Technical field of the invention

The present invention relates to an engine group according to the preamble of claim 1. The invention also concerns a method for recirculating exhaust gases of an engine group according to the preamble of claim 10.

Background of the invention

Nitrogen oxide (NOx) emissions of internal combustion engines are subject to continuously tightening regulations. In general, NOx emissions can be reduced by reducing the temperature in the combustion chamber. An effective way to reduce NOx emissions of an internal combustion engine is to use exhaust gas recirculation (EGR), where part of the exhaust gases are directed back to the cylinders of the engine. Since the heat capacity of the recirculated exhaust gas is higher than the heat capacity of air, the same amount of energy released by combustion leads to lower temperature increase in en- gines with EGR. Also the lower oxygen mass inside the cylinders and reduction of combustion speed help to achieve smaller temperature increase. The effect of an EGR system can be improved by cooling the recirculated exhaust gas. An EGR system can be either internal or external. In an internal EGR system part of the exhaust gas is trapped within the cylinder or back- flow from the exhaust duct into the combustion chamber is utilised. In an external EGR system exhaust gases are recirculated from the exhaust gas duct into the air intake duct. External EGR systems can be further divided into short- route and long-route systems. In a short-route EGR system the exhaust gases to be recirculated are introduced from the exhaust gas duct directly into the air intake duct without going through the turbine or compressor of any turbocharger, whereas in a long-route EGR system the exhaust gases to be recirculated go first through the turbine of at least one turbocharger before being directed through the compressor of the turbo- charger mixed with the charge air.

A drawback of especially external EGR systems is that they are not suitable for being used in engines that are operated on residual fuels or on fuels with high sulfur or ash content, if impurities are not removed from the exhaust gas before introducing it back into the cylinders of the engine. If the impurities are not removed from the recirculated exhaust gas, the exhaust gas recirculation might cause damage to the engine and its auxiliaries, such as intercoolers and turbochargers, for instance in the form of corrosion. On the other hand, exhaust gas purifiers make the EGR system more complicated and ex- pensive.

Summary of the invention

The object of the present invention is to provide an improved engine group and method for recirculating exhaust gases of an engine group, as defined in the characterizing parts of claims 1 and 10, respectively.

The engine group according to the present invention comprises at least a first internal combustion engine and a second internal combustion engine, and means for introducing exhaust gases from the first engine of the engine group into the cylinders of the other engine(s) of the engine group.

According to the present invention, in the method for recirculating exhaust gases of an engine group comprising at least a first internal combustion engine and a second internal combustion engine, exhaust gases from the first engine of the engine group are in- troduced into the cylinders of other engine(s) of the engine group.

When exhaust gases from one of the engines are used for exhaust gas recirculation in the other engines of the engine group, it is possible that a better quality fuel with low sulfur and ash content is used in one of the engines only. Suitable fuels for the engine from which exhaust gases are taken for recirculation are for instance light fuel oil (LFO) and many other distillate fuels or gaseous fuels. Other engines of the engine group can be operated on for instance heavy fuel oil (HFO) with high sulfur content or some other residual fuel, and the benefits of exhaust gas recirculation can still be gotten without costly investments in complicated equipment for removing contaminants from the ex- haust gases. Since HFO with high sulfur content and other low quality fuels are typically much cheaper than for instance LFO with low sulfur content, the invention helps in achieving low operating costs together with low NOx emissions. According to an embodiment of the invention, the engine group comprises an exhaust gas recirculation duct being connected to an exhaust duct of the first engine. According to an embodiment of the invention, the first engine is provided with a turbo- charger and the exhaust gas recirculation duct is connected to the exhaust duct downstream from the turbine of the turbocharger.

According to an embodiment of the invention, the first engine is provided with a turbo- charger and the exhaust gas recirculation duct is connected to the exhaust duct upstream from the turbine of the turbocharger.

According to an embodiment of the invention, the other engines of the engine group are provided with turbochargers and the exhaust gas recirculation duct is connected to the intake ducts of the engines downstream from the compressors of the turbochargers.

According to an embodiment of the invention, the other engines of the engine group are provided with turbochargers and the exhaust gas recirculation duct is connected to the intake ducts of the engines upstream from the compressors of the turbochargers.

According to an embodiment of the invention, the exhaust duct of the first engine is provided with a throttle valve downstream from the point where the exhaust gas recirculation duct is connected to the exhaust duct. The throttle valve increases the pressure difference between the intake ducts and the exhaust duct of the first engine and facili- tates thus the exhaust gas flow in the exhaust gas recirculation duct.

According to an embodiment of the invention, the exhaust gas recirculation duct is provided with a compressor/fan for increasing the pressure of the recirculated exhaust gas. A compressor might be needed especially when the exhaust gases are introduced down- stream from the compressors of the turbochargers, where the intake air pressure is higher than on the upstream side. According to an embodiment of the invention, the exhaust gas recirculation duct is provided with an exhaust gas cooler for cooling the recirculated exhaust gases. The exhaust gas cooler improves the NOx reducing effect of the EGR system. According to an embodiment of the invention, in the exhaust gas recirculation method the exhaust gases for the recirculation are taken from an exhaust duct of the first engine downstream from the turbine of a turbocharger.

According to an embodiment of the invention, the exhaust gases for the recirculation are taken from an exhaust duct of the first engine upstream from the turbine of a turbo- charger.

According to an embodiment of the invention, the exhaust gases are introduced into the intake ducts of the other engines in the engine group downstream from the compressors of turbochargers.

According to an embodiment of the invention, the exhaust gases are introduced into the intake ducts of the other engines in the engine group upstream from the compressors of turbochargers.

According to an embodiment of the invention, the recirculated exhaust gases are cooled before being introduced into the cylinders of the engines.

Brief description of the drawings

Fig. 1 shows schematically an embodiment of the present invention.

Fig. 2 shows schematically another embodiment of the invention.

Fig. 3 shows schematically a third embodiment of the invention.

Fig. 4 shows schematically a fourth embodiment of the invention.

Detailed description of the invention

The invention is now described in more detail with reference to the accompanying drawings. In figure 1 is shown a first embodiment of the present invention. The engine group comprises three internal combustion engines la, lb, lc. The engines la, lb, lc can be for instance large compression ignition internal combustion engines that are used for producing electricity at a power plant. The engine group could also be used in a ship. For example, one of the engines la, lb, lc could be an auxiliary engine of a ship and the other two main engines. The first engine la of the engine group is operated on a fuel having low ash and sulfur content, such as light fuel oil (LFO). Other suitable fuels can be for instance marine gas oil (MGO), marine diesel oil (MDO) or other distillate fuels. Also many gaseous fuels can be used. The range of suitable fuels depends for instance on the components used in the engines la, lb, lc and the operating conditions. According to an embodiment of the invention, the first engine la is operated on a fuel having sulfur content less than 0.05 % (500 ppm) as a percentage by weight. The second engine lb and the third engine lc are operated on a cheaper, lower quality fuel that can contain higher amount of sulfur and ash, such as heavy fuel oil. For reducing NOx emissions of the engines la, lb, lc, the engines la, lb, lc are provided with an exhaust gas recirculation (EGR) arrangement. The EGR arrangement recirculates part of the exhaust gases of the engine group back into the cylinders 11a, l ib, 11c of the engines la, lb, lc. The recirculated exhaust gases lower the cylinder temperatures and reduce thus NOx emis- sions.

Each engine la, lb, lc is provided with an exhaust duct 9a, 9b, 9c and an intake duct 10a, 10b, 10c. The intake duct 10a, 10b, 10c of each engine la, lb, lc is provided with a charge air cooler 5a, 5b, 5c. A turbocharger 2a, 2b, 2c comprising a turbine 3a, 3b, 3c and a compressor 4a, 4b, 4c is arranged in connection with each engine la, lb, lc for pressurizing the intake air. For recirculating part of the exhaust gases, the EGR arrangement is provided with an exhaust gas recirculation duct 12, which comprises three branches 12a, 12b, 12c for connecting the duct 12 to the air intake ducts 10a, 10b, 10c of the engines la, lb, lc. In the embodiment of figure 1, the branches 12a, 12b, 12c of the exhaust gas recirculation duct 12 are connected upstream from the compressors 4a, 4b, 4c of the turbochargers 2a, 2b, 2c. Since sulfur and other contaminants of the exhaust gases can cause corrosion and other damage in an engine and in the parts of the air intake system, such as turbochargers and charge air coolers, exhaust gases from engines operating on lower quality fuels cannot be used for EGR without cleaning. Therefore, the exhaust gas recirculation duct 12 is connected to the exhaust duct 9a of the first engine la, which is operated on a fuel having low sulfur content. In the embodiment of figure 1, the exhaust gas recirculation duct 12 is connected to the exhaust duct 9a of the first engine la downstream from the turbine 3a of the turbocharger 2a of the first engine la.

The exhaust gas recirculation duct 12 is provided with an exhaust gas cooler 7 for cooling the recirculated exhaust gases. Each branch 12a, 12b, 12c of the exhaust gas recircu- lation duct 12 is provided with an exhaust gas recirculation valve 6a, 6b, 6c for regulating the amount of the recirculated exhaust gases. The exhaust gas recirculation duct 12 is provided with a main EGR valve 14 for preventing the exhaust gas recirculation when needed. A throttle valve 8 is arranged in the exhaust duct 9a of the first engine la downstream from the point where the exhaust gas recirculation duct 12 is connected to the exhaust duct 9a. The throttle valve 8 helps to create sufficient pressure difference between the exhaust duct 9a of the first engine la and the intake ducts 10a, 10b, 10c for allowing the exhaust gases from the first engine la to easily flow into the intake ducts 10a, 10b, 10c of the engines la, lb, lc. The embodiment of figure 2 is similar to the embodiment of figure 1. The main difference between the two embodiments is that in the embodiment of figure 2, the recirculated exhaust gases are introduced into the intake ducts 10a, 10b, 10c of the engines la, lb, lc downstream from the compressors 4a, 4b, 4c of the turbochargers 2a, 2b, 2c. Since a higher pressure is needed for mixing the recirculated exhaust gases with the pressurized intake air, the exhaust gas recirculation duct 12 is provided with a compressor 13 for increasing the pressure of the recirculated exhaust gases. In this embodiment, there is no separate exhaust gas cooler. Instead, the exhaust gas - air mixture is cooled in the charge air coolers 5a, 5b, 5c of the engines la, lb, lc. In the embodiment of figure 3, the recirculated exhaust gases are introduced into the intake ducts 10a, 10b, 10c downstream from the compressors 4a, 4b, 4c of the turbo- chargers 2a, 2b, 2c, as in the embodiment of figure 2. Also in this embodiment, the exhaust gas recirculation duct is provided with a compressor 13 for increasing the pressure of the recirculated exhaust gases. Instead of the compressor 13, also a fan could be used for assisting the exhaust gas recirculation flow. A difference to the embodiments of figures 1 and 2 is that the exhaust gases are taken for the recirculation from the exhaust duct 9a of the first engine la upstream from the turbine 3a of the turbocharger 2a.

In the embodiment of figure 4, the exhaust gases are taken for the recirculation from the exhaust duct 9a of the first engine la upstream from the turbine 3a of the turbocharger 2a like in the embodiment of figure 3. The recirculated exhaust gases are introduced into the intake ducts 10a, 10b, 10c upstream from the compressors 4a, 4b, 4c of the tur- bochargers 2a, 2b, 2c. The exhaust gas recirculation duct 12 is provided with a throttle valve 8 for lowering the pressure of the recirculated exhaust gases before introducing them into the intake ducts 10a, 10b, 10c.

It will be appreciated by a person skilled in the art that the invention is not limited to the embodiments described above, but may vary within the scope of the appended claims. For instance, it is obvious that the number of the engines forming the engine group can be different. It is also possible that a separate exhaust gas recirculation duct is arranged for each engine of the engine group for connecting the intake ducts to the exhaust duct of the first engine. Especially in a larger engine group, exhaust gases from more than one engine could be used for the exhaust gas recirculation. The engines can also be provided with two-stage turbocharging, where each engine of the engine group is provided with two turbochargers connected in series. In that case, exhaust gases for recirculation can be taken either upstream or downstream from both of the turbochargers or from between them. Likewise, the exhaust gases can be introduced into the intake ducts either upstream or downstream from both of the turbochargers or between them.