint and the measured second engine parameter, the controller  200  can adjust the blowdown exhaust valve  310  in a manner calculated to affect a change in the second engine parameter and bring it closer to the second engine parameter setpoint for the determined first engine parameter at  510 . The controller  200  can optimize the combustion conditions within the combustion chamber  306  based on pre-determined optimization protocols based on the first engine parameter or other engine system parameters. 
     For example, in certain embodiments, the first engine parameter can be the engine speed and the second engine parameter can be the peak cylinder pressure. In such embodiments, the controller  200  determines the engine speed, then determines the peak cylinder pressure setpoint based on the engine speed. The peak cylinder pressure setpoint is a pre-determined target peak cylinder pressure for the particular engine speed. Through sensors or other known means of acquiring the peak cylinder pressure, the controller  200  takes a measurement of the actual peak cylinder pressure. The controller  200  then compares the measured peak cylinder pressure to the peak cylinder pressure setpoint and adjusts the blowdown exhaust valve  310  to bring the actual peak cylinder pressure to a value nearer to the value of the peak cylinder pressure setpoint. 
     One way to change the peak cylinder pressure is to vary the time for which the blowdown exhaust valve  310  remains open during the second compression stroke. Generally, the longer the blowdown exhaust valve  310  remains open during the second compression stroke, the lower the peak cylinder pressure will be during the second power stroke. The peak cylinder pressure is lower because more combustion products  362  are expelled out of the variable volume  358  when the blowdown exhaust valve  310  is open for a long period of time. Thus, if the measured peak cylinder pressure is greater than the peak cylinder pressure setpoint, the controller  200  can control the blowdown exhaust valve  310  to remain open for a longer period of time to expel more combustion products  362  and decrease the peak cylinder pressure. Conversely, if the measured peak cylinder pressure is less than the peak cylinder pressure setpoint, the controller  200  can control the blowdown exhaust valve  310  to remain open for a shorter period of time to expel fewer combustion products  362  and increase the peak cylinder pressure. The method can be repeated for as long as the engine  302  is operating or for a selected range of engine parameters calculated to optimize efficiency and emissions, as well as to ensure that the engine components operate reasonably within pre-determined mechanical stress levels. 
     The apparatus and methods described herein can be adapted to a large variety of machines. For example, various types of industrial machines, such as off-highway trucks, backhoe loaders, compactors, feller bunchers, forest machines, industrial loaders, wheel loaders and many other machines can benefit from the methods and systems described. 
     It will be appreciated that the foregoing description provides examples of the disclosed system and technique. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for those features, but not to exclude such from the scope of the disclosure entirely unless otherwise indicated. 
     Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context.