An indirect sensing technique for closed-loop diesel fuel quantity control

Despite significant advances in electronic control technology applied to diesel engines, commercially available injection sys­ tems for automotive diesel engines remain limited by the open­ loop mapplng of the injection pump. An initial calibration is relied upon to translate a fuel delivery command to an actual fuel quantity. In practice, however, these two variables may be sub­ stantially different due to the effects of mechanical wear, repair, and the wide range of operating conditions. Possible ramifications of this discrepancy are excessive exhaust smoke due to overfuel­ ing, increased fuel consumption, degraded driveability, and poor idle characteristics. The major obstacle to closing the fuel control loop is the lack of a suitable sensor for instantaneous fuel delivery from the injector. An indirect fuel delivery sensing mechanism based upon the use of the injector needle lift in conjunction with the fuel tempera­ ture is evaluated. An estimation of the injection rate characteristic is determined from real-time analysis of the needle lift signal using a high-spee d sensor processor. Integration of the rate characteris­ tic and temperature correction yields a total mass delivery estimate for use as a feedback quantity for closed-loop fuel control. Signal processing algorithms are derived from computer modeling of the injector and verified experimentally. Possible long-term decalibra­ tion due to nozzle coking is studied. Advantages and limitations of the technique are identified. BACKGROUND AND PROBLEM DESCRIPTION Although diesel engines and lnjection systems represent a mature technology, it is only ln recent years that electronic con­ trols have been successfully applied. The majority of diesel appli­ cations are still mechanicall y controlled, with no electronics involved other than the fuel shutoff solenoid valve control. The potential benefits of microprocessor-based control applied to diesel engines have been well established [Reams82, Kihara83, Martlnsons82, Trenne82, Kawai84]. However, many of the improvements made possible by advanced electronic control This work was supporled by a grant from the National Science Foundation. Electrical and Electronic Engineering Dept. California State Polytechnic Umv. San Luis Obispo, CA are dependent upon exact knowledge by the controller of the hydraulic characteristics of the injection system components. This dependency is particularly important in the case of small-displacement automotive diesel engines, which use low-cost distributor pumps which must accurately meter very small (less than 50 mm3 per injection) fuel quantities at high speeds. Major incentives for more accurate fuel control have ap­ peared in the form of recent regulatory pressures for "cleaner" diesels along with the demands of the automotive market for driv­ ing characteristics more like those of gasoline fueled engines. Gradually increasing concerns about rising gasoline costs may be expected to produce a renewed interest in automotive diesels. Available and currently envisioned electronically controlled distributer-type fuel injection pumps for automotive diesel engines generally operate with a map-based translation between command­ ed fuel quantity and actual fuel delivery. A ROM-stored multi­ dimensional map is typicall y accessed with inputs of commanded fuel volume and pump speed. A third parameter, fuel or pump housing temperature, may be additionally used to modify the table output to yield a corrected fuel control position corresponding to a given commanded fuel mass. The map and correction factor(s} are generated experimentally from pump test data, typically using a reference pump. A fully specified map of adequate resolution and valid correction factors may require the acquisition of a large number of data points on a pump test stand. There are several limitations of this open loop fuel control method. The generation of individual calibration maps for each pump, and subsequent storage of individual maps in ROM, is im­ practical in production. At best, a linear correction for pump misalignment is performed during final checkout of individual pumps, using either an external resistor network [Stumpp83] or final PROM programming procedure. However, minor machining differences, even within manufacturing tolerances, can cause no­ ticeable differences between the calibration maps of individual production pumps and the test pump. This difference is com­ pounded by the synergistic relationship between the injectors and the pump. The injectors fitted to a particular production pump may differ slightly in their flow characteristics from those used with the test pump during the master calibration, thus clumging the overall calibration. One or more injectors might also be replaced or readjusted at a later date. �--Possibly more important is the problem that the injection system often operates under conditions much different than those that existed during the master calibration tests. The calibration also changes over the course of time due to nonnal wear and cor­ rosion of pump and injector internal components, and the accumu­ lation of carbon deposits in the injector nozzle (nozzle coking). In actual service, the delivered fuel quantity may differ sub­ stantially from the mapped quantity. Possible effects of this discrepancy are excessive exhaust smoke due to overfueling, inac­ curate torque limiting, increased fuel consumption, degraded drivcability, and unstable or noisy idle characteristics. If the actual fuel delivery per injection were directly sensible, closed-loop control of the fuel quantity would be possible as a means for improving the fuel control accuracy without the need for further mechanical refinements and tighter tolerances in the pump. The major obstacle to closing the fuel control loop appears to be the lack of a suitable sensor for fuel quantity. Real-time monitoring of the actual fuel delivery per indivi­ dual injection stroke is difficult. A number of factors can be cited in relation to this technical obstacle. For a typical small displace­ ment (i.e., under two liter) diesel engine, the fuel delivery volume is very small (in the range of from 5 to 50 mm3 per stroke), and the duration very brief (on the order of 1.0 ms). The repetition rate per cylinder is typically from 5 to 50 injections per second, with line pressure fluctuating from the delivery valve opening pressure to the injection peak pressure (as high as 100 MPa) at this repeti­ tion rate. The static volume in each fuel injection line and the secondary passages of the pump may exceed the fuel delivery per injection by a large factor. Auid compressibility, inertial effects, tubing strain, and internal leakage in the pump make the process non-ideal, so that the actual fuel delivery often differs significantly from the metered plunger displacement in the pump. Mass transport in this medium is characterized by propagation of a pressure wave between the pumping chamber and the injector nozzle. Computer simulation of the injection pump hydraulics using finite difference methods is often relied upon to predict the delivered quantity and injection characteristics [0ren83, Kumar83, Shanna83]. While suitable sensors for the rate characteristic have been suggested [Bosch66, Komaroft'66, Thoma74] for use in test bench calibration of pumps, a practical sensor suitable for use during actual engine operation as a real-time feedback control device is not, to the best of our knowledge, currently available. Methods utilizing injection pulse duration in conjunction with engine spee d to estimate fuel usage have been investigated [Wolff86]. A thennal convection based fuel flow sensor has also been studied [Challen88]. The advantages of closed loop control in general are well established. Efforts to close the control loop on engine torque [Ribbens81,Aeming82,Sood 84], combustion luminescence [Bunting84], and cylinder pressure [Challen88] have been important recem contributions to diesel control technology. All of these techniques may be considered as indirect indicators of the injected fuel quantity, which for some performance metrics (i.e., emissions) is the target variable of primary importance.