The invention concerns an internal combustion engine having prechambers of the fuel- gas-air-mixture flushed type.

Parameter l characterizes a mixture of air and fuel gas with l = 1 denoting a stoichiometric mixture, l > 1 denoting a lean mixture and l < 1 denoting a rich mixture. The term “fuel gas” can encompass fuel gas such as natural gas, biogas, landfill gas, sewage gas etc...

In internal gas combustion engines having cylinder diameters below 180 mm direct ignition is prevalent. A gas-air mixture present inside the main combustion chamber is ignited by a spark plug protruding into the main combustion chamber. Together with the concept of lean operation (use of a gas-air mixture with l > 1 ) a NOx fraction of the exhaust gas produced during operation can be held below legally proscribed values. It is also known to dilute a stoichiometric gas-air mixture with exhaust gas instead of additional air such that the gas-air mixture has a l about 1 and an exhaust gas fraction is present. The advantage of this measure is the fact that a three-way-catalysator can be utilized and emission of pollutants by the internal combustion engine can be massively reduced.

In internal gas combustion engines having cylinder diameters above 180 mm the concept of a gas-flushed prechamber is used wherein the main combustion chambers are provided with a lean gas-air mixture and pure gas is provided to the prechambers via prechamber gas valves. During compression stroke lean gas-air mixture is pressed from the main combustion chamber into the prechamber to form a gas-air mixture as the prechamber charge with l about 1 at the time of ignition. The prechamber charge is ignited by an ignition source (e. g. spark plug) and ignition torches protruding from the prechamber into the main combustion chamber ignite the lean gas-air mixture present in the main combustion chamber. The concept of a gas-flushed prechamber can be used together with a lean operation concept but not together with an operation concept where a stoichiometric gas-air mixture is diluted with exhaust gas since in the latter concept a very rich gas-air mixture (with a high exhaust gas fraction) is formed in the prechamber and there is not enough oxygen in the prechamber to guarantee a successful and fast ignition of the resulting gas-air-exhaust gas mixture.

EP 3303804 B1 discloses an internal combustion engine having the following features:

- at least one cylinder head

- an intake manifold

- a plurality of piston-cylinder-units in which pistons are movably arranged

- a plurality of main combustion chambers, wherein each main combustion chamber is formed in a cylinder by a piston and at least one cylinder head

- a plurality of intake ports wherein each intake port is connected to one of the main combustion chambers and to an intake manifold

- a plurality of prechambers wherein each prechamber is connected to one of the main combustion chambers and is provided with a prechamber gas valve, an ignition device and a source for a gas-air mixture which is formed by the intake port of the main combustion chamber to which the prechamber is connected and a connection line provided between the intake port and the prechamber gas valve

- an electronic control unit configured to control a lambda l of the gas-air mixture provided to the intake ports or the intake manifold

EP 3303804 B1 discloses an operation concept in which the prechambers are flushed not with pure gas but with a mixture of air and fuel gas taken from an intake port of the main combustion chamber to which a prechamber is coupled. A very lean prechamber charge with a l greater than 1 ,2 or even greater than 1 ,7 is formed resulting in low NOx emissions.

It is an object of the invention to provide an internal combustion engine having prechambers of the fuel-gas-air-mixture flushed type with reduced pollutant emissions.

This object is being achieved by an internal combustion engine having the features of claim 1. Preferred embodiments of the invention are defined by dependent claims. As is well known in the art, stoichiometric mixture of air and fuel gas can be diluted with additional air or with recirculated (cooled) exhaust gas in order to reduce the combustion knocking tendency and thus to increase the maximum specific power output of the engine. The dilution by additional air will result in lean burn combustion, the dilution with exhaust gas however will not change the air-fuel ratio (which will stay at l = 1 ) and therefore enable the usage of a three-way-catalyst for pollutant reduction.

The exhaust gas recirculation rate (EGR rate) of the mixture of air, fuel gas and exhaust gas in the intake manifold (in per cent) is defined by the formula mEGR

^X EGR [%] 100 mtot with m _EGR denoting mass of the recirculated exhaust gas and m _tot denoting total mass of the mixture of air, fuel gas and recirculated exhaust gas.

An internal combustion engine as defined in claim 1 has:

- at least one cylinder head

- an intake manifold

- a plurality of piston-cylinder-units in which pistons are movably arranged

- a plurality of main combustion chambers, wherein each main combustion chamber is formed in a cylinder by a piston and the at least one cylinder head

- a plurality of intake ports wherein each intake port is connected to one of the main combustion chambers and to an intake manifold

- a plurality of prechambers wherein each prechamber is connected to one of the main combustion chambers and is provided with a prechamber gas valve, an ignition device (the ignition timing of which can be controlled by an electronic control unit) and a source for a gas-air mixture which is formed by

• the intake port of the main combustion chamber to which the prechamber is connected and a connection line provided between the intake port and the prechamber gas valve or

• the intake manifold and a connection line provided between the intake manifold and the prechamber gas valve - an external exhaust gas re-circulation to the intake manifold

- an electronic control unit configured to provide the intake manifold with a mixture of fuel gas, air and exhaust gas with

• a lambda l of approximately 1 and · an exhaust gas recirculation (EGR) content of about 10 % to about 45 %

According to the invention a stoichiometric gas-air mixture is diluted with re-circulated exhaust gas to form an ignitable mixture that is supplied to a prechamber via the prechamber gas valve and to the main chamber via the intake valves. The use of a stoichiometric mixture fuel gas, air and exhaust gas allows the use of a three-way- catalysator in an exhaust pipe (usually downstream of a turbine of a turbocharger if one is provided) of the internal combustion engine to decrease emission of pollutants.

The parameter lambda l of the gas-air mixture in the intake manifold and the intake ports and therefore of the gas-air mixture provided to the main combustion chamber and the prechambers via the intake valves and the prechamber gas valves (which can be preferably passive valves, e. g. check valves) can be adjusted in the usual manner, e. g. by adding more or less fuel gas to the intake air in a mixing device. The EGR content X _E GR of the mixture of air, fuel gas and exhaust gas in the intake manifold and the intake ports can be adjusted by an external exhaust gas re-circulation in the usual manner e. g. by adding more or less recirculated exhaust gas to the air or the air-fuel mixture in an EGR mixing device. The ignition device can be in the form of a spark plug.

In a preferred embodiment of the invention there is at least one turbocharger to provide pressurized mixture (air-EGR or air-fuel gas-EGR) to the intake manifold. In this embodiment it is provided for that the connection lines to the prechamber branch off from the intake manifold or the intake ports between the turbocharger and inlet valves of the main combustion chambers. In a preferred embodiment of the invention the electronic control unit is configured to provide the intake ports with a gas-air mixture with a lambda l between about 0,95 and about 1 ,05. In a preferred embodiment of the invention the electronic control unit is configured to provide recirculated exhaust gas to the main chambers and prechambers such that the EGR content is between about 10 % and about 45 %.

It can be provided that the fuel gas is supplied either by means of a gas mixer upstream of the compressor of the turbocharger or by means of port fuel injection valves positioned downstream of the compressor of the turbocharger.

In a preferred embodiment of the invention the recirculated exhaust gas is provided to the main chambers and prechambers by an EGR system that connects the exhaust system to the intake system via an EGR mixing device which can be controlled by the electronic control unit via an EGT control device.

It can be provided that inlet and outlet valves of the main combustion chambers are actuated by an actuator which is controlled by the electronic control unit such that an inlet valve closes before the piston reaches a lower dead center position. This ensures the complete flushing of the prechambers through the still retracting pistons. However it is preferred that this motion of the inlet and outlet valves is controlled not electronically but by a camshaft. In a preferred embodiment of the invention a three-way-catalyst is arranged in an exhaust pipe of the internal combustion engine (usually downstream of the turbine of a turbocharger if one is provided).

In a preferred embodiment of the invention a ratio of a volume of a prechamber to a compression volume of the main combustion chamber to which the prechamber is connected in an upper dead center of a piston movement ranges from about 0.5 % - about 4 %. In a preferred embodiment of the invention the connection lines are formed as cavities in the cylinder head.

A tempering device can be provided for each connection line in order to prevent condensation of the gas-air mixture provided to the prechambers.

An optional aperture can be provided for each connection line in order to decouple the prechambers from pressure pulsations in the intake ports. The present invention can be used irrespective of a piston shape, e. g. in connection with flat pistons or pistons with bowls.

The internal combustion engine is preferably formed as a stationary gas motor which preferably is coupled to an electrical generator to generate electrical power or to a drive shaft to generate mechanical power (e.g. to drive a compressor).

An embodiment of the invention is shown in Figs. 1 and 2. Fig. 1 shows an internal combustion engine 1 according to the invention and Fig. 2 shows a single piston- cylinder-unit 3 of the internal combustion engine 1 of Fig. 1

The internal combustion engine has a cylinder head 2 (in reality two cylinder heads 2 in V configuration but only one is shown), a plurality of piston-cylinder-units 3 in which pistons 4 are movably arranged and a plurality of main combustion chambers 5, wherein each main combustion chamber 5 is formed in a cylinder 6 by a piston 4 (of. Fig. 2) and the cylinder head 2.

A plurality of intake ports 10 is provided wherein each intake port 10 is connected to one of the main combustion chambers 5. A plurality of prechambers 7 is provided wherein each prechamber 7 is connected to one of the main combustion chambers 5 and is provided with a prechamber gas valve 8, an ignition device 9 (the ignition timing of which can be controlled by an electronic control unit 19) and a source for a mixture of fuel gas, air and exhaust gas. The source for a mixture of fuel gas, air and exhaust gas is formed by the intake port 10 of the main combustion chamber 5 to which the prechamber 7 is connected (shown as an example for all of the prechambers 7 but one) or the intake manifold 12 (shown as an example with respect to the prechamber 7 which is situated second from left in the upper region of Fig. 1) and a connection line 11 provided between the intake port 10 or the intake manifold 12 and the prechamber gas valve 9. Of course, other than shown in Fig. 1 , it will be preferred to connect the connection lines 11 of all prechambers 7 either to the intake port 10 or the intake manifold 12.

In Fig. 1 a mixing device 23 for air and fuel gas and port fuel injection valves 26 are shown together. It is of course preferred to use either a mixing device 23 or port fuel injection valves 26.

The electronic control unit 19 (dashed command lines are shown only with respect to a single piston-cylinder-unit 3 but are of course provided with respect to all of them) is configured to provide the intake manifold 12 with a mixture of fuel gas, air and exhaust gas via a fuel gas-air mixing device 23 with a lambda l of at approximately 1 and via an an external exhaust gas re-circulation (external EGR) mixing device (having an EGR control device 24 controlled by the electronic control unit 19 and an EGR cooler 25) with x _EGR of approximately 10 % to 45 %.

The main chamber and prechamber charge is controlled to have a lambda l of approximately 1 (preferably between about 0,95 and about 1,05) and a recirculated exhaust gas content x _EGR of about 10 % to about 45 %, preferably between about 10 % and about 45 %, more preferably about 15 % and 40 %.

If a turbocharger 20 is provided, exhaust gas is provided upstream of a compressor C of the turbocharger 20.

In this embodiment the internal combustion engine 1 is provided with at least one turbocharger 20 (having a compressor C and a turbine T) to provide pressurized mixture (air-EGR or air-fuel gas-EGR) to the intake manifold 12. The connection lines 11 branch off from the intake manifold 12 or the intake ports 10 between the turbocharger 20 and inlet valves 15 of the main combustion chambers 5. Inlet and outlet valves 15, 16 of the main combustion chambers 5 are actuated by an actuator 22 (which might optionally be controlled by the electronic control unit 19) such that an inlet valve 15 closes before the piston 4 reaches a lower dead center position. It is however preferred to control this motion of the inlet and outlet valves 15, 16 without the electronic control unit 19 via a camshaft.

A three-way-catalysator 14 is arranged preferably downstream of the turbocharger turbine T.

As an example, a ratio of a volume of a prechamber 7 to a compression volume of the main combustion chamber 5 to which the prechamber 7 is connected in an upper dead center of a piston movement ranges from about 0.5 % - about 4 %. As an example, the connection lines 11 are formed as cavities in the cylinder head 2.

A tempering device 17 and a throttle 21 can be provided for each connection line 11.

An aperture 18 can be provided for each connection line 11 in order to decouple the prechambers 7 from pressure pulsations in the intake ports 10 or the intake manifold 12.

List of reference numbers:

1 internal combustion engine

2 cylinder head 3 piston-cylinder-unit

4 piston

5 main-combustion chamber

6 cylinder

7 prechamber 8 gas valve

9 ignition device

10 intake port

11 connection line

12 intake manifold 13 exhaust manifold

14 three-way-catalysator

15 inlet valve

16 outlet valve

17 tempering device 18 aperture

19 electronic control unit

20 turbocharger

21 throttle

22 actuator 23 mixing device

24 EGR control device of external exhaust gas re-circulation

25 EGR cooler of external exhaust gas re-circulation

26 port fuel injection valve C compressor of turbocharger

T turbine of turbocharger