Field of technology

The present invention relates to a combustion engine that has two turbochargers connected in series. A combustion engine can, for example, be a diesel engine. In addition, the engine can comprise more turbochargers than said two turbocharges connected in series. Especially the invention relates to diesel engines and more precisely to low or medium speed diesel engines.

Prior art

Load of a combustion engine may change suddenly. A sudden switching of electric load to the generator connected to the engine or a sudden driving instruction of the engine, for example, can cause the rapid load change. These changes are often called load steps or fast ramps . The engine is desired to handle the load step. For example, engines in ships should handle a certain load step within a certain period and to reach a steady state.

US 20020002969 discloses a method that are used when a positive load step occurs. The engine is switched to a special operation to provide the secondary fuel injection in addition to main fuel injection into each cylinder of the engine. This increases exhaust gas enthalpy. In this way an exhaust-gas turbocharger can be brought up to speed faster, so that charge-air pressure is also built up faster. This reduces the so-called turbo lag in response to the positive load step.

Although the prior art solutions provide quite good ways to handle the positive load steps, characters of the responses to the load steps are still not optimal in many cases due to a great variety of running conditions of the engine. Short description of invention

The aim of the invention is to provide an improved way to handle the positive load steps of the combustion engine. The aim is achieved in a way described in the independent claims. The depended claims describe different embodiments of the invention.

The combustion engine according to the invention has two turbochargers connected in series. The first turbocharger is arranged to run in a higher pressure range than the second turbocharger, and the first turbocharger is connected to an exhaust pipeline to provide exhaust gas to a turbine wheel of the first turbocharger. The engine has also a wastegate that is connected to the exhaust pipeline, and an exhaust line between the first turbocharger and the second turbocharger. The combustion engine comprises also a control unit to control the first turbocharger in case a sudden load change of the engine. The control unit is arranged to provide a second fuel injection into cylinders of the engine after main fuel injections to control the first turbocharger to an over speeding state, and to open the wastegate in order to prevent the turbocharger to run in the over speed state.

List of figures

In the following, the invention is described in more detail by reference to the enclosed drawings, where

Figure 1 illustrates an example of a combustion engine according to the invention,

Figure 2 illustrates an example of a diagram illustrating a load step and responses of a turbocharger to the load step, and

Figure 3 illustrates an example of a flow chart illustrating a method according to the invention. Description of the invention

Figure 1 illustrates a schematic example of a combustion engine according to the invention. The combustion engine 1 has two turbochargers 6, 4 connected in series, the first turbocharger 6 being arranged to run in a higher pressure range than the second turbocharger 4. As can be seen in the figure that the turbochargers comprise turbine wheels 18, 16 to rotate compressor wheels 17, 15 that compress intake air 3 to higher pressure. The first turbocharger is often called high pressure turbocharger, and the second turbocharger is often called low pressure turbocharger. The intake air is first supplied to the compression wheel 15 of the low pressure turbocharger 4. The pressurized air by the low pressure turbocharger is transmitted via an air pipe 5 to the compression wheel 17 of the high pressure turbocharger 6. The pressurized air by the high pressure turbocharger is transmitted to the cylinders 2 of the engine 1 via the air intake pipeline 7. Usually there are also coolers at the air pipe 5 and between the high pressure turbocharger and the air intake pipeline 7 that are not showed in Figure 1.

Fuel is supplied through a fuel supply pipeline 21 from a fuel supply 22 to injectors 20 in the cylinders. The injectors inject the fuel into the cylinders. The air/fuel mixture burns in the cylinders providing mechanical power that is delivered outside the engine to power output 19 of the engine. The power output is a part of the crank shaft of the engine, and it is connected to load that consumes the engine's output power. Exhaust gases are transmitted out of the cylinders 2 via the exhaust pipeline 8.

The high pressure turbocharger 6 is connected to the exhaust pipeline to provide the exhaust gas to the turbine wheel 18 of the first turbocharger 6. The output of the turbine wheel 18 is connected to the input of the turbine wheel 16 of the low pressure turbocharger 4 via an exhaust line 13. The output 14 of the turbine wheel 16 of the low pressure turbocharger 4 delivers the exhaust gas outside the whole engine 1.

The engine has also a wastegate 11 that is connected 10 to the exhaust pipeline 8 and the exhaust line 13 between the high pressure turbocharger and the low pressure turbocharger. The wastegate is a valve that can be closed, fully opened or opened partially between the closed and fully open states. The wastegate 11 and it's input line 10 and output line 12 provide a bypass route to the exhaust gas from the side of the high pressure turbocharger 6. The bypass route of the input line, the wastegate and the output line can be provided as an internal structure of the high pressure turbocharger or an external structure in view of the high pressure turbocharger. So, it is clear that Figure 1 is only a schematic figure. To control the wastegate it is possible to adjust the bypass flow of the exhaust gas and therefore the exhaust gas flow going through the turbine wheel 18 of the high pressure turbocharger 6.

The combustion engine 1 comprises also a control unit 26 to control the high pressure turbocharger 6 in case a sudden load change of the engine. The sudden load change may be caused by a sudden connection of load to the power output 19 of the engine. For example, if a generator is connected to the engine's output, the fast electric load change of the generator may cause a rapid load change of the engine. Another way which may cause a corresponding situation as load change is a sudden change of supplying fuel into the engine. Cases where load increases very rapidly are called positive load steps and rapid changes are called fast ramps because they can be modelled as a step or ramp.

When the positive load step occurs the control unit 26 is arranged 29 to provide a second fuel injections into cylinders 2 of the engine after main fuel injections to drive the first turbocharger 4 to an over speeding state, and to open the wastegate 11 in order to prevent the turbocharger 1 to run in the over speed state. The control unit is connected 28 to the power output 19 of the engine for measurements of load / speed, and is arranged to estimate load changes. The engines speed can also be measured form other locations, like the cam shaft. It is also possible to measure load from a generator of the engine, for example. The control unit is also connected 27 to the wastegate in order to control state of the wastegate, and also connected 29 to the injectors 20 to provide the second fuel injections into the cylinders.

In addition, the control unit can be connected 25 to other control devices 23 of the engine, like fuel supply drives, speed/load controllers etc. Figure 1 is only the schematic figure that shows the other control devices in a very simplified way. It should be noted that the other control devices are normal control devices that are used in combustion engines. The additional connections may be useful for monitoring the whole engine's system, reconfiguring the control unit 26, and if needed to deliver drive instructions 24 of the engine to the control unit 26.

So, the said control unit 26 is for driving the first turbocharger 6 to an over speeding state, and for opening the wastegate 11 in case of a positive load step. It is not needed in other cases which are handled by the other control devices of the engine. However, it should be noted that the control unit 26 can be integrated into the other control devices. Because the control unit 26 is needed only when a positive load step occurs it does not need to operate all time. Therefore said functions of the control unit are arranged to set ON when the sudden load change exceeds a threshold value. The threshold value can be a certain ramp value illustrating the shape and speed of the load step. The sharper ramp indicates the faster ramp. A gentle ramp means a slow ramp. The threshold ramp value is exceeded when the measured ramp is the same or sharper than the threshold ramp. In practice the curve of ramp or step is a speed change curve relating to load change. So, the threshold value can be a ramp value illustrating speed change.

Figure 2 illustrates an example of a diagram illustrating a load step and responses of the high pressure turbocharger to the load step. When the positive load step 200 occurs, the control systems 23 of the engine starts to take care of the increased load by producing more power to the output 19 of the engine. The speed of the high pressure turbocharger (HPT) arises as well. Line 201 illustrates how the speed of the HPT arises without the invention.

The load response of a turbocharged engine could be improved by introducing the second later fuel injection after the main fuel injection.

This later fuel injection can be called post injection. The post injection provides more energy to the exhaust gases which will in turn result in higher acceleration of the turbine wheel of the high pressure turbocharger. Higher acceleration of the turbocharger provides faster increase of charge air pressure which in turn enables the possibility for more complete combustion of the fuel and a possibility to inject more fuel, which will result in improved response to fast load changes. The dashed line 202 illustrates the turbocharger's speed response to the load step using the post injection. However, it is not desired to run the turbocharger at too high speed because it may break or damage the turbocharger. The maximum allowed speed of the high pressure turbocharger is 100 % level in Figure 2. It can be seen that the post fuel injections after the main fuel injections arise the speed much faster than the main fuel injections only, and the speed arises to the over-speeding area above the maximum allowed speed. In order to prevent the urbocharger to run in the over- speeding area the wastegate 1 1 is opened, which reduces the exhaust gas flowing through the high pressure turbocharger 6 and thus reducing the top speed of the turbocharger. A thicker dashed line 203 illustrates the effect of opening the wastegate to the line 202. As can be seen the opening of the wastegate prevents the turbocharger to run at over speed, and the combination of the post injection and opening the wastegate provides a much better response to the positive load step than known responses. The response according to the invention is much faster and it is in the allowed speed area of the turbocharger, providing faster and safer load response of the engine to a sudden load change. The response curve 205 of the low pressure turbocharger is also better than without the inventive arrangement providing a faster increase than in the response curve 204 without the invention.

The timing when the wastegate is opened is selected in such a way that it is not too early to decrease the raise of the speed at the beginning and it is not too late when the speed exceeds the over-speeding area. Therefore the control unit can be arranged to open the wastegate on the grounds of detected speed increase rate (acceleration) of the high pressure turbocharger, for example a percentage value of the maximum allowed speed / second), and/or a percentage value of max speed the turbo running at the moment.

For example, if the increase of high pressure turbocharger's speed is 5 % of the maximum speed / s, the wastegate is arranged to open when the speed reaches 90 % of the maximum allowed speed. Or if the increase of high pressure turbocharger's speed is 1 % of the maximum speed / s, the wastegate is arranged to open when the speed reaches 97 % of the maximum allowed speed. The values of the maximum speed / s, and/or the values of the maximum speed can be arrange, for example, in a table format to be used by the control unit.

Further, the control unit can be arranged to control an opening angle of the wastegate 1 1 in relation to the speed of the first supercharger. In this way it is possible to modify the shape of speed response curve 202, 203.

When the load step has been handled the speed of the turbocharger has stabilized. The combustion engine can return to a normal control mode. Therefore the above said functions of the control unit can be arranged to set OFF after a delay from setting said functions ON, or when the threshold value is not exceeded any more. The delay can be, for example 10 seconds.

Figure 3 illustrates an example of a flow chart illustrating a method according to the invention. The method concerns operation of a combustion engine having two turbochargers connected in series. The first turbocharger is arranged to run in a higher pressure range than the second turbocharger, the first turbocharger being connected to an exhaust pipeline to provide exhaust gas to a turbine wheel of the first turbocharger. As described above, the engine has also a wastegate that is connected to the exhaust pipeline and an exhaust line between the first turbocharger and the second turbocharger.

The method comprises two steps. When a sudden load change of the engine occurs second fuel injections are provided 30 into cylinders of the engine after main fuel injections to control the first turbocharger to an over speeding state, and the wastegate is opened 31 in order to prevent the turbocharger to run in the over speed state. Because the method is only for certain situations (positive load steps) it does not need to be running all time. Therefore the steps of the method can be arranged 33 to set ON when the sudden load change exceeds a threshold value. The threshold value is described in the above text. As already described above as well, the response to the load step stabilizes finally, so the method can be arranged 34 to set OFF after a delay from setting the steps on, or when the threshold value is not exceeded any more. In addition, the step of opening the wastegate can be arranged to start when the speed of the first turbocharger is at least a certain level of the percentage of the maximum speed of the high pressure turbocharger. Further, the step of opening the wastegate can be arranged to control an opening angle of the wastegate in relation to the speed of the first supercharger.

Transient operation could be identified in different ways. It could be based on engine speed change, engine load change, or charge air pressure being different from the expected charge air pressure level based on ambient conditions and engine speed & load. The measurement of the engine speed and load change can be obtained, for example, from the power output of the engine.

As already said the invention makes it possible to achieve faster and safer speed response of the turbocharger. In addition, the invention provides greater capability to a larger load step than previously allowed to a combustion engine. Therefore the combustion engine can be driven in a larger range of driving conditions than before. It is evident from the above that the invention is not limited to the embodiments described in this text but can be implemented in many other different embodiments within the scope of the inventive idea.