BACKGROUND AND PRIOR ART New environmental requirements concerning vehicles are setting high requirements with regard to reduced quantities of particles in exhaust gases. Large quantities of particles in exhaust gases arise from poor combustion in the engine, which may itself be due to non-optimum fuel/air ratio at the time of combustion in the engine.

A common cause of non-optimum fuel/air ratio is too little air reaching the engine (so- called charge air). This is often the case at load changes with a large positive derivative for the fuel quantity injected, i. e. when there is a sudden large increase in the amount of fuel injected per unit time, as when a driver of the vehicle quickly presses the accelerator pedal all the way down from a steady driving position. If the air volume between a turbounit of the vehicle and the engine inlet is too great, insufficient pressure is built up in the air volume at the time of such a load change.

The result is that the fuel/air mixture contains too little air for achieving optimum combustion conditions.

Between the turbounit and the engine inlet there is a charge air cooler for cooling the charge air. Cooling the air which has been compressed and thereby heated in the

turbounit helps to increase the density of the air so that more air can be supplied to the engine. A lower charge air temperature also contributes to better combustion conditions and, for example, to less nitrogen oxides in the exhaust gases.

However, a problem in connection with the charge air cooler arises when there is a load change with a large positive derivative for the fuel quantity injected. What happens is that the air volume in the charge air cooler itself contributes to a situation whereby the air volume between the turbounit and the engine inlet at such a load change is too great for sufficient charge air pressure to be built up, which means that the fuel/air mixture then supplied to the engine will contain too little air. Another problem may arise when warm charge air is desired, as when warming up the engine, since the air may then be cooled too much in the charge air cooler.

There are various solutions to the abovementioned problem. One solution is to bypass the whole charge air cooler. Examples of such designs are referred to in patent documents US 4513729, JP 60-8435 and JP 09256915. One problem with that kind of solution is that the number of pipes increases. Such solutions also provide no cooling at all of the charge air in the charge air cooler.

US 5632256 refers to a different design with a charge air cooler whereby the whole cooling element of the charge air cooler can be disconnected. This reduces the air volume between the turbounit and the engine inlet but likewise provides no cooling at all of the charge air in the charge air cooler.

A third type of design is referred to in JP 4-132831 and JP 1-92523, whereby the charge air cooler is divided into two separate portions, one of which can be shut off to reduce the air volume. That design would be expensive and bulky owing to a large number of constituent parts being involved.

SUMMARY OF THE INVENTION

The present invention relates to solving the problem of too slow a build-up of charge air pressure at a load change with a large positive derivative for the fuel quantity injected, and hence to reducing the amount of particles in the exhaust gases, without the disadvantages of the prior art.

This is accomplished by means of a method for controlling the volume utilised in a charge air cooler for a motor vehicle, and a charge air cooler for a motor vehicle, which charge air cooler comprises an inlet nozzle, an outlet nozzle, a cooling element which has ducts for cooling of flowing air and which is arranged between the inlet nozzle and the outlet nozzle, and a control device for control of air flowing through the charge air cooler whereby the control device is arranged so that when there is a positive derivative, exceeding a predetermined value, for the fuel quantity injected, the control device partially shuts off the cooling element so that the air only flows through a part of the cooling element.

This construction makes it easy to reduce as necessary the air volume between the turbounit and the engine inlet. The construction provides a simple, flexible and inexpensive way of reducing said air volume and hence more quickly achieving the correct charge air pressure and less cooling of the charge air, without entailing the disadvantages of the prior art.

In one embodiment, the control device also partially shuts off the cooling element when charge air heating is required. This means that the problem of too powerful charge air cooling when warm charge air is desired is solved without the disadvantages of the prior art.

In one embodiment, the control device, when partially shutting off the cooling element, may divide the cooling element into two separate portions, one of them larger than the other. Creating this situation and allowing the air to flow through only the smaller portion makes it possible to quickly achieve the correct charge air pressure and less cooling of the charge air heated in the turbounit.

In one embodiment, the control device comprises a first damper in the inlet nozzle and a second damper in the outlet nozzle.

The invention also relates to a computer program for implementing the abovementioned method, and a computer program product comprising a computer- readable medium which comprises said computer program. This makes it easy for the control unit to be provided with instructions for regulating the control device. The invention also relates to a control unit comprising a computer-readable medium which comprises said computer program.

DESCRIPTION OF THE DRAWINGS The invention is described below with reference to the attached drawings, in which: Fig. 1 depicts schematically a charge air cooler according to the present invention whereby the whole volume of a cooling element is used; and Fig. 2 depicts schematically the charge air cooler according to Fig. 1 in a situation where the volume utilised has been reduced.

DESCRIPTION OF EMBODIMENTS Figs. 1 and 2 respectively depict a charge air cooler 1 for a truck. The charge air cooler 1 comprises an inlet nozzle 2, an outlet nozzle 3, a heat exchanger portion with a cooling element 4 between the nozzles 2,3, and a control device 5,6, 5', 6'.

The charge air cooler 1, which is usually arranged in front of the truck's radiator, receives air via the inlet nozzle 2, which air needs to be supplied to the engine to form part of a fuel/air mixture for combustion in the engine. Before it reaches the charge air cooler, the air is compressed in a turbounit, thereby increasing the pressure and temperature of the air. In the charge air cooler 1, the temperature of the air drops as it passes through the cooling element 4. The temperature is thus reduced so that the

density of the air is increased, which means that more air can be introduced into the engine. Lowering the temperature also results in better combustion conditions in the engine and hence smaller amounts of harmful discharges of, for example, nitrogen oxides, and lower fuel consumption.

When the driver of the truck effects a load change by depressing the accelerator pedal, this is registered by a control unit 7 which directly orders a larger amount of fuel/air mixture for the engine. Fuel is then supplied relatively quickly via a fuel supply system, whereas it takes longer for the air to be supplied. The reason for the air taking longer to be supplied is that the high air pressure provided by the turbounit has to be built up throughout the volume situated between the turbounit and the engine inlet before the air is supplied to the engine.

At a load change with a large positive derivative for the fuel quantity injected, e. g. in a situation when a driver of the vehicle quickly presses the accelerator pedal all the way down from a steady driving position, it is not possible to build up sufficient air pressure between the turbounit and the engine inlet before the air is released into the engine. This means that the engine receives too little air, resulting in poor combustion and hence a large amount of particles in the exhaust gases.

In the charge air cooler 1 in Fig. 1, a large portion of the volume of the cooling element 4 can be shut off so that the volume in which a high pressure has to be built up is greatly reduced, as depicted in Fig. 2. The air flow through the charge air cooler is represented by the arrows in Figs. 1 and 2. As the correct pressure can be built up more quickly in a smaller, reduced volume than in the larger ordinary volume, the partial shutting off depicted in Fig. 2 results in a quicker response from the charge air cooler 1. At the same time, there is still a certain cooling of the air, which means that the charge air cooler 1 is appropriate for a greater variety of operating situations than a charge air cooler 1 whereby the cooling element 4 is completely bypassed in order to reduce the volume between the turbounit and the engine inlet.

The volume reduction is brought about by the control unit 7, which also orders the fuel quantity injected and detects the prevailing charge pressure. The control unit 7 calculates whether volume reduction is necessary in order to achieve sufficiently quickly the correct charge pressure. When volume reduction is necessary, the control unit 7 orders a control device 5,6, 5', 6'in the charge air cooler 1 to direct the air so that it only passes through one portion of the cooling element 4. The communication between the control unit 7 and the control device 5,6, 5', 6'is represented in the diagrams by broken lines. The control device 5, 6,5', 6'in the charge air cooler 1 takes the form in Fig. 1 of first and second dampers 5,5'in the form of sheetmetal discs (one in each inlet and outlet nozzle 2,3) each having an actuator 6,6'in the form of a pneumatic cylinder which controls the respective damper 5,5'.

When the actuators 6,6'turn the dampers 5, 5'to the position according to Fig. 2, all of the air is led into the cooling element 4 above the damper 5 in the inlet duct 2. In the cooling element 4, which is composed of ducts in the form of tubes or plates, there is no possibility of the air flowing downwards. The air thus also comes out above the damper 5'in the outlet duct 3 so that the volume in which the correct charge pressure has to be built up is diminished by the volume shut off which is situated below the level of the damper 5, 5', which level is represented by a chain-dotted line through the cooling element 4 in Fig. 2. The control device 5,6, 5', 6'thus divides the cooling element 4 into two separate portions 8,9, one of which (8) is larger than the other (9), and the air flows through only the smaller portion 9 during the partial shutoff of the cooling element 4.

The control element 7, which calculates or detects the derivative for the fuel quantity injected, or corresponding parameters which are indicative for the derivative, brings about, when the derivative has decreased sufficiently, a turning back of the dampers 5, 5'to the position according to Fig. 1, so that the total cooling volume of the charge air cooler 1 is used, resulting in optimum combustion conditions. Some cooling is nevertheless maintained even during the abovementioned transient, i. e. the period of time with a large positive derivative for the fuel quantity injected, when part of the volume of the charge air cooler is shut off.

The control unit 7 takes the form of some kind of computer (comprising for example a microprocessor according to the state of the art in vehicle technology) which performs calculations and gives various commands on the basis of what is indicated in a computer program for controlling the volume utilised of the charge air cooler 1. The computer program is stored in a data-readable medium 10, such as a flash memory, an ROM memory or EPROM, which medium 10 is read by the computer 7. The control unit 7 uses sensors situated on the engine to obtain the values which provide the basis for calculations and commands.

The control device 5,6, 5', 6'in the charge air cooler 1 may also be used for regulating the charge air temperature, if so desired, by controlling the size of the portion of the cooling element 4 which the air is allowed to flow through. Warm charge air may for example be desirable when warming up the engine. In such situations the control unit 7 uses temperature values received from the sensors on the engine to reduce as necessary the cooling portion utilised of the cooling element 4.

In an alternative embodiment, the ducts through the cooling element 4 have a larger cross-sectional area in the smaller portion 9 of the cooling element 4, through which smaller portion 9 the air flows when the cooling element 4 is partially shut off as in Fig. 2. A larger cross-sectional area of these ducts reduces the pressure drop of the air flowing through the ducts, thereby facilitating the build-up of the correct charge air pressure. In such an embodiment, the air flows advantageously through either a smaller portion 9 or a larger portion 8 of the cooling element 4 and never through the whole cooling element 4 simultaneously. This prevents the possibility of too large a proportion of air passing through the ducts with the largest cross-sectional area and hence the lowest flow resistance, a situation which would not result in optimum cooling. To make flow possible only through either the larger portion 8 or the smaller portion 9 of the cooling element 4, the arrangement of the dampers 5,6, 5', 6'in the charge air cooler has to be altered, e. g. so that they rotate about spindles situated adjacent to the cooling element 4 instead of spindles situated centrally in the nozzles 2, 3 as in Figs. 1 and 2.

The foregoing are not to be regarded as limiting the invention, but merely as possible embodiments. For example, this kind of charge cooler may be used in all types of motor vehicles and not only in trucks. The actuators in the control device may take the form of, for example, electric shifting motors, and the dampers may be replaced by some kind of valves. Many other alternative embodiments also fall within the scope of the ensuing claims.