Method for operating a reciprocating-piston internal combustion engine which is operated with a fuel which contains at least 50 vol% methanol

Abstract

In the method, the temperature of the combustion air taken in and the pressure inside the intake pipe are measured by sensors. In the event of a temperature below the flash point of methanol, the starting of the reciprocating-piston internal combustion engine is supported by means of an electric drive. In this case, the pressure in the intake pipe is controlled to below 500 mbar absolute and, with decreasing temperature, lower, and then methanol is fed to the cylinder(s) of the reciprocating-piston internal combustion engine. The mixture calorific value being lower than the minimum value required for overcoming the torques of the reciprocating-piston internal combustion engine which arise as a result of friction is accepted. When the torques of the reciprocating-piston internal combustion engine which act as a result of internal and external losses are less than the torque which has been achieved due to combustion of the methanol in the cylinder(s), the pressure in the intake pipe and thus the mixture calorific value of the methanol-and-combustion-air mixture fed to the cylinder(s) are increased and the electric drive is switched off, so that the reciprocating-piston internal combustion engine is then operated in normal operation exclusively with fuel burned in the respective cylinder(s).

Description

[0001] Technical University of Dresden, OBRIST Technologies GmbH P149085PC00

[0002] Method for operating a reciprocating internal combustion engine powered by a fuel containing at least 50% methanol by volume. The invention relates to a method for operating a reciprocating internal combustion engine powered by a fuel containing at least 50% methanol by volume, preferably at least 70% by volume, and particularly preferably pure methanol. In addition to methanol, such a fuel mixture may also contain, for example, conventional gasoline or another fuel that is more readily flammable than methanol. According to the literature, a safe and reliable cold start of a reciprocating internal combustion engine powered by pure methanol is only possible down to a lower limit of ambient temperatures of 10°C (which has also been demonstrated on the test bench). These problems also occur with higher proportions of methanol in fuel mixtures.By applying the method described here, this limit can be significantly reduced, which makes the use of reciprocating internal combustion engines powered purely by methanol or with high proportions of methanol in a fuel mixture very attractive, even at temperatures below 10°C.

[0003] Previous approaches to vaporizing a sufficient amount of fuel for a cold start include preheating the intake air and / or methanol cider, as well as adding another, more flammable fuel. This can be added to the main fuel via an additional fuel system or catalytically separated from the methanol used before mixture formation.

[0004] In summary, all approaches investigated so far involve additional components, and therefore costs and design effort. Heating methanol, in particular, is further limited by the risk of vapor lock in the fuel system. Catalytic decomposition of methanol to hydrogen and carbon dioxide carries the risk of uncontrolled hydrogen ignition.

[0005] Therefore, an application-oriented solution utilizing any existing components and / or by adjusting the engine control is always desirable and preferable.

[0006] It is therefore an object of the invention to provide methods for the safe operation of a reciprocating internal combustion engine with fuel mixtures containing high proportions of methanol or exclusively methanol as fuel, even at low temperatures, without requiring any major additional design modifications to the reciprocating internal combustion engine. According to the invention, this object is achieved by a method that employs the features of claim 1. Advantageous further developments and embodiments can be realized with the features specified in the dependent claims.

[0007] In the method according to the invention, the temperature of the intake combustion air and the pressure inside the intake pipe are measured with sensors.

[0008] When the temperature of the intake combustion air in an intake manifold and inside each cylinder of the reciprocating internal combustion engine is below the flash point of methanol 10 °C, preferably below 10 °C below the flash point of methanol, the starting of the reciprocating internal combustion engine is assisted by an electric drive, whereby the pressure in the intake manifold of the reciprocating internal combustion engine is regulated during this time to below 500 mbar absolute by influencing the intake combustion air mass flow, and lowered with decreasing temperature, and subsequently the fuel containing methanol or pure methanol is supplied to the cylinder(s) of the reciprocating internal combustion engine.

[0009] In this process, a reduction in the minimum calorific value of the fuel-air mixture is accepted to overcome the torques of the reciprocating internal combustion engine resulting from friction. The pressure in the intake manifold is regulated accordingly, and the electric drive is operated, until the combustion of the fuel mixture or pure methanol and / or its temperature increase in the cylinders has increased sufficiently to allow the intake air pressure to be raised. This ensures that the vaporization of fuel or pure methanol still occurs due to the higher temperature in the respective cylinder, until the torques of the reciprocating internal combustion engine resulting from friction are lower than the torque resulting from the combustion of the methanol or the methanol-containing fuel in the cylinder(s) (negative torque at the crankshaft).Subsequently, the pressure in the intake manifold, and thus the calorific value of the fuel-methanol-combustion air mixture supplied to the cylinder(s), can be increased, and the electric drive will be switched off, so that the reciprocating internal combustion engine will then be operated exclusively with fuel mixture burned in the respective cylinder(s) or pure methanol in normal operation.

[0010] Advantageously, after a number of ignitions have occurred in a cylinder where a torque of at least 30% of the maximum torque of the reciprocating internal combustion engine is available, the fuel-air mixture ratio can be increased to a value X > 1 in the cylinder(s).

[0011] Advantageously, the ignition of the fuel-air mixture in the cylinder(s) should be initiated at least 1° crank angle (CA), preferably at least 20° CA, before the thermodynamically ideal ignition point, up to the point at which combustion begins (detected by the measured electric current at the electric drive). After combustion has begun, the ignition point can be set to the temperature-dependent ignition point for warm-up and normalized during the warm-up of the reciprocating internal combustion engine until the thermodynamically optimal ignition point, dependent on the operating point, is reached.

[0012] Then the temperature of the intake combustion air inside each cylinder of the reciprocating internal combustion engine would likely have exceeded the flash point of the methanol.

[0013] It has been shown that even a shift in the ignition timing by more than 10 °KW can lead to an improvement, and that the ignition timing can be chosen more than 20 °KW up to 40 °KW before the thermodynamically ideal ignition timing, as long as no combustion takes place in the reciprocating internal combustion engine.

[0014] The reciprocating internal combustion engine should be permanently coupled to an electric drive, at least during periods when it is operated without methanol combustion, and should drive this drive at the start of the engine's operation. The reciprocating internal combustion engine is started by means of the electric drive. During normal operation, the reciprocating internal combustion engine can also drive the generator-operated electric drive, which can charge a battery. Normal operation can thus be achieved solely or additionally by means of the electric drive, or entirely solely by means of a fuel mixture or methanol combustion. In this case, the reciprocating internal combustion engine can be started redundantly and, if necessary, additionally to increase torque, or on its own, particularly in the absence of electrical power.

[0015] The reciprocating internal combustion engine cannot have a mechanical coupling to the wheels when driven purely electrically.

[0016] The operating point of the reciprocating internal combustion engine can be selected independently of a desired vehicle speed of a vehicle powered in this way or depending on the desired achievable torque.

[0017] A reciprocating internal combustion engine can be supported by at least one electric drive as needed, as long as independent operation is not possible or intended. During this operating phase, sufficient torque is provided by at least one electric drive to power the reciprocating internal combustion engine.

[0018] During the start-up phase of a reciprocating internal combustion engine, when no or insufficient torque is achieved through combustion alone (negative torque), an electric motor, such as a suitably adapted starter or an electric drive powered by electrical energy stored in an energy storage device (e.g., a battery), preferably also functioning as an electric generator, can be used to convert the energy obtained through combustion into electrical energy. The latter type of electric drive can be of the type currently used in so-called hybrid drives. The reduction of the pressure in the intake manifold, which is regulated during the start-up phase, can be achieved through control, particularly using a conventional throttle valve.Their position should be chosen so that the free flow cross-section is less than 1.5% of the cross-sectional area of ​​the throttle valve.

[0019] The speed of the reciprocating internal combustion engine can be regulated by the electric drive during a cold start.

[0020] For clarification, it should be noted below that the flash point of a substance, according to DIN V 14011, is the lowest temperature at which an ignitable vapor-air mixture can form for a given substance.

[0021] The flash point applies to standard conditions: a temperature of 298.15 K = 25°C with a pressure of 1013.25 mbar = 1013.25 hPa.

[0022] Below its boiling point, a liquid, such as methanol in a closed volume, evaporates until a suitable vapor pressure is reached. A decrease in ambient pressure forces more atoms to transition into the gas phase to compensate for the pressure loss (1st effect).

[0023] Surprisingly, the process can also be operated with pure methanol as fuel, thus achieving normal operation of a conventional internal combustion engine.

[0024] The vapor pressure curve for methanol shown in Figure 4 illustrates the boiling point of methanol. At 0.2 bar and -10°C, only a portion of the methanol vaporizes. This continues until a vapor pressure of 0.02 bar / 20 mbar is reached. This is sufficient, in conjunction with the portion that evaporates during compression, to produce a combustible air-fuel vapor mixture within the explosive limits of methanol.

[0025] The explosive limits of methanol lie between 6% and 50% by volume. The absolute pressure required to form a combustible air-fuel mixture is therefore significantly above the boiling point at low temperatures, since only a portion of the fuel needs to vaporize. However, it is below the minimum absolute pressure in the intake manifold required to supply the reciprocating internal combustion engine with sufficient air mass and, consequently, sufficient fuel mass—and thus energy—to overcome the friction-induced torques of the engine.

[0026] The very low fuel mass helps to reduce the significant additional cooling of the fuel-air mixture within each cylinder during methanol vaporization. (2nd effect)

[0027] In a series hybrid powertrain, the electric drive can compensate for the energy needed to overcome friction. The combustion process heats the structure of the reciprocating internal combustion engine, allowing more methanol (as fuel) to vaporize. Consequently, after a brief warm-up phase, the pressure in the intake manifold, and therefore the amount of methanol supplied to the cylinder(s), can be increased until the reciprocating internal combustion engine no longer requires electric assistance.

[0028] The process is independent of engine displacement. However, the amount of electrical energy required to compensate for the frictional effect of the reciprocating internal combustion engine depends on the engine displacement and the system (friction-dependent).

[0029] Compared to pure premium gasoline (RON 95) as a fuel, methanol has a boiling point rather than a boiling curve under typical ambient conditions. Methanol contains no volatile components and has a vaporization enthalpy 3-6 times higher.

[0030] Twice the amount of fuel supplied is required compared to premium gasoline to achieve the same energy content. The flash point is 11°C (at p: 1013 mbar). This can be better understood using the table in Figure 1, which compares the corresponding parameters of premium gasoline RON 95 with methanol.

[0031] Investigations to determine the flash point of a reciprocating internal combustion engine using pure methanol as fuel yielded results as shown in the diagram in Figure 4. The diagram shown in Figure 3 provides information on the influence of temperature.

[0032] For this purpose, a conventional reciprocating internal combustion engine with port fuel injection was operated with the following parameters.

[0033] During the start-up phase, the reciprocating internal combustion engine was driven by an electric drive designed for hybrid operation up to a speed of 2000 rpm.

[0034] The throttle position was adjusted and the injection of the respective fuel was initiated. An ambient temperature of 10°C was maintained, which was also maintained for the intake air and the internal combustion engine. This is represented by the bold black line.

[0035] During this start-up phase, the throttle position was set to 8%, resulting in an intake manifold pressure of 52 kPa (520 mbar). This led to occasional ignitions, but the engine did not start as required for normal operation.

[0036] The curves shown with the dashed line represent a scenario with a throttle valve setting of 6%. This resulted in an intake manifold pressure of 44 kPa (440 mbar).

[0037] With this minimal throttle valve setting, a positive torque was achieved at approximately 2000 rpm, overcoming the friction within the reciprocating internal combustion engine and enabling normal operation. From this point on, the reciprocating internal combustion engine could sustain its own operation, as the torque achievable through combustion had become greater than the frictional torque (positive torque). No misfires occurred, and a reliable engine start was possible.

[0038] A lower intake manifold pressure thus increases cold-start capability at low ambient temperatures.

[0039] Figure 3 shows an engine start at 24°C, 0°C and -20°C. The engine block, engine oil, engine coolant, fuel and intake air were conditioned before the test.

[0040] At temperatures between 24°C and 0°C down to -20°C, the engine block, engine oil, engine coolant, fuel, and intake air are conditioned for a given reciprocating internal combustion engine. As the temperature drops, the pressure in the intake manifold must be further reduced to vaporize enough fuel for combustion to occur in the respective cylinder of the reciprocating internal combustion engine.

[0041] As the mass of combustion air supplied decreases, the energy content of the fuel-air mixture also decreases, and thus so does the achievable torque (down to below the self-sustaining operation of the reciprocating internal combustion engine).

[0042] During this phase, the reciprocating internal combustion engine must be driven, or at least assisted, by the electric drive until the reciprocating internal combustion engine has been sufficiently heated to increase the pressure in the intake manifold, e.g. by means of a throttle valve, and to supply more combustion air and methanol as fuel to the respective cylinder.

[0043] They show:

[0044] Figure 1 shows a table with parameters of premium gasoline RON 95 compared to methanol. Figure 2 shows diagrams with time profiles of torque and coolant temperature of a reciprocating internal combustion engine as a function of the respective throttle valve opening angle over time.

[0045] Figure 3 Diagrams showing the time-dependent curves of torque and coolant temperature of a reciprocating internal combustion engine as a function of power output and the respective throttle valve opening angle.

[0046] Figure 4 shows a vapor pressure curve for methanol with the boiling point curve of methanol.

[0047] The functionality of the method according to the invention could be demonstrated on a reciprocating internal combustion engine intended for series production.

[0048] Engine capacity 999 cc

[0049] Cylinder 2

[0050] Crankshaft drive: two counter-rotating crankshafts

[0051] Bore 86 mm

[0052] Stroke 86 mm

[0053] Power 40 kW

[0054] Compression ratio 12.5 : 1

[0055] Air system naturally aspirated engine

[0056] Intake manifold injection

[0057] Ignition position central

[0058] Operating fluids, intake air, engine block conditioned to cold start temperature

[0059] Intake air remains at cold start temperature during operation

[0060] Fuel continues to reach cold start temperature during operation

[0061] Starting speed 2000 min-1, generator speed-controlled

[0062] Throttle position less than 6%, depending on the starting temperature

[0063] Ignition timing particularly early (up to 40° crank angle before top dead center), normalized after combustion begins

[0064] Injection timing: End of injection when the intake valve is open.

[0065] Injection mass: lambda-controlled (stoichiometric)

Claims

Technical University of Dresden, OBRIST Technologies GmbH P149085PC00 Patent claims 1. A method for operating a reciprocating internal combustion engine powered by a fuel containing at least 50% methanol by volume, wherein the temperature of the intake air and the pressure inside the intake manifold are measured by sensors, and when the temperature of the intake air in an intake manifold and inside a respective cylinder of the reciprocating internal combustion engine falls below the flash point of methanol, the starting of the reciprocating internal combustion engine is assisted by an electric drive, wherein the pressure in the intake manifold of the reciprocating internal combustion engine is regulated during this time to below 500 mbar absolute by influencing the intake air mass flow, and further reduced as the temperature decreases, and subsequently methanol is supplied to the cylinder(s) of the reciprocating internal combustion engine.and in this process, a reduction below the minimum mixture calorific value is accepted to overcome the torques of the reciprocating internal combustion engine resulting from friction, and the pressure in the intake manifold is regulated accordingly and the electric drive is operated until the combustion of the methanol and / or its temperature increase in the cylinder(s) has increased sufficiently to allow the intake air pressure to be increased and the evaporation of methanol still occurs due to the higher temperature in the respective cylinder, until the torques of the reciprocating internal combustion engine resulting from internal and external losses are less than that. The torque (resulting from the combustion of methanol in the cylinder(s)) has been reached, and subsequently the pressure in the intake manifold and thus the calorific value of the methanol-combustion air mixture supplied to the cylinder(s) is increased, and the electric drive is switched off, so that the reciprocating internal combustion engine is then operated exclusively with fuel burned in the respective cylinder(s) in normal operation.

2. Method according to claim 1, characterized in that after the onset of a number of ignitions in a cylinder, in which a torque of at least 30% of the maximum torque of the reciprocating internal combustion engine is available, the fuel-air mixture ratio is increased to a value X > 1 in the cylinder(s).

3. Method according to one of the preceding claims, characterized in that the pressure in the intake manifold is regulated by controlled adjustment of the opening angle of a control element, in particular a throttle valve.

4. Method according to one of the preceding claims, characterized in that a reciprocating internal combustion engine can be operated with negative torque at the crankshaft.

5. Method according to one of the preceding claims, characterized in that a reciprocating internal combustion engine is supported by an electric drive as required.

6. Method according to one of the preceding claims, characterized in that the set speed of the reciprocating internal combustion engine during cold start is controlled by the electric drive.

7. Method according to one of the preceding claims, characterized in that the ignition of the fuel-air mixture in the cylinder(s) up to the point in time when combustion begins is increased by at least an additional 1 0 KW, in particular at least an additional 20° KW, is introduced before the thermodynamically ideal ignition time.

Images