Abstract:
A system for a multi-cylinder compression ignition engine includes a plurality of nozzles, at least one nozzle per cylinder, with each nozzle configured to spray oil onto the bottom side of a piston of the engine to cool that piston. Independent control of the oil spray from the nozzles is provided on a cylinder-by-cylinder basis. A combustion parameter is determined for combustion in each cylinder of the engine, and control of the oil spray onto the piston in that cylinder is based on the value of the combustion parameter for combustion in that cylinder. A method for influencing combustion in a multi-cylinder engine, including determining a combustion parameter for combustion taking place in in a cylinder of the engine and controlling an oil spray targeted onto the bottom of a piston disposed in that cylinder is also presented.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under Contract No. DE-EE0003258 awarded by the Department of Energy. The government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     Gasoline Direct-injection Compression-Ignition (GDCI) is an engine combustion process that shows promise in improving engine emissions performance and efficiency. GDCI provides low-temperature combustion for high efficiency, low NOx, and low particulate emissions over the complete engine operating range. Low-temperature combustion of gasoline may be achieved using multiple late injection (MLI), intake boost, and moderate EGR. GDCI engine operation is described in detail in U.S. Patent Application Publication 2013/0213349A1, the entire contents of which are hereby incorporated herein by reference. 
     The autoignition properties of gasoline-like fuels require relatively precise control of the thermal state within each combustion chamber to maintain robust combustion in each individual cylinder of a multiple-cylinder engine. Due to cylinder-to-cylinder variation in a multiple cylinder engine, improvements in temperature control are desired. 
     BRIEF SUMMARY OF THE INVENTION 
     In a first aspect of the invention, a system for selectively cooling combustion chambers of a multi-cylinder engine is provided. The system includes a plurality of nozzles, each configured to spray oil onto a piston so as to cool the piston. Oil supply to each nozzle is controllable so as to provide the ability to provide cooling to an individual piston independent of cooling provided to a different piston in the engine. 
     In a second aspect of the invention, a method for selectively cooling combustion chambers of a multi-cylinder engine is provided. The method includes controlling oil flow to a plurality of nozzles, each nozzle configured to spray oil onto a piston so as to cool the piston. Oil flow from each nozzle is controlled individually so as to provide individually controllable cooling to each piston in the engine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of an embodiment of an engine control system suitable for controlling a single cylinder of a GDCI engine. 
         FIG. 2  is a block diagram of an embodiment of the gas (air and/or exhaust) paths of an engine system. 
         FIG. 3  is a block diagram of an embodiment of the coolant paths of an engine system. 
         FIG. 4  is a schematic diagram depicting an intake air heater system for a multi-cylinder engine. 
         FIG. 5  is a schematic diagram depicting a piston cooling system for a multi-cylinder engine. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a non-limiting embodiment of an engine control system  10  suitable for controlling a single cylinder portion of a GDCI internal combustion engine  12 . The engine  12  is illustrated as having a single cylinder bore  64  containing a piston  66 , wherein the region above the piston  66  defines a combustion chamber  28 . The system  10  may include a toothed crank wheel  14  and a crank sensor  16  positioned proximate to the crank wheel  14  such that the crank sensor  16  is able to sense rotational movement of the crank wheel teeth and output a crank signal  18  indicative of a crank angle and a crank speed. 
     The engine control system  10  may also include a controller  20 , such as an engine control module (ECM), configured to determine a crank angle and a crank speed based on the crank signal  18 . The controller  20  may include a processor  22  or other control circuitry as should be evident to those in the art. The controller  20  or processor  22  may include memory, including non-volatile memory, such as electrically erasable programmable read-only memory (EEPROM) for storing one or more routines, thresholds and captured data. The one or more routines may be executed by the processor  22  to perform steps for determining a prior engine control parameter and scheduling a future engine control signal such that a future engine control parameter corresponds to a desired engine control parameter.  FIG. 1  illustrates the processor  22  and other functional blocks as being part of the controller  20 . However, it will be appreciated that it is not required that the processor  22  and other functional blocks be assembled within a single housing, and that they may be distributed about the engine  12 . 
     Continuing to refer to  FIG. 1 , the engine control system  10  may include a combustion sensing means  24  configured to output a combustion signal  26  indicative of a combustion characteristic of a combustion event occurring within the combustion chamber  28 . One way to monitor the progress of a combustion event is to determine a heat release rate or cumulative heat release for the combustion event. However, because of the number and complexity of measurements, determining heat release may not be suitable for controlling engines during field use such as when engines are operated in vehicles traveling in uncontrolled environments like public roadways. A combustion detection means suitable for field use may provide an indication of a combustion characteristic that can be correlated to laboratory type measurements such as heat release. Exemplary combustion detection means  24  may include a pressure sensor configured to sense the pressure within the combustion chamber  28 . Another device that may be useful for indicating some aspect of the combustion process is a combustion knock sensor. The combustion detection means  24  may be any one of the exemplary sensors, or a combination of two or more sensors arranged to provide an indication of a combustion characteristic. 
     The engine control system  10  includes one or more engine control devices operable to control an engine control parameter in response to an engine control signal, wherein the engine control parameter influences when autoignition occurs. One example of an engine control device is a fuel injector  30  adapted to dispense fuel  68  in accordance with an injector control signal  32  output by an injector driver  34  in response to an injection signal  36  output by the processor  22 . The fuel injection profile may include a plurality of injection events. Controllable aspects of the fuel injection profile may include how quickly or slowly the fuel injector  30  is turned on and/or turned off, a fuel rate of fuel  68  dispensed by the fuel injector  30  while the fuel injector  30  is on, the initiation timing and duration of one or more fuel injections as a function of engine crank angle, or the number of fuel injections dispensed to achieve a combustion event. Varying one or more of these aspects of the fuel injections profile may be effective to control autoignition. 
     The exemplary engine control system  10  includes an exhaust gas recirculation (EGR) valve  42 . While not explicitly shown, it is understood by those familiar with the art of engine control that the EGR valve regulates a rate or amount of engine exhaust gas that is mixed with fresh air being supplied to the engine to dilute the percentage of oxygen and/or nitrogen in the air mixture received into the combustion chamber  28 . The controller  20  may include an EGR driver  44  that outputs an EGR control signal  46  to control the position of the EGR valve  42 . In a non-limiting embodiment, the EGR driver may, for example, pulse width modulate a voltage to generate an EGR control signal  46  effective to control the EGR valve to regulate the flow rate of exhaust gases received by the engine  12 . In an alternative non-limiting embodiment, the EGR valve may be commanded to a desired position by control of a torque motor actuator. 
     Referring again to  FIG. 1 , the engine control system  10  may include other engine management devices. For example the engine control system  10  may include a turbocharger  118 . The turbocharger  118  receives a turbocharger control signal from a turbocharger control block that may control a boost pressure by controlling the position of a waste gate or bypass valve, or controlling a vane position in a variable geometry turbocharger. The engine control system  10  may also include a supercharger driven by the engine through a supercharger clutch  140 , the supercharger clutch  140  being controlled by a supercharger control block in the controller  20 . The engine control system  10  may also include a valve control block  58  that may directly control the actuation of engine intake valve  62 A and exhaust valve  62 B, or may control the phase of a cam (not shown) actuating the intake valve  62 A and/or the exhaust valve  62 B. 
     Still with reference to  FIG. 1 , the engine control system  10  may include one or more intake air heaters  80  configured to heat air at the intake manifold or intake port of each cylinder. Each intake air heater  80  is controllable by a control signal received from an intake air heater control block in a manner to be discussed in further detail below. 
     Also indicated in  FIG. 1  are a nozzle  82  configured to spray oil onto the bottom of the piston  66  to provide cooling of the piston  66 . Oil flow to the nozzle  82  is provided by an oil pump  86  that supplies oil to the nozzle  82  through an oil control valve  84 . Control of the oil pump  86  and/or of the oil control valve  84  is provided through an oil control block in the controller  20  in a manner to be discussed in further detail below. 
     Although not specifically indicated in  FIG. 1 , the engine control system  10  may include additional sensors to measure temperature and/or pressure at locations within the air intake system and/or the engine exhaust system. Also, it is to be noted that the embodiment depicted in  FIG. 1  may contain components that are not essential to operate a GDCI engine but may offer benefits if included in an implementation of a GDCI engine system. 
       FIG. 2  is a block diagram of a non-limiting embodiment of the gas paths  190  of a GDCI system usable with the engine  12  of  FIG. 1 . This diagram depicts the routing and conditioning of gases (e.g. air and exhaust gas) in the system. It will be appreciated that configurations other than that shown in  FIG. 2 , for example a configuration using a single air cooler, may be feasible. 
     Referring to  FIG. 2 , air passes through an air filter  112  and a mass airflow sensor  114  into an air duct  116 . The air duct  116  channels air into the air inlet  122  of the compressor  120  of a turbocharger  118 . Air is then channeled from the air outlet  124  of the compressor  120  to the air inlet  128  of a first charge air cooler  126 . The air outlet  130  of the first charge air cooler  126  is connected to the air inlet  136  of a supercharger  134 . A first charge air cooler bypass valve  132  is connected between the air inlet  128  and the air outlet  130  of the first charge air cooler  126  to controllably divert air around the first charge air cooler  126 . 
     Continuing to refer to  FIG. 2 , air at the air outlet  130  of the first charge air cooler  126  is channeled to the air inlet  136  of a supercharger  134 , which is driven by the engine  12  through a controllable clutch  140 . The air from the air outlet  138  of the supercharger  134  is channeled to a first port  146  of a second charge air cooler bypass valve  144 . The second charge air cooler bypass valve  144  in  FIG. 2  allows air entering the first port  146  to be controllably channeled to the second port  148 , to the third port  150 , or to be blended to both the second port  148  and to the third port  150 . Air that is channeled through the second port  148  of the second charge air cooler bypass valve  144  enters an air inlet port  154  of a second charge air cooler  152 , through which the air passes by way of an air outlet port  156  of the second charge air cooler  152  to an air intake manifold  158  of the engine  12 . Air that is channeled through the third port  150  of the second charge air cooler bypass valve  144  passes directly to the air intake manifold  158  of the engine  12  without passing through the second charge air cooler  152 . A plurality of air intake heaters  80  is disposed in the air intake manifold  158 , with each air intake heater  80  configured to heat air at the intake port of a cylinder of the engine  12 . 
     Still with reference to  FIG. 2 , engine exhaust gas exits an exhaust port  160  of the engine  12  and is channeled to the turbine  162  of the turbocharger  118 . Exhaust gas exiting the turbine  162  passes through a catalytic converter  170 . Upon exiting the catalytic converter  170 , the exhaust gas can follow one of two paths. A portion of the exhaust gas may pass through an EGR cooler  164  and an EGR valve  42 , to be reintroduced into the intake air stream at air duct  116 . The remainder of the exhaust gas that is not recirculated through the EGR system passes through a backpressure valve  168 , and a muffler  172 , to be exhausted out a tail pipe. 
     It will be appreciated from the foregoing description of  FIG. 2  that the focus of  FIG. 2  is on the transport and conditioning of gas constituents, i.e. air into the engine  12  and exhaust gas out of the engine  12 . Some of the components in  FIG. 2  affect the temperature and/or the pressure of the gas flowing through the component. For example the turbocharger compressor  120  and the supercharger  134  each increase both the temperature and the pressure of air flowing therethrough. The first charge air cooler  126 , the second charge air cooler  152 , and the EGR cooler  164  are each heat exchangers that affect the temperature of the gas (air or exhaust gas) flowing therethrough by transferring heat from the gas to another medium. In the embodiment of  FIGS. 2 and 3 , the other heat transfer medium is a liquid coolant, discussed in further detail in relation to  FIG. 3 . In an alternate embodiment, a gaseous coolant may be used in lieu of a liquid coolant. 
       FIG. 3  depicts an embodiment of coolant paths  180  of the system  100  for conditioning intake air into an engine  12 .  FIG. 3  includes several components such as the engine  12 , the first charge air cooler  126 , the second charge air cooler  152 , and the EGR cooler  164  that were previously discussed with respect to their functions in the gas paths  190  of the system  100  depicted in  FIG. 2 . The coolant system  180  may further include an oil cooler  270 , a heat exchanger  272  to provide cooling for the turbocharger  118  and a heater core  274 , a temperature sensing device, a pressure sensing device, and/or other components not shown in  FIG. 2 . 
     Referring to  FIG. 3 , the coolant paths  180  of the system  100  for conditioning intake air includes a first coolant loop  202 . The first coolant loop  202  includes a first coolant pump  210  configured to urge liquid coolant through coolant passages in the engine  12  and through a first radiator  214 . The first coolant pump  210  may conveniently be a mechanical pump driven by rotation of the engine  12 . The first radiator  214  may conveniently be a conventional automotive radiator with a controllable first air supply means  218  configured to urge air over the first radiator  214 . Preferably the first air supply means  218  comprises a variable speed fan, but the first air supply means  218  may alternatively comprise, by way of non-limiting example, a single speed fan, a two speed fan, a fan of any sort in conjunction with one or more controllable shutters, or the like, without departing from the inventive concept. 
     Continuing to refer to  FIG. 3 , the coolant paths  180  of the system  100  includes a thermostat crossover assembly  242  within which is defined a first chamber  244 , a second chamber  246 , and a third chamber  248 . A first thermostat  250  allows fluid communication between the first chamber  244  and the second chamber  246  when the temperature of the coolant at the first thermostat  250  is within a first predetermined range. A second thermostat  252  allows fluid communication between the third chamber  248  and the second chamber  246  when the temperature of the coolant at the second thermostat  252  is within a second predetermined range. It will be appreciated that, while the first chamber  244 , the second chamber  246 , the third chamber  248 , the first thermostat  250 , and the second thermostat  252  are depicted as housed in a common enclosure, these components may be otherwise distributed within the system  180  without departing from the inventive concept. 
     The embodiment depicted in  FIG. 3  further includes the EGR cooler  164 , one coolant port of which is connected to a four-way coolant valve  216 . The other coolant port of EGR cooler  164  is fluidly coupled to the first chamber  244  through an orifice  254 . 
     Continuing to refer to  FIG. 3 , the coolant paths  180  of the system  100  further includes a second coolant loop  204 . The second coolant loop  204  includes a second coolant pump  220  configured to urge liquid coolant through a second radiator  222 , the second charge air cooler  152 , a three-way coolant valve  224 , and the first charge air cooler  126 . The second radiator  222  may conveniently be a conventional automotive radiator with a controllable second air supply means  226  configured to urge air over the second radiator  222 . Preferably the second air supply means  226  comprises a variable speed fan, but the second air supply means  226  may alternatively comprise, by way of non-limiting example, a single speed fan, a two speed fan, a fan of any sort in conjunction with one or more controllable shutters, or the like, without departing from the inventive concept. Alternately, the second radiator  222  may be positioned in line with the first radiator  214  such that the first air supply means  218  urges air over both the second radiator  222  and the first radiator  214 , in which case the second air supply means  226  would not be required. 
     Coolant communication between the first coolant loop  202  and the second coolant loop  204  is enabled by the three-way coolant valve  224  and a conduit  240 . Control of the four-way coolant valve  216  and the three-way coolant valve  224  may be employed to achieve desired temperature conditioning of intake air. Operation of a similar system is disclosed in U.S. patent application Ser. No. 13/469,404 titled “SYSTEM AND METHOD FOR CONDITIONING INTAKE AIR TO AN INTERNAL COMBUSTION ENGINE” filed May 11, 2012, the entire disclosure of which is hereby incorporated herein by reference. 
     In the preceding discussion relative to  FIGS. 1 through 3 , it will be appreciated that the engine control system  10  and the system  100  for conditioning intake air contain several components and subsystems that can influence the temperature and pressure within the combustion chamber  28 . Of these components and subsystems, there are several that have a global effect on the temperature and/or pressure in all cylinders of a multi-cylinder engine. The turbocharger  118 , the supercharger  134 , the charge air coolers  126  and  152 , the air bypass valves  132 ,  142 , and  146 , the EGR cooler  164 , the EGR valve  42 , the coolant pumps  210 ,  220 , and the coolant valves  216 ,  224  can be considered “global” components in that they each influence the temperature and/or pressure in the combustion chambers  28  of the engine  12 , with the temperature and/or pressure in all combustion chambers  28  of a multi-cylinder engine  12  moving in the same direction as a result of a change in the control setting of one of these “global” components. 
     The GDCI combustion process has demonstrated very high thermal efficiency and very low NOx and particulate matter emissions. The GDCI combustion process includes injecting gasoline fuel into the cylinder with appropriate injection timing to create a stratified mixture with varying propensity for autoignition. Heat and pressure from the compression process produces autoignition of the air/fuel mixture in the cylinder with burn duration long enough to keep combustion noise low, but with combustion fast enough to achieve high expansion ratio for all fuel that is burned. Fuel injection into each combustion chamber  28  is tailored to optimize the combustion achieved in that combustion chamber  28 , as measured by the combustion sensing means  24  associated with that combustion chamber  28 . Unlike the “global” components discussed above, the injection of fuel can be controlled to influence the robustness of combustion on a cylinder-by-cylinder basis. 
     A particular challenge in GDCI combustion is maintaining robust combustion in each combustion chamber. Gasoline fuel has characteristics such that it is resistant to autoignition. As a result, unlike a conventional spark ignition gasoline engine, a GDCI engine requires relatively tight control of the in-cylinder pressure and temperature to robustly achieve and maintain compression ignition. 
     A multi-cylinder engine presents challenges in matching the characteristics that are important to maintaining robust and stable compression ignition with gasoline fuel. It is known that all cylinders of a multi-cylinder internal combustion engine do not operate at precisely the same conditions. Compression ratio may vary from cylinder-to-cylinder due to manufacturing tolerances, wear, or deposits in a combustion chamber. Temperature may vary from cylinder to cylinder due to differences in heat transfer from the cylinder to the coolant and to ambient air, for example with middle cylinders operating hotter than outer cylinders. Air flow into each combustion chamber may differ due to intake manifold geometry, and exhaust flow out of each combustion chamber may differ due to exhaust manifold geometry. Other sources of variability may include differences in fuel delivery amount or spray pattern due to tolerances associated with the fuel injector  30 . While control of the “global” components discussed above may be useful to achieve a desired minimum temperature, desired average temperature, or desired maximum temperature under steady-state conditions, the “global” systems are not able to compensate for the cylinder-to-cylinder differences that impede achieving optimal conditions in all cylinders of a multi-cylinder engine. Additionally, under transient engine operating conditions, i.e. changing engine speed and/or load, the response time of the “global” components to influence combustion chamber temperature may be too slow to allow robust and stable GDCI combustion during the time that the engine is transitioning from one speed/load state to another. 
     To achieve robust, stable GDCI combustion in a multi-cylinder engine, it is desirable to provide means for influencing the temperature and/or pressure in each individual combustion chamber. One way to achieve this is to provide a plurality of intake air heaters  80 , with each cylinder of the engine  12  having an associated intake air heater  80  to increase the temperature of the air entering that cylinder. In a non-limiting embodiment, each heater  80  may be disposed in an intake runner of the intake manifold  158 , as depicted in  FIG. 2 . 
       FIG. 4  is a schematic diagram depicting an intake air heater system for a multi-cylinder engine. In  FIG. 4 , lines with arrowheads at one end are used to indicate air flow, with the arrowhead indicating the direction of air flow.  FIG. 4  includes dashed boxes denoted as a, b, c, and d, each associated with one of four cylinders in a four cylinder engine. Within each dashed box, features introduced above with reference to  FIG. 1  are identified with the reference numeral of  FIG. 1  with a letter appended to the numeral, the letter corresponding to the cylinder identification associated with the feature. For example, “ 80   a ” in  FIG. 4  represents the intake air heater  80  that is associated with cylinder “a”. 
     Referring to  FIG. 4 , an intake air heater  80   a  is configured to heat air entering the intake port of the combustion chamber  28   a . When GDCI combustion occurs in the combustion chamber  28   a , combustion characteristics are detected by the combustion sensing means  24   a . A signal from the combustion sensing means  24   a  indicative of a combustion characteristic in combustion chamber  28   a  is provided to the controller. The controller is configured to provide a control signal to the air intake heater  80   a  in response to the combustion characteristic detected by the combustion sensing means  24   a , thereby enhancing the robustness of GDCI combustion in the combustion chamber  28   a . A corresponding relationship exists between the corresponding components within each of the other cylinders “b”, “c”, and “d”, 
     As indicated in  FIG. 4 , each of the cylinders a, b, c, d is associated with a corresponding intake air heater  80   a ,  80   b ,  80   c , and  80   d  respectively. Each of the cylinders a, b, c, and d additionally has a corresponding combustion sensing means  24   a ,  24   b ,  24   c , and  24   d  respectively. The controller is configured to receive signals from each individual combustion sensing means  24   a ,  24   b ,  24   c ,  24   d  indicative of a combustion characteristic in that cylinder, and to provide an appropriate control signal to an individual intake air heater  80   a ,  80   b ,  80   c ,  80   d  to influence the intake air temperature in that cylinder, where each control signal based on the combustion characteristic measured in the respective combustion chamber  28   a ,  28   b ,  28   c ,  28   d . Accordingly, the temperature in each cylinder can be optimized to maximize the robustness of GDCI combustion in each individual cylinder beyond the capabilities of the “global” components described above. 
     In an embodiment of the invention, a plurality of temperature sensors may be provided, with one of the plurality of temperature sensors associated with each of the heaters  80   a ,  80   b ,  80   c ,  80   d . By way of non-limiting example, a temperature sensor may be disposed so as to directly measure a temperature of a particular heater  80 , a temperature of air in the intake manifold  158  heated by a particular heater  80 , or a temperature in a particular combustion chamber  28  that receives air heated by a particular heater  80 . Information from the temperature sensor may be used to influence the control of power to the particular heater, for example to limit the heater power so as not to exceed a predetermined maximum heater temperature. 
     Control of each heater  80   a ,  80   b ,  80   c ,  80   d  may be achieved, for example, by using solid state relays (not shown) to control current through each heater  80   a ,  80   b ,  80   c ,  80   d . The heat delivered by each heater  80   a ,  80   b ,  80   c ,  80   d  may be controlled, for example, by pulse width modulation of the current through the heater  80   a ,  80   b ,  80   c ,  80   d.    
     In addition to using individually controllable intake air heaters  80   a ,  80   b ,  80   c ,  80   d  to increase combustion chamber temperature on a cylinder-by-cylinder basis, piston cooling by a plurality of individually controllable oil jets may be used to decrease combustion chamber temperature on a cylinder-by-cylinder basis.  FIG. 5  is a schematic diagram depicting piston cooling system for a multi-cylinder engine. In  FIG. 5 , lines with arrowheads at one end are used to indicate oil flow, with the arrowhead indicating the direction of oil flow.  FIG. 5  includes dashed boxes denoted as a, b, c, and d, each associated with one of four cylinders in a four cylinder engine. Within each dashed box, features introduced above with reference to  FIG. 1  are identified with the reference numeral of  FIG. 1  with a letter appended to the numeral, the letter corresponding to the cylinder identification associated with the feature. For example, “ 82   a ” in  FIG. 4  represents the oil nozzle  82  that is associated with cylinder “a”. 
     Referring to  FIG. 5 , a nozzle  82   a  is configured to spray oil onto the piston  66   a  that partially defines the combustion chamber  28   a . Oil supply to the nozzle  82   a  is provided by an oil pump  86  through an oil control valve  84   a . The oil that is sprayed onto the piston  66   a  serves to remove heat from the piston  66   a , thereby lowering the temperature in the combustion chamber  28   a . When GDCI combustion occurs in the combustion chamber  28   a , combustion characteristics are detected by the combustion sensing means  24   a . A signal from the combustion sensing means  24   a  indicative of a combustion characteristic in combustion chamber  28   a  is provided to the controller. The controller is configured to provide a control signal to the oil control valve  84   a  in response to the combustion characteristic detected by the combustion sensing means  24   a , thereby enhancing the robustness of GDCI combustion in the combustion chamber  28   a . A corresponding relationship exists between the corresponding components within each of the other cylinders “b”, “c”, and “d”, 
     As indicated in  FIG. 5 , each of the cylinders a, b, c, d is associated with a corresponding oil control valve  84   a ,  84   b ,  84   c , and  84   d  respectively. Each of the cylinders a, b, c, and d additionally has a corresponding combustion sensing means  24   a ,  24   b ,  24   c , and  24   d  respectively. The controller is configured to receive signals from each individual cylinder indicative of a combustion characteristic in that cylinder, and to provide an appropriate control signal to an individual oil control valve  84   a ,  84   b ,  84   c , and  84   d  to influence the temperature in that cylinder, where each control signal based on the combustion characteristic measured in the respective combustion chamber  28   a ,  28   b ,  28   c ,  28   d . Accordingly, the temperature in each cylinder can be optimized to maximize the robustness of GDCI combustion in each individual cylinder beyond the capabilities of the “global” components described above. 
     Control of each oil control valve  84   a ,  84   b ,  84   c , and  84   d  may be achieved, for example, by using solid state relays (not shown) to control voltage and/or current to each oil control valve  84   a ,  84   b ,  84   c , and  84   d . In the embodiment shown in  FIG. 5 , each oil control valve  84   a ,  84   b ,  84   c , and  84   d  is supplied oil by a common oil pump  86 . As indicated in  FIG. 5 , the oil pump  86  is controllable by a signal from the controller  20 , thereby reducing parasitic losses when full oil flow or pressure is not required. By way of non-limiting example, the oil pump may be a two-step oil pump or a continuously variable oil pump. The viscosity of oil is dependent on its temperature, and the spray characteristics of the nozzles  82   a ,  82   b ,  82   c ,  82   d  are dependent on oil pressure and oil viscosity. In a non-limiting embodiment, as shown in  FIG. 5 , a sensor  88  may be provided to measure the pressure and/or temperature of pressurized oil made available to the oil control valves  84   a ,  84   b ,  84   c ,  84   d  by the oil pump  86 . Alternatively, individual pressure and/or temperature sensors may be provided between each oil control valve  84   a ,  84   b ,  84   c ,  84   d  and its corresponding nozzle  82   a ,  82   b ,  82   c ,  82   d.    
     For GDCI engine operation using a plurality of intake air heaters  80   a ,  80   b ,  80   c ,  80   d  to condition intake air to the combustion chambers  28   a ,  28   b ,  28   c ,  28   d , part-to-part variability between individual heaters  80   a ,  80   b ,  80   c ,  80   d , as well as differences in aging characteristics between individual heaters  80   a ,  80   b ,  80   c ,  80   d , may contribute to further cylinder-to-cylinder variability. In an embodiment of the present invention, the control parameters associated with each individual heater  80   a ,  80   b ,  80   c ,  80   d , or a relationship between the control parameters associated with each individual heater  80   a ,  80   b ,  80   c ,  80   d  that produce the desired combustion characteristics, as described above, may be retained in non-volatile memory, for example in the controller  20 . These “learned” values may then be used as initial values in determining heater control parameters to be used to control individual heaters  80   a ,  80   b ,  80   c ,  80   d  during a subsequent engine operating event. 
     For GDCI engine operation using a plurality of nozzles  82   a ,  82   b ,  82   c ,  82   d , each fed by a corresponding oil control valve  84   a ,  84   b ,  84   c ,  84   d , to provide piston cooling and thereby influence the temperature in the combustion chambers  28   a ,  28   b ,  28   c ,  28   d , part-to-part variability between individual nozzles  82   a ,  82   b ,  82   c ,  82   d  and oil control valves  84   a ,  84   b ,  84   c ,  84   d , as well as aging characteristics of the oil pump  86  and/or differences in aging characteristics between individual nozzles  82   a ,  82   b ,  82   c ,  82   d , and oil control valves  84   a ,  84   b ,  84   c ,  84   d , may contribute to further cylinder-to-cylinder variability. In an embodiment of the present invention, the control parameters associated with the oil pump  86  and with each individual oil control valve  84   a ,  84   b ,  84   c ,  84   d , or a relationship between the control parameters associated with each individual oil control valve  84   a ,  84   b ,  84   c ,  84   d , that produce the desired combustion characteristics at each of a plurality of engine speed and load conditions, may be retained in non-volatile memory, for example in the controller  20 . These “learned” values may then be used as initial values in determining control parameters to be used to control the oil pump  86  and/or to control individual oil control valves  84   a ,  84   b ,  84   c ,  84   d  during a subsequent engine operating event at the corresponding engine speed and load conditions. 
     The combustion sensing means  24  may include a pressure sensor configured to sense the pressure within the combustion chamber  28  and/or a temperature sensor configured to sense the temperature in the combustion chamber. Measurements made by these sensors may be used directly, or may be processed to derive other combustion-related parameters. By way of non-limiting example, control of the intake air heaters  80   a ,  80   b ,  80   c ,  80   d , and/or the oil control valves  84   a ,  84   b ,  84   c ,  84   d , may be based on combustion chamber temperature, combustion chamber pressure, crank angle corresponding to start of combustion (SOC), crank angle corresponding to 50% heat release (CA50), heat release rate, maximum rate of pressure rise (MPRR), location of peak pressure (LPP), ignition dwell (i.e. elapsed time or crank angle between end of fuel injection and start of combustion), ignition delay (i.e. elapsed time or crank angle between start of fuel injection and start of combustion), combustion noise level, or on combinations of one or more of these parameters. 
     In a first operating mode of a GDCI engine system, the “global” components that influence combustion chamber temperature as described above may be controlled so as to establish temperatures in each combustion chamber that, absent a heat contribution from the intake air heaters, would be at or below the temperature corresponding to the optimum temperature for robust combustion in all combustion chambers. The intake air heaters  80   a ,  80   b ,  80   c , and  80   d  may then be controlled to supply supplemental heat to their corresponding combustion chambers  28   a ,  28   b ,  28   c ,  28   d  as appropriate to achieve robust combustion in each combustion chamber  28   a ,  28   b ,  28   c ,  28   d.    
     In a second operating mode of a GDCI engine system, the “global” components that influence combustion chamber temperature as described above may be controlled so as to establish temperatures in each combustion chamber that, absent a cooling effect from oil spray on the pistons, would be at or above the temperature corresponding to the optimum temperature for robust combustion in all combustion chambers. The oil control valves  84   a ,  84   b ,  84   c ,  84   d  may then be controlled to remove heat from their corresponding combustion chambers  28   a ,  28   b ,  28   c ,  28   d  by cooling their corresponding pistons  66   a ,  66   b ,  66   c ,  66   d  as appropriate to achieve robust combustion in each combustion chamber  28   a ,  28   b ,  28   c ,  28   d.    
     In a third operating mode of a GDCI engine system, the “global” components that influence combustion chamber temperature as described above may be controlled so as to establish temperatures in each combustion chamber that, absent a heating effect from air intake heaters and a cooling effect from oil spray on the pistons, would be such that at least one combustion chamber would require supplemental heating to achieve the optimum temperature for robust combustion in that combustion chamber, and at least one other combustion chamber would require supplemental cooling to achieve the optimum temperature for robust combustion in that combustion chamber. The intake air heaters  80   a ,  80   b ,  80   c ,  80   d , and the oil control valves  84   a ,  84   b ,  84   c ,  84   d  may then be simultaneously controlled to achieve robust combustion in each combustion chamber  28   a ,  28   b ,  28   c ,  28   d.    
     The first operating mode, second operating mode, and third operating mode as described above may all be employed in a given GDCI engine system at different times, depending on factors including but not limited to engine speed, engine load, engine temperature, ambient temperature, whether the engine is warming up or fully warmed, and whether engine speed and load are in a steady state or a transient state. Selection of an operating mode may be influenced by other factors, such as the desire to minimize parasitic loads on the engine, such as the need to provide energy to the heaters  80   a ,  80   b ,  80   c ,  80   d , to the oil control valves  84   a ,  84   b ,  84   c ,  84   d , to the oil pump  86 , and/or to the coolant pumps  210 ,  220 . Other considerations may also influence the selection of an operating mode. For example, while the engine is warming up, it may be desirable to operate the heaters  80   a ,  80   b ,  80   c ,  80   d  to provide the maximum air heating that can be accommodated to achieve robust combustion through control of fuel injection parameters, in order to accelerate light-off of the catalyst  170 . In a transient condition, for example when the engine is accelerating, a piston cooling system as depicted in  FIG. 5  may provide improved response time for controlling combustion chamber temperature compared with the response time of the “global” components discussed above. This improved response time may enable enhanced stability of the multi-cylinder engine. 
     While this invention has been described in terms of preferred embodiments thereof, it is not intended to be so limited, but rather only to the extent set forth in the claims that follow.