Patent Application
20120048218
The invention relates generally to a system and method for operating an internal combustion engine and, more specifically, to a system and method for operating an internal combustion engine in accordance with a Miller cycle for a turbocharged system.
In certain applications, the turbocharged engines are used in different environmental conditions. These environmental conditions can adversely affect engine performance, efficiency, exhaust pollutants, and other engine characteristics. For example, diesel engines operating in such environmental conditions are subject to greater loads, lower atmospheric pressures, lower temperatures due to colder climate, lower air density due to lower atmospheric pressure, and so forth. At such conditions, the compressor and the turbocharger speed can increase beyond a preselected limit without suitable control measures.
Surge is one unstable operating condition of the compressor. Most compressors have a stability limit that is defined by a minimum flow rate on a pressure-rise versus flow-rate characteristic curve. Surge margin refers to a margin of safety between a normal operating point and a stability limit of the compressor. Events, both external and internal to the compressor, may occasionally move compressor operation to a point that is beyond its stability limit, causing a surge condition.
A Miller cycle is a modification of a conventional Otto or Diesel cycle in a four-stroke internal combustion engine. The Miller cycle is aimed at reducing the effective compression ratio while maintaining the expansion ratio. This lowers the in-cylinder adiabatic compression temperature, which enables a reduction in nitrogen oxide (NOx) emissions.
The Miller cycle has been traditionally implemented by either closing the intake valve early before the end of the intake stroke, or closing the intake valve late during the compression stroke. In the prior method, the amount of intake charge is reduced, and is expanded to a lower pressure when the piston reaches bottom dead center (BDC), compared to the typical case where intake valve is closed near BDC at the end of the intake stroke. Because of the lower initial pressure, the pressure at the end of the compression stroke is reduced, resulting in a lower effective compression ratio. The latter method can be achieved by keeping the intake valve open until the start of the compression stroke, or closing the intake valve near the end of the intake stroke and reopening it during the compression stroke. Compression starts only when all the valves are closed in the compression stroke, thus the effective compression ratio is reduced. The lower amount of charge in the cylinder due to the Miller cycle would result in a loss in power. Thus a supercharger or turbocharger is typically used to compensate for this loss.
The Miller cycle typically requires increased boosting to maintain sufficient airflow, often times pushing the compressor closer to the surge limit. Redesign of the compressor increases the cost and complexity of implementing the Miller cycle. In another application, the Miller cycle is implemented together with a bypass loop for allowing compressed air to bypass the engine and mix with the exhaust stream. However, exhausting compressed air directly to the turbine results in wastage of energy. Also, such a strategy is not feasible in systems where an exhaust pressure is greater than an intake pressure.
For these and other reasons there is need for embodiments of the invention.
In accordance with one embodiment of the present invention, a method for operating an internal combustion engine in accordance with a Miller cycle is disclosed. The method includes moving a piston from a top dead center position towards a bottom dead center position in an engine cylinder and closing an intake valve of the internal combustion engine when a piston is about the bottom dead center position in the engine cylinder. The method further includes opening an exhaust valve for a predetermined time period when the piston is about the bottom dead center position of the engine cylinder after closing the intake valve so as to exhaust a predetermined quantity of fresh charge from the engine cylinder via the exhaust valve.
In accordance with another embodiment of the present invention, an internal combustion engine operated in accordance with a Miller cycle is disclosed. The engine includes a piston disposed in the engine cylinder and configured to reciprocate between a top dead center position and a bottom dead center position of the engine cylinder. An air intake valve is coupled to the cylinder. The air intake valve is closed when the piston is about the bottom dead center position in the engine cylinder. An exhaust valve is coupled to the engine cylinder. The exhaust valve is opened for a predetermined time period when the piston is about the bottom dead center position of the engine cylinder after closing the intake valve so as to exhaust a predetermined quantity of fresh charge from the engine cylinder via the exhaust valve.
In accordance with another embodiment of the present invention, an internal combustion engine operated in accordance with a Miller cycle is disclosed. The engine includes a controller communicatively coupled to the air intake valve and the exhaust valve and configured to control at least one of the air intake valve, the exhaust valve. The exhaust valve is opened for a predetermined time period when the piston is about the bottom dead center position of the engine cylinder after closing the intake valve so as to exhaust a predetermined quantity of fresh charge from the engine cylinder via the exhaust valve.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
In accordance with certain embodiments of the present invention, a method of operating an internal combustion engine in accordance with a Miller cycle is disclosed. The method includes moving a piston from a top dead center position towards a bottom dead center position in an engine cylinder and closing an intake valve of the internal combustion engine when a piston is about the bottom dead center position in the engine cylinder. The method includes opening an exhaust valve for a predetermined time period when the piston is about the bottom dead center position of the engine cylinder after closing the intake valve during a compression stroke of the piston so as to exhaust a predetermined quantity of fresh charge from the engine cylinder via the exhaust valve. In accordance with a specific embodiment of the present invention an internal combustion engine operated in accordance with a Miller cycle is disclosed. As discussed herein below, the exhaust valve of the engine is re-opened after closure of the intake valve during a compression stroke of the engine. In accordance with the embodiments of the present invention, the Miller cycle enables reduction in nitrogen oxide emissions with little or no penalty in fuel economy. This facilitates further optimization of the engine, whereby nitrogen oxide emissions are maintained within limits and fuel economy is improved. It should be noted herein although the “fresh charge” discussed herein is with reference to “fresh charge of air”, in certain other embodiments; the “fresh charge” may also include “fresh charge of recirculated exhaust gas”.
Referring to
In the illustrated embodiment, the turbocharged system 10 also includes a controller 30 communicatively coupled to the intake valve 17 and the exhaust valve 19 and configured to control at least one of the intake valve 17, the exhaust valve 19. In one embodiment, the controller 30 is an electronic logic controller that is programmable by a user. In another embodiment, the controller 30 is an electronic valve-timing controller for the engine 14 and specifically an exhaust valve-timing controller. A plurality of fuel injection pumps 32 drive a plurality of fuel injectors 34 for injecting fuel into the plurality of cylinders 21 of the engine 14. A piston 36 is slidably disposed in each cylinder 21 and reciprocates between a top dead center and a bottom dead center positions. As discussed in greater detail below, the engine 14 is operated in accordance with a Miller cycle. Instead of manipulating the intake valve closing time, the exhaust valve 19 is re-opened after closing of the inlet valve 17 for a short time period during the compression stroke to reduce the compression ratio.
Referring to
For a typical 2-stroke cycle engine and a 4-stroke cycle engine, a device, such as a compressor, is used to increase the flow of air into the engine cylinder. The compressor compresses the air and forces it into the intake manifold of the engine. Thus, more air at constant pressure is available as required during the cycle of operation. The increased amount of air, as a result of compressor action, fills the cylinder with a fresh charge. During the scavenging process, the increased amount of air facilitates to clear the combustion gases from the cylinder. In a two stroke cycle engine, the scavenging process occurs during a latter part of the piston down stroke (expansion stroke) and the early part of the piston upstroke (compression stroke). In a four stroke cycle engine, scavenging occurs when the piston is about the top dead center position during the latter part of a piston upstroke (exhaust) and the early part of a piston down stroke (intake stroke). The intake and exhaust valves are both open during the scavenging process. The overlap of intake and exhaust valve timings permits the air from the blower to pass through the cylinder into the exhaust manifold, cleaning out the exhaust gases from the cylinder and, at the same time, cooling the hot engine parts.
As discussed previously, surge is one unstable operating condition of the compressor. Events, both external and internal to the compressor, may occasionally move compressor operation to a point that is beyond its stability limit, causing a surge condition. The engine is operated in accordance with a Miller cycle to reduce compressor surge. Conventional implementation of Miller cycle in the engine involved altering the valve closing timing of an intake valve of the engine cylinder. Such implementation of Miller cycle is difficult to achieve with typical two-stroke engines, where the intake valve opening and closing event is often symmetric about bottom dead center. In accordance with the embodiments of the present invention, instead of manipulating the intake valve closing time, the exhaust valve is re-opened after closing of the inlet valve for a short period of time during the compression stroke to reduce the compression ratio. Implementation of Miller cycle in accordance with the embodiments of the present invention is equally feasible on both 2-stroke and 4-stroke engines.
As known to one skilled in that art, the compression ratio of an internal-combustion engine is a value that represents the ratio of the volume of a combustion chamber; from a largest capacity to a smallest capacity. In other words, compression ratio is the ratio between the volume of the combustion chamber when the piston is at the bottom dead center position, and the volume of the combustion chamber when the piston is at the top dead center position. A geometric compression ratio can be changed with a movable plunger at the top of the cylinder head. The effective compression ratio can be reduced from the geometric ratio by using a variable valve actuation (i.e. variable valve timing permitting Miller cycle) or by implementing a fixed-cam Miller-cycle strategy.
Geometric compression ratio (CRGeom) of the engine 14 is represented as:
where VBDC is the volume of the combustion chamber at the bottom dead center position in the cylinder 21 and VTDC is the volume of the combustion chamber at the top dead center position in the cylinder 21.
Miller compression ratio (CRMiller) of the engine 14 is represented as:
where VEVC is the effective volume of the combustion chamber of the cylinder 21 during compression stroke.
In accordance with the embodiment of the present invention, the exhaust valve 19 is re-opened after intake valve 17 is closed. In one embodiment, the exhaust valve 19 is re-opened after the intake valve 17 is closed at the bottom dead center position. In a specific embodiment, the exhaust valve 19 is opened after a predetermined time period after closure of the intake valve 17 during the compression stroke. In another specific embodiment, if the intake valve 17 is closed early, for example, 40 degrees before bottom dead center position; the exhaust valve 19 is opened while the piston 36 is still travelling down (nominally the intake stroke, although the intake valve 17 has closed). In such an embodiment, the exhaust valve 19 is also open during the early part of the compression stroke.
In accordance with another embodiment, if the intake valve 17 is closed early, for example, 40 degrees before bottom dead center position; the exhaust valve 19 is opened at bottom dead center or during the compression stroke so that the pressure in the cylinder 21 is sufficient to prevent exhaust gas from being drawn into the cylinder 21 only to be re-exhausted during the compression stroke. In accordance with yet another embodiment, the intake valve 17 is closed late, for example, 40 degrees after the bottom dead center position and the exhaust valve 19 is opened during the compression stroke after the intake valve 17 has closed.
In accordance with the embodiment of the present invention, the valve timing of the exhaust valve 19 is altered compared to a conventional Otto or diesel cycle engine. As the piston 36 moves upwards towards the top dead center position during the early part of the compression stroke, the fresh charge is partially expelled through the exhaust manifold 18 by opening the exhaust valve 19. Typically, such loss of charge air would result in a loss of performance due to less available air for the combustion reactants. However, in the Miller cycle, the loss of trapped air mass in the cylinder 21 can be compensated by increasing the intake manifold pressure. In accordance with the embodiments of the present invention, the Miller cycle is implemented in such a way that significant compression of the cylinder contents starts after the piston 36 has pushed out the “extra” charge and the exhaust valve 19 is closed. In one embodiment, the exhaust valve 19 may be closed at around 20% to 30% into the compression stroke. In other words, the actual compression occurs in the latter 70% to 80% of the compression stroke.
Referring to
As discussed previously, during the scavenging process, the increased amount of air facilitates to clear the combustion gases from the cylinder. In a two-stroke cycle engine, the scavenging process occurs during a latter part of the piston down stroke (expansion stroke) and the early part of the piston upstroke (compression stroke). In a four-stroke cycle engine, scavenging occurs when the piston is about the top dead center position during the latter part of a piston upstroke (exhaust) and the early part of a piston down stroke (intake stroke). The intake and exhaust valves are both open during the scavenging process. In the illustrated embodiment, the scavenging process is indicated by the area 60. The area 60 is the overlapping area of the projected portion 50 indicative of exhaust valve open position and the projected portion 58 indicative of the intake valve open position. The scavenging typically occurs when the piston is about the top dead center position in the engine cylinder.
In accordance with the exemplary Miller cycle of the present technique, when the piston is about the bottom dead center position of the engine cylinder, the exhaust valve is opened (in other words re-opened) after closing the intake valve so as to exhaust a predetermined quantity of intake air from the engine cylinder via the exhaust manifold. This is clearly evident from
In one embodiment, the closing of the intake valve and the re-opening of the exhaust valve may occur at the same instant. In another embodiment, the exhaust valve is re-opened after a predetermined time period after closing the intake valve. In a specific embodiment, the intake valve is closed at the bottom dead center position of the piston and the exhaust valve is opened during the compression stroke. In another specific embodiment, if the intake valve is closed early before bottom dead center position; the exhaust valve is opened while the piston is still travelling down and also maintained in an open state during the early part of the compression stroke.
In accordance with the embodiments of the present technique, the compression ratio is reduced by opening the exhaust valve early in the compression stroke after closing the intake valve. Under normal (non-Miller) operation, the compression ratio is effectively equal to the expansion ratio. In accordance with the Miller cycle of the present technique, the compression ratio is reduced without changing the expansion ratio. The effective compression ratio is reduced because the mixture in the cylinder does not undergo compression until the exhaust valve is closed during the compression stroke. This effectively reduces gas temperatures, enabling the start of combustion to be advanced without increasing the nitrogen oxide emission levels.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
