Abstract:
A compression ignition engine ( 10 ) has a control system ( 24 ) for processing data, one or more cylinders ( 16 ), a fueling system ( 18 ), and a variable valve actuation mechanism ( 20 ). Control system ( 24 ) develops both fueling data for fueling the engine and timing data representing time during the engine cycle for intake valve closure to a cylinder that will endow the cylinder with an effective compression ratio (ECR) appropriate to current engine operation for causing auto-ignition to occur near or at top dead center in the engine cycle. During a compression upstroke, the cylinder is fueled according to the fueling data and intake valve closure for the cylinder is performed according to the timing data. This creates an air-fuel mixture that is increasingly compressed to the point of auto-ignition near or at top dead center.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to internal combustion engines of the compression ignition type. More specifically it relates to a discovery for improving alternative combustion processes in such engines by selectively varying compression ratio. In a disclosed embodiment this improvement is realized by a variable valve actuation strategy for controlling effective compression ratio in a manner that creates an in-cylinder air-fuel charge that is increasingly compressed during piston upstroke without igniting and finally auto-ignites near or at top dead center. 
   BACKGROUND OF THE INVENTION 
   HCCI is a known process for fueling a diesel engine in a manner that creates a substantially homogeneous air-fuel charge inside an engine cylinder during a compression upstroke of an engine cycle. After a desired quantity of fuel for the charge has been injected into the cylinder to create a generally homogeneous air-fuel mixture, the increasing compression of the charge by the upstroking piston creates sufficiently large pressure to cause auto-ignition of the charge. In other words, the HCCI mode of operation of a diesel engine may be said to comprise 1) injecting a desired amount of fuel into a cylinder at an appropriate time during the compression upstroke so that the injected fuel mixes with charge air that has entered the cylinder during the preceding intake downstroke and early portion of the compression upstroke in a manner that forms a substantially homogeneous mixture within the cylinder without combusting, and then 2) increasingly compressing the mixture to the point of auto-ignition near or at top dead center (TDC). Auto-ignition may occur as the substantially simultaneous spontaneous combustion of vaporized fuel at various locations within the mixture. No additional fuel is injected after auto-ignition. 
   One of the attributes of HCCI is that relatively lean, or dilute, mixtures can be combusted, keeping the combustion temperatures relatively low. By avoiding the creation of relatively higher combustion temperatures, HCCI can yield significant reductions in the generation of NO x , an undesired constituent of engine exhaust gas. 
   Another attribute of HCCI is that auto-ignition of a substantially homogeneous air-fuel charge generates more complete combustion and consequently relatively less soot in engine exhaust. 
   The potential benefit of HCCI on reducing tailpipe emissions is therefore rather significant, and consequently HCCI is a subject of active investigation and development by many scientists and engineers in the engine research and design community. 
   HCCI may be considered one of several alternative combustion processes for a compression ignition engine. Other processes that may be considered alternative combustion processes include Dilution Controlled Combustion Systems (DCCS) and Highly Premixed Combustion Systems (HPCS). 
   By whatever name an alternative system or process may be called, a common attribute is that fuel is injected into a cylinder during a piston upstroke to form an air-fuel charge that is increasingly compressed until auto-ignition occurs near or at top dead center (TDC). 
   If such alternative processes are not be suitable over the full range of engine operation for any particular engine, the engine may be fueled in the traditional conventional diesel manner where charge air is compressed to the point where it causes the immediate ignition of fuel upon fuel being injected into a cylinder, typically very near or at top dead center where compression is a maximum. 
   With the availability of processor-controlled fuel injection systems capable of controlling fuel injection with precision that allows fuel to be injected at different injection pressures, at different times, and for different durations during an engine cycle over the full range of engine operation, a diesel engine becomes capable of operating by alternative combustion processes and/or traditional diesel combustion. 
   The advent of variable valve actuation systems allows timing of engine valves to be processor-controlled in various ways, and with precision. As will be explained by later description, the present invention takes advantage of the capabilities of such fuel injection and variable valve actuation systems to control fuel injection and valve timing in various ways that can improve a diesel engine by significant reductions in engine-out emissions. Some modes of valve actuation are even accompanied by modest fuel economy improvements. 
   Because a diesel engine that powers a motor vehicle runs at different speeds and loads depending on various inputs to the vehicle and the engine that influence engine operation, fueling requirements change as speed and load change. An associated processing system processes data indicative of parameters such as engine speed and engine load to develop control data for setting desired engine fueling for particular operating conditions that will assure proper control of the fuel injection system for various combinations of engine speed and engine load. A variable valve timing system can also controlled in a different ways according to engine speed-load conditions. 
   SUMMARY OF THE INVENTION 
   Modulation (both continuous modulation and controlled increase and decrease) of the ignition delay period for HCCI, DCCS, HPCS, and other alternative internal combustion processes has disclosed, both theoretically and experimentally, the possibility of significant reductions in engine-out emission level, including NOx and soot. One of the factors that can be used effectively for influencing ignition delay is Effective Compression Ratio (ECR), defined as a ratio of in-cylinder pressure at the end of a compression stroke to the in-cylinder pressure at the end of an effective intake stroke. 
   The present invention relates to the use of certain variable valve actuation strategies for effective ECR control when an engine is fueled for alternative combustion processes like those mentioned. While a principal benefit of the invention is reduction in engine-out emissions, it is contemplated that optimization can yield improvement in other aspects of engine performance in a motor vehicle, including gains in fuel economy, noise reduction, and better cold-starting and drivability. Moreover, the invention can be embodied in a cost-effective manner in production vehicles that already have electronic engine control systems and variable valve actuation systems. 
   Various mechanisms that are disclosed in various patents and technical literature may be used to induce change in ECR of an engine. One is a simple phase mechanism that is widely used in gasoline engines and is capable of adjusting the effective compression ratio by phasing or delaying the intake valve timing. Such a phasing mechanism may however have limited use in a diesel engine application due to tight clearance that exists between the engine valves and piston at TDC and due to pumping losses that can occur by delaying the intake valve event in order to obtain significant changes in ECR. 
   Other mechanical systems possess the capability for adjusting valve lift and duration simultaneously by introducing a control shaft with pivot element between the cam and the valve itself, allowing intake valve closing to be modulated while maintaining the intake portion of the valve profile close to baseline. 
   Electro-hydraulic devices may also be used to adjust valve lift, and thus timing of valve profiles. A dual or multiple lift system has been adopted by some automotive manufacturers. 
   A completely electromagnetic device that provides large flexibility to the engine valve motion has been described in published literature but as yet is not believed to be in production vehicles. 
   Similarly, several fully flexible systems based on electrohydraulic mechanisms have been proposed, but like the completely electromagnetic devices, have not yet been introduced into production engines. 
   Commonly owned U.S. Pat. Nos. 6,044,815 and 6,263,842 relate to hydraulically-assisted engine valve actuators that can change individual valves and control individual cylinders for better combustion control and are useful in compensating for different charge temperatures resulting from different cylinder locations in an engine. 
   Principles of the invention can also be applied to engines that have mechanisms for changing geometric compression ratio although such mechanisms may be more complex and difficult to implement in a compression ignition engine. 
   The present invention relates to an engine, system, and method for enhancing the use of HCCI combustion in a diesel engine toward objectives that include further reducing the generation of undesired constituents in engine exhaust, especially soot and NO x , and further improving thermal efficiency. Significant reductions in exhaust soot can have favorable implications for soot control strategies. 
   The invention is embodied partly in the fuel injection control strategy, a strategy that is programmed in an associated processing system, and partly in an engine compression ratio control strategy, a strategy which it is presently believed can be more practically implemented as an effective engine compression ratio strategy using a variable valve actuation mechanism. 
   One generic aspect of the present invention relates to a method of operating a compression ignition engine that has a processor-based engine control system controlling both a fueling system for fueling the engine and a variable valve actuation mechanism that varies timing of intake valves that open and close an intake system to individual engine cylinders. 
   The method comprises processing certain data to develop both fueling data for fueling the engine and timing data representing time during the engine cycle for intake valve closure to a cylinder that will endow the cylinder with an effective compression ratio (ECR) appropriate to current engine operation for causing auto-ignition to occur near or at top dead center in the engine cycle. 
   During a compression upstroke, the cylinder is fueled according to the fueling data and intake valve closure for the cylinder is performed according to the timing data to cause fuel in the cylinder to mix with charge air that has entered the cylinder from the intake system during an immediately preceding intake downstroke and early portion of the compression upstroke. 
   The mixture is then increasingly compressed to the point of auto-ignition near or at top dead center. 
   A further generic aspect relates to a compression ignition engine that operates according to the method just described. 
   The foregoing, along with further features and advantages of the invention, will be seen in the following disclosure of a presently preferred embodiment of the invention depicting the best mode contemplated at this time for carrying out the invention. This specification includes drawings, now briefly described as follows. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic portrayal of portions of an engine relevant to principles of the invention. 
       FIG. 2  is a graph comparing the smoke component of engine-out emissions when an engine was run using two different piston bowl geometries at one effective compression ratio. 
       FIG. 3  is a graph comparing the smoke component of engine-out emissions when the engine was run using the two different piston bowl geometries but at a different effective compression ratio. 
       FIG. 4  is a graph generally indicative of the NOx component of engine-out emissions for the various running conditions of  FIGS. 2 and 3 . 
       FIG. 5  is a graph illustrating the effect on effective compression ratio and basic specific fuel consumption of using various variable valve actuation strategies. 
       FIG. 6  is a graph representing improvements in smoke reduction by applying a variable valve actuation system over the speed and load range of the engine. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows portions of an exemplary internal combustion engine  10  that embodies principles of the present invention. Engine  10  comprises an intake system  12  through which charge air for combustion enters the engine and an exhaust system  14  through which exhaust gases resulting from combustion exit the engine. Engine  10  operates on the principle of compression ignition, not spark ignition, and may be turbocharged. When used as the prime mover of a motor vehicle, such as a truck, engine  10  is coupled through a drivetrain to driven wheels that propel that the vehicle. 
   Engine  10  comprises multiple cylinders  16  (either in an in-line configuration of a V-configuration) forming combustion chambers into which fuel is injected by fuel injectors of a fuel system  18  to mix with charge air that has entered through intake system  12 . Pistons that reciprocate within cylinders  16  are coupled to an engine crankshaft. 
   An air-fuel mixture in each cylinder  16  combusts under pressure created by the corresponding piston as the engine cycle passes from its compression phase to its power phase, thereby driving the crankshaft, which in turn delivers torque through the drivetrain to the wheels that propel the vehicle. Gases resulting from combustion are exhausted through exhaust system  14 . 
   Engine  10  has intake and exhaust valves associated with cylinders  16 . A variable valve actuation system  20  opens and closes at least the intake valves and may also open and close the exhaust valves. Each cylinder has at least one intake valve and at least one exhaust valve. 
   Engine  10  also comprises an engine control system (ECS)  22  that comprises one or more processors that process various data to develop data for controlling various aspects of engine operation. ECS  22  acts via appropriate interfaces with both fuel system  18  and variable valve actuation system  20  to control the timing and amount of fuel injected by each fuel injector and at least the opening and closing of the intake valves, possibly the opening and closing of the exhaust valves, too. 
   In accordance with principles of the invention, ECS  22  causes the engine to be fueled, at least at times, for a form of alternative combustion, such as those mentioned earlier. In conjunction with that fueling, ECS  22  controls variable valve actuation system  20  in a manner that creates an ECR for the particular fueling that will cause auto-ignition near or at piston TDC. While each individual cylinder may be fueled in the same way and have the same ECR, certain cylinders may be fueled differently and operated with different ECR if the fueling system and the variable valve actuation system possess capabilities that allow for such variation. 
   As engine speed and/or load changes, fueling requirements change. Auto-ignition however still needs to occur near or at TDC. ECS  22  takes this into account by processing certain data to yield fueling data for fueling system  18  to provide proper fueling, and data that will cause variable valve actuation system  20  to set a suitable ECR for auto-ignition near or at TDC. ECS  22  may have one or more maps, or look-up tables, that correlate various combinations of engine speed and engine load with correspondingly appropriate fueling values that can be processed in any suitably appropriate manner to yield fueling commands to devices in the fueling system. 
   ECS  22  may also have one or more maps that correlate various combinations of engine speed and torque with correspondingly appropriate ECR values that can be processed in any suitably appropriate manner to yield commands for desired timing of cylinder intake valves that will cause auto-ignition near or at TDC for the particular engine speed and fueling (torque). 
   Effectiveness of the invention is shown by  FIGS. 2 ,  3 , and  4  which contain engine-out emission data for a diesel engine running in DCCS combustion mode. 
   In  FIG. 2 , a first trace  40  represents smoke data when the engine was run at a certain compression ratio (CR 1 ) using pistons whose bowls had a certain shape (BWL 1 ). A second trace  42  represents smoke data when the engine was run at CR 1  using pistons whose bowls had a different shape (BWL 2 ). The bowls having the shapes BWL 2  can be considered baseline bowls. The BWL 1  bowls have geometries that provide improved charge air turbulence and fuel spray mixing, resulting in lower smoke, when compared with the baseline bowls. Compression ratio CR 1  is substantially 18:1. 
   In  FIG. 3 , a first trace  44  represents smoke data when the engine was run at a different compression ratio (CR 2 ) using pistons having the BWL 1  bowls. A second trace  46  represents smoke data when the engine was run at CR 2  using pistons whose the BWL 2  bowls. Compression ratio CR 2  is substantially 15:1. 
     FIG. 4  shows a trace  48  representing NOx data that is essentially representative of NOx generated when the engine was running to generate each trace  40 ,  42 ,  44 ,  46 . This shows that NOx was largely unchanged. 
   Comparison of  FIGS. 2 and 3  discloses that when compression ratio was geometrically varied on two different combustion bowls (BWL 1  and BWL 2 ) within a range 18:1–15:1 (i.e. when dimensions of bowls has been physically modified and pistons with those bowls have been changed in the test engine between related tests) the level of soot emission decreased dramatically—by approximately one order of magnitude-while the NOx emission remained essentially unchanged. Because at least a part of such significant soot reduction is definitely caused by effect of compression ratio on ignition delay and related improvement in charge homogenization, a similar effect can be obtained if the bowl geometry would be kept constant but compression ratio changed by some other means, such as variable valve actuation that can be operated to cause the engine to run at different effective compression ratios. 
     FIG. 5  summarizes results of an analytical investigation of the effects of different variable valve actuation strategies on ECR and engine brake fuel consumption (BSFC) when geometry of combustion chamber remains fixed. In general, the variable valve actuation strategies were based on different means for varying the opening and closing events of both intake and exhaust valves, changing magnitudes of valve lifts, and shifting the intake and exhaust valve lift profiles relative to each other. As can be seen, all of those strategies offer significant range of ECR modulation when combustion bowl geometry is constant, indicating that variable valve actuation can be used as a powerful and consistent ECR controlling parameter. 
     FIG. 5  comprises a graph  60  that illustrates several variable valve actuation techniques that can be effective in accomplishing significant smoke reduction. Over certain ranges of effective compression ratio, modest fuel economy improvements can be achieved. Over other ranges, there is some fuel economy penalty. 
   The generic operational characteristics of the various valve actuation techniques that can vary effective compression ratio are schematically depicted by the associated timing diagrams  62 ,  64 ,  66 ,  68 . All diagrams show common exhaust valve timing  70 . 
   Diagram  62  shows representative intake valve operation  72  where that intake valve lift and open duration (as measured in engine degrees) are constant, but phasing of valve opening is being varied. 
   Diagram  64  shows representative intake valve operation  74  where intake valve open duration and phasing are constant, but lift is being varied. 
   Diagram  66  shows representative intake valve operation  76  where intake valve phasing is constant, but lift and duration are being varied. 
   Diagram  68  shows representative intake valve operation  78  where intake valve, open duration, and phasing are being varied. 
   Trace  80  in graph  60  is a result of the use of IVC to control intake valve operation when a test engine was run at 2000 rpm and 50% load. IVC is represented by diagram  68  using short duration lift  78 A. The vertical axis of the graph measures compression ratio while the horizontal axis measures fuel consumption in % bsfc. Trace  80  comprises two segments  80 A and  80 B. Segment  80 A shows fuel consumption improvement when compression ratio is varied within a range from about 11 to about 18. Hence, that would be a preferred range for steady state engine operation at that speed and load. 
   Trace  82  in graph  60  is a result of the use of IVC to control intake valve operation when a test engine was run at 800 rpm and 50% load. Trace  82  comprises two segments  82 A and  82 B. Segment  82 A shows fuel consumption improvement when compression ratio is varied within a range from about 12.5 to almost 16.5. Hence, that would be a preferred range for steady state engine operation at that speed and load. 
   Trace  84  is a result of using short duration lift control shown in diagram  66  by the reference numeral  76  with the engine operating at 2000 rpm and 50% load. 
   Trace  86  is a result of using same duration lift control shown in diagram  64  by the reference numeral  74  with the engine operating at 2000 rpm and 50% load. 
   Trace  88  is a result of using phasing control shown in diagram  62  by the reference numeral  72  with the engine operating at 2000 rpm and 50% load. Traces  84 ,  86  and  88  are capable to reduced effective compression ratio but it is done at a penalty to bsfc. 
     FIG. 6  shows a diagram  90  where the vertical axis represents engine torque (in foot-pounds) and the horizontal axis represents engine speed (in rpm—revolutions per minute). A trace  92  represents baseline data for an engine, torque output at full throttle also known as lug line. Traces  94  and  96  represent boundaries differentiating the use of cams for optimum effective compression ratio to limit smoke output. Data represented in  FIG. 6  was obtained using three cams, one is the engine baseline cam, and the others with reduced duration per the method described in  68 . Above trace  94 , the baseline cam is used. Between traces  94  and  96 , cam with an earlier intake valve closing with respect to the baseline cam yields smoke reduction. Below trace  96 , smoke reduction is accomplished by further advancing the intake valve closing. 
     FIG. 6  also shows five speed-torque zones  98 ,  100 ,  102 ,  104 ,  106  each representing a range of improved smoke reduction by applying the varying intake valve closing timings as indicated by the traces  94  and  96 . In zones  98  and  106 , the smoke number has been reduced anywhere from 0.0 to 0.5 when compared with the baseline data. In zones  100  and  104 , the smoke number has been reduced anywhere from 0.5 to 1.0 when compared with the baseline data. In zone  102 , the smoke number has been reduced anywhere from 1.0 to 2.0 when compared with the baseline data. 
   Because variable valve actuation offers continuous modulation of valve timing, ECR can be fully optimized in relation to specific demands of boundary conditions. 
   Another application of variable compression ratio is cold starting. Cold start requires higher ECR. From cold-start, higher-ECR regime, variable valve actuation can provide effective transition into lower-ECR regime supporting low emission alternative combustion process. 
   While a presently preferred embodiment of the invention has been illustrated and described, it should be appreciated that principles of the invention apply to all embodiments falling within the scope of the following claims.