ARRANGEMENT AND METHOD IN CONNECTION WITH A PISTON ENGINE HAVING A TURBOCOMPRESSOR

The present invention relates to an arrangement in connection with a piston engine having a turbo-compressor.

The present invention also relates to a method used in connection with a piston engine having a turbo-compressor.

In engine technology it is known to use turbo-compressors for increasing engine power. A turbo-compressor comprises a compressor part by means of which pressurized combustion air is fed to the engine. A turbo-compressor further comprises a turbine part that runs the compressor. The exhaust gases from the engine are directed to the turbine that transforms the energy of the exhaust gases into compressor drive power.

Nitrogen oxides (NOx) are formed in high combustion temperatures in the combustion chamber of a cylinder of a piston engine, the nitrogen oxides being conveyed into the atmosphere with the exhaust gases. Due to the negative environmental effects of the nitrogen oxide emissions it is desirable to avoid their formation (primary methods) or formed nitrogen oxides are removed from the exhaust gases (secondary methods).

Adding water with the combustion air to the actual combustion reduces the nitrogen oxide emissions. Water reduces the combustion temperature in the cylinder, whereby the amount of nitrogen oxides formed is reduced. In practice, adding water to the combustion process of a piston engine is carried out in two alternative ways: Water can be injected either directly into the combustion chamber of a cylinder or it can be mixed with the combustion air prior to introducing it into the cylinder. In both cases the injection of water into the combustion air causes an increase in the mass flow of the exhaust gas passing through the turbine. The increase of

the exhaust gas mass flow increases the rotation speed of the turbine causing the risk of overspeeding. Excessive increase of the rotation speed can be avoided by dimensioning the turbine for a larger mass flow, but thereby the operation of the turbo-compressor is not optimal in situations in which moistening of the combustion air is not activated.

The object of this invention is to provide a solution in which the rotation speed of the turbine of the piston engine can be controlled when the moistening of the combustion air is in use.

The invention is based on that the exhaust gas directed from the piston engine to the turbine is cooled by mixing water into it while moistening the combustion air.

More specifically, the invention is characterized by what is disclosed in the characterizing part of claim 1.

On the other hand, the arrangement according to the invention is characterized by what is disclosed in the characterizing part of claim 7.

Considerable advantages are achieved by means of the invention.

Cooling of the exhaust gas directed to the turbine compensates the turbine rotation speed increasing effect of the moistening of the combustion air, whereby the running of the turbine at too high a rotation sped can be prevented. A turbine optimized for non-moistened conditions can be used in connection with the engine. Cooling of the exhaust gas further reduces the thermic loading of the components of the turbine. In engines installed in ships, so-called grey waste water, i.e. water that has been used for e.g. washing, washing dishes, cooking and for laundry, can be used as exhaust gas cooling water. Thus, the operating expenses of exhaust gas cooling are low. Further the arrangement according to the invention can be

made as a simple construction and it can also easily be arranged in connection with the engine.

In the following, the invention is described in an exemplary way, with reference to the appended drawing, schematically showing one arrangement according to the invention.

The drawing illustrates a piston engine 1 having a turbo-compressor 2. The turbo- compressor 2 comprises a compressor 3 and a turbine 4 connected to each other via a drive shaft 5. The drive shaft 5 is supported by bearings on the body of the turbo-compressor 2. The task of the compressor 3 is to pressurize air and introduce it as combustion air for the engine 1. The compressor 3 comprises a rotatable rotor having blades for pressurizing the combustion air to be introduced to the engine 1. A flow channel 6 is arranged on the high-pressure side of the compressor 3, i.e. between the compressor 3 and the combustion chambers 10 of the engine for introducing the pressurized combustion air into the cylinders 10. A heat exchanger 7 is arranged into the flow channel 6 for cooling the combustion air. The flow channel 6 further comprises a charge air receiver 8 arranged at a place subsequent to the heat exchanger 7 in the flow direction of the combustion air. The flow channel 6 also comprises inlet channels 9 arranged between each cylinder 10 and the receiver 8 for introducing combustion air from the receiver 8 to the cylinders 10.

An exhaust gas channel 1 1 is arranged between the cylinders 10 and the high- pressure side of the turbine 4 for directing the exhaust gas from the engine 1 to the turbine 4. The first end of the exhaust gas channel 1 1 is attached to the turbine 4. The exhaust gas channel 1 1 is arranged parallel with the line formed by cylinders 10. The second end of the exhaust gas channel 1 1 extends to the vicinity of the cylinder most distant from the turbine 4. Each cylinder 10 is connected to the exhaust gas channel 1 1 by means of a branch tube 17. The turbine 4 comprises a rotor having blades, the rotor being rotated by exhaust gas from the engine 1. The

engine 1 additionally comprises fuel feed means (not shown) for introducing fuel into the cylinders 10.

The engine 1 is provided with a combustion air moistening apparatus by means of which the moisture content of the combustion air can be increased while the engine 1 is running. Moistening the combustion air reduces the amount of nitrogen oxides (NOx) formed in the combustion. The combustion air is moistened by spraying water to mix with the combustion air. Water is mixed with the combustion air in the flow channel 6 prior to introducing combustion air into the heat exchanger 7. The moistening apparatus of the combustion air comprises a feed tube 12 connected to the water treatment system. Injectors 15 opening into the flow channel 6 are connected to the feed tube 12, from which injectors water is introduced in drop or mist form into the flow channel 6 to a point prior the heat exchanger 7 in the flow direction of the combustion air. The feed tube additionally comprises a valve 16 by means of which the flow of water to injectors 15 can be allowed or prevented. The combustion air can alternatively be moistened by means of an apparatus feeding water directly to the cylinders 10.

The turbine 4 comprises feed means 18 for introducing cleaning agent into the exhaust gas flow channel located in the turbine 4. The cleaning agent is used for removing carbon deposits and/or other impurities accumulated on the surfaces of the turbine 4. Carbon is removed specially from the surface of the nozzle ring located in the turbine 4 and from the flow channel enveloping the rotor blades. The nozzle ring comprises vanes deflecting the flow direction of the exhaust gas so as to suit the rotor blades and for increasing the flow speed of the exhaust gas. The cleaning agent feed means 18 comprise nozzles from which the cleaning agent is injected into the exhaust gas flow channel to a place before the nozzle ring and an arrangement for directing cleaning agent to the nozzles. The arrangement disclosed in US Patent 5,944,483 can, for example, be used as feed means 18 of the cleaning liquid, the arrangement allowing the cleaning 4 with wet cleaning method, i.e. either with a heat shock type of cleaning or with a wash-type cleaning.

The exhaust gas to be directed into the turbine 4 can be cooled by mixing water into it by means of feed means 14 arranged in connection with the exhaust gas channel 11. The feed means comprise a tube 14 opening into the exhaust gas channel 11. Preferably the tube 14 opens into a place at the exhaust gas channel 11 being between the branch tube 17 nearest to the turbine 4 and the second end of the exhaust gas channel 1 1. In an embodiment according to the drawing the tube 14 opens into the place between the two branch tubes 17 located farthest away from the turbine 4. The tube 14 is provided with a valve 13 for allowing or preventing the injection of water into the exhaust gas channel 1 1 for adjusting the injected amount of water. A nozzle is provided at the end of the tube 14 for forming the water to be introduced into the exhaust gas channel for facilitating vaporizing. Water is introduced at such a distance from the turbine 4 that the amount of water to be injected has time to totally or essentially totally vaporize or before arriving at the turbine 4.

As the engine runs, combustion air is directed into the compressor 3 for increasing its pressure above ambient pressure. The pressurized air is directed into the flow channel 6 located in the high-pressure side of the compressor 3. Water is injected into the air in the flow channel 6 by means of injector nozzles 15. The amount of the water to be mixed with the combustion air is adjusted to be such that the combustion air is saturated or essentially saturated. The amount of water introduced into the combustion air is adjusted by opening and closing a suitable amount of water injection nozzles 15 as necessary, while the injection pressure at each nozzle is about constant. Thus, both a preferable atomising or misting of the water and a correct amount of water in the combustion air, are achieved.

The moistened combustion air is directed to the heat exchanger 7 and warmed or cooled by means of a heat exchanger 7 into a temperature of typically about 70 - 75 °C. Combustion air can be heated or cooled by means of a low-temperature cooling liquid of the engine 1 directed into the heat exchanger 7. Subsequent to

heating/cooling the combustion air is directed into the charge air receiver 8. From the receiver 8 the combustion air is directed along inlet channels 9 into cylinders 10. Fuel, such as heavy fuel oil, is injected into the cylinders 10, the fuel being combusted with combustion air in the combustion chambers 10 of the cylinders. The exhaust gas formed in the combustion is directed along the exhaust channel 1 1 to the turbine 4. As the flow traverses the turbine 4, it rotates the rotor of the turbine 4, the rotation of which is transmitted via drive shaft 5 to the rotor of the compressor 3.

When the combustion air is being moistened, i.e. when water is injected from the feed tube 12 to mix with the combustion air, water is simultaneously directed from the tube 14 to the exhaust gas channel 1 1. Water is introduced into the exhaust gas channel 1 1 , especially when the engine 1 is running at full speed or at nearly full speed. Water is vaporized in the exhaust gas channel 1 1 and it travels along with the exhaust gas to the turbine 4. The injection of water 1 1 is begun simultaneously with the moistening of the combustion air. When the moistening of the combustion air is ended the introduction of water into the exhaust gas channel 1 1 is stopped as well.

Introduction of water into the exhaust gas channel 1 1 reduces the temperature of the exhaust gas being directed to the turbine 4. This will prevent an increase of the rotation speed of the turbine 4, caused by the increase of mass flow due to moistening the combustion air. The mass flow of the water to be introduced into the exhaust gas channel 1 1 is determined case by case to suit each application and the load conditions of the engine. The mass flow of the water to be introduced into the exhaust gas channel 1 1 is adjusted so that the increase of the rotation speed of the turbine 4 is compensated wholly or essentially wholly. The mass flow of the water to be introduced into the exhaust gas channel 11 is adjusted by means of valve 13. The valve is controlled by the engine control system on the basis of the rotational speed of the turbine 4.

When it is desired to remove the carbon deposit and other impurities accumulated in turbine 4, the turbine cleaning sequence is started. Thus, cleaning agent, such as water, is injected into the turbine 4 while the engine 1 is running. Cleaning agent is conveyed by the injection pressure and with the exhaust gas flow to the surfaces having carbon deposits, such as the surface of the nozzle ring and the surface of the exhaust gas channel surrounding the rotor. The cleaning effect of the cleaning agent can be either of the washing type or the heat shock type. In a washing type cleaning the cleaning effect is based on the mechanical effect of the cleaning agent droplets and the dissolution of dirt matter in the cleaning agent. In heat shock type of cleaning the cleaning effect is based on the different heat expansion properties of the dirt layer and its gripping surface and the release effect due to sudden cooling.

In order to achieve an optimal cleaning result the temperature of the exhaust gas to be directed to the turbine 4 must be at a suitable level during the cleaning sequence. The temperature of the exhaust gas going to the turbine 4 is measured with a measurement device located at the exhaust gas channel 11. In a washing type cleaning the temperature of the exhaust gas being directed to the turbine 4 must typically be under 450 -C and in heat shock cleaning the temperature must be under 500 -C. In heat shock cleaning the temperature of the exhaust gas must, however, be over 430 -C. As the engine 1 runs at full speed, the temperature of the exhaust gas arriving at the turbine 4 is typically 530 - 600 -C, and therefore the power of the engine must be lowered for reducing the temperature of the exhaust gases or the exhaust gases must be cooled during the cleaning sequence. The cleaning of the turbine 4 can be carried out when the engine 1 runs at full speed or at nearly full speed, when the water injection into the exhaust gas channel 1 1 is in use. Thus, the amount of water injected from the tube 14 into the exhaust gas channel 11 is adjusted so that the temperature of the exhaust gas going to the turbine 4 is suitable for the cleaning method. The cleaning of the turbine 4 can be carried out while the combustion air is being moistened.

Ships are usually provided with separate waste water systems for so-called grey waste waters (water used e.g. for washing, washing dishes, cooking and for laundry) and brown waste water (waste water from the toilets). In engines used in ships the exhaust gas is cooled by mixing it with the grey waste water of the ship. The cooling water of the exhaust gas can partly be grey waste water and partly other water or grey waste water only.