It is a known fact that it is not possible to provide piston ring sealing between the pistons and the surrounding cylinder walls in an internal combustion engine which 100% seals off the combustion chambers from the crank case of the engine. A certain small quantity of combustion gases, here called blow-by, therefore always flows past the piston ring and down into the crank case of the engine. In order that a too high overpressure partially caused by the blow-by gases will not occur in the crank case the crank case must be ventilated and the more effective the ventilation is then the lower the overpressure in the crank case is and therefore the lower the engine pumping losses become.

In modem engines closed crank case ventilation is used in order that the environmental effects shall be as small as possible. Normally, the blow-by gases are led out from the crank case via a hose to the inlet manifold of the engine before the throttle and mixes with the intake air. In order to separate oil out of the oil mist which unavoidably is mixed with the blow-by gases, different types of filters and oil traps are used in the crank case ventilation. All hitherto known crank case ventilation systems have it in common that they have not made it possible to minimize a certain overpressure in the crank case which becomes higher the higher the power demand is. This also means that a much higher pressure exists in the crank case of the engine than in the combustion chamber during the intake stroke and this pressure attempts to press the oil mist into the crank case past the oil

scraper ring of the piston and into the combustion chamber of the engine. In order to prevent in the greatest possible extent the oil flow to the combustion chamber the ring tension must be high for the oil scraper ring and the oil scraper ring is therefore the component which alone causes the greatest internal friction in the engine. The oil which even so unavoidably penetrates into the combustion chamber of the engine does not just cause pollution in the engine exhaust gases with consequential strain on the catalyzer. It also lowers the octane number of the fuel which in modem engines with knock sensors and automatic ignition advance leads to a retarding of the ignition with consequential increased fuel consumption. Last but not least the oil consumption of the engine itself and the costs of replacing used oil are directly dependent on how much oil penetrates into the combustion chamber because of the pressure difference between the crank case and the cylinder space above the piston.

An object of the present invention is generally to produce an engine of the type mentioned in the introduction in which the difference between the pressure in the crank case of the engine and the pressure in its intake manifold, that is the pressure difference which strives to press the lubricating oil past the piston rings and into the combustion chamber during the intake stroke of the engine during all operating conditions is lower than in hereto known engines in order to minimize the engine pumping losses and oil consumption.

This is achieved according to the invention through the cylinder block being designed with an evacuation channel for each cylinder, which channel opens out in the cylinder barrel and forms a communicating connection between the cylinder barrel and the intake channel and that the outlet of the evacuating channel and the piston are so adapted to each other that the piston holds the evacuation channel open in order to maintain the connection between the crank case and the intake channel during the movement of the piston from its upper dead centre to a position at a predetermined distance from upper dead centre and to thereafter break the connection of the evacuation channel with the crank case by its continued movement down to bottom dead centre.

In a preferred embodiment of an aspirating engine according to the invention the evacuation channel opens out directly into the inlet channel in the cylinder head.

The piston is in its basic construction a conventional cylindrical piston which according to the invention, on the side which is facing towards the opening of the evacuation channel is provided with a shield which forms a screening-off towards the opening of the evacuation channel in order to limit the intrusion of oil splashes.

The screen has a greater clearance towards the cylinder barrel wall than the piston cylinder so that the crank case, via the gap formed through the greater clearance, and the evaucation channel are joined with the inlet channel during a predetermined part of the path of movement of the piston.

The design according to the invention makes it possible to maintain very closely the balance between the pressures in the combustion chamber and the crank case. This means that an underpressure is present in the crank case during the inlet stroke. The pressure difference over the piston rings becomes so negligible that the ring tension in the oil scraping ring can be reduced to a fraction of what is normal today without the risk of oil penetration from the crank case to the combustion chamber. In addition to the direct effects which the pressure balancing gives in the shape of lower oil and fuel consumption, important secondary effects are attained. Lower ring tension, which gives a lower interal friction with consequential lower fuel consumption leads to lower starting power for the starting motor, i. e. smaller starting motor and starting battery. A smaller quantity of pollution in the exhaust gases caused by oil in the combustion chamber means a lower load on the catalyzer which can be made smaller. Finally, the need for external components such as oil traps and hoses with associated connection parts can be eliminated which leads to important cost savings.

It is unavoidable that a small quantity of the blow-by gases, unburnt fuel and other pollutants which reach the crank case cannot be ventilated out but remain as a suspension in the oil in the crank case and contribute to an accelerated ageing

process of the oil and impairs its lubricating qualities. This in turn influences the length of the life of the engine.

A further development of the engine according to the invention which, in addition to further contributing to hold the pressure low in the crank case, also aims to reduce the quantity of pollutants in the engine oil, is characterized in that the skirt of the piston has a collection chamber between the piston ring groove for unburnt fuel-air mixture and combustion gases which pass the upper piston ring, and that the opening of the evacuation channel is so orientated in relation to the collecting chamber that, after a predetermined movement of the piston from its upper and lower dead centre, respectively, a communicating connection is maintained between the collecting chamber and the inlet channel via the evacuation channel. In this way unburnt fuel-air mixture and combustion gases are prevented from reaching the crank case. Instead they are ventilated out directly into the evacuation channel and flow to the inlet channel because an overpressure occurs in the collection chamber, while an underpressure occurs in the evacuation channel. Unburnt fuel-air mixture trapped under the first piston ring would otherwise flow back to the combustion chamber during the expansion stroke as soon as the cylinder pressure fell below the pressure of the mixture but this would occur too late for the mixture to be able to be burnt.

The invention is described more closely with reference to the embodiments shown on the accompanying drawings, where Figs. 1 and 2 show cross sections through a cylinder block of one embodiment of an engine according to the invention, Fig. 3 a cross section through a cylinder head of an embodiment of an engine according to the invention, Fig. 4 a schematic representation of the evacuation channel of a four cylinder aspirating engine and Fig. 5 a schematic representation of the evacuation channel of a four cylinder turbo-compressor charged engine.

In Figs. 1 and 2 1 denotes a cylinder block and 2 a cylinder barrel in which a piston 3 is displaceably mounted. The piston 3 is via a connecting rod 4 connected with a

crank shaft 5 which is mounted rotatably in the crank case 6 of the cylinder block 1.

A lower frame bearing bridge and an oil pan which together close the crank case are omitted from Figs. 1 and 2. In the cylinder block 1 between the wall 7 of the cylinder barrel 2 and an adjoining cooling water jacket 8 is an evacuation channel 9.

The channel 9 has a lower incline end part 20 which opens out into the wall 7 of the cylinder barrel 2. The second end of the evacuation channel 9 opens out into the inlet channel 11 in the cylinder head 12 of the engine (see Fig. 3). As is evident from Fig. 3 the evacuation channel 9 opens out into the inlet channel 11 relatively close to the inlet valve 13 in the combustion chamber 14. In the combustion chamber 14 there also opens out an exhaust channel 15 with an exhaust valve 16.

The piston 3 has a first and a second piston ring groove 17 and 18, respectively, for a pair of not shown compression rings and a third piston ring groove 19 for an oil scraper ring. The distance between the two piston ring grooves 17 and 18 is somewhat greater than that which is normal in pistons for a conventional multi- cylinder petrol engine. In the part of the piston between the piston ring grooves 17 and 18 is a further groove 20, the width and depth of which is considerably greater than the depth and width of the piston ring groove 17,18. In the embodiment shown the groove 20 is approximately three time as wide and twice as deep as the groove 17. The piston is further designed with a shield 21 on the side which is facing towards the opening of the evacuation channel 9. The axial extent of the shield 21 is approximately the same as the length of the piston 3 from the lower edge of the oil scraper groove to the lower edge of the piston. The smallest width of the shield 21 at the lower part 2 la is approximately four times the diameter of the evacuation channel 9. The width of the lower part may be chosen up to the greatest width of the upper part 21b, which may be up to approximately one sixth of the circumference of the piston. As is evident from Figs. 1 and 2 the shield is so arranged in relation to the skirt area of the piston that a gap 22 is formed between the wall of the cylinder barrel 2 and the shield wall 21c. The width of the gap 22 should be approximately a fourth of the diameter of the opening of the evacuation channel 9. However, the sizes should be chosen so that this gap 22 in combination with a stepped shield 21

are dimensioned so as to give the crank case vapour constricted passage into the evacuation channel 9, as well as preventing oil splashes from reaching the evacua- tion channel. When forming the shield 21 with a step there will be an upper portion with a small clearance between the shield surface and the cylinder wall and a lower portion with a larger clearance.

The piston 3 with the shield 21 functions as a moving valve element which connects the crank case with the inlet channel from the upper dead centre of the piston and approximately a half effective piston stroke. In this way the pressure difference between the crank case and the inlet channel is reduced. Closing of the evacuation channel causes a reduction of the inner cyclic pressure pulse effect in the crank case which otherwise would lead to an increased oil consumption through transferring of oil in suspension to the combustion chamber. The relatively low pressure in the crank case with a low and medium throttle opening also leads to reducing the negative effects of these inner pressure pulses which makes it possible to dimension the engine with a smaller crank case volume than that which has been common up to now.

All the evacuation channels 9 are pre-warmed continuously by the cooling fluid in the adjacent cooling jacket 8 which eliminates the requirement of expensive heated pipes or tubes. This reduces the costs and the risk for freezing at extremly low temperatures. Oil from the oil scraper ring is prevented from being sucked into the inlet channel via the evacuation channel 9 through pratically the same underpressure existing in the crank case under the piston as in the inlet channel. Oil in the oil mist which reaches the evacuation channel 9 is separated out by means of an oil trap as shown in the embodiment in Figs. 1 and 2 comprising a helically coiled metal band 23. the width of which is equal to the diameter of the evacuation channel.

Blow-by gases which during the early expansion stroke of the piston stream past the first compression ring in the groove 17 are retained in the collection chamber formed by the groove 20 in the second compression ring in the groove 18. When the

piston has completed the main part of the expansion stroke the collection chamber 20 is connected with the evacuation chamber 9 (see Fig. 2). Blow-by gases under pressure can now expand and be evacuated to the inlet channel via the evacuation channel 9. No further air or gas is used to push out the blow-by gas but the gas i evacuated through its own pressure. When the piston after having passed the lower dead centre moves upwards during the exhaust stroke, any possible remaining quantity of gas is evacuated as the collection chamber 20 is still connected with the evacuation channel 9 at the piston's initial movement upwards. If blow-by gas should still remain in the chamber 20 during the final part of the exhaust stroke and the main part of the inlet stroke, this quantity of gas can be evacuated to the inlet channel when the chamber 20 and channel 9 are again joined to each other.

Thus, during all working strokes the collection chamber 19 at times is connected with the evacuation channel 9 which ensures that the collection chamber is properly emptied at the beginning of each expansion stroke.

The part of the HC emission which must be neutralized in a conventional engine's catalyzer is caused by the quantity of unburnt fuel-air mixture which is pressed pasat the first compression ring during the compression stroke and trapped between the compression rings. This mixture normally flows back to the combustion chamber, so-called reverse blow-by, when the pressure there during the expansion stroke is less than the pressure in the mixture between the rings. However, this fuel- air mixture can be accumulated and comes back to the combustion chamber too late for burning and for contributing to the output of the engine. With the help of the evacuation channel 9 the unburnth fuel-air mixture can be evacuated from the chamber 20 before the pressure in the combustion chamber becomes so low that the mixture can flow past the first piston ring and back into the combustion chamber.

Through eliminating the"reverse blow-by"the quantity of HC emission in the exhaust is reduced which means that the size, weight and thereby even the price of the catalyzer can be reduced, at the same time as its length of life can be increased.

Also cold start emissions can be substantially reduced.

Figs. 4 and 5 illustrate schematically two four cylinder engines with four cylinder barrels 2, which each is communicating via an evacuation channel 9 with an inlet channel 11 for each combustion chamber. The engine in Fig. 4 is an aspirating engine and has each cylinder barrrel 2 directly connected with the respective inlet channel 11. The engine in Fig. 5 is turbo compressor charged and the evacuation channel 9 is divided into a suction channel 9a and a pressure channel 9b which each opens out in their own collection channel 9c and 9d, respectively. The collection channel 9c opens out into an inlet to a not shown turbo compressor, while the collection chamber 9d is connected to an outlet in the compressor.

In Figs. 1 and 2 the piston 3 and the shield 21 are illustrated as a single detail, for example they can be cast in aluminium as one piece. In a preferred embodiment, however, the shield is manufactured of a plastic mateiral, for example PTFE, which makes the shield flexible and eliminates the risk of fatigue fracture through a possible uneven load. The shield 21 can be riveted to the piston or the piston can be designed with some sort of groove or cavity and the shield with a resilient hook which is snapped tight into the groove.

Within the scope of the invention it is also conceivable to arrange two outlets from the cylinder barrel 2 to the evacuation channels 9, for example an outlet corresponding to the existing sloping channel section 10, primarily intended to drain back oil and balance the pressure in the crank case and in the inlet channel, and a corresponding outlet higher up, primarily intended to evacuate the blow-by gases.