Abstract:
A turbocharger ( 32 ) creates intake manifold boost for a diesel engine ( 10 ). At times, exhaust valve opening is increasingly retarded in relation to the engine operating cycle to cause the turbocharger to increase boost, engine fueling is also increased in relation to the increased boost, and in response to any incipient surging of the compressor resulting from such increasingly retarded exhaust valve opening and such increased engine fueling, compressed charge air is bled from the intake manifold to counter the incipient surging and thereby avoid any significant turbocharger surge.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to turbocharged diesel engines that propel motor vehicles and are equipped with variable valve actuation, and in particular to a control strategy for increasing engine torque without undesirable consequences on tailpipe emissions, such as diesel engine exhaust smoke, especially at lower engine speeds where turbocharger boost is relatively low. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     A turbocharger is one type of device that is used to supercharge an internal combustion engine. A diesel engine that is supercharged by a turbocharger is sometimes referred to as a turbocharged diesel. A turbocharger comprises a turbine that is powered by engine exhaust gas and coupled by a shaft to operate a compressor that boosts pressure in the engine air intake system downstream of the compressor. Boost is controlled by controlling turbine operation. 
     A strategy for controlling turbine operation needs to consider the particular type of turbocharger. One type of turbocharger has a variable geometry, or variable nozzle, that is capable of changing the manner in which exhaust gas that flows through the turbocharger interacts with the turbine. Movable vanes are selectively positioned to control the nature of exhaust gas interaction with the turbine, and hence control boost. The turbocharger includes an electromechanical actuator for interfacing an electric control with the movable vanes. That actuator comprises a solenoid for setting vane position according to a control signal from the electric control. The control signal is developed according to a desired control strategy. 
     A waste-gate type turbocharger controls the proportion of exhaust gas that is allowed to interact with the turbine by controlling the extent to which a waste gate valve that diverts exhaust gas from the turbine is allowed to open. The waste gate valve may be operated by an electric actuator to which a control signal is applied. 
     It is believed fair to say that a turbocharger is generally considered to be a device for improving engine performance. A turbocharger is typically designed for higher engine speeds, because the amount of engine exhaust that is available to act on a turbine of a turbocharger at low engine speeds is usually insufficient for the turbocharger compressor to develop sufficient boost to render it effective in contributing to improved performance at those low speeds. 
     It has been discovered however that certain turbocharged diesel engines, especially engines that have variable valve timing, can develop increased low speed torque without undesirable consequences on tailpipe emissions, such as smoke in the engine exhaust. This improvement is achieved by certain conjunctive control of: 1) time at which the engine exhaust valves open during an engine operating cycle and 2) engine fueling. In general, the conjunctive control comprises retarding, i.e. delaying, the opening of the exhaust valves while increasing the fueling to maintain a desired air-fuel ratio in the combustion chambers. The improvement can provide a significant increase in engine torque during low speed operation of the engine without significant adverse effect on tailpipe emissions. 
     The process of exhausting products of combustion from a combustion chamber of a diesel engine may be considered to comprise two phases: 1) a blow-down phase where the exhaust gas pressure is large enough to induce exhaust gas flow through an open exhaust valve; and 2) a pump-out phase where the moving engine mechanism is reducing the swept volume of the combustion space to an extent that forces exhaust gases out through the open exhaust valve. The blow-down phase will commence immediately upon opening the exhaust valve while the pump-out phase will occur later. For example, if the exhaust valve for an engine cylinder is opened as a piston is completing a power downstroke within the cylinder in advance of the piston&#39;s arrival at bottom dead center (BDC), the blow-down phase will commence in advance of BDC. It may also continue into the ensuing exhaust upstroke of the piston until the pressure drops to an extent insufficient to induce continued exhaust flow or until the upstroking piston has reduced the swept volume sufficiently to create pressure that forces the exhaust gases out through the open exhaust valve. Testing has shown that retarding the timing of exhaust valve opening can create more effective exhaust blow-down that is beneficial to turbocharger operation, particularly at low engine speeds where a turbocharger may have heretofore been considered relatively ineffective in improving engine performance. 
     Because certain principles of the present invention include changing the time in the engine operating cycle when the exhaust valves open, the engine must have an appropriate mechanism for each exhaust valve. An example of such a mechanism comprises an electric actuator for opening and closing an exhaust valve in accordance with an electric signal applied to the actuator. Such an engine is sometimes referred to as a camless engine, particularly where the engine intake valves are also controlled by electric actuators. When the inventive strategy is invoked, the timing of the opening of each exhaust valve during the engine cycle is increasingly retarded. 
     By retarding exhaust valve opening, the in-cylinder burning time for particulates is increased, and this reduces particulate emission. Retarding the exhaust valve opening has also been discovered to provide increased energy input to the turbocharger compressor, thereby increasing boost, and it is believed that this discovery is a departure from presently prevailing knowledge. As boost increases and smoke decreases, engine fueling is also increased to develop increased engine torque so that the additional fueling is not adverse to tailpipe emissions in any significant way. In this way, the turbocharger is forced toward operating at its performance limit, thereby enabling the engine to develop a corresponding torque that is greater than the torque that would otherwise be achieved. 
     The consequence of retarding exhaust valve opening in conjunction with increasing engine fueling may however affect turbocharger operation. One possible consequence is undesirable surging of the turbocharger compressor that may occur should the exhaust flow acting on the turbine force the turbocharger to operate beyond its performance limit. In order to avoid such surging, a bleed valve at the compressor outlet operates at, or in anticipation of, incipient compressor surging to bleed compressed charge air from the intake system sufficiently to counteract, or prevent, the incipient surging. The compressed charge air is bled from the intake system in a manner that allows intake manifold pressure to increase without turbocharger surging. Because of this ability to achieve increased intake manifold pressure without accompanying turbocharger surging, the turbocharger is enabled to operate at or near its performance limit, even during low-speed engine operation, and in addition, the basic construction of the turbocharger, which is typically designed with high speed, rather than low speed, operation in mind, does not have to be modified or altered in order to implement principles of the present invention in an engine. Association of a bleed valve with the engine intake system to bleed compressed charge air from the intake system is sufficient, possibly with an additional sensor or sensors, and incorporation of an appropriate algorithm in the engine control processor. 
     A primary aspect of the present invention relates to a novel strategy for controlling exhaust valve opening in a turbo-diesel engine that has a variable valve actuation apparatus. When the engine is running at less than peak torque speed, the engine control system causes the exhaust valves to open at a later time during the engine cycle than they would in an engine that has a camshaft operating the exhaust valves. The extent to which the control system retards exhaust valve opening is a function of one or more selected variables, such as engine speed, engine load, boost, brake specific fuel consumption (BSFC), and vehicle acceleration. 
     One aspect of the present invention relates to a novel strategy for a turbocharged internal combustion engine, especially a turbocharged compression ignition, or diesel, engine that has variable valve actuation. The disclosed strategy is implemented via a processor-based engine control and utilizes data relating to certain engine operating parameters to control the bleed of compressed charge air from the engine intake system. The data is processed according to a software algorithm that is executed by the processor to develop data for a control signal that is applied to an electric-operated bleed valve at the outlet of the compressor of the turbocharger. The controlled bleeding counters any incipient surging of the turbocharger resulting from increasingly retarding the timing of exhaust valve opening and accompanying increased fueling. 
     One general aspect of the claimed invention relates to an internal combustion engine comprising an intake system through which charge air is delivered to an intake manifold of the engine, including a turbocharger that comprises a compressor operated by exhaust gases from the engine for creating compressed charge air that provides boost in the intake manifold. A bleed for bleeding some of the compressed charge air away from the intake manifold allows the intake manifold pressure to increase without turbocharger compressor surging. A control controls the opening of engine exhaust valves in relation to an engine operating cycle, fueling of the engine in relation to the engine operating cycle, and the bleed. At times, the control increasingly retards exhaust valve opening in relation to the engine operating cycle to cause the turbocharger to increase intake manifold pressure, increases engine fueling in relation to the increased intake manifold pressure, and in response to any incipient surging of the compressor resulting from the effect on engine exhaust gases of such increasingly retarded exhaust valve opening and such increased engine fueling, operates the bleed to counteract such compressor surging. 
     Another general aspect of the claimed invention relates a method for an engine as just described wherein at times, exhaust valve opening is increasingly retarded in relation to the engine operating cycle to cause the turbocharger to increase intake manifold pressure, engine fueling is increased in relation to the increased intake manifold pressure, and in response to any incipient surging of the compressor resulting from the effect on engine exhaust gases of such increasingly retarded exhaust valve opening and such increased engine fueling, the bleed is operated to counteract such compressor surging. 
     Further aspects of the claimed invention relate to a software algorithm that is embodied in the engine control processor for accomplishing the method just described. 
    
    
     The foregoing, along with further aspects, features, and advantages of the invention, will be seen in this disclosure of a presently preferred embodiment of the invention depicting the best mode contemplated at this time for carrying out the invention. This specification includes drawings, briefly described below, and contains a detailed description that will make reference to those drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a general schematic diagram of an engine, in accordance with principles of the present invention. 
     FIG. 2 is a flow diagram of an algorithm used in practicing the invention, 
     FIGS. 3-11 are various graphs useful in understanding the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates an internal combustion engine  10  that powers a motor vehicle. An example of such a vehicle is a truck having a chassis containing a powertrain in which engine  10  is a fuel-injected turbocharged diesel engine operatively coupled through a drivetrain to driven wheels for propelling the vehicle. The engine has variable valve actuation that allows the time of exhaust valve opening to be controlled according to engine operation. 
     Engine  10  comprises an intake system  12  through which charge air is delivered to an intake manifold  14  of engine  10 . Charge air enters each engine cylinder  16  from manifold  14  via a corresponding intake valve  18 . Individual fuel injectors  20  inject diesel fuel into individual engine cylinders in properly timed relation to engine operation. Engine  10  also comprises an exhaust system  22  for conveyance of exhaust gases created by combustion within the engine cylinders from the engine. Exhaust gases pass out of each cylinder via a respective exhaust valve  24 . 
     Engine  10  may be a camless engine, meaning one where each of the normally closed intake and exhaust valves is opened at the proper time in the engine operating cycle by applying an electric signal to a respective electric actuator. 
     An electronic engine control  30  that possesses digital processing capability is associated with engine  10 . Control  30  may comprise one or more processors that process data from various input data signal sources in accordance with programmed algorithms to develop certain data for signals used in the performance of various functions associated with operation of engine  10 . The data processed by control  30  may originate at external sources (input variables) and/or be generated internally of control  30  (local variables). Control  30  develops the data for the signals that operate the intake and exhaust valve actuators and for the signals that operate fuel injectors  20 . 
     Turbocharging of engine  10  is accomplished by a turbocharger  32  which comprises a turbine  34  connected in exhaust system  22  coupled via a shaft  36  to a compressor  38  connected in intake system  12 . Compressor  38  is operated by exhaust gases from engine  10  that act on turbine  34  to create compressed charge air that provides boost in intake manifold  14 . 
     A bleed valve  40  comprises an inlet communicated to the compressed charge air in intake system  12 . Bleed valve  40  may for example be mounted at the outlet of compressor  38 . Bleed valve  40  comprises an electric actuator that controls the extent to which bleed valve is allowed to open. The actuator is electrically connected with engine control  30 . When bleed valve  40  is open, it bleeds compressed charge air out of intake system  12 . The extent to which the valve is open determines the extent of bleeding. 
     Engine control  30  contains a software program that implements an algorithm for control of bleed valve  40 , in conjunction with control of engine fuel via fuel injectors  20  and control of exhaust valves  24 . That algorithm is presented in FIG. 2 where it is designated by the reference numeral  50 . 
     When algorithm  50  is executed, it performs a series of steps designated by the general reference numeral  50 , the first of which is a start step  52 . Once the start step has concluded and the engine has started, a subsequent step  54  determines a) if the control  30  (ECU) is on, i.e. powered up and running, and b) if the engine is running at a speed less than peak torque speed. If these two conditions a) and b) are not satisfied, then the timing of exhaust valve opening is reset to a baseline value (step  55 ), after which steps  52  and  54  repeat. When the two conditions a) and b) are satisfied, then step  56  causes the ECU to retard the timing of the opening of exhaust valves  24  in relation to the base line timing value. In the example of algorithm  50 , exhaust valve opening is retarded by an additional five degrees of engine crankshaft rotation from the baseline value in the engine cycle. 
     The next step  58  causes the control to increase engine fueling in accordance with the increased boost resulting from retarding the timing of exhaust valve opening. The increased fueling serves to maintain a desired fuel-air ratio. The next step  60  of algorithm  50  determines if turbocharger  32  began to surge as a result of the delay in opening the exhaust valves. If the turbocharger did not begin to surge, the algorithm loops back to step  54 , and if conditions a) and b) continue to be satisfied, steps  56 ,  58 , and  60  repeat. As long as the two conditions a) and b) continue to be satisfied, exhaust valve opening is increasingly delayed by every ensuing iteration of steps  56  and  58 . 
     Eventually however, the retardation will become enough to cause turbocharger surging. Hence, when step  60  determines that turbocharger  32  is beginning to surge, control  30  then begins opening valve  40 , as indicated by step  62  of algorithm  50 . Valve  40  is initially opened one increment. Step  62  is again performed to determine whether the incipient surging is being counteracted. If not, the algorithm executes step  62  again to cause the valve to open more by applying an additional increment to the control signal for the valve. Step  62  will continually repeat to increasingly open valve  40  until step  60  determines that the incipient surging has been counteracted. 
     When such a determination has been made, the algorithm returns to step  54 . 
     FIG. 3 illustrates an example of a compressor speed map for a known turbocharger. A surge line  100  divides a zone of stable turbocharger operation  102  from a zone of unstable operation  104 . Within zone  102 , known relationships exist between the three parameters presented, namely pressure ratio, reduced mass flow rate, and speed. Let it be assumed that the turbocharger is operating with stability at the operating point marked by the reference numeral  106 . If the opening of the exhaust valves is now increasingly delayed or/and engine fueling increased beyond full load fueling, the pressure ratio will increase faster than the air flow which the engine demands. As a result the turbocharger operating point will begin to migrate along a line segment  108  from point  106  toward surge line  100 . As the opening of the exhaust valves continues to be increasingly delayed or/and engine fueling continues to be increased, the engine operating point will move along a line segment  110  that crosses the surge line and enters zone  104 . The turbocharger will therefore begin to surge as the operating point moves across the surge line at location  112 . By bleeding some of the compressed charged air from intake system  12  via bleed valve  40 , the operating point can be returned to the stable zone  102 , such as along a line segment  114 , instead of along line segment  110 . In this way, the compressed charge air is bled from the intake system so as to further increase intake manifold pressure without compressor surge. As a result, turbocharger stability is achieved at a pressure ratio and a mass flow rate that are both increased relative to point  112 . 
     FIG. 4 comprises a plot  120  defining a relationship of the extent of bleed valve opening to the time during the engine cycle at which the exhaust valves begin to open, as measured in degrees of engine crankshaft rotation. It also comprises a second plot  122  defining a relationship of turbocharger speed to the time at which the exhaust valves begin to open. FIG. 4 suggests that as the beginning of exhaust valve opening is increasingly delayed, more compressed charge air needs to be bled through valve  40  in order to prevent turbocharger surging. Plot  120  represents a minimum flow area for bleed, as a function of beginning of exhaust valve opening, that is needed to prevent turbocharger surging. Although retarding exhaust valve opening may result in an engine pumping loss due to compression of gases in a cylinder which is not yet open to exhaust system  22 , those gases will have increased effectiveness on turbine  34  when they do enter the exhaust system and pass through the turbocharger toward the tail pipe if the valve opening is delayed sufficiently to allow the pumping out phase to be effective on the exhaust gases. 
     FIG. 5 comprises a plot  130  defining a relationship of air-fuel ratio to the time during the engine cycle at which the exhaust valves begin to open, as measured in degrees of engine crankshaft rotation. It also comprises a second plot  132  defining a relationship of smoke to the time at which the exhaust valves begin to open. FIG. 5 shows that air-fuel ratio increases and smoke decreases in consequence of the increased turbocharger boost created by increasingly retarding the beginning of exhaust valve opening. 
     FIG. 6 comprises a plot  140  defining a relationship of brake specific fuel consumption (BSFC) to the time during the engine cycle at which the exhaust valves begin to open, as measured in degrees of engine crankshaft rotation. It also comprises a second plot  142  defining a relationship of engine torque to the time at which the exhaust valves begin to open. FIG. 6 shows that the useful benefits shown by FIG. 5 come at the expense of increased fuel consumption and reduced engine torque occasioned by the increased engine pumping loss. 
     Because of the reduction in smoke however, additional fueling can be introduced into the engine. FIGS. 7,  8 , and  9  comprise six plots  150 ,  152 ,  160 , 162 ,  170 , and  172  defining relationships of engine torque, turbocharger speed, air flow rate, smoke, power output, and tailpipe, or exhaust stack, temperature respectively to the time during the engine cycle at which the exhaust valves begin to open, as measured in degrees of engine crankshaft rotation. The relationships are given for the same fixed fueling. They show that retarding the beginning of exhaust valve opening from the baseline (around 135 deg. ATDC in this case) increases the torque up to a point with increase in turbocharger speed. Thereafter, torque tails off slightly as turbocharger speed continues to increase. 
     At the point where torque begins to increase as a result of the beginning of exhaust valve opening being increasingly retarded, exhaust energy is more efficiently used by the turbocharger, increasing turbocharger speed even more. The resultant increased boost increases the air flow rate. Smoke is reduced because of the lengthened burn-off time inside the cylinder before the exhaust valve opens and a larger air-fuel ratio. 
     FIG. 10 shows a plot  180  of engine torque as a function of percent over-fueling and a plot  182  of smoke as a function of percent over-fueling at an increased retardation of the beginning of exhaust valve opening. For the same smoke emission as baseline smoke, FIG. 10 shows that 30% more fuel can be added to the engine due to the retardation, resulting in a similar increase in engine torque. 
     FIG. 11 shows four traces  190 ,  192 ,  194 ,  196  of exhaust pressure versus engine crankshaft angle for four different timings of the beginning of exhaust valve opening. Trace  190  represents exhaust pressure versus engine crankshaft angle for a reference timing. Trace  192  represents exhaust pressure versus engine crankshaft angle for an advanced timing. Trace  194  represents exhaust pressure versus engine crankshaft angle for a retarded timing. Trace  196  represents exhaust pressure versus engine crankshaft angle for an over-retarded timing. 
     The exhaust process from a cylinder assumes the blow-down and pump-out phases described earlier. When timing is advanced from the reference timing, as shown by comparing trace  192  to trace  190 , energy is added to the exhaust gases and thus turbocharger speed increases. Because the energy is added to the rising edge of the exhaust pressure pulse where the pressure is already increasing, the energy use is of lower efficiency to the turbocharger. Energy is taken from the engine and thus engine torque is reduced. 
     When timing is retarded from the reference timing, as shown by comparing trace  194  to trace  190 , a smoother transition appears in the exhaust pressure from the blow-down phase to the pump-out phase. To the turbocharger, this means more efficient use of exhaust energy. In addition, because the retarded beginning of the exhaust valve opening delays the blow-down into the exhaust stroke, the upstroking piston in the engine cylinder slows down the pressure drop in the blow-down period. For the engine, this means additional pumping loss as the piston works against a higher gas pressure. Nevertheless, the gain in expansion work outweighs the pumping loss so that torque output increases and eventually reaches a peak which indicates an optimized beginning of exhaust valve opening for maximum torque output with minimum BSFC. For the turbine, this means additional energy, leading to increased turbocharger speed, boost pressure, and intake flow rate as shown by FIGS. 7-9. As energy is added to the falling the edge of the pressure pulse, the energy use is of high efficiency to the turbocharger. 
     When timing is over-retarded relative the reference timing, as shown by comparing trace  196  to trace  190 , pumping loss outweighs the gain in work and engine torque begins to drop. As turbocharger speed continues to increase, so does the intake air flow rate. This is because the pumping loss to the engine becomes energy added to the exhaust, reflected as a widened and taller exhaust pressure pulse in trace  196 . Because of smoke limiting at low engine speeds due to insufficient intake air mass, over-retarding the beginning of exhaust valve opening may provide more air and thus permit more over-fueling for torque improvement. In other words, there is a potential for torque improvement by over-retarding, but such torque gain may be accompanied by an increase in BSFC, when compared to optimized retardation for maximizing torque while minimizing BSFC, as discussed above. 
     While a presently preferred embodiment of the invention has been illustrated and described, it should be appreciated that principles of the invention are applicable to all embodiments and uses that fall within the scope of the following claims.