Method for operating an internal combustion engine with heat recovery and decoupled compressor and expansion unit

Abstract

Method for operating an internal combustion engine with a separately designed compressor and expansion unit, wherein the ignition of the fuel-gas mixture is effected by an ignition spark, and wherein the compressor and expansion unit are of different designs and are coupled or decoupled depending on the load case, wherein the expansion unit is designed in two stages, and wherein the exhaust gas, after leaving the first expansion stage, is fed to a heat exchanger which in turn heats the compressed air coming from the compressor.

Description

Prior ArtAccording to the prior art, heat engines for mobile use are predominantly designed as diesel engines or gasoline engines (with 2 or 4 operating cycles).Common to these engines is that in the same working chamber (cylinder) firstly a working medium is compressed, which is subsequently expanded with the supply of heat and thereby performs mechanical work. In this construction, the efficiency and thus the efficiency of the concept are limited in that, owing to the construction, the working space cannot be optimally executed for the compression on the one hand and the expansion process on the other hand.Therefore, compromises must be made in the design of the working space, which lead to losses in efficiency. In addition, the traditional design of the internal combustion engine has the disadvantage that a recovery of system-related waste heat (exhaust gas heat) is possible only with the aid of additional units (e.g. turbochargers).A further disadvantage of the traditional design is that the coupling of an energy store to the process is technically very complicated (e.g. generator-battery-electric motor)The publications DE3112 290 A1, DE10 2004 005 518 A1, DE60 2004 013 230 T2, DE10 2010 061 239 A1, DE10 2007 053 829 A1, EP 1 640 598 A1 each disclose internal combustion engines with exhaust gas heat exchangers and / or turbochargers.The object is achieved according to the invention in that the compression unit and the expansion unit are designed decoupled from one another for the design of a heat engine. That is, the compressor unit is operated at a different rotational speed than the expansion unit. The compressor can be designed, for example, as a piston machine which is coupled or uncoupled to the expansion unit depending on the load case. With this configuration, it is possible to use the waste heat which arises after the expansion has been carried out in order to preheat the compressed working medium via a heat exchanger in order to thus improve the efficiency of the process. This is achieved in particular by the thermodynamic optimization of the compression process achieving only a comparatively low final temperature after the compression of the gas. Thus, the gas can absorb a relatively large amount of heat via the heat exchanger.In addition, the provision of a pressure accumulator on the connecting line between the compressor unit and the expansion unit opens up the possibility, for example, of utilizing kinetic energy which arises during the braking of a vehicle in order to operate the decoupled compression unit via the drive shaft and thus charge the pressure accumulator (energy accumulator). The quantity of gas contained in the pressure accumulator can be used to increase the gas throughput through the expansion unit when there is a peak load demand, in order to thus temporarily increase the overall power of the heat engine.The method is essentially based on the following principle (see FIG. 1 ): a compressor B draws in gas from the intake connection A. The compressed gas is conducted past a pressure accumulator D via a connecting line. If the valve C is opened, compressed gas can be conducted into the pressure accumulator or can be removed. Subsequently, the compressed gas is passed via the heat exchanger E to the expansion unit F. In this case, the usable waste heat from the exhaust gas is supplied to the gas. After entering the expansion unit F, fuel is fed to the compressed gas, which is subsequently burned and leads to an increase in the gas temperature. Mechanical energy is obtained during the expansion of the gas. The remaining thermal energy in the exhaust gas is used to preheat the compressed gas via the heat exchanger E. The gas then reaches the exhaust system G.AdvantagesThis construction has the following advantages, for example, compared with the traditional construction of engines in motor vehicle technology:• The working space of the compressor can be optimized for the conditions of use in order to achieve a high efficiency. Thus, for example, by optimizing the cooling and the geometry, a distinct increase in the efficiency during the compression of the gas can be achieved.• The working space of the expansion unit can be optimized for the specific application, i.e. the heat losses can be minimized and the expansion ratio optimized.• The waste heat can be used to preheat the working medium processed by the compressor unit before it is further heated by further energy supply and finally expanded.In addition, excess energy can be used to charge an energy store integrated into the system (hybrid properties). Thus, for example, in the case of use in a motor vehicle, the kinetic energy of the vehicle can be used during the braking process in order to fill a gas pressure accumulator with the aid of the compressor unit. The stored work may be returned to the process as needed. No further aggregates are necessary for this purpose. Thus, for example, the compressed gas present in the energy store can be used to cover short-term peak power requirements or to save drive power in the compressor or also to start the heat engines.Functioning mode:Air is drawn into an open circuit via an intake connection A of compressor B and compressed. In this case, a method is used which enables efficient, energy-saving compression with low thermodynamic losses (for example quasi-isothermal compression). A preferred solution would be to use a screw compressor as the compressor unit B, since this achieves efficient compression with a relatively low gas temperature at the end of the compression process. An embodiment of the compressor unit B as a piston machine with an optimized method for cooling the medium during the compression process would also be conceivable.The compressed air is then conducted past a pressure accumulator D or through it. This pressure accumulator is designed such that compressed air can be stored or discharged as required. If there is excess drive line, the valve C may be opened to fill the accumulator while simultaneously throttling or stopping the supply of compressed gas to the expander F. Conversely, if the drive power of the expansion unit F is not sufficient (peak load requirement), the quantity of gas contained in the pressure accumulator is used to increase the gas throughput through the expansion unit, in order to temporarily increase the overall power of the heat engine. It is conceivable that the pressure accumulator is operated at a pressure level that lies above the normal operating pressure. It is also conceivable for the pressure accumulator to be designed in one unit with the heat exchanger E.In the heat exchanger E, the compressed gas is heated. The waste heat of the exhaust gas is used in this case in order to bring the compressed gas to the highest possible temperature level before it enters the expansion unit F. For this purpose, a preferred solution uses a shell-and-tube heat exchanger designed for countercurrent operation, through which exhaust gas flows on the shell side and compressed gas flows on the tube side.Subsequently, the compressed gas is conducted into the working chamber (e.g. pre-chamber or the piston chamber) of the expansion unit F. This expansion unit (working machine) is operated initially independently of the compressor unit. In this expansion unit F, fuel is supplied to the compressed gas and the latter is burned. As a result, the working medium is heated further and finally expanded with the release of work. An efficient expansion process with low thermodynamic losses is used. The expansion unit F can be designed, for example, as a single-stage or multistage displacement machine. An embodiment as a piston machine with highly temperature-resistant materials and minimized cooling is conceivable, for example. In the case of, for example, two-stage expansion, the exhaust gas, after leaving the first expansion stage, is conducted through the exhaust system G (and its exhaust gas treatment system) and is subsequently conducted on the secondary side through the heat exchanger E.After the expansion has been concluded, the hot gas is supplied to the exhaust system G and flows through the primary side of the heat exchanger E. Features of the internal combustion enginePermits compression ratio >1:10 or operating pressure after compression stage >10 barignition of the fuel-gas mixture by means of ignition sparks- Decoupled compaction and expansion unitThe compression unit can be designed as a screw compressor or a further design of a positive displacement compressorThe design of the compressors and expansion unit can be designed to be different according to thermodynamic requirementsThe use of a pressure accumulator between the compression and expansion units is possibleAn optionally usable heat exchanger allows the use of the waste heat of the exhaust gas for preheating the compressed airThe expansion unit can be designed in two stages, in which case the exhaust gas, after leaving the first expansion stage, can be fed to an exhaust-gas treatment system and, if appropriate, to a heat exchanger and then expanded in the second stage.Example MadeAn internal combustion engine (see FIG. 2 ) is shown in which the compression unit is designed as a screw compressor B with a compression ratio of 1:15. The compressed gas (160° C.) is passed over a heat exchanger E and heated in the process (297.7° C.). The decoupled expansion unit is realized as a piston machine with two-stage expansion (F 1 / F 2). In this case, the exhaust gas, after leaving the first expansion stage F 1 (788° C.), is passed through an exhaust gas treatment system and then, after passing through a heat exchanger, is fed to the second expansion stage for preheating the compressed gas (170° C.). The efficiency of the internal combustion engine is calculated on the basis of a calculation method for thermodynamic cycles and compared with the efficiency of a standardized spark ignition engine.Variable ExplanationQ 12: compression work (heat) of the compressor B m air mass k adiabatic coefficient c" isochoric specific heat capacity c p isobaric specific heat capacity R s specific gas constant V 1 volume before compression V 2 volume after compression compression ratio T 2a final temperature compressor B T 1 ambient temperature n polytropene exponent T 2b temperature before 1st isochoric heat input T 3 temperature after isochoric heat input T4temperature after 1st adiabatic Expansion t 5 Temperature after the 2nd isochronous heat input T 6 Temperature after the 2nd adiabatic expansion P 1 Ambient pressure P 2 Pressure after the compression P 3 Pressure after the 1st isochronous heat input P 4 Pressure after the 1st adiabatic expansion P 5 Pressure after the 2nd isochronous heat input P 6 Ambient pressure P 6= P 1 q12specific heat during the compression qinsupplied specific heat winsupplied Specific work w out Discharged specific work w net Net work η EfficiencyThe following starting conditions are assumed: The following starting conditions are assumed:T 1°C20293,15T 2a°C160433,15V 1m^30,001515P 1bar bar bar bar bar bar bar bar1P 2bar bar bar bar bar bar bar bar15P 4bar bar bar bar bar bar bar bar3,62T 3°C2126,852400T 3°C170443,15R sJ / kg*K287,058c vJ / kg*K0,718c pJ / kg* K1,005k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k1,40m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m mkg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg kg0,001782509Theoretical Thermodynamic CycleTheoretical Thermodynamic Cycle1→2 apolytropic compaction2 b →3Supply of Energy to Isochoric Heat3→4adiabatic expansion4→5Isochoric Heat Dissipation5→6adiabatic expansion6→1Isochoric Heat DissipationIn the compaction process we start from polytropic compaction. The following values are obtained:Calculation of Temperatures and PressuresEnergy balanceEfficiency EfficiencyThe efficiency of the internal combustion engine shown in the example is 68.87%. This is the theoretical, not the real, efficiency.Comparative calculation of a spark ignition engine with a compression ratio of 1:8The following starting conditions are assumed:Theoretical Thermodynamic Cycle Theoretical Thermodynamic Cycle1→ 2isentropic compression2→ 3Supply of Energy to Isochoric Heat3→4isentropic expansion4→1Isochoric Heat DissipationCalculation of Temperatures T1 and T2Energy balanceEfficiency EfficiencyThe efficiency of the comparative spark ignition engine is 56.5%. This is the theoretical, not the real, efficiency.The internal combustion engine described in the patent application thus has an efficiency which is increased by about 22% compared with the standard spark ignition engine.

Claims

Method for operating an internal combustion engine with a separately designed compressor and expansion unit, wherein the ignition of the fuel-gas mixture takes place via ignition sparks, and wherein the compressors and expansion unit are designed in different designs and are coupled or uncoupled depending on the load case, wherein the expansion unit is designed in two stages, and wherein the exhaust gas, after leaving the first expansion stage, is fed to a heat exchanger, which in turn heats the compressed air coming from the compressorInternal combustion engine according to Claim 1), wherein the compression unit is designed as a screw compressorInternal combustion engine according to Claim 1), wherein an energy store in the form of a compressed air store is mounted between the compression unit and the expansion unit.Internal combustion engine according to Claim 1), wherein the exhaust gas is fed to an exhaust-gas treatment system after leaving the first expansion stageThe method according to claim 1, wherein the working pressure after the compression stage is at a value of >10 bar

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