This invention relates to an exhaust gas recirculation (EGR) system for use in a diesel engine for recirculating a controlled rate of exhaust gases to the engine and, more particularly, to such an EGR system employing an electronic control unit adapted to calculate a target EGR ratio based upon engine operating parameters for controlling the rate of exhaust gases recirculated to the engine so as to obtain the calculated target EGR ratio.
In order to minimize emission of noxious pollutants discharged from a diesel engine to the atmosphere, it is the current practice to suppress combustion by recirculating a controlled rate of exhaust gases to the engine through an EGR passage having therein an EGR valve and connecting the engine exhaust passage to the engine intake passage downstream of the throttle valve. The rate of exhaust gases recirculated to the engine, which has a significant effect on both emission of nitrogen oxides and production of carbon fine particles, is determinative on not only the position of the EGR valve but also the position of the throttle valve across which a pressure differential exists in aid of introducing exhaust gases into the intake passage from the EGR passage. For example, the rate of exhaust gas flow through the EGR passage increases as the EGR valve moves in an opening direction for the same throttle valve position or as the throttle valve moves in a closing direction for the same EGR valve position. The position of the throttle valve, which also determines the rate of air flow to the engine, should be controlled properly to maintain optimum engine output performance in accordance with engine operating conditions.
Exhaust gas recirculation (EGR) control systems are well-known which involve an electronic control unit for providing accurate EGR ratio open-loop control. Such an electronic control unit calculates a target value for the EGR ratio meeting with requirements relating to engine output and exhaust performances as close as possible based upon engine operating parameters such as engine speed, accelerator pedal depression (indicated by fuel injection pump control sleeve or control rack position), fuel injection timing, engine coolant temperature, engine oil temperature, and the like and controls the EGR valve and throttle valve positions to obtain the calculated target EGR ratio. Such EGR ratio open-loop control has the distinct advantage in extremely fast response to engine operating condition changes.
In case where a deviation occurs between the calculated target EGR ratio value and the actual EGR ratio requirement due to errors in measurement in making and assembling engine parts such as the EGR valve and the throttle valve, changes in the engine part characteristics with the passage of time caused by mechanical wear and accumulated carbons on the engine parts, and the like, however, the EGR open-loop control system cannot correct the target EGR ratio value for the deviation. In addition, with an emission control device such as a soot collector located in the engine exhaust system for purifying engine exhaust emissions, the EGR open-loop control system cannot provide accurate EGR ratio control due to exhaust pressure changes caused by soot collected in the emission control device.
The present invention provides an improved and novel exhaust gas recirculation control system which open-loop controls the EGR ratio in accordance with a target EGR ratio value calculated based upon engine operating parameters and which, each time the vehicle travels a predetermined distance, calculates an actual EGR ratio value based upon measurements of the rate of air flow to the engine and the rate of exhaust gases recirculated to the engine and corrects the calculated target EGR ratio value for the deviation between the actual and target EGR ratio values, thereby eliminating the limitations and drawbacks inherent in previous EGR open-loop control systems.