GB2498222A - Reducing fuel consumption of a coasting vehicle - Google Patents

Abstract

A vehicle energy saving strategy is implemented upon detection of coasting of a vehicle. A slip device (for example a clutch) between motor and transmission allows motor speed to fall to e.g. idle speed but without fuelling. This arrangement reduces frictional and pumping losses in the engine, yet avoids stalling. The method comprises the steps of (i) detecting coasting, (ii) shutting-off fuel to the engine and (iii) controlling the slip device to maintain engine speed 21 at a control speed at or above the fuel cut-in speed 24. Preferably the method includes the steps of detecting cessation of coasting; re-fuelling the vehicle engine; controlling the slip device to match the engine speed and the transmission input speed and controlling the slip device to prevent engine speed exceeding the transmission input shaft speed.

Description

Reducing Energy Consumption of a Vehicle This invention relates to a method and system of reducing energy consumption of a vehicle, in particular, but not exclusively, by disengaging a rotating mass, such as the vehicle motor, whilst the vehicle is coasting. Aspects of the invention relate to a method, to a system and to a vehicle.

Internal combustion engines of vehicles rely upon fuel injection for effective management of fuelling. Fuel must be generally admitted to the engine at all times to avoid stalling thereof.

In one strategy for minimizing fuel consumption, the engine may be deliberately stopped when the vehicle is temporarily halted, and re-started when the accelerator pedal is advanced.

During coasting, or over-run, the vehicle is allowed to roll, generally without application of the accelerator pedal. In this condition fuel may be cut-off (so called deceleration fuel shut-off) and the vehicle slowly decelerates due to motor braking and other external factors.

Coasting' includes the condition in which the vehicle can maintain speed without assistance from the motor, as well as the zero throttle condition in which speed is deliberately permitted to fall without braking of the vehicle wheels.

During coasting motor speed is a function of the speed of the vehicle and the gear ratio of the transmission, so is typically much higher than idle speed. In an i.c. engine relatively high friction and pumping losses cannot be avoided, notwithstanding that fuel is cut-off.

One known method of reducing fuel consumption is to shift the vehicle transmission into neutral, so that motor speed can drop to idle. This technique may result in the driver having less control of the vehicle, and is generally not recommended by vehicle manufacturers.

Although possible with a manual transmission, the technique may not be possible, or not appropriate for an automatic transmission. Most importantly however, in an i.c. engine, fuel must be admitted to the engine to keep it running at idle speed, so that from a fuel economy viewpoint, the advantage of deceleration fuel shut-oft is lost, but lower friction and pumping losses may apply.

According to one aspect of the invention there is provided an energy saving strategy for a vehicle having a motor, a transmission, and a slip device for selectively coupling the motor and transmission, the strategy comprising detecting coasting of the vehicle and controlling the slip device to allow motor speed to fall from a full coupled condition to a control speed.

The motor is not permitted to stop rotation, as would be the case if the slip device were completely de-coupled. The control speed is maintained so that when coasting is ceased, the motor can respond quickly to driver demand.

In the invention, during coasting, the stored momentum of the vehicle is more efficiently used since less momentum is wasted in rotating the motor at higher than idle speed, with the resulting losses due to for example friction, pumping and windage. As a result the vehicle will coast significantly further than would otherwise be the case. Put another way, the vehicle will loose less speed over a given distance when employing the invention, and furthermore will require less energy to reach a higher speed when coasting is terminated.

The motor may be of any suitable kind, and in one embodiment is an internal combustion engine. In this embodiment, fuel is cut-off completely during coasting, and engine speed is maintained approximately at idle speed, but without fuelling. Upon termination of coasting (and resumption of fuelling) the vehicle engine is immediately able to produce power, and the slip device is fully re-engaged so that engine speed is increased to match transmission speed.

In one embodiment the invention is activated in response to detection of an unadvanced accelerator pedal, and de-activated if the accelerator pedal is advanced.

The fuel cut-in speed may be broadly equivalent to normal idle speed, and may be the minimum engine speed at which fuelling can be smoothly re-commenced without being noticed by the vehicle driver. The fuel cut-in speed can be determined empirically by reference to any particular engine.

The slip device is typically a friction clutch (wet or dry), but may be a torque converter, a fluid flywheel, a vee-belt or any other device suitable for progressively connecting a motor to a transmission. Typically the slip device is the main drive connection between motor and transmission.

The method of the invention may be selectable by a vehicle driver so as to maximize fuel economy upon coasting, or may be automatically enabled with other systems upon selection by the diiver of a vehicle economy mode. Typically, the driver may be insensible to operation of the energy saving strategy of the invention once enabled. In contrast the strategy may be inhibited or disabled in certain circumstances, such as where engine braking is required, for example upon selection of a sport' mode.

In embodiments of this invention the motor continues to idle, and accordingly engine driven ancillaries continue to operate. A degree of motor braking is provided owing to the partial coupling to the transmission.

The cut-in speed of the motor may vary according to conditions of use or ambient conditions, and appropriate values retained in a look-up table or the like.

In a refinement the strategy may further include the steps of detecting operation of ancillary motor driven devices, such as power assisted steering, air conditioning, generator and the like, and maintaining motor speed at an appropriate level for effective working thereof. A control input from an ancillary device may provide for an increase in motor speed whilst the strategy of the invention is enabled; thus a steering angle sensor may trigger an increase of motor speed in order to provide power steering assistance from a motor driven pump. This motor speed which may be higher than idle, is maintained by controlling the slip device at a level above idle (or fuel cut-in speed of an i.c. engine) but below the normal coupled speed of the transmission. Energy saving at a lesser rate may be obtained by this strategy, whilst also allowing for a variable driving torque demand of one or more motor driven ancillary devices.

The method of the invention may include the further steps of determining one or more operating conditions of the vehicle, and inhibiting the energy saving strategy. Thus the strategy may be inhibited during engine or transmission warm-up (as defined by minimum temperatures of engine or transmission fluids) or when ambient temperature is below a pre-determined minimum, or when a dynamic stability control is active.

In a further refinement, the method may include the step of determining when coasting of the vehicle is possible, for example when being driven downhill at a constant speed, and enabling the energy saving strategy of the invention if the gradient is sufficient to maintain vehicle speed without the assistance of the engine.

In such a circumstance, in a vehicle with i.c. engine, the vehicle driver may normally maintain an advanced accelerator position, and thus be fuelling the engine, unnecessarily.

This feature of the invention allows a further energy (fuel) saving during, for example, downhill sections of highways.

The strategy of the invention may be automatically disabled upon actuation of the vehicle wheel brakes, so as to provide increased engine braking and thereby reduce overall braking distance. Wheel brake application may for example be detected by a signal of brake pedal movement, or by a rise in hydraulic pressure in the service brake line. The strategy may also be disabled in the event of sensing of a steering input exceeding a pre-determined value, or a predetermined vehicle inclination or certain cruise control conditions, such as autonomous cruise control. Signals indicative of these and other conditions are typically available from the vehicle CAN-BUS, or equivalent.

Aspects of the invention may also be implemented within an adaptive cruise control, whereby a vehicle is allowed to exceed a pre-set cruise control speed when travelling downhill. The cruise control speed may be allowed to fall below the pre-set speed when travelling uphill. This aspect may provide that a vehicle journey is completed at an approximately average speed equivalent to the pre-set cruise control speed, but vehicle momentum is maximized on downhill travel. Observance of highway speed limits is required, which may be pre-determined in automatic application of the strategy. Thus in a simple example a cruise control speed of 90 kph may in this aspect allow downhill speed to rise to 100 kph. Momentum is increased at no energy cost during downhill travel because motor speed is reduced. Uphill speed may be limited to say 80 kph so that less energy is used on the upgrade, whilst ensuring that overall journey time is substantially unaltered.

Such adaptive cruise control may be implemented in conjunction with terrain information giving the likely gradients to be encountered. Such information may be provided in any suitable manner, for example by reference to OPS and topographical data.

Embodiments of the invention may be implemented in any suitable manner, and may for example be incorporated in an electronic control unit of the motor or transmission, and operable in response to input signals of e.g. road speed, accelerator pedal position, motor speed, transmission speed and clutch pressure. The control unit may include a memory having thresholds and stored characteristics or algorithms, to ensure that motor speed is maintained at the lowest possible value during implementation of the strategy. Such characteristics can be determined empirically by suitably qualified staff to the intent that the strategy can be automatically enabled and disabled without affecting driveability of the vehicle or being otherwise noticeable to the vehicle driver.

Within the scope of this application it is envisaged that the various aspects, embodiments, examples, features and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings may be taken independently or in any combination thereof. For example features disclosed in connection with one embodiment are applicable to all embodiments unless there is incompatibility of features.

The present invention will now be described, by way of example only, with reference to the accompanying drawings in which:-Fig. 1 illustrates a vehicle having a conventional engine, transmission and selectively engageable coupling therebetween; Fig. 2 illustrates graphically the effect of embodiments of the invention; Fig. 3 illustrates graphically an enablement of a strategy embodying the invention; and Fig. 4 illustrates graphically a disablement of a strategy embodying the invention.

Fig. 1 illustrates a vehicle (10) having a typical i.c. engine/transmission combination (11, 12) in which a shp device (13) allows engine speed to be progressively matched to transmission speed. The slip device (13), which is typically a clutch or torque converter, is principally provided for moving the vehicle from the stationary condition, but also allows the engine to smoothly change speed as different transmission ratios are engaged on the move.

In the case of a torque converter, a lock-up clutch is also typically provided to minimize pumping losses in the circulating fluid thereof once the input and output members are substantially coupled. Such a lock-up clutch positively engages the input and output members and is typically a hydraulically de-activated, spring engaged, friction clutch.

The following description refers to a torque converter with lock-up clutch.

Upon detection of coasting, typically by detecting zero accelerator pedal advance and a minimum road speed, sufficient hydraulic pressure is supplied to the lock-up clutch to cause a partial release thereof whereby the clutch enters a regulated slip control condition to permit engine speed to fall below turbine speed of the torque converter. A suitable control system is implemented in a controller of the lock-up clutch, and may take into account factors such as oil viscosity, oil temperature and maximum allowable speed differential between the engine and the torque converter turbine. Engine speed and turbine speed are sensed to permit feedback control of slip to the intent that engine speed is typically maintained at or close to idle speed, but without fuelling.

In more detail, the method of the invention may be implemented as the vehicle driver backs off the accelerator pedal. As the vehicle begins coasting, the torque converter is commanded to go from the normal closed (or locked) condition, towards an open state. At a calibrated point, typically individually determined for each transmission ratio, lock-up clutch pressure is regulated with the aim of allowing engine speed to reduce as far as possible whilst remaining above a fuel cut-in value.

As vehicle speed falls, turbine speed inevitably decays towards engine speed, and the pressure applying to the lock-up clutch changes to reduce the relative slip thereof, thus allowing engine speed to be maintained at or above the fuel cut-in value.

Falling vehicle speed may induce a change of gear ratio in the transmission, in which case turbine speed will increase. The system of regulation acts accordingly on the lock-up clutch to again increase slip, so that engine speed continues to be maintained at or close to idle speed, but without fuelling.

In this embodiment, control of clutch pressure is the primary control function, and is by reference to a controller memory containing a look-up table of one or more target characteristics, or by reference to an appropriate algorithm.

The effect of the invention is illustrated graphically in Fig. 2.

In this example engine speed (21) falls as the accelerator pedal is progressively released, and the system detects coasting at point (22). In the absence of the present invention, engine speed would I all at a low rate, as indicated by the solid line portion (23).

According to the invention engine speed falls to idle speed, but without fuelling at point (22), and accordingly follows the dotted characteristic line (24). The lower the speed of the unfuelled engine, the greater the conservation of momentum, and the longer the coasting distance.

Road speed is indicated by solid line (25), and falls over time during coasting. The chain-dot line (26) shows that according to the invention, decay of speed is at a slower rate, as a result of the reduction in frictional and pumping losses due to the lower engine speed.

Certain refinements to the sequence of clutch regulation are possible. Thus as soon as coasting is detected (lift-off of vehicle accelerator pedal), the clutch pressure may be immediately reduced at the fastest possible rate in order to permit the quickest possible decay of engine speed. Clutch pressure is then increased to hold engine speed at the desired minimum. Fig. 3 illustrates this arrangement and shows rapid decrease of the clutch pressure characteristic (solid line) from a fully engaged condition, followed by an increase to hold engine speed at about the fuel cut-in value. Engine speed is indicated by the dotted line.

In the event that a driver advances the accelerator pedal during the coasting regime of the invention, there is a momentary risk of high engine speed whilst proper regulation of the clutch is effected (typically full engagement thereof). Thus risk can be reduced by a rapid increase of clutch pressure, so as to prevent engine speed momentarily exceeding turbine speed, and thereby increasing fuel consumption unnecessarily. Fig. 4 illustrates this arrangement and shows a momentary over pressure of the clutch (solid line) to ensure rapid biting thereof to prevent too rapid a rise of engine speed (dotted line) in response to an instant demand for acceleration.

The control system of the invention is implemented in an electronic control module, typically the transmission control module. Input signals of road speed, accelerator pedal position, engine speed, transmission input shaft speed and clutch pressure are provided whereby coasting is recognized, and clutch pressure controlled to maintain engine speed at the pre-determined minimum whilst the fuel saving strategy is implemented.

Generally the input signals are provided from a CAN-BUS or the like, and are reviewed at a refresh rate exceeding 10 Hz Although described with reference to an internal combustion engine with a torque converter and lock-up clutch, it will be appreciated that the invention may be applied to any vehicle in which the coupling between motor and transmission can be progressively engaged and disengaged. Dual clutch transmissions, automated manual transmissions and the like are generally provided with such a coupling.

Claims (1)

1. <claim-text>Claims 1. A method of reducing fuel consumption of a coasting vehicle having an internal combustion engine, a transmission and a slip device for selectively engaging the engine and transmission, the method comprising: detecting coasting; shutting-off fuel to the vehicle engine; and controlling the slip device to maintain engine speed at a control speed at or above the fuel cut-in speed thereof.</claim-text> <claim-text>2. A method according to claim 1, wherein the slip device maintains engine speed above the idle speed thereof.</claim-text> <claim-text>3. A method according to claim 1 or claim 2, and including the steps of: detecting cessation of coasting; re-fuelling the vehicle engine; and controlling the slip device to match engine speed and transmission input shaft speed.</claim-text> <claim-text>4. A method according to claim 3, and including the step of controlling the slip device to prevent engine speed exceeding transmission input shaft speed.</claim-text> <claim-text>5. A method according to any preceding claim, wherein the slip device is controlled by varying a fluid control pressure thereof.</claim-text> <claim-text>6. A method according to any preceding claim, wherein upon detecting coasting, the slip device is momentarily substantially released and then re-engaged to maintain engine speed at said control speed.</claim-text> <claim-text>7. A method according to any preceding claim, and further including the steps of: detecting operation of one or more engine driven ancillary devices, and controlling the slip device to maintain engine speed above said control speed to ensure effective working of said one or more ancillary devices.</claim-text> <claim-text>8. A method according to any preceding claim, and further including the step of detecting an operating condition of the vehicle, and inhibiting or enabling said strategy upon said condition meeting a pre-determined threshold. l0</claim-text> <claim-text>9. A method according to any preceding claim, wherein said strategy is immediately disabled upon application of vehicle wheel brakes.</claim-text> <claim-text>10. A method according to any preceding claim, and further including the steps of determining that coasting of the vehicle is possible, and in consequence automatically enabling said strategy.</claim-text> <claim-text>11. A method according to claim 10, and including the steps of: determining that the vehicle is travelling downhill at a gradient sufficient to maintain speed thereof without assistance of the engine, and enabling said strategy.</claim-text> <claim-text>12. A method according to claim 11, wherein said strategy is automatically implemented during cruise control, and includes the step of permitting vehicle speed to exceed a pre-set cruise control speed when coasting downhill.</claim-text> <claim-text>13. A method according to claim 12, and including the step of permitting vehicle speed to reduce below a pre-set cruise control speed when travelling uphill so as to substantially maintain said pre-set cruise control speed over the period of a journey.</claim-text> <claim-text>14. A control system for a vehicle adapted or configured to implement the method of any of claims 1-13.</claim-text> <claim-text>15. A vehicle incorporating the control system of claim 14.</claim-text> <claim-text>16. A method, a control system or a vehicle constructed and/or arranged substantially as described herein with reference to any of Figures 2-4 of the accompanying drawings.</claim-text>