Abstract:
An internal combustion engine is disclosed in which at least one cylinder is adapted to run in homogeneous-charge compression-ignition (HCCI) combustion mode and at least one cylinder is adapted to run in spark-ignition (SI) combustion mode with the HCCI cylinder having a compression ratio at least 2 ratios higher than the SI cylinder. The intakes and exhausts of the HCCI and SI cylinders are separated with a throttle valve disposed in each intake and exhaust aftertreatment devices tailored to each combustion mode disposed in the exhausts.

Description:
FIELD OF INVENTION 
   The present invention relates to an internal combustion engine in which a portion of cylinders exclusively operate under a spark-ignition combustion mode and remaining cylinders exclusively operate under a homogeneous-charge, compression-ignition mode. 
   BACKGROUND OF THE INVENTION 
   Homogeneous-charge, compression-ignition combustion is known to those skilled in the art to provide high fuel efficiency and low emission operation in internal combustion engines. However, HCCI operation is feasible in a narrow range in engine torque, approximately one-third of the torque range of a conventional spark-ignited engine. Thus, most HCCI engines being developed are dual mode engines in which HCCI is used at low torque conditions. When a higher torque is desired, operation is transitioned to an alternative combustion mode, such as spark-ignition combustion or heterogeneous, compression-ignition combustion (diesel). Challenges accompanying such transitions include: torque matching (providing driver demanded torque during the transition interval), maintaining emission control, and robustly returning to HCCI combustion, to name a few. Another difficulty encountered in engines which transition from one combustion mode to another is that the combustion system geometry cannot be optimized for either combustion mode, but is instead a compromise. For example, a desirable compression ratio for HCCI combustion is about 15:1 and about 10.5:1 for spark-ignition combustion. 
   A disadvantage of HCCI combustion is its inferior transient response to a demand for a change in torque, orders of magnitude slower than SI combustion. The inventors of the present invention have recognized that HCCI operation cannot provide a vehicle operator with the responsiveness that they have come to expect from a SI engine. 
   In U.S. patent application 2004/0182359, an 8-cylinder HCCI/SI engine is described in which HCCI to SI transitions are made one cylinder at a time, i.e., at a lower torque demand all 8 cylinders are operating in HCCI combustion mode and as torque demand exceeds what HCCI combustion can provide, cylinders are individually switched to SI operation. The inventors of the present invention have recognized that it would be desirable to have an engine which provides the desired range in output torque at the high efficiency of HCCI combustion without having to undergo a combustion mode transition in any given cylinder because of the compromises inherent in designing a cylinder to operate robustly and efficiently in both HCCI and SI combustion modes over a wide operating range. 
   SUMMARY OF THE INVENTION 
   According to the present invention, some cylinders operate in SI combustion mode only and some cylinders operate in HCCI combustion mode only, which overcomes the disadvantages of transitions between operating modes in a single cylinder and engine design compromises. In particular, an internal combustion engine is disclosed which has at least one higher compression ratio cylinder; and at least one lower compression ratio cylinder, with a compression ratio difference of at least 2 compression ratios. As an example, the higher compression ratio cylinder is greater than 14:1 and the lower compression ratio cylinder is less than 12:1. The higher compression ratio cylinder is adapted to operate under homogeneous charge compression ignition combustion and the lower compression ratio cylinder is adapted to operate under spark ignition combustion. 
   The engine has separated intakes and exhausts so that the gases to and from the HCCI cylinder remain separate from the gases to and from the SI cylinder. Both intakes have a throttle valve for individual control of intake air. By separating the exhaust gases, exhaust aftertreatment devices are selected based on the aftertreatment specific to each combustion type. For the SI cylinder, a three-way catalyst is employed. Various embodiments include an oxidation catalyst, a lean NOx trap, a lean NOx catalyst, or a combination of these aftertreatment devices for the HCCI cylinder. Exhaust gas sensors are coupled to the exhaust, upstream, downstream, or both with respect to the aftertreatment device. 
   Preferably, the engine has valve deactivators coupled to the lower compression ratio cylinder. The lower compression ratio cylinder can be deactivated when not needed for providing torque, to further improve fuel efficiency. 
   The engine has an exhaust gas recirculation duct coupling an exhaust of the low compression ratio cylinder with an intake of the high compression ratio cylinder and an exhaust gas recirculation valve in the exhaust gas recirculation duct. 
   In one embodiment, a compressor is coupled to the higher compression ratio cylinder intake to overcome a deficit in peak power due to HCCI combustion. 
   In one embodiment a combustion sensor is provided in the higher compression ratio cylinder. From the sensor output crank angle of peak pressure can be determined. 
   Additionally, an internal combustion engine is disclosed which has a first portion of engine cylinders comprising every other cylinder in the engine&#39;s firing order and a second portion of engine cylinders comprising every other cylinder in the engine&#39;s firing order. The first and second portions are mutually exclusive and the first portion of engine cylinders has a compression ratio at least two ratios higher than the second portion of engine cylinders. 
   The present invention overcomes the problems of designing a HCCI-capable engine in which the cylinders operate in both HCCI and another combustion mode depending on the demand for engine speed/torque. In the present invention, at least one cylinder is dedicated to HCCI operation and at least one cylinder is dedicated to SI operation. In this way, the fuel efficiency of HCCI is achieved without the mode transition difficulties of prior art HCCI engines. By deactivating the SI cylinders when the HCCI cylinders can provide the desired torque, the fuel efficiency is further enhanced. 
   Another advantage of the present invention is that exhaust aftertreatment can be optimized for each combustion mode since the exhaust ducts are separated. In the prior art, the exhaust aftertreatment system handles both the very lean, low temperature HCCI exhaust as well as stoichiometric SI exhaust, which greatly complicates exhaust aftertreatment, particularly during transitions. 
   By pressure charging the HCCI cylinders, the torque produced by the HCCI cylinders can more closely match that of the SI cylinders, thus overcoming a possible torque deficit with naturally aspirated HCCI operation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The advantages described herein will be more fully understood by reading an example of an embodiment in which the invention is used to advantage, referred to herein as the Detailed Description, with reference to the drawings wherein: 
       FIG. 1  is a schematic of an internal combustion engine according to an aspect of the present invention; 
       FIGS. 2   a - 2   c  are graphs of torque vs. fuel efficiency for prior art SI and HCCI engines and an engine according to an aspect of the present invention; 
       FIG. 3  is a flowchart of a cold start strategy; 
       FIGS. 4   a  and  4   b  are graphs of driver demanded torque over time for an engine according to the present invention; and 
       FIG. 5  is a schematic of the exhaust gas recirculation according to an aspect of the present invention. 
   

   DETAILED DESCRIPTION 
   In  FIG. 1 , a multi-cylinder internal combustion engine  10  is shown. By way of example, engine  10  is shown to have six cylinders, two banks of 3 cylinders each. One bank of cylinders  12  is adapted to operate in a conventional spark-ignition (SI) mode. The other bank of cylinders  14  is adapted to operate in a homogeneous-charge, compression-ignition (HCCI) mode. Combustion air is provided to the cylinder banks via an intake manifold  30 , which is separated such that air for cylinder bank  12  does not mix with air for cylinder bank  14 . Each bank is provided with a throttle valve  24 , or other means to control flow. SI combustion characteristically occurs at stoichiometric proportions of air and fuel, meaning that if combustion were complete, all the fuel and oxygen would be completely combusted to H 2 O and CO 2 . To control the amount of torque produced in SI combustion, the amount of air is controlled by throttle valve  24 . The amount of fuel added to the air, via injector  26 , is metered to provide a stoichiometric mixture. For clarity, only one fuel injector  26  is shown in  FIG. 1  for the 3 cylinders of bank  12 . However, each cylinder is provided a fuel injector. Similarly, each cylinder in bank  14  is provided with a fuel injector like fuel injector  28 . In  FIG. 1 , bank  12  is shown with a port injector in which fuel is sprayed outside of the cylinder and is brought into the cylinder with the combustion air and bank  14  is shown with a direct fuel injector in which fuel is sprayed directly into the combustion air which has been inducted into the cylinder. These types of fuel injection systems are shown by way of example. Both banks could be provided with port injection or both with direct injection according to the present invention. I.e., both HCCI and SI combustion can be accomplished with port, direct, or a combination of port and direct injection. Alternatively, fuel is provided by central body injection for either combustion mode. 
   Again, for clarity, only one of the three spark plugs for each of the cylinders of bank  12  is shown. Bank  14  cylinders may also have spark plugs. Although the bank  14  cylinders are HCCI cylinders, which indicates that combustion is initiated by compression ignition, it is known to those skilled in the art that at some operating conditions, it is useful to employ spark assist to initiate combustion. Alternatively, another ignition assist device such as glow plugs, plasma jet igniters, catalytic assisted glow plugs, as examples, could be used for ignition assist in HCCI. In SI combustion, a spark initiates a flame kernel and a flame front travels throughout the cylinder. In spark assisted HCCI, a spark initiates a flame kernel at the location of the spark plug. However, the mixture in the cylinder is too weak (not enough fuel or too much burned gases in the mixture) to sustain a flame front traveling through the cylinder gases. The flame kernel combusts the fuel-air mixture near the spark plug. The release of energy by the combustion of the mixture near the spark plug increases the pressure in the cylinder thereby causing the gases away from the spark plug to attain their ignition temperature and to self-ignite. When spark assist HCCI is contemplated, all HCCI cylinders are provided with a spark plug  58 . 
   Engine  10  is shown to be a 6-cylinder with bank  12  being SI and bank  14  being HCCI by way of example. This is not intended to be limiting. Engine  10  has any number of cylinders greater than one and in any configuration: in-line, V, W, radial, opposed, or any other cylinder arrangement. The HCCI and SI cylinders need not be segregated by banks. There could be HCCI and SI cylinders on any given bank. However, as mentioned above, the intake gases to the HCCI cylinders and SI cylinders remain separated and exhaust gases coming from HCCI cylinders and SI cylinders also remain separated. Thus, such arrangements may require complicated manifolding to maintain the separation. An expected arrangement is that every other cylinder in the firing order is alternately HCCI and SI. 
   SI engines are typically produced with a 9.5-10.5:1 compression ratio, which is the ratio of the volume in the cylinder above the piston when the piston is at the top of its travel divided by the volume in the cylinder above the piston when the piston is at the bottom of its travel. HCCI combustion occurs more favorably with a higher compression ratio: 13-15:1. In prior art engines in which combustion mode is transitioned, the compression ratio selected is a compromise between the two compression ratios. According to the present invention, however, because each cylinder is optimized for a single combustion mode, the engine is produced with the compression ratio appropriate for the particular combustion mode. Thus, unlike prior art engines, the engine according to the present invention has some cylinders with a substantially higher compression ratio than other cylinders. 
   HCCI combustion occurs in a dilute mixture, either very lean of stoichiometric with excess air and/or with a very high level of exhaust dilution. It is well known to those skilled in the art to provide exhaust dilution by either recirculating exhaust gases into the engine intake, known as exhaust gas recirculation (EGR) sometimes referred to as external EGR, or to retain exhaust gases in the cylinder from a prior combustion event to mix with the combustion gases of an upcoming combustion event, commonly known as internal EGR. The latter is often achieved by valve timing adjustments. Typically exhaust gases are routed from an exhaust duct to an intake duct via a control valve (EGR valve). The present invention provides for an alternative configuration for EGR in which gases exhausted from the SI cylinder bank  12  are routed to the intake of the HCCI cylinder bank  14  via valve  39 . In  FIG. 1 , the exhaust gases are collected downstream of exhaust gas aftertreatment device  20 . This is shown by way of example and not intended to be limiting. The exhaust gases can be taken from any position in the exhaust duct. There are two advantages for circulating exhaust gases from the SI bank  12  to the HCCI bank  14 . Typically, SI combustion occurs with a stoichiometric mixture, which provides combustion gases containing primarily CO 2 , H 2 O, and N 2 . In contrast, HCCI cylinders combust lean mixtures which have excess air. Thus, HCCI exhaust gas has significant levels of O 2  and more N 2  than a SI exhaust gas. To obtain a desired diluent fraction, a greater amount of HCCI exhaust gas is recycled compared with the SI exhaust gas quantity. It is well known to those skilled in the art that to achieve ignition in HCCI, it is common to heat the intake air. Because the exhaust gas temperature is higher with SI combustion, less intake heating is required when the EGR employed comes from SI combusting cylinders. In particular, with reference to  FIG. 1 , gas from the exhaust duct coupled to cylinder bank  12  is drawn through EGR system  39  and supplied to the intake of cylinder bank  14  via a control valve. 
   Continuing with  FIG. 1 , each cylinder bank is provided with an exhaust gas aftertreatment device,  20  and  22 . In one embodiment, device  20  is a three-way catalyst, which efficiently oxidizes CO and hydrocarbons and reduces nitrogen oxides (NOx) when provided a stoichiometric exhaust gas. As mentioned above, obtaining higher fuel efficiency motivates HCCI development. Another advantage of HCCI combustion, which occurs in a very lean or dilute mixture, is that it produces very low levels of NOx, particularly compared to SI operation. In one embodiment, the HCCI cylinders require no NOx aftertreatment and aftertreatment device  22  is an oxidation catalyst to process unburned fuel and CO. In another embodiment, a lean NOx aftertreatment device is employed to process the low levels of NOx when very low NOx levels are required or when HCCI operation is extended into regions at which the NOx produced is somewhat higher than typical HCCI combustion. In this embodiment, the lean NOx aftertreatment device is either a lean NOx trap or a lean NOx catalyst. A lean NOx trap stores NOx during lean operation. When the trap is no longer able to store additional NOx, the trap is purged by operating lean for a period of time. During the lean operation, the NOx is desorbed from the trap and reacted to N2 and O2. To accomplish a rich excursion with HCCI, one alternative is to operate with a very high level of EGR to displace excess air. A lean NOx catalyst processes NOx in the presence of a reductant, either fuel or urea. 
   In  FIG. 1 , an indication of exhaust gas constituents is provided by an exhaust gas sensor,  60  and  62 , situated in the exhaust ducts exiting each cylinder bank. Only a single sensor is shown in  FIG. 1 . Exhaust gas sensor  60  is an oxygen sensor, either heated or unheated, which provides an indication of whether the exhaust gas is near stoichiometry. In another embodiment, sensor  60  is a wide-range oxygen sensor provides a measure of exhaust gas stoichiometry. Exhaust gas sensor  62  measures NOx concentration. Alternatively, sensor  62  is a wide-range oxygen sensor. Only one exhaust gas sensor is shown in each of the exhaust ducts of engine  10 . However, it is known to use multiple exhaust gas sensors. In one embodiment, both a wide-range oxygen sensor and a NOx sensor are placed in place of sensor  62 . Furthermore, it is common practice to provide a sensor both upstream and downstream of an exhaust aftertreatment device. The inventors of the present invention contemplate any known exhaust gas sensor type in any location in the exhaust ducts. 
   The signal from an exhaust gas oxygen sensor  60  is commonly used for air-fuel ratio feedback control of SI combustion. Analogously, HCCI combustion timing is controlled by adjusting intake temperature, according to one alternative embodiment. Adjustment of intake temperature is feedback controlled based on a combustion parameter such as crank angle of peak pressure. Examples of sensors from which crank angle of peak pressure can be ascertained include: head bolt strain gauge, in-cylinder pressure sensor, ionization sensor, a head gasket sensor, sensor measuring instantaneous flywheel speed, etc. For stoichiometric SI combustion, it is well known by those skilled in the art that the crank angle of peak pressure corresponding to peak efficiency operation (at a given speed/torque condition) occurs roughly at 15 degrees after top dead center. Alternative combustion systems, particular lean burn, tend to have the crank angle of peak pressure occur at a somewhat earlier time, e.g., 12 degrees after top dead center to achieve peak efficiency. Furthermore, there are other objectives, besides achieving peak efficiency, such as emission control, which cause the desired crank angle of peak pressure to be other than that providing peak efficiency. It is expected that a desired crank angle of peak pressure is in a range of 8 to 20 degrees after top center. Various combustion control parameters, such as: intake temperature, EGR valve position, throttle valve position, flow through an intake heat exchanger, and pressure charging, can be feedback controlled based on crank angle of peak pressure, particularly for the HCCI cylinders. 
   Because HCCI combustion is dilute, the peak torque capable from a given cylinder is much less than peak torque from a SI cylinder. To increase the amount of torque from a HCCI cylinder, compressor  34  increases the intake manifold pressure on the HCCI cylinders, allowing for an increased amount of fuel delivery while maintaining a high dilution. As shown in  FIG. 1 , compressor  34  is connected by a shaft to turbine  32 , a device known as a turbocharger. The unconventional ducting of  FIG. 1  has turbine  32  extracting work from SI cylinder exhaust gases which compress intake gases of HCCI cylinders via compressor  34 . HCCI combustion is known to provide superior fuel efficiency to SI combustion. Thus, HCCI exhaust gases have a lower enthalpy than SI exhaust gases because HCCI allows more of the energy release of combustion to be extracted. Thus, it is desirable to extract the SI exhaust gas energy to pressure charge the HCCI inlet. In another embodiment, exhaust turbine  32  is coupled to the HCCI exhaust duct. The turbocharger of  FIG. 1  is a variable geometry turbocharger. In yet another embodiment, a supercharger is provided in place of turbocharger (comprising elements  32 ,  34 , and  36 ). A supercharger is a compressor, like compressor  34  of  FIG. 1 , which is driven by engine  10 . A supercharger is not coupled to a turbine. 
   In  FIG. 1 , an intake gas heat exchanger  38  is contained with the exhaust duct coupled to the SI cylinders. It is known in the art that one of the methods to control ignition timing of HCCI combustion is by controlling the intake temperature. Diverter valve  37  allows adjustment of the quantity of HCCI intake gases passing through heat exchanger  38  and the quantity passing through bypass duct  35 , thereby providing control of HCCI intake temperature. 
   Continuing to refer to  FIG. 1 , electronic control unit (ECU)  40  is provided to control engine  10 . ECU  40  has a microprocessor  46 , called a central processing unit (CPU), in communication with memory management unit (MMU)  48 . MMU  48  controls the movement of data among the various computer readable storage media and communicates data to and from CPU  46 . The computer readable storage media preferably include volatile and nonvolatile storage in read-only memory (ROM)  50 , random-access memory (RAM)  54 , and keep-alive memory (KAM)  52 , for example. KAM  52  may be used to store various operating variables while CPU  46  is powered down. The computer-readable storage media may be implemented using any of a number of known memory devices such as PROMs (programmable read-only memory), EPROMs (electrically PROM), EEPROMs (electrically erasable PROM), flash memory, or any other electric, magnetic, optical, or combination memory devices capable of storing data, some of which represent executable instructions, used by CPU  46  in controlling the engine or vehicle into which the engine is mounted. The computer-readable storage media may also include floppy disks, CD-ROMs, hard disks, and the like. CPU  46  communicates with various sensors and actuators via an input/output (I/O) interface  44 . Examples of items that are actuated under control by CPU  46 , through I/O interface  44 , are commands to fuel injectors  26  and  28  such as fuel injection timing, fuel injection rate, and fuel injection duration. Additional parameters under CPU  46  control are position of throttle valves  24 , timing of spark plug  58 , position of control EGR valves  39 , other control valves  37 , variable geometry turbocharger nozzle position, intake and exhaust valve timing, and others. Sensors  42  communicating input through I/O interface  44  may include piston position, engine rotational speed, vehicle speed, coolant temperature, intake manifold pressure, accelerator pedal position, throttle valve position, air temperature, exhaust temperature, exhaust stoichiometry, exhaust component concentration, and air flow. Some ECU  40  architectures do not contain MMU  48 . If no MMU  48  is employed, CPU  46  manages data and connects directly to ROM  50 , RAM  54 , and KAM  52 . Of course, the present invention could utilize more than one CPU  46  to provide engine control and ECU  40  may contain multiple ROM  50 , RAM  54 , and KAM  52  coupled to MMU  48  or CPU  46  depending upon the particular application. 
   Referring to  FIG. 2   a , a graph of thermal efficiency as a function of torque is shown for a conventional SI engine as curve  100 . Curve  104  indicates the higher thermal efficiency that is possible when operating the same displacement engine in the HCCI combustion mode. The thermal efficiency is markedly improved. However, HCCI does not provide the same range in torque as a SI engine of the same displacement. To provide the same torque level, either the displacement of the engine has to be roughly doubled or combustion mode is changed from HCCI at low torque and then SI when higher torque is demanded. 
   Also shown in  FIG. 2   a  are curves  102  and  106 , which is only half of the engine cylinders operating on SI and HCCI respectively. The peak thermal efficiency is the same as the corresponding curves  100  and  104  when the engine is operated with all cylinders on SI and HCCI, respectively. The range in torque, is half that of the running the whole engine. 
   According to an aspect of the present invention, half of the cylinders are operated with HCCI combustion and half of the cylinders are operated with SI combustion, the effect of such operation on torque and thermal efficiency being shown in  FIG. 2   b . Note that  FIG. 2   b  relates to a naturally aspirated engine in which there is no supercharger or turbocharger to pressurize intake gases. Because of the high efficiency of HCCI combustion, it is desirable to operate only the HCCI cylinders at low torque demands. Thus, curve  106  of  FIGS. 2   a  and  2   b  are identical, i.e., half of the cylinders operating on HCCI and the other cylinders being deactivated. When a higher demand in torque is desired, SI cylinders are activated and torque is being provided by both the SI and HCCI cylinders, shown as curve  108  in  FIG. 2   b . Because HCCI cylinders cannot provide the same torque range as SI, the peak torque in  FIG. 2   b  is less than what is shown in  FIG. 2   a . The efficiency shown in  FIG. 2   b  exceeds that of the SI engine efficiency, curve  102 , of  FIG. 2   a  at all values of torque. 
   To make up for the lesser torque of the engine illustrated in  FIG. 2   b , either the displacement of the engine can be increased or boosting applied. Boosting can be applied to the HCCI cylinders or all engine cylinders. However, as discussed above, since HCCI provides less torque, an approach for HCCI cylinders to match torque is to boost only HCCI cylinders. Curve  116  of  FIG. 2   c  shows thermal efficiency where boost is provided by a supercharger. The torque range of curve  116  is wider than that of curve  106  (of  FIG. 2   b ) due to the increased amount of air delivered to the HCCI cylinders by the supercharger. The torque level at which half HCCI and half SI (curve  118  of  FIG. 2   c ) is invoked it higher than in  FIG. 2   b . The maximum torque in  FIG. 2   c  is about that of  FIG. 2   a.    
   If a turbocharger were employed in place of a supercharger, no increase in torque range with HCCI only operation is possible because the SI cylinders are deactivated, thus no exhaust to drive the turbocharger. 
   Because achieving a sufficiently high temperature to cause autoignition is paramount in HCCI combustion, providing a robust cold start presents a serious hurdle for HCCI combustion. Those skilled in the art discuss starting on SI combustion and transitioning to HCCI combustion after the engine has achieved a suitable operating temperature. However, with the present invention, the cylinders are adapted to operate in only one combustion mode. To overcome, the inventors of the present invention contemplate starting on SI cylinders. During the period of SI combustion, air can be delivered to HCCI cylinders through heat exchanger  38 . By blowing warm air through the HCCI cylinder bank  14 , the engine surfaces can be preheated and ready for HCCI combustion. In addition, the engine coolant is heated by the SI cylinders and preheats the HCCI cylinders. 
   According to an aspect of the present invention, the SI cylinders are equipped with valve deactivators (not shown). During HCCI only operation, the SI cylinders are deactivated by closing off the intake and exhaust valves. The piston continues to reciprocate, but the gas in the cylinder at the last combustion event remains trapped in the cylinder. If the valves were allowed to remain active, the flow of air through SI cylinder bank  12  would flow into exhaust aftertreatment device  20 . If device  20  is a three-way catalyst, oxygen would be absorbed onto the surfaces and when the SI cylinders were reactivated, the three-way catalyst would be unable to reduce NOx until such oxygen is removed from device  20 . Furthermore, the flow of air through SI cylinder bank  20  cools the engine down, thereby making restart more difficult. 
   In one embodiment, valve deactivators are provided for the HCCI cylinders (not shown in  FIG. 1 ). However, because HCCI combustion is more efficient than SI combustion, it is desirable to operate the HCCI cylinders whenever possible. When the HCCI cylinders have not attained a suitable operating temperature, the HCCI cylinders can be deactivated. However, according to an aspect of the invention discussed above, exhaust waste energy from SI cylinders is transferred to HCCI induction air via heat exchanger  38 . Thus, it may be preferable to allow valves in HCCI cylinders to operate as normal to allow heating. 
   Referring to  FIG. 3 , a cold start is initiated in block  150 . The engine starts operation on SI cylinders only, block  152 . Induction air flows through heat exchanger  38  and then through HCCI cylinders to warm up the HCCI cylinders, block  154 . In block  156 , it is determined whether a sufficient temperature has been achieved. If so, operation in the HCCI cylinders is initiated in block  158 . If the torque that can be produced by the HCCI cylinders is sufficient to meet torque demand (determined in block  160 ), control is passed to block  162  in which the SI cylinders are deactivated. If not enough torque can be produced in the HCCI cylinders, the engine is operated with all cylinders active, block  164 . 
   In  FIG. 4   a , a hypothetical driver torque demand as a function of time is shown as line  200 . Between time o and time a, the driver demands a relatively low torque, at time a, the driver pushes down the accelerator pedal, which indicates demand for higher torque. The driver again demands increased torque at times b and c. Between times o and a, only the HCCI cylinders are active. Since the HCCI combustion mode provides superior fuel economy and the HCCI cylinders are capable of providing the desired torque, the SI cylinders are deactivated. As the torque demand isn&#39;t changing, there is no concern about the slow transient response of HCCI combustion. When the transition to a higher torque is demanded at time a, the HCCI cylinders cannot respond quickly enough to attain the new, higher torque. At time a′, the HCCI cylinders have attained the desired torque level. However, between time a and a′, if only HCCI cylinders were active, the torque response of the vehicle would be unacceptable to the operator of the vehicle. Thus, according to the present invention, the SI cylinders are activated at time a so that the operator demanded torque can be more closely followed than with HCCI cylinders alone. As the HCCI cylinders ramp up their torque production, the SI cylinders are ramped down and eventually deactivated at time a′. At time b, another rapid torque increase is demanded. Again, the SI cylinders are activated to fill in and the torque produced by the HCCI cylinders is ramped up. As at time a when a rapid torque increase is demanded, the HCCI cylinders are too slow to provide the transient response requested. Thus, the SI cylinders are reactivated at time b. At time b′, the HCCI engines have attained their maximum torque output condition. Thus at time b′, the SI cylinders remain active. At time c, the further increase in demanded torque is provided solely by the SI cylinders because the HCCI cylinders are already operating at their peak capacity. 
   In  FIG. 4   b , a similar increase in torque is shown, however with a much slower demand for transient torque response. From time o to time b′, the driver demand is slow enough that the HCCI cylinders can provide the desired torque demand. At time b′, the HCCI cylinders have reached their capacity and the SI cylinders are activated. Further increases in torque are provided by the SI cylinders. 
   In another embodiment, the SI cylinders remain active at all times. In one example of this embodiment, an 8-cylinder is started on 2 SI cylinders. The remaining 6 cylinders are HCCI cylinders, which are turned on when they reach a suitable temperature which supports robust HCCI combustion. In this embodiment, the SI cylinders remain operational even after HCCI combustion has been achieved in the 6 HCCI cylinders. 
   Referring now to  FIG. 5 , an HCCI bank  14  and a SI bank  12  are incorporated in engine  10 . (This is simply for ease of illustration. As discussed above, various arrangements are contemplated by the inventors of the present invention, in one example each bank has one HCCI cylinder and three SI cylinders with complicated manifolding to keep their intake and exhaust gases separated.) Bank  12  is provided with exhaust  16  and bank  14  is provided with exhaust  18 . A portion of exhaust gases from bank  12  may be drawn off by an EGR system through EGR valve  70 . Similarly, exhaust gases are drawn from an exhaust from bank  14  through EGR valve  72 . Both EGR ducts flow into the intake to bank  14 . The intake to bank  14  is also supplied with throttle valve  24 . Throttle valve  24 , EGR valve  70  and EGR valve  72  are controlled by electronic control unit  40 . 
   Because HCCI combustion is very dilute, HCCI combustion gases are at a much lower temperature than SI combustion gases. By controlling the proportion of EGR gases coming from bank  12  and from bank  14 , the temperature in HCCI cylinders is controlled. As mentioned above, one of the ways, known by those skilled in the art, for controlling HCCI combustion timing is by adjusting the temperature of the gases in the HCCI cylinder. By continuing to operate SI cylinders while HCCI cylinders are operating, the exhaust gases from SI cylinders is available for recycle to the HCCI cylinders for controlling temperature in HCCI cylinders. 
   While several modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize alternative designs and embodiments for practicing the invention. The above-described embodiments are intended to be illustrative of the invention, which may be modified within the scope of the following claims.