Abstract:
Exhaust gas recirculation is provided in a turbocharged diesel engine by adding a separate EGR manifold and a secondary exhaust valve for each combustion chamber that permits passage of exhaust gas from the combustion chamber to the EGR manifold. The secondary exhaust valve is opened during the expansion stroke of the engine cycle and sometime after the combustion process has been completed while the pressure in the combustion chamber is still greater than the pressure in the intake manifold. One or two EGR valves can be opened to admit the high pressure exhaust gas from the EGR manifold into the intake manifold. Decompression braking can be provided by an additional valve between the EGR manifold and the exhaust manifold that is opened to dump the gas from the EGR manifold to the exhaust manifold while the secondary exhaust valve in the cylinder head is opened at the beginning of the expansion stroke of the piston whereby the compressed air in the cylinder escapes before performing work on the piston.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an exhaust gas recirculation (EGR) system for a turbocharged, internal combustion engine. 
     2. Description of the Related Art 
     Exhaust gas recirculation is well known for internal combustion engines where part of the exhaust gas discharged from an engine is recirculated to the intake passage and injected back into the combustion chambers, along with air and fuel, to decrease the combustion temperature thereby reducing the amount of nitrogen oxides in the exhaust gas. 
     Two principle means of implementation of exhaust gas recirculation have been suggested for application to turbocharged diesel engines. In the first method, known as “low-pressure” loop, exhaust gas is routed from the turbine outlet to the compressor inlet. This method suffers from the drawbacks of fouling the compressor wheel and housing with exhaust deposits, possible overheating of the compressor wheel and the potential for severe fouling of the air-to-air charge air cooler. In the second method, known as “high-pressure” loop, exhaust gas is routed from the exhaust manifold, before the turbocharger turbine, directly into the engine&#39;s intake manifold (thereby eliminating the fouling potential of the low-pressure loop). This method suffers from the drawback that the pressure in the exhaust manifold must be greater than the pressure in the intake manifold. Most well-developed, heavy-duty, turbocharged diesel engines operate with intake manifold pressures that are higher than the exhaust manifold pressures. This partially accounts for the diesel engines&#39; excellent fuel economy characteristics. To cause the engine&#39;s exhaust pressure to be higher than the intake pressure requires that a relatively inefficient turbocharger configuration be fitted to the engine or a back pressure device be fitted following the turbocharger&#39;s turbine stage to cause the exhaust pressure to be higher than the intake manifold pressure. Poor fuel economy will be expected for either of these types of high-pressure loop arrangements. 
     SUMMARY OF THE INVENTION 
     The EGR system of the present invention uses a high-pressure loop in a manner that enables the intake manifold pressure to remain higher than the exhaust manifold pressure. This is accomplished by adding a separate EGR manifold and an additional exhaust valve for each combustion chamber, referred to herein as a ‘secondary exhaust valve’, that permits passage of exhaust gas from the combustion chamber to the EGR manifold. The secondary exhaust valve can be actuated by a mechanical, hydromechanical or electro-hydromechanical actuator in such a way that the valve can be opened and closed as a function of the rotational position of the engine&#39;s crankshaft. The opening of the secondary exhaust valve occurs during the expansion stroke of the engine cycle, after the combustion process has been completed. The valve is closed at a point near the opening of the primary exhaust valve or valves. 
     Exhaust gas exits from the EGR manifold to the intake manifold, or other conduit for pressurized air, downstream of the turbocharger compressor. Exhaust gas exit is provided by an EGR valve that is controlled so that a pressure can be created in the EGR manifold and controlled to a higher level than exists at any moment in the intake manifold. The EGR valve can be controlled so that the amount of exhaust gas being fed into the intake manifold can be controlled over a wide range of flow rates as desired for optimum levels of emissions reduction and minimal fuel consumption penalty. The EGR valve is controlled by the engine&#39;s ECU (engine control unit). 
     A decompression brake valve can be provided with the EGR system that connects the EGR manifold and the engine&#39;s exhaust manifold. When the decompression brake valve is held open and the EGR valve is closed, the secondary exhaust valve in each combustion chamber can be opened at the beginning of the expansion stroke and closed at the end of the expansion stroke such that engine decompression braking is achieved. 
     With the EGR system of the present invention, the engine&#39;s exhaust manifold and turbocharger can operate in the same type of efficient manner as is characteristic of current turbocharged engines without an EGR system. It is believed that with such a system, the engine will operate with better fuel economy as compared to engines equipped with any of the EGR systems known in the art at this time. 
     A further feature of the invention provides two EGR valves at the outlet of the EGR manifold. One valve allows the exhaust gas to be directed into the intake manifold without passing through a cooler. This will enable the diesel engine to start at lower ambient temperatures and warm up more quickly than engines without such a controllable exhaust gas recirculation system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an engine having the exhaust gas recirculation system of the present invention. 
     FIG. 2 is a schematic sectional view through the cylinder head and one combustion chamber of the engine shown in FIG.  1 . 
     FIG. 3 is a graph of the combustion chamber pressure versus crankangle illustrating the valve opening and closing during exhaust gas recirculation. 
     FIG. 4 is a graph of the valve lift versus crankangle for the three different valves of the engine. 
     FIG. 5 is a graph of the combustion chamber pressure versus crankangle illustrating the valve opening and closing during decompression braking. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A multi-cylinder turbocharged diesel engine having the exhaust gas recirculation system of the present invention is shown schematically in FIG.  1  and designated generally at  10 . The engine  10  includes a cylinder head  12  that contains at least one primary exhaust valve  16 , two intake valves  18  and a secondary exhaust valve  20  for each of the cylinders or combustion chambers  14 . While the illustrated engine  10  has two intake valves  18  and one primary exhaust valve  16 , it will be understood that the EGR system of the present invention can be used with engines having any number of intake and primary exhaust valves. 
     Exhaust gas flows from each combustion chamber through a primary exhaust port  22  in the cylinder head to an exhaust manifold  24 . From there the exhaust gas flows through an exhaust inlet  26  to a turbine  28  of a turbocharger  30 . The turbocharger  30  includes an air compressor  32  that delivers fresh air under pressure, through a cooler  34 , to an intake manifold  36  through a connecting pipe  38 . From the intake manifold  36 , the air passes through intake ports  40  in the cylinder head into the combustion chambers  14 . 
     A secondary flow of exhaust gas from each combustion chamber is created through the secondary exhaust valves  20  and the associated exhaust ports  42  in the cylinder head  12 . The flow of exhaust gas through the secondary exhaust valves  20  is accumulated in a second exhaust manifold, referred to herein as an EGR manifold  44 . The secondary exhaust valve  20  is shown in FIG. 2 together with the cylinder head, a cylinder block  46  and a piston  48  reciprocal within the cylinder or combustion chamber  14 . 
     With reference once again to FIG. 1, the exhaust gas accumulated in the EGR manifold  44  can be introduced into the intake air flow in the connecting pipe  38  or the intake manifold  36  by operation of either or both of the EGR valves  50  and  52 . The EGR valves  50 ,  52  can one of various known types of EGR valves. They may be on/off valves or linear valves capable of variable gas flow rates depending on the EGR flow control scheme that is utilized. Exhaust gas flowing through the EGR valve  50  also passes through an exhaust gas cooler  54  that is cooled either by air or by the engine coolant. This cools the temperature of the exhaust gas before the exhaust gas is mixed with the intake air. 
     The EGR system can also be provided with an additional EGR bypass or braking valve  58 . The bypass valve  58  is a flow control valve such as a butterfly valve. The valve  58 , when opened, allows exhaust gas to pass directly from the EGR manifold  44  to the exhaust manifold  24 . The braking valve  58  is used in conjunction with the operation of the secondary exhaust valve  20  and closure of the EGR valves  50  and  52  during the engine operating mode known as ‘decompression braking’ as described below. 
     The secondary exhaust valves  20  are actuated by actuators  56  that allow opening and closing of the valves as a function of the rotational position of the engine crankshaft. Furthermore, the engine&#39;s electronic control unit (ECU)  60  electronically controls the actuators  56 . The valve actuators  56  may be any of various mechanical, hydromechanical or electro-hydromechanical actuators that enable the opening and closing of the secondary exhaust valve to be variably controlled as described below. For example, the actuator  56  may be an electromechanical variable valve actuator of the type shown and described in U.S. Pat. No. 5,515,818 or a hydraulically operated valve actuator of the type shown and described in U.S. Pat. No. 5,058,614. The EGR valves  50  and  52  and the EGR bypass valve  58  are also electronically controlled by the ECU  60 . Each of these valves  50 ,  52 ,  58  can be independently regulated by the ECU. 
     In a first engine operating mode, the valves are controlled to achieve exhaust gas recirculation. This results in delivery of exhaust gas from the exhaust system to the engine&#39;s intake manifold for mixing with intake air and delivery to the engine&#39;s combustion chambers  14 . The opening and closing of the secondary exhaust valves  20  is a function of crankshaft position as shown in FIG.  3 . Opening of the secondary exhaust valve  20  occurs during the expansion stroke of the engine cycle; i.e., while the piston  48  is moving downward toward what is described in the art as the bottom dead center position. This is shown in FIG. 3 by the two vertical lines designated ‘SEVO’ for ‘secondary exhaust valve opening’ and ‘SEVC’ for ‘secondary exhaust valve closing.’ The secondary exhaust valves are open during the range of crank angles labeled A. The secondary exhaust valve is opened prior to the opening of the primary exhaust valve(s)  16 . The secondary exhaust valve is open when the pressure of the exhaust gas in the cylinder is greater than the pressure in the intake manifold as shown by the horizontal line P int  in FIG.  3 . The opening and closing of the primary exhaust valve is designated by the lines ‘PEVO’ and ‘PEVC’ while the opening and closing of the intake valve is designated by the lines ‘IVO’ and ‘IVC.’ The valve lift of all three valves is shown in FIG.  4 . 
     The secondary exhaust valve  20  will be held open a specified amount of time and then be closed by the ECU. In relation to the opening of the primary exhaust valve, closing of the secondary exhaust valve can be as shown in FIG. 3 in which the secondary exhaust valve closes after the opening of the primary exhaust valve, or the secondary exhaust valve can close at the same time or prior to the opening of the primary exhaust valve. Flow of exhaust gas from the combustion chamber to the EGR manifold  44  may continue during the period of time while the secondary exhaust valve is opened or exhaust gas may cease flowing if the instantaneous pressure in the manifold  44  equals the instantaneous pressure in the combustion chamber. 
     The ECU also controls the opening and closing of the EGR valves  50  and  52  as well as the EGR bypass or braking valve  58 . The position of the valves  50 ,  52 ,  58  can be programmed in such a way that a pressure is created, maintained and controlled in the EGR manifold  44 . The pressure created in the manifold  44  will be dependant on the rate of gas entering the manifold versus the rate at which gas is allowed to exit the manifold based on the instantaneous positions of the valves  50 ,  52 ,  58 . By maintaining a pressure in the EGR manifold that is greater than the intake manifold pressure, exhaust gas will flow into the intake manifold when one or both of the EGR valves  50 ,  52  are opened. 
     During the exhaust gas recirculation operating mode, the EGR bypass valve  58  is held in the closed position to prevent the flow of exhaust gas from the EGR manifold  44  to the exhaust manifold  24 . In the first case of implementation of exhaust gas recirculation, without use of EGR cooler  54 , the EGR valve  50  is held in the closed position. The pressure in the EGR manifold  44  is controlled by the ECU by control of the position of the EGR valve  52 . The valve  52  allows the exhaust gas to bypass the cooler  54  for cold temperature starting and for a faster engine warm-up. The EGR valve  52  is thus also known as the cold start EGR valve. In the second case of implementation of exhaust gas recirculation, including the use of exhaust gas cooler  54 , the EGR valve  52  is held in the closed position. The pressure in the EGR manifold  44  is controlled by the ECU by control of the position of the EGR valve  50 . 
     In a second engine operating mode, the valves are controlled to achieve decompression braking. Implementation of decompression braking is accomplished by holding both the EGR valves  50 ,  52  in the closed position while the EGR bypass or braking valve  58  is held in the open position. The secondary exhaust valve  20  for each cylinder is then opened and closed as a function of rotational position of the crankshaft as shown in FIG. 5, in the range B. Opening of the secondary exhaust valve occurs at a rotational position of the crankshaft corresponding near to the “top dead center” position of the piston travel at the end of the compression stroke. 
     Closing of the secondary exhaust valves  20  is controlled by the ECU to occur nearer to the bottom dead center position of the piston&#39;s travel. The opening and closing action of the secondary exhaust valve  20  will cause the pressure built up in the cylinder during the engine&#39;s compression stroke to be expelled through the EGR manifold  44  to the exhaust manifold  24  before the gas can be expanded in the combustion chamber. Thus, the amount of work done by the piston to compress the gas during the engine&#39;s compression stroke greatly exceeds the amount of work done by the gas on the piston during the engine&#39;s expansion stroke, thereby resulting in the ability of the engine to absorb a significant amount of kinetic energy. 
     In FIG. 5, the dashed line  62  shows a typical combustion chamber pressure during normal motoring operation of the engine. The solid line  64  illustrates the combustion chamber pressure during the decompression braking operating mode. The closing of the secondary exhaust valves  20  can occur anytime during the expansion stroke after the combustion chamber pressure has dropped to or below the intake manifold pressure. This may occur before the bottom dead center position of the piston. 
     Electronic control of the secondary exhaust valves  20  enable the opening and closing of the secondary exhaust valves to occur at different times in the engine cycle based on the engine operating mode. Furthermore, the timing of the opening and closing of the secondary exhaust valves  20  during EGR operation can be varied to optimize the emissions reduction while minimizing the fuel consumption penalty. 
     The EGR system of the present invention enables the engine to be controlled for optimum emissions reduction while minimizing the fuel consumption penalty. The disadvantages of prior EGR systems for turbocharged diesel engines have been avoided. The EGR system of the present invention may also be applicable to turbocharged spark-ignition engines as well. 
     The invention should not be limited to the above-described embodiment, but should be limited solely by the claims that follow.