Patent Grant
7254478
The present invention relates to the injection of secondary reaction air into the exhaust manifold of an internal combustion engine for emission control purposes, and more particularly to a method of estimating the magnitude of secondary airflow.
A known technique for reducing hydrocarbon emissions in internal combustion engines involves injecting secondary air into the engine exhaust manifold. This technique, usually referred to as Air Injection Reaction or simply AIR, is particularly important during engine warm-up following a cold start. Under such conditions, the air/fuel mixture supplied to the engine cylinders is relatively low (rich) to ensure stable combustion, and the injected air promotes rapid light-off of the exhaust system catalytic converter to enable early initiation of closed-loop fuel control. The secondary air promotes light-off of the catalytic converter by reacting with unburned hydrocarbons in the exhaust gas to produce heat, and by raising the air/fuel ratio in the exhaust system to an optimum level for converter light-off. In practice, however, it is difficult to optimize the injection of secondary air because the delivered airflow varies considerably depending on environmental factors such as ambient temperature and barometric pressure. This makes it difficult to take full advantage of AIR and can also lead to incorrect diagnosis of an AIR failure under conditions which severely restrict AIR airflow. Accordingly, what is needed in a method of accurately and reliably estimating AIR airflow, both for control and diagnostic purposes.
The present invention is directed to an improved method of estimating the secondary reaction airflow supplied to an internal combustion engine by modeling airflows in an AIR pump, an AIR valve, and a plenum coupling the pump to the valve. The pump flow is modeled based on the pump speed (or pump motor voltage), the pump inlet air temperature and the pressure ratio across the pump, and the valve flow is modeled based on the pressure ratio across the valve and the valve inlet air temperature. The plenum pressure used in the pump and valve pressure ratios is modeled based on the net plenum airflow and the plenum air temperature. The estimated AIR airflow is useful for managing the air/fuel ratio in the engine exhaust system and enhancing the reliability of AIR diagnostics.
Referring to
Alternatively, the pump 24 may be driven by engine 10; in such case, the AIR system additionally includes either an electrically activated clutch for selectively activating pump 24, or an electrically activated diverter valve for selectively coupling the plenum 26 to manifold 20 or atmosphere. However, the illustrated arrangement is preferred because it facilitates speed control of the pump 24 for the purpose of regulating the flow of secondary air. This may be particularly important during an engine warm-up period following a cold start, since the catalytic converter 16 lights-off more rapidly when the air/fuel ratio of the exhaust gas supplied to its inlet is maintained in a given range (which is typically leaner than the engine combustion products during cold start and warm-up).
Since ECM 32 also manages the engine air/fuel ratio, the desired flow of secondary air can be easily computed, and ECM 32 activates motor 30 at a corresponding speed. However, the actual secondary airflow (that is, the flow through SOV 28) is subject to significant variability depending on conditions that influence the backpressure in exhaust manifold 12, and it is difficult to verify that the desired flow is actually being achieved. The present invention addresses this issue with a model-based method of accurately estimating the secondary airflow (F28) through SOV 28, based on known or easily determined parameters such as the pump speed PMP_SPD (or the motor voltage), the air temperature T22 in pump inlet 22, the air temperature T26 in AIR plenum 26, and the atmospheric pressure Patm.
The block diagram of
The Pump Flow Calculation blocks 54, 58, 62, 66 develop an estimate F24_std of the airflow through pump 24. As indicated, the block 58 incorporates a look-up table based on empirically determined data (under standard temperature and pressure conditions) for the flow through pump 24 as a function of PMP_SPD and a ratio PR24 of the pump inlet and outlet pressures. The pump inlet pressure is the atmospheric pressure Patm on line 48, while the pump outlet pressure is the modeled AIR plenum pressure P26 on line 52. The pressure ratio P24 is computed in block 54 as shown. The blocks 62 and 66 compensate the output F24_std of block 58 for actual pressure and temperature conditions of the air entering pump 24. The block 62 develops a pump air density correction ADCp based on inlet pressure Patm and inlet temperature T22, relative to the standard pressure and temperature, and the correction ADCp is multiplied by F24_std at block 66 to produce a density corrected pump flow estimate F24 on line 68.
The System Flow Calculation blocks 70, 74, 78, 82 develop an estimate F28_std of the airflow through SOV 28. As indicated, the block 74 incorporates a look-up table based on empirically determined data (under standard temperature and pressure conditions) for the flow through SOV 28 as a function of a ratio PR28 of the inlet and outlet pressures of SOV 28. The inlet pressure is the modeled AIR plenum pressure P26 on line 52, while the outlet pressure is the exhaust manifold pressure P12 on line 50. The pressure ratio P28 is computed at block 70 as shown. The blocks 78 and 82 compensate the output F28_std of block 74 for actual pressure and temperature conditions of the air entering SOV 28. The block 78 develops a valve air density correction ADCv based on the modeled inlet pressure P26 on line 52 and the inlet temperature T26 on line 44, relative to the standard pressure and temperature, and the correction ADCv is multiplied by F28_std at block 82 to produce a density corrected system flow estimate F28 on line 84. As indicated above, ECM 32 uses this flow estimate to verify that the desired secondary airflow is being achieved; this is useful both for control purposes (that is, for regulating the speed of motor 30 or compensating the quantity of fuel supplied to engine 10 to achieve the optimal air/fuel ratio at the inlet of catalytic converter 16) and diagnostic purposes (that is, for verifying correct operation of the AIR system 18). Other uses of the estimated secondary airflow F28 may occur as well.
The blocks 86, 92 and 94 use the flow estimates F24 and F28 and the AIR plenum temperature T26 to update the modeled AIR plenum pressure P26 on line 52. The summation block 86 subtracts the system flow estimate F28 from the pump flow estimate F24 to form the net plenum flow estimate F26 on line 88, and the blocks 90 and 92 convert the flow estimate F26 to a corresponding change in plenum pressure (P26_DOT) based on the equation:
P26_DOT=(F26*R*T26)/V
where R is a gas constant for air, and V is the volume of AIR plenum 26. The plenum temperature T26 is applied to block 90, and the constant Kair_plenum of block 90 includes the terms R and V. Finally, the integrator block 94 integrates P26_DOT over the fixed update rate of ECM 32 to form the new estimate of the modeled plenum pressure P26 for application to the blocks 54, 70 and 78.
While the present invention has been described in reference to the illustrated embodiments, it is expected that various modifications in addition to those mentioned above will occur to those skilled in the art. Thus, it will be understood that methods incorporating these and other modifications may fall within the scope of this invention, which is defined by the appended claims.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3785152 | Pozniak et al. | Jan 1974 | A |
| 3872666 | Bentley | Mar 1975 | A |
| 5845627 | Olin et al. | Dec 1998 | A |
| 6155043 | Zhang et al. | Dec 2000 | A |
