1. Field of the Invention
The present invention relates to an improved supercharged internal combustion engine which operates using the four stroke cycle and has a compressed air tank for the purpose of bridging turbo lag.
2. Discussion of Related Art
Reciprocating piston engines were invented more than 100 years ago and have been developed continuously since then, but their maximum degree of efficiency is subject to thermodynamic limits. If a reciprocating piston machine were operated constantly at its maximum degree of efficiency, approximately only half as much fuel would have been burned worldwide as is actually the case today. The main reason for the inefficient utilization of contemporary reciprocating piston engines is that the engine is usually operated only at part load (city traffic, constant speeds, . . . ), where the degrees of efficiency are poor. Since car drivers demand vehicles with a high maximum performance, the result is a high proportion of part load operation and therefore relatively high fuel consumption figures.
The simplest and least expensive solution concept for this problem is the use of reciprocating piston engines with a smaller cubic capacity, since they have a high degree of efficiency in most driving situations. In order that the desire for maximum performance by the car drivers is taken into consideration, said small reciprocating piston engine can be supercharged with the aid of a turbocharger which utilizes the exhaust gas enthalpy; the performance of a reciprocating piston engine with twice as large a cubic capacity can thus be achieved without it being necessary again to sacrifice the achieved gains in the degree of efficiency in the load range described.
This concept is simple and inexpensive and is already also partially used by car companies. However, in the case of this concept, specifically if applied to gasoline engines (it is standard in diesel engines), there is a reason why it is used rarely: what is known as “turbo lag”: when the driver requests acceleration at low rotational speeds of the supercharged reciprocating piston engine, there is a lack of air in the supercharged system, which results in poor response behavior.
This problem can be eliminated by the connection of a compressed air tank to the cylinders (=combustion chambers): if a great torque is requested by the driver, additional air can be introduced directly (!) into the combustion chambers by opening of the charging valve, in addition to the air which has already been introduced via the inlet valves. This additional air makes it possible to inject more fuel for the corresponding cycle (the 4 strokes of the reciprocating piston engine: intake of fresh gas—compression—combustion/expansion—ejection of the burned gases). As a result, not only is a higher torque produced in an instant for combustion, but also an increased exhaust gas enthalpy flow which drives the turbine and therefore also the compressor of the turbocharger. The compressor therefore compresses more air to a higher pressure level. The increased pressure on the inlet side of the reciprocating piston engine leads to more air entering the reciprocating piston engine during the inlet operation, whereby the “vicious circle of the lack of air” (engine first of all produces little torque; produces little exhaust gas enthalpy as a result; as a result, the turbocharger is not driven sufficiently; the compressor of the turbocharger delivers little fresh air; as a result, only a limited air mass passes into the combustion chambers; only a limited fuel quantity can thus be injected and, as a consequence, only little torque can be produced; . . . ) is interrupted. As a result, the additionally introduced air from the compressed air tank is necessary only for a short time period. As soon as the turbocharger has reached high rotational speeds, it supplies sufficient air for the production of the maximum torque.
The connection of a pressure tank to the combustion chambers of a reciprocating piston engine (called “pneumatic hybridization” in the following text) serves the purpose in many patents and scientific publications of it being possible to recuperate energy which becomes available during braking and is dissipated in conventional vehicle brakes by pumping and storing air in the compressed air tank (without the introduction of fuel). This compressed air can be used at another instant, at which the reciprocating piston engine is driven exclusively by air (without the introduction of fuel). A pneumatic start is thus also made possible. However, these advantages are secondary, as numerous examinations have shown (publication list at www.hpe.ethz.ch). The concept described in the preceding paragraph is responsible for a fuel saving of approximately 25%, whereas the advantages which are described in this paragraph raise the saving merely overall to approximately 32%. The present invention therefore concentrates on the pneumatic hybridization for bridging the turbo lag.
Overview of relevant existing patents with respect to the prior art in the field of pneumatic hybridization, sorted in time terms from old to new:
U.S. Pat. No. 1,013,528 is the first patent about a pneumatic hybrid engine. Here, a reciprocating piston engine is additionally used as compressed air expansion engine. The pneumatic engine start is likewise described. The compressed air tank is charged as soon as two cylinders are combusting and two cylinders are pumping; charging of the tank by utilization of the energy released during braking is not provided.
U.S. Pat. No. 3,765,180 likewise describes a reciprocating piston engine which can be operated both as an internal combustion engine and as a pneumatic motor. An external electric compressor is used to produce compressed air.
U.S. Pat. No. 3,963,379 is based partly on U.S. Pat. No. 1,013,528, but the brake energy can be used here to produce compressed air (pumping in the two stroke cycle). The patent provides axially adjustable camshaft profiles for all inlet, outlet and charging valves which make operation possible as an internal combustion engine, pump and pneumatic motor. Here, the charging valve is always actuated via one of three cam profiles: a zero cam which keeps the valve closed, a double cam for the two stroke pumping cycle and a single cam for the engine start, which single cam opens the charging valve when the expansion or combustion stroke takes place in the combustion cycle. Moreover, the described reciprocating piston engine functions in both rotational directions.
U.S. Pat. No. 5,529,549 is the first patent for a pneumatic hybrid engine, in which the use of an engine control unit is provided for actuating the valves on the basis of sensor signals. In addition, the patent describes (as first patent) a “supercharged” mode which is possible for this (non-turbocharged) engine. Here, air from the pressure tank is used exclusively to fill the engine; no air from the surroundings (normal inlet path) is used, and the fuel is injected directly into the combustion chamber. More air can therefore pass into the cylinder and more fuel can be injected compared with a normal internal combustion engine. The construction provides controllable valves which connect the inlet channel either to compressed air, to the inlet path (from the surroundings) or even not at all; a volume therefore exists which is used alternately by the inlet path and by the high pressure path on the way to the combustion chamber. The engine additionally provides a pneumatic motor mode, a normal combustion mode, a pump mode and a cylinder deactivation mode. No fluid-dynamic device for charging the engine is provided for the engine.
GB2402169 describes a reciprocating piston engine which, in addition to the combustion mode, also makes a pump mode and a pneumatic motor mode possible (two stroke and four stroke modes). For the pneumatic motor mode, a method is described, in which the expanded air is pushed out toward the inlet, in order that the air does not cause any problems for the catalytic converter operation. All the valves are actuated in a fully variable manner (electrohydraulically) and make it possible that various cylinders are operated in different modes. In addition, it describes what is also called “air power assist (APA) mode” in scientific publications. During an inlet stroke, first of all air is introduced via the inlet valve and subsequently air from the pressure tank is added during the same inlet stroke.
FR2865769A1 describes a reciprocating piston engine which is supercharged with a turbocharger and in which the pneumatic hybridization is used to bridge the turbo lag. The “supercharged” mode used for this purpose differs here from the “supercharged” mode in U.S. Pat. No. 5,529,549 in that the inlet path and the pressure tank path do not jointly utilize any volume on the way to the combustion chamber. Moreover, first of all here air is let out of the inlet and subsequently air is let out of the pressure tank. In contrast to GB2402169, the air from the pressure tank is not introduced here during the inlet stroke, but rather during the compression phase of the cylinder. Fully variable valve control systems are used for all valves; the pneumatic motor mode and pump mode are thus made possible in each case in the two stroke process.
WO2009/036992 describes, like FR2865769A1, a reciprocating piston engine which is supercharged with a turbocharger and in which the compressed air tank is connected directly to the combustion chambers with the aid of fully variable, preferably electrohydraulically actuated charging valves. In contrast, the inlet and outlet valves are driven via camshafts which always have the same lift profile during every four stroke cycle. The pump mode and pneumatic motor mode can therefore be operated in the four stroke process. In addition, the charging valve serves to make the “supercharged” mode possible analogously to FR2865769A1, in order to bridge the turbo lag. In a further embodiment, an exhaust gas pressure tank is used in addition to a compressed air tank. The exhaust gas tank is used for a new combustion cycle, in which gases burned at the end of the expansion are transferred into the exhaust gas tank. The pressurized exhaust gases can be used for a pneumatic motor mode in the four stroke process.