METHOD AND SYSTEM FOR TRANSMISSION CONTROL AT LOW POWER

REQUIREMENT

TECHNICAL FIELD The present invention relates to a method and a system for control of a gearbox connected to a combustion engine. The invention relates in particular to a method and a system for control of a gearbox connected to a combustion engine when it is determined that there is, or will soon be, a situation of low power requirement.

BACKGROUND Controlling the engine and gearbox in a motor vehicle entails engine speed and gear selection being controlled partly by commands from the driver and partly by control units adapted to optimising the engine's operating performance with regard to fuel economy, exhaust emissions etc. The control unit, usually called ECU (electronic control unit), receives sensor signals from various parts of the driveline, and the engine and the gearbox are controlled on the basis of these signals and of commands from the driver.

In long-distance driving it is particularly important to optimise fuel consumption. To this end, so-called long-haulage vehicles have been specified which have an optimised engine speed for a certain cruising velocity. Typical cruising velocities may be 80, 85 or 89 km/h. To achieve good fuel economy, the vehicle is adapted to having an engine speed which is in the middle of the torque plateau when running at cruising velocity.

As fuel consumption optimisation is always desirable, there is a need to reduce the fuel consumption of vehicles which are powered by a combustion engine.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method and a system which reduce the fuel consumption of a combustion engine which powers a motor vehicle.

The present invention provides a method for control of a gearbox connected to a combustion engine which powers a motor vehicle. The method first determines whether there is or will soon be a situation of low power requirement below a certain threshold value for running the vehicle. If such a situation occurs, the gearbox is switched to a high gear so that the vehicle

runs at an engine speed which is below the torque plateau for the high gear. The result is that no fuel, e.g. diesel oil in cases where the engine is a diesel engine, need be supplied when the engine is running with the high gear engaged, i.e. the vehicle is coasting.

In an embodiment of the invention, a look-ahead function is used for predicting when the high gear can be engaged or needs to be disengaged to enable the vehicle to run at cruising velocity with minimal fuel supply.

The invention relates also to a vehicle and a computer programme product adapted to effecting control according to the method described above.

Using the method, the vehicle and the computer programme product according to the invention results in lower parasitic losses in the running of the vehicle, thereby leading to lower fuel consumption of a vehicle powered by a combustion engine.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described in more detail below on the basis of non- limitative examples with reference to the attached drawings, in which:

- Fig. 1 is a general partial depiction of an engine,

- Fig. 2 is a torque curve indicating the operating range for a gear, and

- Fig. 3 is a flowchart illustrating steps in the control of an engine in a motor vehicle.

DESCRIPTION OF PREFERRED EMBODIMENTS

Fig. 1 schematically depicts selected parts of a driveline 100 in a motor vehicle 10. The driveline depicted in Fig. 1 may for example be intended to form part of a truck or some other heavy vehicle, e.g. a bus or the like. An engine 101 forms part of the driveline 100. The engine is also connected to a semi-automatic gearbox 105.

The engine 101 and the gearbox 105 are controlled by at least one control unit 107, e.g. an electronic control unit (ECU). The control unit is adapted to receiving sensor signals from various parts of the vehicle, including the gearbox and the engine. The control unit 107 is also adapted to supplying control signals to various parts and components of the vehicle, e.g. the engine, including but not limited to control signals for the gearbox. The control unit 107 is further adapted to receiving data from a so-called "look-ahead" (LA) function 103. The look-ahead function provides the control unit with information inter alia about the nature of

the road ahead of the vehicle. For example, information about the gradient of the road is obtainable via the look-ahead function.

The functioning of various parts and components of the vehicle, particularly gear selection, is controlled by preprogrammed instructions in the control unit. The preprogrammed instructions typically take the form of a computer programme product 109 stored on a digital storage medium such as a working memory (RAM), flash memory (EPROM, EEPROM) or a read-only memory (ROM) and are executed by the control unit. The vehicle's behaviour in a specific situation may be adapted by altering the preprogrammed instructions.

The gearbox 105 in the vehicle 10 is provided with at least one high gear adapted to having an operating range below the torque plateau for the gear. The vehicle thus cannot be driven with such a high gear engaged, since the engine speed would be too low to deliver the necessary torque at such an operating point. The object of such a gear is to minimise parasitic losses and hence fuel consumption in operating situations where there is a low power requirement. For example, if the vehicle is travelling on a downgrade, the high gear is selected and the engine saves fuel, while at the same time the engine speed drops to a very low value and the noise level from the engine decreases. If the control unit is provided with data about the nature of the road ahead of the vehicle, the high gear can be engaged at an optimum point in time without detection of engine signals for determining that there is a low power requirement. For example, if the control unit is connected to receive data about the road from a look-ahead function, data from that function can be used for determining when the high gear should be engaged.

Fig. 2 depicts a torque curve and operating range for a high gear as above. It shows that the gear is intended to be used at an operating point below the plateau of the torque curve. Thus no power can be extracted from the engine when this gear is engaged. For example, if the vehicle is travelling at a cruising velocity of 80 km/h and there is a low power requirement, the high gear can be selected and the engine will save fuel. The engine will then run at a very low speed below the torque plateau, as depicted in Fig. 2. When power subsequently needs to be extracted from the engine, the control unit switches down to a lower gear.

Fig. 3 depicts a flowchart of steps performed during control of an engine as above. A first step 301 determines that there is a low requirement for engine power. A low power requirement may typically be identified as a power requirement below a certain threshold

value. The threshold value may for example be a proportion, e.g. 10-15%, of the maximum power. For example, control signals from the engine may be used for determining that little engine power is for the time needed to run the vehicle. According to an embodiment, data about the road ahead of the vehicle are used for determining that little engine power is for the time being needed for running the vehicle. In particular, data from a look-ahead function may be used. When it is determined that little engine power is needed for running the vehicle, the gearbox is switched at a step 303 to a high gear which is not operable, i.e. the engine speed is below the torque plateau for the gear when the vehicle is travelling at cruising velocity, e.g. 80 or 90 km/h.

Thereafter there is continuous checking at a step 305 as to whether there is a need for power from the engine. For example, depression of the accelerator by the driver and/or control signals from the engine may be used to determine that there is a need for power. If the control unit 107 has access to data about the road, these data can be used for determining whether power is or will soon be needed. If at step 305 it is determined that power is or will soon be needed, the engine is switched down at a step 307, otherwise the engine will continue to run with the high gear engaged.

Using the method, the vehicle and the computer programme product as herein described results in lower fuel consumption by a vehicle powered by a combustion engine. This is because no fuel need be supplied to the combustion engine when it is running at a low speed below the plateau of the torque curve, i.e. the vehicle is coasting.