Method and apparatus for controlling a crankcase scavenged two-stroke engine

Abstract: A method and apparatus are described for increasing engine braking, when a crankcase scavenged two-stroke engine (10) is operated in a deceleration fuel cut-off mode. In response to the detection of operating conditions indicating engine operation in the deceleration fuel cut-off mode, the quantity of air inducted into the engine (60) is decreased, for a predetermined period of time. After this period of time elapses, the quantity of air inducted into the engine (10) is then increased. This is preferably accomplished by regulating the degree of opening of a throttle valve (24) disposed within the engine air intake system (22). The initial closing of the throttle valve (24) restricts air flow through the engine (10) for a predetermined time, to maintain efficient catalyst operating temperatures in the engine exhaust system. During extended operation in the deceleration fuel cut-off mode, the throttle valve (24) is opened, after the lapse of the predetermined time, to increase the quantity of air inducted by the engine (10). This results in increased engine braking, due to the additional work that must be performed in compressing the larger quantity of inducted air.

Description: This invention relates to a method and apparatus for controlling a crankcase scavenged two-stroke engine, particularly during a deceleration fuel cut-off operating mode to achieve increased engine braking. It is customary to interrupt the supply of fuel to all cylinders of a four-stroke engine during a mode of engine operation commonly known as deceleration fuel cut-off. The deceleration fuel cut-off mode is generally initiated when an engine powered vehicle is decelerating, by positioning its engine output control element (throttle valve or accelerator pedal) in the position for engine idling, i.e. at the position when there is no driver demand for additional engine output. Recovery from the deceleration fuel cut-off mode typically occurs when either the engine rotational speed drops below a predetermined minimum speed near idle or the engine control element is moved from the idling position to accelerate the engine and increase output torque. The purpose of the deceleration fuel cut-off operating mode is to reduce fuel consumption and maximize the engine braking that results from frictional drag and negative torque applied to the engine by its load. In addition, the customary closing of the air throttle valve during deceleration fuel cut-off reduces exhaust emissions from engines equipped with catalytic converters. Restricting engine air flow during deceleration fuel cut-off decreases the cooling rate of the exhaust catalyst, thereby extending the period of time that the catalyst remains at efficient operating temperatures. As is well known, the structure and operation of a conventional crankcase scavenged engine differs significantly from that of a four-stroke engine, see for example, U.S. Patent Nos. 4,920,789 and 4,932,371. In a crankcase scavenged engine, each cylinder has a separate crankcase chamber into which air is inducted during a portion of the engine operating cycle. This inducted air is compressed during that portion of the engine cycle, when a crankcase chamber is decreasing in volume due to the downstroke of a piston within its cylinder, and is then transferred to the cylinder combustion chamber where it mixes with fuel for combustion. Also, a crankcase scavenged engine does not include intake or exhaust valves. Instead, inlet and exhaust ports open directly into the walls of the engine cylinders. These inlet and exhaust ports are covered and uncovered by the movement of pistons within their cylinders. As combustion is initiated, a piston moves in its downstroke, uncovering an exhaust port to release burned fuel and then uncovering an inlet port to enable the entry of a fresh charge of air, which assists in driving out the burned fuel. In crankcase scavenged two-stroke engines, roller type bearings are generally used in place of the higher friction, planer type crankshaft bearings usually present in four-stroke engines. The use of these low friction crankshaft bearings and the absence of intake and exhaust valves in crankcase scavenged, two-stroke engines can significantly reduce the frictional forces acting to brake the engine during deceleration fuel cut-off operation. This is particularly noticeable when a vehicle equipped with a crankcase scavenged two-stroke engine is operated on extended negative gradients. Here, the reduced engine braking provides inadequate deceleration, which significantly degrades the drivability of vehicles employing crankcase scavenged two-stroke engines. DE-A-39 28 406 discloses an engine brake system of a two-cycle engine for a motor vehicle, the engine having at least one cylinder, a scavenge port, an intake passage communicated with said scavenge port, a fuel injector provided for injecting fuel in the cylinder, a scavenge pump provided in the intake passage for supplying intake air to the cylinder and a bypass provided around the scavenge pump, the system comprising detector means for detecting deceleration of the vehicle and for producing a deceleration signal; a valve provided in the bypass so as to close the bypass; and means responsive to the deceleration signal for closing the valve and for stopping fuel injection from the fuel injector. The present invention seeks to provide an improved method and apparatus for controlling a crankcase scavenged two-stroke engine. According to an aspect of the present invention, there is provided a method of controlling a crankcase scavenged two-stroke engine as specified in claim 1. It has been found that for a crankcase scavenged, two-stroke engine, a relationship exists between frictional torque and the quantity of air inducted into the engine. The present invention can provide a method for regulating the quantity of air inducted by a crankcase scavenged two-stroke engine to effectuate increased engine braking when the engine is operated in a deceleration fuel cut-off mode. In a practical embodiment, this is accomplished by first detecting engine operating conditions that indicate engine operation in the deceleration fuel cut-off mode, and decreasing the quantity of air inducted into the engine for a predetermined period of time in response to these detected operating conditions. After this period of time elapses, the quantity of air inducted into the engine is then increased. By initially decreasing the quantity of air inducted by the engine, the cooling rate in the engine catalytic converter is reduced, to extend the period of time that the catalyst remains at efficient operating temperatures. After the predetermined time has elapsed, the efficiency of the catalyst will have diminished due to cooling, so the quantity of air inducted by the engine can then be increased, without significantly increasing engine exhaust emissions. This results in additional engine loading, since the crankcase scavenged engine must perform more work to compress the larger quantity of inducted air. Consequently, during extended operation in the deceleration fuel cut-off mode, this additional loading acts to increase engine braking and improve vehicle drivability. Preferably, the quantity of air inducted by the crankcase scavenged, two-stroke engine is regulated by the degree of opening of a throttle valve disposed in the engine air intake system. After detecting engine operation in the deceleration fuel cut-off mode, the throttle valve is completely closed, to restrict the quantity of air inducted by the engine. After the lapse of a predetermined time, the throttle valve is opened, at a defined rate, to its fully open position. Consequently, the engine is gradually loaded by the additional inducted air to increase engine braking smoothly to a maximum during extended deceleration fuel cut-off operation. According to another aspect of the present invention, there is provided apparatus for controlling a crankcase scavenged two-stroke engine as specified in claim 4. An embodiment of the present invention is decribed below, by way of illustration only, with reference to the accompanying drawings, in which: Figure 1 schematically illustrates a crankcase scavenged, two-stroke engine and an embodiment of control system for regulating the quantity of air inducted by the engine during deceleration fuel cut-off; Figure 2 graphically illustrates the difference in frictional torque acting on a crankcase scavenged, two-stroke engine when the engine air throttle valve is in the completely closed and wide open positions; Figure 3 is a flow diagram representative of an embodiment of routine executed by the control system of Figure 1 when detecting engine operating conditions indicating engine operation in the deceleration fuel cut-off mode; and Figure 4 is a flow diagram representative of an embodiment of routine executed by the control system of Figure 1 when regulating the quantity of air inducted by the engine during the deceleration fuel cut-off operating mode. Referring to Figure 1, there is shown schematically a crankcase scavenged, two-stroke engine 10 having three cylinders CYL1, CYL2, and CYL3. Each engine cylinder is individually fuelled by a conventional pneumatic direct fuel injection system, which includes selectively operable, solenoid actuated fuel injectors 12, 14, 16, and associated conduits, namely fuel rail 18 and air rail 20 which deliver fuel and compressed air for injecting metered quantities of fuel into the cylinders. Engine 10 further includes an air intake manifold 22, with a throttle valve 24 therein, for controlling the quantity of air inducted by the engine. As illustrated in Figure 1, the operation of engine 10 is controlled by a conventional electronic control unit (ECU) 26, which derives input signals from several standard engine sensors. The ECU 26 includes a central processing unit, random access memory, read only memory, analog-to-digital and digital-to-analog converters, input/output circuitry, and clock circuitry. The ECU 26 is supplied with a POS input signal which indicates the rotational position of engine 10. The POS input can be derived from a standard electromagnetic sensor 28, which is capable of detecting the passage of teeth on wheel 30 as it is rotated by engine 10. A MAF input signal indicates the mass air flow into engine 10 and can be obtained by any known means, for example a mass air flow (MAF) sensor 32 disposed in the engine intake manifold 22. In addition, a temperature sensor 34 provides the ECU 26 with an input signal TEMP related to the engine coolant temperature; and a potentiometer 36 is employed to provide an input signal PED, which indicates the position of accelerator pedal 38. During normal operation, the ECU 26 looks up the quantity of fuel to be injected into each cylinder from a table stored in memory, based upon the position of the accelerator pedal 38 (as indicated by the PED input signal). At appropriate rotational positions of the engine 10 determined from the POS input signal, the ECU 26 generates pulsed signals F1-F3 and A1-A3, for actuating respectively the fuel and air solenoids (not shown) within injectors 12, 14, and 16. The width of the fuel pulses F1-F3 determines the metered quantity of fuel per cylinder (FPC) injected into the respective engine cylinders. The air pulses A1-A3 are timed in relation to the fuel pulses F1-F3 to supply the injectors with the appropriate volume of compressed air to drive the metered fuel into the engine cylinders. To achieve the correct cylinder air-fuel ratio, ECU 26 computes a desired mass air flow in a conventional fashion from a look-up table based on current engine speed and the amount of fuel injected per cylinder. The actual mass air flow to the engine, as indicated by the MAF signal, is then controlled to the desired value, in a closed loop fashion, by opening or closing throttle valve 24. The ECU 26 accomplishes this by generating an appropriate throttle position output signal TP to drive a stepping motor 40, which is mechanically coupled to the throttle valve 24. Many of the additional sensor, actuator and ECU input and output signals that are generally present in a conventional engine control system have not been specifically shown in Figure 1. It is customary to interrupt completely the injection of fuel to all cylinders of a four-stroke internal combustion engine during the deceleration fuel cut-off mode. Entry into the deceleration fuel cut-off operating mode is generally initiated when an engine powered vehicle is decelerating, with its engine output control element (throttle valve or accelerator pedal) positioned for engine idling, with no driver demand for additional engine output. Recovery from the deceleration fuel cut-off mode typically occurs when either the engine rotational speed drops below a predetermined minimum speed near idle or the engine control element is moved from the idling position to accelerate the engine and increase output torque. Referring now to Figure 2, there is shown a graph representative of the measured difference in frictional torque acting on a crankcase scavenged, two-stroke engine as a function of engine speed when the engine air throttle valve is in the completely closed and the wide open positions. It is believed that the illustrated difference in frictional torque results from the additional work performed by the engine in compressing the larger quantities of air inducted into the engine crankcase chambers when the throttle valve is wide open. As applied to the engine 10 illustrated in Figure 1, this embodiment first detects engine operating conditions, which indicate that engine 10 is operating in the deceleration fuel cut-off mode. In response to these detected operating conditions, the quantity of air inducted into engine 10 is decreased, for a predetermined period of time. After this period of time elapses, the quantity of air inducted into the engine 10 is then increased. Preferably, the quantity of air inducted by the engine 10 is regulated by the degree of opening of the throttle valve 24 disposed in the engine air intake system 22. After detecting engine operation in the deceleration fuel cut-off mode, the throttle valve 24 is completely closed to decrease the quantity of air inducted by engine 10. Assuming that the exhaust system of engine 10 includes a catalytic converter (not shown), the closing of throttle valve 24 decreases engine air flow, thereby extending the period of time that the catalyst remains at efficient operating temperatures. After a predetermined time has elapsed, the efficiency of the catalyst will have diminished due to cooling, so the quantity of air inducted by the engine can be increased without significantly affecting engine exhaust emissions. Throttle valve 24 is then opened, at a defined rate, to its fully open position. As a result, the load on engine 10 is gradually increased by the additional inducted air, to increase engine braking smoothly to a maximum, during extended deceleration fuel cut-off operation. Referring now to Figure 3, there is shown a flow diagram representative of the steps executed by ECU 26 in detecting engine operating conditions, which indicate the initiation of the deceleration fuel cut-off mode of engine operation. After engine startup, all counter, flags, registers, and timers within the ECU 26 are initialized to the appropriate values. After this initialization, the deceleration fuel cut-off (DFCO) enable routine 42 of Figure 3 is repeatedly executed as part of the main looped engine control program stored within ECU 26. The routine is entered at point 42 and proceeds directly to step 44. At step 44, a decision is made as to whether the engine coolant temperature, as indicated by the TEMP input signal, is less than a predetermined temperature DFCOTEMP (for example, 70°C), below which entry into deceleration fuel cut-off is prohibited because the engine has not adequately warmed up. If TEMP is not less than DFCOTEMP, the routine proceeds to step 46; otherwise, it proceeds to step 48. When the routine proceeds to step 46, another decision is made as to whether RPM, the rotational speed of the engine, is less than a predetermined speed DFCORPM. A value for RPM can be derived from the POS input signal to ECU 26, by counting the number of equally spaced tooth pulses occurring in a specified time period. When the engine speed is less than DFCORPM (for example, 1500 revolutions per minute), the engine is so near idle that substantially no benefit would be realized by enabling deceleration fuel cut-off. Thus, if RPM is less than DFCORPM, the routine proceeds to step 48. However, if RPM is not less than DFCORPM, the routine then proceeds to step 50. At step 50, a decision is made as to whether the engine is operating at idle. For the embodiment illustrated in Figure 1, this is accomplished by determining whether the PED input signal is equal to zero (i.e. the accelerator pedal is in the idle position). If PED equals zero, entry into deceleration fuel cut-off is not prohibited and the routine proceeds to step 54. If PED does not equal zero, indicating driver demand for engine output above idle, the routine proceeds to step 52. At step 52, the quantity of fuel injected per cylinder (FPC) is checked to determine if it exceeds a predetermined amount DFCOFPC (for example, 2 milligrams per cylinder per cycle). If FPC is not greater than the relatively small amount represented by DFCOFPC, the engine is considered to be substantially at idle, even though the accelerator pedal is not positioned exactly for idling, and the routine proceeds to step 54. However, if FPC is greater that DFCOFPC, the engine is considered not to be at or near idle and the routine proceeds to step 48. When the routine proceeds to step 54, a decision is made as to whether the engine rotational speed RPM is less than the predetermined engine speed DFCORPM used at step 46, plus a hysteresis factor HYST (for example, 200 revolutions per minute). If RPM is less than DFCORPM + HYST (in this example, 1700 revolutions per minute), the routine proceeds to step 56, where the routine is exited. However, if RPM is not less than DFCORPM + HYST, the routine proceeds to step 58. At step 58, a decision is made as to whether a value of the TIMER exceeds a predetermined time DFCOTIME (for example, 500 milliseconds). If TIMER is greater that DFCOTIME, the routine proceeds to step 60, where a DFCOFL flag is set to a value of one, to indicate that engine operating conditions exist for initiating the deceleration fuel cut-off mode, and the routine is then exited at point 56. However, if the value of TIMER does not exceed DFCOTIME, at step 58, the routine proceeds to step 62, where the value of the TIMER is incremented, prior to exiting the routine at point 56. If the routine proceeds to step 48 from either of steps 44, 46, or 52, the DFCOFL flag and the TIMER are set to values of zero, indicating that the engine operating conditions necessary for initiating deceleration fuel cut-off do not exist. After setting these values, the routine is exited at point 56. In summary, when the engine operating conditions are such that (1) the coolant temperature is less than DFCOTEMP; (2) the engine rotation speed is not less than DFCORPM + HYST; (3) either the accelerator pedal is at the idle position, or the quantity of fuel injected per cylinder per cycle is not greater DFCOFPC; and (4) these conditions have not changed for a period of time defined by DFCOTIME, the routine will proceed to step 60, where the DFCOFL flag is set, to initiate operation in the deceleration fuel cut-off mode. When the engine operating conditions are such that either (1) the coolant temperature is not less than DFCOTEMP; (2) the engine rotational speed is not less than DFCORPM; or (3) the accelerator pedal is not at idle and the fuel injected per cylinder per cycle is is greater than DFCOFPC, the routine proceeds to step 48, where the DFCOFL flag is set to initiate recovery from deceleration fuel cut-off, when the engine is operating in that mode. When operating conditions exist for initiating the deceleration fuel cut-off mode, except when engine rotational speed falls within the range between DFCORPM and DFCORPM + HYST (step 44 in combination with step 54), the routine is exited without setting the DFCOFL flag at step 60, or incrementing the TIMER at step 62. This prevents oscillations that would otherwise occur between setting the DFCOFL flag at steps 48 and 60 if the engine rotational speed swings slightly above and below the defined DFCORPM speed. When the DFCOFL flag is set to one, the ECU 26 interrupts the injection of fuel into the cylinders of engine 10. This may be accomplished by either masking or gating the fuel and air output signal pulses (such as, F1-F3 and A1-A3), so they do not arrive at their respective fuel injectors. Alternatively, the widths of these pulses could be set to a value of zero, which will effectively eliminate them. It has been found advantageous to interrupt the air pulses as well as the fuel pulses to pneumatic type injectors, during deceleration fuel cut-off. This prevents the air pulses from drying out the residual fuel remaining in the injectors, after the fuel pulses are interrupted, and assures that the proper quantity of fuel will immediately be injected into the cylinders, when the DFCOFL flag is set to zero and the fuel pulses are again applied to the injectors. Referring now to Figure 4, there is shown a flow diagram representative of an embodiment of routine executed by ECU 26, in regulating the quantity of air inducted by engine 10, during deceleration fuel cut-off operation. The air regulation routine 64 represents a part of the main looped engine control program repeatedly executed after engine 10 is started. The routine is entered at point 64 and immediately proceeds to step 66. At step 66, a decision is made as to whether the DFCOFL flag is equal to one. If the DFCOFL flag has been set to a value of one, this indicates that the engine is operating in the deceleration fuel cut-off mode in response to the detected engine operating conditions illustrated in the routine of Figure 3. If DFCOFL equals one, the present routine proceeds to step 68; otherwise, the routine passes to step 70. When the routine passes to step 70, from decision step 66, the value of a timer ATIMER is set to zero, prior to exiting the routine at point 78. When the routine passes to step 68, from step 66, a decision is made as to whether a predetermined period of time, represented by ATIME (for example, 9.7 seconds), has been exceeded by the timer ATIMER. If the value of ATIMER is not greater than ATIME, the routine proceeds to step 72. When the value of ATIMER exceeds ATIME, the routine proceeds to step 80. If the routine proceeds to step 72 from decision step 68, the value of ATIMER is incremented prior to passing to step 74. Then, at step 74, a decision is made as to whether the throttle valve 24 is in the completely closed position. The ECU 26 maintains a value representative of the current position of the throttle valve 24, based upon the past movement induced by the output throttle position signal TP. If the throttle valve 24 is completely closed, the routine exits at point 78. However, if throttle valve 24 is not in the completely closed position, the routine passes to step 76, where the appropriate TP output signal is generated to close throttle valve 24 completely, before exiting the routine at step 78. When the routine proceeds to step 80 from the step 68, an additional decision is made as to whether or not the throttle valve 24 is in its wide open position. If throttle valve 24 is in the wide open position, the routine exits at step 78. However, if throttle valve 24 is not in its wide open position, the routine passes to step 82. At step 82, the appropriate output signal TP is generated to open throttle valve by an amount designated as ΔTP (for example, 0.4 % of the available movement between the completely closed and wide open positions), without exceeding the wide open limit position. Thus, the rate at which the throttle valve 24 is opened is defined by the amount of opening ΔTP, divided by the time between consecutive executions of the routine of Figure 4 (for example, approximately every 8 milliseconds). In summary, the air regulation routine of Figure 4 determines whether the engine is operating in the deceleration fuel cut-off mode, based on the value of the DFCOFL flag (step 66), which is set in accordance with the detection of the specific engine operation conditions, carried out in the routine of Figure 3. When operation in the deceleration fuel cut-off mode is detected, the throttle valve 24 is moved to its completely closed position for a predetermined period of time, represented by ATIME (step 68). After the predetermined period of time has elapsed, the throttle valve 24 is gradually opened to the wide open position at a rate defined by the amount ΔTP (step 82) and the time between consecutive executions of the air regulation routine. In the above described embodiment, the amount of fuel injected into the engine was determined by the movement of the accelerator pedal 38, with the quantity of inducted engine air being controlled by ECU 26, to achieve a desired air-fuel ratio. This embodiment would be equally applicable to an engine control system where the quantity of air inducted by the engine is determined by the movement of an accelerator pedal, with the amount of fuel injected being controlled by ECU 26 to achieve the desired air-fuel ratio. For this alternative, the quantity of air inducted by the engine during deceleration fuel cut-off could be regulated by using a stepping motor under the control of ECU 26 to open and close the throttle, just as illustrated in Figure 1. Alternatively, for applications where the engine throttle valve is mechanically linked to the accelerator pedal, a large air passage and air bypass valve can be provided to bypass the throttle valve, and regulate the quantity of inducted air, when the throttle valve is closed to an idle position.

Claims: A method of controlling a crankcase scavenged two-stroke engine, which engine comprises an air intake system (24,40) for adjusting the quantity of air inducted by the engine, and a fuel supply system (12-16) adapted to operate in a deceleration fuel cut-off mode when the engine is decelerating under defined operating conditions; the method comprising the steps of detecting the defined operating conditions in which the fuel supply system is in the deceleration fuel cut-off mode; and during the fuel cut-off mode (a) decreasing the quantity of air inducted by the engine for a predetermined period of time when the defined operating conditions are detected, and (b) increasing the quantity of air inducted by the engine after the predetermined period of time has elapsed. A method according to claim 1, wherein the quantity of air inducted by the engine is decreased by closing an air intake throttle valve (24) and is increased by opening the throttle valve. A method according to claim 2, wherein the throttle valve is completely closed when the fuel supply system is in the fuel cut-off mode and, after the predetermined time has elapsed, the throttle valve is opened at a defined rate to a fully open position. Apparatus for controlling a crankcase scavenged two-stroke engine, which engine comprises an air intake system (24,40) for adjusting the quantity of air inducted by the engine, and a fuel supply system (12-16) adapted to operate in a deceleration fuel cut-off mode; the apparatus comprising sensing means (28,36) adapted to sense defined engine operating conditions which cause the fuel supply system to be in the deceleration fuel cut-off mode; and regulating means (24,40) operative during the fuel cut-off mode (a) to decrease the quantity of air inducted by the engine for a predetermined period of time when the defined operating conditions are detected, and (b) to increase the quantity of air inducted by the engine after the predetermined period of time has elapsed. Apparatus according to claim 4, wherein the regulating means is adapted to reduce the quantity of air inducted by the engine by closing an air intake throttle valve (24) and to increase the quantity of air inducted by the engine by opening the throttle valve. Apparatus according to claim 5, wherein the regulating means is adapted to close the throttle valve completely when the fuel supply system is in the fuel cut-off mode and, after the predetermined time has elapsed, to open the throttle valve at a defined rate to a fully open position.

Inventors: BUSLEPP KENNETH JAMES, REINKE PAUL EDWARD, TROMBLEY DOUGLAS EDWARD, BUSLEPP, KENNETH JAMES, REINKE, PAUL EDWARD, TROMBLEY, DOUGLAS EDWARD
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1992
Main classification: F02D41
IPC codes: F02B75, F02D41, F02D9