The present invention relates to a method for reducing nitric oxi emissions in the exhaust gases from diesel engines by means of co ducting water into the engine and by recirculating the exhaust gases.

A drawback in the use of combustion engines such- as diesel engin arises in the form of harmful compounds contained:; in- exhaust gase Among them , the most dangerous and the most difficult o eliminate, a nitric oxides. The amount of created nitric, oxides is .cosraparabte.to th temperature prevailing in the combustion, chamber ,-- as,' ell- as. to th oxygen content of the explosion gas.

A previously known method for reducing the amount of nitric oxides i to lower the combustion temperature by adding water into the combus tion chamber of the diesel engine. The water addition can be carrie out in various ways : according to the DE Patent Application No. 31 4 472 , water is sprayed separately into the combustion chamber. In con nection to the water addition , the water condensed from the exhaus gases can also be utilized; this water is conducted into the combustio chamber by means of spraying , as is the case with water additions i the apparatus introduced in the DE Patent Application No. 28 51 232 The supplied water is fed directly into the suction duct (DE Paten Application No. 28 31 694) , or the water is first dispersed in a fine form by feeding the fresh combustion air through a liquid volume an further into the suction duct (DE Patent Applications No. 26 01 45 and 26 33 639) . The water to be conducted into the combustion cham ber can also be first evaporated by means of a thermal exchanger, s that the heating is carried out by conducting the hot exhaust gase through a water container. The resulting steam is then conducted int the suction duct, whereas the exhaust gases are discharged into th open air. The evaporation method is introduced for instance in the S Patent Publication No. 428 587 and DE Patent Applications No. 21 5 755 and 26 04 050.

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The drawback in the aforementioned methods for feeding water into the combustion chamber is the fact that the purchase expenses of the equipment needed for adjusting the water supply are very high. More¬ over, its maintenance requires special care, which partly also in¬ creases the operational costs.

Another prior art method for diminishing the nitric oxide emissions of a diesel engine is partial recirculation of the exhaust gases. In certain prior art methods , for instance in those introduced in the SE Patent Publication No. 404 068 and DE Patent Application 2 443 897, the ex¬ haust gas is recirculated back into the engine in dry form. The recir¬ culated exhaust gas , the quantity whereof is advantageously 20% of the suction air volume, functions as an inert gas and thus reduces the oxygen content of the gas contained in the combustion chamber. Simul¬ taneously the production of nitric oxide is also diminished . However, the regulation of recirculated exhaust gases is also difficult to control . Moreover, it is preferable that the recirculated exhaust gases are cooled off for instance in order to reduce thermal strain.

The method of recirculating damp exhaust gases is utilized in the fuel evaporator according to the US Patent No. 2 430 852. In that case, however, the recirculated damp exhaust gases are fed, together with the gasified fuel , into the suction space of a combustion engine. Con¬ sequently , the suggested arrangement cannot be applied for instance for diesel engines, where the fuel is sprayed into the combustion chamber.

In an apparatus according to the DE Patent Application 2 504 308, a part of exhaust gases is conducted to the wet cleaner wherefrom the washed and cooled exhaust gases are recirculated . The amount of these exhaust gases recirculated according the DE Application is in no way maintained constant and this recirculation is not utilized , either, when the motor is operating with idling or with full load . Further, a safety valve is required in the apparatus of the DE Patent Application 2 504 308 and in that apparatus distilled water is used and the deliv¬ ery of that kind of water is in many cases difficult.

The object of the present invention is to eliminate some of the draw backs of the prior art and to realize an improved , more secure metho for reducing the nitric oxide emissions of diesel engine exhaust gases by means of applying the recirculation of cooled exhaust gase combined to the aqueous vapours resulting from the wet cleaning o the exhaust gases. The essential novel features of the invention ar apparent from the appended patent claim 1 .

According to the invention , the exhaust gases from the engine ar conducted through a catalyzer, advantageously for reducing th amount of carbon monoxide and hydrocarbons , before directing th gases into the wet cleaner. In the wet cleaner, the exhaust gases ar simultaneuously washed and cooled . The cooled exhaust gases are fur ther conducted , humidified by the vapours created in the washing , back into the suction duct of the diesel engine. The recirculated ex haust gas , the quantity whereof is about 20% of the whole exhaust ga volume , is mixed in the general suction air flow by means of a mixe valve and a nozzle, whereby the mixture ratio can be maintained con stant irrespective of the charge and the speed of rotation .

When employing the method of the invention , the recirculation of the exhaust gases , the cooling of the recirculated exhaust gases , and th water additions into the combustion chamber , as wel l as the regulation of the volume of the returned gas, can thus all be combined.

In the following the invention is explained with reference to the ap¬ pended drawing , where Figure 1 is a schematical illustration of a preferred embodiment of the invention , and Figure 2 illustrates the superiority of the damp recirculation method of the invention , as compared to the prior art dry recirculation method.

According to figure 1 , the exhaust gases from the diesel engine 1 are conducted , through the catalyzer 2 , into the wet cleaner 3. In the wet cleaner 3, the exhaust gases are cooled and washed . The major part of the exhaust gases are discharged through the exhaust pipe 4. The

damp exhaust gas to be recirculated according to the invention is con¬ ducted, through the return pipe 5, into the suction system 10 of the diesel engine. The recirculated exhaust gas supplied from the return pipe 5 , which gas is humidified by the steam created in the cleaner 3, is conducted, via the recirculation valve 6, back to the recirculation nozzle 7. The suction air of the diesel engine, drawn in through the air purifier 8 , is also conducted to the nozzle.7. The. mixture of recir¬ culated exhaust gas and pure suction air thus created - tn the nozzle 7 is further conducted , via the suction duct TO? of the-:- diesel- engine into the combustion chamber 9, wherefro ^" the created? .'-exhaust gases are further conducted to the catalyzer 2. By means", of ^' the- recircula¬ tion valve 6 and the nozzle 7, there is achieved an. advantageous reg¬ ulation of the volume of the recirculated exhaust gas.,; -so that a profit¬ able mixture ratio between the suction air and the- recirculated exhaust gas is maintained irrespective of any possibTe changes- in the charge or in the speed of rotation.

In figure 2 , where the symbol NO refers to ^' the sum of the nitric oxides NO and NO- ^' , the damp recirculation of the exhaust gases according to the invention is compared to the prior art dry recircula¬ tion , when the recirculated portion is 20% of the total exhaust gas vol¬ ume. The nitric oxide content curves, drawn in figure 2, are illus¬ trated as a function of the mean pressure of the engine. It is appar¬ ent from figure 2 that while employing the method of the invention (curves A) , the total amount NO of the nitric oxides is essentially reduced compared to the so-called dry recirculation method (curves B) of the prior art. The superiority of the damp recirculation method is also obvious when comparing the content of curves A to the curves C representing such prior art method where water is conducted into the diesel engine, but the exhaust gases are not recirculated. According to curves A, the ^{^} total amount NO of the nitric oxides is reduced , and the component NO- , considered to be the most dangerous among the nitric oxides , is almost completely eliminated from the exhaust gases when applying the method of the invention , while the mean pressure of the engine is increased.