Ion-current combustion control for compression-ignition engines

Abstract

FUTURE STRINGENT DIESEL EMISSION STANDARDS REQUIRE A drastic reduction in tolerance areas in the particulate matter and nitrogen oxide limits. A control system, which periodically calibrates the ageing diesel fuel injector and controls, in real-time, the start-of-combustion (or location of the 50% Mass Fraction Burned) in diesel and Controlled-Auto Ignition (CAI) petrol engines, becomes an invaluable asset. The costs of implementing on-board closed-loop control obviates the need for vehicle servicing for this purpose. Optimum fuel consumption and emissions will result which are beneficial both to the customer and to the environment in the long term. The system investigated here is based on ionisation sensors, which are currently used in high-end, spark-ignited petrol engines to detect misfire and knock. This thesis focuses on the use of Ion-Current technology for combustion control in petrol and diesel compression-ignition engines. The cycle-to-cycle fluctuations of the Ion-Current signals are studied with a transparent engine. Results show that for conventional diesel combustion, averaging of the Ion-Current signals is required, while in the case where a hulk combustion takes place i.e. for homogeneous charge compression-ignition diesel or CAI petrol engines, both direct and averaged Ion-Current signals give useful results. The averaged Ion-Current signals strongly correlate with the averaged heat-release rate signals for diesel engines in all operating points on the engine map i.e. when varying the exhaust gas re-circulation, common rail pressure, injection timing and dwell time. This is also the case for CAI petrol engines for variations in air mass, duration of injection, Pilot/Main-Injection ratios, swirl and exhaust valve timing. If soot is present on the glow-plug and cannot be removed by glowing, its effect can be filtered out by multiplying the inverse soot characteristic in the frequency domain. The effect on the ignition delay by using different diesel fuel qualities is also presented here. By varying the injection timing of low-cetane fuels, a NOx reduction of up to 40% is obtained without the need of any extra measures. The diesel fuel injector can be calibrated periodically by measuring the injected fuel quantities and by controlling the actual injected quantity. The total fuel quantity can be calibrated by integrating the Ion-Current signal (for quantities above 15mm3) or in the frequency domain. Moreover it is possible to calculate the total fuel injected within 1mm3, by using multi-layer perceptron artificial neural networks for normalised Ion Current signals. Pilot-Injection quantities (below 1mm3) can be calculated indirectly by detecting the start of the main combustion. Using neural networks, the Pilot Injection quantity can be calculated with an accuracy of 0.66mm3. Since it is possible to calibrate both large and small injection quantities, the diesel injector characteristic can be calibrated at regular intervals to keep performance and emissions under control. The location of the 50% mass fraction burned can be calculated with less than 1° crank angle error, by using a threshold method or artificial neural networks. The variation of emissions in CAI engines is studied and a linear relationship to the actual location of the 50% Mass fraction Burned is obtained.