Abstract:
The invention relates to a method for heating a catalyst. When the engine is started, engine speed is maintain at high rpm. This high rpm is maintained even if the driver shifts the transmission out of neutral, at time (A). Engine speed is lowered if an indication that the driver wishes to drive off is detected, at time (C), e.g. release of the brake pedal. The shaded area shows the additional contribution to catalyst warm up. The high rpm may be disabled if catalyst temperature is high enough or if the vehicle is parked on a slope. The invention also relates to a vehicle carrying out the method.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to improvements in the control of emissions of internal combustion engines, in particular to achieve more rapid initiation of treatment of exhaust gases upon cold start of an engine. Aspects of the invention relate to a method and to a vehicle. 
       BACKGROUND 
       [0002]    Vehicle emissions legislation prescribes certain limits for the emission of noxious gases in the exhaust stream of an internal combustion engine. Typically a catalytic converter is provided in the vehicle exhaust system, and operates to chemically convert noxious gases to relatively benign alternatives. 
         [0003]    A catalytic converter requires a minimum temperature for effective operation, and this is achieved in a short period by heat transfer from the hot exhaust stream. When a vehicle starts in relatively cold conditions, the effective operation of the catalytic converter may be delayed. It would be desirable to reduce this delay so as to minimize emissions of untreated exhaust gas upon cold start. 
       SUMMARY 
       [0004]    Aspects of the invention provide a method and a vehicle as claimed in the appended claims. 
         [0005]    According to an illustrative embodiment there is provided a method of initiating treatment of the exhaust stream of an internal combustion engine of a vehicle having electronically sensed wheel brakes and an electronically sensed drive line, the method comprising the steps of:
       starting the vehicle engine;   running the vehicle engine at a first idle speed;   detecting movement of a drive line selector from a neutral to a drive condition while vehicle wheel brakes are activated;   delaying enablement of said drive line until at least one indication that a driver desires that the vehicle starts moving is detected, so as to prolong duration of said first idle speed; and   reducing engine speed from said first idle speed to a second, lower idle speed for drive line enablement subsequent to the delaying.       
 
         [0011]    The method of the illustrative embodiment thus postpones enablement of the drive line until release of wheel brakes is detected. This is in contrast to conventional drive line enablement in which the drive line is enabled in response to movement of a drive line selector. The illustrative embodiment is intended for application to any vehicle drive line having non-manual enablement, and thus excludes a conventional manually selected and engaged transmission with a manual clutch. In such vehicles movement of the drive line selector and/or enablement of the drive line is generally inhibited unless the foot brake is applied. Furthermore starting of the vehicle engine may be inhibited unless the drive line selector is in neutral or park positions. Typically a drive line of a vehicle that includes idle speed control according to an embodiment of the invention will comprise automatically controlled enablement via an actuator, for example a hydraulic automatic transmission or an automated manual transmission. 
         [0012]    In this specification ‘drive line’ means torque transmitting components linking the vehicle engine to the driven wheels of the vehicle. 
         [0013]    Immediate enablement of the drive line upon selector movement may necessarily impose a drag on the vehicle engine which may in turn result in the engine speed dropping from high idle to normal idle. The reduction in engine speed may however be electronically commanded upon detection of selection of a drive condition, so as to ensure smooth drive line engagement. Normal idle speed is generally required in order to give acceptable launch characteristics to the vehicle, and to give a smooth, high quality experience to the vehicle occupants. 
         [0014]    In some example embodiments, the postponement of enablement of the drive line permits a high idle speed to be maintained for a longer period, thus ensuring a greater quantity of hot exhaust gas streams through the catalytic converter before idle speed is reduced to normal, and with the consequence that the catalyst temperature is raised more quickly to the operating point. 
         [0015]    The illustrative embodiments thus permit more rapid light-off of the catalyst in a manner which is not noticeable to the vehicle driver. It will be understood that enablement of the drive line can be very rapid in present kinds of electronically controlled vehicle transmission. 
         [0016]    Typically the drive line will be enabled by engagement of a clutch to permit torque to be transmitted from the vehicle engine to the vehicle wheels. The clutch may be wet or dry, and form part of a multi-speed transmission commanded by a suitable electronic control unit (ECU). The enablement sequence of the drive line is selected to ensure smooth take-off can be achieved. In some circumstances for example a multi-speed transmission may temporarily engage a sequence of clutches before the launch clutch, so that the launch clutch can itself engage smoothly. This sequence should not be noticeable to the vehicle driver, and timing thereof should be accommodated by the control system so that idle speed is reduced at the appropriate time. 
         [0017]    Drive line enablement may comprise two-stages, whereby for example an actuator is filled with working fluid in a first stage. In a second stage pressure in the filled actuator is increased to effect movement thereof. Illustrative embodiments accommodate such a first stage whilst high idle speed is maintained, since it is the second stage which enables torque transmission. This arrangement may reduce the time for drive line enablement in certain circumstances. 
         [0018]    The vehicle wheel brakes typically comprise an electronically controlled ABS (anti-lock braking system) arrangement adapted to sense brake pressure, and to detect or predict wheel movement. 
         [0019]    As will be well understood, most vehicle drivers do not drive-away immediately upon cold start of a vehicle engine, or upon selection of a drive condition (typically a low forward or a reverse speed ratio). Accordingly the benefit of the disclosed embodiments can be realised for most circumstances of use. In the event that immediate and rapid drive-away is required, the ECU will permit rapid drive line enablement; the period of high idle will be reduced—however in those circumstances the engine is generally producing a greater quantity of hot exhaust gas, which will itself reduce the delay before effective operation of the catalytic converter. 
         [0020]    In a modification of the method of the illustrative embodiment, a temperature of the exhaust catalyst may be monitored, and the method may be disabled in the event that correct operation of the catalyst is determined. Thus the method can be confined to cold and cool starts only, and have the shortest appropriate duration of high idle. 
         [0021]    In some embodiments, the method may take account of vehicle attitude. It will be appreciated that a vehicle may be parked on a slope, and in such circumstances may require a higher brake pressure to prevent movement than if on substantially level ground. The brake pressure to hold the vehicle against movement will increase as the slope increases. 
         [0022]    In some embodiments the method may further comprise the steps of:
       detecting vehicle attitude;   determining a corresponding minimum brake force to hold the vehicle against movement;   detecting reducing brake force as the wheel brakes are released; and   implementing a strategy to hold the vehicle wheels against movement at said minimum brake force pending drive line enablement.       
 
         [0027]    These further steps prevent vehicle movement in the event that, due to gradient, there is a short period during which movement of the vehicle may be possible before the drive line is enabled. Whilst said movement may be very slight, it is preferable that it is eliminated. 
         [0028]    The strategy to hold the vehicle wheels against movement may comprise for example implementing operation of a handbrake function, implementing a hill hold function of the wheel brakes, engaging an output component of a multi-speed transmission to ground (for example a transmission casing), locking a multi-speed transmission by simultaneous engagement of two speed ratios, or commanding drive line enablement; other solutions are possible, but in each case should be susceptible of electronic control under command of the relevant ECU. 
         [0029]    In an embodiment, drive line enablement is blended with movement of the vehicle wheels (i.e. release of wheel brakes or disablement of the strategy to hold the vehicle wheels against movement) so that drive-off is smooth, and without perceptible roll-back. 
         [0030]    Progressive take-up drive corresponding to automatic release of a conventional electronic handbrake or hill hold function provides one optional arrangement. 
         [0031]    The method may include the step of detecting reducing brake force as the wheel brakes are released, and commanding drive line enablement at a predetermined brake force. The predetermined brake force may be selected by reference to vehicle attitude. 
         [0032]    In this method, the drive line is enabled so that engine torque is transmitted to the vehicle wheels in order to counter any tendency for uncontrolled vehicle movement. The predetermined brake force may for example be found in a look-up table by reference to vehicle attitude, or calculated according to an appropriate algorithm. 
         [0033]    The vehicle drive line may include a transmission susceptible of electronic control. Many kinds of transmissions are suitable, including conventional multi-speed hydraulic transmissions, and dual clutch transmissions. 
         [0034]    Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. For example, features described in connection with one embodiment are applicable to all embodiments unless such features are incompatible. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0035]    Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
           [0036]      FIG. 1  is a graphical representation of engine speed during a conventional cold start sequence; 
           [0037]      FIG. 2  corresponds to  FIG. 1  and shows a cold start sequence according to an embodiment of the invention; 
           [0038]      FIG. 3  illustrates a cold start sequence according to another embodiment of the invention; 
           [0039]      FIG. 4  is a flow chart of a typical engine start sequence in accordance with embodiments of the invention, and 
           [0040]      FIG. 5  illustrates graphically a cold start sequence according to embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0041]    With reference to  FIG. 1 , a conventional cold engine start for an electronically controlled internal combustion engine typically has initial engine speed of 2000 rpm, which drops to a fast idle of 1350 rpm almost immediately. Fast idle is maintained until about 15 seconds from start, in order for example for combustion to stabilize. Idle speed then reduces at A to around 850 rpm as the drive line is selected and engaged, which is a suitable speed for the driver to move-off smoothly and progressively. Vehicle launch, that is to say drive-away, is at B, typically after a further period of about 5 seconds. 
         [0042]    In any automatic or automated drive line, a controller will select an appropriate launch ratio in response to a driver command. Typically the driver will select a low ratio or DRIVE, from PARK or NEUTRAL. Commonly, starting of the vehicle engine is inhibited unless PARK is selected, and enabling of the drive line is inhibited unless the foot brake is applied. 
         [0043]    Thus a typical cold start sequence comprises engine start with transmission in PARK, fast idle of the engine, foot brake on, selection of low speed ratio, and drive-away as the foot brake is released. The sequence may differ slightly according to the particular kind of transmission, which may for example be a conventional hydraulic automatic gearbox, an automated manual transmission or a twin clutch transmission. 
         [0044]    The drive line controller is typically a transmission ECU in command of actuators within the transmission. These actuators can be electric, hydraulic or pneumatic, and effect engagement of one or more speed ratios and/or one or more torque transmitting friction elements. 
         [0045]    In order to ensure smooth engagement of drive to the vehicle wheels, fast idle may be ceased upon selection of a drive condition. Such ceasing may be as a result of transmission drag, for example of a torque converter, or upon command of the ECU as selection of the drive condition is detected. 
         [0046]    It will be appreciated that in this example it is assumed that the driver starts the engine, and that there is a brief delay before the drive line is enabled for drive-off—this delay may for example allow a seat belt to be engaged or for the driver to check an instrument display. 
         [0047]    It will also be understood that engine idle speeds and times will vary according to engine type and specifications, and according to ambient conditions, and that the figures quoted above are merely illustrative. 
         [0048]    The effect of the invention is illustrated in  FIG. 2 . Drive is selected at point A, as in the prior art, but the drive line is not engaged. Accordingly there is no drag upon the engine and a high idle speed can be maintained until point C at which releasing of wheel brakes is sensed. Idle speed then reduces to point B as the drive line is engaged, ready for drive-away. This engagement is represented in  FIG. 2  by trace D which illustrates a gradual rise in clutch pressure associated with drive engagement, and over the period during which idle speed is reducing from 1350 to 850 rpm. Trace D represents any suitable arrangement which progressively moves from a disengaged to an engaged condition. 
         [0049]    The additional high speed idle contribution is represented by the shaded area of  FIG. 2 . 
         [0050]    In a further enhancement idle speed may be maintained at a higher level (e.g. 1750 rpm), as illustrated in  FIG. 3 , since the drive line is not engaged at the point of selection of drive. In consequence the engine need not be at a speed suitable for drive line selection (1350 rpm according to  FIGS. 1 and 2 ) until releasing of wheel brakes is sensed. This embodiment blends the eventual reduction in idle speed with drive line engagement (in the manner of trace D of  FIG. 2 ), so that for example such engagement commences as engine speed reduces from higher idle at 1750 rpm across the 1350 rpm threshold. The additional higher idle speed contribution is represented in the shaded area of  FIG. 3 . 
         [0051]    In the event that the vehicle driver commands instant drive-away, the controller will typically terminate fast idle early so as to ensure rapid smooth enablement of the drive line. In such conditions the engine will quickly be commanded to generate significant power, in which case the exhaust gas will rapidly rise in temperature so as to achieve rapid light-off of the catalyst. 
         [0052]    Fast (high) idle may also be terminated after a predetermined time period in the event that the driver does not command enablement of the drive line, or in response to light-off of the catalyst being detected or predicted. An exhaust gas temperature sensor at the inlet of the catalyst device may permit light-off to be assumed at a predetermined temperature threshold. 
         [0053]      FIG. 4  illustrates a typical engine cold start with neutral maintained (CSN) to permit fast idle—i.e. drive line selected but not enabled—in accordance with the invention. An electronically controlled hydraulic multi-speed transmission is assumed. 
         [0054]    At step  11  the validation routine of the invention is implemented and the vehicle ignition switched on (step  12 ). Other systems may be interrogated to ensure that the vehicle is ready to move (step  13 ), and confirmation is obtained that the engine is running (step  14 ). 
         [0055]    CSN is assumed non-active (step  15 ) and the catalyst temperature is determined (step  16 ); if less than the threshold the routine continues, if not CSN is disabled (step  31 ). Step  16  prevents activation of CSN if the catalyst is at or above operating temperature. CSN is disabled until the next engine start event (step  32 ). 
         [0056]    At step  17  a diagnostic check ensures relevant communications are working, for example control inputs from the braking system and transmission selector. A fault disables CSN. 
         [0057]    At step  18  the temperature of automatic transmission fluid is checked. CSN may for example be inhibited (step  33 ) if ATF temperature is below a predetermined minimum of less than zero, say −6° C. 
         [0058]    At step  19  a check is made that the vehicle is stationary, for example by signal from a motion sensor; if no CSN is inhibited. 
         [0059]    At step  20  a check is made of vehicle attitude, for example to determine a minimum brake pressure which will hold the vehicle stationary. Above a threshold, CSN is inhibited. CSN may also be inhibited if the vehicle attitude exceeds a predetermined value, i.e. the vehicle is strongly inclined, say above 15°, or above 20 ° fore and aft. 
         [0060]    At step  21  a check is made that brake pressure applied by the driver or a hill hold function is above a minimum to hold the vehicle; if no CSN is inhibited. 
         [0061]    At step  22  a check is made for selection of a drive condition; if no CSN is inhibited. 
         [0062]    At step  23  accelerator position is checked to determine if vehicle launch (drive-off) is imminent; if no CSN is inhibited. Launch may be indicated by movement of the accelerator from the rest condition. or beyond a pre-determined threshold. 
         [0063]    At step  24  a hill hold feature of the anti-lock braking system is implemented to prevent roll back during CSN; if no CSN is inhibited. 
         [0064]    At step  25  CSN is implemented, having passed validation, to permit fast idle to be maintained according to the invention. 
         [0065]    The skilled man will of course select the necessary check functions in the routine of  FIG. 4 , and the steps quoted are merely examples appropriate to one selected embodiment. 
         [0066]    In  FIG. 5 , a typical CSN routine is illustrated by way of example. At cold engine start, engine revolutions (N) are maintained at high idle whilst brake pressure (B) is applied, via driver brake application or hill hold functionality. 
         [0067]    At time t 1  brake pressure begins to fall, and vehicle movement is accordingly predicted. At time t 2  engine speed begins to drop from fast to normal idle to permit smooth engagement of a pre-selected transmission ratio. At t 3  brake pressure reaches zero, but between t 2  and t 3  a transmission clutch is engaged to enable the drive line, and prevent vehicle roll-back. 
         [0068]    At t 4  the accelerator (A) is depressed, and engine speed rises to permit drive-away. 
         [0069]    Selection of times t 1 -t 4  will be determined according to acceptable vehicle launch factors, the torque characterization of the transmission clutch, and other parameters relating to the vehicle brakes and accelerator—to the intent that prolongation of raised engine idle speed is substantially imperceptible to the vehicle driver. 
         [0070]    It will also be understood that the engine speeds corresponding to normal, high and higher idle will be selected according to the technical specification of the engine and the required duty, and furthermore that the reduction of idle speed will blend with drive line engagement at a rate which gives acceptable take-up of drive with appropriate smoothness and quality. Selection of these factors is within the skill of a suitably qualified technician.