DE102018211890A1 - Method for operating an internal combustion engine, device for carrying out the method and internal combustion engine - Google Patents

Abstract

The invention relates to a method for operating an internal combustion engine of a motor vehicle with natural gas, which is stored in liquid form in a pressure container (1) on board the motor vehicle. According to the invention, natural gas is removed from the pressure vessel (1) or from a fuel line (2) connected to the pressure vessel (1) for pressure reduction in the pressure vessel (1) and introduced into an exhaust tract (3) of the internal combustion engine, in particular blown in, and burned there The invention further relates to an apparatus for performing the method and an internal combustion engine.

Description

The invention relates to a method for operating an internal combustion engine with natural gas according to the preamble of claim 1. Furthermore, the invention relates to a device for carrying out the method and an internal combustion engine with such a device.

State of the art

Developments in engine technology deal with the direct injection of natural gas into the combustion chamber of internal combustion engines and the ignition of the gas-air mixture with diesel fuel. This type of combustion promises advantages in terms of lower pollutant emissions, in particular CO ₂ emissions, and lower costs due to the use of natural gas as a fuel. The natural gas can be stored on board a motor vehicle in liquid (LNG, ie "liquefied natural gas") and / or gaseous (CNG, ie "compressed natural gas").

The storage in liquid form has the advantage that the volume is reduced. For this purpose, the natural gas is cooled down to temperatures of around -160 ° C and stored in a special pressure container ("cryogenic tank"). The initial pressure in the pressure tank is determined during refueling by the pressure at the gas station. However, due to the external supply of heat, liquid natural gas present in the pressure vessel can evaporate, so that a gas phase forms above the liquid phase. At the same time, the pressure in the pressure vessel rises. Gas leaks and / or amounts of gas that are specifically returned to the pressure vessel, for example when starting the engine cold or driving a fuel feed pump cold, can further heat up the natural gas present in the pressure vessel. In order to prevent overpressure in the pressure vessel, the pressure vessel is usually secured with a pressure relief valve. This opens when the pressure in the pressure vessel exceeds a predetermined limit. The excess gas is then released to the environment via the open pressure relief valve.

Since resources are wasted by releasing gas to the environment, the aim is to keep the levies as low as possible. In some countries, the supply of gas is even regulated by legislation. Because the gas, which is primarily methane, contributes to global warming and is therefore harmful to the environment.

The invention has for its object to make the operation of an internal combustion engine with natural gas more efficient and at the same time more environmentally friendly.

To achieve the object, the method with the features of claim 1, the device with the features of claim 9 and the internal combustion engine with the features of claim 11 are proposed. Advantageous developments of the invention can be found in the respective subclaims.

Disclosure of the invention

A method is proposed for operating an internal combustion engine of a motor vehicle with natural gas, which is stored in liquid form in a pressure vessel on board the motor vehicle. According to the invention, natural gas is extracted from the pressure vessel or from a fuel line connected to the pressure vessel for pressure reduction in the pressure vessel and introduced, in particular blown, into an exhaust tract of the internal combustion engine and burned there.

In the proposed method, therefore, a quantity of natural gas that is taken directly or indirectly from the pressure vessel for pressure reduction is not simply released into the environment, but is instead fed to an exhaust tract of the internal combustion engine and burned in the exhaust tract. In this way, no harmful gas is released into the environment, so that the environment is protected.

The method also contributes to the thermodynamic state of the two-phase mixture of liquid and gaseous fuel in the pressure vessel, that is to say the pressure and the temperature, being as far as possible below the boiling line of the liquid fuel.

In addition, the excess gas is used. Exhaust gas cleaning can be optimized by introducing or blowing the extracted gas into the exhaust tract. In particular, the heat released during the combustion of the gas can be used to heat the exhaust gas in the exhaust tract. In this way, the efficiency of the components of a downstream exhaust gas cleaning system of the internal combustion engine can be increased. For example, when the internal combustion engine is operating close to idle (in a traffic jam or temporarily in city traffic), the exhaust gas temperature can be raised to more than 180 ° C, 200 ° C or 250 ° C, so that the conversion rate in a downstream catalytic converter, in particular a NOx storage catalytic converter or SCR catalytic converter, increases to a value above 80%. In the event of a cold start, the starting time of the catalytic converter can also be shortened. The proposed procedure therefore contributes to compliance with the legally prescribed emission limit values (e.g. Euro 6 standard). If the exhaust gas temperature is raised to more than 600 ° C by the combustion of the gas in the exhaust tract, this contributes to particle filter regeneration. This can lead to the fact that other measures for the regeneration of the particle filter, such as the injection of diesel fuel into the exhaust system, can be dispensed with.

In the case of a thermodynamic state near the boiling pressure line, natural gas is advantageously removed from the pressure vessel or from a fuel line connected to the pressure vessel. Such a condition can occur due to heat input from the outside and / or through a targeted return of natural gas into the pressure vessel. The quantity returned to the pressure container can in particular be a cold run quantity, by means of which at least one pump of a fuel delivery device connected to the pressure container via the fuel line is flushed during a cold start. The originally cryogenic liquid fuel heats up so that it evaporates and returns as gas to the pressure vessel.

The removal can take place directly from the pressure vessel or indirectly via a fuel line connected to the pressure vessel, the fuel line preferably connecting the pressure vessel to a fuel delivery device. Depending on the thermodynamic condition in the pressure tank, there may be an undefined two-phase mixture (LNG and methane) in the fuel line, which affects the efficiency of the fuel delivery system. By removing the two-phase mixture from the fuel line, an increase in efficiency can thus be brought about. However, this also means that it is not only mandatory to withdraw gaseous natural gas and to incinerate it in the exhaust system.

The natural gas removed from the pressure vessel or from the fuel line is preferably introduced into the exhaust tract upstream of a catalytic converter arranged in the exhaust tract, in particular upstream of a diesel oxidation catalytic converter. “Upstream” refers to the main flow direction of the exhaust gas in the exhaust tract. The natural gas is thus available for oxidation and heat release in a downstream exhaust gas aftertreatment system, so that the efficiency of the exhaust gas aftertreatment system is increased. The introduction or injection of the natural gas into the exhaust tract is preferably carried out downstream of an exhaust gas turbocharger of the internal combustion engine.

Furthermore, the natural gas introduced, in particular blown, into the exhaust tract is preferably ignited - at least when the internal combustion engine is cold started - with the aid of an electrical heating element, for example with the aid of a spark plug or a glow plug. In this way, the combustion of the gas is ensured even in a cold, non-reactive environment, for example in the case of an inactive diesel oxidation catalytic converter. For this purpose, the electric heating element is preferably arranged downstream of the point at which the natural gas is introduced or blown in.

Advantageously, the natural gas removed from the pressure vessel or from the fuel line is heated before being introduced, in particular blown, into the exhaust gas tract. In the case of large methane mass flows, the heating prevents the system from undercooling and thus ice formation. The temperature of the natural gas when it is introduced or blown into the exhaust tract is preferably from 20 ° C. to 80 ° C. For heating, the natural gas removed from the pressure vessel or the fuel line can be passed past a component of the exhaust system of the internal combustion engine for the purpose of heat exchange. Alternatively, the heating can also be effected with the aid of an external heat exchanger, for example with the aid of an EGR cooler. In this way, the cooling capacity of the EGR cooler can be increased at the same time.

Furthermore, it is proposed that the temperature in the exhaust tract be increased before natural gas is introduced, in particular, into the exhaust tract, for example by post-blowing natural gas into a combustion chamber of the internal combustion engine during combustion or by diesel injection into the exhaust tract. These measures promote optimal combustion of the gas in the exhaust system. Ideally, the natural gas is only introduced or blown into the exhaust tract when the operating state of the internal combustion engine in terms of engine speed and load creates favorable conditions for the combustion of the gas in the exhaust tract. This means that the gas is introduced or blown in in a controlled manner, preferably independently of the amount removed from the pressure vessel or the fuel line. This quantity may have to be temporarily stored. In a development of the invention, it is therefore proposed that the natural gas withdrawn from the pressure vessel or the fuel line is temporarily stored in an intermediate store before being introduced, in particular blown, into the exhaust gas tract.

The natural gas introduced, in particular blown, into the exhaust tract is preferably burned in a separate combustion chamber integrated into the exhaust tract. The combustion chamber creates a protected space that locally limits the combustion of the gas and helps to stabilize the combustion process. For example, the risk of flame extinguishing due to flame extension is reduced by an exhaust gas flow present in the exhaust tract. The aim is in addition, combustion that takes place without combustion residues (e.g. ash and / or deposits) and without combustion intermediates (e.g. UHC) under all foreseeable operating conditions of the overall system.

A flow straightener is advantageously connected upstream of the combustion chamber. This also contributes to stabilizing the combustion process, since it generates a flow field that is as uniform as possible, preferably one-dimensional, downstream of the point at which the gas removed from the pressure vessel or the fuel line is introduced or blown in. The flow straightener preferably has a conically widening entry area for calming the gas flow, which is adjoined by a cylindrical area, which preferably forms the combustion chamber. The cylindrical region preferably terminates with a conically tapering outlet region which opens into the exhaust tract.

According to a preferred embodiment of the invention, the natural gas is withdrawn from the pressure vessel or from the fuel line via a first valve, in particular a proportional valve or a pressure relief valve, and introduced, in particular blown, into the exhaust tract via a second valve. The two valves enable a controlled removal or a controlled introduction / injection of the gas.

The device also proposed for pressure reduction in a pressure vessel, which is used to store natural gas in liquid form on board a motor vehicle, comprises a first valve, in particular a proportional valve or a pressure relief valve, for connection to the pressure vessel or a fuel line connected to the pressure vessel, and a second valve for connection to an exhaust tract of an internal combustion engine of the motor vehicle. The first valve and the second valve are connected via a fuel line.

The first valve, in particular in the version as a proportional valve, enables quantity control or regulation, so that a controlled removal of natural gas from the pressure vessel or from the fuel line is possible. The first valve must be suitable for gas as well as for two-phase mixtures. A proportional valve, for example, which is designed analogously to a metering unit of a diesel fuel pump, has proven to be suitable.

The second valve of the device according to the invention is used for the controlled introduction or injection of natural gas into the exhaust system. Due to the arrangement directly on the exhaust tract, the valve must have the required temperature resistance. The second valve can therefore be designed analogously to a metering valve which is used to inject an aqueous urea solution into the exhaust tract.

The fuel line that connects both valves is preferably designed as a pressure-resistant connection line. Ideally, it enables heat exchange between the gas or two-phase mixture in the line and the environment in order to preheat the gas or two-phase mixture. In this case, the fuel line can be guided past components of the exhaust system of the motor vehicle, so that the gas or two-phase mixture heats up even more.

According to a preferred embodiment of the invention, an intermediate store is arranged in the fuel line. The intermediate store enables the gas to be retained, so that it can be introduced or blown into the exhaust tract specifically at times at which the operating state of the internal combustion engine ensures optimal combustion of the gas.

The proposed device for reducing pressure in a pressure vessel is used in particular when carrying out the method according to the invention.

In addition, an internal combustion engine for a motor vehicle with a device according to the invention and a pressure vessel and an exhaust tract is proposed. The first valve of the device is arranged on the pressure vessel or a fuel line connected to the pressure vessel and the second valve is arranged on the exhaust tract, preferably upstream of a catalytic converter, in particular a diesel oxidation catalytic converter. The internal combustion engine can thus be operated using the method according to the invention. This means that the advantages of the method are also realized by the proposed internal combustion engine. In this respect, reference is made to the description of the method.

An electrical heating element, for example a spark plug or a glow plug, is preferably arranged in the exhaust tract downstream of the second valve for igniting the natural gas blown into the exhaust tract. “Downstream” here also refers to the main flow direction of the exhaust gas. The electrical heating element ensures reliable ignition and thus the combustion of the natural gas in the exhaust tract, even if the cold and non-reactive environment prevails in the tract, for example when the internal combustion engine is cold started.

Furthermore, a separate combustion chamber is preferably integrated into the exhaust tract in the exhaust tract downstream of the second valve. The combustion chamber forms a protected space that stabilizes the combustion process. As a further development measure, it is proposed that the combustion chamber be preceded by a flow straightener for calming the gas flow. This measure also helps to stabilize the combustion process.

The invention described above can be used in many gas drives / applications. It is particularly suitable for use in a dual-fuel powertrain application in the commercial vehicle sector.

The invention is explained in more detail below with reference to the accompanying drawing. This shows a schematic representation of a device according to the invention in connection with a fuel system and an exhaust system of an internal combustion engine.

Detailed description of the drawing

The internal combustion engine shown comprises an engine block 14 with several combustion chambers 15 , Every combustion chamber 15 is an injector 16 for injecting fuel into the combustion chamber 15 assigned. The fuel in the present case is natural gas, which is in liquid form in a pressure vessel 1 is stored at about -160 ° C. The pressure vessel 1 is through a fuel line 2 with a fuel delivery system 13 connected, which in the present case is an LNG high-pressure pump. The means of the fuel delivery device 13 The amount conveyed is first an evaporator 17 and then a storage tank 18 fed. The storage tank 18 is with a gas rail 19 connected, which supplies the injectors with fuel.

In the pressure vessel 1 is formed due to external heat input above a liquid phase 20 a gas phase 21 out. The gas phase that forms 21 leads to an increase in pressure in the pressure vessel 1 , so that an overpressure can arise. The pressure vessel 1 is therefore through a safety valve 22 secured, that opens when the pressure in the pressure vessel 1 exceeds a predetermined limit. With opening of the safety valve 22 gaseous natural gas, which consists primarily of methane, is released into the environment. This process contributes to global warming and is therefore harmful to the environment. Furthermore, the amount of fuel removed is no longer available for the operation of the internal combustion engine.

The internal combustion engine shown therefore has a device 11 for reducing pressure in the pressure vessel 1 on which is a first valve 9 and a second valve 10 includes. Both valves 9 . 10 are through a fuel line 12 connected with each other. The first valve 9 is on the fuel line 2 connected to the pressure vessel 1 with the fuel delivery system 13 combines. Opens the first valve 9 becomes natural gas from the fuel line 2 removed and via the fuel line 12 the second valve 10 fed. This is on an exhaust tract 3 arranged of the internal combustion engine, namely upstream of a catalyst 4 , which in the present case is a diesel oxidation catalyst. With the help of the second valve 10 is the amount of natural gas withdrawn into the exhaust tract 3 blown in to be burned as residue-free as possible. To create optimal combustion conditions, is in the exhaust system 3 a separate combustion chamber 7 trained of a flow straightener 8th is connected upstream. The latter calms the gas flow, so that it is in the downstream combustion chamber 7 forms a diffusion flame with as flat a flame front as possible. To ignite the gas safely, is in the combustion chamber 7 an electric heating element 5 arranged in the form of a spark plug.

The combustion of natural gas in the exhaust system 3 leads to heating of the exhaust gas of the internal combustion engine derived therefrom. This has the positive effect that a downstream exhaust gas treatment system can work more efficiently. As a result, the pollutant emissions of the internal combustion engine discharged via the exhaust gas can also be reduced.

Claims (13)

Method for operating an internal combustion engine of a motor vehicle with natural gas, which is stored in liquid form in a pressure container (1) on board the motor vehicle, characterized in that natural gas from the pressure container (1) or from one with the Pressure tank (1) connected fuel line (2) removed and introduced into an exhaust tract (3) of the internal combustion engine, in particular blown, and is burned there. Procedure according to Claim 1 , characterized in that the natural gas withdrawn from the pressure vessel (1) or from the fuel line (2) is introduced, in particular blown, upstream of a catalytic converter (4) arranged in the exhaust tract (3), in particular upstream of a diesel oxidation catalytic converter, into the exhaust tract (3) becomes. Procedure according to Claim 1 or 2 , characterized in that the natural gas introduced into the exhaust tract (3), in particular blown in, at least when the internal combustion engine is cold started, with the aid of an electric heating element (5), for example with the help of a spark plug or a glow plug. Method according to one of the preceding claims, characterized in that the natural gas withdrawn from the pressure vessel (1) or from the fuel line (2) is heated before being introduced, in particular blown, into the exhaust tract (3). Method according to one of the preceding claims, characterized in that before the introduction, in particular blowing, of natural gas into the exhaust tract (3), the temperature in the exhaust tract (3) is increased, for example by post-blowing natural gas into a combustion chamber of the internal combustion engine during the combustion or by diesel injection into the exhaust system (3). Method according to one of the preceding claims, characterized in that the natural gas withdrawn from the pressure vessel (1) or the fuel line (2) is temporarily stored in an intermediate store (6) before it is introduced, in particular blown, into the exhaust tract (3). Method according to one of the preceding claims, characterized in that the natural gas introduced, in particular blown, into the exhaust tract (3) is burned in a separate combustion chamber (7) integrated in the exhaust tract (3), which is preferably connected upstream of a flow straightener (8) is. Method according to one of the preceding claims, characterized in that the natural gas is withdrawn from the pressure vessel (1) or from the fuel line (2) via a first valve (9), in particular a proportional valve or a pressure relief valve, and via a second valve (10 ) is introduced into the exhaust tract (3), in particular blown in. Device (11) for reducing pressure in a pressure vessel (1), which is used to store natural gas in liquid form on board a motor vehicle, comprising a first valve (9), in particular a proportional valve or a pressure relief valve, for connection to the pressure vessel (1) or a fuel line (2) connected to the pressure vessel (1) and a second valve (10) for connection to an exhaust tract (3) of an internal combustion engine of the motor vehicle, the first valve (9) and the second valve (10) via a fuel line (12) are connected. Device (11) after Claim 9 , characterized in that an intermediate store (6) is arranged in the fuel line (12). Internal combustion engine for a motor vehicle with a device (11) Claim 9 or 10 and a pressure vessel (1) and an exhaust tract (3), the first valve (9) of the device (10) on the pressure vessel (1) or a fuel line (2) connected to the pressure vessel (1) and the second valve (10) is arranged on the exhaust tract (3), preferably upstream of a catalytic converter (4), in particular a diesel oxidation catalytic converter. Internal combustion engine after Claim 11 , characterized in that an electrical heating element (5), for example a spark plug or a glow plug, is arranged in the exhaust tract (3) downstream of the second valve (10) for igniting the natural gas blown into the exhaust tract (3). Internal combustion engine after Claim 11 or 12 , characterized in that in the exhaust tract (3) downstream of the second valve (10) a separate combustion chamber (7) is integrated in the exhaust tract (3), which is preferably connected upstream of a flow straightener (8).

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