METHOD AND SYSTEM FOR BALANCING THE CYLINDERS OF A DIESEL ENGINE

The invention relates to a method of balancing the cylinders of a diesel engine. The invention also relates to a system for balancing the cylinders of a diesel engine.

In piston engines, there are differences in power generation between the various cylinders of the engine. The differences are, e.g., due to the wear of the components of the injection system resulting in changes in their operation in the course of their service life. Differences in cylinder outputs have a negative impact on the engine operation, e.g. by increasing the load on the crankshaft and other components as well as engine vibrations. Therefore, efforts are made to balance the cylinders, i.e. to keep the combustion process as similar as possible between the cylinders. Problems related to the output differences between the cylinders occur in abundance in diesel engines that are provided with a common pressure supply system, in which heavy fuel oil is used as a fuel.

An object of the invention is to provide an improved solution for balancing the cylinder loading in a diesel engine.

The objects of the invention are achieved primarily as disclosed in the appended claims 1 and 11. In the invention, the starting moment of the combustion process in each cylinder is defined and the defined starting moment of the combustion process is compared to a specific set value. The starting moment of fuel injection is changed, if the defined starting moment of the combustion process differs from the set value. In addition, the duration of fuel injection into the cylinders is changed in order to equalise the outputs produced by the cylinders.

Considerable advantages are achieved by the present invention. By adjusting both the starting moment of the combustion process and the duration of fuel injection into each cylinder individually the output differences between the cylinders will be balanced. The output differences between the cylinders due to the wear of the components of the injection system can be compensated and thus the operation of the cylinders kept optimal during the whole lifetime of the components. Also, the loading changes caused by the quality variations of the fuels of different types or the fuel can be compensated, whereby they have as small an impact on the engine operation as possible. The system may also be utilised in the maintenance of the components, e.g. injectors, of the fuel injection system. The control unit of the system may be set to monitor the changes in the set values of the components, whereby the control unit informs about the need to change a component, if the set value exceeds the limit value indicating the need to replace the component. Further, less expensive injectors can be utilised in the injection system, since there is no need to find individual injector trim values for each common pressure supply system.

In the following, the invention will be explained in more detail, by way of example, with reference to the appended drawing, which illustrates schematically one system according to the invention.

The drawing depicts an example of the system 1 according to the invention for balancing the cylinders 3 of a diesel engine. The system 1 is arranged in conjunction with a piston engine 2. The engine 2 is a large diesel engine, which is used for instance as a main and an auxiliary engine in vessels or in power plants. The engine 2 is provided with a common pressure supply system 4 for supplying fuel into the cylinders 3. For instance, heavy fuel oil is used as a fuel in the engine 2. The injection system ,4 is electrically controlled.

The engine comprises several cylinders 3, of which each one is provided with an injector 5 for injecting fuel into the combustion chamber of the cylinder. The supply system comprises a common rail 9 for pressurised fuel. The injectors 5 are connected to the common rail 9. The fuel supply system 4 comprises a fuel source 6, for instance a fuel tank, and a low-pressure pump 11 and a high-pressure pump 12 for feeding fuel from the fuel source 6 into the common rail 9. Each injector 5 is in flow connection with the common rail 9 via a fuel channel 10.

While the engine 2 is running, fuel is pumped from the fuel tank 6 by the low- pressure pump 11 along a feed channel 13 to the high-pressure pump 12 and further, by the high-pressure pump 12 via the feel channel 13 into the common rail 9. From the common rail 9, fuel is led to the injectors 5. Fuel is injected by the injectors 5 into the cylinders 3 at a desired moment.

The engine comprises the system 1 , by which the cylinders 3 are balanced, i.e. the combustion process is kept as similar as possible between the cylinders 3. The system 1 comprises a control unit 14, which controls fuel injection from the injectors 5 into the cylinders 3. The control unit 14 defines the starting moment of fuel injection into each cylinder. The control unit 14 adjusts the starting moment of fuel injection. Moreover, the control unit 14 controls the duration of fuel injection.

The control unit 14 defines the starting moment of the combustion process, i.e. the crankshaft angle in each cylinder 3 corresponding the start of the combustion. The starting moment of the combustion process can be defined on the basis of the position measurement of cylinder pressure (the crankshaft angle corresponding to the cylinder pressure indicating the start of the combustion process) or on the basis of the measurement of the torsional vibration of the crankshaft. In both methods, the angle of rotation is

measured by an angle sensor 16 suitable for the purpose. The measuring data of the angle sensor 16 is conveyed to the control unit 14.

In the method based on the position measurement of cylinder pressure, the cylinder pressure in each cylinder 3 is measured by a measuring means 15 suitable for the purpose, such as a pressure sensor, a knock sensor (an acceleration transducer) or a strain gauge, adapted in conjunction with the cylinder 3. Cylinder pressure can also be measured by using ionisation measurement. The measuring data on cylinder pressure and the angle of rotation of the crankshaft is conveyed to the control unit 14. The control unit 14 defines the starting moment of the combustion process in each cylinder 3 on the basis of the measuring data on cylinder pressure and the angle of rotation of the crankshaft. As soon as the measured cylinder pressure reaches the value, which indicates that the combustion- process in the cylinder 3 has started, a corresponding crankshaft angle will be defined. The start of the combustion process is indicated for instance by a change in angular coefficient occurring in the cylinder pressure increase curve. The starting moment of the combustion process may also be defined by measuring the maximum cylinder pressure.

When the starting moment of the combustion process is defined on the basis of torsional vibration measurement, the system comprises measuring means for measuring the torsional vibration of the crankshaft. The angle sensor 16 measuring the angle of rotation of the crankshaft may be used as a measuring means, whereby measuring data on the magnitude of torsional vibration and the angle of rotation, at which torsional vibration occurs, is received. The measuring data is conveyed to the control unit 14. On the basis of the measuring data, the control unit 14 defines the starting moment of the combustion process in each cylinder 3.

Once the starting moment of the combustion process is defined, the control unit 14 compares the defined starting moment of the combustion process to a specific set value. The set value may be predetermined for instance on the basis of engine load and the pressure of the common rail 9. If the defined starting moment of the combustion process differs from the set value, the control unit 14 changes the starting moment of fuel injection for each cylinder individually. The starting moment of fuel injection is changed so that the starting moment of the combustion process approaches the set value. Since the starting moment of fuel injection corresponds quite precisely the starting moment of the combustion process, the starting moment of fuel injection can be changed to be identical with the set value of the starting moment of the combustion process.

Further, the control unit 14 changes the duration of fuel injection into the cylinders 3 in order to equalise the outputs produced by the cylinders. The duration of fuel injection is changed for each cylinder individually. The duration of fuel injection is changed as soon as the above-described adjustment of the starting moment of fuel injection has been made, i.e. the starting moment of fuel injection is such as desired. The duration of fuel injection into the cylinders can be changed so that the outputs produced by the respective cylinders 3 are equal. The duration of fuel injection can be changed on the basis of the exhaust gas temperatures of the cylinders 3 or on the basis of the measurement of the torsional vibration of the crankshaft.

In case the duration of fuel injection is adjusted on the basis of exhaust gas temperatures, the system 1 comprises cylinder-specific temperature sensors 18 for measuring the exhaust gas temperatures of the cylinders. The temperature sensors 18 are mounted in the exhaust ducts 19 of the cylinders. The measuring data of the temperature sensors 18 is conveyed to the control unit 14. The control unit 14 changes the duration of fuel injection on the basis of the measured exhaust gas temperatures. The duration of fuel

injection is changed in order to equalise the outputs produced by the cylinders 3. The control unit 14 may change the duration of fuel injection so that the exhaust gas temperatures in the respective cylinders 3 are equal. The measured exhaust gas temperatures are compared to the set value. If the measured exhaust gas temperature of one of the cylinders 3 differs from the set value, the duration of fuel injection into said cylinder is changed so that the exhaust gas temperature is at its set value. The mean value of the measured exhaust gas temperatures or a predetermined value can be used as a set value for the exhaust gas temperature, the magnitude of which value may depend for instance on the engine load or on the fuel that is used. The duration of fuel injection is prolonged, if the exhaust gas temperature is too low. Similarly, the duration of fuel injection is shortened, if the exhaust gas temperature is too high.

In case the duration of fuel injection is defined on the basis of the measurement of the torsional vibration of the crankshaft, the system comprises measuring means for measuring the torsional vibration of the crankshaft. The angle sensor 16 measuring the angle of rotation of the crankshaft may be used as a measuring means, whereby measuring data on the magnitude of torsional vibration and the angle of rotation, at which torsional vibration occurs, is received at the same time. The measuring data is conveyed to the control unit 14. The control unit 14 changes the duration of fuel injection into each cylinder individually so that the torsional vibration of the crankshaft is minimised.

The duration of fuel injection can be changed by utilising both the above- mentioned methods, i.e. on the basis of the exhaust gas temperatures of the cylinders and on the basis of the torsional vibration of the crankshaft. Then, the duration of fuel injection may be adjusted by utilising both measurements simultaneously or by using one measurement primarily, for instance that of the exhaust gas temperatures, while the other measurement is a backup

measurement. When required, the other measurement may be taken into use, for instance in case the primary measurement fails.

The invention is not limited to the shown embodiments, but several variations are conceivable within the scope of the appended claims. The starting moment of the combustion process can be defined by measuring the current in the solenoid valves of the injectors 5 or by measuring the fuel injection pressure. The fuel injection pressure can be measured by a pressure switch or a pressure transducer, which is adapted in the common rail 9 or in the fuel channel 10 of the injector.

For instance, according to one embodiment, the duration of fuel injection into the cylinders 3 is changed on the basis of the exhaust gas temperatures and on the basis of torsional vibration in order to equalise the outputs produced by the cylinders. In this manner, it is possible to further increase the accuracy of the adjustment as far as the duration of the injection is concerned.

Further, the method comprises a stage, at which the correctness of the measurement of the exhaust gas temperature is evaluated and the change in the duration of fuel injection is determined on the basis of torsional vibration, in case the measurement of the exhaust gas temperature of the cylinder is not valid. Such situations, in which the measurement of the exhaust gas temperature is not valid are, for instance, the failure of the sensor that measures the exhaust gas temperature or the case when the exhaust gas temperature 18 of the cylinder deviates sufficiently from the preset temperature value or from the mean value of the exhaust gas temperatures of the cylinders. This provides the advantage that the error conditions of the sensor affecting the adjustment of the duration of injection can be identified and the engine does not necessarily have to be stopped. In this case, one way to define the change in the duration of fuel injection into the cylinders 3 is based on torsional vibration, if the exhaust gas temperature 18 of the

cylinder deviates more than 50 % from the preset temperature value or from the mean value of the exhaust gas temperatures of the cylinders. The preset temperature value for a specific load may be for instance a reference value for that specific load and speed recorded in the adjustment system. Another way to define the change in the duration of fuel injection into the cylinders 3 is based on torsional vibration, if the exhaust gas temperature 18 of the cylinder deviates more than 10 % from the preset temperature value or from the mean value of the exhaust gas temperatures of the cylinders. This provides the advantage that the determination method affecting the adjustment of the duration of injection can be changed momentarily, whereby one determination method to be used primarily can be selected, in this case the method based on the exhaust gas temperatures. Both practices can be realised by varying the functionality of the control unit 14 of the system.