Abstract:
Engine surge includes oscillations in engine torque resulting in bucking or jerking motion of a vehicle that may degrade driver experience. The present application relates to increasing reformate entering an example engine cylinder in response to engine surge.

Description:
TECHNICAL FIELD 
     The present application relates to hydrogen-rich reformate and more particularly, to preventing or mitigating engine surge. 
     BACKGROUND AND SUMMARY 
     Engine surge (one example of engine combustion instability) includes oscillations in engine torque. Such oscillations in engine torque result in reduced drive feel. 
     In one approach a wheel brake pressure is controlled in response to measurements of engine torque. By increasing wheel brake pressure on one or more wheels in response to engine surge, vehicle traction may be improved during surge. Consequently, drivability may be improved. 
     The inventors herein have recognized issues with the above described approach. Controlling wheel brake pressure does not address the underlying engine conditions leading to engine surge. Without addressing the underlying engine conditions, engine surge may persist. 
     Consequently, systems, devices and methods are disclosed for engine control for a reformate engine. As one example, a method for an engine includes reforming fuel at a catalyst into reformate; and adjusting a supply of reformate to a cylinder of the engine in response to an engine surge, the surge including an oscillation in torque produced by the engine. The fuel to be reformed may include, for example, ethanol, another alcohol, gasoline, diesel fuel, or a combination of fuels. 
     One advantage of the example is that surge may be mitigated. Further, the present example allows for a smaller and lower cost reformer, if desired, because the supply of reformate to the engine cylinder is adjusted in response to surge, rather than continuously maintained at an unnecessarily high level during engine operation. By adjusting reformate in combination with further vehicle operating parameters, like charge dilution, wheel brake and spark timing, engine surge is mitigated while achieving increased engine efficiency due to aggressive use of lean burn, exhaust gas recirculation (EGR), and/or variable valve timing (VVT). 
     It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic representation of a vehicle illustrating example locations wherein longitudinal acceleration sensors may be placed on the vehicle. 
         FIG. 2  shows a schematic diagram of an example internal combustion engine. 
         FIG. 3  shows a block diagram of systems and devices related to sensing acceleration in a vehicle. 
         FIG. 4  illustrates an example routine for adjusting a supply of reformate to a cylinder of the engine in response to an engine surge. 
         FIG. 5  illustrates an example subroutine for determining a feasibility of increasing a percentage of reformate in an example engine cylinder. 
         FIG. 6  illustrates an engine map with respect to engine speed-load. 
     
    
    
     DETAILED DESCRIPTION 
     Initially, an example vehicle, including an engine and further systems, such as a reformate system and a stability control, are described with respect to  FIGS. 1 and 2 .  FIG. 3  is then discussed, which shows a block diagram further describing some of the systems and devices related to sensing acceleration in a vehicle. A first example routine is described with respect to  FIG. 4  as one example of a method for controlling reformate use in an engine, such as reformed ethanol. Further, a subroutine shown in  FIG. 5  is discussed as one example of a method for determining a feasibility of increasing a percentage of reformate in an example charge. 
       FIG. 1  is a schematic illustration of a vehicle  150 , and  FIG. 2  is a schematic illustration of a system  200  that may be included in the vehicle  150 . The vehicle  150  and the system  200  may have one or more longitudinal acceleration sensors (which are all example accelerometer sensors) in accordance with various embodiments. Various numbers and configurations of acceleration sensors may be used. One or more longitudinal acceleration sensors that may already be present on the vehicle  150  may be used, or one or more longitudinal acceleration sensors may be added to the vehicle  150 . Three longitudinal acceleration sensors are illustrated in  FIGS. 1 and 2 . One longitudinal acceleration sensor  202  may be included as part of a stability control  204  for the vehicle  150 . The stability control  204  may be an electronic stability control (ESC) or a rollover stability control (RSC), or the like. Another longitudinal acceleration sensor (e.g.,  206  of  FIG. 3 ) may be included as part of an airbag system  208  for the vehicle  150 . Another longitudinal acceleration sensor  210  may be an added to the vehicle  150 . 
     The processor  212  may be operatively coupled with an engine controller  12 . The system  200  may include an ignition system  88  that may be configured to provide an ignition spark to combustion chamber  30  via spark plug  92  in response to a spark advance signal SA, or a spark retard signal SR from engine controller  12 , under select operating modes, and in accordance with instructions from the processor  212 . 
     Alternatively, the processor  212 , and/or functions described herein may be included as part of the engine controller  12 , and may in particular be included as part of a microprocessor unit (CPU)  102 . 
     Engine controller  12  is shown in  FIG. 2  as a microcomputer, including microprocessor unit  102 , input/output ports  104 , an electronic storage medium for executable programs and calibration values shown as read only memory chip  106  in this particular example, random access memory  108 , keep alive memory  110 , and a data bus. 
     Engine controller  12  may receive various signals from sensors coupled to engine  10 , in addition to those signals previously, and hereinafter, discussed, including measurement of inducted mass air flow (MAF) from mass air flow sensor  120 ; engine coolant temperature (ECT) from temperature sensor  112  coupled to cooling sleeve  114 ; a profile ignition pickup signal (PIP) from Hall effect sensor  118  (or other type) coupled to crankshaft  40 ; throttle position (TP) from a throttle position sensor  62 ; a measurement of reformer tank pressure from pressure sensor  85 ; and a measurement of reformer tank temperature from temperature sensor  87 ; and absolute manifold pressure signal, MAP, from sensor  122 . Engine speed signal, RPM, may be generated by engine controller  12  from signal PIP. Barometric pressure may also be sensed (sensor not shown) for processing by controller  12 . 
       FIG. 2  illustrates one cylinder of multi-cylinder engine  10 , which is included in a propulsion system of vehicle  150 . Engine  10  may be controlled at least partially by a control system including the engine controller  12  and by input from a vehicle operator  132  via an input device  130 . In this example, input device  130  includes an accelerator pedal and the pedal position sensor  134  for generating a proportional pedal position signal PP. Engine  10  includes combustion chamber  30  and cylinder walls  32  with piston  36  positioned therein. Piston  36  may be coupled to crankshaft  40  so that reciprocating motion of the piston is translated into rotational motion of the crankshaft. Crankshaft  40  may be coupled to at least one drive wheel of a vehicle via an intermediate transmission system. Further, a starter motor may be coupled to crankshaft  40  via a flywheel to enable a starting operation of engine  10 . Combustion chamber  30  is shown communicating with intake manifold  44  and exhaust manifold  48  via respective intake valve  52  and exhaust valve  54 . 
     In this example, intake valve  52  and exhaust valves  54  may be controlled by cam actuation via respective cam actuation systems  51  and  53 . Cam actuation systems  51  and  53  may each include one or more cams and may utilize variable valve timing (VVT) which includes one or more of cam profile switching (CPS), variable cam timing (VCT), and/or variable valve lift (VVL) systems that may be operated by controller  12  to vary valve operation. The position of intake valve  52  and exhaust valve  54  may be determined by position sensors  55  and  57 , respectively. In alternative embodiments, intake valve  52  and/or exhaust valve  54  may be controlled by electric valve actuation (EVA). For example, cylinder  30  may alternatively include an intake valve controlled via electric valve actuation and an exhaust valve controlled via cam actuation including CPS and/or VCT systems. 
     Intake manifold  44  is also shown coupled to the engine cylinder having fuel injector  66  coupled thereto for delivering liquid fuel in proportion to the pulse width of signal FPW from controller  12 . Fuel is delivered to fuel injector  66  by a fuel system including fuel tank  91 , fuel pump (not shown), fuel lines (not shown), and fuel rail (not shown). The engine  10  of  FIG. 1  is configured such that the fuel is injected directly into the engine cylinder, which is known to those skilled in the art as direct injection. Alternatively, liquid fuel may be port injected. Fuel injector  66  is supplied operating current from driver  68  which responds to controller  12 . In addition, intake manifold  44  is shown communicating with optional electronic throttle  64 . In one example, a low pressure direct injection system may be used, where fuel pressure can be raised to approximately 20-30 bar. Alternatively, a high pressure, dual stage, fuel system may be used to generate higher fuel pressures. 
     Gaseous fuel may be injected to intake manifold  44  by way of fuel injector  89 . In another embodiment, gaseous fuel may be directly injected into cylinder  30 . One example of gaseous fuel is reformate. Gaseous fuel is supplied to fuel injector  89  from storage tank  93  by way of pump  96  and check valve  82 . Pump  96  pressurizes gaseous fuel supplied from fuel reformer  97  in storage tank  93 . Check valve  82  limits flow of gaseous fuel from storage tank  93  to fuel reformer  97  when the output of pump  96  is at a lower pressure than storage tank  93 . Fuel reformer  97  includes catalyst  72  and may further include optional electrical heater  98  for reforming liquid fuel (such as ethanol) supplied from fuel tank  91 . Fuel reformer  97  is shown coupled to the exhaust system downstream of catalyst  70  and exhaust manifold  48 . However, fuel reformer  97  may be coupled to exhaust manifold  48  and located upstream of catalyst  70 . For example, fuel reformer  97  may use a catalyst and exhaust heat to drive an endothermic dehydrogenation of alcohol supplied by fuel tank  91  to promote fuel reformation. 
     Distributorless ignition system  88  provides an ignition spark to combustion chamber  30  via spark plug  92  in response to controller  12 . Universal Exhaust Gas Oxygen (UEGO) sensor  126  is shown coupled to exhaust manifold  48  upstream of catalytic converter  70 . Alternatively, a two-state exhaust gas oxygen sensor may be substituted for UEGO sensor  126 . 
     Converter  70  can include multiple catalyst bricks, in one example. In another example, multiple emission control devices, each with multiple bricks, can be used. Converter  70  can be a three-way type catalyst in one example. Further, in the present example engine  10  includes an EGR conduit  80  to direct exhaust gases, upstream of converter  70  and/or downstream of converter  70  back to the intake manifold  44 . In further examples, EGR conduit  80  may not be coupled to intake  42  upstream of throttle  64 . Further, EGR conduit  80  includes an EGR valve  81  which meters flow through the EGR conduit, and may be a continuously variable valve or a two position on/off valve. 
     In a preferred aspect of the present description, engine position sensor  118  produces a predetermined number of equally spaced pulses every revolution of the crankshaft from which engine speed (RPM) can be determined. In one embodiment, the stop/start crank position sensor has both zero speed and bi-directional capability. In some applications a bi-directional Hall sensor may be used, in others the magnets may be mounted to the target. Magnets may be placed on the target and the “missing tooth gap” can potentially be eliminated if the sensor is capable of detecting a change in signal amplitude (e.g., use a stronger or weaker magnet to locate a specific position on the wheel). Further, using a bi-dir Hall sensor or equivalent, the engine position may be maintained through shut-down, but during re-start alternative strategy may be used to assure that the engine is rotating in a forward direction. 
     In some embodiments, the engine may be coupled to an electric motor/battery system in a hybrid vehicle. The hybrid vehicle may have a parallel configuration, series configuration, or variation or combinations thereof. 
     During operation, each cylinder within engine  10  typically undergoes a four stroke cycle: the cycle includes the intake stroke, compression stroke, expansion stroke, and exhaust stroke. During the intake stroke, generally, the exhaust valve  54  closes and intake valve  52  opens. Air is introduced into combustion chamber  30  via intake manifold  44 , and piston  36  moves to the bottom of the cylinder so as to increase the volume within combustion chamber  30 . The position at which piston  36  is near the bottom of the cylinder and at the end of its stroke (e.g. when combustion chamber  30  is at its largest volume) is typically referred to by those of skill in the art as bottom dead center (BDC). During the compression stroke, intake valve  52  and exhaust valve  54  are closed. Piston  36  moves toward the cylinder head so as to compress the air within combustion chamber  30 . The point at which piston  36  is at the end of its stroke and closest to the cylinder head (e.g. when combustion chamber  30  is at its smallest volume) is typically referred to by those of skill in the art as top dead center (TDC). In a process hereinafter referred to as injection, fuel is introduced into the combustion chamber. In a process hereinafter referred to as ignition, the injected fuel is ignited by known ignition means such as spark plug  92 , resulting in combustion. During the expansion stroke, the expanding gases push piston  36  back to BDC. Crankshaft  40  converts piston movement into a rotational torque of the rotary shaft. Finally, during the exhaust stroke, the exhaust valve  54  opens to release the combusted air-fuel mixture to exhaust manifold  48  and the piston returns to TDC. Note that the above is shown merely as an example, and that intake and exhaust valve opening and/or closing timings may vary, such as to provide positive or negative valve overlap, late intake valve closing, or various other examples. 
     Turning now to  FIG. 3 , processor  212 , stability control  204 , airbag system  208 , and longitudinal acceleration sensor  210  are shown in further detail. Example spark timing adjustments and charge reformate concentration adjustments may occur upon recognizing a steady state of the vehicle while the vehicle is in operation on a driving surface. The system  200  may include various sensors, in addition to the one or more longitudinal acceleration sensors  202 ,  206 ,  210  that may be configured to recognize the steady state. For example, a wheel position sensor  300  may be coupled to the processor  212 , and configured to sense a wheel position that is substantially unchanged for more that a predetermined amount of time; an accelerator position sensor  302  may be coupled to the processor  212 , and configured to sense an accelerator position being substantially unchanged for a predetermined length of time; a lateral acceleration sensor  304  may be coupled to the processor  212 , and configured to sense changes in lateral acceleration being below a predetermined threshold for more than a predetermined amount of time; and a vehicle yaw sensor  306  may be coupled to the processor  212 , and configured to sense changes in yaw of the vehicle being below a predetermined threshold for more than a predetermined amount of time. The accelerator position sensor  302  may be the same, or different than the pedal sensor  134  discussed above. 
     In the present example, an additional longitudinal acceleration sensor  206  is included as part of an airbag system  208  for the vehicle  150 . Another longitudinal acceleration sensor  210  may be added to the vehicle  150 . Each of the longitudinal sensors may be coupled with a processor  212 . The processor  212  may be configured to affect increasing or decreasing a percentage of reformate in a charge of air and fuel flowing to one or more example engine cylinders based on an output from one or more of the longitudinal acceleration sensors  202 ,  206 ,  210 . Further processor  212  may be configured to advance or retard spark timing of an internal combustion engine  10  configured to power the vehicle  150 . The processor  212  may further be configured to effect a spark timing adjustment of the engine toward a peak torque timing. 
     In the present example, the processor  212  includes a logic unit  214  configured to adjust charge reformate concentration, as discussed above. Further, logic unit  214  may be configured to output a spark timing control signal to the engine controller  12  to adjust the spark timing of the internal combustion engine  10  of the vehicle  150  in a first direction. The logic unit  214  may be further configured for further adjusting the spark timing in the first direction in the case of a positive acceleration or to adjust the spark timing in a second direction in the case of a negative acceleration. The processor  212  may also include an input/output module  216  configured to receive a signal from the longitudinal acceleration sensor and configured to pass the signal to the logic unit  214 . 
     Turning now to  FIG. 4 , a routine  400  is shown. Routine  400  may be a set of instructions on a read-only memory included in an example controller. Routine  400  may be carried out in an example vehicle (e.g.  150  described above) including an engine, a catalyst for reforming liquid fuel into reformate and one or more accelerometers. Further, routine  400  in may be included in a method, the method including adjusting a supply of reformate to a cylinder of the engine in response to an engine surge, the surge including an oscillation in torque produced by the engine, an accelerometer sensor indicating the oscillation in torque. 
     In the present example, it will be appreciated that measurements of engine conditions and parameters are assumed to take place when needed or be stored in a memory readily accessible to routine  400 . Engine conditions and parameters include valve timing, engine coolant temperature, acceleration along one or more axis, air to fuel ratio, percentage opening of an example EGR valve, etc. 
     In the present example, routine  400  begins at  410 ,  410  including determining if an engine is surging. In one example, an acceleration is detected by one or more longitudinal acceleration sensors. The example acceleration may be oscillating, as a result of oscillating torque brought on by surge. Further,  410  may include band pass filtering an acceleration signal to frequencies above and below a surge window of frequencies. In some examples, amplitudes, intensities, and/or a strength of frequencies in the surge window above a surge threshold determines that there is surge (e.g., the example engine is in a surge state). In the present example, if surge is not present, routine  400  ends. 
     When routine  400  ends the example engine may continue operating nominally. In this way, a method including routine  400  may include a first operating mode of supplying a first charge reformate concentration to a cylinder in the engine during a nominal engine combustion state. 
     If engine surge is present, routine  400  continues to optionally apply a wheel brake at  412  or to determine the feasibility of increasing a percentage of reformate entering one or more engine cylinders at  414 . In this way, routine  400  includes a second mode. The second mode may further include supplying a second charge reformate concentration to the cylinder in the engine, the second charge reformate concentration greater than the first charge reformate concentration (e.g., at  418 ). The second mode also includes increased engine surge as compared to the first mode, discussed above. The second mode may include an engine surge state monitored by an example accelerometer sensor coupled in an example vehicle body. 
     In further examples,  410  includes more generally, determining if combustion is stable. Such further examples may include one or more determinations based on charge motion, dilution, knock detection, a compressor speed (such as in a turbocharger or supercharger), MAP, MAF, etc. In such examples, if combustion is stable, routine  400  ends. 
     Continuing with routine  400 , as discussed above in some examples after  410  the routine includes at  412  applying a wheel brake to one or more wheels of the example vehicle. Further  412  may include increasing pressure to the example wheel brake, selectively and/or repeatedly. Further, applying the wheel brake may be done without a request from an operator to do so. Applying the wheel brake in response to engine surge or combustion instability is well known to those of skill in the art and is optional in routine  400 , hence the dashed line at  412 . After  412  and  410 , routine  400  may continue to  414 . 
     After  412  or in response to determining that engine surge is present at  410 , routine  400  continues to  414  which includes determining if increasing a percentage of reformate in one or more cylinders of the engine is feasible.  414  may include determining if there is enough reformate available. Routine  500 , discussed below with respect to  FIG. 5 , is one example of determining if increasing charge reformate concentration is feasible. 
     If at  414 , routine  400  determines that it is feasible to increase charge reformate concentration, then routine  400  continues to  418  to increase the percentage of reformate in a charge entering at least one example engine cylinder. Optionally, routine  400  may include reforming liquid fuel at the example catalyst into reformate at  416  before continuing to  418 .  416  is in dashed lines to indicate its optional nature. By reforming liquid fuel at the catalyst before increasing the percentage of reformate in charge entering an example engine cylinder at  418 , routine  400  may ensure that reformate in a storage tank stays above a reformate threshold, being a quantity of reformate desired for nominal operation of the engine. 
     Continuing with routine  400 , in the present example  418  includes increasing the supply of reformate to the cylinder of the engine in response to surge. In some examples, increasing the percentage of reformate includes incrementing an amount of reformate injected into an intake manifold and/or an example engine cylinder. In further examples,  418  includes maintaining a consistent air to fuel concentration and therefore an amount of non-reformate fuel injected into the intake manifold and/or the engine cylinder is decremented or decreased. After the percentage of reformate is increased, routine  400  ends. 
     If at  414 , routine  400  determines that it is not feasible to increase charge reformate concentration, and then routine  400  continues to  420 .  420  includes improving combustion. In the present example, improving combustion includes decreasing charge dilution, (e.g., via reducing EGR, advancing or retarding variable valve timing such as VCT, and reducing lean burn) and/or advancing or retarding spark timing. In additional examples,  420  includes further actions to increase charge combustibility. 
     Additionally at  420 , combustion may be improved via adjusting charge dilution or spark timing. Engine conditions may be adjusted enough at  420  to lead to probable stable combustion during the next ignition event in the example engine. However, in further examples charge dilution and/or spark timing may be adjusted, but intentionally not enough to lead to probable stable combustion during the next ignition event. In such example, routine  400  includes  428 , discussed below. 
     In the present example, improving combustion at  420  includes determining if an engine speed-load is above first s-l threshold and the engine speed-load is below second s-l threshold at  422 .  422  is one example of determining a condition of the example engine (e.g., as discussed in further detail below, with respect to  FIG. 6 ). In a first condition charge dilution may be adjusted (e.g., at  424 ). In a second condition, spark timing may be adjusted (e.g., at  426 ). In further examples, both spark timing and charge dilution may be adjusted. The condition of the engine, such as a high engine speed and high engine load, may lead to a preferred or effective set of actions for improving combustion. 
     In the present example, if the engine has a speed-load above a first s-l threshold and below second s-l threshold at  422 , routine  400  continues to  424  to reduce charge dilution. Reducing charge dilution includes adjusting VVT, an EGR valve, etc. In further examples of routine  400 , if the example engine has a speed-load above a first s-l threshold and below second s-l threshold, the engine may additionally or alternatively adjust spark timing at  424 . 
     If the engine has a speed-load below a first s-l threshold or above second s-l threshold at  422 , routine  400  continues to  426  to adjust spark timing. Adjusting spark timing at  426  includes adjusting toward a best torque or away from a best torque. Adjusting timing at  426  includes adjusting timing based on feedback from one or more example longitudinal acceleration sensors to minimize engine surge. 
     After either  426  or  424 , routine  400  may optionally continue to  428 . In further examples of routine  400 , the routine may end after either  426  or  424 . 
       428  includes determining the feasibility of increasing the percentage of reformate in the charge in one or more engine cylinders, as described above with respect to  414 . The  428  is shown in dashed lines to indicate its optional inclusion in routine  400 . If increasing a percent of reformate is feasible, routine  400  continues to  416  to reform liquid fuel into reformate (optionally) or routine  400  continues directly to  418  to increase reformate percentage in a charge, as described above. If not, routine  400  ends. 
     By inclusion of  428 , routine  400  includes adjusting at least one of charge dilution level and/or spark timing in combination with increasing the supply of reformate to the cylinder of the engine. In further examples, where  428  is not included, and routine  400  may repeatedly run to carry out continuous control of reformate concentration, charge dilution and spark timing. 
     Additional examples of routine  400  may include decreasing the supply of reformate to the cylinder in response to a dissipation of engine surge, where the dissipation of engine surge includes engine torque oscillations in a surge window below a surge threshold. 
     By inclusion of  422  and  414 , routine  400  may include determining a condition of an example engine to effect how to response to engine surge. In this way, routine  400  includes a method for the engine including during a first condition, adjusting charge dilution in response to engine surge, the first condition including a speed-load above a first s-l threshold and the speed-load also below a second s-l threshold, during a second condition, adjusting spark timing in response to engine surge and during a third condition, adjusting reformate delivered to the engine in response to engine surge, the third condition including at least one of a reformate amount above a reformate threshold and a rate of reformate production above a production threshold. Engine surge may include torque oscillations indicated by an example longitudinal acceleration sensor, as discussed above. 
     Further, reformate delivered to the engine may be adjusted before adjusting spark timing or charge dilution are adjusted (e.g., by repeated iterations of routine  400 ). Further still, charge dilution may be adjusted before spark timing (e.g., via repeated iterations of routine  400  and at  420 ) and additionally, reformate delivered to the engine in response to engine surge may be adjusted after adjusting at least one of charge dilution and spark timing (e.g., at  420  and then at  428  and then at  418 ). 
     One advantage of routine  400  is that the feedback controls described above allow a smaller and cheaper reformer, because reformate concentration is adjusted in response to surge. Further, surge is mitigated and engine efficiency may be improved due to aggressive use of lean burn, EGR and/or VCT. 
     Turning now to  FIG. 5 , a routine  500  is illustrated for determining if increasing charge reformate concentration is feasible. Routine  500  is one example of a subroutine included in routine  400  at  414  and optionally again at  428 . As discussed above, with respect to routine  400 , routine  500  may be a set of instructions on a read-only memory and be implemented on an example engine, including a reformate catalyst and an accelerometer. 
     Routine  500  starts at  510 , which includes determining if a reformate amount is above a reformate threshold. The reformate amount in the present example is a quantity of reformate in an example gaseous fuel storage tank. The reformate threshold may be a mass, pressure or a volume. Inclusion of  510  is one example of how a method may increase the supply of reformate to the cylinder of the engine in response to surge and a reformate amount in a reformate storage tank above a reformate threshold. If the reformate amount is above the reformate threshold, then routine  500  continues to  516 ; if not then routine  500  continues to  512 . 
     At  512 , method  500  includes determining if a rate of reformate production is above a production threshold. The rate and production threshold may be measured in mass per unit time, pressure per unit time or volume per unit time. Further, a rate of production may be inferred from a reformate catalyst temperature, a surface area of the catalyst and an amount of fuel in contact with the catalyst. Inclusion of  512  is one example of how a method may increase the supply of reformate to the cylinder of the engine in response to surge and a rate of reformate production at the catalyst above a production threshold. If reformate production rate is above a production threshold, then routine  500  continues to  516 ; if not, then routine  500  continues to  514 . 
     At  514 , the routine  500  includes flagging the increase in percentage of reformate entering one or more example engine cylinders as not feasible. In one example of routine  500 ,  514  includes setting a variable equal to false (e.g., incrs %=0). In the present example, after  514 , the routine ends, however, in additional examples routine  500  includes adjusting at least one of EGR, VCT, lean burn and spark retard (e.g., as at  420  described above with respect to  FIG. 4 ). 
     In the present example, if reformate amount is above the reformate threshold at  510  and the rate of reformate production is above the production threshold, then routine  500  may continue to  516 . At  516 , routine  500  includes determining if an air to fuel ratio is above an A/F threshold. Inclusion of  516  in routine  500  is one example of determining if the example engine is running in a lean burn mode. Further  516  may include determining if enriching an air and fuel mixture entering an engine cylinder will improve engine combustion and mitigate surge (e.g., if air to fuel is above the A/F threshold) or lead to further surge (e.g., if air to fuel is below the A/F threshold). If the air to fuel ratio is above the A/F threshold, then routine  500  continues to  518 ; if not, then routine  500  continues to  514 , discussed above. 
     At  518 , routine  500  includes flagging the increase in percentage of reformate entering one or more example engine cylinders as feasible. In one example of routine  500 ,  518  includes setting a variable equal to true (e.g., incrs %=1). In the present example, after  518 , the routine ends. In additional examples, routine  500  includes increasing the percentage of reformate entering the example cylinder(s) of the engine (e.g., as at  418  described above with respect to  FIG. 4 ). 
     As discussed above, routine  500  is one example of a subroutine for determining the feasibility of increasing reformate charge concentration. One advantage of routine  500  is that an amount of reformate may be maintained above the example reformate threshold, thereby ensuring an amount of reformate is available to be combusted in the engine later, for example in response to knock. Another advantage of routine  500  is that intake air is not enriched to saturation which may increase hydrocarbon emissions and lowering fuel economy. Further examples of routine  500  include additional processes and determinations and may be arranged differently, (for example, determining if reformate amount is above the reformate threshold (presently at  510 ) after determining if the rate of reformate production is above the production threshold (presently at  512 ). 
     Turning now to  FIG. 6 , a map  600  illustrating engine operating conditions with respect to engine speed-load is shown. In the present example, three engine conditions  610 ,  620 , and  630  are shown. The boundary of each condition (e.g., solid line boundary of second engine condition  620 ) includes all of the points on the boundary and within the boundary, including additional engine conditions (e.g., first engine condition  610 ). In additional examples, engine operating conditions may be exclusive to other operating conditions, and not contain the same engine speed-loads as the other operating conditions. 
     The boundary of first condition  610  is illustrated by a dashed line. First condition  610  includes intermediate engine loads, and low to intermediate engine speeds. First condition  610  may be above a first example s-l threshold and below a second s-l threshold, the second threshold having a greater speed and/or load than the first. Furthermore, in the present example first condition  610  includes engine speeds and loads during which charge dilution via EGR, VVT, boost, etc. may be used. Therefore, an engine operating in first condition  610  which experiences surge may decrease charge dilution and effectively increase charge combustion quality. 
     An engine operating in the first condition may prioritize engine surge mitigation by first increasing reformate, then decreasing charge dilution and finally adjusting spark timing. 
     The boundary of second condition  620  is illustrated by a solid line. In one example, second condition  620  includes an entirety of stable engine operating speeds and loads. Second condition  620  may be below the first example s-l threshold and above the second s-l threshold. Second condition  620  may be above a third example s-l threshold and below a fourth s-l threshold, the fourth threshold having a greater speed and/or load than the third. Furthermore, in the present example second condition  620  includes engine speeds and loads during which charge dilution may, or may not, be used. Further still, such speeds and loads may not facilitate the adjustment of charge dilution without decreasing combustion stability or drive feel. 
     An engine operating in the second condition may prioritize engine surge mitigation by first increasing reformate, then adjusting spark timing. 
     The boundary of third condition  630  is illustrated by a dash-dot line. In further examples, the boundary of third condition  630  may depart radically from the present example. Third condition  630  may be below the first example s-l threshold and above the second s-l threshold. Furthermore, third condition  630  may be below the third example s-l threshold and above the fourth s-l threshold. In the present example at least one of a reformate amount is above an example reformate threshold and a rate of reformate production is above an example production threshold. Adjusting reformate amount provided to an example engine in the third condition allows for mitigation, prevention, or limiting of engine surge across a wide range of engine loads and speeds. Further, adjusting reformate amount may enable a continued use of aggressive charge dilution and spark timing, thereby increasing engine performance and efficiency. 
     Note that the example control and estimation routines included herein can be used with various engine and/or vehicle system configurations. The specific routines described herein may represent one or more of any number of processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various acts, operations, or functions illustrated may be performed in the sequence illustrated, in parallel, or in some cases omitted. Likewise, the order of processing is not necessarily required to achieve the features and advantages of the example embodiments described herein, but is provided for ease of illustration and description. One or more of the illustrated acts or functions may be repeatedly performed depending on the particular strategy being used. Further, the described acts may graphically represent code to be programmed into the computer readable storage medium in the engine control system. 
     It will be appreciated that the configurations and routines disclosed herein are exemplary in nature, and that these specific embodiments are not to be considered in a limiting sense, because numerous variations are possible. For example, the above technology can be applied to V-6, I-4, I-6, V-12, opposed 4, and other engine types. The subject matter of the present disclosure includes all novel and nonobvious combinations and subcombinations of the various systems and configurations, and other features, functions, and/or properties disclosed herein. 
     The following claims particularly point out certain combinations and subcombinations regarded as novel and nonobvious. These claims may refer to “an” element or “a first” element or the equivalent thereof. Such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and subcombinations of the disclosed features, functions, elements, and/or properties may be claimed through amendment of the present claims or through presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure.