METHOD FOR OPERATING A COMBUSTION ENGINE WITH GAS MIXTURES, HIGH FLAME SPEED AND LOW IGNITION ENERGY, AND A CORRESPONDING COMBUSTION ENGINE

Description

[0001] The present invention relates to a method for operating an internal combustion engine with gas mixtures of high flame speeds and low ignition energy according to claim 1 and to an internal combustion engine for operation with gas mixtures of high flame speeds and low ignition energy according to claim 7.

[0002] Internal combustion engines and methods for operating internal combustion engines with natural gas-air mixtures are well-known. The main component of natural gas in this process is methane. At the pressures, temperatures, and air-fuel ratios typically found in the intake manifolds of such internal combustion engines, the flame speeds of methane-air mixtures are below 50 cm per second. The minimum ignition energy for methane-air mixtures is on the order of 0.3 mJ. Typical natural gas-air mixtures exhibit comparable behavior.

[0003] Within the scope of the present patent application, natural gas-air mixtures in internal combustion engines powered by natural gas constitute gas mixtures with a low flame velocity and high ignition energy. In such internal combustion engines, the gas mixture is supplied via a gas mixer arranged in the intake manifold of the engine. This represents a safe operating mode for internal combustion engines powered by natural gas.

[0004] Other internal combustion engines are operated with gas mixtures exhibiting high flame speeds and low ignition energies. Such gas mixtures are produced, among other things, when hydrogen is used as a fuel gas. Exhaust gases from the chemical industry with high proportions of ethene, ethyne, or ethylene oxide also form gas mixtures with high flame speeds and low ignition energies. A high proportion of these gases can be present even at low volume percentages of the substances, provided these substances dominate the combustion behavior of the gas mixture. The flame speeds of gas mixtures with high flame speeds are in the range of >30 cm / s, particularly >50 cm / s, and especially in the range between 1 and 4 m / s. Low ignition energies are in the range between 0.01 and 0.25 mJ, particularly in the range between 0.02 and 0.1 mJ.Such gas mixtures are highly flammable, so even ignition sources with relatively low energy output, such as glowing soot particles in a cylinder or hot spots on a cylinder intake, can be sufficient to ignite such a gas mixture. Due to the high flame speed, the combustion of the gas mixture propagates at high speed through the intake manifold of the internal combustion engine. This can destroy the internal combustion engine and injure people in the vicinity.

[0005] To minimize this risk, when using suitable fuel gases such as hydrogen, the gas mixture is supplied close to the combustion chamber. The fuel gas is only introduced immediately before the cylinder is filled with the gas mixture, in front of the intake valve of the respective cylinder. Alternatively, the fuel gas can be supplied close to the combustion chamber by injection directly into the combustion chamber of a cylinder. Both methods are referred to here as combustion chamber-adjusted fuel gas metering. Combustion chamber-adjusted fuel gas metering ensures that only the amount of fuel gas required for the next cylinder to be filled with the gas mixture is present in the intake manifold. If premature ignition of the gas mixture occurs at an ignition source on or in the cylinder, the effects are limited by the small amount of fuel gas in the intake manifold.

[0006] Internal combustion engines with a fuel metering system located near the combustion chamber, as well as methods for operating such internal combustion engines, are known from the prior art.

[0007] US Patent 2012 / 0041665 A1 discloses an internal combustion engine with multiple cylinders and a method for operating the internal combustion engine. The engine features fuel metering close to the combustion chamber (direct injection) and fuel metering away from the combustion chamber (intake pipe injection). According to the disclosed method, at low to medium loads, the fuel is supplied by the fuel metering system away from the combustion chamber. At high loads, the fuel is supplied by the fuel metering system close to the combustion chamber.

[0008] German patent application DE 199 45 544 A1 discloses a fuel supply system for a spark-ignition internal combustion engine and a method for operating such an engine. The fuel supply system comprises an injection valve for fuel metering close to the combustion chamber and an injection valve for fuel metering further away from the combustion chamber. According to the disclosed method, at low to medium loads, the fuel is supplied by the fuel metering system close to the combustion chamber. At high loads, the fuel is additionally supplied by the fuel metering system close to the combustion chamber. DE 199 45 544 A1 deals with the metering of a liquid fuel, in particular gasoline. The use of gaseous fuels, especially those that lead to gas mixtures with high flame velocities and low ignition energy, such as hydrogen, is not disclosed.

[0009] US 2014 / 0114552 A1 discloses a method for supplying gaseous fuels such as natural gas or propane to an internal combustion engine, as well as a corresponding internal combustion engine. The use of natural gas or propane results in gas mixtures with, in the terminology of the present patent application, low flame velocities and high ignition energy. The internal combustion engine has a fuel metering system located near the combustion chamber and a fuel metering system located further away from the combustion chamber. Both fuel metering systems are always used for fuel supply. By using the fuel metering system located near the combustion chamber in addition to the fuel metering system located further away from the combustion chamber, the method can respond better to load variations.

[0010] The object of the present invention is to provide a safe and cost-effective method for operating an internal combustion engine with gas mixtures of high flame speeds and low ignition energy, in particular hydrogen-air mixtures, as well as a cost-effective internal combustion engine for carrying out the safe method.

[0011] In the inventive method for operating an internal combustion engine, the engine has a plurality of cylinders and an intake manifold through which intake air enters the cylinders. The engine further comprises a first gas metering device for the pulsed, combustion chamber-adjacent metering of a fuel gas with several gas injectors assigned to the cylinders. Each cylinder is assigned one gas injector. The intake manifold has an intake port for each cylinder. The gas injector meters the fuel gas into the intake port belonging to each cylinder. If the engine has cylinder pairs that are arranged and pulsed in a particularly advantageous manner, one gas injector can handle the metering of the fuel gas for both cylinders of such a cylinder pair. The fuel gas is then metered at the point where the intake ports of the two cylinders diverge.The fuel gas is typically metered by the gas injectors using a timer. The amount of fuel gas is determined by how long a gas injector is open to allow fuel gas to pass through.

[0012] Furthermore, the internal combustion engine features a second gas metering device for the continuous, central metering of a fuel gas into the intake manifold, away from the combustion chamber. This second gas metering system provides a gas mixture in the intake manifold, supplying all cylinders of the internal combustion engine. In a turbocharged internal combustion engine, this can be achieved by individual injections into an intake manifold downstream of a compressor, particularly an exhaust gas turbocharger. Alternatively, the metering can be carried out, particularly via a gas mixer, in the intake manifold upstream of a compressor in an exhaust gas turbocharger. In the second gas metering device, the quantity of fuel gas supplied is preferably controlled by a proportional valve. The amount of fuel gas entering the intake manifold per unit of time is regulated by the variable cross-section of the proportional valve.

[0013] In the method according to the invention, the fuel gas is metered exclusively by the first gas metering device in a first low-load range and at least mainly by the second gas metering device in a second high-load range. At high load, the flow velocity in the intake tract, particularly at an exhaust cross-section of the intake tract, is higher than the flame velocity of the gas mixture, in particular at least 1.5 times higher, and more preferably at least twice as high. The exhaust cross-section is a specific, defined cross-section of the intake tract located downstream of the intake tract components to be protected.If the gas mixture were to ignite at an ignition source on a cylinder, it cannot spread beyond the exhaust cross-section of the intake tract against the flow direction because, at high load, the flow velocity in the exhaust cross-section is higher than the flame velocity of the gas mixture. Any premature ignitions of a gas mixture with a high flame velocity and low ignition energy are thus controlled. A high load range is defined as, in particular, a range above 50% of the rated power at rated speed. A low load range is defined as a power output of 10% of the rated power at rated speed or less. In this case, the fuel gas is metered via the first gas metering device. The fuel gas is metered close to the combustion chamber and supplied in pulses shortly before the gas mixture is drawn into the cylinder, or injected directly into the combustion chamber.This reduces the amount of gas mixture present in the system to the exact quantity required. Reducing the amount of gas mixture minimizes the risk of damage to the internal combustion engine due to premature ignition of the gas mixture, even if the flame speed of the gas mixture is higher than the flow velocity in the intake manifold, particularly in the exhaust cross-section. This allows the internal combustion engine to operate safely at low loads.

[0014] Since the first gas metering unit only supplies fuel gas in low load ranges, specifically 10% of the rated load at rated speed (on its own), it can be designed to meter relatively small quantities of fuel gas. The gas injectors of the first gas metering unit can therefore be significantly smaller compared to internal combustion engines, where metering takes place near the combustion chamber across the entire power range. Gas injectors, especially for hydrogen, must be particularly large due to hydrogen's comparatively low energy density. Large gas injectors for intermittent operation are custom-made components and expensive. Furthermore, these gas injectors are subject to increased wear due to the regular opening and closing. The gas injectors are typically replaced several times during the service life of the corresponding internal combustion engines.The high costs of large gas injectors are thus incurred multiple times over the lifetime of a corresponding internal combustion engine. If the gas injectors only need to handle the sole metering of fuel gas up to a load range of approximately 10% of the rated load, they can be dimensioned at least 50%, preferably around 75%, and especially between 75% and 90% smaller than gas injectors that have to manage gas metering across the entire load range. Smaller gas injectors are more cost-effective and, due to their lower moving masses, also more durable.

[0015] Under high load, fuel gas is primarily metered by the second gas metering device. At high load, the flow velocity in the intake tract, particularly in the exhaust cross-section, is higher than the flame velocity of the gas mixture. In the event of premature ignition of the gas mixture, the flame front cannot propagate across the exhaust cross-section. This ensures safe operation even when fuel gas is metered by the second gas metering device. The second gas metering device features a proportional valve, the passage of fuel gas through which depends on the valve's opening cross-section. Fuel gas metering via the second gas metering device is continuous. Therefore, the second gas metering device is subject to minimal wear.

[0016] According to the invention, the second gas metering device dispenses the fuel gas in such a way that the flame speed of the gas mixture in the intake tract is always lower than the flow velocity in the intake tract, particularly in an exhaust cross-section of the intake tract. In particular, the ratio of flow velocity in the intake tract to flame speed of the gas mixture is at least 1.5, and more preferably at least 2.0. The flame speed of the gas mixture depends, in particular, on the air-fuel ratio. It can therefore be influenced by the amount of fuel gas metered. The fuel gas can be partially or completely metered into the intake tract by the second gas metering device. If only part of the required amount of fuel gas is metered by the second gas metering device, the remaining amount is metered by the first gas metering device.This ensures safe operation of the internal combustion engine, preferably across the entire power range of the internal combustion engine.

[0017] Preferably, during the start-up phase of the internal combustion engine, the fuel gas is metered exclusively by the first gas metering unit. During the start-up phase, the load is low. Consequently, the flow velocity in the intake manifold is also initially low. Because the fuel gas is metered exclusively by the first gas metering unit during this phase, the risk of premature ignition of the gas mixture is manageable, as only the required amount of fuel gas is present in the internal combustion engine. The amount of gas mixture in the engine that can ignite prematurely is thus limited. The switch from fuel gas metering exclusively by the first gas metering unit to fuel gas metering by the second gas metering unit can, for example, be time-controlled after the start-up phase has elapsed. The time until the switchover can be, in particular, one minute.Preferably, the switching is load-dependent, so that after reaching a minimum power output of, in particular, 10% of the maximum power output of the internal combustion engine, the switch is made from the first gas metering device to the second gas metering device.

[0018] When the internal combustion engine is switched off, the fuel gas metering can be completely shut off. The engine then coasts to a stop. During this process, it draws in fresh air from outside the engine through the intake manifold. The intake manifold is purged. The engine is automatically brought into a safe state for restarting.

[0019] Preferably, the amount of gas flowing through the intake manifold is measured. This measurement can be performed, in particular, by an air mass meter. From the gas quantity, the volumetric flow rate can be determined, and from this, given a known cross-section of the intake manifold, especially an exhaust cross-section within the intake manifold, the flow velocity in the intake manifold or in the exhaust cross-section can be calculated. If the flame velocity profile of a gas mixture as a function of the air-fuel ratio is stored in an engine control unit, the metering of the fuel gas by the second gas metering device can then be controlled such that the flame velocity of the gas mixture is sufficiently lower than the flow velocity in the intake manifold, especially in the exhaust cross-section of the intake manifold.

[0020] Alternatively or additionally, the flow velocity can be determined from the engine speed. From the engine speed, together with the engine displacement and the number of filling cycles per revolution, the volume flow rate of the gas mixture through the intake manifold can be calculated. The flow velocity is then obtained by dividing the volume flow rate by the relevant cross-sectional area of ​​the intake manifold.

[0021] Preferably, the pressure and temperature of the gas mixture in the intake manifold are measured. Using the volume flow rate, temperature, and pressure in the intake manifold, the amount of gas flowing through the intake manifold can be determined. From this data, it can be determined how much fuel gas needs to be metered to achieve the desired air-fuel ratio of the gas mixture.

[0022] Preferably, when a threshold value is reached, particularly at a specific percentage of the rated power at rated speed, e.g., 10%, the fuel gas metering is switched from the first gas metering unit to the second gas metering unit. This method enables particularly simple control of the internal combustion engine. Upon reaching or exceeding a threshold value, the fuel gas metering is completely switched from the first to the second gas metering unit. In this case, the first gas metering unit is no longer used during further operation of the internal combustion engine in the operating range above the threshold value. The gas injectors of the first gas metering unit are not subject to any further wear. The service life of the gas injectors relative to the overall service life of the internal combustion engine is thus improved.

[0023] In an alternative embodiment of the process, under high load, fuel gas is metered by the first gas metering unit in addition to metering by the second gas metering unit. The majority of the fuel gas is supplied by the second gas metering unit. However, the first gas metering unit with its gas injectors can be used to supply individual cylinders with additional fuel gas on a cylinder-specific basis. Such cylinder-specific control of the gas metering allows the combustion-relevant parameters of all cylinders to be equalized. This applies in particular to the equalization of combustion temperatures, the position and duration of combustion, especially the combustion centers, as well as the power output of the individual cylinders. In this process, only a small amount of fuel gas, specifically less than 10% of the total fuel gas quantity, is metered via the gas injectors.The gas injectors can therefore be made correspondingly smaller and are thus more cost-effective.

[0024] The invention further relates to an internal combustion engine for operation with gas mixtures of high flame speeds and low ignition energy according to claim 7. The internal combustion engine comprises a plurality of cylinders and an intake manifold through which intake air enters the cylinder of the internal combustion engine. The internal combustion engine has a first gas metering device for intermittently metering a fuel gas near the combustion chamber with several gas injectors, each assigned to an individual cylinder. The internal combustion engine further comprises a second gas metering device for continuously metering a fuel gas far from the combustion chamber centrally to all cylinders in the intake manifold. In this context, metering a fuel gas near the combustion chamber also includes direct injection of the fuel gas into a combustion chamber of a cylinder. Preferably, each cylinder is assigned a gas injector.In special cases, however, a gas injector can also be used for two cylinders if the spatial arrangement of the cylinders and their timing allow this.

[0025] According to the invention, the intake tract of the internal combustion engine has a discharge cross-section in which the flow velocity of the gas mixture reaching the cylinder is higher than the flame velocity of the gas mixture at high engine load. In particular, the discharge cross-section is arranged downstream of the intake tract elements that are to be protected from ignition of the gas mixture by the discharge cross-section. In particular, the discharge cross-section is arranged upstream of the cylinders that constitute the primary ignition source. More preferably, the discharge cross-section is arranged between the cylinders and a mixture formation point of the second gas metering device. The intake tract elements to be protected from ignition of the gas mixture can be a throttle valve, a compressor, an intercooler, and / or sensors arranged in the intake tract.By designing the intake manifold's exhaust cross-section to ensure that, under high engine load, particularly at loads exceeding 50% of rated speed, the flow velocity is greater than the flame speed of the gas mixture, thus guaranteeing reliable engine operation. Typically, at power outputs above approximately 10% of rated power, the flow velocities in the intake manifold's ports leading to the individual cylinders are sufficiently high to exceed the flame speed of the gas mixture.

[0026] Preferably, a compressor is arranged in the intake manifold to generate boost pressure. In particular, the compressor is part of an exhaust gas turbocharger. Internal combustion engines operated with increased boost pressure exhibit higher power and / or efficiency.

[0027] In a further particularly preferred embodiment, the second gas metering device for metering a fuel gas into the intake manifold has a gas inlet arranged downstream of the compressor in the direction of flow. This configuration is particularly advantageous when the fuel gas is available at high pressure. High pressure here refers to a pressure at least one-tenth of a bar above the boost pressure. The fuel gas at high pressure can be introduced directly into the intake manifold at the high boost pressure. Efficiency losses due to expansion and subsequent recompression of the fuel gas are avoided. If the fuel gas is available at high pressure, the first gas metering device can also be supplied directly with the fuel gas at high pressure.

[0028] In an alternative preferred embodiment, the second gas metering device for metering fuel gas into the intake manifold has a gas inlet arranged upstream of the compressor in the direction of flow. The internal combustion engine also includes a fuel gas compressor for compressing the fuel gas for metering via the first gas metering device. This configuration is advantageous when the fuel gas is available at low pressure. Low pressure is defined as less than one-tenth of a bar above the boost pressure, and particularly lower than the boost pressure. The fuel gas is introduced into the intake manifold upstream of the compressor and compressed together with the intake air in the compressor. However, a fuel gas compressor is necessary for the operation of the first gas metering device, which allows for metering of the fuel gas close to the combustion chamber.Otherwise, no fuel gas can be metered into the intake manifold area where the increased boost pressure exists via the first gas metering device. However, the fuel gas compressor only needs to compress a portion of the fuel gas, namely the portion intended for metering via the first gas metering device. It can therefore be smaller and does not need to operate continuously.

[0029] Preferably, the internal combustion engine is designed as a stationary engine. Stationary engines typically operate with a load close to their design point. They are also frequently operated at lower fuel gas pressures than internal combustion engines for mobile applications, since energy density is not as critical for stationary applications as it is for mobile applications. This allows for large tanks to be provided for stationary engines, which can be operated at comparatively low fuel gas pressures, particularly up to 30 bar. The lower the minimum fuel gas pressure required to operate an internal combustion engine, the better the tank volume can be utilized. To be able to operate at a low fuel gas pressure, the gas injectors of the first gas metering unit must be correspondingly large or have a large cross-section.In stationary engines, the advantage of using a primary gas metering unit with gas injectors and a secondary gas metering unit for the continuous metering of fuel gas becomes particularly evident. The gas injectors for the primary unit, which only needs to meter a portion of the fuel gas supplied to the internal combustion engine, can be significantly smaller and therefore more cost-effective than if they had to ensure fuel gas metering across the entire power range.

[0030] Further advantages and details of the invention will become apparent from the following description of the figures. They show: Fig. 1 shows an internal combustion engine according to the invention in a first embodiment for fuel gas available at high pressure; Fig. 2 shows an alternative embodiment of the internal combustion engine according to the invention for fuel gas available at low pressure; Fig. 3 shows an exemplary representation of the operation of an internal combustion engine with a representation of the power output and the fuel gas flows via the first gas metering device and the second gas metering device over time; Fig. 4 shows the laminar flame speed of a gas mixture as a function of the air-fuel ratio.

[0031] Identical or similarly functioning parts are provided with identical reference numerals, where appropriate. Individual technical features of the embodiments described below can be combined with the features of the independent claims as well as with the features of individual embodiments described above for the invention.

[0032] Fig. 1 Figure 1 shows an internal combustion engine 2 with four cylinders 4. The internal combustion engine 2 has an intake manifold 6. A compressor 8 is arranged in the intake manifold 6. Other elements in the intake manifold 6, not shown, can be a throttle valve and an intercooler. The compressor 8 can be designed as part of an exhaust gas turbocharger. Downstream of the compressor 8, the intake manifold 6 has an intake section 10, which divides into cylinder-specific intake ports 12 upstream of the cylinders. Temperature sensors 14, pressure sensors 16, and flow meters 18 are arranged in the intake section 10. The internal combustion engine 2 has a first gas metering device 20. The first gas metering device 20 comprises four gas injectors 22. Each gas injector 22 is assigned to one cylinder 4. The first gas metering device 20 is designed to meter fuel gas into the respective intake channels 12 of the respective cylinders individually via the gas injectors 22.The gas injectors 22 open and close in a pulsed manner during operation. This pulse is synchronized with the engine cycle. The amount of fuel gas is determined by the opening duration of the respective gas injectors 22, controlled by time.

[0033] The internal combustion engine 2 also has a second gas metering device 24. This metering device supplies fuel gas to the intake manifold 10 via a proportional valve 26. The quantity of fuel gas introduced into the intake manifold 10 of the intake tract 6 by the second gas metering device 24 via the proportional valve 26 is determined by the opening cross-section of the proportional valve 26. The opening cross-section of the proportional valve 26 required for the passage of a specific quantity of fuel gas per unit of time depends on the boost pressure prevailing in the intake manifold 10 and the pressure at which the fuel gas is available. The first gas metering device 20 and the second gas metering device 24 are supplied with fuel gas via a common fuel gas supply 28.

[0034] The internal combustion engine 2 according to Fig. 1 This is suitable for fuel gas supplies that can provide fuel gas at high pressure. The pressure should be at least one-tenth of a bar higher than the boost pressure prevailing in the intake manifold 10. Therefore, if the internal combustion engine is operated at rated power with a boost pressure of 3 bar, the fuel gas supply must be able to provide the fuel gas at a pressure of at least 3.1 bar.

[0035] This embodiment is suitable, for example, for use with hydrogen produced via high-pressure electrolyzers. Typical high-pressure electrolyzers provide hydrogen at a pressure of up to 30 bar.

[0036] Fig. 2 Figure 2 shows an alternative embodiment of an internal combustion engine 2 for fuel gas supplied at a low pressure. Low pressures here are defined as pressures less than one-tenth of a bar above the boost pressure. This internal combustion engine 2 also comprises four cylinders 4. The cylinders 4 are supplied with air or a gas mixture via an intake manifold 6. Fuel gas is introduced into the intake manifold 6 via a first gas metering device 20 and a second gas metering device 24. The first gas metering device 20 comprises four gas injectors 22. Each of the gas injectors 22 is assigned to a cylinder 4 of the internal combustion engine 2. The gas injectors 22 supply the fuel gas cylinder-specifically in the respective intake ports 12 of the respective cylinders 4. The second gas metering device 24, with the proportional valve 26, supplies the fuel gas, which is available at low pressure, upstream of the compressor 8.The compressor 8 thus compresses the gas mixture and not just the intake air. A fuel gas compressor 30 is provided in the fuel gas supply 28 for the operation of the first gas metering device 20. The fuel gas compressor 30 pressurizes a portion of the fuel gas flow to a pressure sufficient to meter the fuel gas via the gas injectors 22 into the pressurized part of the intake tract 6, specifically the intake ports 12. The temperature sensors 14, pressure gauges 16, and flow meters 18 are also arranged in the intake tract 10.

[0037] The amount of gas mixture entering cylinders 4 can be determined via the temperature sensors 14, pressure gauges 16, and flow meters 18. Based on this data, the amount of fuel gas that must be supplied via the first gas metering unit 20 or the second gas metering unit 24 is determined.

[0038] The number of cylinders in Fig. 1 and Fig. 2 The embodiments shown may also vary. Internal combustion engines with more than four cylinders, in particular with 6, 8, 10, 12, 16, 20 or 24 cylinders, are also included.

[0039] Fig. 3 Figure 1 shows the power output 32 of an internal combustion engine 2 over time, as well as a representation of the fuel gas flow 34 supplied by the first gas metering device 20 and the fuel gas flow 36 supplied by the second gas metering device 24. During start-up of the internal combustion engine 2, fuel gas is supplied exclusively by the first gas metering device 20. When the engine power reaches approximately 10 percent of its rated power, additional fuel gas is supplied by the second gas metering device 24. The quantity of fuel gas metered by the first gas metering device 20 remains constant during this phase.When the amount of fuel gas supplied by the second gas metering unit 24 roughly corresponds to the amount supplied by the first gas metering unit 20, at approximately 40 percent of the engine 2's power output, the fuel gas flow through the first gas metering unit 20 is reduced, and the fuel gas flow through the second gas metering unit 24 increases more steeply. At approximately 80 percent of the engine's power output, no further gas metering occurs via the first gas metering unit 20 in this example. Further increases in power output are accompanied by an increase in the amount of fuel gas supplied by the second gas metering unit 24. When the engine 2's power output is reduced again, the amount of fuel gas supplied by the second gas metering unit 24 is initially reduced. At approximately 80 percent of the engine's power output, a certain amount of fuel gas is again supplied by the first gas metering unit 20.The amount of fuel gas supplied by the first gas metering unit 20 no longer increases at a power output of approximately 40 percent. The amount of fuel gas supplied by the second gas metering unit 24 is further reduced. At a power output of approximately 10 percent, no more fuel gas is metered by the second gas metering unit 24. Subsequently, the amount of fuel gas metered by the first gas metering unit 20 is further reduced until the internal combustion engine 2 is switched off.

[0040] As the power output of the internal combustion engine 2 increases, the amount of gas mixture passing through the intake tract 6, in particular the intake section 10 and the intake ports 12, also increases. Fuel gas can be supplied via the second gas metering device 24, provided that the flame speed of the resulting gas mixture is below the flow velocity of the gas mixture in the intake tract 6 or at an exhaust cross-section of the intake tract 6. Preferably, the flow velocity in the intake tract is at least 1.5 times the flame speed, and particularly preferably at least 2.0 times the flame speed of the gas mixture.

[0041] The flame speed of a gas mixture is in Fig. 4 The following is an example showing the relationship between the combustion air-fuel ratio (CAFR) and the air-fuel ratio (λ). The flame speed reaches a maximum at a CAFR of 1. Starting from this maximum, the flame speeds decrease with increasing CAFR. The second gas metering device 24 can therefore begin metering small quantities of fuel gas even at relatively low flow velocities in the intake tract, provided the flame speed of the resulting gas mixture is below the flow velocity in the intake tract. The remaining amount of fuel gas is metered near the combustion chamber via the first gas metering device 20. If the flow velocity in the intake tract 6 is sufficiently high, the fuel gas can be metered entirely via the second gas metering device 24.Because the flow velocity in the intake tract 6, particularly at an exhaust cross-section of the intake tract, is higher than the flame velocity of the gas mixture, it is prevented that, in the event of premature ignition of the gas mixture, the flames can propagate against the flow direction in the intake tract. Typical ignition sources for a prematurely igniting gas mixture are found in the area of ​​the cylinders. These include, in particular, hotspots in the area of ​​the cylinder inlet or glowing particles inside the cylinders 4. Since the ignition sources are typically associated with the cylinders 4, the smallest cross-section of the intake duct 10 is used as the exhaust cross-section for the sake of simplicity.

[0042] Besides the one in Fig. 3In addition to the exemplary operating sequence shown, other operating modes of the internal combustion engine 2 are also possible. For example, the system can switch from metering by the first gas metering unit 20 to metering by the second gas metering unit 24 when a threshold value is reached, with all further metering then taking place exclusively via the second unit. This simplifies the control of the internal combustion engine 2.

[0043] The internal combustion engine 2 can be shut down by completely stopping the fuel gas metering. The engine then coasts to a stop, while simultaneously being purged by the fresh air drawn in. Switching back to metering via the first gas metering device 20 is then unnecessary. The control of the internal combustion engine 2 is simplified.

[0044] Even under high load, the first gas metering unit 20 can partially meter the fuel gas to allow for the individual addition of small amounts of fuel gas to each cylinder. This ensures that the individual cylinders 4 are equalized even under high load. Since only small amounts of additional fuel gas need to be metered via the first gas metering unit 20, the gas injectors 22 can still be designed to be small.

Claims

1. Method for operating an internal combustion engine (2) with gas mixtures of high flame velocities and low ignition energy, in particular hydrogen-air mixtures, comprising a plurality of cylinders (4), an intake tract (6) through which intake air enters the cylinders (4) of the internal combustion engine (2), a first gas metering device (20) for intermittently metering a fuel gas close to the combustion chambers with a plurality of gas injectors (22) assigned to the cylinders (4), and a second gas metering device (24) for continuously metering a fuel gas into the intake tract (8) for all cylinders (4) at one point placed at a distance from the combustion chambers, wherein the fuel gas is metered in a first load range of low load exclusively by the first gas metering device (20) and in a second load range of high load at least mainly by the second gas metering device (24), characterized in that the fuel gas is metered by the second gas metering device (24) in such a way that the flame velocity of the gas mixture is lower than the flow velocity of the gas mixture in the intake tract (6), in particular at a blow-out cross-section of the intake tract (6).

2. Method according to claim 1, characterized in that the fuel gas is metered exclusively by the first gas metering device (20) during a start-up phase of the internal combustion engine.

3. Method according to one of the preceding claims, characterized in that the amount of gas flowing through the intake tract (6) is measured.

4. Method according to one of the preceding claims, characterized in that the pressure and temperature of the gas mixture in the intake tract (6) as well as the rotational speed of the internal combustion engine (2) are measured, and the amount of gas flowing through the intake tract (6) is determined from this data together with the displacement of the internal combustion engine (2).

5. Method according to one of the preceding claims, characterized in that when a threshold value is reached, the metering of the fuel gas is switched from the first gas metering device (20) to the second gas metering device (24).

6. Method according to one of the preceding claims, characterized in that, at high load, fuel gas is metered by the first gas metering device (20) in addition to metering by the second gas metering device (24), wherein the injection timings of the first gas metering device (20) are controlled individually for each cylinder in such a way that the combustion-relevant parameters of all cylinders (4) are equalized.

7. Internal combustion engine (2) for operation with gas mixtures of high flame velocities and low ignition energy, in particular hydrogen-air mixtures, comprising a plurality of cylinders (4), an intake tract (6) through which intake air enters the cylinders (4) of the internal combustion engine (2), a first gas metering device (20) for intermittently metering a fuel gas close to the combustion chambers with a plurality of gas injectors (22) assigned to the cylinders (4), and a second gas metering device (24) for continuously metering a fuel gas into the intake tract (6) for all cylinders (4) at one point placed at a distance from the combustion chambers for carrying out the method according to one of claims 1 to 6, characterized in that the intake tract (6) has a blow-out cross-section in which the flow velocity of the gas mixture reaching the cylinders (4) is higher than the flame velocity of the gas mixture at high load.

8. Internal combustion engine according to claim 7, characterized in that the blow-out cross-section is arranged in the flow direction behind elements of the intake tract (6) which are to be protected from a through-ignition of a gas mixture by the blow-out cross-section.

9. Internal combustion engine according to one of claims 7 to 8, characterized by a compressor (8) arranged in the intake tract for generating a boost pressure.

10. Internal combustion engine according to claim 9, characterized in that the second gas metering device (24) for metering a fuel gas into the intake tract (6) has a gas inlet arranged in the flow direction behind the compressor (8).

11. Internal combustion engine according to claim 9, characterized in that the second gas metering device (24) for metering a fuel gas into the intake tract (6) has a gas inlet arranged in the flow direction upstream of the compressor (8) and the internal combustion engine (2) has a fuel gas compressor (30) for compressing fuel gas for metering via the first gas metering device (20).

12. Internal combustion engine according to one of claims 7 to 11, characterized in that the internal combustion engine (2) is a stationary engine.