Method for operating a reciprocating-piston internal combustion engine

Abstract

In the method, an air / hydrogen mixture is supplied to a cylinder of the reciprocating-piston internal combustion engine while maintaining an air / hydrogen ratio of at least 4, preferably at least 5, and at a crank angle starting as of at least 55°CA before the top dead centre is reached at which an ignition takes place (ZOT) and / or after one or more inlet valves of a respective cylinder of the reciprocating-piston internal combustion engine have been closed, is ignited by means of an ignition device. As a result of the compression and combustion of hydrogen, a locally limited volume region, heated to above 1100 K, is reached in the combustion chamber, and subsequently ammonia is injected into this heated volume region within the combustion chamber and ignited. The mechanical energy released as a result of the combustion of ammonia is used to drive the reciprocating-piston internal combustion engine.

Description

[0001] Method for operating a reciprocating piston internal combustion engine

[0002] The invention relates to a method for operating a reciprocating piston internal combustion engine that can be operated in two- or four-stroke mode. Ammonia is to be used as the primary fuel.

[0003] Compared to conventional fuels typically used in reciprocating piston internal combustion engines, ammonia exhibits a longer ignition delay and a correspondingly lower combustion and flame propagation speed. This, in turn, leads to significantly uneven combustion, if such combustion is even possible at all. Therefore, ammonia has been used as a fuel for reciprocating piston internal combustion engines only in those with very low rotational speeds and very large volumes, which severely limits the use of such internal combustion engines. Its use has so far been ruled out, particularly in mobile applications and where high power is required.

[0004] To counteract these disadvantages, experiments have been conducted with the direct addition of hydrogen during the injection of ammonia into the respective cylinders. However, this leads to frequent misfires or backfires, which damage the respective internal combustion engine and hinder its operation.

[0005] One could also increase the temperature of the intake air. However, this option is very ineffective and requires a high energy input for the necessary heating. It is also questionable whether the required increase in the intake air temperature can even be achieved with an external heater.

[0006] Raising the compression ratio can also be considered to counteract poor ignition, excessive ignition delay, or severely uneven ammonia combustion. However, raising the compression ratio simultaneously increases peak combustion pressure, which sometimes results in very large forces and torques, particularly on the pistons, connecting rods, crankshaft, and bearings, and leads to high mechanical stress on all components.

[0007] The object of the invention is therefore to provide possibilities for improved operation of reciprocating piston internal combustion engines powered by ammonia as fuel, even at higher speeds, which can, in particular, take into account problems related to the poorer ignition and combustion behavior of ammonia compared to conventional fuels. Higher speeds are understood to mean speeds above 1000 rpm. According to the invention, this object is achieved by a method having the features of claim 1. Advantageous embodiments and further developments of the invention can be realized with features defined in the dependent claims.

[0008] In the method according to the invention, an air-hydrogen mixture is supplied to a cylinder of the reciprocating internal combustion engine during the intake stroke, maintaining an air-hydrogen ratio (combustion air ratio) of at least 4, preferably at least 5. In the compression stroke, at a crank angle (CA) starting at at least 55° CA before reaching top dead center (TDC) during piston movement and / or after closing one or more intake valves of a respective cylinder of the reciprocating internal combustion engine, the compressed air-hydrogen mixture is ignited with an ignition device, and the compression and combustion of hydrogen result in a locally limited volume area in the combustion chamber being heated to over 1100K. Ignition should preferably always take place after the intake valve(s) have closed.

[0009] Ammonia is injected into the respective cylinder in this heated volume area in the combustion chamber and at a crank angle that is at least 5°, preferably at least 10° greater than the crank angle at which ignition occurred.

[0010] This process leads to a reduction in the ignition delay and the accelerated conversion of ammonia into kinetic energy to drive the reciprocating piston internal combustion engine.

[0011] Ammonia can be injected conventionally using an injection nozzle, preferably a multi-hole injection nozzle. Hydrogen can be added to the intake air in a manner analogous to conventional manifold injection. The combustion air required to burn the ammonia can be supplied to the respective cylinder in a conventional manner via the intake manifold and, if necessary, at least one intake valve. An air-to-hydrogen ratio of at least 4 and a maximum of 19, preferably a maximum of 9, and particularly preferably 5, should be maintained, resulting in a combustion air ratio in the intake air of between 4 and 10, preferably between 5 and 8.

[0012] In each compression stroke, ammonia should be ignited with an ignition device, e.g. a spark plug, after the ignition of the air-hydrogen mixture and, if possible, at a crank angle starting at least 55° CA before top dead center (BTDC) (crank angle before ignition TDC), preferably from 50° CA before TDC, the compressed air-hydrogen mixture should be ignited using an ignition device, e.g. a spark plug. Ignition should occur at a crank angle of 30° CA before TDC at the latest. The combustion of the hydrogen after ignition essentially serves to increase the temperature in at least one volume region of the respective cylinder in order to improve the ignitability of the ammonia added later as a result of the increased temperatures. The contribution to torque generation can be neglected.

[0013] During or after compression, ammonia should be injected directly into the respective cylinder with an air-to-ammonia ratio of at least 0.8, preferably at least 0.9.

[0014] Preferably, the air-hydrogen mixture should flow tangentially or radially off-center through at least one intake valve into the respective cylinder. This allows a predetermined swirling, circular flow of the air-hydrogen mixture to be achieved, inclined at a specific angle relative to the central longitudinal axis of the respective cylinder and circulating within the cylinder. Alternatively, a tumble flow (rotation of the charge around an axis of rotation perpendicular to the cylinder axis) can be used. This allows the ignited air-hydrogen mixture to spread in a targeted manner within the cylinder during combustion, and the injected and at least partially vaporized ammonia to be successively ignited with the spreading flame of the burning hydrogen, in order to achieve uniform combustion of the ammonia.

[0015] Ignition can be carried out with the respective spark plug with an energy in the range of 20 mJ to 40 mJ over a period of 0.5 ms to 1.5 ms. The spark plug can be positioned off-center in the cylinder in a nearly standard arrangement, preferably in such a way that the air-hydrogen mixture flowing into the respective cylinder moves toward the spark plug.

[0016] The reciprocating piston internal combustion engine should be operated at an average minimum speed of 1200 rpm.

[0017] The injected ammonia is ignited by the flame of the ignited hydrogen during its oxidation and the mechanical energy released in this process moves the piston of the respective cylinder and thus drives the reciprocating internal combustion engine.

[0018] With the invention, hydrogen obtained by catalytic decomposition of ammonia can be supplied to the respective cylinder with the intake air. This eliminates the need for an additional tank, which preferably stores liquid hydrogen, which, in mobile applications, must be carried in a suitable vehicle and refilled separately at regular intervals.

[0019] Catalytic decomposition can be carried out, for example, as described by QF Lin in "Instant hydrogen production from ammonia by non-thermal arc plasma combining with catalyst"; Energy Reports 7 (2021); pp. 4064-4070. A low-temperature plasma of ammonia is ignited using an electrode, and the species activated by the plasma are brought into the sphere of influence of a catalyst formed with NiO / AhCh as the catalytically active component and decomposed to produce free hydrogen. The hydrogen then only needs to be separated and, if necessary via an intermediate storage device, can be added in metered form to the intake air with the above-mentioned air-hydrogen ratio before this mixture flows into the respective cylinder.

[0020] Of course, it is also possible to use hydrogen from a separate storage tank, preferably in liquid form, which can then be carried on a vehicle powered by a reciprocating piston internal combustion engine.

[0021] By supplying hydrogen to each cylinder with the intake air in the appropriate ratio, the ignition delay of the subsequently injected ammonia can be advantageously influenced after the ignition of this mixture. The hydrogen in the cylinder, after ignition by the spark plug, generates a flame that spreads through the combustion chamber, influenced by the inflowing air-hydrogen mixture. The lean combustion of this mixture, which, in addition to the increase in pressure in the cylinder that occurs as a result of compression during a compression stroke, leads to a further temperature increase that has a beneficial effect on the ignition behavior of ammonia. This can cause a hot ring to form around an ammonia injection nozzle, which further contributes to the even combustion of ammonia in the combustion chamber.

[0022] With this invention, ammonia can be used as a fuel even in small-scale reciprocating piston internal combustion engines operating at relatively high speeds, thus avoiding uneven or insufficient combustion. Since the combustion process takes place entirely without the release of carbon dioxide, the operation of the engine does not adversely affect the climate.

[0023] Since the supplied hydrogen only serves the purpose of improving the ignition and combustion of ammonia, the performance of the reciprocating piston internal combustion engine operated in this way is essentially determined by the amount of ammonia injected and burned. Control can therefore be implemented in the same way as in conventional internal combustion engines.

[0024] The invention will be explained in more detail below by way of example.

[0025] It shows:

[0026] Figure 1 shows a diagram of the cylinder and calorific value curves for different air-hydrogen ratios. For the basic investigations, a 4-stroke, four-cylinder in-line diesel engine with common rail direct injection, a compression ratio of 16, and a single-cylinder displacement of 0.5 liters was used. As an example, the new combustion process was evaluated at an operating point with an indicated center pressure (pmi) of 14 bar and an engine speed of 2280 rpm. -1 The operating point is a typical acceleration point in the medium partial load range, as found in current approval cycles. It can be operated with or without exhaust gas recirculation, although the investigations in this paper are limited to the consideration without EGR. The boundary conditions are taken from comparative measurements or calculated (charge air temperature 60 °C, charge air pressure 1825 mbar, oil and coolant temperature 90 °C).

[0027] In a direct comparison, it can be seen that a) without pre-combustion, the ammonia is not converted b) with a higher EU share, a significantly faster energy release of the ammonia takes place (shorter ignition delay) Ammonia ignites during injection c) a targeted control of the ignition delay via the intake air-hydrogen ratio is possible.

[0028] The process involves introducing a small amount of hydrogen into the intake manifold. The lean mixture is drawn in and ignited during compression. Subsequently, as in the conventional diesel process, ammonia is directly injected via a centrally mounted multi-hole nozzle, igniting at least some of the injection jets. The increase in pressure and temperature subsequently leads to the ignition of the remaining jets. The functionality of the operating process has been demonstrated for both an air-to-hydrogen ratio of X = 5 and X = 8.

Claims

Patent claims 1. A method for operating a reciprocating internal combustion engine, in which an air-hydrogen mixture is supplied to a cylinder of the reciprocating internal combustion engine during the intake stroke, maintaining an air-hydrogen ratio of at least 4, preferably at least 5, and is ignited with an ignition device at a crank angle starting from at least 55°CA before reaching top dead center at which ignition occurs (TDC) and / or after closing one or more intake valves of a respective cylinder of the reciprocating internal combustion engine,whereby, through the compression and combustion of hydrogen, a locally limited volume area heated to over 1100 K is achieved in the combustion chamber, and subsequently ammonia is injected into this heated volume area within the combustion chamber and ignited, and the mechanical energy released by the combustion of ammonia is used to drive the reciprocating internal combustion engine.

2. Process according to claim 1, characterized in that an air-hydrogen ratio of a maximum of 18, preferably a maximum of 9, particularly preferably a maximum of 5 is maintained.

3. Method according to one of the preceding claims, characterized in that in the respective compression stroke at a crank angle beginning at least 55° KW before the top dead center, the compressed air-hydrogen mixture is ignited with an ignition device for Ignition is carried out and ammonia is injected after the air-hydrogen mixture has been ignited.

4. Method according to one of the preceding claims, characterized in that during or after compression, ammonia with an air-ammonia ratio of at least 0.8, preferably at least 0.9, is injected directly into the respective cylinder.

5. Method according to one of the preceding claims, characterized in that the air-hydrogen mixture flows into the respective cylinder in a tangential direction off-center or in a radial direction through at least one inlet valve.

6. Method according to one of the preceding claims, characterized in that the ignition is carried out with the respective ignition device with an energy in the range of 20 mJ to 40 mJ over a period of 0.5 to 1.5 ms.

7. Method according to one of the preceding claims, characterized in that a compression ratio of at least 12 is maintained in the respective cylinder.

8. Method according to one of the preceding claims, characterized in that the reciprocating piston internal combustion engine is operated at an average minimum speed of 1200 rpm.

9. Method according to one of the preceding claims, characterized in that hydrogen obtained by catalytic decomposition of ammonia is supplied to the respective cylinder with the intake air.

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