Nitrogen oxides (NOx) are formed in high combustion temperatures in the combustion chamber of a cylinder of a piston engine, the nitrogen oxides being released into the atmosphere. Because the nitrogen oxide emissions are detrimental to environment there is a tendency to try and minimise the emissions.

It is known that adding water to the combustion together with the combustion air reduces the formation of nitrogen oxide emissions. This phenomenon is based, among others, on the cooling effect of water. In practice, the introduction of water into the combustion chamber of a piston engine is carried out in three alternative ways so that water is introduced directly into the combustion chamber of the engine, water is introduced with fuel as fuel emulsion or water is introduced via inlet duct along combustion air.

Publication US 5,758, 505 discloses a method for introducing water into the combustion air subsequent to the supercharger. The water is first heated with the engine cooling water and fed into a separate moistening column, where the water, sprayed into mist, is vaporised. Such an arrangement is impractical due, among others, the space requirement of the moistening column and it also causes a certain safety risk, as the large moistening column is operated under high pressure.

Introducing water into the combustion chamber with the combustion air is preferable, because it does not reduce the shaft efficiency of the engine. However, the temperature of the combustion air must in some conditions be elevated to a relative high level in order to maintain the air humidity on a level high enough for reducing the nitrogen oxides. The maximum amount of water introduced in

vaporised form into the combustion chamber can therefore be the amount that stays in gaseous form in the pressure and temperature conditions of the combustion air, and therefore a solution, preferable in its entirety, is needed for increasing and maintaining the temperature of the combustion air.

The aim of the invention is to provide an improved method and arrangement of reducing the nitrogen oxide emissions of a supercharged piston engine, but with the drawbacks of prior art essentially eliminated. In more detail the aim if the invention is to provide a method and arrangement for increasing and maintaining the inlet air temperature at a suitable level, higher than the ambient temperature.

The aims of the invention are mainly achieved as disclosed in the appended claims 1 and 9 and disclosed in more detail in other claims.

In a method according to the invention for reducing the nitrogen oxide (NOx) emissions of a supercharged piston engine the pressure of the combustion air is during the operation of the engine elevated to a level higher than that of the ambient pressure by means of a compressor apparatus and, further, the moisture level of the combustion air is increased and, additionally, heat is introduced into the combustion air prior to feeding it to the combustion chamber of the engine. The heat which is introduced into the combustion air is obtained from the exhaust gases of the engine i. e. the heat is transferred to the combustion air.

Preferably heat is transferred into the combustion air in a temperature approximately corresponding to the temperature of the exhaust gases prior to feeding them into the turbine part of the compressor apparatus. Thus, the temperature is as high as possible and the combustion air is heated by means of a heat exchanger having a relatively small area.

According to one embodiment the exhaust gas flow of the engine is divided into two parts so that a partial flow is separated therefrom prior to directing the exhaust gases into the turbine part of the compressor apparatus, whereby the heat transferred into the combustion air is taken from the separated partial flow. The separated partial flow is combined with the exhaust gas flow subsequent to the

turbine part of the compressor apparatus. The efficiency of the heat transfer is controlled by adjusting the volume of the partial flow, which in turn is controlled by adjusting the throttling of the partial flow. The volume of the partial flow is controlled according to the load of the engine so that the volume of the partial flow is proportional to the load of the engine. For example, with very light loads the volume of partial flow is small.

Preferably the moisture of the combustion air is increased in a number of stages, but preferably at least a part of that is carried out subsequent to heat being transferred from the exhaust gases to the combustion air.

According to another embodiment heat is transferred from the engine exhaust gases by means of a heat exchanger, arranged in the exhaust channel, first to a medium from which heat is transferred further to the combustion air by means of the heat exchanger arranged in the combustion air channel.

In a piston engine arrangement according to the invention comprising a number of cylinders, a turbocharger comprising a compressor part arranged in flow connection with each cylinder through the combustion air channel and a turbine part arranged in flow connection with each cylinder through the exhaust gas channel, there is a nozzle arrangement for introducing water to the combustion air channel and a heat exchanger arranged in heat transfer connection with the combustion air channel. The said heat exchanger is in heat transfer connection with the exhaust gases. Thus, in the arrangement the heat source is within the same apparatus, very close, whereby the solution is a compact one.

In the arrangement the heat exchanger is arranged so that it is in flow connection with the exhaust gas channel prior to and subsequent to the turbine part of the turbocharger, thereby the pressure difference caused by the turbine part can be used for exhaust gas flow via the heat exchanger as well. The flow connection between the heat exchanger and the exhaust gas channel is arranged by means of a channel and a control valve attached thereto, whereby the volume of flow can be adjusted by throttling and opening the said control valve.

According to another embodiment the heat exchanger is connected to a separate heat exchanger medium circuit, in which a separate heat exchanger medium flows and which comprises a second heat exchanger arranged in the exhaust gas channel.

Many advantages can be achieved by means of the invention, such as a considerably more effective reduction of nitrogen oxides from the exhaust gases.

Further, the invention provides a space-saving solution effectively utilising the heat loss of the engine is achieved.

In the following the invention is described by way of example and with reference to the appended drawings, of which figure 1 schematically illustrates an embodiment according to the invention, figure 2 schematically illustrates another embodiment according to the invention, and figure 3 schematically illustrates a third embodiment according to the invention.

In the figures, reference number 1 refers to a piston engine connected to a turbocharger 2 in a way known as such. A fuel injector nozzle 7 is arranged in connection with each cylinder for introducing fuel into the engine. A nozzle arrangement 4 for introducing water into the combustion air is arranged into the air channel 3 connecting the turbocharger 2 and the air channel 3, by means of which air is fed to the engine.. A corresponding nozzle arrangement 4'is also arranged prior to the compressor of the turbocharger. The nozzle arrangement 4,4'is connected to the water system, shown in the figures with reference number 10.

Depending on the application it will also comprise a water source and it can also comprise a water pre-heater apparatus, the heat source of which preferably is the heat generated in the engine. A heat exchanger arrangement 5 is also arranged in the air channel 3 for increasing the temperature of the combustion air. The channel can comprise other heat exchangers as well. The channel 3 is connected to the air chamber 8 of the engine 1, wherefrom air is introduced into each cylinder of the engine via channel 3. The figures also show a droplet separator 6 used in the arrangement, if necessary. The nozzle arrangement 4,4'is used for increasing the

water content of the combustion air prior to introducing combustion air to the combustion chamber of the engine. The amount of water introduced into the air is preferably selected so that the air is essentially saturated. Typically the amount of water introduced with the combustion air is about 0.5 to 5 times the amount of fuel.

According to the invention water is introduced into the air channel 3 as a liquid.

Thus, the vaporisation of water in the air channel 3 uses the heat of air, and to avoid the temperature of the air falling too low and for simultaneously being able to vaporise a sufficient amount of water into the air, heat is introduced into the engine combustion air flowing in the air channel by means of a heat exchanger arrangement 5.

In the embodiment of figure 1 in the exhaust gas channel 7 of the engine, prior to the point wherefrom the exhaust gases are directed to the turbine part of the compressor apparatus 2, there is arranged a branch channel 8 through which the partial flow of the exhaust gas flow is directed to flow via the heat exchanger arrangement 5. Thus, a part of the heat energy of the exhaust gas coming from the engine combustion chamber can be transferred to the combustion air and thus such a combustion air temperature can be maintained that a sufficient amount of water can be vaporised therein and that a sufficient amount of water stays in vapour phase. Thus, the energy of the exhaust gases is used for vapourizing the water in combustion air. The branch channel 8 is thus in connection with the heat transfer apparatus 5, through which hot exhaust gas is arranged to flow. The branch channel is arranged to join back to the exhaust gas channel 7 subsequent to the turbine part of the compressor apparatus, and it is provided with a valve apparatus 11 for controlling the volume of the partial flow. The flow flowing through the branch channel 8 is initiated by the pressure difference caused by the turbine part of the compressor apparatus 2, and the volume of the flow can be adjusted by means of controlling the position of the valve apparatus 11. Preferably, the control is carried out on basis of the current load of the engine and also on the comparison between the measurement value and the set value of the combustion air temperature.

Figure 2 illustrates an embodiment otherwise corresponding to that shown in figure 1, but in which the heat exchanger 5 is connected to a separate heat transfer medium circuit 5'comprising a second heat exchanger 5.1 arranged in the exhaust gas channel. In such an arrangement the exhaust gases transfer heat first to the heat transfer medium flowing in the heat transfer medium circuit 5'. The heated heat transfer medium then transfers its heat to the combustion air and that way to the water introduced in the combustion air as the water vaporises.

In the embodiment shown in figure 3 the arrangement comprises both a heat exchanger 5 in the exhaust gas channel for directly transferring heat from the exhaust gas and a second heat exchanger 5, connected to the separate heat transfer medium circuit 5'further comprising a second heat exchanger 5.1 arranged in the exhaust gas channel. Thus, the embodiment of figure 3 combines the solutions of figure 1 and 2.

Preferably, the amount of water introduced into the combustion air is adjusted on the basis of the load of the engine, of which the pressure of the combustion air, for example, could be a suitable indicator. The amount of water introduced into the combustion air is adjusted by opening and closing a suitable amount of water injection nozzles 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 invention is not limited to the embodiments described here, but a number of modifications thereof can be conceived of within the scope of the appended claims.